The Stovebolt Site Forums The Shop Area Engine Shop Stumbles bad off idle

I have done everything I know to do. And I think I am still having an issue with the carburetor. Float works and new kit has been installed and the whole thing has been cleaned and every hole blown out! It's a Rochester B on a 1961 235 engine. The truck will crank and run very well on idle. Where I am having trouble is trying to get it off idle. It loses power and I can hear it sucking air abnormally loud when it is losing power and stumbling. I tried to drive it up and down the road and it did fine in 1st gear but 3 out 4 times it stumbled bad when shifting to 2nd gear. Timing is set right, valves adjusted properly, get steady good vacuum at idle, rpm was around 550 at idle. I was able to read the metering jet and it has 52 stamped on it which would mean it is a .052. From everything I read this engine and carburetor (also a 1961) calls for a metering jet of .064. Could this be my problem? I wonder if the engine is starving for fuel when I try to accelerate! Any help would be much appreciated.

***I already know some will say go buy a Carter. I may eventually do that but this thing ran great with me for 2 years without any issues on the Rochester B.

1) Does it smooth out once the rpm stabilizes a bit? Just stumbles on acceleration? Or does it fuss at any RPM above idle?

2) You say timing is set correctly….you mean initial timing, I assume? Have you checked to see that the vac advance is working? Does the distributor rotate and return quickly and smoothly when the throttle is goosed? Might be worth checking “total timing” at RPM with a light, if you have not done so already.


(***** the fact that this same carb ran fine for two years with that same jet (even if it’s the wrong jet) makes me highly skeptical that jetting is your problem. It still might be a fuel issue of some kind, but I’d suspect the jetting is not creating your current symptom.)

3) Will the truck stay running (even poorly) if you hold it at the RPM where it starts fussing? If so, use the hand throttle or a partner to help hold it at that RPM. Fill some sort of container that will allow you to squeeze some fuel down the throat one a fairly controlled way (a ketchup bottle works well). While the truck is running at its “bad spot” RPM, squirt some fuel down the carb and observe the change.

Does it get better? You can safely assume it needs more fuel and keep exploring that avenue. Don’t forget fuel pump or obstructed filter/line in that equation.

Does it get worse or little change? Some circuit on the carb is dumping too much fuel (common B issues) or you still might have not the right spark at not the right time.

The 52 jet is more suited to a 216. 64 jet is for a 261. The 235 uses a 58 jet. It will be running lean on a 52 jet.

Matt, when I got my truck it had a Frankencarb on it. It ran "pretty good" for a while before I started having problems.

When fully inspected, it turned out to be a carburetor cobbled up from several different components originally specified for early 1950s and mid 1960s Jeeps. Who knows what the metering rod was spec'd for?

I purchased a correct rebuildable Carter core and put it all back together with the correct parts specified for my particular engine. That was 3 years and 8000+ miles ago and I have had no carburetor problems since then. The truck runs perfectly at all speeds from 0mph to 65mph.

I guess my point is that incorrect carbs will sort of work but when you get the proper one set up correctly, it is a night and day difference in performance.

True. But this does NOT explain how the OP’s truck ran great for the previous two years with the same setup.

It should have about a .058 jet like Jethro stated.
Before he o-hauled it the float level may have been too high or the power valve stuck open, compensating for the lean condition.

George

PasterMatt - 4 things:

(1) Rochester used a number of different sizes of jet in the Rochester B for the 235. Your last post stated it was a 1961 carb which should be tag number 7015011, and the Rochester specified size for this carburetor for the 235 was 0.055. Repeating: different versions had various size jets from (memory) 0.054 to 0.059. The size of the FUEL jet corresponds to the size of the AIR BLEED in the specific carburetor. This is why for best results, it is imperative that one knows EXACTLY what carburetor with which one is dealing. The earlier (1958 and before) carbs were jetted richer.

(2) NEVER (and I detest the use of the word "never") believe what is stamped on ANY jet from any company. Folks have been drilling jets oversize since 5 nanoseconds after the first carburetor with jets (the first carburetor didn't have jets, it has a wick) was released.

