holly 390 is rich...

72bajascout

72bajascout

New member
I have a holly 390 on a 345. It idles a bit Rich, does anyone know the original stock jet size? It has 52s in it now. Timming is at 3btdc, and idles @ 750 thank you
 
Ok, I'll lean it out a tad and turn the idle up a touch, what sould the timming be at? 3btdc ok? Thank you
 
Last edited:
You can safely bump the timing up to 8 or 10 degs, but that also won't change your air/fuel mixture if it is out of adjustment. Your idle speed is already on the verge of being too fast. If you advance the timing any further, that will cause the idle speed to go up too. If anything, you should actually lower your idle speed. 600 is good for a manual trans and 725 in park is about right for an auto. Your Holley carb is a model 4150 square bore rated at 390 cfm and it May well be time for a rebuild.
 
The carb got a kit recently,last 6 months and it was nite and day difference. It got the usual, points, cap, rotor, wires, plugs, condenser. And a new coil too. After I did the tune up, it helped alot more. It runs much better. But it won't fine tune. Thank you I'll try the timming and idle after work. Maybe an ignition upgrade is in the near future too. The davis unfied ignition looks good. Thanks again
 
Last edited:
Do you have a dwell meter at your disposal? Dwell for a v8 engine should fall between 28 and 32 degrees. Setting the point gap with a feeler gauge gets you close enough for rudimentary knuckle-draggin' gummint werk, but your actual dwell May be just enough outside that sweet range to be causing you some issues. What did you gap your spark plugs to? Made damn certain that you didn't get any plug wires mixed up, did ya? Have you verified that your mechanical advance and your vacuum advance are functional? Do you have a vacuum gauge that you could connect to manifold vacuum to verify that your vacuum is around 20 hg and steady at idle? What's the elevation where you live? You aren't trying to burn high test/octane fuel are you? Its a complete waste of $$ if you are. No need to go above 87 octane with these low compression engines. I've actually witnessed them run noticeably worse on high grade shit than they do on the regular. Kinda flies in the face of assumptive conventional wisdom, but if you really stop and analyze for a minute, it ends up making good sense.
 
I took a vacation day today, and worked on it... I took a compression test and it was 100-110 in all 8, plugs are nice and light brown. Gap is .035 vacuum gauge said 16 in and it's solid, no movement. Put the timming @ 8 , it idles @ 600, I got the dwell to 26 and it runs pretty good, much better than it did. The vac and mech advance both work. The idle moves around about 50 rpm though, on the digital tach dwell meter... Huh
well anyway I took it for a spin afterwards.... Smooth , and happy it is!!!!!!!! And so am I yeehaw!!!!! Thank you scoutboy74. I'm @ 600 above sea level in redding . Seth :cool:
 
High test fuel is for higher compression engines so they run cooler and not ping?and to run more advanced timming too?
 
Ah yeah...redtucky. I been buzzin' through there on the 5 a lot lately. Sounds like you made some good progress. Yes, high comp, high revving engines require higher octane fuel for a variety of reasons. Some thoughts...when you did your comp test, did you have all the spark plugs removed and the throttle blocked wide open? If not, you wasted your time and can ditch your results. 100-110 while acceptable, is approaching the low end of things, but if you didn't do the test in the manner I just mentioned, that would explain the borderline low figures. 16hg at your elevation is a tad low also. At 6000 feet I'd say dandy, but at near sea level, you should be seeing roughly 19-21 steady on the gauge at idle. A blip of the throttle should peg the gauge towards 0 with a quick recovery to 20 as engine speed returns to idle. 16 is indicative of an overly Rich air/fuel mixture at idle, or a manifold vacuum leak. The latter is often accompanied by erratic gauge needle movement, so the steady low number says Rich mix to me more than vacuum leak. Also, your adjustment screws should be the nearly the same number of turns out from gentle seating on either side. Next time you mess with it, try this...get it warmed up good. Then shut it down. Run each screw in until they gently bottom counting the number of turns each screw was out. Jot down the numbers just for later giggles and grins on this thread. Then run each screw out an equal 2 full turns. Get your handy dandy vacuum gauge hooked up again. Slowly turn first one screw in while watching the gauge and then the other. While watching your gauge, you also want to notice if the engine begins to stumble. Stop turning the screw in and back off a smidge if it begins to stumble. The goal is to get that vacuum number to peak near 20 and still have the engine idle smoothly. Known as lean best idle. You May have to adjust your idle speed slightly both during and after you get done with the mixture fiddling. If you're satisfied with your dwell at 26 degs, you can probably leave it be. As the rubbing block wears down, the gap will close slightly and the dwell will increase to the desired range. If you were at 34 degs instead of 26, I'd be suggesting an adjustment there as well, 'cause it would only be getting worse as the rubbing block wore down.
 
Well, ah, I got to dial it in pretty good! It likes the timming at 8, the dwell at 30. I ran the air fuel screws in next and brought them out 2 turns. The vac gauge was moving from 15 to12. Crap, crap, crap ok, I got it to idle at @600, I turned turned the air/fuel screws in 1/8th turn at a time to get the lean idle to be the best at 1 3/4 turns out each. The vac gauge moves still between 14 and 15 with a weird almost miss... I'm going to check plug wires maybe a header tube cooked one. It's pouring rain so not today. Thank you guys, all you guys helped alot. It runs alot better now, I hope its just a plug wire. :thumbsup: seth
 
Last edited:
Well it's not a plug wire... Egr block off plate had a vacuum leak, I fixed that still idles rough. And the off idle miss is still there. I'm putting in new plugs today maybe one of these autolites are bad. Maybe it has low voltage to the coil , @ 9 volts rite? Im checking for more vacuum leaks today, l thought I fixed them all, guess not. And doin a real compression test today too, thank you scoutboy74!!!! On the good side the dwell is good at 30 and timming at 10 that hasn't changed yeehaw. Headin over there in a bit seth
 
You're welcome. Couple other things you can try...connect your timing light to each plug wire one at a time to see of you notice erratic or non flashing from any of them at various engine speeds and make note. With the engine idling, carefully pull one plug wire at a time listening and watching for a subtle stumble. If you come to one that doesn't make any difference whether hooked up or disconnected...oopsy! Better find out why. Your vacuum at idle is still too low even for my less than stellar, "good enuff fer gummint werk" seal of approval. If you could get to 18hg steady, then I'd shut up about it. So you're still leaking somewhere.
 
Rain sucks..... But we need it!!! I got alot of stuff done, and I found the intake leaks in many places around the intake bolts and I'll wait and do that in a week or two when we do the main seals. I'm going to replace the rubber brake lines, I got the right side done, and left side almost done then I broke a bleeder off a caliper. Crap, I guess it was time to call it a day. Lol you start to fix 1 thing and I found a loose wheel bearing, fixed that and broke the bleeder!!! That's gonna be a bear to extract!!!!!! At least it's moving forward 1 thing at a time !!!!!!!! I mite be better off to get a reman caliper, ah any extraction ideas????? Ah yeah good times!!!!!! Seth
 
You must log in or register to reply here.
Back
Top