Tuning Nightmare! Fixing Big Cam Big Block Chevy Idle Issues
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- Опубліковано 21 січ 2023
- After rebuilding my Holley 750 Double Pumper, I contemplate my life decisions as I fight to tune the engine and drill holes in the butterflies on the carburetor.
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Sounds like you need more timing. Some cams may require as much as 20+ initial.
You need more initial timing aka timing at idle. Big cams need it.
Drill them from the bottom and you won’t have to hold the throttle shut, set the idle screw at one full turn from contact and open the idle bleed holes until it runs at RPM. Plus you need a distributor recurve in the worst way. You have to take timing out of the centrifugal advance in the distributor and increase the initial timing on the crank. Just having total isn’t enough. It all depends on how you achieve the total.
I had almost the same problem😢, i solved by just locking the distributer and set initial to 32 in my small block
Needs more timing. Set initial at 12, put the distributor vac hose from ported to manifold vacuum at base of carb and plug ported vac nipple, this will give additional timing at idle which it needs. It may need even more than 12, vac advance to idle well. Did this on a 454 big cam with 850 Holley. worked great idled well and had MUCH more power everywhere. If it is flooding after taking the carb apart,check for a bad power valve by applying a vacuum to it, even by mouth works to see if it leaks. Check the power valve gasket as it may have moved and is not sealing that happened to me ran horrible and flooded the plugs.
Funny I been there. drilling the butter flies works. 6.5 power - duration of the cam is it high. May have to go smaller on power valve.
I'm late to the party. Rather than drilling holes, you can turn the secondary throttle stop screw to get more idle.
Your power valve should be 4.5, 5.5 max. Float bowl setting should be to the bottom of the opening and dribbling out when you shake the car.
Hey, what’s up man thank you for making your video and sharing it. Currently, as I am commenting on your video I am looking at my 402 big block because I am having the same problem. I have a big cam in here and I just can’t seem to get the vacuum, right. Prior to me taking off the carburetor and cleaning it put in new gaskets etc. it was running decent but not OK. Anyway, don’t want to share the whole big story here but hopefully I could get this up and go and I’m using an Edelbrock 650 CFM. Thanks again man.
I hope you've already figured out this issue, but I'll throw my 2 cents in. I'll have to agree with the other comments on here. It seems like Timing. Now i dont have much experience with big cammed engines, so maybe you do need more initial timing as some comments have said. I did notice you were adjusting and trying things on the Carb, then going back to the Distributor, then Carb, then Distributor. Thats kinda like chasing your tail. If you get your carb 1/2 way decent, then go to the Distributor and don't leave it until its dialed in the best it can be before you go back to the Carb. Also, big cam engines usually dont use vacuum advance at all, from what ive seen, vacuum is too weak to be usable. Anyways, i hope you've already figured this issue out, but yup, sounds like it needs more timing. My small block Chevelle sounded like that when i 1st fired it up, it sounded restricted, smoked from rear engine area too (maybe raw fuel dumped out exhaust?), good luck to you man!
Bigger cams need more advance at idle...then you need to limit your mechanical advance.
Did you work it, I'm still working on mind. The motor is 358, top end is great just idle and few pops through carbs. I had to run 18 initial I did drill holes on the plates. Blue springs in the distributor.
900 th subscriber!
I would not have drilled the throttle blades, Now your transition slot is useless , like a vacuum leak. I've seen this done but on some full out twin dominator set-up's. also my timing is locked at 38.
My Roadrunner 383 I just bought runs but will not idle unless I keep my hand on the throttle. I think the person I bought it from put a bigger cam in it. I have an Edelbrock on it. It runs so rich it burns my eyes. If I bump up idle screw it gets into the transfer slots.
My 454 is not that much different than your 402 except for the extra cubes. I have a holley 750HP double pumper with 4 corner idle and down leg boosters, but my idle setting is 1/2 turn out and there's no power valve in the secondaries, you should have no power valve in the secondary metering block.
What's your fuel pressure? I run an electric fuel pump ahead of the fuel tank with a fuel pressure regulator and toggle switch. It gives me fuel pressure before I go to start. Carburetors like volume not pressure, 5-6 psi is perfect.
Hook up that vacuum advance.
nothing ain't nothing with air / fuel mix if your plugs ain't hot enough.....
1st off those power valves are way too High of a vacuum rating for a large cam. You'd need to make 13lbs of vacuum at idle in gear. The power valve raring needs to 1/2 of what it is at idle in gear to work right.