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post #1 of 8 (permalink) Old 06-01-2018, 10:42 PM Thread Starter
 
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Bogged down?

I've got a matching numbers 1968 GTO, 400ci with a M21 4-speed trans. Had a stock rebuild on the engine with the exception of a mild high performance cam that was done last year. It has the original Quadrajet 4-barrel carb that was rebuilt 3 years ago. All this was done by reputable shops, including the tranny. I have put on 3,000 miles since the break in was completed, was running unbelievably good. Now I notice when I get on it and running thru gears 1-2-3 its has a slight hesitation before I hit the next gear as if the carb is pumping too much gas. Any advice on a good starting point? I thought about taking it to a shop that specializes with dyno tuning, am I on the right path?
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post #2 of 8 (permalink) Old 06-01-2018, 11:07 PM
 
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If the factory transmission is a M21 then you most likely have a steep gear ratio. Does your car have factory A/C? How many fan blades?

Check the fuel filter. That could cause the symptoms you describe.
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post #3 of 8 (permalink) Old 06-02-2018, 07:43 AM
 
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drg44: "was running unbelievably good. Now I notice when I get on it and running thru gears 1-2-3 its has a slight hesitation before I hit the next gear as if the carb is pumping too much gas."

PJ: What RPM do you shift at? I suspect it is either a fuel delivery problem because it is not getting enough gas if shifting at higher RPM's or an ignition problem.

What RPM does it begine to do this? Try a wide open blast to 4,000 RPM's, then 4,500 RPM's, then 5,000 RPM's, then 5,500 RPM's.

Crappy gas from sitting or water in the gas.

Air cleaner needs replacing.

Could be a fuel filter, fuel pump going bad, rubber fuel line going bad, carb issue/adjustment from lack of use.

Could be an ignition problem. You did not say if points or electronic. If points, gap or dwell closed up, points burned/pitted, corrosion on rotor/cap, a bad spark plug wire, coil going bad, timing is off, stuck/sticking mechanical weights in distributor, timing curve incorrect, bad engine to body ground, spark plugs improperly gapped, spark plugs fowled, spark plugs worn.

Could be a vacuum leak - hose, carb gasket, or intake gasket, bad PCV valve.

Oil pressure low at upper RPM's not providing enough oil pressure to the lifters. Lifters going bad. Rocker arms not adjusted correctly if you have poly locks or one has backed off loose if you torque them down. Incorrect valve springs for your cam swap and springs are weak or have weakened with use. Valve retainers bottoming out on valve guides due to incorrect clearances. Valve spring coil bind.

Exhaust problem. Collapsed muffler. Heater valve stuck closed if factory cast iron manifolds.
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post #4 of 8 (permalink) Old 06-02-2018, 11:55 AM Thread Starter
 
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The car is none A/C and has 5 fan blades. This weekend I'll pick up a new fuel filter. I can start there.
I had a pacemaker implant 2 days ago so I'm limited to what I can do for awhile. At 64 and 20 years of dealing with atrial fib it was time to override the original equipment, haha. Thanks Shake-N-Bake.
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post #5 of 8 (permalink) Old 06-02-2018, 12:14 PM Thread Starter
 
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PJ, I run a high grade premium gas, usually from Arco. After I replace the fuel filter then I can take it out somewhere that I can get on it while I'm keeping a eye on the tach. I'll do the 4,000-4,500-5,000-5,500 blast that you recommended. I'll take note of the RPM's when it does this. I do have the pertronix ignition in it, I'll also check for vacuum leaks. I'm probably in the category of weekend mechanic, however a firm believer in the more a person knows about the workings of things the better off they are. Again, Pontiac Jim, thank you for this information.
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post #6 of 8 (permalink) Old 06-02-2018, 04:16 PM
 
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The car is none A/C and has 5 fan blades. This weekend I'll pick up a new fuel filter. I can start there.
I had a pacemaker implant 2 days ago so I'm limited to what I can do for awhile. At 64 and 20 years of dealing with atrial fib it was time to override the original equipment, haha. Thanks Shake-N-Bake.
The FT code (M21 close ratio 4 speed) was only available with 3.90 and 4.33 HD axles. Not available with A/C....which makes sense since your car is a non A/C car. The 5 blade fan rules out the ram air engine so most likely your axle ratio is 3.90 (but could have easily ordered with the 4.33 just the same...). Either of those gear sets is capable of draining the fuel bowl on a quadrajet if the filter is just slightly clogged so that is the first thing I would check. 67 and 68 had a slightly different float design so they are even more susceptible to fuel flow issues than the later style carbs. If that is good then look for a loose or damaged plug wire. Jim's suggestions are great items to check.

A cam upgrade could stress other parts of the valvetrain so a bent pushrod or broken rocker stud is also a possibility. These engines will run great even on 7 cylinders if all else is okay....just tossing out ideas...
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post #7 of 8 (permalink) Old 06-02-2018, 04:47 PM Thread Starter
 
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It has the safety track rear end with 3:55 gears. Back in February took it on a 4 1/2 hour trip down highway 101 and back a few days later to visit family, had it up over 100 mph numerous times, all kinds of room left on the accelerator with the oil gauge and temp gauge staying right where they needed to be. I was so impressed by the performance. Hopefully it all comes down to a carburetor issue. Thanks again, will keep you guys posted as to what gets resolved.
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post #8 of 8 (permalink) Old 06-02-2018, 06:50 PM
 
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drg44: "I do have the pertronix ignition in it"

PJ: Is it the electronic conversion or a Pertronix distributor? The Pertronix ignition can be a sore spot in some cases. I have had good luck with them and even got the billet distributor for my present 455 build. Others have had bad luck.

One of the keys if it is the electronic conversion is to make sure you use the correct and recommended coil. The correct coil recommended has the minimum 1.5 Ohms needed to work with the conversion. With the correct coil, you need to eliminate the factory resistor wire and run 12volts to it. Many run the resistor wire which is factory for points and only passes about 8-9 volts so it does not burn points. Pertronix needs 12volts or it can be damaged.

Instructions also show you how to get the correct air gap on the pick-up ring - .010" to .060". A problem could arise in getting these numbers IF your distributor has too much play - worn shaft or too much up/down end play travel. So you may want to verify this.

Another thing I seem to read a lot of is that the units fail, BUT, I also noticed this as being earlier units. I am sure Pertronix has bettered their product and eliminated those failure prone areas. As I said, I had no issues and ran it 20,000 hard miles, but had the 12volts going to it and the correct coil. If they were bad units, they would not still be selling them nor expanding their line of products using them.

That said, you could have an earlier conversion? or it may not be correctly set-up. Something else to check on your list of possible causes.
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