The Stovebolt Site Forums The Shop Area Engine Shop I am stumped

Hello all,
I will try to be brief with my description. My 235 in a 53 truck will only run with the choke on. Tonight, I installed 2 bigger jets, a 60 and a 63. I started with a 57. It will run with the 63 but only at 1/3 throttle or more. It will run with the choke wide open but only at 1/2 throttle. There is a popping sound coming from the throat of the carb. Carb is a Rochester B.

Here is what I have done so far. Newly rebuilt engine with maybe 1 hour on it. New head and valves. I have tried 2 new fuel pumps. Valves are set on 8 and 18. I have searched for a vaccum leak using starting fluid, WD 40, carb cleaner, and a propane torch. I cannot find one. Timing is spot on. The distributor has a pertronix ignition on it. AC plugs, new coil that matches the distributor, new plug wires, cap, etc.

With the choke 3/4 closed, the engine will take throttle like it should when the choke is open. Open the choke and it acts like it starves for fuel. The carb has had 3 rebuild kits in it.

Any ideas or suggestions greatly appreciated.
Thanks, Richard

Big vacuum leak - vacuum line disconnected, intake manifold not seated - gasket - crack, Carb gasket - spacer?

The engine is getting too much air. When you can add enough gas to make the mixture rich enough it runs.

Unless there is a big crack in the head, your problem is between the face of the head and the bottom of the throttle plate.

Perhaps you have a vacuum leak where the intake manifold meets the head.

You sure you don,t have open valves. If timing is correct there should be no popping. Perhaps valve lash is to tight?

Put a vacuum gauge on it. You might have many places to check and need to isolate them one at a time. I had a vacuum line crack at the grommet in the firewall. Could not see it but when I pulled it off and plugged that line off the problem went away. Start with the basics. Are the bolts on the manifold tight? The nuts on the carb base snug? Are all vacuum lines good and snug? Is there a leak at the vacuum advance? At the place where the vacuum advance goes in to the carb? And that is just off the top of my head. Put a gauge on it and fix them 1 at a time. You will get it. Then go to other issues. Yes it will run at idle with out the vacuum advance hooked up.

Richard

GWWirth poses two good questions. You might as well get them out of the diagnosis.

- What procedure did you use to set the timing?
- What procedure did you use to set valve lash?

It sounds like a vacuum leak. As posted above, disconnect the vacuum line at the advance unit and seal the end of the tube. Put a vacuum gauge on the wiper port on the intake manifold.

How did you check for vacuum leaks?

One more thing to check if you have popping in the carb is the spark plug wires. Make sure they are in the right order. A couple of plug wires crossed will make it pop.

I don't know that a vacuum advance line leaking would cause you problems with idling, but worth a look. But seems to me the only time vacuum is applied to the advance is when the throttle is opened, so shouldn't be any at the distributor at idle.

As mentioned, valve adjustment could be off. A bent push rod on a newly rebuilt engine is possible too due to a valve guide being too tight. If it's on a exhaust valve it will pop back through the carb.

I would start with the intake manifold alignment rings, one of them is not seated properly causing a big vacum leak. If not that keep looking for vacum leak, though I think if you just disconnect vacum hose from the intake that should just about eliminate all source of vacum from the system. Then you can check carb and intake better.
Otherwise check pushrods and vavle settings, then timming. Good luck, let us know what you find.

Guys, Thank you for all your responses. It helps me to redirect my efforts. I have the intake plugged and the base of the carb plugged. There is no vaccum on it due to the electronic igniton. My dad has been helping me set the valves (cold). We rotate the engine until the intake is loose and set it to .008, the same with the exhaust .018. Timing is set with a light and looks to be spot on. The engine cranks over easily and revs freely as long as the choke is on.

I guess my next project will be the intake manifold to see if I can tell anything there.

If you can get the engine running with the vacuum gauge connected, adjust the timing to achieve maximum vacuum (at idle).

What kind of carburetor(s) do you have?
How did you adjust it/them?
Adjusting the mixture to maximum vacuum at idle is one technique.

Open valves will act like a vacuum leak, yet you will not find them with the normal ways of checking. Any time you hear popping from the carb, you have something miss firing or back firing in the intake side of the engine. A very lean condition can also cause a miss fire, but usually only under a load.

