The Stovebolt Site Forums The Shop Area Engine Shop Rochester B running rich

I rebuilt the Rochester B on my 235 a few years ago, and it has very little cumulative run time. Now that I’ve done a decent amount of running the engine, I can tell it’s running rich. I can smell it, I can see it on the plugs.

I know the real answer here is to ditch the Rochester and I do have the proper Carter YF waiting in the wings to be rebuilt. But I have plenty enough other tasks to keep me busy and the engine is running well in spite of the sub-optimal carb.

Are there any quickie tricks a guy can use to get by for a while? This carb doesn’t seem to be visibly warped. Can I cheat the float drop and/or setting a bit? Any other easy function checks I can do?

(1) Compression test
(2) Ignition test (dwell, timing, and if an electronic whizbang has been installed, upgrade to alternator)
(3) Check that the flange vacuum passages on the B are open to vacuum

Then install the Carter

Jon

No whizbangs in the system. I do have a dwell meter and that’s a good reminder to get it out.

#3 was an issue some time ago. I think I actually might have punched my own hole in the gasket after getting frustrated trying to find the right one.

Likely that the power piston is stuck.

I’m familiar with what the power piston is...but how would a guy diagnose it and if it’s faulty....correct it?

JW there are a few things that are very likely to wreck your power piston. Ethanol-laced gas, a warped air horn/float bowl (or anything that causes a lack of vacuum), the wrong strength of spring (really common on those that gone through the rebuilder mill a few times because the same parts do not get matched with the same carburetor) to begin with. The best way to diagnose the problem is to remove the air horn and and make certain the piston moves freely in the bore. If it does, then make certain your carb isn't warped to the point vacuum is being lost. You can see what I posted a while back about straightening: https://tinyurl.com/8pydzfd6

There's not much I can add that hasn't been discussed before in this forum about this carburetor. My 2 cents.

I have seen that the power-piston well can be worn to a slight taper, some wells have ridges, some wells were pitted and some wells were scratched by containments. The lower and upper edges of the power piston (annular rings?) can wear a little but form a sharp edge like a honed knife (you can feel the edge with fingers). After cleaning and fine polishing sometimes the piston moves freely again but I suspect this is a short term solution. The only way to find out is to run the rebuild on an engine. My take on a well working carburetor is that the piston-well wall needs to be straight and smooth and the power-piston to be at the original diameter. I doubt there is an economical way to bore, bush and hone the well and turn a new piston. The better alternative is to find a carburetor that has less wear.

Have we mentioned carb base gasket having the correct slot to line up with the vacuum feed port to the power piston?

I've learned ethanol gasoline is incompatible with certain soft metals...including brass (the power piston) and some pot metals, aluminum, zinc (the air horn) and copper. Ethanol sets up corrosion and as I understand will cause some dissimilar metals to bond in the corrosion process...which obviously spells doom for the whole power circuit in the Rochester. From what I was led to believe when discussing this with several folks, unless you made the sleeve and the power piston out of 316 stainless you don't stand much chance of ever making the original design work very long today.

OP stated he rebuilt the carb "a few years ago, and it has very little cumulative run time". I was in the same situation and rebuilt it again. The power piston was OK - I just needed to clean off the white powdery residue on it.

I think that these things need to be run regularly to keep them from getting stuck. Maybe something like Sta-bil 360 would help.

I pulled it apart and degunked it before my recent round of work. The piston is not seized or stuck. It moves, just wondering if it moves freely enough.

It is very likely there isn't enough vacuum to lift the piston off of the valve. There is a spring which pushes the piston down if there isn't enough vacuum. I'm talking about direct manifold vacuum here and that vacuum goes through a passage in the carburetor to the top of the power piston. When there's not sufficient vacuum and the piston is pushed down by that spring, the rod at the bottom of the power piston pushes the spring loaded ball at the base of the mast open and this floods that area with fuel. Fuel rushes in there and is available for the carburetor to suck up through the power restriction hole. When the power circuit is open, the carburetor runs much richer, and it can even do this at idle. More on that in a moment. The whole idea is that when the engine is under load, the manifold vacuum will drop. Naturally the engine will need more fuel when it is under load, however lacking manifold vacuum, it isn't easy to get enough fuel without help. That's what the power system is supposed to do. But...when your Rochester gets warped, then vacuum passing from the throttle body and up through the float bowl is lost where the float bowl connects to the air horn. How much vacuum needed to keep the power piston off of the power valve depends entirely on the strength of the spring in there. I've seen some which only needed about 7 or 8", more of them which needed roughly 9" and others which needed about 11". Honestly 11" is pretty doggone extreme. You may only have 15" or so at idle, so any bit of vacuum lost anywhere in your carburetor will let the power circuit be open...at idle. This is what causes the Rochester B to run rich all the time. The power piston isn't doing anything for you.

