The Stovebolt Site Forums The Shop Area Engine Shop rochester b off idle tuning

Has anyone here drilled their crossbar idle restriction in attempt to enrich low throttle settings?

This is what I mean:
https://1.bp.blogspot.com/-SS4acBSiF-4/WCnnC6FLw9I/AAAAAAAAMsc/Qb1P1LTCSJkipoQS40pcA6qgCReqRU8cACEw/s1600/carb%2B%2Baa106.jpg
https://www.inliners.org/tech/tech4.html

I'm hoping to send more gas to my 'upper idle port' when the throttle is slightly cracked. Currently my wideband o2 gauge goes quite lean, about 17:1, while cruising at low throttle settings.
If the throttle is opened just a little more, it enrichens down to 14. Sometimes this impacts drivability by jolting the drivetrain, and making the clutch harder to manage starting off.

I just drilled it from .07" up to .078" per Sam's instructions but didn't notice a big difference. Has anyone here tried this?

Unlike me, he's running double carbs... so I wonder if I should drill even bigger.
This runs contrary to his logic when he states that you need bigger jets in double carbs since the vacuum signal is reduced?
To me, it seems like a single carb would need bigger jets to equal the metering capacity of dual carbs??

Maybe it depends on idle vs full throttle?

What do the readings from your flow bench and exhaust gas analyzer show? Are there any changes in the midrange torque numbers on your dyno?
Jerry

Hey Jerry!
Good to hear from you. I wish I could say I was using those, but this is all based on an oxygen sensor in my exhaust that's hooked to a mixture gauge. I just drive around the neighborhood watching the gauge.

Test drives before and after drilling did not indicate a significant mixture change. It might have been slightly richer on startup, but it only took 1/4" turn of the mixture screw to restore original idle mixture ~12.5


I just tried drilling the upper idle port itself and will do another test drive after re-assembly

I've gotten measurably different dyno runs from morning to afternoon with no modifications, until we figured out there had been a big change in the relative humidity. It was hard to believe that something as simple as a thunderstorm passing by during our lunch break could skew the numbers that much! While your O2 sensor is better than nothing for evaluating modifications, don't forget to factor in things like ambient temperature, atmospheric pressure, and humidity when analyzing your data. Have fun!

It night be a good idea to find a slightly stiffer spring for the power piston, so the power valve opens up a bit sooner. The guy with the dual carb setup is probably going to make a massive change toward the rich side somewhere, to compensate for the loss of venturi velocity by splitting the airflow between the two carbs.
Jerry

Whew... I've removed and installed this carb 4 times today!
1) Corrected the float level
2) drilled the crossbeam idle restriction from .07 to .08"
3) drilled the upper idle port to .04" (it was probably ~.03" to start)
4) drilled the middle idle port to .052 from .043" (idle ports shown in Jon G's picture attached)

Each time I test drove while watching the mixture gauge and it seems like nothing changed!
I was hoping to enrich the mixture at times when throttle is slightly above idle to make the clutch more beginner friendly on uphill starts.
2nd gear starts (in the 420 trans) need all the help they can get to start rolling my 32" tires and 3.38 gears.
Attached is a graph of the mixture behavior when I get rolling.

Tomorrow, I'm thinking about drilling even bigger on the crossbeam restrictor... but am starting to wonder if the real reason it's tough to get rolling is a limitation of the accelerator pump at light throttle? It's the kind without an intake check ball, just a gap in the upper stroke for the float bowl to slosh in. However, the presence of the extra-lean cruise plateau indicates more room for idle circuit enrichment?

Interestingly, none of these bigger holes seem to effect my heavy throttle mixtures either, they are ~13.5-14.5.

I'm starting to like Jerry's idea of a stiffer power piston spring. I had trimmed a little off while adjusting the float bowl... maybe I'll add a spacer or stretch it some tomorrow.
Since main jet is only .053", it seems silly to go much bigger than my .08" on the idle restriction.

If the power piston is feeding the idle circuit (along with the main circuit)...? Then I'd expect that central 'valley' in the graph to get wider and hopefully smooth out the initial peak a bit.

