chevy 312 on webers - but its beng awkward!
Discussion
Can any of the brainy peeps on this forum help?
its a bit of an odd one - I was helping out a friend who has a formula F5000 car which runs a 312ci chevy block and 48 webbers. hes just rebuilt it and we got it going today but only on 5 cylinders. wierdly its not firing on 6,7,and 8.
the plugs are all sparking on the relevant cyls and on removal the plugs are wet with fuel - for some unknown reason though on those cyls spark+fuel=no bang.
as far as im concerned it SHOULD be running on those cyls. the cyls have fuel and spark - is there any way wayward ign timing could be responsible?
no amount of blippage or other coaxing will bring the other cyls to life.
even without the 3 cyls the engine runs clean and starts easily enough.
have any of you guys got any ideas?
TIA
Paolow
its a bit of an odd one - I was helping out a friend who has a formula F5000 car which runs a 312ci chevy block and 48 webbers. hes just rebuilt it and we got it going today but only on 5 cylinders. wierdly its not firing on 6,7,and 8.
the plugs are all sparking on the relevant cyls and on removal the plugs are wet with fuel - for some unknown reason though on those cyls spark+fuel=no bang.
as far as im concerned it SHOULD be running on those cyls. the cyls have fuel and spark - is there any way wayward ign timing could be responsible?
no amount of blippage or other coaxing will bring the other cyls to life.
even without the 3 cyls the engine runs clean and starts easily enough.
have any of you guys got any ideas?
TIA
Paolow
Thanks for the replies guys,
compression is high across the board and the ign is a double pointed dizzy and the leads are deffo in the right order. so in short im still stuck! I just dont understand why it wouldnt be firing on the last 3 cyls. is it just a coincidence that its the last 3 cyls that arent firing? to have all the iffy cyls in a neat little bundle at one end of the engine is a bit odd...
any other ideas? im still scratching my head on this one!
compression is high across the board and the ign is a double pointed dizzy and the leads are deffo in the right order. so in short im still stuck! I just dont understand why it wouldnt be firing on the last 3 cyls. is it just a coincidence that its the last 3 cyls that arent firing? to have all the iffy cyls in a neat little bundle at one end of the engine is a bit odd...
any other ideas? im still scratching my head on this one!
GreenV8S said:
What do the iffy cylinders have in common?
nothing I can put my finger on - the cyls that arent firing are 6,7+8 and are all at the rear of the engine. theres nothing obvious to differentiate them from the working cyls - ie - no different carb, leads or plugs or anything like that.
Could be they share a common choke on the carb, or petrol dribbles inside the manifold are sent that way under acceleration, or a water jacket leak allowing water into that part of the intake manifold, or the plugs are awkward to get to so they weren't changed when they should have been, or they run hotter and the plugs have overheated, ...
GreenV8S said:
Could be they share a common choke on the carb, or petrol dribbles inside the manifold are sent that way under acceleration, or a water jacket leak allowing water into that part of the intake manifold, or the plugs are awkward to get to so they weren't changed when they should have been, or they run hotter and the plugs have overheated, ...
All fairly sensible ideas but alas i dont think theres a fix there. the engine was firing on all 8 and then the right hand head was removed to check the bore and stroke of the cylinders to confirm engine cc. The head was then replaced and the completed carb/inlet manifold unit whacked on.
Wierdly 2 of the down cylinders are on the side where the head WASNT removed - nothing has changed - just for some reason they wont fire.
the weber 48s that it runs on means that every cyl has its own choke - but the carbs are fuelling - that much i know becuase the plugs come out wet when its run for a bit. the trumpets are all sucking and a hand over the top of any trumpet gives a dose of black smoke so im happy that theres fuel. im also happy that theres a spark and no amount of dizzy wigglage will make the others come alive so i think the timing is ok.
really mystified by this one!
ngr said:
Have you tried a new set of plugs? Some plugs don't react very well after the have been soaked in fuel
I havent but then ive had them out and know for a fact that they spark ok - ive got others though - do you think its worth swapping them in case they arent sparking when in the cylinder?
Hi guys - bit of an update for you about this engine. the changing of the plugs and a checking of the leads has now given us 6 running cylinders but unfortunately 5+7 still dont want to know. the plugs are definitely sparking and i feel its more than a coincidence that both cyls are fed from the same weber carb. im then pointing the guilty finger at this carb - the plugs that come out arent noticeably wet - surely if its not firing theyre going to be soaking?
the wierd thing is that the carb was working fine when removed (in a complete unit with the other 3) - what could have changed?
Ive tried running it with a hand over the mouths to try and force fuel in thinking it was being starved but to no avail. have any of you got any tips?
paolow
the wierd thing is that the carb was working fine when removed (in a complete unit with the other 3) - what could have changed?
Ive tried running it with a hand over the mouths to try and force fuel in thinking it was being starved but to no avail. have any of you got any tips?
paolow
You may just have a stuck float in the carb after they were removed etc. you can see if you have fuel in the carb simply buy looking down the inlet with a light and fully depressing the throttle. You will get a squirt of fuel from the excel jet. Compare the suspect carb to the others.
ngr said:
You may just have a stuck float in the carb after they were removed etc. you can see if you have fuel in the carb simply buy looking down the inlet with a light and fully depressing the throttle. You will get a squirt of fuel from the excel jet. Compare the suspect carb to the others.
thanks for the tip! will do - im going round there tomorrow so will have a look then and update
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