Firing but no combustion


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Psycheonabike
Posts: 200
Joined: Sun Feb 10, 2019 5:00 am
Location: England UK
Motorcycle: 1985 Aspencade

Firing but no combustion

Post by Psycheonabike » Sun May 19, 2019 4:04 pm



Hey,

Carbs been off and even managed to get the idle jet out of the carb that was over the cylinder that has a Spark, but isn't firing. I thought cleaning the carbs may have sorted this. Here's whats happening...

Idling ok now, starts first time and after cleaning carbs (again!!) throttle seems more responsive. But when riding she seems sluggish and unless I open the throttle wide she doesn't really respond.

Main thing I've noticed is pulling the plug on all cylinders has a noticeable effect dropping revs. But pulling on #2 makes no difference at all...but there is definitely a spark there as I can both hear it and felt it once (ouch!)

Got me stumped.



User avatar
Maz
Posts: 673
Joined: Tue May 19, 2015 4:43 am
Location: Kent, UK
Motorcycle: 1975 GL1000K1
1976 CB500T
1979 CB750L
1990 Yamaha FJ1200
1993 Suzuki GS500E

Re: Firing but no combustion

Post by Maz » Mon May 20, 2019 1:58 am

Psycheonabike wrote:
Sun May 19, 2019 4:04 pm
Hey,

Carbs been off and even managed to get the idle jet out of the carb that was over the cylinder that has a Spark, but isn't firing. I thought cleaning the carbs may have sorted this. Here's whats happening...

Idling ok now, starts first time and after cleaning carbs (again!!) throttle seems more responsive. But when riding she seems sluggish and unless I open the throttle wide she doesn't really respond.

Main thing I've noticed is pulling the plug on all cylinders has a noticeable effect dropping revs. But pulling on #2 makes no difference at all...but there is definitely a spark there as I can both hear it and felt it once (ouch!)

Got me stumped.
You know what's required for combustion ..Spark, Fuel, Air and compression. It can only be one of those missing. Do you have a Colortune? it's a very useful tool for seeing whats happening inside a combustion chamber whilst the engine is running. Does turning the mixture screw out while idling, make any difference to the non firing cylinder? I'm still leaning towards lack of fuel at idle/slow running.
One other thought, though, have you checked the idle air jet on that carb (It's inside the little brass elbow on the OUTSIDE of the carb)? It could be blocked.

Maz
Ironically, Common Sense is the LEAST common of all senses!

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