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Author Topic:   Carb problem
Dirt Freak

Total posts: 233
posted April 11, 2004 09:26 PM  
I have a Holley HP 4150 750 cfm methanol carb. When I crank the car it runs fine, but when I rev it up, it will not go back down to idle. I mean when I rev it up 2 or 3 times, it stays running at about 3500 rpm's. If I let it do this for about 2 minutes, it goes back down slowly. When it is doing this, I can look on the 2 back barrels and see fuel doing a very fast drip out of the two round nozzles (not the squirt nozzle, but the 2 that are sticking outward). Please help me, I am tired of this problem. I have changed power valves, and am currently thinking about putting in some power valve block offs and jetting it up a little to make up the small difference.

Dirt Forum Champ
Total posts: 1261
posted April 11, 2004 09:52 PM  
Float level to high or the neddle and seat are dirty...but i would think float level.

Dirt Freak

Total posts: 233
posted April 11, 2004 10:07 PM  
I have checked and double checked the floats, and cleaned the needle and seats. Still have the same problem.

Dirt Roller

Total posts: 10
posted April 11, 2004 10:27 PM  
We have a similar problem. When we start up it goes to 3000 rpm & won't idle down. Any suggestions?


racer 4
Dirt Forum Racer

Total posts: 77
posted April 12, 2004 12:55 AM  
be sure your valves are set properly, a tight intake valve will allow air into combustion chamber acting like a throttle venturi causing high idle.

Dirt Freak

Total posts: 303
posted April 12, 2004 02:14 AM  
check and be sure float dosent have a leak,could be filling up and sinking?also make sure back barrels are closing,could be stuck?

Dirt Forum Champ
Total posts: 1019
posted April 12, 2004 08:02 AM  
The rear booster pullover problem is probably not float level related. And DON'T block off the power valve(s) and jet up. That will cause more problems than it ever solves.
Your rear throttle shaft is probably sticking open slightly, which is what is causing the fuel pullover/dribbling problems in the rear.
Aside from a full blaseplate freshening, the quick fix is to install an extra return spring JUST on the secondary shaft so it is forced shut when the throttle closes.

Let us know,

Dirt Freak

Total posts: 270
posted April 12, 2004 08:23 AM  
Your secondaries are probably hanging up slightly.When it does this can you manually (by hand)close the secondaries and slow it down? Sometimes a warped base plate will cause this or corrosion due to running methanol with out a fuel lube additive designed for alcohol/methanol. Try cleaning or lubricating the throttle shafts.If that doesnt work check for base plate warping due to too many gaskets and/or over tightening.
If thats not the problem check the gasket between the base plate and the main carb body (in the carb). Sometimes the screws loosen up and a back fire or just racing will suck the gasket out of shape. Is this carb new to you? Possibly the secondary butterflies are drilled too large for your application.Check that they are not bigger than 1/8 in. There is an adjustment for the amount that the secondaries are held open at idle.Make sure that is correct.Call the carb builder or manufacturer for that info.Carb trouble can be really frustrating.I hope I have given you some help. Dman

Dirt Freak

Total posts: 233
posted April 12, 2004 03:00 PM  
I bought the carb brand new last year. It used to only do this a little bit. However, I sheared off 4 teeth on the distributor gear, and had to replace that. It cranks up and runs fine, at idle just when I get on it, it will not go back down for about 2 minutes. I looked pretty good (was in a little hurry) to make sure the butterflies were closing, and they appeared to be. I would push with my finger on the secondary throttle linkage, but that did not help. I am still open to all suggestions, I am going to look at all replies and go down the list checking them all. I do not think the distributor problem I had caused it, because it back fired 2 times and I killed it. It was doing this out of both the front and back 2 barrels, but I replaced both power valves and now it only does it out of the back. My dad said one of the power valves looked like it could have been opened previously. Kind of thinking we may have gotten a bad power valve. I run methanol that comes with the top lube already in it. Keep giving me suggestions, and comments, they are all appreciated.

David Roper #23

Dirt Freak

Total posts: 370
posted April 12, 2004 06:32 PM  
If not throttle shaft then I would estimate it to be baseplate warpage, leakage, ect. Backfiring is hard on carbs. Your problem is small, look closely.

Dirt Freak

Total posts: 233
posted May 06, 2004 10:47 PM  
Fixed the problem. I found that I had one bad needle and seat. Also my two floats felt like they had absorbed a lot of fuel, so I changed them. I WD-40'd my carb and throttle and it now closed fully. My main problem I think was that it was not closing fully.

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