The Stovebolt Site Forums The Shop Area Engine Shop low manifold pressure ?

Besides vacuum leaks what are causes of low vacuum at the intake. The truck runs fine but will not idle unless the choke is on. I don't think it is pulling enough vacuum to get the gas up when the choke isn't on. I'm dealing with an updraft carb. I'm on my third carb, the first two were used and rebuilt by me. The last was NOS. Problem still exists. No vacuum leaks.

I'm going to test it tomorrow, it's to late tonight.

Jr

What is the vacuum at idle? Have you done a compression test? Are your valves adjusted correctly? Let us know what you have and we can help.
Mark

Valve adjustment and general engine condition, and by that i mean worn out rings or cylinder bores. Check the valves first, this is a case of a little loose is better then a little tight.

Joe

I know you think the leak and idle circuit concerns can be ruled out but I think that is where the trouble will be found if the engine seems to run well off idle as you say.
Vacuum leak probably.
Consider the valve guides also when searching for source of leak.Probably not enough wear for a leak of this volume but I once saw a guide come loose and break up causing a massive leak.

A vacuum leak is only one of several causes of low vacuum. Ignition timing is one of the big ones. Valve adjustment is another, and as has been mentioned, worn piston rings can cause the problem. Do a cylinder leakdown test to try and spot any compression leak, valves or rings, and try advancing the ignition timing to see if it's just late timing. Also, use a Propane torch (unlit) to check all the manifold-to engine gasket areas for leaks. If you find one, the engine should smooth out or speed up as the gas enters the leak.
Jerry

So I ran my test and vacuum does not appear to be my problem. I'm a bit disappointed, thought I had a avenue to go down I haven't messed with yet. Was 16 lbs with the choke on and 16 lbs with it off. It runs maybe 5 seconds with the choke off then dies quickly. I was hoping that the vacuum would suddenly drop when the choke was off. I guess I'm back to my idle circut.

Am I correct that the fuel pump's only purpose is to keep the carb bowl fuel. Assuming that the bowl is full the fuel pump is not going to effect the quanity of gas entering the engine. The quanity of gas is determined by vacuum? I'm basically not getting enough gas with the choke off.

When the choke is on does the gas get pulled from somewhere other than the idle circut?

The fuel pump's job is to keep the bowl full. However, it is actually air rushing through the carb that creates a venturi effect. The main metering jet determines how much gas is injected into the air stream. I'm not sure vacuum has anything to do with your problem. It could very well be crap in your main jet. I would take the air cleaner off, rev her up, and put your hand over the carb. Quite often this will suck the trash through the jet and it will run better.

I think your correct about my vacuum not being my problem. What you describe regarding putting you hand over the throat of the carb I think is exactly why it will only run with the choke on. The choke is basically doing the same thing. What I am wondering is that when the choke is on is the carb pulling gas from the main jet along with the idle jet. And when the choke is off is the gas only coming from the idle circut.

I've been around car many years but I don't have much of an idea of how carbs work

Applying the choke pulls gas through all the circuits of the carb, low speed, high speed, and even the power valve under sone circumstances. It sounds like your idle circuit is clogged up, and the only way you're getting any gas at idle is by strangling the airflow and creating vacuum everywhere in the carb. Try tearing the carb down and tracing the idle circuit passageways with a piece of soft copper wire to check for obstructions. Use a spray can of carb cleaner to flush out all the idle circuits, followed by massive volumes of high pressure compressed air. Then gently bottom out the idle mixture screw and back it out about 2 1/2 turns for an initial adjustment. Once the engine warms up, it should idle without the choke applied. Then you can adjust the idle mixture for the smoothest idle and/or the best manifold vacuum.
Jerry

I pulled the carb and cleaned out the idle circut. Pulled the 2 jets and blew the upper and lower carb out. One problem I see is the the hole in the lower idle jet is about the thickness of a human hair. I think even under the best situations that is going to be a reocurring problem. the upper jet is a bit bigger. The carb is NOS, never been mounted until a few days ago.