Please take a look below at the first image. In it 1 is the float/float pin, 2 is the main metering jet, 3 is the power valve cap, 4 is the bowl mast, 5 is the power piston assembly, 6 is the vacuum passage for your power piston (with a brass tube I added), 7 is the main nozzle, 8 is the vacuum passage lip for the idle circuit, 9 is an internal vent with an air bleed hole above it. And please take a look at the second image. That vacuum passage as you will see has no tube coming up. When I was doing all the study work on the Rochester B I decided it might help to add the brass tube, but unless the air horn and float bowl seal well, I'm not sure it matters. If there is any warpage in your air horn, it is very likely you're losing vacuum right where that passage is. I should also mention there is a vacuum break in the bore of the air horn. It is there to relieve any vacuum that might build up at the piston. If the vacuum break passage wasn't there, fuel vapor from the float bowl could be sucked around the piston and through the vacuum passage and into the manifold...and this would result in your carburetor running richer. But (a large "but" in the case of the Rochester B), you have to have a perfectly sealed air horn/float bowl for this to happen. And in my observation that's pretty rare unless the carburetor has been straightened.

And in the third image below you can see how fuel enters. Yellow arrow is your main jet. Red arrow is the power valve. If it is open you're getting much more fuel than you need. And the fourth image shows the power restrictor...if you look hard you can see it. It is a hole which allows more fuel to run up the mast along with what the main jet has allowed in there. So you can appreciate the need to keep that power valve closed as much as possible.

Hope this is helpful. Good luck!

If you look above at the second image, you'll see how the Rochester B came from the factory. The hole to the left and below the power piston is the vacuum passage...and I suspect you'll understand why I was trying anything to increase vacuum transfer with my added tube. There is no screw there to keep that part of the air horn pushed down on to the float bowl and you can appreciate that any bit of warping of the body (which is exacerbated by only having 4 screws holding this all together) will make that all worse. 2 images are attached here. This is what I discovered when I took one of the carburetors apart...2 gaskets had been used. A common trick which can make things worse. And as you can see, one of them had been sucked out of place leaving a special little vacuum leak between those two gaskets and also back into the float bowl. In the second image you can see where the power piston vacuum passage should be sealed. And it might have worked if there hadn't been about .007" space between the 2 gaskets leading back to that area. But additionally, every time this vehicle went around a corner, it was likely to slosh fuel up and into the bore.

Jon, thank you for your detail regarding the Rochester B's. I appreciate the time you spend putting the posts together.

You're welcome. About 3 years ago I wrote and posted several articles about this (and some about the Carter YF) however when Photobucket died the digital images no longer could be accessed and I haven't moved them to another hosting service. I need to do that.

Yeah, Jon G....that was really extensive and insightful.

Been a while since I started this thread. Good time for a status update and some follow up question. Decided to take Carbking’s advice and double check the ignition system was all squared away. That was a quite a little journey in itself but long story short....

New set of plugs, new coil, dwell is now set at 32 degrees. Timing is set precisely to the flywheel ball. The ignition system is as squared away as I can get it. Engine pulls a steady 22 in of vacuum at idle. Set the idle down to 650ish.

Truck is idling as well as it ever has. Still have some valve noise I can’t quite explain but I think that actually got a bit better after I did the “running engine with diesel added to crankcase” flush.

I can’t say for certain that it’s still running rich. No black smoke (plenty of oil smoke still). I don’t get as much reaction out of the idle mixture screw as I think I should. But I don’t know what “normal” is for a B either.

That was a lot of words...onto the current symptom and my questions....

Under acceleration, and/or RPMs above some level (I’m gonna guess 1500-2000 and up), you get what I’d describe as a miss. I did some extended running and fiddling/tuning at 1000 RPM. The miss/shudder is not present there. I even validated that by pulling one plug wire at a time. Each cylinder reacted the same...run just a little rougher and RPM dropped by about 50. I should have replicated that test at a higher RPM but didn’t think of it.