If your light throttle cruise mixture is that lean, why not try opening up the main jet a little? Until the power valve opens up, all the fuel has to pass through the main jet, so that's where the idle system is getting its fuel. That deep dip just off idle makes me think the accelerator pump is not doing its job, also. If you can mitigate the stumble by using a bit of choke, that would also point an accusing finger at the pump. Final suggestion- - - -try dialing in a couple of degrees more initial spark advance. A late timing stumble is almost identical to a lean one. Together, they really make performance suck like a Dyson vacuum cleaner!
Jerry

I was going to suggest the choke, but jerry beat me to it. Apply enough choke to get the mixture down to 10 - 11 to 1 and see how it acts. The attached photo shows how I adjusted my carburetors. You need a combination of gauges to figure out what is happening. The throttle position on the far right is a simple home made gauge with a string tied to the throttle pedal, from idle to full throttle. After driving around for a while, I quickly figured out I only use the first 1/4 of the gauge, so when shopping for metering rods ( carter W-1's ) I concentrated on the first steps off idle. Full throttle was always with in range on the A/F gauge. I also replaced the linkage running the accelerator pumps and brazed the lever holes shut. I then drilled the holes back out so I eliminated all slop. The pumps move with just the slightest throttle movement.
Your problem sounds like you are past the idle circuits, why the hole drilling is not effecting it, and are in the run circuit. A vacuum gauge might show how low it's dropping when it goes lean, this comes back around to the power valve spring. If you could measure when the power valve opens, you could match it's opening to the vacuum level at the time of lean running. Not sure how to measure it, but I would bet you could cure the hesitation with a spring.

If you study the power piston, power circuit and power valve and how it feeds fuel, you'll see it can affect many things...including idle as I'll explain in a moment. A stiffer power piston spring will keep your piston down longer and will make it need more vacuum to stay open. This will make everything richer. In your graph, it appears it is turning on/off. You're in NM, but we don't know where. If you're in Ruidoso, Taos or Red River, you're at a pretty high altitude and your vacuum at idle might be as low as 11~14...depending on the condition of your engine. If you're in Roswell, you've lost a lot of that elevation and your vacuum might be 15~16, but not higher and maybe measurably lower. So, depending on where you are you might not have too much vacuum from the start. If that is the case and if your Rochester is losing vacuum from typical Rochester problems (a completely different conversation), your power piston won't be working nearly as well as if you lived in New Orleans. As for the size of your main jet, I'll return to that in a bit. You might also remember with gasoline (good old gasoline) the stoich was 14.7, but with ethanol gas it will be less than that and if you're using 15% ethanol, it will be less still. Most data I've seen says to figure 13~13.5 or so for 10% ethanol and more like 9.5 or 10 for E85. The reasons involve the hydrogen/carbon bonds in the fuel and this can and will change between "winter" blends and "summer" blends. So there are a few factors to consider.

Because of the design of the B series, the fuel level is important in determining whether you run rich or lean. As you'd expect atmospheric pressure plays a part here (you probably have less and maybe considerably less) and the level naturally affects the distance the fuel has to travel for the idle circuit and also the main circuit. Higher fuel level = richer. Lower level = leaner.

In the B series, the idle scheme is simple and given the right circumstances will work well. At idle there isn’t enough air flow to make the venturi do anything, so the carb relies on the main jet, a vacuum passage, an idle tube in the float bowl, four air bleeds, discharge ports and a mixture needle. Vacuum pulls fuel up through the float bowl via the idle tube and into a cross shaft a couple of inches below the top of the carburetor in which air is mixed with fuel via 3 air bleeds and also an internal air bleed that is sort of hidden. Then the mixture is drawn back down through a passage in the fuel bowl and fed into the engine below the throttle valve. There are only 3 things you can change or adjust in a correctly working idle circuit: the size of the main jet, the level of fuel in the bowl and the position of the idle adj. needle. All the air bleeds and idle tubes and other things are fixed.

Located above the idle discharge hole are other discharge holes known collectively as the "off-idle" ports. They're activated as the throttle is opened, drawing air across them and increasing the same fuel/air stream already in play at the idle discharge hole. As the throttle is opened further and air flow increases, air pressure in the boost venturi begins to drop and fuel starts being sucked out of the main nozzle (which interestingly also works as one of the air bleeds in the idle circuit). This fuel travels through the main venturi down into the engine and this almost completes the idle/off-idle scheme. I say almost because the idle port will still be feeding fuel into the engine at this time. Rochester engineers believed this would create a smooth transition from idle to driving speed, and it wasn’t a bad thought. Or at least I'll say it works most of the time.

But unless the Rochester B or BC has been straightened and brought back pretty much to like new condition in other regards, I doubt it is ever going to work as well as a worn-out Carter. And I'm not just criticizing the Rochester because it is an easy target. The design characteristics were really that different, and where the Rochester's performance relies heavily on wear/tear/age and the number of "rebuilders" who monkeyed with it over the decades, the Carter YF isn't affected by those things nearly as much.