I will adjust the idle mixture screw out 2 1/2 turns. I was reading a service manual of some sort that said 3/4 to 1 1/2, so I have been using 1 turn.

What vehicle and what kind of carb are you talking about? NOS doesn't mean that it doesn't have trash in it.

46 GMC COE, carb is a Carter BB1. I disassembled and blew out the carb prior to installing. I've taken apart and blew out again. It's currently 18 degrees so I may not make it out into the shop today. When I do I have a couple things I'm going to try, I'll report back.

Have you checked fuel pressure between the pump and carb?

Yes. I replaced the fuel pump with a new one, thinking I wasn't getting enough. Didn't make a difference, would still run only with choke on. Then I checked the pressure and it was a couple lbs high so I put an adjustable pressure regulator on and set it from 1-5 lbs, no difference. I've gravity feed the carb bypassing the tank, lines and fuel pump entirely. No difference.

I'm going to readjust my valves, check timing, set points, clean carb out again, blow out the gas lines, bypass gas tank, new fuel filter, new rubber gas lines, and hope for the best. If it still runs with only the choke on I think I will sell.

Just curious, but where is the fuel tank located? In my COE, it is behind the seat. Sitting at this height it is easily high enough that the fuel pump really isn't needed.

Add an inline fuel filter directly under the tank, before the fuel pump. If you have any crap in the tank it will continue to be a problem with clogging the carbs passages.

I had dealt with a similar instance with my truck, it would run, but pretty much any time it was turned off, crap would settle and plug the carbs passages.

As above stated, check your timing, adjust your valves and with an inline fuel filter, you should be set.

Jeff

Behind the seat as well. At this point I am bypassing the tank. I have gravity fed the carb before. Ran equally bad with no choke whether I used the fuel pump or not.

In your 2nd sentence of the original post you say; "The truck runs fine but will not idle unless the choke is on".Then we talk about things to help overcome insufficient fuel feed to the carburetor.I think fuel feed deficiency that would effect idle could not allow the engine to run "fine" off idle.In fact my experience has been just the opposite that an engine experiencing insufficient fuel feed to the bowl would idle quite well but could not run when the throttle is opened.
Updraft carburetors are different animals when it comes to getting the idle fuel to the orifice at the throttle where it enters the airstream.The emulsion of fuel and air passes through the joint of the carb upper and the lower which contains the bowl, and sealing of the gasket at this joint is critical to the idle circuit.
Be sure the venturi has not been put in upside down.My picture shows the larger part of the bore toward the throttle plate.
Did you take the main discharge nozzle out when cleaning the idle passages in the lower body?My picture shows some complex passages in this area where the idle orifice tube is fed from the main nozzle area.
If it runs well off idle but won't feed fuel to idle the trouble lies in the idle circuit.

Spent a few hours in the shop today.
- Adjusted the valves
- Checked the timing
- Tore the carb apart and cleaned all passages. Didn't find any issues. Set the float and reassembled with a new gasket.
- new fuel lines, with new filter. Fuel pump has prescreen as well.

Will run only with choke on or with idle screw turned all the way in. I'd guess RPM's are at 2k.

I'm out of ideas. Any ideas I have had I have tried 3 or more times. My dad who is a hard core car guy 50+ yrs is stumped as well.

Lack of fuel to the carb is not the problem.

I wonder if you have a weak spark.
A weak spark will light a rich mixture, where it won't a lean.
I have even seen where plug gaps being to close would cause a 235 to buck and run ruff.

