carburetor gurus....

rightwinged

New Member
Joined
May 7, 2010
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new bern
So heres the dilema. 86 k5, high output crate 350, with now close to 2k miles on it. Has a holly 660DP avenger on it. It runs like its 1986 again in every position except WOT. In WOT, it boggs down, similiar to the effects of main jets clogged, but then regains power a few seconds into full throttle. Im at a brick wall here, have adjusted and re adjusted everything, idle, fast idle, mixture, choke(electric by the way), secondaries.etc... still same problem, runs like a champ except in wot. hopefully some carb masterminds would love to chime in and lend me a hand.
 
Sounds like what an old pickup of mine did when my accelerator pump was going bad. Have you adjusted the pump shot on it?

It's been 10 years so I searched around the Holley web site.


QUESTION: How do I adjust the pump shot on my carburetor?

ANSWER: Look down in the carb without it running move the throttle and see if the two sprays of fuel shoot as soon as the throttle moves if it is not instant back off the nut on the pump arm 1 full turn counter clockwise then check it again do this until it is instant.
 
Accelerator pump wouldn't cause this problem. Its only job is to shoot a quick shot into the motor when you first crack the throttle, because when you do that at first you lose signal to the boosters because of the momentary loss of vacuum and lack of velocity, the engine will starve for fuel if it didn't have that shot to pick it up before it stumbles.

Under WOT or any steady throttle position the accelerator pump isn't operating or providing anything to the engine though. So in short, accelerator pumps are pretty much there to keep the engine from stumbling off an idle, or when going from off the throttle to on when moving.

You didn't say whether or not you have set/adjusted the floats, if you haven't, remember, its important to set the top out and the droop on them.

Another thing is to check and double check your timing, I see lots of people go after the carburetor when its a timing issue, you running a vac advance HEI unit?

You say it picks up after a couple seconds, during those couple seconds are the rpms rising normally with lack of power, or are the rpms being limited and then pick up after it cleans up? Also, does it act this way at a specific rpm? Like a happy spot in the powerband where it starts to run better, or is it just bogging until you're on the gas for a second at any rpm?
 
4x4 ranger...lots of questions in that post, so ill do my best to answer them. It only boggs down at full throttle, and the rpms stay the same for that 2 or 3 seconds, and it drops rpms during that time. Then it "catches" up and acts like it never happened. Floats are both absolutely perfect, according to holleys recommendations.
Could the culprit be having no emissions?(ie, canister, all vaccum lines) not sure how that works with vaccum operated secondaires, something might be a little fishy...
 
First did it always run like this? Or did it just start? Vacuum secondaries run of venturi vacuum. They have nothing to do with emissions etc.

You say WOT. Is it punching it off idle or at half throttle then punching it? Or both?

Also once it's in WOT it catches up. What power valve is in the carb? What is your manifold vacuum?

I also agree you need to set your timing. What is our distributor setup?
 
when we first put the motor in, it ran perfect for id say the first 1000 miles, then it did it a little, and it has gotten progressivly worse since that. If i floor it at a dead stop, 50% of the time it will just shut off, the other 50% it will bogg down for a few seconds, then take off. It happens prob 82% of the time when punching it in any other position then idle.
it has a HEI distributor, power valve is stock-singe stage....
The timing is the only thing im not 100% on, i had a buddy help me with it, and even he seemed a little weary.... would love to learn the proper way though.
 
Yeah, the more you describe it, the more it sounds like timing. First I would check your vac advance canister, you can get a cheap vacuum pump at harbor freight for like $20. Put the pump on the canister with the cap off and pump some vacuum into it to see if its actuating the advance arm, or if it holds vacuum at all. When these things aren't working they will cause a condition like you are describing because the engine is getting no timing until its gets spinning enough to engage the centrifugal advance. Also a good digital timing light is what you need to set the timing, you can make due just fine with the old analog guns, I just like the convenience and accuracy of the digital gun. After you have that you can look up instructions on google, its pretty simple.
 
You say canister, emissions canister? ill tell youright now that that has no vacuum, because its non existant. Or do you mean to the back of the ventrui to see if secondaries open up? I have a nice digital timing light, so i guess ill be giving it a whirl this weekend. thanks.
 
There should be a vacuum advance diaphragm on the dist. Get a manual vacuum pump and check to see if it holds. This needs to be connected to ported vacuum no manifold vacuum. So a vacuum line tot he bottom of the carb. At idle there should be no vacuum on the on the advance diaphragm. When you give it gas it'll advance via the vacuum quickly until the spring and weights pic up the slack. Like he said above.

If it ran perfect before then started getting bad that leads me to belive it's not a hard part: jets, pv etc.

I say go after the timing first. Then work on the carb. Also check for vacuum leaks around the manifold and carb. Or a bad vacuum line.
 
Power valve. That is it's job and it's not doing it. Even a small backfire thru the carb will kill it. It enriches the fuel circut durring WOT.
 
nctom said:
Power valve. That is it's job and it's not doing it. Even a small backfire thru the carb will kill it. It enriches the fuel circut durring WOT.
Click to expand...

Most power valves built in the last 15 years have a safety to keep them from blowing out. It either works or doesn't. There really is no in between. Due to the fact that he says it "catches up" tells me it's not the PV.

