My engine runs rich at low RPM - above idle, but runs great at higher RPMs. During long cautions the headers will start glowing red and the engine will start misfiring. When we go back to green, the engine will bog down in the turns. My carb is a Quick Fuel 750 CT and my ignition timing is set at 36 degrees. I run a Falcon transmission, so I can't down shift or step on the clutch and rev up the motor to try to keep the exhaust temps down. I'm looking to fix the problem, not find a work-around. I bought a book on Holley Carbs would be great for explaining the basic principles of how carbs work, but actual tuning or repair - Nope! Thanks!
355 sbc 9.5 to 1 compression (rules), jets are 74, 91 octane pump gas (rules), 4.5 power valves, HEI distributer not locked - 36 degree at full advance. Have checked the timing several times, even tried different timing lights. Had a 25 lap feature go caution free - no problems. Next 20 lap feature, one long caution - engine stumbles on the restart and is flat coming off the turns.
Take 2 numbers of jets out and lock the distributor timing.Probably need to adjust the 4 corner idle screws in some.They need to be 1 1/2 turns out from bottom.
I'd be checking the idle restrictions and air bleeds for dirt and debris. Glowing headers tell me there's is fuel still burning as the exhaust valve opens and the black plugs tells me there is too much of it. I'd surmise the excess fuel is still burning when it leave the valve so figuring out why there is too much near idle and tip in is where I'd start looking beside the timing is where is should be. Just my thoughts.
A few things need to be done. First, the transition slots in the Quick Fuel baseplates are too large, to fix this you need transition slot restrictors in the metering block face of the main body. 8-32 brass set screws, drilled to .082. And second is lower the idle feed restrictions, it will make idle and transition fuel more stable. For a lowered idle feed I would start with a #66 drill, .033" start with a .070 idle air bleed, tweak as needed. Timing should be locked out. And then take the main air bleeds to .030. Instructions for the idle jet relocation at the link below.
A few things need to be done. First, the transition slots in the Quick Fuel baseplates are too large, to fix this you need transition slot restrictors in the metering block face of the main body. 8-32 brass set screws, drilled to .082. And second is lower the idle feed restrictions, it will make idle and transition fuel more stable. For a lowered idle feed I would start with a #66 drill, .033" start with a .070 idle air bleed, tweak as needed. Timing should be locked out. And then take the main air bleeds to .030. Instructions for the idle jet relocation at the link below.
A few things need to be done. First, the transition slots in the Quick Fuel baseplates are too large, to fix this you need transition slot restrictors in the metering block face of the main body. 8-32 brass set screws, drilled to .082. And second is lower the idle feed restrictions, it will make idle and transition fuel more stable. For a lowered idle feed I would start with a #66 drill, .033" start with a .070 idle air bleed, tweak as needed. Timing should be locked out. And then take the main air bleeds to .030. Instructions for the idle jet relocation at the link below.
I think I'd check some basic stuff first. What spark plugs? What gap? Are the fuel lines close to any heat source like headers, lower radiator hose, or the engine? Have you tried another known good carburetor to see if the problem stays or goes? The fact that is doesn't do it all the time and it's worse after a long caution could indicate that the carb is rich on the idle circuit or transition but there may be other things going on there like boiling the fuel or just simply too cold a spark plug. Where are the idle mixture screws and float level set?
And yes, as mentioned your timing should be locked. One reason is to pull more vacuum at idle and close the throttle blade down in the base plate to show less t-slot if it's showing too much now. Locked timing is more reliable too.
I've been chasing this gremlin for a long time. Several carbs, distributers, and headers. The biggest improvement came from replacing the 180 headers with over the top style headers that I was running before this problem started. They were creating so much heat under the hood that the engine would vapor lock - even melted the distributer cap. I've made so many adjustments to the carbs, that I'm fairly sure that I caused this problem. I've found one problem -showing too much T-slot on the secondaries. I'm going to put it on the track and see how it runs before I do anything else. Jmarkaudio, I have no doubt that you know what you're talking about, but mods like that, I'm going to leave to a professional. It might seem simple to you, but trust me, I can screw it up. Thanks for all the advice. I'll let you know how it goes.
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Carb tuning question
Originally Posted by jmarkaudio
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