(3) I don't believe an improper sized jet (if in fact it is improper) is the cause of a stumble on switching gears. If the carburetor is the culprit, then a stuck power valve or trying to use a non-leather accelerator pump with ethanol fuel would be two possibilities. Another possibility would be the advance mechanism in the distributor sticking. Too small a jet would cause the engine to run poorly, not "did fine in 1st gear".

(4) Another option, should you not wish to go with a Carter, would be to remove the hood, and the Rochester. And UPGRADE to an unemployeed 15-year-old standing on the running board and pouring fuel into the engine from a leaky boot!

Jon

Could be caused from a warped upper half of the carburetor. I once observed an internal leak at the parting line where the top of the carb meets the air horn. As I opened the throttle to raise the RPM, gas would pool up near the gasket and as I let off of the throttle, I could watch the gas evaporate before my eyes. There were not leaks on the outside.
The biggest problem with a warped Model B is not external leaks, but rather, the internal leak. These internal leaks are pulling gas away from where it belongs and putting it where it does not belong. This will cause a stumble upon acceleration.

What is weird here is that my spark plugs are sooty black after letting it idle for 5 minutes. Even with the small jet size it runs rich at idle anyway. I am missing the insulator that goes on top of the intake. Looks like my grandfather had stacked a bunch of gaskets to create something like an insulator. I do have the proper gasket on the bottom of carburetor with the slot matched up to the hole for it. I wonder if missing that insulator makes a big difference or not.

I don’t know what this tells me if anything but I had it idling and sprayed a little carb cleaner at the base of the carburetor and three times it cut off immediately when I did that.

That means there is a leak at the base of the carburetor and the intake vacuum is sucking in the carb cleaner and conking out. The base is warped or the manifold surface is warped, or both.

Most likely your carburetor needs to be repaired (if possible) or replaced.

Otto,

You don’t think just getting the proper insulator/riser would fix it? I’ve got a composite of like 15 gaskets stacked up there in place of the insulator/riser.

Additionally… what I have determined is that it stumbles when I really give it gas with the choke off. If I slowly rev it up, it does just fine. I can give it plenty of gas if I have the choke pulled slightly. But if the choke is off completely and I give it a hard pump of gas it stumbles and keeps stumbling till I let off.

You definitely need to lose the stack of gaskets and get the proper phenolic spacer. Use one gasket.

You might have other issues, but fix that one before chasing anything else.

Oh yes. I didn't know you had a gasket stack. That's almost certainly the source of the vacuum leak. You need to buy a spacer.

Otto and JW51,

I have now replaced the gasket stack with the proper spacer and using one gasket between spacer and throttle body. Had it running and sprayed carb cleaner all around that area and it kept running just fine. No conking out, no revving up, just keep running at the same rpms. I've also sprayed carb cleaner around the intake and exhaust areas that connect to the engine and it keeps running fine.

However, I am STILL having the issue of the engine stumbling and continuing to stumble when I try to rev it up. If I slowly increase the throttle it does fine but when I give much at all it stumbles! What else should I check for?

Are there other common areas where I should be looking for possible vacuum leak?

I will add that I am thinking it is not getting enough fuel. I did hold the throttle by hand in the stumbling position and sprayed a little starter fluid in the carburetor which immediately made it clear up. That makes me think it isn't getting enough fuel. The fuel pump was replaced 2 years ago. The valve (or Petcock) that connects the fuel tank to the fuel line is leaking ever so slightly. It isn't dripping but it does always have a little drop of gas on the bottom side of it. Could that be causing a problem of where the fuel pump can't get enough fuel to the carburetor?

I am planning to take the fuel tank petcock out this evening and just connecting the fuel line to the tank with a small rubber fuel hose to see if that fixes the problem. Seems like an easy issue to trouble shoot.

One other thing... I believe that the metering jet on this carb is too small (.52) and should be (.58). When I rebuilt the carb I did not order a metering jet and change that because I didn't know about that at the time. I did replace the float and set it to the proper height and depth according to the manual. I am wondering if the old float could have been adjusted differently to compensate for the a metering jet 6 sizes too small. Is that possible? If so, I could put the old float back in or order the right size metering jet.

