I'm at a complete loss and wonder if anyone has any suggestions. Sorry this is a little long but I want to detail the fatcs.

I'm trying to get a basically stock 400 motor in a 69 442 restoration project running. The engine was completely rebuilt and assembled by a reputable shop. Only deviations from stock are a Mondello JM20/22H cam and cooresponding springs/lifters. Also has roller tipped rockers installed. The carb was rebuilt/restored by Sparky's to factory settings.

The engine runs rough, RPM is 750, timing is 12-14 BTDC and vacuum is 12 inches and the needle is fairly steady. If I restrict the air into the carb, the rpms just up a few hundred and the engine smooths right out. When it runs rough, the exhaust will burn my eyes but with a rag over the carb and it running smoother, the exhaust is fairly normal and will not burn my eyes.

I have disconnected everything that uses vacuum from the motor. Actually, I removed all of the fittings and installed pipe plugs into the manifold. So there is no vacuum going to anything except for the PCV line from the carb to the PCV fitting. But I did cap that at the carb to see if it made a difference and it did not. I also disconned the choke pull-off on the carb to be sure that wasn't leaking. And the vacuum fitting on the carb is also plugged. All the carb bolts have thread sealer since they go into the intake plenum as well as the bolt that holds the dashpot bracket to the intake. I tried another quadrajet (70 455 7040251) and I had the same result. The intake was removed and a new gasket/tray was installed and silicone was used on all ports. The rubber ends were not used...only a bead of silicone to make sure they weren't holding up the intake any. That didn't produce any change. I have run propane around the intake, base of the carb and all of the plugged fittings and nothing changed, I also tried spraaying starting fluid around all those areas and that didn't uncover anything either. All indications are telling me there is a vacuum leak causing the engine to run lean but I can't find it.

At this point I am at a complete loss as to what to do next. I have an intake from a 70 455 that I can put on there to try, thinking maybe that mine has a crack on the underside but I hate to have to tear the manifold off again on a guess.

Anyone have any ideas?

 

First thought is the drivers side valve cover have the breather installed or a plug engine needs to have a little air coming in or will be like a clogged pvc valve. just a thought

 

Well i believe you've established that you don't have a vacuum leak!! I don't think internally the manifold is leaking either.

Here's what I would do:

First make sure you have fresh fuel in the tank. Ethanol blends tend to attract moisture over short periods of time.

Make sure all your spark plug wires are connected in the proper order. Make sure you have the correct voltage at the coil based on whatever ignition system you are running.

Do you have points or electronic? If you have points recheck your dwell to ensure your @ 30 deg btdc. (I would swap out condesor with a known good one at this time.) Then reset your timing back to your 12-14 with vacuum adv disconnected and plugged.

Are you running your vac advance off of manifold or ported vacuum? I would go to ported for now.

Look down into the carb and make sure you are not dripping fuel at idle which would be a carb issue!

With your tach hooked up, set your idle speed back to 750 rpm. Perform the following on each air/ fuel mixture individually. Turn each of your idle fuel mixture screws c/w and see if the engine stumbles and runs rough, once you you reach the rough idle, turn the scews cc/w until your reach the highest rpm on your tach. After you have adjusted the pair, readjust idle, and repeat two more times. On the 3rd time turn each screw in 1/8 turn and reset idle speed back to 750 rpm. If the engine does not stumble and run rough there is a vacuum leak somewhere in the carb, or your metering rods are hanging up.

Good luck!

 

Thanks for the thoughts.

I'm running an electronic points conversion system and the voltage to the coil is good. Gas is only 2 weeks old and tank, pump lines etc are all new. The carb will cause the engine to stumble if I turn the mixture screws in but I cannot get the egine to smooth out at all no matter what I do to the mixture screws. The engine is very responsive when I open the throttle but it just won't idle worth a darn and the vacuum gauge is only reading 12 inches. There is no vacuum running to anything other than the PCV valve. All other vacuum sources are plugged so there is no vacuum to the advance on the distributor.

