SF1 Misfire

C

chrissoz

Senior member
Location
Sydney, Australia
SF1 is back together after the rebuild.

Have been out for a few shakedown runs and most things are looking good - think I have tracked down the last oil leak which was from the cam cover on the LHS (and then running down the fins and dripping out around the exhaust).

Runs strong when revs are up a bit and idles OK - but it has an intermittent but persistent backfire on the RHS cylinder. On idle and at low revs get backfires every few seconds which is annoying the neighbours and local dogs.

I swapped the output from the electronic ignition and the spark plug cables and the backfire was still on RHS so that eliminates coils and associated wiring.

I have checked the inlet manifold and sealing of the exhaust and that looks OK. Have triple checked the choke is seating properly and the throttle cables are all adjusted OK.

Any suggestions on what next to look at.

Thanks
 
Reads like a lean out, though remove air cleaner coupling and see if backfire is out of the carbies, otherwise if exhaust random ignition, possible built up un-ignited mixture ( too lean to combust at compression ) builds up and then random ignition of exhaust unburnt fuel load. If carby, ignition mistime or cam issues. If exhaust then install fresh new known quality spark plugs, easy things first. HTH j
 
iis_iis said:
Reads like a lean out, though remove air cleaner coupling and see if backfire is out of the carbies, otherwise if exhaust random ignition, possible built up un-ignited mixture ( too lean to combust at compression ) builds up and then random ignition of exhaust unburnt fuel load. If carby, ignition mistime or cam issues. If exhaust then install fresh new known quality spark plugs, easy things first. HTH j
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I am pretty sure it is igniting in the exhaust - I see a bit of flame out of the end of the exhaust. Swapped spark plugs and still issue with RHS.

Checked exhaust tappets and they are OK.

Next step is to look at carbs - not really my area of expertise but I can certainly check the float heights and read up on syncing.
 
Use new spark plugs, unless you have test equipment to assess the actual spark plug specifications, cost less than $15 and removes one of the oft overlooked causes of mystery motor misoperation. j
 
iis_iis said:
Use new spark plugs, unless you have test equipment to assess the actual spark plug specifications, cost less than $15 and removes one of the oft overlooked causes of mystery motor misoperation. j
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And swapping good and bad side plugs will also be a quick test to see if the plug is involved. But new plugs are the cheapest and easiest go-to, as Jon rightly says.
 
All good experienced indicators and test procedures, from the lads. Additional process was doing many moons ago, exhaust header temperature, at idle and road running ( later was done with K type thermocouples on three headers, initial bend(s) ). May or may not be relevant in this case, does give an indicator of fuel load burn, cylinder to cylinder. j
 
The plug have <50 km on them. And I think I have eliminated them as a souce with swapping them over.

Carbs are the original 36mm Dellortos - they seem in OK shape but carbs are really my weak spot in knowledge. I pulled them apart and ran them through the ultrasonic cleaner - then reassembled them with everything set up as before. I have yet to play with the mixture screws.

Temp swap of the carbs seems like a good Idea.
 
I would focus on the carbs to make sure they both have the same jets and mixture screw settings that’s 1.5 turns out from closed and synching them will help. best to use new carb O rings and gaskets. I have a comprehensive del orto rebuilding and synching pdf if you email me: ted@tedgadecki.com
 
Mixture screws get richer as they screw out. Do not hesitate to play with them. With correct (for your engine) pilot jets anywhere between one and two and a half turns out is fine. If you can take them more than that out and the revs don't drop due to over richness either your pilot jets are too small or it is sucking air somewhere. The o-rings on mixture screws are good and in correct place (furthest into carb with washer and spring behind)? You are certain all the wiring to (what type? If Ignitech those fucking plugs can have trouble with wire connection to the pins) ignition are good?
 
Carbs swapped and issue has move to LHS cylinder so that is good troubshooting. I think I will take to opportunity to pull both carbs off and go through all the setting to see how much they have been messed around with. Check they have standard jets and needle and check the mixture o-rings. I did replace all the o-rings on the rebuild so just need to make sure I have not missed anything. Then just start from scratch on them. All good learning stuff.
Will get TeddyG's doco and throw myself into learning about carbs the way I did about gearboxes recently.
 
You hear about multiple O rings stacked on top of each other stuck in the carb. The method I have used on a few twins for setting mix screws, not Laverdas but they all ran Dels. Disconnect one plug from the engine but put a spare plug in, something about EIs not liking to run without plugs firing. Make sure the cables are set evenly, up the idle so it doesn't stall and set the mix screw halfway between the 2 extremes of out where it runs poor and in till it runs poorly. Do the same for the other carb and then go in a tiny bit on both, they run better slightly rich. There is a spring effect on the connect point of the float, it must not be depressed. The float level with this just touching.
 
chrissoz said:
Carbs swapped and issue has move to LHS cylinder so that is good troubshooting. I think I will take to opportunity to pull both carbs off and go through all the setting to see how much they have been messed around with. Check they have standard jets and needle and check the mixture o-rings. I did replace all the o-rings on the rebuild so just need to make sure I have not missed anything. Then just start from scratch on them. All good learning stuff.
Will get TeddyG's doco and throw myself into learning about carbs the way I did about gearboxes recently.
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Good for you. I only recently came to carbs myself, previously it seemed like whenever I touched them I made them worse. I've since learned that it takes a certain deftness that seems to only come with experience. Patience and small adjustments help. I'll never be able to get them as good as a proper mechanic but if I give myself enough time then I can get it pretty close.
There is some good carb info in the library here too.
 
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