Recheck the valve adjustment,
pull out all six plugs
rotate the engine till both intake and exhaust valves are closed on the first cylinder,
if you align the timing marks, that would be even better,

adjust both valves per manual settings,

rotate engine till the next two valves are closed and adjust them

do all six this way, do not jump around, go cylinder by cylinder.

Double check top dead center, timing marks, and where your rotor is pointing.
use piece of wire or wood down number one cylinder and rotate by hand till the piston reaches the top. At this point, the timing mark, and the rotor with #1 wire wire should all line up.

Attach your vacuum gauge, plug any port you see on the manifold or carb, and fire it up.
Let us know what the gauge says.

joe

When you say 'there is no vacuum on it due to the electronic ignition', are you saying that you don't have a vacuum advance unit on the stock distributor?

Maybe had intake valves too tight during cam break in? bent rods?

I just rebuilt my 54 235. set valves according to Oly's method on this site: http://www.stovebolt.com/techtips/valveadjustment.htm
worked like a charm.

However, my rochester that I had been using forever, I rebuilt it and then it was not happy-couldn't adjust and was seeping out the bottom after the rebuild. The bottom butterfly was wallered out and needed to be sleeved. Why it showed up after the rebuild, I don't know. It was fine before. But check it out. you can get a serious vaccum leak there. And a leak that close to the venturi really screws with fuel atomization.

I bought a vacuum gauge yesterday and will be trying it tomorrow. To answer 1 question, there is a vacuum advance on the bottom of the dist. But there is no hose connecting it to the manifold. I have all manifold and carb vacuum ports plugged. I will be going over valve settings, timing, and using my new gauge. Thanks again for the responses and I will post my findings soon.

I would guess that if GM put a vacuum advance on the engine, then I would think that it is safe to say that you need to have it hooked up. You mentioned that you have electronic ignition. Is it Pertronix or something similar? If so, then you need the hook up the vacuum advance imho. I don't know if that will solve your problems, but it is where I would start if it where mine.

Ok, I got a chance to work on the truck some today and here are the results. I re-adjusted the valves according to the procedure on this site. Then I started the motor with the vaccum guage hooked up below the carb on the intake. With the choke on, I have a steady 17-19 lb vaccum. I started opening the choke and it would run until the choke was about 1/2 open. The vaccum was still holding steady @ 14. I then advanced the distributor and the popping sound mostly disappeared. The distributor is almost at full advance. It will now run at a fast idle with about 15 lb vaccum. It is much better with the valves adjusted and the dist. advanced. However, even though it is warm, it will not start back unless I choke it.

I feel like I still have a vaccum leak and did not get time to look any further today. To answer 1 question, I do have the vaccum advance still on the stock dist. but the vaccum ports on the intake and carb are plugged. Do I need the vaccum advance with the electronic conversion? It seems to rev freely. It will now run with the choke about 90% open.

Thanks again for all of the help and your suggestions are still most welcome. I have made progress, but there is still a way to go.

Yes you need the vacuum advance. You may need to reposition the distributor so you have more room to adjust it. One tooth should be enough.

As for the vacuum leak, keep working on the timing, as you advance it more, the vacuum will continue to rise. When you see the vacuum start to reverse and go down, you have advanced to far. Get the timing right, and you may find most of your problems will go away.

For information only, the vacuum advance only works when the carb is open, this is whats called ported vacuum. The air going through the carb creates vacuum pulling the advance in. Manifold vacuum is created from the pistons sucking air into the cylinders and the throttle blade blocking the air flow creating a low pressure in the intake manifold. Don't confuse the two. Manifold vacuum drops as the throttle is opened, ported vacuum rises. Ported vacuum is above the throttle blade, manifold vacuum is below the throttle.

Mechanical advance is a system of weights and springs that advance and retard the timing according to rpm. You should see this working using a timing light and changing rpm. As the engine increases in rpm, the advance also increases.

As mentioned above, a often overlooked place for a vacuum leak is the throttle shaft bushings on the carb base. If you can wiggle the shaft, the base or shaft itself is worn and will need some repairing.

You should still have the vacuum advance connected with the electronic ignition. What type of ignition did you install? I've got a car that has been converted using Pertronix stuff and you need to make sure that your getting enough voltage to the distributor. Electronic ignition is affected by low voltage issues.

