80 lf backfiring through carbs

[buddyc]

I have a 80 lf put a 82 engine in it.Rebuilt engine new pistons,seals,gaskets.Put the 80 ignition in the 82.Didn't want to use pj carbs got some vm36's don't know what jetting should be for them.The engine is stock with no airbox and aaen pipes.Have uni sno filters.I've started out with 30 pilot,330 main and 6dh3 needle q2 needle jet,2.5 slide,viton tip needle and seat 1.5 brand new.Fuel pump is mikuni 35 liters an hour.Don't know if this is right or not?I have a hard time getting it to start,backfires through the carbs.When i do get it to start will run ok and get it warmed up shut it off go to start again and it will not start.Will start backfiring again.Checked plugs for spark,it has good spark to plugs.Was wondering i've alway's heard that the cdi work's or it doesn't work is this true??Engine is timed properly.I just don't understand why it would be backfiring through the carb's.Pipes are both open no obstruction's.The only thing i didn't replace yet was the choke plunger's,i noticed they were hard on the bottom and there adjusted correctly.Don't know if it is in the jetting of the carbs or something in ignition,but like i say i have decent spark.Got some choke plunger's coming for it.I'm puzzled!! Any help would be appreciated!

[nick80lf]

Backfiring out the carb can be an indication of a lean mixture. Normaly the stock VM36 carbs with power jets use a 40 pilot. I had this problem 12 years ago or so when using a 30 pilot on VM36 with power jet (I now use 40). Also check the passage that delivers fuel from the pilot jet between the slide and the engine. Assuming it backfires when starting with the throttle closed.

Just my opinion, sounds like it wants more fuel at idle/startup

Nick

[nick80lf]

I should also add that leaking carb flanges, crank seals, ect would cause a lean condition. I assume the engine was pressure tested after the new seals were installed.

Nick

[buddyc]

Hey thanks for the info nick! Put the 40 pilots in today got it started warmed it up ran pretty decent.Went to shut it off to check plug color,and went to fire it back up to see if it would start and i couldn't get it to fire back up.I tried to choke it but it still wouldn't fire.it would act like it wanted to fire but it wouldn't.I don't think the main jet being a 330 would have anything to do with it firing would it?I've got a new coil coming for it and choke plungers i'll try those to see if they help.Still has me baffled.Engine was pressure tested too.no leakes

[nick80lf]

I would think that as long as you are not holding the throttle open when trying to start it the main jets would not matter too much. The stock mains for the power jet carbs are 150. I have never tried non power jet carbs on the LF engine but I would guess that you would need to increase the main jet size to compensate for not having the power jets. You might also have to chose I needle combo that has a steeper than stock fuel delivery curve. When you tried to restart it were the plugs wet or dry? Also maybe it's just me but I think my LF starts easier when using non-resister type plugs. I use NGK B9ES.

I don't claim to be a carb/tuning expert but I've fiddled with my carbs quite a bit and learned from my mistakes.

Nick

[buddyc]

I've got the stock pj carbs i think i'll clean them and give them a try.I know i'll have to richen them a little to compensate for pipes and air filters.Something just isn't right with it.I think i'm running resistor br9es at .025 gap.Will try b9es. thanks!!