cylinder not firing

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Sep 14, 2003
I need some assistance if someone could help me.PROBLEM: 1 cylinder is not firing there is lots of spark however spark plug is soaked with fuel and does not fire. Carb checked ok even changed position of clip so needle @ lowest setting.Spark plug always fouls and will not burn.Changed carb to a spare I had in case I missed something no effect same problem spark plug soaked.Keep in mind that adjustment of throttle stop screws and pilot air screws were made while trouble shooting.Even cutting back fuel with adj screw has no effect what up with that. Valve clearences ok carbs ok timing ok changed plugs, checked carbs so much I can remove and replace with my eyes closed.Fuel, spark, Timing. ??? hmm anything else.At this time with a visual inspection the v/v appears to be opening and closing as observed by looking into plug hole while turning engine so the spring maynot be busted.I'd like to solve this problem without getting into removal of the head however I'm at a dead end if anyone has any suggestions please lay it on me because its a mystery to me. tks ROGER
Sounds like you have eliminated fuel. How are you checking for spark? Is it possible you have a bad plug wire, giving you an intermittent fire? When you pull the plug and ground it you have spark, when you twist the plug wire back around to put the plug back in you don't?
firing on one cylinder


If the plug is sparking and you're getting fuel, the timing is off. A possibility is the alternator rotor may be loose on it's center causing the timing mark to move and the timing to be incorrect. Pre '73 machines are prone to this. Later rotors were welded. What ignition system are you using - points or electronic (which electronic)?

Don't mess with the carb idle mixture screws or the needle height. Set to original spec then fine tune after the engine works. If you have moved the idle screws, turn them in all the way and back out 1.5 turns - good starting point.
I have a 68 750 with a rita electronic ign and 2 930 series amal carbs I have had the bike since 1978. The right hand cyl fires excellent and runs with a nice even tick over speed with the left hand side plug wire removed.I strobed the cylinder @ 2000 rpm and adj ever so slightly to 20 I did not strobe @ 6500. Even getting timing close u would think both cyl would fire. left hand side exh pipe cold compared to right side. About the alt rotor it is a keyed fit to the crank and when the timing is set is not real important except for voltage.It could fall off I think all I would have to do is put on another one and not touch the timing.So tks for the thought but I do believe the timing is on the mark. the opposite cyl fires nicely and with the electronic ign there is spark @ the plug and lots of it but so much fuel??
re cyl no fire

How about flange leaks from carb to head would this cause too weak a mixture instead of my problem of way too lean.Ya I would think not my problem. Thinking out loud... how about a cracked head hmmm think I'm getting carried away don't want to go there, but would that cause the one cylinder not to fire?Stumpped
Just because the sparkplug fires in the open does not mean it will fire inside a cylinder at 9 atmospheres. I have had this probem which was an internal breakage in the low tension circuit wire, you could have a coil on its way out give the same problem.

The best way is to swop the HT leads and sparkplugs over but leave the HTs still connected to their original coil , if the missfiring moves with the plug the fault is electrical, if it stays in the same side its mechanical or fuel.
Switching HT leads

TKS again good thought tried it no change RICKAMAFRAT I dont know I been looking and looking just doesn't make sense everything is there it should work.Well I have a spare rita that wasn't the problem.My old one was near 15 years so....... I thought maybe even though it tested ok on the bench maybe flashing it up and getting hot anyway I have a new RITA now eliminate that chance...... next.
ALT rotor

Heres one if someone knows.. is an alternator rotor interchangable between a 750 and 850?I have had part shops say that it is however I have both engines and the 850 rotor I have will not fit on my 750.The inside diameter hole of the 850 rotor is too small to fit on the 750 shaft?? go figure anyone know whats up with that.Are the crankshafts the same size will they fit in either bottom end or not.Just a thought as to know what I have here.
The rotors should swap around, I can put a rotor from a C15 on an 850, you may have a rotor on bottom tolerance combined with a 750 crank where the diameter is on top tolerance. This may give a tight fit or an interference fit.
Sounds like the ignition is not the problem if you swapped leads on the plugs. I would put in a new plug just to make sure but you may have a torpedoed float or its hanging up on the gasket. I would think though that it would at least run for a short time before loading up but you know how Murphy works.

I may have the answer to your problem.Thesame thing happened to my norton last weekend,e.g.there appeared to be no spark on l.h cyl,petrol fouled spark-plug.
The usual checks showed a good strong spark,And that the timing was spot on.New plug fitted,which fouled with petrol as soon as i turned the engine over on the starter hmm head scratch time!!
The carb wasn't flooding,but removing the plug and starting the engine as a 414cc single produced a continous spray of petrol from the plughole.
I removed the carb top to check throttle slide and needle,all APPEARED to be everything The engine started first kickand sat ticking over on both cylinders quite happily.After the engine had warmed up i started to rev it up a bit, At first there were no problems,i revved it to 4000rpm, then let the engine drop to idle.Back to one cylinder again!!!!!!!!! pulled the plug out and it was soaked in petrol.
I then decided to take off the float bowl and have a look at the jets.I removed the jet holder,no problem found.
I was about to re-fit the jet holder assy when the penny dropped,i could not see the needle! carb top off again the needle was tangled up in the throttle return spring. As you all know,the needle is located by a circlip retained by the throttle return spring,Ihad given this a quick look the first time i took the top off the carb On careful checking i found it had broken where it clips round the needle.FItted a new one, re-assembled carb no more problem.Healthy bike happy owner!! Hope this is of some help
Roger, Here's one that had me baffled. I bought a Trident cheap that had been through the hands of a mechanic who failed to repair the bike for it's owner. I'm sure I went through the same process that the mechanic did before giving up. I was also baffled. The bike had the exact symptoms on the middle cylinder as your bike. After checking compression, timing, going through the carbs twice, the gloves finally came off. Third time was a charm. The main jet had developed an almost imperceptible crack in the final thread app. 1/3 around. After replacing the jet it ran great.-Rob
Both plugs fire at same time. If one is firing good, igintion is probably O.K.

If it still happens after you have swapped over the plugs, leads and coils, it's fuel.
Something is wrong in the float bowl. Stuck float, non seating float needle (gunked up), float too high? It's in there somewhere, you are getting too much raw fuel.

Fiberglass tank caused me all kinds of grief.
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