The Stovebolt Site Forums The Shop Area Engine Shop Rochester 'B' - no idle after rebuild

Rebuilt my Rochester 'B' carb, which is actually a very simple rebuild, but I'm stumped with an issue.....It will not start unless gas is poured down the carb, then in order to keep it running, poorly I might add, I need to pump the gas pedal rapidly....It will stall if I stop pumping the pedal.....Now as best as I can figure there's a blockage symptom of gas not getting to the main jet, and the only way its running is from the accelerator pump drawing gas down the carb....I took the carb off twice disassembled it, and did not find anywhere where there was a blockage.... The power valve tower was perfectly clean, as with the screen & jet.....Is there somewhere else I should be looking at ?

Rochester B or BC? Year of engine, etc? Was it working ok before? If the pump is working and gas is making it into the float bowl (and assuming the float is adjusted correctly), then gas is definitely making it to the main jet. I'm sure you know this already but in the B or BC, gas is sucked up through the main jet and through the crossbar over to the other side of the carb body where it goes down through a passage to the idle adjustment screw where it is metered and fed into the manifold below the throttle valve flap. If you have a loss of vacuum, then nothing works right. If you have a plugged up vacuum passage, then nothing works right.


Were all gaskets installed correctly (not plugging up any vacuum passages)? You mentioned a screen. Which screen? Are air bleed holes (top of crossover piece) free and clear? Is the idle adjustment screw out 1.5 turns? Please give us more information and we can give you some cogent thoughts.

Thanks for the comeback...1967 Rochester 'B' manual choke....There's a tiny circular screen in the 'support assy' I call it the pedestal,in the same tube, or cavity where the main jet screws into... There is an illustration of it, in the instruction sheet of the rebuild kit.... I believe the float set is correct as the bowl is 3/4 full when I take the top of the carb off....All the springs & balls are in the correct spots, as I did the rebuild while viewing the 'Mike carburetor vid'. Your comment regarding vacuum, and especially the air bleed holes at the cross over piece, is leading me to believe there might be a restriction in these area's...I had a friend of mine clean/soak the carb in an ultrasonic carburetor cleaner machine and perhaps some of the passages might still have debris. I double checked the gasket and confirmed that it wasn't restricting any holes.... What should the vacuum be coming off the engine? - I will check that as well.......

Soaking in carburetor cleaner of any kind simply softens the deposits. The true "cleaning" is done with high pressure compressed air- - - -LOTS of it! An instructor at the General Motors training center in Memphis where I spent a lot of time put it this way- - - -"Put 55 gallons of air through every hole you can find in that carburetor!" His point was this- - - -in order to function properly, the carb passages must be CLEAN and the only way to assure that happens is to use copious amounts of air to blow the crud out of them. Use a needle-point air gun and spend a ridiculous amount of time blowing the air through those passages, after softening the deposits with solvent AND doing a hot (almost boiling) water rinse of all the parts to neutralize the solvent. One of my auto mechanics students learned that last procedure the hard way. He didn't rinse- - - -just started to reassemble the carb while it was still wet with the dip cleaner. His hands started to tingle, so he went to the restroom to wash up. "Hmmmm- - - -gotta pee"- - - - - - -wrong move! Did you ever notice that the FIRST thing a mechanic does in a restroom is wash his hands? There's a very good reason for that!
Jerry

Can't dispute what you say here, and its not news....knowledge in the back of my cob web mined, along with far to long not having hands on....and the first & most important rule is, "If you want it done right, do it yourself"...A 'friend' did me the favor of cleaning it, in his ultrasonic cleaner, perhaps he didn't do anything other than dunk it.....I'll take it from here....

If you're interested, you can go back to around this time last year and read my posts on the Rochester B and BC problems and some solutions. There are several things that can go wrong with a Rochester...and some of them can be rather serious.

Given that you say you have fuel in the bowl, your problem is either a blocked vacuum passage (most likely) or a serious vacuum leak or your idle mixture screw is screwed in too far...and I will add that if you didn't remove the mixture screw and clean it and the passage and hole well, this could also be the problem. I've seen people clean and "rebuild" a carburetor leaving that idle screw sitting right where it was. The throttle body is iron and it will rust and that passage can be blocked.

At the heart of it, you're not sucking gas through to the idle hole and into the manifold. It is just that simple.

When you take the carburetor apart again, dry it well and then start checking all the vacuum passages to make sure air is passing through them. I'll attach an image that has been circulated and used by lots of people. It shows the Rochester idle circuit (in red). All these passages must be nice and clear. You can use compressed air or you can suck on them while plugging the other end up with a finger. Either way you ought to be able to tell if they're open or not. In the case of the idle screw you can remove the screw and see if the passage into the throat is open or not, and if you see any rust or crud in there, you know you have work to do. Good luck!

Advise & suggestions well taken...with in the next day or two, I will set time aside for an uninterrupted focus on this task....I'm confident with the gained knowledge here, that this issue will be resolved....No matter what the hiccups are, I truly enjoy playing with my trucks.....note I did not say work.....When it becomes work, its no longer fun......Will keep you posted....Thanks again for your time & interest......