OK Jason, So I'm ready to quit thinking idle circuit problem and go to your earlier thought of vacuum leak.
Your recorded reading of 16 (you said pounds but I think inches/mercury)is not good at all running with the choke or throttle closed---but spraying stuff around potential leak sites showed nothing leaking from outside air.Could the manifold be leaking from the heat box?--where the exhaust manifold and intake manifold are bolted together.There would have to be a crack or perforation inside but I have seen some terrible deterioration in old tractor and stationary engine manifolds.the combination of time and corrosive deposits can make metal go away.
Do you have access to another manifold even a downdraft for a temporary run?
When the valve cover was off did you shake the inlet valves in their guide or visually inspect for a guide terribly loose or missing?

Something bothers me here. With the Rochester model B, the idle mixture gets richer as the mixture screw is turned out. With this carb, it will only run with the screw turned in. Am I mistaken here or is this opposite to what one might expect?

With the screw out should be the way it works, make no sense the other way.

Time to start looking at the spark as mentioned above you could be chasing the wrong tail. Try a different coil wired directly to the battery and distributor, bypassing the ignition circuit. Also check the plug gap and condition of each one. If it's a 12 volt system, wire direct 12 volts to coil, no resistor wire, same for 6 volts, you are looking for maximum spark. Try adding a 7th plug grounded to the block and one of the plug wires hooked to it, the engine will miss, but you can get an idea of the spark color and size.

Does it smoke? cracked head sucking oil,

Try plugging the exhaust pipe, all the vacuum ports, and carb inlet. The blow air in through a vacuum port and listen for leaks. Not real sure how to do all that, but a fellow I work with had a late model Chevrolet truck tested using a smoke machine very much like that. They found a small crack in the intake manifold not found using the standard methods. A new intake solved all its problems, by the way, the truck ran good till the intake heated up and the crack opened up.

Joe

All, I too wondered about the statement "with the idle screw turned all the way in" and thought he must mean the idle STOP screw not mixture screw, as he is in the same sentenance talking RPM.
There are carburetors that enrich the mixture when turning the mixture screw in.The updraft Strombergs we used on Chrysler industrial and many Japanese motorcycle carbs come to mind.Stromberg says the idle needle controls admission of more or less AIR to the idle system.
However my book on the Carter BB updraft says the mixture is made richer by screwing OUT the needle.

I think I'll tackle the ignition. It has been converted to 12v. The wiring to the coil is currently bypassing the harness. I have a wire on a toggle switch directly from the battery to the coil, no resistor. Will try new plugs, coil & points. And redo my temp wiring. It is on temp wiring because I have my dash out.

Regarding the intake & exhaust there was some corrosion on the face of the exhaust manifold and I had to have it machined. The two are no longer connected. I brazed close the two holes that are on the exhaust manifold that feed the heat box. It makes taking the intake off easier and I see all side of the intake good. Can see no issues with the intake.


The idle screw I referenced was the idle speed screw. I had the idle speed screw in all the way. I've had the idle mixture screw anywhere between 3/4 turn out an 3 turns out.

I did not wiggle the valves to see if the guides were loose. I'll do that. What should the vacuum be ? When it is running at high idle (2k rpms ?) no choke the vacuum is at 18 inches. When at low idle with the choke on around 15 or 16. When I give it gas it drops.

The engine sounds really good when it is running, no smoke, nice and smooth and relatively quiet.

I'm taking a few days off for the holidays will report back when I make some progress.

Not using a resistor with 12 volt system is only acceptable if the coil is designed for 12 volts and would destroy the points in a short time.The only automotive coils supplied originally using points and no resistor may have been american motors(that I am aware of).
I think high idle vacuum about right, but why lower @ low idle?
Vacuum leak? spark retarded from port to dist closed?
When the manifold was refaced does it now cause the sealing rings to hold the manifold slightly off the gasket?
I think the leak is not massive but generally diluting the supplied idle mixture beyond the ability of the carb idle circuit to supply---small leak might NOT respond to things sprayed near it.
Maybe could pressure test manifold on engine by; plug threaded ports--remove pushrods on intake valves that are open--cover carb inlet with? hand?--apply air/smoke through threaded port--look/listen for issue ????--smoke from a cigar could be allowed in suspect areas to watch for draft
Someone else will give us some leak testing ideas.