Several questions in there but any help you can give would be greatly appreciated.

Does the stumbling clean up once you get ABOVE a certain RPM? Or does it remain in a rough running condition in all RPMs above the point at which it stumbles.

Answers to those questions would help answer some of the others you have asked.

Petcock? Doubt it, but fix the leak for your own safety.

Fuel pump? Maybe, but it would depend on your answers to the questions I just posted and even then would require some more testing.

Metering jet? Carbking made an excellent point earlier about measuring the jets rather than simply reading the stamped number. And I have my doubts that is your issue, regardless.

Try disconnecting and plugging the vacuum advance line at the distributor and revving the engine with the advance disabled. If the problem goes away, find the open or short circuit between the distributor housing and the condenser, and/or replace the wire between the coil (-) terminal and the side of the distributor. You might be chasing an electrical gremlin by working on the carburetor- - - -which can be VERY frustrating!
Jerry

Perhaps I missed it but have you checked the fuel from the accelerator pump discharge?

BC59,

Yes. That works fine.

JW51,

No the stumbling does not clean up above a certain rpm. It continues to stumble. I can't just push through the stumble and it resolve at a higher rpm. Once it starts stumbling it will continue until I let off the accelerator and very gently bring it up. If I push down the accelerator very slowly it will increase rpm without any stumbling but I have to be very gentle in moving it off idle to get that which is not possible when driving it.

Carburetor needs to be rebuilt. Same symptoms on mine before I bought and rebuilt a correct carburetor. 3 years and 8000 miles later, no problems.

Of course make sure your points etc are in perfect shape and gapped correctly. Take them out and look at them. All talk about the carb assumes your ignition system is in perfect operating condition.

Watch the describbler when you rev the engine by hand. Is the vac advance working? (does dizzy rotate when revved?)

Otto,

Distributor moves and advances timing when revved up. Points have been checked and gapped properly and I’ve put in new coil and condenser. I have also rebuilt the carburetor. It’s a single barrel Rochester B.

Check the wiring- - - - -you're describing a classic "broken wire strands under the insulation" scenario. I've fixed at least two dozen similar problems by finding the broken, and/or frayed wires, either inside the distributor, or from the coil to the dizzy. That constant motion of the housing by the vacuum advance working back and forth is rough on primary wiring. I use high flex voltmeter test lead wire for that one, with soldered connectors on both ends.

www.ebay.com/itm/352887552664?

Jerry

Jerry,

I've replaced both wires coming off the coil. I'm not familiar with any wires inside the distributor. Any more guidance on that?

BTW... I did disconnect and plug the vacuum advance line at the distributor and revved it up but had the same issue!

There's a wire from the condenser to the points. It can get chafed or damaged and short out the ignition intermittently. A bad condenser can also cause the problem you're having. The offshore made condensers we're getting these days have a ridiculous failure rate- - - -sometimes as much as 50% or more.
Jerry

Jerry,

I replaced the condenser yesterday and both wires coming off the coil two weeks ago. Still having same problem.

Given that you can clear the stumble by squirting additional fuel, I’m a little skeptical that ignition is to blame here. And I say that as someone who has learned more than once, that it’s almost never the carb.

Perhaps the add’l fuel can cover up an underlying ignition problem. Jerry might have to weigh in there.

We have a lot of forum members in NC. Would be nice if someone could loan you a different carb just to bolt on and see what happens. I would, if I were closer. A pressure/volume test of your fuel pump might be a next logical step.

If choking a little solves, or at least minimizes the problem, chances are that it's actually a lean problem- - - -but making sure the ignition is up to snuff is always "job one" before concentrating on the carburetor exclusively. One more test- - - -check the voltage coming to the coil from the ignition switch- - - -it should be within a small fraction of a volt as what can be measured at the battery posts with the engine running, unless there's a ballast resistor in the circuit. Once you're sure the ignition is doing its job properly, try raising the float level in the carb a little. It should almost, but not quite allow fuel to drip into the venturi at idle speed. As soon as the RPM rises to around 750-800 or so, you should be able to observe a mist of fuel coming from the venturi as the airflow increases above idle.
Jerry

Pastor,

I think you just found your problem. Check the metal surfaces between carb and manifold. If they are smooth, replace the gasket and torque properly.

