Left Cylinder too rich

Veteran experience needed. I have my new Brandsen electronic unit working, new plugs installed, timing is correct - 31 degree TDC, rebuilt/sleeved Amal carbs set up correctly. But...the left side cylinder is quickly fouling - black, sooty sparkplug. It seems like the piston top has a lot of black carbon build-up on it. What should I be doing to get the bike running smooth (acceleration is fine, but during cruise it is surging)? Decarbonize? is there a good chemical cleaner, Is it a coil problem? I not sure where to focus my effort.

Thanks

Several scenarios. Weak battery, put a voltmeter to it and report back. HT lead loose. Swap spark plugs. Carb needle hung up in spring. Bad old float needle. Internally leaking old float. Check these and report back.

I hate to generalize from one sample, but when mine was showing those symptoms, it turned out to be an oil seal on the valve not where it should be, instead acting like an oil pump. The soot from the extra oil looked like an over rich mixture at first.

Torontonian said:

Several scenarios. Weak battery, put a voltmeter to it and report back. HT lead loose. Swap spark plugs. Carb needle hung up in spring. Bad old float needle. Internally leaking old float. Check these and report back.

Click to expand...

In addition, can you remember if the surge that you have crusing is more like a "blubbering sound" and sluggish or a more crispy intermittent staccatto-like skipping/misfiring? The reason I ask is surging is associated with a lean condition. The exhaust note can tell you alot.

I know you only asked for a starting point, but this is a situation where it can be carb, ign or even valve adjustment - a lean mis-fire and intermittent spark are symtomatically alike. So you have to go in two directions at the same time!

A very easy mistake to make is hanging the needle on the edge of the nozzle when fitting the slide back into carb body. The needle and retaining clip gets pushed up into the spring. Depending on how far its pushed up, you can get different mid-range rich symtoms. Take the top of the carb off and inspect needle/clip position. look close, it might not be much. I have seen the needle get pushed just a notch and everything look fine.

With that said, I would start with the needle check first and a good ole poke and wiggle test next (wiring connections). If that's not it, before we speculate any further you need to provide more details (single or dual coils, for instance) Is it worse when hot or does it do it only when hot. Is it temperture sensitive at all? How is the idle? (fresh plug of course) What does it do when you blip the throttle? Is there a hesitation before it will take the gas?

Then again, it could be as simple as a tight intake valve.

Hope this gets you on the right path, good luck.

Mike

If it is just the left cylinder, why not wind the mixture screw out 1/4 of a turn ?.

You could swap coils and leads over, see if that makes any difference.

What sort of plugs do you have ?, BP7ES, Try a pair of Iridium BPR7EIX. I recently fitted these and needed to richen up the mixture, these plugs are really worth the money.

Just note the "R" ( BP versus BPR ) is the plug has a resistor/suppressor, if your leads and/or plugs are suppressed, you can replace them with non resistive/non suppressed, you only need one source of resistance/suppression per cylinder.

Oil in cyl possible cracked ring or valve seal I'd start with a compression test to see if cylinder pressure is low if ok more than likely valve seal and oil leaking into cylinder when valve opens

Black sooty powder w. carbon on piston crown not an oily situation ,renorton I believe you may be off track on this one.Surging lean condition . Lloyds- son report please.

Surging at cruise usually means that my needle jets are worn out. Did you check the size of the jets you put in when you rebuilt the carbs ? Sometimes they are only close to the state size...

Greg

I'm thinking the same as gjr
lloyds-son did you change the needle jets and needles when you got the slides re-sleeved
When the slides start to wear too much the needle will start to rub up against the jet causing it to go oval. I've seen it on my needle jets looking at the bores thru a shadowgraph.
Cheers
CNN

I hope your right but running lean usually from my understanding the engine will run hotter on that cylinder usually making plug run whiter than nice grey brown plug should be if motor running to rich I don't think carbon would build up that quickly usually takes a fair while to build up maybe I missinterprided this but I still think a compression test will quickly rule out internal state of motor preferably leak down test too from what I read I had the impression by rebuilt carb ign etc he has thrown a fair bit of time trying to sort this and I think comp test well worth while more info would be good miles motor has done etc apologize if I'm wrong

Fair comment renorton.
Lloydes-son, check you float level. Too high can alloy fuel to "flood" into carby through the needle jet.
Worn Needle jet, and or Needle.?
Lean = Too much air. Flat spot. Surging. White plug, high engine/pipe temp.
Rich = Too much fuel. Trying to burn it but not succeding, leaving black deposits on piston and plugs.
How bad is it,? Can you coax it up to more revs with full throttle, to a stage where it starts to clear itself.
However you did say " rebuilt/sleeved "Amals.
New Needle jets AND Needles.?
If they are, then perhaps it is the possibiblty of a Needle not sitting correctly in the slide.
Dual air cleaners.? One dirty.? Probably not, remove air cleaner for testing.
Slides out of sync.
One weak plug or lead.
One weak coil.
All of the above.
Good luck.

I know it hard to take photo of piston but could you get a shot of plug electrode end and tell us how far you traveled before it fouled would give all of us a more educated guess it would be good if we could see how fouled it is

Thanks to all for lots of good advice. I want to follow up on a couple things yet such as compression test. With the resleeved carbs I rebuilt the carbs with new vitron needle, floats seem fine, needles are new. I did not like the way the throttle cable was run under the frame rail so I did some rerouting and also noticed that my left side carb cable had some play in it it. With the float bowls off and needle jets off with carbs still mounted I could see that the left side needle did not rise with the first little bit of twist on the throttle. I removed the play with the adjuster on the top of the carb then rechecked the movement which is now in sync. I still need to do a ride and set the air screws. I hope I have solved the problem.

In my previous post the fouling occurs after a ride of 10 miles or so. When I pull the left side plug it is very black - kind of a powder like. A quick clean with a small wire brush easily cleans the soot. Meanwhile the right side plug will be grayish white and clean when compared to the left side. I will do some more work tomorrow and report back.

Lloyds-son