I have an1953 8BA, bored 80, .425 Offenhauser Heads, Isky Max1, Mallory electronic distributor (no vacuum advance), Red's Headers. I put a Holley 390CFM 4160 on a Offenhauser single plane intake. The engine has about 600 miles on it and the idle characteristics are unacceptable. It an attempt to tune the carburetor I installed an A/F meter in the passenger header. The idle starts at 12-1400 RPM and after the choke opens drops moves to 800-1000RPM. This is fine, but when driven for a short distance and coming to a stop it will either return to 12-1500 or stall and become difficult to restart. I have a dedicated vacuum gauge on the dash and can find no vacuum leaks. I have read Holley 4160s are not very street able and am considering a Performer Edlebrock. I chose the Holley because the Flathead racers in the area suggested it, but they don't do much stop and go driving! And suggestions will be appreciated.
Are you sure it is a carb issue? That is a fresh motor, valves may be tight, if they are adjustable. Also see what is going on when it wont restart, could be vapor locked or a fuel delivery problem. Hope this helps
I haven't messed with Holleys in a long time, but this is what I would check: When it's hard to start, do you smell gas? Could be a bad power valve or your floats are off Is your throttle return spring stiff enough? I just bought an Edelbrock 500cfm for my Flathead that I'm building. I haven't installed it yet so I can't speak to it's performance, but it's nice quality.
I've got a 600 Cfm 4160 on an OT 71 f250 and it runs/idles very well. You idle rpm seems high, even on the step for the choke. Does it stumble running constant part throttle down the road or does it only have an issue at idle? What is the resting blade position in relation to the idle slots in the throat? Tyson Posted using the Full Custom H.A.M.B. App!
Thanks to everyone for the quick responses.. Valves are OK (14 cold) and the blade position is not into the idle slots and the secondaries are set slightly open. No problem other than idle. I carefully insulated the fuel line when I built the truck and vapor lock dosen't seem to be a problem. Good fuel pressure (electric pump and moon liquid FP gauge shows 6-8 PSI)
May be showing my ignorance,But if its not a 4 corner idle carb why have the secondaries open at all?
Actually that sounds like a pretty good place to start looking. I wonder if the secondaries are open just enough to start pulling a bit of fuel thru the secondary nozzles? Also, if you have access to one, I'd try a distributor with vacuum advance. That can make a big difference in idle quality and driveability. And you can back the initial and total timing down a bit so you don't have hot-start problems.
As another comment, I've never been a fan of open, 360* intake manifolds on anything other than a full competition engine, tuned only for high rpm use only. Agree also about the "slightly open" secondaries.
Shut the secondaries and reset the idel mix and rpm look for best vac. make sure timing is correct. I build and have run 4160 for over 40 years and they will work well if they are set up correctly.... don't give up and you will be happy with it once you get it sorted out....
With a non-stock cam you'll most always have to have the secondary throttle plate slightly open to get the best idle. But it shouldn't be open to the point where it shows more than about .020" of the transfer slots. At that point, the portion of the slot below the plate will look like a square hole. If it looks longer than it is wide, then the plate is open too far. Why does the secondary plate need to be opened a bit? To allow the engine to get enough air to idle without having to open the primary plates to the point where they expose too much of their transfer slots. If the primary plates are opened too far then the idle screws have little effect since you're now bypassing the idle circuit. The goal is to let it have the air it needs to idle while keeping both primary and secondary plates closed far enough to keep from activating any fuel circuit other than the idle circuit. If you end up with the secondary plate adjusted as above and the engine gets enough air to idle at the speed you want, its okay if the primary plates are fully closed. If it still idles too fast, then the secondary plate needs to be closed a bit more. Since the OP's engine has a hotter than stock cam he should also check the power valve. As a general rule of thumb on engines that won't pull a lot of vacuum at idle, the power valve rating should be about half the idle vacuum reading else the power valve is going to be open too much of the time and dumping in extra fuel unnecessarily. FWIW, all the above info on how to set primary and secondary throttle plate openings for idle conditions comes straight from Holley's published info on their carbs. Learned how all that stuff works together after chasing my ass for a few months trying to get a 3310 on a medium warm 350 to idle decent about 40 yrs ago. After all else failed, I tried the old RTFM trick and things worked a lot better. Agree with the above suggestion re using a vac advance distributor if possible. Straight mechanical advance distributors and difficult hot starting are often delivered together in the same box