I can get the engine to run very nicely by throwing a rag over the top of the carb or close the choke part way or hold my hand over the primaries. When this is done, the engine smooths right out, the engine jumps up to about 1000 rpm from 750 and the vacuum immediately jumps to 18 inches. If it ran like this all the time I would be golden. But as soon as I remove the restriction from the inlet air to the carb, it runs rough and the vacuum drops back to 12 inches.

Since I can make it run nicely or like crap by playing with the air coming into the carb, I still feel like there is a vacuum leak somewhere. The carb is completely rebuilt and I also installed a second carb that causes the engine to run exactly the same way. To me, this tells me its not a carb problem although I guess it is possible that both carbs are screwed up. But I doubt it.

The engine in this car had water in it at some point as when I received it 2 cylinders had things rusted in place. So I know there was water into the carb and intake. I was thinking that maybe the water froze and may have caused a crack in the intake. But I just want to be sure I'm not missing something obvious before removing the intake again.

Any other thoughts? If I'm looking at this problem the wrong way, I'd love to have someone tell me what I'm doing wrong. Sometimes you get so close to a problem that the obvious is missed.

 

Check for an air leak by spraying car startin' ether around at all possible places (slowly, so you can tell if you find the place). If there's a leak, the engine speed will go up when you spray into it.

It sure sounds like vacuum leak symptoms, but I can't give you any good suggestions yet.

Maybe some photos of the engine would help someone to notice something out of place.

- Eric

 

Vacuum leak will cause a lean condition, this sounds more like a lean condition with no apparent vacuum leak.

If it were mine, I'd pull the top off of the carb, blow all the fine ports out with compressed air, and raise the float level 1/16-1/8" for starters. Even though it was rebuilt, the engine has some camshaft, and may need more help drawing fuel in, especially at idle.

JMO
Jim

 

Here's a few photos although they won't show too much. The metal line on the passenger side goes to the trans modulator and the vacuum line next to it comes from the distributor advance. As you can see, nothing is hooked up to anything that uses vacuum except for the PCV valve. The line in the back that runs up the firewall goes to my vacuum gauge. Everything else is plugged or capped off.

I used an entire can of starting fluid and went all over the manifold, carb base and pipe plugs and couldn't find anything. A little spray into the carb and it picked up the RPM and smoothed out but went back to being rough when I stopped.

I keep coming back to the 12 inches of vacuum when this problem exists. That seems much too low but is easily corrected to 18 inches when I restrict the air to the carb. That carb is clean as a whistle and since it does the same with another carb I don't think the carb is the issue.

 

I've had situations where the metering rods were not being drawn back into the idle circuit due more radical cam grinds and lack of vacuum. Outside of that and whats stated above concerning float level...Man I'm stumped, normally you throw a rag on it and it will stall or damn well near stall, you may be right on with the cracked manifold if after chasing all the joints with propane or ether with no leaks detected.

 

Jeez, don't you ever clean the grease off that old thing?

- Eric

 

It's lean, no matter what.
It seems that you have chased down the vacuum sources.
Give Sparky's a call on Tuesday. A call is cheap.

Looks damn nice.
Jim

 

I ran it by Sparky's a little over a week ago and we ran through a few things. He was bummed because I had tried most everything he could think of. That ended up in the thinking that the intake gasket was leaking so I replaced it. I'll give him a call on Tuesday as a last resort as I'm holding out hope that I may be able to keep that intake on there. Tore it down once and repainted the intake when it was put back together last week. But I'm running out of options. I don't think this car likes me much because everything with it seems to be one step forward and two steps back. I guess this is the way it goes sometimes.

Thanks for all the suggestions and I'll post something when I get it squared away to let you know what was causing the problem. My money is on a bad intake manifold but I do hope I'm wrong.

If anyone on this thread has an original 69 400 intake, let me know as if this goes in the direction I think it will, I'll be looking for a manifold. The 70 manifold I have will work for test purposes if I go in that direction but that's it.

 

could try spraying it again while holding the choke part closed,maybe the higher vacume will then reveal the leak,

 

I am thinking the factory jets (spec) do not provide enough fuel since you changed the cam.

Adding fuel is the same as reducing air flow(rag on carb), as far as mixture cares.

Squirt a little extra fuel in the carb while running and see if it improves.

