Running weak then rich!
Thread Starter
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Freshman User
Joined: Nov 2005
Posts: 42
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From: Littleborough UK.
Running weak then rich!
Hi help needed, at £5 a gallon you have to help! my 73 302 fitted with a holley 600 cfm carb, was running very weak plugs were almost white, wouldnt pull, so i got some bigger jets, everything was fine 200 miles plugs were grey, no problems, till today, it started back firing so when i got home i checked the plugs they were black, how can that be! all help apreciated, regards Alan.
Elder User
Joined: Mar 2004
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If your ignition advance isn't working properly or if your timing has slipped or moved it will cause these problems, particularly back fire.
On the carb, check your power valve, and also check to make sure none of the little air ports arn't blocked by dirt. Make sure your choke is working properly and adjusted.
On the carb, check your power valve, and also check to make sure none of the little air ports arn't blocked by dirt. Make sure your choke is working properly and adjusted.
Thread Starter
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Freshman User
Joined: Nov 2005
Posts: 42
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From: Littleborough UK.
How do you check a power valve! went out today plugs grey again, but still a misfire, checked the timing and have it at 12deg btdc, with the vacuum advance pluged, i can advance it loads more and it won't pink, what do you think, regards Alan.
Hotshot
Joined: May 2004
Posts: 12,170
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From: The hills of No. Calif.
Alan, you need to pull the power valve out of the carb
and apply vacuum to the diaphragm through the hole in the center (left side in pic) and see if the spring-loaded plunger on the right moves in and stays that way as long as vacuum is applied. In a pinch I have done it with just my mouth (tastes like gas, for some reason!) but I usually use a small suction cup attachment on my Mitey-Vac hand-held vacuum pump. It doesn't take a very strong vacuum to move it.
Power valve blow-outs are common with older Holley carbs. New ones have a power valve protection circuit built in, but there is a kit you can buy for older ones to convert them. When the power valve goes, it allows raw gas to enter the main curcuit of the carb enrichening the mixture. It can also merely come loose if not installed properly or be installed with the wrong gasket causing the same problem.
and apply vacuum to the diaphragm through the hole in the center (left side in pic) and see if the spring-loaded plunger on the right moves in and stays that way as long as vacuum is applied. In a pinch I have done it with just my mouth (tastes like gas, for some reason!) but I usually use a small suction cup attachment on my Mitey-Vac hand-held vacuum pump. It doesn't take a very strong vacuum to move it.
Power valve blow-outs are common with older Holley carbs. New ones have a power valve protection circuit built in, but there is a kit you can buy for older ones to convert them. When the power valve goes, it allows raw gas to enter the main curcuit of the carb enrichening the mixture. It can also merely come loose if not installed properly or be installed with the wrong gasket causing the same problem.
Thread Starter
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Freshman User
Joined: Nov 2005
Posts: 42
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From: Littleborough UK.
Checked the power valve, all ok, went for a spin the car ran better, checked the plugs, now they are grey again, and a slight miss at about 3,500rpm, it will pull in 5th gear from a standing start, with a slight slip of the clutch just to get it going, cannot be too much wrong! regards Alan.
