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Discussion Starter #1
I have a 68 GTO, 400 motor with TH400 transmission. Car will not idle below 1100, stalls when shifted into drive. I have replaced cap, rotor, wires, pulled carb apart and cleaned it out. Had no fuel filter when I bought it a couple weeks ago so I installed one before I cleaned out he carb. Can't find any vac leaks, adjusted fuel/air mixture. Need help!!
 

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I had a vacuum leak that I couldn't hear. Same symptoms. Friend of mine brought over a Pontiac engine guy. First words out of his mouth were "I don't know what else is wrong but you have a vacuum leak". Turns out the nipple for the PCV line in front of the carb was loose in the manifold. Not much, but enough. JB Weld'd it in and no more leak. A few years later, a little "T" connector that supplied the a/c dashpot worked loose. Milder symptoms. Found it and snugged it up. Good luck.
 

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IF you have a Pertronix or MSD electronic distributor and a poor performing coil, that would happen. Even if you have factory distributor, your coil could be weakening at lower idle and not supplying enough charge. I had this happen with a Pertronix distributor, swapped the coil (to their Flamethrower coil) and never had another issue.
 

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Not sure what brand distributor but have Accel HEI, If i set idle timing to 12 degrees I only have 5 in of Vac. I have to advance the timing up to 20+ at idle just to get 10-12" of vac. Not sure if the timing is jumped. I will check compression today and see where I stand there. Ran acetylene around the intake and carb and found no apparent leaks, RPM's didnt even jump when I ran acetylene direct into the carb. I will pull valve covers today as well to see if I have and broken valve springs or if anything looks f'ed up in there. I have a 67 tri-power manifold and carbs that I am going to rebuild to see if that helps but I am waiting on rebuild kits to show up. This thing is kicking my ass. If the distributor is set off a tooth, would that cause a low vacuum condition? I only seem to have decent power if I am on the highway, Low end seems to be lagging.
 

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Hi Brian,

Unfortunately, a vacuum expert, I am not. I am dealing with that in my 69 now. From what you have said and assuming over 1000 rpm it runs good, I would suspect the carb. I know you said you took it apart and cleaned it out, but it seems it is not delivering the fuel it needs at idle.

I doubt it is the fuel pump, but you can install a clear filter in line and validate the flow. I still think the coil could be letting you down at idle, BUT from what you said, it dies immediately if the idle is set below 1000 rpm. My experience with my coil was like all electrical, it was not an immediate response. It was intermittent. If you are dialing down the rpm at idle and it quits (and as you said its a dog on low end) it seems like the carb to me.

But I am not an expert, just been around the block. Finally, your "67 tri-power manifold" isn't a 67. I don't think any Pontiac had trips in 67 and I know the GTO did not. That's not important, just so you know :)


Good luck, keep us posted.
 

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Brian. a low vacumn reading is consistent with a vacumn leak at intake...

somewhere,...likely spots are where you checked, around intake gasket,..at base of carb, vacumn hoses to blend doors through firewall to brake booster and to PCV. ///

you can just clamp those off for a few minutes as a test....and see if there is any changes...if so remove one by one and zero in....

just make sure that you remove them so you have brakes and crankcase ventilation.

also your throttle valve may be open too much allowing too much air right there,....mixture may be off..

the best way to find a vac leak is with a smoke machine and most garages have them....they will show up very small leaks.

as for distributor being off a tooth, can easily happen and distributor gera can e put on wrong as well and that knocks it off a tooth....

have you had the dist out? or the Gear off? if so it may bear rechecking...

Also is your wiring to distributor correctly powered. Here is why, on that car it was points, a white cloth covered "Ballast" wire that runs at firewall to Dizzy is the coil is the "run " wire, but the "Ballast" aspect means it drops the voltage from 12 to 9 or even 8.5......so the points don't burn...

The HEI's take the full 12 volts and coils operate between 8 to 18 give or take a volt.....if you did not correct that, or PO did not it will make the car quit or run bad....just a place to make sure, ...

