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Holley Carb Experts, please help! FIXED!

73k5blazer

73k5blazer

End the H1B Program!
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In my '86 C10, with brand new 4.3l V6, I put a brand new Holley 390 on it. (Brand new motor too). I started with the old Q-Jet (this truck came from factory with non-computer 4bbl q-jet carb), but it had leaky fuel bowl wells, fixed that, ran ok for a little while, then started blowing gas up the top. Scrapped it and put the new Holley on. It starts perfect, runs great, really brings out the best in the motor, and I get 21mpg. My problem is, when I shut it down, it tends to run-on, or it stops immediatly and backfires once out the tailpipe (I have people running for cover in the parking lots). The Q-Jet did not do that, at all, never, so I don't think it's timing or ignition. Timing is set to 2btdc (stock was 0, it's a manual 3sp trans)
I can crank the choke forward to stop the run-on thing, and it still starts good cold, but runs really really bad until it warms up, I live in MI, so this isn't good. If I crank the choke back, the fast idle barely kicks off, and it runs on when I shut it off. So I leave it in the middle, fast idle works as it should, and runs good while cold, but runs-on or backfires when I shut it off. Only change I made after putting it on was a 1 size up accell. pump shooter to rid a slight bog on accelleration.

Any ideas on what to do here? Do I have some other problem? I hate to give it up, it runs really well, way better than the Q-jet did on this engine.
BTW, it's an electric choke.
 
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Idle is 750, fast idle is 1200. Stock (sticker) calls for 600, 1200 but the engine has 2004 vortec heads, 1-up cam, etc. it idles at 600, but feels shakes. (The 4.3 v6 is shaker anyway)
 
Try backing the idle down a little, and lean the primary jets out about 1-2 sizes. (Smaller no.) Have you checked timing?
 
I've checked the timing again. It's right where I left it, 2*btcd.
Lean the jets out..hmmm... I could try that.
Boy how I wish Holley would use thier non-stick bowl gasket from the box. I hate scraping that stock gasket off. Last time I took a Holley bowl off (about 6 years ago), it took me an hour to clean all that gasket off.
I'll have to order jets, I have the jet kit from years before, but the kit starts at 60. The 390 carb comes with no. 50's in it, they only make down to 48 I think, all order stuff, no store has that in stock.
 
73k5blazer said:
I've checked the timing again. It's right where I left it, 2*btcd.
Lean the jets out..hmmm... I could try that.
Boy how I wish Holley would use thier non-stick bowl gasket from the box. I hate scraping that stock gasket off. Last time I took a Holley bowl off (about 6 years ago), it took me an hour to clean all that gasket off.
I'll have to order jets, I have the jet kit from years before, but the kit starts at 60. The 390 carb comes with no. 50's in it, they only make down to 48 I think, all order stuff, no store has that in stock.
Click to expand...

just try droping the idle down first, between 650 and 700

you have a vac advance distributor? vac line manifold or ported?
 
Yep, vacuum dist. Hooked to the port on side, front right of carb. (I belive that ported?) I've tried manifold, Runs really bad hooked to port below throttle plate (manifold?).
The OEM vac. line setup (according to sticker in engine compartment) had pulled both vacuum and manifold for distributor, and selected between the two via a vac-thermo valve screwed into a coolant passage (which also served 4 other emmisions things, that are gone now). That valveh is gone now. Mabey, for an experiment, I could try running manifold vacuum when it's cold?
 
I would readjust your idle speed using the idle mixture screws and a vacuum gauge. then reset the idle screw to get you correct idle speed.
You can tune the idle w/out a gauge if you listen for the highest idle achieved while turning the screws in or out. I usually start w/ 1 1/2 turns out from tight and do one side at a time. Your float could be set wrong too allowing fuel to drip down after you kill the ign.
 
I have a good vac gauge. I used it to adjust the idle screws when I first put the carb on. I can check again. Float..that's interesting, I did check the level, but that's always a challange on these holleys, you have to check "while running" and the engine is shaking.
 
I have spent a LOT of time with those 8007 carb's. It's not your main jets, the float setting ?... possible, but as others have said, lower that idle, experiment a bit w/ the timing and is there a chance you have a lot of combustion chamber carbon deposits ? What ignition system are you running? A MSD by any chance ? Also buy the clear sight glasses for your fuel bowels and dont run more than around 5 lb. fuel pressure. good luck.
 
maybe...

Holley might have an add on Idle Stop Solenoid for that carb..that will end all your "dieseling" problems..its probably running on because the idle speed is too high..closing the butterflies completely by using the solenoid,not the idle speed screw,to set the idle speed will likely make it shut right off...it was a common dilema on stock carbs in the 70's..

I'd guess its jetted pretty close to perfect if your getting 21 MPG,and its not "pinging" or blowing black smoke at lower speeds..sounds like you can only go one step leaner anyway..doubt that would change much..

Might double check for vacuum leaks..they encourage dieseling too..and trying to swap over to manifold vacuum might help,or make it worse..usually the more advanced the timing,the more likely it is for dieseling to occour,but not in all cases..:crazy:
 
Stock HEI ignition(new pickup,module,rotor,cap). Stock (brand new) mech. fuel pump (w/return). No black smoke. Carbon deposits? Is that possible on a new motor. I think I saw an idle stop solenoid from holley. I'll have to look again. I might try to adjust the timing too. The clear sight plugs are a good idea, but do they really work? Can you really see the gas through them, they look pretty clouded in the pictures. I can try 'em I guess, they are cheap enough.
Thanks alot for all the suggestions. I've got a few things to play with now with hope.
 
It was the float level. I checked it when I put it on and it wasn't high, at least I thought it wasn't.:dunno: But this time I got the see through float plugs, and checked it when it was warm (which I'm pretty sure I checked warm before, I know I did the idle mixutre adjustment warm) and it was high. Backed it down a touch, and no more run-on. It's a real fine line there though. Took a couple of times to get it right. I bumped the idle down to 700 after that too and checked the idle mixture (no change needed). Seems less shaky now that it has 5000mi on it. I did the initial adjustments with 100mi on the motor (first 100 were the gas blowing Q-Jet).

Thanks for the help!
 
Glad to see you got her running better.Here's a tip to keep on file w/ that style carb, I've had probs with fuel sloshing out of the vent tube and killing the engine driving slow over just speedbumps or into driveways, especially in hot weather. I cured that with carefull float settings and I found that some, not all of those carbs come with a little plastic baffle gizmo that fits in the top of the primary bowl and helps limit that fuel slosh. make sure yours has that baffle, If you dont holley makes a fuel bowl vent screen part # 26-39 that does the same job. Keep us posted.
 
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