What about fuel pressure? Hows the pump. Again I say start with timing and go from there.
 
have you pulled the bowls and made sure you have not gotten a piece of trash in the transfer slot ?? Also check around the base and make sure you don't have a vaccum leak @ the base plate pulling air.
 
Lots of good info, im going to check for vac leaks this weekend first, then attempt to mess with the timing. As far as a vac leak at the base plate, i highly doubt it. The first motor i had in it, had that problem, and the motor ran like complete poop in every position, at all times. really dont think its that.

fuel pressure is 5-7psi, got a gauge right before carb. Fuel pump has 2k miles on it with the engine.

Will try and take pictures of vacuum setup this weekend, maybe i dont have something hooked up how it should be. thanks all.
 
You have checked all the simple stuff right?
Air cleaner filter, fuel filter etc.
Even tho' you haved 5-7 lbs. of pressure, you could still have in adequate fuel flow.
Make sure all fuel supply and return lines are OK.

Matt
 
Alright, got a few hours to work on it today, and at least i got somewhere(i think). Timing is 7* advanced, which is what we set it at first startup, so it hasnt changed. Checked all vacuum lines, checked to make sure the vac advance was working(timing light, 3k rpms, watch mark increase, that whole sharade) and it all went smooth. put everything back together(all 1 vacuum line) and went for a drive, leading me to think its starving for fuel. anytime throttle was depressed more then 1/4 of the way, in 1 motion(not easing it down) it would bogg down pretty bad, nearly shutoff. I wouldnt imagine it being the fuel pump(mechanical) because it has 2k miles on it. Changed both fuel filters(inline near tank and after pump). Ive hit a brick wall here, not sure what else to check.
 
You have a stock fuel pump trying to meet an increased demand with the dual feed carb...shouldnt be enough to cause what you are seeing but dont eliminate it.
 
I dont think the good folks at gm performance would put an inadequate fuel pump with the recommended carb for the engine...just my thoughts.....

Now for the timing, not 100% i did it right. Started engine, put clamp over #1 wire,shined on harmonic balancer(it has a timing tab). At idle was a little above 5* mark, so 7* looked pretty solid to me. I disconnected the line, this may sound stupid, but which port do you plug? the line or the diaphram? i plugged the line, thinking back on it, it may have been the wrong one.

Anyone know of a solid write up or the proper way to time? Searched google and here, but everyone seems to have a different method( some say plug the vac adv., others say dont).
 
OK so you should ask them what your total advance should be.

I'm guessing it should be in the 36 range. That means at WOT you should be reading 36. Adjust your base timing from there.

Then hook the vacuum back up and and if there is pinging back off 2 deg.

And you adjust the timing with the vacuum line unhooked and plugged. Otherwise you have a vacuum leak.

Also I am still wondering about fuel. I'd consider getting a new pump from GM. I wonder if it may have been a bad pump and your starving for fuel. Also do you think you may have gotten some dirty fuel. Clogged ports in the carb will be a pain.
 
Ok i understand, timing is adjusted in more then 1 throttle position. Talked to gm performance guys, they recommend no more then 34* advanced at wot.

But im still not convinced its a timing issue...if it was a timing issue, i dont think that it would catch up like it does, i think it would run poorly all the time. if someone could validate that, id appreciate it.

The engine is still under gms 100k crate warranty, but any replacement parts have to be put on by gm dealer.. free of charge though.
 
Just throwing this out there but could it the fact it's a double pumper rather than a vac sec. I assuming it's auto? I think when your going wot the motors just falling on it's face for sec bacause of to much air at once.
 
rightwinged said:
Ok i understand, timing is adjusted in more then 1 throttle position. Talked to gm performance guys, they recommend no more then 34* advanced at wot.
But im still not convinced its a timing issue...if it was a timing issue, i dont think that it would catch up like it does, i think it would run poorly all the time. if someone could validate that, id appreciate it.
Click to expand...

Well a distributor has wieghts in it tha control how fast the timing increases in relation to engine rpm. Just check it to make sure it's right. You can then 100% eliminate this as a problem. Once you've eliminated stuff you can then move onto other things.

Redneck92 said:
Just throwing this out there but could it the fact it's a double pumper rather than a vac sec. I assuming it's auto? I think when your going wot the motors just falling on it's face for sec bacause of to much air at once.
Click to expand...

He said it didn't do this when he first got it. So something could have come out of adjustment.
 
I would say check the accelerator pump on the secondary side or possibly the pump cam. If you have the pumps adjusted to much then they may be bottoming out at WOT or they have failed. If your getting a stumble when you get in the gas then maybe your secondary pump is out of adjustment or the squirter is to small
 
Well problem seems to be fixed, and for those of you who said timing, you were correct. Did ALOT of research, felt confident, and did it my self. Dug up the paperwork on the motor, and it is 100% designed to run with the vac advance for distributor plugged. It mentioned it like 6 times, must have missed it the first 5. So, plugged the line, re-timed it to 10* BTDC at idle, and 32* at 4000rpms. Runs like a champ once again. Still not quite sure what caused it all of the sudden. Maybe loose cap? anyway, problem fixed, thanks for all the advice.
 
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