If the rpm comes up and or you smooth out, increase the main jet size two points at a time

and see if that cures it.

Good luck, Dave

 

This sounds much like the problem I am having with a 57 Olds 371 with a Rochester 4bbl that was just installed. We worked through firing order etc and now it starts very well. However, it stalls as soon as it is put in drive.

My "fix-that-is-not-a-fix" is that I put a tin air-baffle with slots cut into it on top of the front half of the carburetor and that makes it run decently at lower speeds.

That said, it does not upshift or downshift quite as it should, is prone to occasional stalling at low rpm, and once the engine is fully warmed up, seems to be getting very little fuel upon gas pressure.

To me, this seems like at least partly a vacuum leak or a poorly adjusted carburetor.

 

A higher than stock lift cam requires a higher idle speed to keep it running....smooth, 750 RPM may not be high enough to sustain a smooth idle with the cam. Throwing a rag on top the carb will cut air and increase fuel....so you may need a richer mixture. The power valve operates on low vacuum as when you floor the pedal so if the RPM's cannot sustain the engine at 750 the loss of vacuum will cause the power valve to go rich adding to the problem.... I would try and increase idle speed with mixture screws and or idle adjust screw and live with a 850 0r 900 rpm idle speed.... I am guessing your problem is not a vacuum leak. Note thouigh that primary jet fuel supply does not come into play till you are off idle circuit and that is when the throttle plates are starting to open.

 

Thanks everyone for all the thoughts so far. Let me give an update on where I am with this. I spoke to Sparky's this morning and we had a good discussion about all of the things that have been done to try to find the problem. At this point, he wants to get the carburetor back as he feels the problem could be with the power piston. He told me the power piston setting is factory set, is non adjustable and normally there is not a need to play with it. But occasionally, they do get played with over the years by someone who knows what they are doing and those who don't. He mentioned something about drilling out a locking screw to allow it to be adjusted. He said this doesn't happen often but does every so often. He feels the power piston is causing the idle circuit to be much too lean due to the factory setting being modified or off from day one. He said he will check the calibration on it and will also make it so it can be adjusted. He said he just had this same issue last week and the person's car was displaying the same exact symptoms as I have. He modified the setting and the problem was corrected. I'll pull the carb tonight and get it packaged and will send it out tomorrow. He said he would jump on it right away when it arrived. I hope this does it.

 

I've got a similar problem, although without the added complications of non-stock camshaft.

I just swapped intakes, from stock 2-bbl to old Offenhauser 360º and a later model Q-Jet. Same issues you describe in your post, basically, and I'm hoping you found a solution that I can check for.

 

One thing that hasn't been mentioned is plugging off the breather to the one valve cover and hooking a psi/vac gauge to the other(with the pcv disconnected from the valve cover.). If the crankcase shows any fluctuation towards the vacuum side, there is a vacuum leak from the intake to the crankcase.

 

The issue with the engine has not been resolved and became more involved. I decided to replace the Mondello cam with a stock relacement by Comp Cams and decided I was going to let the engine shop do it. So the motor was pulled and dropped off. A few days later the shop called and asked what I had done to the engine as it had a lot of damage. They tell me it looks like the engine was washed down with fuel from a bad carb or severly over rich condition. This also thinned the oil. As a result, the rings were shot, the bearings were worn and the valve stems were galled badly to the point the valves were starting to hang up in the guides. #3 exhaust valve had tapped the top of the piston and fortunately the piston was fine. And all this with roughly 2 hours of run time. The engine is being repaired and should be done soon. The shop is helping me with the cost of this quite a bit as they have done several engines for me and were really upset with what they saw, even though they know this was a fuel wash down problem. This really makes me wonder about the carb and if a problem did exist when I returned it to Sparky even though I was told no problems were found. I hate to blame anyone without proof but I am suspicious.

Erinyes, if your engine was running fine before the manifold/carb swap, it would appear that something didn't go well with the parts exchange. I would look closely at the intake manifold seal to the heads and try using propane or starting fluid along that gasket area to see if the engine runs differently. I'm afraid my engine issues may not give a good indication of what is causing your problems given the damage that was found.

 

Bummer about your engine. Nothing like a major issue after just getting something back together!

