Technical low vacuum engine problem

Was the distributor rebuilt???? I had a problem once where someone install a thick shim in between the distributor body and drive gear.... It ended up being the wrong shim...

 

I agree with some of the others that mentioned valve adjustment. True, the Pontiac valve train is non-adjustable but there are a lot of variables that can affect the lifter adjustment. With the so called “bottle-neck” stud that the rocker nut is torqued to 20 ft lbs against if things are not to factory spec the lifters might have too much preload and be keeping the valves off the seat a bit. What head gaskets were used? Heads or block milled? Pushrod length correct?
I think I’d pull the valve covers and follow some of the on line procedures to take the lash out of the pushrod and then see how much a rocker nut can be turned until the 20 ft lbs torque is achieved. Proper preload on a Chevy with adjustable valve train is 1/2 turn, that equates to about .030, is your amount of rotation appreciably more?

 

This may seem like a dumb statement but, Does it have the stock Balancer on it? If it does, hav you verified that TDC is actually what it say on the Balancer and timing pointer.
Years ago I had a 68 Firebird, first motor was a 69 400, swapped in a new distributor, ran like shit. put the old one back in, ran exactly the same. after beating my head for a while, I figured out the the outer ring on the balancer had slipped and I was setting everything up wrong. It had no vacuum would away start right up but ran like shit. swapped the balancer set all the timing up again, worked perfect.

Since that happened I have known of 3 other 350 and 400 Pontiac motors that have had the balancer slip on them and the timing would be off when you checked it.
just something to look at.

 

ok, here is where i am right now. i verified top dead center with the timing mark. stuck a copper wire in the #1 cylinder and cranked the engine using the crank bolt. rod stopped moving just short of 0, then started dropping just after 0. if it is off it could only be a tiny bit. i removed the carb and made a plate to cover the carb mounting holes with a gasket, hooked the vacuum gage up to a port and plugged off any others. i then cranked the motor over with the starter and watched the needle bounce to 10. flicker is a better word for the needle movement. i then removed the intake and studied the gaskets and surfaces. surfaces are not perfect, but look no different than any other old casting i have seen. gaskets appear to be covering the important stuff. i then took the front timing cover off. timing dots line up as per the motors manual. i ordered all new gaskets. parts guy said there are two different gasket sets for the motor, i ordered both. does anybody know what is different.
problem is low vacuum, i think, which is causing it to run bad. i don't think the running bad is causing the low vacuum, or am i wrong?
car ran horribly before the carb rebuild, actually would not stay running enough to move it without a "neutral drop" type launch and coast after it stalled. i don't have another carb to try at this time.
next step pull the rocker covers, valley pan and watch the cam/lifter/valve action. remove the rockers and fill the cylinders with air and listen for leaks. OR?

 

A long shot here. Many years ago I had an AMC 360 engine that ran with much the same symptoms you describe here. Low vacuum, very poor running, no power. Compression checked good on all 8 cylinders, but yet after 2 days of checking everything you've already checked, I found a broken intake valve spring. It would keep the valve closed at cranking speed but not at idle or above. After replacing that one spring it ran like new again. Might not hurt to have a look. Good luck.

 

Manifold vacuum is affected by different things. It indicates mechanical condition and state of tune. Poor cylinder compression will do it, but you checked that. Late or retarded ignition timing. Tight valve lash, vacuum leaks, restricted exhaust, etc.

A hot cam will run far less manifold vacuum than a stock setup.

I think by pulling some valve covers you can eyeball the valve timing. When #1 cylinder for example is at TDC #6 valves should be exactly at overlap. The reverse is also true. This should hold true for any V8 except for Buick Nailhead as I understand it.

 

To check valve timing pull valve cover turn engine over until the exhaust valve on #1 is closing watch until the intake starts to open the are both open the same amount the timing mark should be very close to top dead center.

 

Which way do the idle screws make it stall? Here's an interesting write up on the idle circuit.

http://www.73-87.com/7387garage/drivetrain/qjetidle.htm

 

Ted or Mark...did the air noises at carb, at idle, sound strange or not? ..and I never got an answer if it still has a Qjet.

Ok, so explain why the car would barely run before carb rebuild, but is much better but still not right?

I don't mind eating crow, but if you guys do now say the intake air noise was somewhat odd, and if it is a Qjet, I still say a warped airhorn could be causing a "major" internal vac leak. You are not looking for a small leak like a 1/8" hose off, or a slight gasket leak at the manifold-to-head. It has to be a fairly large leak..."if" it truly is a leak that is causing that low reading.

You can run(idle) that or any engine on whatever carb you have there, even a one barrel. Just make a 3/4" pine board adapter, and lag screw the smaller carb on, JUST to get another vac reading "if" the Qjet might be suspect..

I'm not saying your carb guru is at fault, but you assumed the carb is mint only because "he" did it. Never go down the rabbit hole without properly testing each component first.


and, see post number 5 on this thread below:
https://www.hotrodders.com/forum/quadrajet-idle-strangeness-120243.html

.

 

This is getting better than watching HBO. Sorry!
Valve spring issue sounds interesting.