Personally I think before I began modifying too many things, I'd try different main jet sizes. If I hadn't sold all the Rochester parts I owned a few years ago I would be happy to mail you some. Here in Dallas, most of the B and BC models always seemed to work best with a jet size between .057" and .059", the most popular being the size of .058". In higher altitude, some will tell you to use one size smaller at 2000 feet, two sizes smaller at 5000 feet and three sizes smaller at 10,000 feet. How accurate that is, I couldn't say, but trying a jet sized 55 or 56 could accomplish what you're wanting and would be interesting. Also...even if you re-jet your carburetor for altitude, you'll still lose somewhere between 3 and 4% of your power with each increase of 1000 feet of elevation. This also is somewhat dependent on the condition of your engine. But let's say your engine produced 100 horsepower at sea level. At 7000 feet, if you figure a loss of 3% per 1000 feet, you'll have only about 79 horsepower. At 10,000 feet, you'll have only around 70 horsepower. So that does make a notable difference...especially in an engine designed to produce lower power from the start.

Excellent analysis, Jon- - - - -for the Cliff's notes version, I'll just quote Carter's slogan from back in the early 1950s- - - - -"The ABC's of carburetion- - - -Always Buy Carter!"
Jerry

I second that Jon, good job on the explaining all workings of the carburetor. I will also second Jerry's ABC, since switching to Carters, I haven't had a single issue with them and mileage is well above any Rochester I have tried.

Awesome replies everyone!
I've got some bigger jets on order... i guess I'll hold off with the drill until they get here.

I wonder how those original carburetor designers kept their job without AFR gauges!?


Elevation here in Los Alamos is ~7000 ft... I haven't tried a vacuum gauge, but it does seem that there is enough to turn off the power piston at light cruise. But the way I drive, it's almost always on. I was also thinking about making the main jet bigger and soldering the power piston orifice so I can drill it a bit smaller since dropping by 4 ratios when piston engages seems a bit extreme.

This 'rottenchester' is a good challenge, I'd say worth a few more chances.

Thanks!

One thing I will mention...when I was absorbed in the Rochester B and BC study a few years ago I learned that if your Rochester is straightened and sealing correctly and if the power piston is cleaned and polished smooth so that it moves freely in the bore then the amount of vacuum needed to lift it off the power valve can be as low as 7 pounds. The strength of the spring is the determinant and I found 4 different strengths of spring plus one more in my box of saved Rochester parts. Mostly the amount of vacuum needed was around 9, however. So you can understand how a working power circuit will function if you're driving using a good vacuum gauge. Step on the gas (say starting on a hill or passing another vehicle) and the vacuum drops to single digits and the power circuit kicks in and feeds more gas to the engine. Reach cruising speed on level ground and vacuum increases to where it stays (we'll say 14 for grins) and the power piston is off of the ball and the engine is only running off gas provided by the main jet. Going downhill of course will increase vacuum greatly. You can also appreciate what happens when one of these is driven infrequently and the ethanol gas causes the power piston to be firmly stuck in the down position.

During this I also tried adding tiny O-rings to the power piston (image below) and greasing the center section to help keep the thing from seizing in the bore due to the sticky gummy residue ethanol gas invariably leaves. And I tried the same thing with HDPE rings. I also tried making retainers and rubber boots to keep gas from being sloshed or sucked up into the power piston. These schemes sadly didn't work, but it was worth trying. I actually think the retainer and rubber boot idea would have the best chance of working, however there are some challenges to that and it might not be cheap to produce. I was searching for some way to either improve on the power piston or maybe even to remove it from inside the carburetor and put it outside where it couldn't be gummed up and might work more precisely and predictably...like the Carter YF does.

Cool experiments. I guess the gas here must not be quite as gummy. Despite the ethanol horror stories, I have not personally experienced problems with stale gas or stuck parts. Maybe our low humidity helps.

I'll report back with vacuum readings soon!

We had a log splitter (in Ruidoso) which I traded for some work done. I got the idea the ethanol gas was about the same as we get here although it was 86 octane (which I understand the need for). I couldn't leave it in over the winter, however or the thing wouldn't start until I cleaned the carburetor out well. Lots of carburetor work going on, eh?

32" tires, and 3.38 gears? No wonder your truck isn't performing the way you desire. You need gears in the neighborhood of 4.11 with 32"tires with that transmission.
I would be shocked if a Carter fixed your problem. You are asking too much for a single barrel carburetor to do IMHO.
There is another famous saying. "Its never the carburetor."
Carl

Do you know what's worse than texting and driving?... vacuum gauge + afr while driving!
Just now was my first time watching vacuum while driving. It is addictive.