Some theories below that I need advice to test. Also welcome any other theories.

1) Some sort of ignition related miss - Guess I could run the engine in the dark and look for arcing or sparks? BTW, the vac advance does function.

2) A fuel-related miss. I can’t discern a transitional stumble between the various carb circuits. The accelerator pump gives a good shot. The miss is present both during the acceleration and as you hold it at an elevated RPM. (Possible clue: I do have a leak at the throttle shaft. You won’t see it while running, but definitely will if you open the throttle not running)

3) A weak valve spring? Is there an easy way to test that theory or just have to go measuring them?

If the engine sat for awhile before you got it running when you got the truck, I would suspect a valve related issue.

Perhaps your carb is now running lean and your are getting "spitting". My carb just picked up that habit. It will run better (stops spitting and noticeable improvement in smoothness and power) if you close the choke partially.

The carb is on the workbench at the moment. I found that the main metering jet was partially plugged and cleaned it with a welding tip cleaner. Blowing air through it was not sufficient.

Try running at about 1500 to 2000 rpm. Pull the choke knob. Does it smooth out?

Quote "2) A fuel-related miss. I can’t discern a transitional stumble between the various carb circuits. The accelerator pump gives a good shot. The miss is present both during the acceleration and as you hold it at an elevated RPM. (Possible clue: I do have a leak at the throttle shaft. You won’t see it while running, but definitely will if you open the throttle not running)" End quote

Billions of pixels have been unnecessarily disturbed on the internet on this subject.

AS A GENERAL RULE ON SINGLE BARREL CARBURETORS:

Original clearance from throttle shaft to throttle body would have been 0.004~0.006 inches. WEAR HAPPENS! Again, the general rule being that an additional 0.003 is acceptable. Measure the clearance WITH A DIAL INDICATOR, NOT CLICK CLICK BETWEEN YOUR THUMB AND FOREFINGER!

If the clearance is less than 0.010, forget it.

If the clearance is 0.010 or greater, then what???

Excessive clearance MAY influence idle quality (set the idle 50 RPM higher, unless you have a dogmatic transmission, in which case change it for a real transmission ). It may also cause a hesitation (again, with a dogmatic transmission) when transitioning from idle to off-idle.

Excessive clearance will have very little to no effect once the engine is running on the main circuit of the carburetor.

When I was still restoring carburetors, have had several tractor carburetors with 5/16 inch throttle shafts come in where at least half of the shaft was worn away. That is correct, more than 0.100 clearance. The tractor would not idle, but ran at P.T.O. speed just fine. Tractors have governors, and one with a fatigued governor spring will just beat the throttle shaft to death.

Personally, I think Martin is correct; but I believe in testing. How about a compression test, followed by a leakdown test, to check internal engine condition?

Jon

Excessive clearance will have very little to no effect once the engine is running on the main circuit of the carburetor.

Perfect explanation. That knocks one thing off the list. My symptom is the opposite. Good idle and mediocre running on the main circuit.

I did a compression check back when I first got the truck. That’s been a while (although not much run time) and wouldn’t be a big deal to repeat. I will do that ASAP. Never have done a leak down test

However, in the meantime....wouldn’t the good vacuum reading and ability to idle smoothly at low RPM tend to point away from a major compression issue?

Easy enough to try. I guess the theory being there that if the artificial “richening” from the choke smooths it out....it’s evidence of running lean.

If your idle is at 650 rpm, the idle mixture screw won't have as much effect as it would at 450 or 500 rpm. At 650, you're beginning to move away from the idle circuit and into the off-idle circuit. The idle mixture screw has less effect on that. "Normal" for a Rochester B is not always easy to quantify, however. Sort of like "normal" for Dr. Frankenstein. Try lowering the idle back to around 450 and see what happens. It is important to remember the idle circuit still feeds fuel in as you move into the off-idle zone. The Rochester engineers thought this would provide a smoother transition from idle speed to driving speed and it was at least as good as some of the other thoughts they had. Under some circumstances it will work.