I agree running a coil on 12 volts might wear points faster, but at this point, you need to find the problem. Get maximum spark, if it runs better continue down that road, if it doesn't make any difference unhook the bypass and keep looking. Sometimes you need to sacrifice parts to get a final result.

From what I have read so far,

three different carbs have been on,
timing checked numerous times,
intake not sealed to exhaust,
ignition bypassed,
valves adjusted a few times,
fuel feed from overhead can,

engines runs on its own @ 2000rpm
engine only idles with choke plate closed
engine idles and runs with 16" vacuum
no change when spraying around intake, at what rpm was this done? and what was used?

What is the condition of the engine, rebuilt, refreshed, original running, original unstuck and running? How long has been running?

What is the condition of the intake to head surface? Does it have sealing/locating rings in place? what about the gasket, new, used, sealer?
What about carb to intake, hows it sealed?

When adjusting the valves, are all the adjusting screws even? or, is one or two set different then the others?

Add to the list if I missed something, I am trying to get all the facts for anyone that might of missed something.

What is the weather conditions where you are at? could the carb be icing up at idle since you no longer have heat to the intake? It can happen real easy if the temp and humidity are just right. Try adding a hose to the carb inlet wrapped around the exhaust pipe so it heats the air going in.

Joe

I got back at it this evening after a few days off. took the dist. off and rebuilt it and cleaned up my temp. wiring. New points, condenser, cap & rotor. dist. looked fine before but now it looks great. Unfortunately the problem is still the same, no change.

Is there a way to check cam timing without tearing the front of the engine off. Maybe the timing gears skipped.

The truck was running when i got it and had been for many years. It came with the problem I am currently fighting. the old farmer I bought it from used it for rocking his field once a year. I'm guessing he just ran it with the choke on.

53 or 54 Chevy 235 in a 46 GMC.

Intake surface to head is good, new gasket with rings with a light coating of sealer. Same with carb to intake.

Weather has been cold, but today was a heat wave at 40.

Jason,
The correct cam/valve timing can be verified by just removing the valve cover and spark plugs (for easy rotation) and observing that overlap position for 1 or 6 occurs when the TDC marker shows TDC.
Overlap is the position on top center end of exhaust stroke where the inlet is just opening and exhaust is just closing.If perfect both will be off the seat equal amounts at TDC.
Moving the crank BACK and FORTH from about 5 degrees BTC to 5 degrees ATC should show the valves equally open.
The results will vary a little if the valve adjustment is much too loose.
One tooth off will make a difference that is easily discerned.
And yes if it is a tooth off the vacuum will be low and performance affected but it will run pretty good--at some speeds and loads--if the spark timing has been optimized.
I think you are looking at your next best test in anylizing your problem.

Timing gears do not skip and keep running. You would have to know the cam timing to check against top dead center and opening and closing cycles. You will need a degree wheel and dial indicator also, so really, NO you can't tell with out looking. It should start and run even being a tooth off, but it would have been installed that way. You can advance or retard the distributor enough to get them running, but power level should be off, not a fuel related problem.

With a good hot spark now, you should be able get back to fuel related or vacuum related problems.

Does this have hydraulic lifters or solid lifters? What clearance did you adjust them to? You might try adding some more to be 100% sure the valves are closing all the way, even if they are hydraulics if thats possible.

It sure keeps coming back to a vacuum leak or low compression.

Hows the exhaust smell? really lean smells bad and full rich smells bad and smokes, also really lean makes a bunch of heat at the manifold and down pipe.

Can you install a vacuum gauge to the intake, then close off the carb opening 100%, crank the engine and see if you get a vacuum in the manifold? It might lead you to a leak or be more noticeable. Due to firing order it might not show on a gauge, but it should do something.

Joe

I made correction to the statement defining overlap.
Original post was made too late at night for this old man.But I can assure you that observing the overlap at top center will easily show if cam timing is correct within acceptable range.Degree wheel and dial indicator are not needed.