JW51 and Jerry,

I am wondering about my distributor wiring. I pulled out another carburetor from building. It was another rochester B. I checked it out, put it on and still had the same problem. Upon further examination of my distributor I noticed that every time I have the problem the distributor will rotate but then slip back into the beginning position. Whenever I moved the accelerator up slowly the distributor will rotate and stay. I don't have the problem when that happens. As I have reviewed some diagrams of distributors online, I noticed that spark plug wiring does not match. What I am seeing online puts the number 1 spark plug wire at about 5 o'clock on the dizzy. My number 1 spark plug wire is more in a 1 o'clock position on the dizzy. Is this a problem? Or can some wire up in a different order?

When I get the BB on the flywheel to line up with the needle on the bell housing the rotor does face the number 1 plug position (as I have it now). The number one cylinder is also top dead center on the compression stroke at this point.

Do I have my wiring and timing all out of whack? Do I need to adjust all of it to follow the order of putting the number 1 plug wire in the 5 o'clock position on the dizzy? Should my spark plug order then be 1,5,3,6,2,4 moving clockwise?

Mine is currently in the order of 1,5,3,6,2,4 moving clockwise. The difference seems to be just that my 1 is around 1 o'clock instead of 5 o'clock.

If the #1 cylinder is at TDC (per the BB) of the compression stroke and if the rotor in the distributor is pointing at the number 1 plug wire (no matter what o'clock the wire is plugged into) and if the order of the plug wires on the distributor is 153624, look elsewhere for your problem.

Thanks Bill.

Every time I experience the issue it’s when I pull up on the accelerator quick. When this happens, the dizzy rotates counterclockwise but the rotates back. When I gradually bring the accelerator up the dizzy rotates and stays forward and I don’t have the issue. Is it normal for the dizzy to rotate and then “slip” back when you pull up accelerator fast?

One more question guys... Well... Who am I kidding... It's probably going to be many more questions... Anyway... My heat riser does not have the spring and it is rusted into a stuck position. I have driven the truck like this in the past with no issues. How big of a problem is it? I have another intake and exhaust in my building. That exhaust manifold riser is not stuck but I would need to get a spring for it. I am thinking about cleaning those up, painting them, and putting them on the truck but don't want to go through all that the heat riser isn't really related to this issue. As it is now, I can't get it on the road. Not enough power to switch gears unless going down hill.

The distributor rotating is the vacuum advance working. Note the distributor's position when the engine is off (no vacuum). Then start the engine and let it idle. The distributor should move CCW (advancing the timing) because idle vacuum is typically around 18 to 20 inches of mercury. Then rapidly open the throttle for just a second. You should see the distributor rotate CW because the vacuum drops, then go back CCW when you let go of the throttle.

If the distributor rotates CCW at idle the vacuum line is connected incorrectly. There should be NO distributor movement at idle. As the RPM increases, the distributor housing should rotate CCW (opposite the rotor motion) and stay there as long as a steady speed is maintained. If the throttle is opened quickly, the housing will rotate CW and stay there until the speed increases and stabilizes- - - -then it will move CCW and stay there again. If your timing is being set to the "ball and pointer" with the vacuum advance operating, you're about 10 degrees late on the initial timing. That will cause terribly poor performance. Disconnect and plug the vacuum line at the distributor- - - -THEN align the ball and pointer at idle speed.

Yes, the heat riser flap valve needs to function properly- - - -no, it will not cause the problems you're having.
Jerry

Jerry,

I most certainly set the timing to the ball and pointer with the vacuum advance operating. I did not even realize I needed to disconnect it in order to properly set the timing! I hope that fixes my problem. I'll update once I do this.

The "ported vacuum" fitting at the carburetor is where the distributor line should connect- - - -and if it has vacuum on it at idle, your idle speed is set too high. DO NOT use the windshield wiper port in the intake manifold for distributor vacuum.
Jerry