This is a good observation Carl... although I'm using a torquey cam and big flywheel, the engine does bog easily when starting against such a ratio, especially uphill.
It seems like double pain when it bogs, manifold vacuum is reduced which takes the carb a while to react. If only it had a vacuum controlled accelerator pump like my toyota carb.

Here's some vacuum observations from a short drive just now:
Idling ~530 rpm = 16" vacuum, 12.5afr
Vacuum increases with idle rpm & choke ~17"
Vacuum goes up around 22" or more when engine braking... afr remains ~12
PP engages right around 5"... i had trimmed 4 coils from the spring, maybe that's why
Leanest AFR 16.5 to 17.5 occurs between 5 and 10" vacuum... light throttle
PP enriches it to 13.5 to 14.5 below 5" vacuum
Strangely it's a little richer ~4-5" vacuum but leans out to 14.5 at WOT

it's a cheap vaccum gauge that was laying on my shelf for years, take those numbers with a grain of salt!

Not that much carburetor work here... but I was surprised that my power piston wasn't stuck when I first opened the rochester. Also I've used the same can of gas to mow my lawn ~2x a year for the last 3 years! And, my infrequently used chainsaw finally got through one gallon jug of saw gas after 4 years with no problems.
Now I've jinxed myself!

At 7000 feet and 16" vacuum at idle, it tells me your vacuum gauge might be reading a bit high. That's more along the lines of what I'd expect at 2500~3000 feet, but still it is very encouraging. It suggests you're not losing much vacuum inside the Rochester. Yes, definitely...if the PP is opening around 5, then the removal of 4 coils would be the answer. Of those springs I found, one of them was notably weaker and a couple were pretty strong. I wondered about this but the other Jon told me he thought Rochester had used something like 7 or 8 different springs and any time these went through a "rebuilder's" hands, any sort of mixing and matching of parts would happen. The leanest AFR between 5 and 10" vacuum says your power piston is off the valve (a very good thing and not something I'd be inclined to change). 16.5 to 17.5 is slightly on the lean side, but probably not worrisome. A slightly larger main jet should bring that back to 15 or so (where I'd be a bit more comfortable). You don't want it too lean, but you also need to know your AFR (particularly the sensor) could be giving questionable readings unless it is brand new. Oh, and you probably know this, but don't leave the AFR sensor in the exhaust pipe if the gauge isn't being used. Even a little bit will mess them up. And those are sensitive things. They need to be positioned and angled just so to get the right readings and most of them want to be something like 24" from the junction of exhaust pipe and manifold. But it sounds like yours is pretty close to right. As for leaning out at WOT, that isn't surprising to me. With that 53 jet and the throttle wide open, your fuel is limited but your air intake has been increased, so as I see it, you reached a max with the jet somewhere before WOT and now you're just catching up with air to fuel.

I'm beginning to think that the problem has much more to do with gearing than carburetion. That 3.23 rear end is usually used behind an automatic transmission, where the torque converter effectively doubles engine torque during breakaway from a stop. That's why Powerglide-equipped cars used a 3.55 gear instead of the much lower ratio used for 3 speed cars. I once drove a 1963 AMC Rambler Classis with a 2.88 rear end, and it was quite a performer, even with a small 287 cubic inch V8, but only once if got rolling. From a stop, it was a lead sled!. Top end speed with that gear was outrageous! Combining a high gear ratio with an OD 5 speed is a sure recipe for performance problems.
Jerry

It's always fun to compare tuning notes!

As for gear ratio... I never truly appreciated how tall this 3.38 axle is until a couple weeks ago I had a chance to drive a 1950 stovebolt car.
It had 3 on the tree and the carb was super rich. But drivability was amazing! It felt like a modern automatic, a novice clutch driver could use it no problem.

This is what motivated my latest round of tuning... my gear ratio is here to stay, so hopefully tuning can help a little.

It's fun chasing gremlins that don't exist, I suppose. It's not my cup of tea, however. I mis-spoke on the 3.23 gear, but a 3.38 isn't much better. I'd be looking for a 3.55 or maybe even a 3.73, particularly since you've got an OD gear for getting up to highway speed. Stovebolt engines lug pretty well, but getting down below the torque curve is a sure way to have problems with bogging and flat spots.
Jerry

I've posted this maybe 100 times, but here it is again. There are ways to make the T5 and a 3.07~3.08 rear work together. Here's one:

OEM 3 speed ratios were 2.94, 1.68 and 1.00. With a 4.11 rear, the overall ratios were 12.08, 6.90 and 4.11 (naturally). And this worked very well with the 3 speed up to the point where the 235 got uncomfortable.