Could be an ignition related miss. This might be from a worn-out distributor (allowing the shaft to wobble which causes odd dwell variations and possibly other problems). A couple of years ago Jerry (Hotrod Lincoln) was working on modifications to the original distributor to fix this...and he posted something about this just today. And I will tell you it could be a phantom problem from a seriously worn camshaft retainer. There should be a gap of only .001 to .005" between that retainer and your cam gear. That is to keep your camshaft as stable as possible. Apx 50 years ago I had one that absolutely would not stop misfiring and it turned out the retainer had worn so much over time (maybe .020" or more) it was allowing the cam to travel in/out as the engine ran (which caused the timing to advance and retard enough to make hiccups). After replacing every ignition component completely and checking the distributor on the machine three or four times, a fellow mentioned that had happened to him. It was not fun to replace (nor really cheap), but it stopped the problem, and it isn't anything you can visually observe.

A fuel related miss is always a possibility. Running too rich will always cause misfires and if the carburetor is constantly running rich the misfires will be at all speeds. The leak at the throttle shaft probably will make you run a bit leaner...as air will be sucked in while the engine is running, however. Try wiggling it up/down/back/forth and see how much wobble there is. The thing is with the Rochester B there are a few important things that will cause vacuum loss inside the carburetor and that isn't good. There are also parts of the air horn that could have been knocked off by clumsy or inebriated "rebuilders" and this can/will cause problems if you have a vacuum loss. If you have a vacuum loss, it can/will allow your power piston to go all wonky. This will invariably make your carburetor run rich because it will keep the power valve open---possibly constantly. What happens is there isn't enough vacuum to overcome the power piston spring (which by design forces the power piston down...which is the very last place you want it to be) and when you're accelerating you cause this to happen by virtue of vacuum drop also. And while it is a long shot, there's a vent hole for the power piston (going back into the throat of the carburetor) that must be open. If it isn't open, vacuum can suck fuel up into the passage where it will head right back into your intake manifold. Isn't the Rochester B a wonderful design? Look at the last image below. That vent is shown by the yellow arrow. The red arrow is the power piston, the blue arrow the vacuum passage that operates it and the green arrow...well the green arrow is a silent arrow, just like the letter h in John (which was omitted from my name to avoid further confusion).

A weak valve spring? Maybe, but sort of a longer shot in my mind. I've seen sticking valves be a more frequent problem than a weak valve spring. Particularly in #5 and #6 cylinders. Since 1965 I've had exactly 3 valve springs in 235s break, also. But you know that instantly.

I'd focus on the Rochester and try to study it well. Is the air horn warped (check without gasket as it sits on the float bowl)? Are there pieces missing? 6 and 8 shown below should be there. If not and you have a vacuum leak these will lead to problems. 6 is the vacuum passage that operates your power piston. 8 feeds your idle circuit. Check also the ports in your throttle body. They're shown below. One image below shows the Rochester B all straight and nice again. Yours probably won't look like that, but hopefully it will.

Good stuff, Jon G and Carbking Jon.

I’m kinda over the Rochester. I do have what seems to be a rebuildable YF 2100 just waiting for me to buy a kit.

Another reason I have avoided tearing into this B (which would be like the 4th time)...just want to eliminate as many other issues and let a near perfect carb be the cherry on top. That probably doesn’t make sense. But my limited experience with other types of engines validates Carbking’s theory that most carb problems aren’t carb problems at all.

JW51, I don't believe I have run across any info regarding The Rochester model B on this site that has not been helpful. In my much younger years when asked to rebuild or diagnose a carburator by friends or other customers I would reply " I know enough to leave them alone". I had the greatest LUCK making B's to run great. That was years ago and I was working on model B's that had probably never been rebuilt. So there it is...... I was a wizz kid. Not so anymore, these old carbs have been rebuilt way too many times. It takes someone like Hotrod or Jon G, Carbking to figure this stuff out. Many times the surfaces are warped or the power piston shaft is corroded which makes indentations, etc.,etc. Jon G got me going. I'm going to contact him and see if I can get tolerances etc. from his threads. While the B is easy to work on, it can be a horror chamber of missed items. Low vacuum is the major culprit with any of the straight 6 engines that GM made which is my opinion. Doc

A question for the "B" experts. Have any of you ever checked the actual float bowl fuel level with a correctly set float? I did not have anything to measure it with looked close to 1/2 inch from the parting surface.
I would like to know what it should be?
Running rich so I'm going to use a transmission valve body vacuum tester on that power piston vacuum circuit to see what the leakage is. And thank you for this thread good info here.
Whats your method to straighten warped parts?