I do have the Petronics igniton and I have converted to 12 volt. I am also running the 12 coil they suggested. Because all of this was unhooked before I got the truck, does the vaccum line run from the vaccum advance to the bottom of the carb near the mixture screw? Thanks again to all. I finally feel like I am making progress.

Yes, that is the correct routing for the vacuum advance.

Hello again,

Sorrry for the slow update but I have a friend who is a Chevy mechanic from way back and I got him to come over and listen to my truck. He checked a few things and said I have a bad carb. He said there is something internally wrong with it, he had seen it happen before.

I have another friend who let me use a carb from a 250 six to see how that would do. I had to change the base and the linkage is all wrong, but it would bolt to the intake. We worked the throttle and choke by hand and it almost immediately began to run better. After it warmed up for a minute or so, it idled and ran like it should.

Now I need to find another rochester model B. Anybody got a good one? Anybody know a good source?

Thanks again everyone!

I did not have a carb with purchase of my 1958 235. I purchased one from Parts wanted , Stovebolt. When I started engine after renew, I had similiar symptoms. After pulling my hair out adjusting and readjusting, I called my mechanic (Old Timer)and he said, Pull it off and bring it in. I watched him as he disassembeled the Rochester "B', examine each part, give a blast of air thru its passage. When he reached the bottom, he said, "Your problem is NOT in this carb, It is somewhere else .(Vac leak, valve setting, timing etc) I watched him reassemble the carb, then took it home, reinstalled it and fired the engine. It started right up, Purred like a kitten at idle, and ran like a Cat with turpentine on its butte when I put my foot thru the carb. To this day, I do not know what was screwed up in that carb but there must have been a particle of dirt or gasket somewhere as that engine continues to SCREAM today. If you want to throw out that carb, I will gladly take it off your hands and "Assist" you in finding a different one! Good Luck, Jerry

Thanks for the reply Jerry. I have had this carb to our local guy who has rebuilt GM carbs probably 50 years. Everyone in this area will tell you he is the go to guy. He put 3 brand new kits in it with the same results each time. I then had my old line Chevy certified mechanic put another kit in it with the same results. We tried kits from Oreilly, Advance, and NAPA. It has been dipped in carb cleaner, sprayed with carb cleaner and even ultrasonically cleaned. ( I have a cleaner. ) I have used compressed air thru each passageway as well as carb cleaner. It seems to be perfectly clean, it just will not draw fuel at an idle without the choke. My father, who is 81 has rebuilt his share of carbs also. I can find no external cracks or leaks. I am at a loss to what its problem is, but I know when I switched carbs the difference was immediate. So, yes I would like to have another Rochester B.

Thanks again,
Richard

I hear you, just relating my experience. I wish you the very best. It would be enlightening to take your carb to my mechanic and see if he finds anything. He is a classmate of mine and we graduated back in 1962. I will pm you the source for Rebuilt Carbs that I believe to be straight forward. Good luck, Jerry

Look for a rochester monojet( Improved version of Model B) found on 250 cu in. engines.Should bolt up and the linkage work.

.....has anyone ask or sent Carb King a PM? seems I remember him being the carburetor guru around here. Might be worth a try or conversation with him, Carb King, you listening?

Thanks again guys for all your help. I will keep you posted.

Try to find a Carter. I had a similar experience with a Rochester B and it leaked just enough air to cause a bad hesitation, I believe the throttle shaft was worn. Replaced with a Carter YF2100S...which is year appropriate for my 235 and it runs great.

Well, I guess I got an early Christmas. Another guy on the forum had a rochester and I bought it from him. Bolted it up and it runs and idles like it should. I want to thank everyone again for all of their help!!

Glad you found the problem. I had a similar problem with mine. It would not attempt to run without the choke completely closed. This told me it was starving for gas. I thought fuel pump, but plenty of gas was coming out the line to the carb.
It turned out todays ethanol gas had caused my carb. needle to stick to the seat. I actually had to use pliers to pull the needle out.

I see lots of carbs 'rebuilt' but the lead/aluminum plug were never removed. If new ones come in the kit then they should be replaced as they can leak... plus its the only way to clean the passages thoroughly.

Lead shot or fishing weights also work well as plugs.. as do rivets.

Not sure about the carb you used, but if you don't use the proper carb gasket it blocks a vacuum passage to the governor (you probably don't have a governor, but some carbs do have a passage on the bottom).