Will Not Run When Warm

Carbs or more likely coils. The change in behaviour when hot makes me think that.
As much imfo as possible please.

History, last start, enviroment its kept in, New owner? miles, age of fuel.
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Sounds like a blocked pilot jet on the near side carb a usual problem if she's been standing

 

I am more inclined to suspect an electrical issue. The reason I say that is it does it when the engine is warm. Something electrical is breaking down under heat. Just one old guy's opinion ...J.D.

 

Partially blocked jet pull the carbs apart and clean all jets and take note of the needle positions.

 

Run ...i dont think i have managed anything more than a leisurely trot in the last ten years.

 

Quite possibly a tight tappet if not carb related.

 

Yes i would say tappets as well. When the bike heats up it is leaving a tappet slightly open.
Regards
Joe.

 

I do not see where an engine warming up is going to effect whether a jet is plugged or not. Someone explain. If it runs OK cold but not when it is hot ??? ...J.D.

 

Out on a limb here but 78 was changeover year from Mk1 to mk2's and i know nothing about later carbs but they did have 2 idle jets i believe as one was for cold start and the other for tickover when warm ??

 

Yes they went to the Mk2 in 78 and it did have completely separate cold start jet and there are also two different locations for the pilot jet which can affect how it runs...but i agree it could also be a tight tappet but the rich looking plug could also point to an ignition problem...but i would eliminate the carbs and tappets first.

 

I can buy the tight valve argument. Some engines the valves get tighter when they warm. Some engines, they loosen up as the temperature rises. It has to do with the expansion of the various valve train pieces, and how that effects valve train geometry. [Really, they should be checked anyhow]. But I think you would be stretching the realm of believability blaming this on carburetors. That is if it runs OK when cold, and not as good when it is hot.
…J.D.

 

As i know nothing about the mk2s i can't comment other than bike starts on choke jet then choke off but a second idle jet is blocked and plugs already sooty from cold start so engine dies ? don't you just hate not having the bike in front of you as it's far easier

 

New coils new problem ,or, problem then new coils?

 

My thoughts on the problem was just that a thought
Starts on choke with one carb so misfire not noticeable but then ridden and warm choke off it will not idle with one pilot blocked or partially blocked
For the time the carbs take to take off it's a no brainer to clean the jets

 

I think you are on the right track. An old machine like that, it could be more than one problem. I was taught in a class on engine dianostic way back in the 1960's, there are 4 things to make an engine run. 1-Fuel. 2-Air. 3-Compression. 4-Timing. When I have gotten in this kind of deliema, I have always gone back to that. ...J.D.