Car doesn't want to run after carb rebuild

[Ralph S.]

Hello everyone
I'm hoping I can get the wealth of experience here to maybe point me in the right direction. Car is a 41 coupe with a 42 100hp Merc motor, stock Holley carb. Car did run pretty good with the exception of being little hard to start once warm. Before I got the car here it sat barely driven for about 3 years. I decided to rebuild and clean the carb using the kit from Mac's and I installed a new fuel pump along with a fuel filter before the pump. All said and done, now the car won't stay running. It will run with a bit of throttle but feels like it's not firing on all cylinders and smokes a lot, like it's way rich. Soonest I let go of the throttle it dies. I went to a smaller fuel filter, got rid of the filter all together and still the same problem. Thought maybe that was it since it didn't have a filter before. I also put a new coil in and changed the plugs after the carb rebuild, no change. I'm thinking something is not right with carb so I should probably take it apart again and retrace my steps. Do you guys agree? I'm still new to the flathead world and still trying to figure this car out. I can probably post a video if that would maybe help identify what's going on.

[JWL]

From your description the power valve could be leaking.

[Ralph S.]

I'll take it apart and have it again. Can you recommend a replacement power valve? I'm 99% positive I installed it correctly with right size gasket. Is there a way to test it before I try to replace it?

[hook00pad]

I had something like that happen with my Holley 4 bbl. once and the rear float was sticking down. The result of course, was extreme flooding. I had lost the little keeper snap ring off the float and it was jamming against the wall of the float chamber.

[Ralph S.]

I'll pull the carb off and go through it again. Float was moving freely, and I didn't change it since the car was running pretty good before the carb rebuild, I think I put everything back correctly. I'll double check.

[tubman]

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Take the top off of the carb and fill the bowl with fuel. Check it about 8 hours later to see if it has leaked out. The first Holley I ever rebuilt had the wrong power valve which leaked excessively and I had the same symptoms you are reporting.

[JSeery]

There are members here on the Barn that sell the correct parts (such as power valves) for your carb.

[SofaKing]

I had great difficulty getting my 21 stud to run following 40 year nap and rebuild of everything. Similar performance to what you describe. After much help here, was directed to known problem of replacement power valves not sealing to carb body. Charlie NY sold me one he machined flat, works perfectly. It may help your troubleshooting to know when mine was leaking, the float bowl would drain leading to prolonged cranking.

[dbtenner]

I’ve never experienced the smoking symptom but when I’m starting my 34’ and I’m having to hold the accelerator to keep it running (because if I let my foot off it dies) it’s usually either my

(A) stop screw is not set correctly set or the metal plate is not touching the screw (usually due to vibration allowing the choke bolt to loosen, make sure it is tightened)

(B) or my idle adjustment screws on the back of carb are set to forward restricting the idle gas flow chamber. Suggest backing off of them each one full turn. I have a Holley 94 carb btw. If your getting smoke, it’s running lean right? (Not enough gas). For some reason in the winter, I need to allow more idle gas to flow (back off the idle adjustment screws) to combust with the denser cold air - that is just my opinion.

[Ralph S.]

So to give everyone an update. I took the carb apart again and switched back to the old power valve. Comparing the new one and the old one, it felt like the new one has a bit stiffer spring in it. Double checked everything and put the carb back on. Same issue, car starts but runs rough, like it's not firing on all cylinders, and smokes a lot. Really harsh smelling smoke, as soon as you let off the gas pedal it just dies. I took the top of the carb off and it had fuel in the bowl, so I guess the fuel pump is doing it's job. What is the purpose of the power valve, and when does it come into play or when does it provide fuel? I assume it works off of vacuum. Could one of the gaskets that came with the rebuild kit be bad and it's creating a vacuum leak somewhere not allowing the power valve to operate properly? I started the car from under the hood and giving it throttle to keep it running poorly you can hear the carb sucking air in, almost too loud, but I guess it's not pulling any fuel in with the air. Also I put new plugs in yesterday and I just pulled couple out to check them and they're pretty fouled up. Could that indicate an ignition issue instead of a carb problem. Pretty lost at this point.

[B-O-B]

Not sure this is your problem but how much fuel pressure do you have at the carb? 2-2 1/2 lbs. is best.

[JSeery]

Originally Posted by Ralph S.