the proper way to mod the Holley carb for the low vacuum big cam setups is NOT to open the secondary but to drill small holes in the primary throttle blades to allow more air to enter without opening the blades onto the transfer slots .NEVER xxx NEVER open the secondary's for more air as it also brings in more fuel. It was never intended to be used that way. if you need more air do the drill trick and if you find you need more fuel you adjust the idel mixture screws richer.. and the Power valve only feeds fuel into the main jet circuit only when needed under load and when running on the main circuit only, not at idel (.40 year Holley builder and tuner)
I'm gonna assume Holley knows a fair bit about their own carburetors, and they say to open the secondary plates. They also say to drill holes in the primary plates only as a last resort if you can't get enough air by opening both primary and secondary plates until you begin to expose the transfer slots. No it does not so long as you don't expose more of the slot than Holley says is allowable. Holley thinks it was. Maybe you oughta write 'em a letter and set 'em straight. Who said anything about the power valve opening at idle? And no, the power valve opens anytime the vacuum drops below the valve's rating. It has no idea whether additional fuel is needed, or if the engine is under load. Its not an intelligent device. I mentioned the power valve because you don't just assume the factory one is right when you have a cammed up motor. Often it will open too soon, and that's why Holley makes them with different vacuum ratings. Maybe you missed the fact that the OP is having driveability problems as well as idle problems. And maybe in another 40 you'll learn the basics of how a Holley works.
First check the float levels through the sight plugs,try swapping the power valve to a 5.5 or 4.5 and see what happens it should have came with a 6.5 probably with the cam you have your vacuum is lower and this should help,the 390 Holley should be a better bet than a 500 CFM Edelbrock.
metalmuncher! No disrespect to you intended hear.... Book education is intended as a reference to start. Real world education is where it is fine tuned. You are welcome to your opinion and do what ever you want no skin off my back. I have read everything Holley and others have printed but it does not make it all gospel. Don't be afraid to try new ideas on your own or your book education will never grow into the real world. If we were still doing it the way old books say we would still be back in the day. JMHO.....
I had a similar idle problem on a 350 with a mild cam and a Holley vacuum-secondary carb. Turned out the linkage was slightly bent, not allowing the secondaries to close fully and properly. My first clue was that the idle mixture screws were only mildly responsive, the second being that the car would continue to idle (though poorly) with the mixture screws almost completely shut and the idle speed screw backed all the way off. The carb was getting enough mixture through the poorly-controlled secondaries to allow it to continue to idle with everything pretty much shut down. The carb in question was an old one (probably been dropped on the linkage), so I simply replaced it with a newer used one and the problem went away entirely. Good luck!
UPDATE::: First, thank you everyone for the help!! I pulled the carb off, removed the 3" spacer reset the accelerator pump, closed the secondaries, and replaced the gasket on the secondary sight plug. Then I set the A/F screws to 1 1/2 turns and fired it up. After a little fiddling (including a 1/2 turn on the secondaries) everything settled in and it idles great at about 900 RPM. Funny thing is now the A/F is 16+ at idle! The Flathead racers I talked to gave me great advice, trouble is race cars don't idle that much. I also checked with Holley and was told the secondaries need to be adjusted a little to get a good idle.
Secondary's should only be opened just enough to keep fresh fuel flowing through the bowls unless it has 4 corner idle circuit's and open plenum intake. Holley's have idle bleed circuits in the secondary's for that reason. they are not to be opened but just a small amount... 16 to 1 AFR is to lean....around 13 to 1 would be what you want
Thanks John.. My 390 has vacuum secondaries. I agree I am too lean so I plan to back off the secondaries a little and try tuning by the vacuum gauge.
LjH! What's your AFR just above idle , at cruses speed and WOT. Just curious. after reading your org post.. once warmed up can you add some fuel with the idle mix screws or is it all it's got also as a newly built engine gets broke in it will pick up some RPM,s you will need to fine tune the carb again after break in (aprox 500 miles)
All good suggestions. Also check your timing advance. I had a distributor once with a weak (from age) advance spring and it would kick in a bunch of advance around 800-1000 rpm so at that point it would speed up a lot. If you lowered the idle speed below 800 rpm the timing would retard by like 8 degrees and it would stall.
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