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Firstly, apologies for the long winded explanation.
I have a major misfire problem in my ’74 Corvette. The engine is an LS4, 454cui. The carburetor is a Holley 4165 spreadbore 800 cfm (list: 6212).
I recently rebuilt the engine and had enjoyed a trouble free 400 miles running the engine in. Around this time I switched fuels to a new lead substitute (I had been running the engine on 98-octane premium unleaded with an additive for leaded engines). I had used about half a tank of this lead substitute fuel when the engine started to run rough, it seemed to deteriorate over about 30 or so miles. Subsequent bad reports on this fuel led me to believe that this was the cause of my rough running engine. I drained the fuel tank and mopped out the remnants then refilled it with the premium unleaded I had used previously, this time without the additive.
I drove about 40 miles on the fresh fuel but the misfire was no better. I decided to remove the plugs believing that I may have fouled them on the old fuel. Once removed, I noted that the front cylinders (1/3 & 2/4) appeared normal, plugs 1 and 4 looked the best (good color), plugs 2 and 3 looked okay (a little darker). The back cylinders were a different story, plugs 5 and 8 were sooty and wet, plugs 6 and 7 were the worst, heavier deposits and wet also. I cleaned them up, checked the gaps (spot on), and reinstalled them in reverse order (rear plugs to the front and vice a versa) so as to rule out the four rear plugs as bad actors. Off for another drive, no change, back home, pull the plugs, same story, front cylinders light and dry, rear cylinders darker and damper.
In light of the apparent flooding/richness of the rear cylinders, I decided to pull the carby. After stripping the carby down I noted that the rear float was set very low. I recalled earlier having trouble setting the rear float level, it took quite a number of turns to try and stop the fuel from spilling out the inspection hole, in the end I ran out of adjustment and I still thought the level was a little high. Suspecting a faulty needle and seat, I decided to replace both as well as re kiting the carby (new power valves, ac/pumps and so on). I reinstalled the carby and fired the engine, no improvement. I went ahead and reset the float levels, the front float was no problem (both floats were set statically when the bowls were off) and only required a small adjustment. The rear float, again, presented the same problems as it had earlier. I found myself winding the adjustment down turn after turn with what appeared to be minimal response from the fuel level, I stopped when it seemed like I ran out of adjustment, the fuel level was just at the inspection hole.
At this point I decided to eliminate as best I could, the electrical system. I hooked up my trusty Sunpro analyzer and checked the points, cap, leads and ignition intensity, all seemed fine and I was happy to put them to one side for the time being. Similarly, having just rebuilt the engine, and acknowledging the fact that there was no loud metallic, clunking or grinding noises, I will assume for now that the compression is fine also.
This brings me back to the carby. The back four cylinders appear to be getting more fuel than they can handle; assuming spark and compression are adequate. Visibly, there does not appear to be any fuel spilling into carby at idle, the only thing that appears unusual is the float adjustment on the rear bowl. When the bowl was off I didn’t notice anything amiss with the float, if these things are holed they generally hold fuel for quite some time, the float appeared dry/empty.
Any assistance greatly appreciated.
I have a major misfire problem in my ’74 Corvette. The engine is an LS4, 454cui. The carburetor is a Holley 4165 spreadbore 800 cfm (list: 6212).
I recently rebuilt the engine and had enjoyed a trouble free 400 miles running the engine in. Around this time I switched fuels to a new lead substitute (I had been running the engine on 98-octane premium unleaded with an additive for leaded engines). I had used about half a tank of this lead substitute fuel when the engine started to run rough, it seemed to deteriorate over about 30 or so miles. Subsequent bad reports on this fuel led me to believe that this was the cause of my rough running engine. I drained the fuel tank and mopped out the remnants then refilled it with the premium unleaded I had used previously, this time without the additive.
I drove about 40 miles on the fresh fuel but the misfire was no better. I decided to remove the plugs believing that I may have fouled them on the old fuel. Once removed, I noted that the front cylinders (1/3 & 2/4) appeared normal, plugs 1 and 4 looked the best (good color), plugs 2 and 3 looked okay (a little darker). The back cylinders were a different story, plugs 5 and 8 were sooty and wet, plugs 6 and 7 were the worst, heavier deposits and wet also. I cleaned them up, checked the gaps (spot on), and reinstalled them in reverse order (rear plugs to the front and vice a versa) so as to rule out the four rear plugs as bad actors. Off for another drive, no change, back home, pull the plugs, same story, front cylinders light and dry, rear cylinders darker and damper.
In light of the apparent flooding/richness of the rear cylinders, I decided to pull the carby. After stripping the carby down I noted that the rear float was set very low. I recalled earlier having trouble setting the rear float level, it took quite a number of turns to try and stop the fuel from spilling out the inspection hole, in the end I ran out of adjustment and I still thought the level was a little high. Suspecting a faulty needle and seat, I decided to replace both as well as re kiting the carby (new power valves, ac/pumps and so on). I reinstalled the carby and fired the engine, no improvement. I went ahead and reset the float levels, the front float was no problem (both floats were set statically when the bowls were off) and only required a small adjustment. The rear float, again, presented the same problems as it had earlier. I found myself winding the adjustment down turn after turn with what appeared to be minimal response from the fuel level, I stopped when it seemed like I ran out of adjustment, the fuel level was just at the inspection hole.
At this point I decided to eliminate as best I could, the electrical system. I hooked up my trusty Sunpro analyzer and checked the points, cap, leads and ignition intensity, all seemed fine and I was happy to put them to one side for the time being. Similarly, having just rebuilt the engine, and acknowledging the fact that there was no loud metallic, clunking or grinding noises, I will assume for now that the compression is fine also.
This brings me back to the carby. The back four cylinders appear to be getting more fuel than they can handle; assuming spark and compression are adequate. Visibly, there does not appear to be any fuel spilling into carby at idle, the only thing that appears unusual is the float adjustment on the rear bowl. When the bowl was off I didn’t notice anything amiss with the float, if these things are holed they generally hold fuel for quite some time, the float appeared dry/empty.
Any assistance greatly appreciated.
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