ET: Engine (Twin) Uneven Running in Lower Gears

[Jim Bush]

Tickover speed is said to be allright, so the pilot jet will not be blocked. What´s more, if the pilot jet was blocked you get less of fuel into the engine as air through the pilot system is still available. So rich running at quarter carb open is not the typical symptom then. No, my bet is an oversize needle jet after a million miles, slide cutout next try, when having the needle one step lower does not get you satisfaction.

Vic

I find that having physically clear pilot jet/passages is the foundational pre-requisite before moving on to needle jet wear and cutaway / needle adjustments. Tick over speed can be ok with a partially blocked pilot jet when the air is metered down, but when transitioning off idle the mixture is too weak and stumbles.

 

[nigsey]

I find that having physically clear pilot jet/passages is the foundational pre-requisite before moving on to needle jet wear and cutaway / needle adjustments. Tick over speed can be ok with a partially blocked pilot jet when the air is metered down, but when transitioning off idle the mixture is too weak and stumbles.

Thanks Jim, I’ll check the pilot jets, it sounds like good practice.

 

[oexing]

A partially blocked pilot jet and metered down idle air mixture would be same effect like a very small pilot jet. You´d notice that condition at once as kickstarting the thing will be a pig. The idle mixture would be reduced so the engine will not want to run at idle speed.
Basically with correct idle jetting the picking up from idle is the job for the slide cutout. After that stage the needle jet and needle come into the business at say quarter open, typically at town and overland speed. Here the reported engine stumble from too rich part throttle can be noticed. That is why I believe the root of the problem may be. The float chamber and float have been checked already ?

Vic

 

[chankly bore]

A 76 number drill bit or a single straightened strand of throttle cable may help.

 

[bmetcalf]

Too bad a .5 mm bit is 1.6% bigger. Or is that close enough?

 

[greg brillus]

0.020" MAX........any larger and it will run rich........18 to 20 is what I do on all of the new Shadow carb's. We still don't know if it did this before the carbs were removed........

 

[nigsey]

A partially blocked pilot jet and metered down idle air mixture would be same effect like a very small pilot jet. You´d notice that condition at once as kickstarting the thing will be a pig. The idle mixture would be reduced so the engine will not want to run at idle speed.
Basically with correct idle jetting the picking up from idle is the job for the slide cutout. After that stage the needle jet and needle come into the business at say quarter open, typically at town and overland speed. Here the reported engine stumble from too rich part throttle can be noticed. That is why I believe the root of the problem may be. The float chamber and float have been checked already ?

Vic

Thanks Vic, you’ve described that in very plain English so that even I can understand it. I have plastic floats which I’m reliably informed are set on the correct groove. I do suspect that richness may be playing a hand in all this, not least because the plugs have been on the black sooty side, mostly on the front cylinder and a recent bore scope inspection of an open exhaust valve showed quite a lot of black coke deposits. Oddly enough though, it only emits a tiny amount of black smoke from the exhaust when I accelerate.

 

[Chris Launders]

Any black smoke from the exhaust is over rich.

 

[nigsey]

I noticed today that the slide on the rear carb is sticking quite badly, so much so it doesn’t close down to the stop unless I snap it shut. I’ve stripped out everything including the jet block and tried it with the slide on its own and it’s still very stiff in the carb body. I tried the same slide (it’s a chrome one) in the other carb and it slides absolutely fine, so I’m not sure what’s going on there. Unless I can free off the offending carb body I guess I’m in for a complete new carb because I don’t think you can buy a new body on its own.

 

[oexing]

I am not quite clear what type of Amals you got. The flanged type gets distorted easily from overtightening the flange nuts when some thick gasket is used there. So you should try to measure up the carb bore for oval shape. How to rectify this is next question.

Vic