Update.
Checked compression - 125, 135, 130, 130, 130, 135. Great.

Got engine tdc with rotor pointing to #1 wire. #6 cylinder valves are overlapping. I think that was what I was looking for.

Do the mechanical advance springs wear out? Mine looked fine but maybe. I keep coming back to a carb issue but I have been down that road quite a few times and my new carb is so nice and clean and new.... new fuel pump, lines, filter, etc.. and my distributor is nice with new parts. This sucks.

Actually, its not sucking enough!
I am out of ideas, sure hope you find something soon, I can't see that you missed anything. Thats a good tip on the valve overlap and how to check it, I had heard that before but forgot all about it, thanks Bob.

Joe

Does your engine have a PCV valve that may be stuck open?

What type of spark plugs do you have and what is their gap set at?

I think the plugs were autolite 303, but I'm not looking at them right now. Gap would have been whatever the manual said. I think it was .035.

I'm going to replace my plugs and cut this old muffler off and see what that does. Maybe this old muffler is plugged and giving me problems. I have know idea why a plugged up muffler would correspond to an engine idling only with the choke on. But it is nice to try something new.

What sort of vacuum reading should I be seeing. I reading 16 inches which I thought was good.

I don't have a PVC valve. I do have a contraption that is hanging off the oil filler tube that I don't realy know what it does. Maybe it isn't the oil filler tube, it comes out of the crankcase by the distributor.

Sure wish I had a reason to drive to Spokane!

Is the air cleaner on it? Is it still the oil bath type? The air cleaner will actually help richen the mixture some too. I can't remember if you mentioned intake leaks or not.

Well I picked up a set of plugs and dropped the exhaust system. No difference. At that point I was back to sq one, gas problem.

So, I decided to pull the carb and remove the smaller of the 2 idle circut jets. Put carb back on and the engine RUNS GREAT!!!! It doesn't seem to be running rich either. I set the idle mixture screw in needle further than recommended. Its running nice and smooth with no choke. Vacuum is at 13 inches, with no choke 700 rpms, not good. That is the first time I have been able to get a non choke vacuum reading. I'm going to reset the valves and get the timing dialed in. I would think with compression figures in the 125-135 range my vacuum would be better. At this point I don't care, it is running nice and smooth with no choke.

Thanks for all the help and suggestions.

Glade to hear you are making head way.
13" is a little low at idle, but I bet you can get that up some with a little tuning.

Keep us posted, Joe.

Good news Jason, your correection has given us some positive results and some new questions.You enrichened the idle mixture and helped.Does that mean now we can compensate for a small air leak?
Manifold vacuum readings are a measure of the engines ability to take air in and the throttle plate trys to prevent that intake so a vacuum is created.If there is air coming in other than at the throttle the reading will be lower than normal, such as valve guides, gaskets, flaws in castings, valve leakage, etc.Since you have eliminated most of these now what?
I had a tractor engine once that operated a lot in an industrial situation--low load, much idling,low operating temperatures that developed serious idling problems.The usual tune up procedures and external parts replacement, compression check etc revealed nothing so a head removal was in progress not knowing what else to do.
When the inlet manifold was removed it revealed a great buildup of carbon in the area under the inlet valves.The passage was choked down to about a 3/8 inch diameter passage.Opening this area up brought back decent performance.
I don't remember a vacuum test but a restriction like this would have resulted in a lowered manifold vacuum reading with other conditions being normal.
Also a lot of cam wear would result in a lowered reading but it would have to be fairly uniform to not have fluction observed in the guage reading.
Cam wear can get well advanced and still allow good low speed performance.
Observe the valve action with the engine idling and see if valve lift appears uniform.
Another thought to remember.We are used to sometimes using vacuum reading to set spark timing-BUT, spark timing only changes vacuum readings because it changes idle speed at a given throttle opening.