SF1 Misfire

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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Make sure that the slides aren't worn and loose in the bores. That causes an air leak at low lift.
Triple check float heights.
Paul
 
Andy said:
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.
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Maybe why Japanese manufacturers decided to build 'tamper-proof' idle mixture screws into their carbs in the late 70s Andy. Although the actual reason was to do with emissions regs in the US I think. They had plastic covers on the screws with stops built in that only gave you around 2/5 of a turn - bloody annoying for mechanics (me) if you were trying to diagnose a symptom.
 
Very odd that the bike actually misfires due to worn slides, Chris. And if both slides show equal wear, why is only one side misfiring? I wore my SF1 slides down to the point where the lower section had a deep arc of worn alloy and there were large cracks visible at the base. Bike never misfired, but it probably ran better after I fitted new ones!

You can't discount other issues with the errant carby. for eg, the air port for the idle mixture circuit (one of the holes on the filter side of the carb mouth) can be blocked with crap.
 
If the slides are really close to binning there is a Hail Mary fix that worked once and failed once for me. There is a place in Victoria that does Teflon Coating, adding approximately a millimetre to the size and reducing friction. I first heard about this from a bloke running trail bike tours to Cape York who was wearing out slides from the dust up there. I did the Del slide off my 92 350 Husky, took up the wear and smoothed out the throttle action, it was jerky as hell just off idle and it worked very well. I tried the same to the 3 sides out of my 3c and they blistered, I couldn't hone these lumps off so got replaced. Try ringing first. I used Hot Jet Coating but these blokes look interesting. But it is odd it's only one side. Whats the accelerator pump squirt look like, are both squirting equally?

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Vince said:
If the slides are really close to binning there is a Hail Mary fix that worked once and failed once for me. There is a place in Victoria that does Teflon Coating, adding approximately a millimetre to the size and reducing friction. I first heard about this from a bloke running trail bike tours to Cape York who was wearing out slides from the dust up there. I did the Del slide off my 92 350 Husky, took up the wear and smoothed out the throttle action, it was jerky as hell just off idle and it worked very well. I tried the same to the 3 sides out of my 3c and they blistered, I couldn't hone these lumps off so got replaced. Try ringing first. I used Hot Jet Coating but these blokes look interesting. But it is odd it's only one side. Whats the accelerator pump squirt look like, are both squirting equally?

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Surely not 1mm the slides won't even fit the carb body.
Paul
 
Worn slides cause air leaks and popping at tick over and low revs. Carb bodies wear as well so will need renewing, or sleeving after 3 slide sets.
Paul
 
The businesses that put ceramic coatings on piston crowns usually put teflon coatings on the piston skirts as well. Perhaps they would do it for other automotive applications as well?
 
I had a set of Ross Pistons ceramic coated and anti friction on the skirts by Jet Hot in melbourne.
Had them do my Motodd exhaust. Sure they could apply to carbi slides as well. Might help with friction but I think it wear off on the wear points
 
Hate to say it but maybe time for a compression test on each side. I went EI on all my Bonnies, for this reason, removes variable timing effects from the equation.
 
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Two cents worth, have to agree with the ignition thing. A loud backfire like that does answer to ignition missing a beat then firing, like hoons used to do with cars turning off and on the ignition in the old days, and some quick shifters do today. Carb problems are usually lean or rich and would expect to be very evident at idle which this bike has no problem with, and lean mixture can have popping on overrun but not actual backfire, but this happens when just trundling along. Maybe the carb swap threw in a red herring.
 
I'm off to the UK for a couple of weeks so will have to pick this up again when I return - I think the next step is to swap that slides back over and really confirm the problem follows the slide (so eliminating ignition etc).

Just going to document where I am at the moment:

Rebuilt SF1 - new pistons and rebore - full engine strip and gearbox repair
Bike was running OK before - smokey on startup and running - compression was about 110 in each cylinder
Choke to start from cold but not usually needed when warm
No backfire before
Sachse electronic ignition (has been on bike for 10 years and working well)

Carbs stripped and ultrasonic cleaned - reassembled with same setting as they had before but new seals everywhere.
Sliders are pretty worn and there is some side to side movement
New B8ES plugs

Bike now runs OK above maybe 3000 RPM but has an intermittent backfire in RHS cylinder at idle and at low revs while riding. Been ridden maybe 120KMs since rebuild. Compression is now up 140 (ish) in each cylinder and a lot less smokey. Will recheck compression at some point.

Things I have rechecked:
Ignition timing - this is pretty easy on Sachse and looks good
Cam timing - pretty sure this is OK - rechecked and everything lines up
Exhaust sealing - looks OK
Inlet isolators and sealing rings all look ok
Tappets are at correct clearance

Diagnostics performed
Swapped ignition output, plugs and leads - problem stayed on same side
Swapped carbs - problem moved to LHS - swapped back and problem is back on RHS
Swapped slides - problem moved to LHS.


Thanks for all the help and ideas.
 
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from your last diagnostic posting it appears your issue is w the RHS carb- have you checked the float height and the amount of gas from each fuel pump ( if your carbs have fuel pumps). Could be a worn float valve / needle
or choke plunger
How scored are the slides compared to each other? Are all the jets the same size on each carb? Obviously the RHS carb has something wrong compared to the other.
 
TeddyG said:
Obviously the RHS carb has something wrong compared to the other.
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Incorrect - 'Swapped slides - problem moved to LHS'. Unless you meant RHS carb SLIDE? but I still have real trouble with a carb slide causing this issue - but hard to ignore the facts. New slides are obviously required so worth doing anyway. If still no good, continue searching.
 
One quick easy float check is to drain each carb bowl into a buret, or beaker with volume-measuring lines to ensure both have equal fuel. Sometimes it can be hard to clear blocked idle passages.
 
Not a bad idea Vince, but the measurement could be affected by the fuel in the hoses from the tap to the carb if they're of uneven lengths. Or if connected together, all of the fuel in the hose system is likely to drain through one carb.

Best to disconnect the fuel hoses at the carbs before measuring the volume from each float bowl.
 
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