Silvertone

I've been chasing this for months but spare time is an issue. Got a "new" 383 crate from ATK with a Quick Fuel HR780VS. Probably getting all of that dyno rated 503 ft/lbs at WOT, but can't get it to idle down unless I blip the throttle or twist the throttle shaft by hand. Even with the curb idle screw all the way out. Also getting run on (dieseling) after shutdown, but covering the secondaries by hand will kill the motor cleanly. Also, #1 header port runs about 40* cooler and this is the only plug looking more black than gray. Plug wire passes the ohm test.

Already ruled out the carb base gasket, throttle cable action, high idle cam and butterfly interference as possibilities. Total timing was set at 36. Initial is 12, using full manifold vacuum. No vacuum leaks found, assuming I've got the right PCV valve. Idle mixture adjusted for max idle vacuum which is about 10.5"hg. (.231/.234 @.050 Howards roller cam with .510/.530 lift)

Primary transfer slots look fully covered with the choke open, of course because the curb idle screw is all the way out. I'm guessing I need to close the secondary butterfly a little so more air can be drawn through the primaries? Will that give me more control over idle screw adjustments?

Jebbysan

Yes.....close the secondary butterfly completely.
Adjust the primary to taste and if the idle is erractic...slowly close the primary and open the secondary some....

I take it this carb has four corner idle?

Edit: I see it does......fine tune it as cracked secondaries at shut off can cause the dieseling you describe too....

Jebby

Silvertone

Thanks! Will try it. My first Holley style carb so I have a negative review to post for whomever designed this lame and well hidden adjustment. My hex key isn't budging it so far.

Just to be contrarian to all the Q-Jet and Holley fans, I never had to do anything like this with all three of the Carter AFB and Edelbrock carbs I've run the last 38 years.

Jebbysan

Yeah....you may have to pull it off to do it.....but even so you need to verify when it is off that they are in fact shutting completely....primary and secondary.....if not....Quick Fuel should send you a call tag for it...

Jebby

OMF

This is super basic....you do have two linkage return springs.....right??

Jebbysan

Never overlook the super basic!!!!
Good call!

Jebby

Silvertone

That would be extraordinarily basic, but yes, I've got two springs and I tried mounting them out with a lot more pull but didn't change anything.

I did get the secondaries to close some and ran it. Slowly put the pliers on my poor little 3/32 hex key and it moved. First test: Much better! Got it to idle down far enough to not diesel, and manifold vacuum is up to 12"hg after an idle mixture adjustment. The odd thing is the idle mix screws don't make much difference and I can run them all the way in (one at a time) and it still runs fine. Most efficient is just less than 1/2 turn out.

Well, I did all this while installed so I'll see if I can go shut the secondaries more and test it again. Seem to be on the right track. Are you guys sure they need to be completely shut?

Next, need to know why #1 cyl runs cooler than the rest. Then I can finally move on to trailing arms, suspension and make room for the new wheels I've got on order.

Jebbysan

Yes....unless necessary for a very radical cam....a vacuum secondary carb should be shut on the secondary side.....
Remember....you have a four corner idle....a half turn in a piece is like one turn on a normal carb....
Number 1 is most likely running cool due to plug wire or plug...
Start there....

You are close!

Jebby

Silvertone

This already built, run, and tested new crate motor has been the hardest thing to get running right. Warmed up it runs well, but try starting it cold and it barely stays lit no matter where the choke is set and sometimes blows a little oil smoke. After a couple minutes of chugging it will smooth out and run great. I'm running out of ideas here. What's up with that?

diehrd

Your idling off the transition slots , your primary butterflies are open to far . A good idle with no hesitations and no dieseling is obtained by setting the transition slots correctly and if the idle is to low opening the back barrels a tad to give it that little extra it needs to maintain idle speed.

Jebbysan

Fix #1 cylinder.....place the car in a dark room and run it....look for arcing....it will make a car hard to start.
Then run the hell out of it under load to seat the rings....it may have been run....but rings may not be fully seated.

Jebby

Silvertone

Soon I will post whether or not I found the solution, but Quick Fuel tells me the secondary plate needs to be open a crack. I suspect that is because of the four corner idle feed. My problem is likely needing larger idle air bleeds. I'm about to install .73's to replace the four .70's. Then I'll get more function out of the idle mixture adjustment screws. That's the theory, will update in a couple days.

Silvertone

I changed out the Quick Fuel idle air bleeds from .070 to .073 this weekend and this made a huge difference smoothing out and cleaning up the cold start operation. Still have occasional dieseling effect at shut down but I'll try closing the secondary plate a little more since that's the only thing that has helped this condition so far. As it is now, they're only open less than 1/2 turn of the adjustment screw, not much! But the dieseling is less frequent and much weaker now.

Must be getting close now.

Karsten

I have the ATK 375 HP 350 with a QF 680 CFM......Mine runs rough, wont idle and is hard to start. We have found out the OEM style Fuel Pumps are putting out 9.5 plus pressure and QF recommends no more than 6.5.

I have a pressure reg and gauge setting in the shop just no time right now.



Karsten

Silvertone

Which pump did you get? I have the Holley 110gph #12-327-11 that came with the motor from ATK. It's supposed to have an automatic internal bypass between 6.5 and 8.

I'd like to hear what readings you get, but you might also want to ask QF about using bigger air bleeds.

Silvertone

Looks like you're running with a 1/2" spacer? I've heard that only helps the really big cam single plane intakes. Might not matter, but have you run without it just for kicks?

Karsten

I had ordered the Eldebrock pump to start but plumbing it would had been a nightmare or so I was told. Installed just an OEM Spec pump for a 350 CI picked up locally. After calling was told it was a 9.5-10 PSI pump.

I did call QF and wass told about the max PSI for the 680 CFM (not sure if it applies to all QF Carbs).

After searching for a pump in the 6.5 range and ordering one that was posted to work even that pump is rated at 10 PSI.

So I ordered the inline reg.....Do Not go the cheap route and get the Mr Gasket Dial reg as they tend to blow gaskets and spray fuel everywhere).

If QF has designed their Upgraded Holley around a max PSI of 6.5 that may be your issue.

Also I see ATK recommends HEI ignition which they didn't when I ordered my engine. Summit has a HEI Ignition that is Tack Drive compatable rather than aoo the MSD boxes.

The pic above was before I switched out the stock distributor for the HEI unit.

Karsten

Karsten

The spacer was added as I needed another vacuum source as the QF didn't have one like a Q-Jet of Edlebrock carb so the vacuum comes off the back of the spacer.

As for the cam,

Compression Ratio: 10.1:1 Compression

Camshaft: Hydraulic Roller

Camshaft Specifications: 220/224 @050 .528in/.536ex 112LSA

Your cam,

Compression Ratio: 9.7:1 Compression
Camshaft: Hydraulic Roller
Camshaft Specifications: 220/224 @050 .528in/.536ex 112LSA

Check your Fuel PSI is what QF told me. At Idle I can see excess being forced passed the jets so I am getting way to much fuel.

Karsten

AirBusPilot

I thought I read not to use full vacuum for the vacuum advance, and instead run it off ported vacuum off the carb? This reduces high timing at idle, which usually creates a high idle.

hugie82

I had a demon 625 that wouldn't idle down. It was the butterfly brushing against the barrel. I had to crack the screws loose that hold the butterfly, let it close all the way and re tighten. That solved most of it and a little machinist dye told me where it was still hitting. I touched it up with a machinist file and it would drop right down until it stalled.