Too Much Fuel

Michael Owner/Technician Colorado Posted   Latest  
Unsolved
Driveability
1980 Chevrolet Corvette 5.7L (8) 3-spd (THM350)
Runs Very Badly Fuel Very Rich!!!

Vehicle has a new 355ci/400hp engine and a new (less than a year old) Holley QuickFuel 680 double pumper carburetor. Fuel mix is very rich and runs very poorly. Disassembled carburetor to check power valve but it tests fine…holds vacuum. Reassembled and vehicle ran well with huge vacuum leak but when there is no vacuum leak it ran poorly. Any ideas on where to go from here?

Thanks.

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Steve Owner/Technician
Georgia
Steve
 

Check vacuum, power valve may not be compatible with camshaft. If memory serves me, I believe a 125 power valve opens at 12.5" vacuum and a 105 opens at 10.5" vacuum and so on.

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Guy Educator
Illinois
Guy
 

This is correct in terms of sizing. Most aftermarket carbs come with a 6.5 power valve. Check the vacuum rating on yours and compare it to manifold vacuum. If vacuum is too low, it could be opening the powervalve at the wrong time.

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Michael Owner/Technician
Colorado
Michael
 

hello Guy, sorry so long in reply. this enging is only making about 10 in of vacuum. should i go with a 5.5 power valve? also why would this engine make such low vacuum? its all less than a year old low miles. would valve adjustment make low vacuum? thanks for your time

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Gary Owner/Technician
Colorado
Gary
 

Hello, Michael. 10"Hg is pretty low unless the cam has a lot of valve overlap. And, yes, valve adjustment can tighten up during break-in and cause a major loss of intake vacuum. I'm guessing your intake vacuum should be in the 12" Hg-14"Hg range in Fort Collins. (see my post above)…

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Glenn Owner/Technician
Texas
Glenn
 

Hi Michael, You're only around 5000 feet above sea level, your vacuum is definitely too low. Calculations I looked up show you should be around 17Hg. of vacuum. Pull the valve covers and back the rockers off each ¼ of a turn. See if the vacuum improves. If they tick, find which ones and turn them 1/8 turn at a time until it quiets down. If you have a radical cam, that can also lower the…

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Guy Educator
Illinois
Guy
 

I would go with a 5. My starting point is always half of engine vacuum at warm idle. A 5.5 should be close enough too. If it does not have a radical cam in it, you may have valves out of adjustment, a vacuum leak, or the cam timing may be off. Those are the areas I would check first.

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Gary Owner/Technician
Colorado
Gary
 

Michael, unless the old power valve is leaking, I don't think installing a 5.5 power valve is going to help. With the engine running, see if fuel is flowing from the venturis. If you have fuel dribbling from the venturis, either the float level is too high or the throttle opening is past the transfer slots, which are located just above the throttle plates in the throttle body. Pull the fuel…

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Glenn Owner/Technician
Texas
Glenn
 

Hi Gary, I have to ask Michael: How familiar are you with Holley carburetors? Is this a used project carburetor? I am not picking on you, but just trying to understand better what you are up against. I know Gary is very good with Holley and this vintage of vehicle. I am also well versed and trying assist. Carburetors are dying off more every year and the people who use to work on them on a…

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Guy Educator
Illinois
Guy
 

I would agree with Gary. If you have 10" at idle, the valve should still remain closed, so I do not think this is your immediate issue. Just something to look at when tuning for drivability.

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Guy Educator
Illinois
Guy
 

Is it only rich at idle, or cruise and WOT too? Does it only have idle adjustment screws in the primary block, or is it a 4 corner idle system.

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Bentley Manager
California
Bentley
 

As others have said check the power valve and match it to the idle vacuum so it stays closed. Check both float levels and make sure air bleed passages are open.

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Gary Owner/Technician
Colorado
Gary
   

Hello, Michael. Good advice so far, but everything depends upon the camshaft valve timing and cam idle characteristics. I don't know what type of cam you have, but: Test intake manifold vacuum. Your exact reading depends upon baro pressure, Generally minimum 16"-18" Hg at sea level and can be as low as 10" at 8,000 feet. You need at least 10" Hg vacuum to make the carb to meter fuel correctly…

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Helpful
Glenn Owner/Technician
Texas
Glenn
 

Hi Michael, First thing I would recommend is loosen the rear float seat lock screw, then turn the nut below it to adjust the needle down. This shuts the fuel to the rear of the carb off. This will cause the carburetor to only run on the front 2 barrels and allow you determine if it is the primary side or the secondary. If I am not mistaken this has a power valve in the front and the…

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Michael Owner/Technician
Utah
Michael
 

This carb has a sight glass in each fuel bowl. Check the fuel level in each fuel bowl. Easy to adjust with the externally adjustable floats. Should be dead center in the sight glass. HTH, Mike

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Kenneth Mechanic
Indiana
Kenneth
 

There is one thing that wasn't said or asked! What cam is in it a roller or a flat tappet? Then from there a person can make a better conclusion. Either way GG is right about going to the 6.5 or 5.5 numbers for opening the power valve. And one thing that wasn't included is the front butterfly's need to have a 1/16th hole drilled to help the idle. I'm sure have watched down the carb while idling…

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Kody Educator
Oklahoma
Kody
 

Make sure both the secondary and primary throttle plates are closed all the way so the transfer slots are covered until the Throttle is actually opening. You may have to drill a small hole in the primary throttle plates if you can’t get the idle speed up enough without opening the idle speed screw too much to uncover the transfer slots. Also if you get it at idle speed and it is still rich try…

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Allen Owner/Technician
Texas
Allen
 

Been awhile, but if I recall there is also a seconday plate air adjustment screw for “big cams”

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