With the T5 (correct ratios) and a 3.07 rear, I have 4.03, 2.37, 1.49, 1.00 and .86. And the overall ratios are 12.37, 7.27, 4,57, 3.07 and 2.64. And if you look, you'll see the first three speeds are very close to the original ratios the GM engineers wanted the AD truck to have. And so to drive it you'll feel almost exactly like you're in the same old 3 speed setup...until you get to 4th. At that point your engine rpm will drop by 25%. And then in 5th if you're on a relatively flat highway without too much of a headwind, you'll drive comfortably at around 2000~2100 rpm at 65 mph. When you get into hills or a notable headwind, drop back to 4th. You're still just fine.

Sorry guys... I meant to mention that my transmission is an SM420 4 speed. I haven't heard of anyone using the T5 in a NAPCO?
4th gear is 1:1, and the 3.38 rear with 32" tires is already like an overdrive. I hit 60mph ~2200 rpm
1st gear is a granny only used offroad or on a really steep stop sign.

I can't wait to try a bigger main jet, and will post the results later this week. I'm starting to wish I'd have tried that before drilling the idle ports!

OOPS! I got your rig confused with another bolter's, who is trying to run a high axle ratio with a T-5. Is the 3.38 an original NAPCO ratio? That still seems to be too much gear, especially with tall tires. I assume both axles are geared the same, which would be the reason for not going to a lower ratio final drive.
Jerry

Original was 3.9 and the tires were probably only 30-31"
So it is a bit taller than intended. 3.38 is the only ratio available for replacement these days.
It does work... but I get greedy trying to tune everything 'better'

I would go back to the original 3.90 gears. With the 32' tires (compared to the original NAPCO 28" tires), you will be right where you need to be.
I can't help but feel sorry for the poor threads in your Model B for all of the times you have taken them out and put them back in.
Your rig doesn't need more fuel, it needs more RPM to perform well on grades. The only thing which will fix that with what you have is different gears (3.90)

That's why the rock crawler crowd loves those 4.56 (or 5.13) gears that we can't wait to get rid of- - - -they run tires tall enough that they need a ladder and some airsick pills just to climb up into the cab. Taller tires need shorter gears to compensate for the increased circumference of the rubber!
Jerry

I still have the old gears... though I'm skeptical how long they'd last considering the front yoke had about 1/8" of thrust & radial play in it!
The 3.9 ratio would definitely be nice.

Fortunately the SM420 comes with a secret weapon: the 7.05 granny gear... among the lowest of grannies ever produced!
This, along with the transfer case gives it acceptable offroad ratios.

The rock crawler bros can definitely outdo my rig... however, I can out wheel a stock Tacoma!
I just updated my gallery thread in case you're interested.
https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1336833/4/mighty-morphin-suburban.html

Alright! I finally tried a bigger main jet.

Here's some tuning notes:
-As usual, idle mixture any leaner than 13:1 causes occasional misfires. I set mine ~13
-Idles smoothest between 530 to 560 rpm
-Going from 53 to 56 main jet made put my mid throttle (5-10"hg) mixture ~15:1 (it was 17:1) and made a noticeable difference in drivability:
-Driveline jolting is reduced by smoother transitions between mid-throttle, idle, or power piston
-The mixture can still lean a bit if the clutch isn't finessed, however it stays in a more reasonable range, go going no leaner than ~15.5.
-Power piston kicks on ~5" hg.
-Mixture between 2-5"hg is ~11.5 and 12.5 at WOT

The only thing still annoying me is the 11.5:1 mixture just below WOT.
I could probably live with it... but I'm thinking about trimming another coil from the power piston spring so that it only activates right at WOT (maybe 0-3" hg)
Alternatively, I could solder the power piston restriction and then re-drill it a bit smaller. Presumably shifting my PP mixture from 11.5-12.5 where it is now up to 12.5-13.5.

Thanks for the help guys.

Sounds encouraging! Did you try 57 or 58 jet?

Thanks Jon, I tried the 58 at first. It was really nice for clutch work however the mid throttle ~13 was a bit fat for my taste.
And it dropped down to ~10 with the power piston and I could just smell the carbon buildup!

I didn't order a 57, but I think this 56 should do the trick!
I'm under the impression that anything richer than 12-12.5 is a waste of gas and as you mentioned 15 is good mid range.

So I trimmed another two coils from the PP spring (for a total of 6 including the closed end) and now the piston comes on ~3-4"hg.
This will probably satisfy me since I still get 12.5 at WOT and a tiny regime between 2-3"hg where it is still 11.5
I could probably trim another coil but I'm starting to feel for the treads too!

That sounds like a workable solution. I was curious about larger jets but sounds like 56 is going to be the right one. We're in NM for a bit. I'm doing rain dances.