Here is another suggestion. If you're in upstate NY or central NY or any rural area where you'd have access to a hardware store go to a True Value or Ace and find one of those boxes of various metal pieces. Buy a length of the smallest piano wire they have...probably about 1mm diameter or so. You can run this carefully through vacuum passages and even passages you think are clean may have junk in there. Especially if this carburetor sat in some shed or junkyard for a long time, it is likely small insects made their way inside and if they did, there could be who knows what in there.

In the passage way in the top part of the carb there is a small plug in one end. Sometimes when you are blowing air through all the passages that plug gets blown out. It will cause a humonges vacuum leak & the symtoms you describe. If the plug is lost you can seal the end with epoxy.
George

Do you have the correct flange gasket between the carb and manifold? Some of the rochester flange gaskets do not have the vacuum hole that allows manifold vacuum to the power valve. If this hole in the bottom of the carb is covered, it will open the power valve and your engine will not idle and die as long as the throttle blades are closed.

OK - Took carb apart...checked all the passages with thin piano wire, carb cleaner & compressed air...No blockages anywhere....Running fresh, non ethanol gas, with a good dose of Starthron.. Pulled out manual choke....long cranking before starting....after initial warm up runs good on 1/2 choke...can rev engine, no hesitation...with choke wide open idles good however stumbles and wants to die upon acceleration........

Try watching this to double check, plus there are some other checks too:

sstock mentioned checking for the correct flange gasket. I had the wrong gasket installed which blocked the carb's vacuum port and the engine exhibited the same symptoms you describe. In my case, there is a Bakelite* insulator between the manifold and the carb. There is no gasket used between the carb and the insulator - not all carb kits tell you this.

* I believe it is a Bakelite (phenolic) material

Correct Gord, with the phenolic spacer, normally you do not need a flange gasket. The phenolic (heat) spacer has the vacuum hole drilled into it, but if one uses a gasket between the spacer and carb. flange then they need to make sure it has the vacuum hole in it too.

This is getting interesting....There is no hole in the manifold for a vac port.....To shed some clarity on this truck...I recently purchased it (1967 K20 - 250 cu in) When I showed up to look at it, I could tell it was already up to temperature and the air cleaner was on the ground....the seller mentioned it had a carburetor issue....putting that aside, I focused more on the remainder of the truck....Once the deal was made the seller suggested flat bedding to my house about 25 miles away....I convenience him to deliver it by driving, which he did....I followed and the truck appeared to run well...I saw no smoke coming from the tail pipe.....As to date, I never drove the truck, other than in his driveway to check the clutch, brakes & 4x4 operation...

The hole is not on the intake manifold, but on the phenolic spacer and base of the carb flange, tip the carb upside down and look for the vacuum hole on the mating flange.

See the divot to introduce vacuum on this spacer

https://www.chevsofthe40s.com/detai..._Bakelite_Insulator_Carb_Base_235ci.html

sstock...You are correct.....I should have realized this, upon exaiming the manifold more closely, I see a faint trace where this spacer was at one time.....The curious thing is the studs are to short when the carburetor is installed, as the studs are basically flush with the top of the nuts....leaving no room for the spacer??? I'll have to replace the studs with longer ones, and obtain the spacer...Hopefully that will correct the problem... And yes the carb does have the vacuum hole.......

You might be able to get your vacuum in by using a thick base gasket like this one,instead of a phenolic spacer, that way you dont have to put longer studs in

https://www.carburetor-parts.com/Carter-YF-Rochester-B-BC-BV-Flange-Gasket_p_1410.html

The spacer also serves as a heat isolator, preventing "heat soak"- - - - -fuel boiling in the float chamber and flooding during a short shutdown like refueling, etc. Even if you need to install longer studs, the spacer should really be there for correct operation.
Jerry

Agreed....but I wish the solution was that simple....I found a thicker base gasket in my parts boxes and installed it....I confirmed all holes line up....No difference....My next step is to check vacuum.....There's a port just below the carb on the manifold, which would be the best place to get the reading....Gotta find a fitting to fit the gauge & port....

How thick is it? One of the B models (later years...maybe mid to late 1960s) did not have a phenolic spacer but used a 1/4 inch thick carb to manifold gasket.

I'm going to say ts about 1/8 thick...I was talking to a friend and he suggested the PCV valve...but it doesn't have that either....I'm really stumped......

1/8 isn't the one I was expecting you to have, but if the holes all match vacuum holes, then you should be ok there. Maybe I missed it, but you did clean out the idle port and clean the idle adj screw, yes?

Yup...The carburetor is surgically cleaned...I'm starting to think its more than the carb...the vacuum test will shed more light on the issue....

Sounds like you have a sucking chest wound of a vacuum leak somewhere.

