#1 cylinder intermittently back fires.

paulson_shawn

paulson_shawn

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I'm trying to get my bike up and running after overhauling the top end. It's running on #2 cylinder but only intermittently backfires on #1. Plug looks and smells rich. I'm guessing carb or ignition since an exhaust problem would probably make it run lean. Any advice?
 
I'd start by checking your float settings and float valves and seats . Sounds like an intermittant flooding.
How does it idle ?
This might be helpful
 
thank you. It might seem a bit dumbed down but I'd rather it was clear to everyone even a complete novice (not that you are ).

What happens when you throttle up ? does No1 cylinder begin to fire normally ?

if so its the problem is likely to be in your pilot circuit on cylinder No1 carburettor.
Clean the pilot air intake jet on the mouth of the venturi and also the tiny pilot fuel delivery holes in front of the butterflys on the roof of the venturi.
use some compressed air to blow it through .
With the engine off lift each of the slides in turn and with your thumb placed over the top vent on the mouth of the carb see how long it takes each slide to fall. They should stay up indefinitely if your diaphrams are sealed properly.

Have you tried swopping the HT leads over ? or replacing the spark plugs ? just to eliminate an ignition problem
 
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If you are going to be replacing the float seat, I recommed buying them from boats.net not mikesxs. The ones from Mikes XS has a terrible terrible reputation. Bought three sets for my 3 XSs to find out none worked right. Bought from boats.net and they work perfect.

Thanks - Vince
 
excellent ! :thumbsup:

now you could have eliminated the entire ignition system which will save you some time and help you zero in on the problem. Just a couple of other things to check.

I see that you also have a Boyer ignition system .Do you know if it includes an electronic advance ? or are you still using the original mechanical advance ?
Worth checking that the white dot on the Boyer plate still lines up with the hole in the mica board at full advance . (under the points cap)

I take it the back firing is in the exhaust ?
I would reset your valve lash for both cylinders but use a slightly looser setting. The early XS1 XS2 used 0.006" inlet and 0.012" exhaust . This is slightly looser and will ensure that your valves are definitely seating properly. Many folk run their valve lash this loose now as it leaves the valves seated longer and it transfers heat better away from the exhaust valves and head and improves compression .

Put the piston at TDC on its compression stroke for the cylinder you are setting then turn the engine over until the other cylinders piston is at TDC and then set that one.
 
it will be a bit tappety but it will be easier to set and you can be pretty sure that all the valves are seating properly whilst you check and test everything else.

The Boyer Micro Digital ignition system is a brilliant bit of kit. Just fit and forget. My 79 goes like stink.
 
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I have usually found Backfires through the carb is from a lean condition, OR advanced timing state, backfire through exhaust lean carb and retarded timing.... You didn't forget to turn on your petcock did you? LOL just kidding...kind of..
BTW I'll jump on the video bandwagon, you just fixed an issue I have had with mt Left carb..... overfilling the bowel when parked. I cant count how many times I have come out from work to find a wet puddle of gasoline under the bike from it dripping.... BTW you didn't get your carb sync messed up did you?
I wish you luck tracking down what must be a frustrating issue..
 
I set the cam chain tensioner at 1.5mm, valves to .006 int 0.12 exh, checked boyer timing marks they were still on. Runs on #2 cylinder. #1 had cold exhaust and intermittent backfire still.
 
When starting from cold what does the plug look like in the cold cylinder after the engine has run a while ?
It should be wet with fuel especially if you are using choke and it isn't firing.
When you get the carbs off check all over the carbs for potential air leaks especially the choke bodies. I had to use some threebond on mine to seal some minute air leaks .
 
well thats a clue .;)

I know its commonly said but its true. An engine only needs compression, fuel and air and a correctly timed spark in order to fire so if the cylinder has fuel and air.
You've tried switching the other working spark plug, high tension lead,and cap over to the dead cylinder and its getting fuel and it still doesn't work so it has to be either a duff spark plug or lack of compression in the dead cylinder.:shrug:

Remove the spark plug and turn the engine over until the dead cylinder piston is rising on its compression stroke and place your finger over the spark plug hole before the piston reaches TDC . You should feel the compression building under your finger and it should lift your finger off the spark plug hole.
Try swopping spark plugs around .
Check to see that you are getting a spark at the dead cylinder plug cap.
 
It was idling good on both cylinders and would die when I gave it throttle before I changed th needle jet and pilot screw orings so It should have good compression but I'll check anyways.
 

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