On mine, I figure either there's a vacuum leak somewhere that spraying is not detecting, or the carb is simply not a bolt-on match, in terms of idle circuitry.

I used a carb from the latest year that did not use computer controls (1978? something like that), but with the APT control that could be reached with some fiddling. Drilled out the cap, threaded it for a plug and fashioned an adjuster, but want to work out the lean idle before making any adjustments.

I did find vacuum leaks at the throttle plates, but as lean as it is, any leak at all would noticeably change the idle. The throttle valves did not appear to be extremely loose.

I have to pull the manifold off anyway, since the front oil seal is leaking a bit, so am taking it in for a pressure test to see if there are any invisible leaks. If the manifold seems solid and I can't find anything else at all, I'll start looking into drilling out the idle tubes.

It does run nice, once it is no longer idling. Ran it long enough to warm up thoroughly, took it on the highway to open up the secondaries and got a nice push-back - not bad for a 4000 pound tank with 3.23 gears! Once I've got this ironed out, I'll start messing around with adding the dual exhaust...

 

Did the engine run better or worse when cold as compared to how it ran at operating temp?

I just found this thread, but I really wonder if your cam timing was correct? One or two teeth off CAN cause these problems. This is not as uncommon as you might think with several aftermarket timing gear sets on the market with adjustable key-ways. Circles look like dots, triangles look like arrows, squares look like key-way markings. Sometimes people make mistakes, etc, etc.

 

Another thing to look into is the carb specs. I would compare the specs for a quadrajet for your model year/engine to that of the quadrajet you used. That later carb may be jetted a little leaner, may have different primary metering rods and may have a leaner idle circuit. If the difference is significant, you might be better off using a carb that is set up like you need or change the one you have, if you can find the parts. Check the throttle shaft busings for vacuum leaks too.

 

We just finished a 350 Olds engine and had a problem where the machine shop milled the heads and manifold but used a surfacer on the manifold and got the angle wrong, a couple of degrees off. We noticed the problem before completely installing the manifold and the shop fixed it. However if the gap is at the bottom of the intake ports you might miss it and have a leak that would not be detected with starting fluid. I would remove the manifold and check that the mating surface is 45 degrees to the bottom at the front and rear of the manifold. If it's good, then set the manifold on the engine without gaskets and make sure the mating surfaces don't have gaps using feeler guages. If all is good reinstall using silicone on the ends of the manifold instead of rubber gaskets making sure that the intake gaskets don't move during the installation. Mark the heads with a pencil where the intake gaskets are when they line up with the ports, then after the manifold is set on the engine make sure the gaskets haven't moved using these marks as reference. After doing these steps you should have corrected any potential problems with the manifold. If the problem persists you can at least cross this off the list.

 

Fresh build with nearly identical symptoms listed by OP right down to the following suggestions for elimination of possible leaks.
Engine went together with a thick Mr Gasket intake set and Loctite high temp sealer on the front and rear block area.
Leak down has been performed and front end has been taken down to verify camshaft alignment.
Cam is a Comp 308-4 and vacuum is 12 inches steady in P but hunting just short of stall in D.
Close up the choke a bit and it straightens right out telling me vacuum leak.
As suggested, all vacuum sources have been capped including PCV with no vacuum in crank case.
Hearing from some builders that this is a common problem.
Going to aluminum is gonna mean a new carb with elec choke by the looks of it, but even the aluminum manifolds are on BO.
Any assistance is greatly appreciated, chased this all the way down the rabbit hole to the point of a mental meltdown.
Original carb has been done and redone and came.off a running 350 with no such issues.

 

Rick, there are tons of folks here to assist you. However, you should create your own thread - read the Newbie thread(s) and create your own thread. We need to maintain organization within the forum and this thread belongs to 69442C and members assisting on his issues. Create your own thread so we can keep things in their respective buckets when assisting others. Thanks and Welcome to CO.

 

Get a can of carb cleaner and check around the carb base and intake gaskets. Propane will not react as fast as carb cleaner.
If you find nothing pull the carb as there is a problem with the rebuild as even the best of us screw up, it could be a leaking power valve in the carb if it’s a holley