There's another thread on here for a 292 with a similar issue...good idle, but coughs & stumbles to stall upon acceleration.......It seems he's into valves & springs at this point....Most opinions I'm reading is a vacuum issue... In general an inline 6 cyl of early vintage is about as simple as you can get, so I'm having a difficult time realizing where all the vacuum failures can take place?....A running engine creates vacuum at idle....comes up through the intake, gets diverted into a port at the base of the carb, then follows its path throughout the carb....I'm 99.9 % confident I have no restriction in my carburetor, so I'm not understanding where this 'Sucking Chest Wound' of a leak would be??? Perhaps if I understand where the starting & ending point of this vacuum trail is, I can isolate my issue.....

I had a similar experience when I first started my truck this past March. It would start and run only by pumping the accelerator then stall maybe five seconds later. Convinced it was a starving fuel issue, I rebuilt the carburetor then disassembled/reassembled it three additional times with similar results. At that point I’m wasn’t sure where to go from there. I read a few posts on this forum and one post hit me to keep it simple. I had not done much with the distributor aside from checking the points and checking for spark so I figured I would start there. After replacing the points and the condenser the truck started and continued to run. After testing the removed condenser I confirmed it was bad. I screwed with that carburetor for over a week and the real cause was the condenser. I did however learn how to properly rebuild a Rochester type”B” carburetor. Just a thought!

Vacuum leaks can be as simple as a bad vacuum wiper motor, or a bad seal between the intake manifold and the head.

Well, - There's an electronic distributor on the truck so that rules out points & condenser.... Intake manifold leak sounds like a real possibility........will check torque on the bolts.....

If there is an electronic distributor; is there also an alternator?????

Generators do not provide a sufficiently stable voltage at idle for proper spark. Have had lots of customers with "p" conversions and carburetor idle issues solve the issue by either upgrading to points and condenser or adding an alternator.

Jon.

Vacuum leaks can be as simple as a bad vacuum wiper motor, or a bad seal between the intake manifold and the head, or sloppy throttle shaft holes.

Its a 1967, so it had an alternator from new...The electronic distributor was added I would say in the last few years as its a GM distributor.....

Or a little more invasive like bad valves/ seats or worse yet piston rings.
If you dont already have buy a vacuum gauge and learn how to use it as a diagnostic tool. Reading and needle fluctuations can mean a whole host of things to point the finger too. Probably the most valuable diagnostic tool in the tool chest.

Thank you Tim,
I was looking for that to post. Kudos
Steve

Have a gauge...haven't used it in 25-30 years...& just put together the fittings today...to take reading from the port on the intake, just below the carb....will take reading tomorrow....I'm not thinking the problem is with the internals....It doesn't smoke out the tailpipe...and nothing coming from the valve cover with the oil cap off....No misses, seems like its firing on all 6.... 52 Carl made a mention of a leak between the intake & the head...which is a good possibility.....I remember several years ago when I replaced the exhaust manifold on the 292, I had to have the intake planed down due to warping.....I snugged up the bolts a tad... so we'll see.....Thanks for the readouts on the gauge....most helpful.....

OK - The good news is a 'steady' 18/19 vacuum at idle, taken from the intake manifold at the base of the carburetor.....Pondering this, my attention went to the vacuum advance on the distributor....I took a reading from the port of the carb. vacuum advance, and ZERO... no vacuum coming from that port....I am 99.9% all the passages on the carb were clear. My question is at what point does the vacuum advance arm start to move? (just want to confirm my answer) and secondly if the distributor vacuum advance is not working would that be the cause of the stumbling going from idle to acceleration?

It should be zero it is ported vacuum only there because the vacuum comes from above the throttle blades, so you are good there. Open the throttle blades a little and you should see vacuum. You can easily check your vacuum distributor can by using a mighty vac and apply vacuum to it or just use a piece of vacuum hose to go to full vacuum port on the manifold.

Ported vacuum should start happening at speeds just a little above idle, and should be very close to full manifold vacuum when the engine is running at a steady cruise speed with light load. On a hard pull or quick acceleration the vacuum advance will go away, leaving nothing but initial and centrifugal timing. As soon as the load eases off, the vacuum advance will return. There should be little, if any vacuum advance while the engine is accelerating, so blaming hesitation or poor performance on the vac. advance unit is barking up the wrong tree. Late initial timing is much more likely to cause a bog on acceleration, or possibly a too-lean idle mixture setting could be to blame. Put a T fitting into the distributor vacuum line and attach a vacuum gauge to watch ported vacuum at work.
Jerry

Oh Boy....Never thought a simple Chevy 6 could be so complicated......The mixture on the carb is a tad to the rich side, so a lean is out.... turning to timing....I got all ready to check the timing and guess what...I cannot find a timing mark on the pulley.....Someone installed a HEI distributor and I'm guessing they just set it, until they got the truck to run fairly well. I'm really wondering why they replaced the distributor as this is the only modification I can see..I hardly feel it was for performance ......Turning to the pulley.....The truck has factory P/S... and the pulley closest to the radiator must have been for the snow plow that was once installed....its no longer there..... The tin timing tab indicator was caked with grease, so it seems no attempt was ever done to check for the timing.....I'm really stumped.....