Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - 1977 Chevy K20: Engine Suffers from Hesitation/Power Loss When In Drive
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Old 02-22-2012, 11:00 AM
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lt1silverhawk lt1silverhawk is offline
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Hey BigRoy,
Originally Posted by BigRoy1978

If it was the vacuum to the trans either the engine would...

1. Idle fast...cause of a vacuum leak


2. the trans wouldn't shift until you had the engine up around 3500-4000 rpm

power loss like that, where it will go to one rpm and doesnt want to get anymore is an ignition/timing problem.
Well it does take some time and/or revving before the transmission shifts. What's strange is that there have been times (during the past few weeks since the truck began exhibiting this problem) where I've spent time letting the truck idle for quite some time while revving it up to 4000 rpm here and there. I would then take it for a spin and everything would work fine. But other times, like in the most recent videos, it doesn't want to run well no matter what I do.

The first time the truck began running fine after first displaying these problems, it was after I checked and cleaned all the plugs, tested out the ICM, and installed a new ignition coil. See post# 9 of this thread for more details.

After that, two days later, the truck exhibited the same symptoms, but they went away after revving the engine to 4,000 rpm a few times. The same thing again happened the following day. See post #15 and #17 of this thread for details.

After that, the truck has not run well in Drive no matter what. Now the added mystery is why there is the ever so slight hesitation at the 2,500 rpm mark when revving the engine in Park and Neutral.

Hey Chet,
Originally Posted by T-bucket23
Disconnect the choke and see how it runs. Without a heat riser that style choke will not work well. The heat riser forces some of the exhaust back through the manifold to warm it up.
I do not want to sound as if I am disagreeing, but I am wondering if that would have been a problem before too. Because the truck has been running just fine in the past, save the dirty fuel / carburetor rebuild issue last year.

Originally Posted by T-bucket23
Using the wrong intake gaskets can also cause issues.
Ah those intake gaskets. Did 'em twice last year. You can see which ones on page 9. Since both you and sbchevfreak have brought up the manifold gasket issue, maybe its time to do it a third time. The timing can be tackled from the beginning too, like I should have done.
Originally Posted by sbchevfreak
Is it just me, or do the "even" plugs look to be badly oil fouled? If so, I would be looking at intake gasket sealing on that side....
I can disconnect the choke and fire it up next time, and also open up the choke plate by tying it down.


Hey Augusto,
Originally Posted by Augusto
q-jet choke's are crap, just get a piece of wire and fix it wide open or remove the butterfly, try this first.
I will be trying this, along with disconnecting the choke on the next try.

Originally Posted by Augusto
about timing, set your timing at 8 deg BTDC with the vaccum lines disconected and pluged at the carb, this will give you a good starting point, once done start the engine and check if the timing mark advances when you rev up the engine, should go to something like 20 degrees BTDC.
I've seen various numbers on the what the timing should be set at, ranging between 12 BTDC to 6 BTDC. I read somewhere that 10 BTDC was a safe number to start with. I'll start with 8 next time. It seems like the manifold may be getting yanked again.

Originally Posted by Augusto
after this connect the hose to a ported vaccum source, the one with no vaccum at idle, this will give you a good average performance and no issues.
Ok, so the one where the vacuum advance was previously connected to.

Originally Posted by Augusto
drive it and if still gives you problems replace the carb.
I've got another core I can rebuild, though I hope it doesn't come down to that.

Last edited by lt1silverhawk; 02-22-2012 at 11:25 AM.
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