Be careful when setting timing "to the ball". Some balls represent 5 BTDC which is a fine starting point for timing. Others denote 2 ATDC (which is the case with my flywheel). I have found the engine runs best between 8 and 10 BTDC on 87 octane 10% ethanol fuel. Not sure how prevalent the 2 ATDC ball flywheel is......

As for the power piston, I use a handheld vacuum pump with a conical tip. With the carb apart and holding in proper orientation I make sure 10"mercury will move the piston. It will not stay due to leakage around piston, but you will see it pull up.

As for diagnosing a rich running condition from power valve, I found the mixture screw will be about 1 1/2T out at about 500rpm for a proper idle mixture +/- 1/2 T. If it falls outside of this range I look for leaks if on the lean side and check power piston if on the rich side. This of course assumes carb is thoroughly clean.

Yes it would be nice to accurately verify the timing ball. Pretty much need to take the head off for that, as I don't think my timerite will work on a 235.

As for straightening the castings, sadly we are not adding tech tips any longer, but if you're patient, have a 3/8" thick piece of metal and a gas grill, this may help you: https://tinyurl.com/4uskmd5k
Please don't try to sand them straight...you'll only mess them up worse. Good luck. I tried for nearly 6 months to figure some way to make the power piston/power circuit work better but sadly there wasn't a way that worked. Would have been much better if Rochester had located the whole she-bang outside the float bowl, but they didn't.

I just checked with a straight edge it looks good to me. And actually it looks like the venturi bore area is a smich higher. The gasket had good contact. So the middle of land area vent to airhorn for power valve piston is fine, the piston fits okay and with my transmission valve body vacuum testers looks just fine, and using hand type vac pump 5 to 10 inches to move it all the way hard to tell. I will have to figure out a way to test it after its all back together. The check ball seals good.
Any verification on actual fuel level in the bowl? The needle valve

So reread this thread, I will assemble the bowl and airhorn without the gasket, (that I wish was a bit thicker) to better check for warpage. So I did the vacuum check on the power piston with the pump system, it takes 7" to hold it all the way to the stop. So far I have not found a reason for this to run fat. One day it sort of ran okay and the next it would barely run. Hmmm the person that owns the truck said the gas is good?
It looks to me that fuel level of a 1/2 inch or more is needed to fill the accelerator pump? I have also vacuum checked the gallery that runs down to the throttle body or base that feeds the power piston, it checked great no leaks. So again I'll make sure the float settings to the book, and make sure the mounting gasket is all good and no warpage there.

If 7" will pull it up and hold it up, that's actually surprising. I found somewhere between 9 up to 11 or so was common. Any drop in vacuum below 7 will open the power valve and immediately make your mixture richer. That could happen when leaving a stop sign, climbing a hill, shifting gears, etc. Warping of the castings will cause vacuum leaks and also damage to the castings can exacerbate that...please see where I used brass tubing to try to help restore damaged casting pieces knocked off by "professional" rebuilders along the way. The power circuit isn't the worst idea I've ever seen...that is to say it is an okay idea on paper and then only if you have real honest gasoline. If you have unleaded gas it will not work as well. If you have ethanol-laced unleaded, it will gum up rapidly and the piston will get stuck in the down position...constantly from what I've seen. In a couple of the Rochesters I studied, I had trouble removing the piston things were gummed up and stuck so badly.

If your power circuit is (currently) working, then I'd wonder what size main jet you had? Float level is important. I mentioned why in another post. Float condition is important...for obvious reasons. Years ago service stations had an in-exhaust pipe monitor they used for state inspection tickets. Those were really just a modified air/fuel ratio monitor. If you could find one of those, you could get a good idea what was coming out of the engine. One can be running rich and seem like it is running ok...at times, that is.

Truckrolet, I know of one professional carburetor restorer that refuses to take on the Rochester B/BC/BV.
There was no profit in rebuilding them. He couldn't offer a warranty because they may simply fail sitting on a shelf.
Like the rest of us, he has a family to feed.

Float level? Doesn't always mean correct fuel level. Depends on the buoyancy of the float, like who made it, its weight, its volume etc.
That is why I'd like a good idea of where it is suppose to be. Though I do not want to ruin another gasket to check it again. I had the float dimension right on spec, and yes will be checking it all again.

