So just this summer I finished up the head gasket and timing component replacement on my 85 Pickup, and it does run pretty good, but I've been having a really annoying problem that's making me start to hate driving the truck. Once it starts up (sometimes takes a few tries of starting then dies, then start and dies) the truck will actually run pretty good, pretty loud valves but other than that not too bad. Once the truck gets warmed up, especially in stop and go traffic, the engine will hesitate and kind of buck starting off in first and letting off of the clutch, and sometimes again going into second. Once it gets moving it will hesitate when lightly pressing on the gas, but runs fine when you really put your foot down. It also has a kind of rough idle where I'll be sitting at a light and it drops to 600RPM then kind of fluctuate between 400 and 600. It has never died, but it's just unsettling.

Anyways, along with the head gasket and timing components I have done the spark plugs and wires, distributor cap and rotor, had the carb rebuilt and tuned by the same guy (a reputable rebuilder), I have also replaced pretty much all of the vacuum hoses. A local shop recommended replacing the carburetor mounting gasket, and I have also heard that the EGR vacuum modulator going bad can cause similar issues, but I haven't replaced these parts yet.

I'm just seeing if anyone has any ideas, thanks!

 

When i had this problem it was one of two things..
1) carburetor was not sealed completely to adapter(weber)
2) Accelerator pump inlet and outlet clogged, or bad diagphram

 

Okay, thanks! I think I'll start with the carburetor gasket then, but do you know if I would need to replace that entire plate, or just the gasket? I know the rebuilder gave me the gasket that came with the set, but told me not to use it unless the plate was damaged, or something like that. And with the accelerator pump diaphragm, is that something that would have been replaced when the carb was rebuilt?

 

Yea the pump diagphram should have been repplaced when rebuilt...however i dont know how your rebuild was done, some kits might not have it. Are you running a weber? If so id bet its your plates! get a 1 piece adapter and call it a day!! or do the lce spiral...seals better than the stock weber plates.

 

No, I'm just running the stock Aisin. I ordered the gasket insulator part today, so hopefully I'll get that in over the weekend. But the problem seems to be getting worse. Now it is having trouble starting up on a cold start (have to give it gas) and the hesitation seems to be happening just about whenever I touch the gas. So I'll see what happens once I replace the gasket, but could the accelerator pump cause these symptoms?

 

Check for carb operation. the service manual has ways to diagnose your issues. check if you have fuel squirting when pushing the throttle.Take off the air cleaner cover and turn the throttle plate. you should see fuel squirt. this checks if accelerator pump is working.

Next check if your auxilry acceleration pump (aap) is working. A quick way to check is to take off the vacuum line to it and see if fuel drips out. if it does drip its bad.

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Check if your dizzy is advancing under acceleration. A vacuum leak would cause high idle. If you hold your hand over the top of the carb to cut off air flow and the idle raises you have a bad vacuum leak.
Just check everything. The manual is great.

 

Thanks, that helps a lot! I'll check them out and see what happens, thanks

 

Okay, well the acceleration pump squirts fuel, and as far as I can tell no fuel came out of the vacuum hose on the AAP, nor did it smell like fuel. But one thing I did notice was that I couldn't see any gas in the float bowl through the sight, and I had just driven it about an hour and a half ago.

 

Well last week I put in a new insulator with gaskets for the carb, and also replaced some vacuum hoses and tightened up a couple loose intake bolts. After all that it seems to be running pretty well! I'm having some issues with the high idle on cold starts now that it has started getting colder here, but other than that I'm pretty happy with it

 

Good to hear!

 

glad to hear it! high idle can be from a sticky butterfly to a temp sensor being bad.

 

These trucks are made to high idle on cold start otherwise the choke on / rich mixture would kill the converter. Check the FSM for details.

 

My 22R has the stock OE carb on it. Vacuum hoses are good, and if I pump it twice it starts right up. Believe it or not, it runs better cold than after it warms up. It hesitates slightly like the OP's truck when starting off the line once it's warmed up. Like the OP, mine has noisy valves, but only when warm. Sounds great cold. Once it warms up though, it rattles and clacks, and if I floor it with the RPMs too low, it starts missing out and stumbles. Maybe it's a different issue with mine, but it sounds familiar.

 

Sounds like a lean condition on acceleration. Could be a vacuum leak, or dirty carb jets and passages or a worn / faulty accelerator pump not functioning 100%. This is assuming the timing components are in good working order and timing is set correctly

The choke on @ cold start will mask a vacuum leak. Vacuum leak @ choke off = lean (too much air not enough fuel).

A cheap halloween smoke machine rigged up into you carb / breather or even your break booster vacuum line could show you where any vacuum leaks are.

There are a good many You-tube videos showing how to do this fog machine vacuum leak test quickly and easily. On trucks of this age there are frequently other non hose related vacuum leak.


To me the most important thing is to prove the malfunctioning part through technical deduction with out spending $$$ on needless repairs.



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