Wandering Misfire, Knocking, Hard Starting
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Joined: Jul 2007
Posts: 58
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From: San Antonio
Car: 1984 Z28 L69
Engine: Cammed 350
Transmission: Tremec Tr-3550
Axle/Gears: 3.42
Wandering Misfire, Knocking, Hard Starting
I recently rebuilt a 350 for my car (first engine build) and its recently developed some odd issues. After i first built the car i ran a set of beehives for my car which eventually led to a bent valve. After a head job, it was again up and running with no issues other than a lean condition which has been addressed. I recently upgraded the factory HEI system with a summit Racing Mechanical Advance Billet distributor and Ignition box. Since the upgrade i have hit several issues. Here are the engine speck as i built it.
350 Standard Bore, L31 Vortec Shortblock
World S/R Heads Milled to 66cc 1.94/1.5 Valves
Crane 228/228 @ .050 286/286 adverised cam, 1.6 Aluminum Roller Rockers
Edelbrock Torker II Intake
Holley 670 Truck Avenger 72/78 jets 6.5 power valves.
First thing i noticed is a weird wandering misfire. When i first set the timing everything was great, then within the first hour of tuning in the carb i developed this misfire, it seems to go away at higher rpms but its definitly noticeable at low rpms. I checked all the plugs, if at most they are a hair lean but not rich. I set base timing at 16* @ 750-800 rpm with the distributor just as it came out of the box. With the timing set like this the car is now hard as hell to start at times. When i crank it over, if i dont give it gas the thing turns just fine, but after i pump the gas a couple of times it hangs up while im cranking it over. Now if i advance the timing from here to 20* it hans up some but not as much, but it will now pop back at me through the carb, which tells me its too far advanced. Just to see what the motor would do i advanced it even more, i went to around 30* and the hang was completely gone, as well as the wandering misfire. This confused me a bit. I went ahead and set base timing at 18* this time to try to find a happy medium, this eliminated the popping through the carb, but the wandering misfire is still present and now i have noticed a knock. With my mechaninc stethoscope i started listening to everything, i cant hear the knock in either valvecover, nor can i hear it in the intake. To me this sounds like its coming from the bottom end. I then decided to try it back at 30* to see if i still heard the knock.... i did not. Not only did this confuse the hell out of me, it made me wonder if i hearing spark knock, but i thought that would come from too much timing and not the reverse.
Anyone have any thought/input on this? This is not a daily but i would like to drive it again lol.
350 Standard Bore, L31 Vortec Shortblock
World S/R Heads Milled to 66cc 1.94/1.5 Valves
Crane 228/228 @ .050 286/286 adverised cam, 1.6 Aluminum Roller Rockers
Edelbrock Torker II Intake
Holley 670 Truck Avenger 72/78 jets 6.5 power valves.
First thing i noticed is a weird wandering misfire. When i first set the timing everything was great, then within the first hour of tuning in the carb i developed this misfire, it seems to go away at higher rpms but its definitly noticeable at low rpms. I checked all the plugs, if at most they are a hair lean but not rich. I set base timing at 16* @ 750-800 rpm with the distributor just as it came out of the box. With the timing set like this the car is now hard as hell to start at times. When i crank it over, if i dont give it gas the thing turns just fine, but after i pump the gas a couple of times it hangs up while im cranking it over. Now if i advance the timing from here to 20* it hans up some but not as much, but it will now pop back at me through the carb, which tells me its too far advanced. Just to see what the motor would do i advanced it even more, i went to around 30* and the hang was completely gone, as well as the wandering misfire. This confused me a bit. I went ahead and set base timing at 18* this time to try to find a happy medium, this eliminated the popping through the carb, but the wandering misfire is still present and now i have noticed a knock. With my mechaninc stethoscope i started listening to everything, i cant hear the knock in either valvecover, nor can i hear it in the intake. To me this sounds like its coming from the bottom end. I then decided to try it back at 30* to see if i still heard the knock.... i did not. Not only did this confuse the hell out of me, it made me wonder if i hearing spark knock, but i thought that would come from too much timing and not the reverse.
Anyone have any thought/input on this? This is not a daily but i would like to drive it again lol.
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