Adding this later after the post above (edit)

Okay the main jet is a 56. There are 2 different gaskets in the kit. I used the same that was used previously. I don't have the original old one. Both gaskets in kit are 1/32 thick. The main well support assembly has a platform that is lower than the contact for fuel channel (or higher if installed in proper orientation and not with airhorn upside down) that protrudes .018 depth mic checked. One type gasket covers the resting area for that platform the other does not, the gasket that I used same as original does not. So with the covered area the contact for the fuel passage will not be on the gasket and have a gap depending on the gasket crush, the other non supported gasket allows the main well support to cock at a slight angle, no I have not torqued on the main well screw for the check. So with either gasket to let the fuel channel contact the gasket, the gasket needs to be crushed by the main well supports screw by over .013, when using a 1/32 gasket. So the gasket used was likely the correct one. Hard to say if a leak at that point would cause rich or lean? I would think lean but ?

Add to your dependency list, fuel valve orifice size, ID of fuel valve, fuel valve height, fuel valve gasket, steel vs Viton tip needle, Ethanol vs Non-Ethanol gas, temperature and fuel pump pressure. I'm probably missing others. To make an even bigger headache, most of the carburetor kits on the market come from Asia. Quality and precision are suspect. As an example, some fuel valves don't seal well because of incomplete finish work during manufacturing. Staking may be required.

The float spec in the Rochester book will basically get you started. It is not uncommon to do trial and error on this setting. The air horn is usually vented to atmosphere in one manner or another. You can reuse the same gasket to do your bowl fuel level setting and measurement. It may leak a little at the gasket. Once you have a setting you want to test on the road, install a new gasket. Bowl gaskets can be purchased separately.

Kit from Mikes carburetor, I have got many from them and never had a problem. I added more to my post above. I was trying to not over crowd this thread. All dimensions for float were on the money. But again its the established fuel level that is important.

There are other Rochester posts kicking around right now and it is hard to recall what I've posted and when. Size of fuel inlet valve (some call it the seat) is very important. Look below at the 3 different sizes I found in 5 Rochester B and BC carburetors. Compare please the smallest to the largest. It is sort of like a horse needle compared to the needle used for a flu vaccine. Carter knew very well the implications of the size differences. I'm not certain about Rochester. Honestly I've said it here before but I think whoever designed the B and all variants was the son or grandson of some GM exec that flunked out of engineering school and needed a job.

I wouldn't rebuild these either. For one thing it is almost impossible to find one that didn't go through the commercial rebuilder's mixmaster and therefore you have no idea what sort of Heinz 57 carburetor you have now.

Needle seat hole diameter if small would just be at high power levels and speed. So which do you recommend ? So are you saying a too large hole would just splash in too much fuel and cause a rich condition?
So I used a good magnifying glass to look at the removed gasket, it looks like it was sealing just fine around the vacuum port and the main well support fuel passage port.

Yes, I am. Too large of an orifice is maybe like comparing a garden hose to the little tube in your toilet bowl that irrigates the overflow tube. It will allow more volume and that volume may be too much for your float & needle to handle correctly...even if your fuel pump pressure is okay. If you're pushing too much fuel into your float bowl through too large of an orifice, you're going to have a higher level than you should...not just for a moment but constantly. It is all a balance, too. If your fuel level is too high, then in the B, you're going to have too much fuel up in the jet mast and that means you'll have too much fuel too close to the suction which means you'll run richer because you'll have more fuel to air than you need. Carter understood this very well and if you study Carter YF "seats" you will see they also incorporated a standpipe to alter flow dynamics and provide a more correct fuel supply.

I mentioned your toilet bowl. The regulator in there operates not simply as an on/off float activated mechanism but also as a flow regulator. Watch the water going into the overflow tube for an example. It has nowhere near the amount of pressure an open faucet would have nor nowhere near the flow. But if you got in there and drilled things out and modified your cutoff mechanism, I'd be willing to bet you could make that hose have so much volume (and pressure) it would fly off of its little perch and spray water everywhere it wasn't supposed to be going rapidly. Your carburetor should be trying to do the same thing. It should all be balanced so that your float is performing its job comfortably and not having to try to slam shut against too much input.

And some of the kits out there are made for what we can call the largest possible use so the maker doesn't have to offer the correct stuff. Look at some of the YF kits. Carter in most of the models designed for the 235 used a fuel inlet of .081". I mentioned I had seen some in cheap YF kits that were .091" the other day and Jon (CarbKing) said some he had seen had .101". That will cause flooding in the 235.