If you have a garage nearby you can have it smoke tested might be a good idea, chasing vac leaks is very frustrating....

hang in there you will get it perseverance pays off!:nerd::nerd::nerd:
 

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Discussion Starter #9
Ok, got into it today. Removed distributor and reset, I thought maybe it jumped time but seems like it was OK. Ran compression test and had 120psig
on all cylinders except 5 which was 112. Pressure tested the #5 cylinder and was passing air through the carb with valves closed, opened the valve covers and found all the rocker bolts made up all the way, adjusted the rocker bolts and it held air much better, didn't feel anything passing through the carb anymore, loosened up the all the rocker bolts until they started knocking, then tightened it up a half turn each. Next thing it was like whack-a-mole and had to keep adjusting all rockers until they were once again tightened up all the way. I am going to order new lifters and remove the intake and valley. Hopefully the cam is in good shape still. In my head I'm thinking with the rocker bolts tightened up all the way, the springs were tightened up too much causing the valves to not close properly which in turn is only giving me 12' of vacuum with the timing advanced to over 20 degrees BTDC just to keep idle. Any feedback will be greatefully appreciated as I am a plumber, not a mechanic.
 

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I think you may be right on it as your plumbing diagnostic skills have to be good as well! just instead of water you have air, and I agree thst if valves were not totally shut poor vac at the intake is a result.

you will get a very smooth idle on these cars at 20 BTDC, but that should be with 10 base and 10 from the vac can....

so if you test and your smoothest idle is at 20 degrees at base with vac disconnected you are on the right track, don't leave it there as your centrifigal and base should total 36......34 to 38 ok...

but you dial in the timing after you get that vac right and the idle running.

super car!.....and great plumbers are smarter than the rest of us! lots of knowledge and technique in there....
 

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Are you using Polylocks? If you have stock heads and have the stock style rocker arm nuts, they get torqued down. If you adjust the stock rocker arm nuts, they will back off again.

Having air blow out the intake with both valves closed and then it stopped with the rocker arm nut backed off, sounds as the rocker arm was too tight and the valve was open like you found out. So if you feel the adjustment of the rocker arm nuts worked, you want to have those Polylocks and not the factory rocker arm nuts.

Don't torque down the nuts as you will be back at square 1. Someone may have milled the heads/block or installed the wrong pushrods.
 

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Discussion Starter #12
I pulled it all apart, replaced the lifters and installed poly locks. It is running much better as to where I can idle and drop it into gear without it stalling but I still have real low vacuum (checked with 2 different gauges) and seems to be lacking low end power. and still has hesitation. I think I may just pull the motor after the season and rebuild it from scratch.
 

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I pulled it all apart, replaced the lifters and installed poly locks. It is running much better as to where I can idle and drop it into gear without it stalling but I still have real low vacuum (checked with 2 different gauges) and seems to be lacking low end power. and still has hesitation. I think I may just pull the motor after the season and rebuild it from scratch.

Ok, you are on the right path.

It could be the cam grind. A cam with a lot of duration (hi-performance/race type cam) will have low vacuum, lack low end, and exhibit hesitation when you first take off. If you have a cam like this, it will really begin to turn on/pull hard around 2,500 and up. Often a cam like this also likes a higher than stock idle (and a higher idle speed will increase the vacuum reading) to help with the hesitation - but this is why a higher stall converter (2500 RPM's) and gearing like 3.55,3.73. & 3.90's are recommended so as to pull the engine quickly through the low RPM's and get it right up to its operating range of 2,500 RPM and above. But with a stock converter, increasing the idle may then cause braking problems as the higher idle has to be overcome by the brakes - which means your car is harder to stop or requires more pedal pressure. Gas mileage also suffers a bit.

You may want to try adjusting the carb idle screws and back them out a little to "fatten" up the air/fuel mixture. This may help a little with the hesitation, but again, it'll eat more gas. Might be worth a try just to see if it improves anything.

Next would be playing around with the timing/distributor/advance curve/vacuum advance. This has been covered a number of times with great info on how to do this. If you do a search using the search button in the upper right, this may help to dial in the ignition and may improve things as well. Timing & the advance curve are important on a Pontiac engine.

:thumbsup:
 
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