Double checked everything and put the carb back on. Same issue, car starts but runs rough, like it's not firing on all cylinders, and smokes a lot. Really harsh smelling smoke, as soon as you let off the gas pedal it just dies.

Usually smoke is a sign of a very rich mixture. Excess fuel gives you black, sooty smoke. Gray is normally oil burning and Whitish is water/antifreeze. Because you changed the fuel pump, I would start there. There have been reports of new pumps providing excess fuel pressure. As already stated, you want somewhere in the 2 - 2 1/2 psi range. Might also be worth while to pull a plug or two. Black and sooty is running rich. If it has been running rich the plugs may be misfiring by now and need cleaned or replaced.

[Mike in AZ]

Still got your old fuel pump? Might put it back on and see if that helps. Mike

[Tinker]

Holley powervalves have been a issue for a while. It can be fixed. Have to mill out the seat.

Fingers crossed, I moved to strom 97s, good so far for 3 yrs. Not correct for my 38 but I have two chandler-grove carburetor that I might run. correct for my 36.


Contact charlieNy or others for a rebuild or info to the 94.





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[Flathead Fever]

Another one of my long answers. I just like people understanding how things work. None of it is complicated if your interested in it.

You asked how the power valve works? The power valve provides extra fuel while accelerating or climbing a hill. There is high vacuum below the throttle plate when the throttle is closed. The further you open the throttle plate the lower the vacuum becomes. Its just like sucking on a straw with your finger over the end. You have high vacuum until you remove your finger. At idle the throttle plate is closed and there is high vacuum pulling the valve down and cutting off the fuel flow. At cruising the throttle plate is not opened very far so there is still enough vacuum to hold the valve closed. When you step down far enough on the gas pedal the throttle plate opens up further and the vacuum drops off. The spring in the power valve is now stronger than the vacuum that was pulling it down. The spring can now lift the power valve off of its seat and allow the extra fuel to flow into the engine. The power valve should not have any effect on an engine at idle unless the diaphragm is bad so that vacuum cannot suck it down against its seat. Then the engine floods out from getting way more fuel than it needs at idle. It could definitely be your problem.

Look down in the carb when its running and see if fuel is dumping into it. At an idle it should only be getting fuel from below the throttle plate where you cannot see.

The fuel pump is probably not the problem. If you have enough fuel to run the engine off of idle, even though its running bad, you should definitely have enough fuel to run it at idle. It would not mediately die when you closed the throttle.

If it sat for three-year's drain all of the gas out of it and replace it with new fuel. This modern gas lasts about six-months before it starts to go bad. If you have small gardening engines that started fine when you used them last and now they will not start it usually because of old fuel. Write down the date each time you put fuel in your old cars if you don't drive them often. I have a chalkboard in the garage to keep track of the fuel in my vehicles. I was just invited to take a '28 roadster pickup to the 70th Hot Rod Magazine reunion. The car appeared in Hot Rod Magazine in 1950. According to the chalkboard it has the same fuel that was in it when I took it to the 60th Hot Rod Magazine reunion. The last time I started it (I have too many projects).

You might have more than one problem going on here. You replaced the plugs. Go back and make sure you did not mix up the plug wires. My dad did it on his '32 roadster and when he got done it would not start. He swore to me he replaced them one at time and there was no way he could have mixed them up. I said, "Dad, did it run before you touched it, if it did then you probably mixed up the wires". I checked and they were all screwed up. I don't what he did? That could be your misfire and idle problem. Looking down from above the engine, #1 is always the furthest forward cylinder on a V8 engine, "except" on a flathead Ford, #1 is on the opposite side. People that do a lot of engine work that are not use to flatheads will assume #1 is the furthest forward cylinder and they would be wrong.

The question is, is it getting too much fuel or not enough fuel at idle? The smoke suggests its getting too much fuel but just for fun when you return the throttle to idle see if you can keep the engine running by squirting starting fluid into it. If it keeps running then the idle jets are probably plugged or they are adjusted too lean. Even though you rebuilt the carb sometimes it takes a lot of air pressure to open those passages up. I use a blow gun with a removable tip. Then I put an adaptor on the end and a short 1/8" metal brake line. That way I can stick the brake line against the hole and give it a blast of air. Just a few months ago it took 120 lbs. of pressure for me to unplug the idle jet on the neighbor's nearly new chipper shredder after he let the fuel go bad in it. Check the jets with carb cleaner sprayed into the orifices to see if it comes out in the throttle body when you rebuild carbs.

