Holley 4150 355 won't stay idling! - Hot Rod Forum : Hotrodders Bulletin Board
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Old 05-23-2006, 10:50 AM
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Holley 4150 355 won't stay idling!

I have a completely fresh 355 in my 75 Pontiac Astre (Vega). The shortblock is all rebuilt, with new KB flattop pistons, balanced, everything. The heads are new Dart Pro1's, the cam is a Comp Cams 268H. The intake is a Weiand Xcelerator that I had laying around, and I have a Holley 750 4150 from the last 350 transferred to the new engine.


So I fire the engine for the first time last night. The engine will not stay running on it's own without having to touch the throttle. At idle, it will idle for a few seconds, then it starts to stumble and die. Same at higher RPMs. Holding the throttle at 2500 rpms, the engine runs fine for a second, stumbles, BUT drops to 2000 rpm, catches itself, revs to 2500, drops to 2000, and so on....

The carb was the only thing I thought would remain constant. It ran great on my last 350. It was last ran in September, but I drained the bowls, and it has since sat, with nothing changing. I'm not so sure it's the carb. I played with the idle mixture screws, and it seemed to help very slightly.

Are there any other possibilities? Is is possible that my used intake may have a warped flange causing my problems? Is this a situation possibly caused by a vacuum leak? Thanks in advance, Josh.

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Old 05-23-2006, 01:39 PM
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Sounds carb related to me. Vacuum leaks can cause this but it's gotta be one big leak.
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Old 05-23-2006, 01:43 PM
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carb

Sounds carb related to me... Take a flashlight and look close at the air bleeds to make sure they are not dirty. You can spray some carb cleaner in them and blow them out with a air hose...

Where is you timing set????

I hope you realize that a fresh engine should not idle at all until the cam is run-in???? Keep it above 2000rpm for the first 20/30 minutes of operation...


Keith
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Old 05-23-2006, 10:28 PM
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I should have mentioned that I did break in the cam first... though my right foot was killing me after having to feather the throttle for 20 minutes! lol


So I think it was a big vacuum leak. I was gonna take the carb off to check things out, and while doing so, the right rear stud was loose cause the mounting boss had been broken. I don't know if it was like that after I put the carb on, or if it was right when I was taking it off. So anyways, I guess this gives me a reason to buy the Performer RPM intake I was thinking of.
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Old 05-27-2006, 11:47 AM
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i had a similar problem a few weeks back and in my case it was the air bleeds. Too big on primary side. swapped out larger bleeds for a smaller set and voila, car idled as it should. also had holes drilled in butterflies and swapped them for new ones. just and idea.
good luck.
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Old 05-27-2006, 01:25 PM
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Thanks for the info, but I got my new Performer RPM intake today, so I'm gonna swap that out. I'm pretty sure that was my problem... I kinda thought it would be when I was installing it, but I didn't have money for a new intake at the time, so all is well.

I was anticipating the carb not having to be tuned at all. It ran great on the 350 I had in there 6 months ago. We'll see this afternoon.
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Old 05-27-2006, 07:45 PM
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rebuild the carb buddy

you can rebuild it without a rebuild kit if it was recently rebuilt. Just take it apart and clean it. If it was never rebuilt, spend the $25 for a rebuild kit. I can rebuild and clean a holley 4150/4160 in 2 hours (unless it's in really bad shape, then add another hour) anyway, you will spend more time than that trying to figure it out.

it might be a vacuum leak but why not rebuild the carb and rule that out. make sure you blast carb cleaner into all the air and fuel ports in the main body, throttle body, metering block, etc. the rebuild kit will include all new gaskets, washers, accelerator pump, needle/seat, etc. they come with instructions also to set the floats, idle mixture screws, etc.

let us know what happens with it.

Lee
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