 

-----------------------------------------------------------------------------------------------------------------

Actually, any 4 cyl, 6 cyl, and 8 cyl. engines can be checked this way. I'm sure a V10 or V12 as well, and maybe a 2 cyl. or 3 cyl. as well. You just have to know how.

Anyway. On a 4 cyl engine, with #1 cyl at TDC the 3rd cyl. in the firing order is the cylinder you would check. With #1 at TDC the valves on the 3rd. cylinder should "rock" if you roll the engine one way, one valve should start to open. If you roll the crank back the other way at TDC the other valve should start to open. If those valves rock open and closed at TDC your valve timing is correct.

The same is true for a 6 cyl. engine, only you use the 4th. cylinder in the firing order to check. Also on an 8 cylinder engine, the same principal holds, you just use the 5th cylinder in the firing order to check (in a lot of cases that would be # 6 cylinder). On a Nailhead it would be cyl. #4.

 

No strange noises from carb when I was there.

 

Like a lot of people have said valve timing is the main suspect.

Pontiacs have fixed valve gear, a rebuilt engine with mixed, Unknown’s parts, is a worry. Heads could of been swapped, deck milled a lot, wrong push rods, lifters, or rocker gear, is all going to upset the geometry. Another one, valve springs can collapse and float

Wrong gasket somewhere, allowing exhaust egr into intake, intake vacuum into the sump. Not likely on a Pontiac tho

Blocked exhaust heat riser, the little flap usually on one exhaust manifold. Had one a caddy totally blocked, exhaust from one bank was pushed back into the intake, it had a twin exhaust with a crossover pipe, so at the back both pipes had exhaust coming out. It would start fine, then run like shit...low vacuum

Happy hunting


Sent from my iPad using H.A.M.B.

 

Right. But it won't be cyl. #6, that's what I was trying to get across. It will be interesting to see what ends up being the culprit!

 

There were two Ram Air III cams 068 and 744. You probably won't want the 744. If you still have the t/c off, pull the fuel pump eccentric off and look on the nose of the cam. It just might have a letter on it. If it's an S, you may be in luck..
Let's see...
Difference in the gasket sets could be the intake gaskets. Most sets come with the early and late ones. Intakes rarely ever leak on Pontiacs , which you found, and they don't leak into the lifter valley. There's a cover there.
They don't have heat risers in those years.

 

Okay, when you get it running again, try this:
See the vent tube towards the front? With the engine off, stick a small tune up screwdriver in there and see if you can push down on the primary rod hanger. If it pushes down and springs back, then start the engine. See if the screwdriver drops down and stays there. Wing the carb open quickly. It should jump up momentarily ,with the lack of vacuum. If it doesn't do all those things, you definitely have a vacuum issue. It'll never run right that way, no matter what else you end up fixing.

 

Well?

 

Frank, it is a quadra jet. i have not ruled it out but out of respect for Kurt's past work i am giving him the benefit of doubt.

 

it would only stall when screwing them in. no change until it hit that point.

 

Mark, i will try that when i get it back together. i did stick the screw driver into the vent and pushed down on the power valve, it pushed in and the spring popped it back up. as it should.

 

i will check the cam tomorrow thanks. is the 744 a big lift cam?

 

this is the type of testing i needed. this should determine if i have a weird gear/chain set or the wrong cam.
i am a couple days into this and feel like an idiot that i can't find the problem. fixing it will be easy, finding it is hard.

 

No ..About .404 / .407 but more duration.. More of a stick shift cam.

 

Just watch number one the timing mark doesn't care if it's TDC compression or exhaust.

 

This is almost certainly not your problem, and it might have come up already, but Pontiacs aren't on #1 cylinder when the timing dots are lined up on the cam, it's on #6. I had to double check that on my OT Firebird when I changed out the timing chain.

 

this one the dots line up on top dead center #1, like the book stated.

 

The 744 was Macs try at a "roller like" flat tappet cam, it didn't turn out too well. I had one in a 69 GTO 400. It idled lumpy and sounded like it would rev to the moon but it would hit 4500 and brake up. The car was mint and had a M-22 with 3:90 cogs, it had 9000 original miles when I got it ( I paid $1000 for it, oh the good old days lol). We put a Nunzi version of the 068 in it and wow did it come alive and idled much smoother..

 

i think i found the problem, or a major contributing factor. i verified the timing test as stated above, marks lined up, then rotated the crank back and forth while watching #6 , as @Aaron65 stated. one valve would open in one direction past alignment, the other would open in the other direction. then with the dots lined up again i checked the intake lifter of number 1, it was slightly starting to open. ok
i needed to find out what this cam was as i was told it was a stock cam for this car. i took it out and found a competition cam # xe-268-h-10 which the machine shop said is too much for a street driven, stock [show restoration], automatic car, without doing mods to the distributor and carb.
so this is the one "race" part amongst a herd of stock parts.......................the zebra at the horse farm.
when i get it back together with a stock cam i will report back.
thanks for the help, is was all helpful in one way or another.

 

Okay..Don't just get a replaced Sealed Power or Mellings cam. It would be a fits-all deal , and short on lift and duration.
I'd get a blueprint Crane 068 or similar. No reason why you can't make it work with just a few tweaks. It will be real torquey and pull hard. Maybe stick in a 2 or 4 deg. advance key. You'll need some sticky tires though.