Did you adjust the idle mixture screws when you rebuilt the carb? Turn them in until they "lightly" bottom out (you don't want to damage them by turning them too tight) then back them out 1 1/2 turns. Start the engine and turn them both in a 1/4 turn at a time until the rpm starts to drop then back them out until it idles at its highest rpm. You can give them an extra 1/4 of a turn out (rich) for all around driving conditions. Make sure they are both turned the same number of turns. Then make your final idle rpm adjustment with the idle speed screw. Always adjust timing first, air fuel mixture second, and idle speed last.

The best way to check that the idle mixture is correct is with a small propane bottle. When you pass the propane over the carb the idle should stay about the same. If it goes up you have it set to lean and you need to back out the screws. If it falls on its face its set to rich and you need to turn the screws in. The Sate of CA required smog repair shops to all have these propane enrichment bottles. Every time you did a carb adjustment it was mandatory you adjust the idle mixture to the lean side. If you had it adjusted correctly when you added the propane the idle speed would come up 50 rpm. The problem was a lot of the four cylinder engines would idle really rough with the AC turned on with the carbs set so lean. Anyway, that's where using the propane bottle came from. Its a good tool for adjusting air fuel mixtures and you can also use it to find vacuum leaks

[drolston]

Did you replace the carb float needle valve and seat during the rebuild? If so, it could be bad, or have a bit of debris on the seat. Take the carb top off and hold it upside down so the float closes the float valve. If you can blow through it at all like that, the float valve is bad. If you removed or replaced the needle and seat, you must check the float level. If the float level is set to high, it will dump fuel into the carb while running and cause extreme rich mixture.

[Ralph S.]

Hi everyone
To revive this old thread. I finally got my distributor back so it was back to trying get the car running. With rebuilt distributor I still had the same issue. Car would start but it was hard to keep it running, and it smoked a lot, harsh smelling smoke. I also noticed that the carb was leaking a little. Somebody mentioned before those newer fuel pumps put out too much pressure so I thought that might have been the issue. I put the old fuel pump back on and also made sure the float was set correctly. Put little gas in the carb and started it. It ran great until it burnt the gas out and stalled out. Now the old pump wasn't even pumping gas at all. Put the other pump back on again(one I bought from Mac's a while ago) and gave it another try. Car was actually running, not perfect but it would idle. However, as soon as i put the air cleaner on it dies. What would that indicate? Trying to narrow it down to either fuel pump or the carb again. At this point I eliminated plugs, wires and distributor since all that has been either rebuilt or is new.

[JSeery]

Really need to figure out what the fuel pressure is, sounds like a fuel pressure issue. The new pumps are know to have excessive fuel pressure. May need to get the pump fixed or add a pressure regulator, but start with a pressure gauge.

[ford38v8]

Ralph, You are running way too rich, as evidenced by the installation of an air cleaner killing the engine. I'm going to guess that you have multiple issues contributing to this condition, but will limit my advice to the power valve:

Flathead Fever gave you an excellent explanation of the function of a power valve. Now, here is further info on your particular problem:

Your checking operation of a power valve by compressing it's spring is not only inconclusive, but can also be destructive of the diaphragm in the valve. To check the function of a power valve, With the diaphragm hole facing you, put it to your lips and suck. A functional valve will hold your tongue to it by suction, a bad diaphragm will not hold your tongue.

Now that you know the valve is in good condition, determine that it is compatible with your carburetor. Confirm that the face of the gasket surface is flat, and does not radius up to the threaded area. Replace it or machine it flat to conform with the mating surface of the carburetor. This is critical to the operation of the valve, just as it is critical to have a good diaphragm.

Install it tightly to the carb with a good gasket, and reinstall the carb on the engine. Start the engine as before, and you may note a need to adjust the idle jets, as well as the idle speed.

The air cleaner should not now make a difference in how the engine runs.

[fortyfords]

I thought 42 Merc engines ran a Stromberg 48 carb? Once you get it running, unhook the vacuum hose to the wiper motor, hook up a vacuum gauge to the fitting where the wiper hose was and set the idle screws to get the highest vacuum reading for each one, then the idle screws are set correctly. This isn't your problem but needs to be done when you get the rich mixture figured out as everyone suggested.