Help please! Weak number 3, stumbles when revving.

Hi all

My lovely XJ 750 (-83) is making me a pedestrian (or almost)

She's loosing power when accelerating. Seems like number 3 cylinder exaust is a bit colder than the rest. I have checked the electrics and found no problem. Switched the coils from right to left without any change on number 3. The carbs has been cleaned, synced and tuned by a proffessional.

What could be the problem?

brgds
Dev

 

Has a compression test been done on the engine? The engine needs to be in good condition to ignite the fuel air mixture. A valve adjustment will also be necessary. If all those are in order I still would not rule out some carb issues.

 

Are you running the stock airbox or pod filters? What kind of exhaust system? Some jetting problems could be there.

 

* Compression is very good (tested).
* Stock airbox (clean filter)
* Spark on plugs okay (plugs changed)
* Recently (2 weeks ago) I adjusted the valve clearance on all 4. (these are okay)

The problem of it loosing power was apparent earlier this fall, before I started doing anythin with the bike. It is slightly better now when the valves are okay and the carbs has been cleaned and synced.

I did not do the carbs myself, could it be the emulsion tube on number 3 causing it?

It runs nice on high rpm's, a rough on low (like it's not running on all 4) and stumbles when accellerating on about 3.500 - 4.500 rpm's (maybe higher).

:-(

 

hmmm sounds like possibly spark coil or something cus the charging system is stronger at the higher rpms, but def worth checking to make sure its not the carbs.

 

I moved the left coil to the right side and vice verse. number 3 cyl was still cold compared to the other cyl's. Doesn't that rule out the coils?

thanks
Dev

 

Looks like switching the coils probably eliminated the possibility of a bad coil since the problem stayed in the #3 cylinder. I suspect a problem with the #3 carb, float level , stuck carb slide or clogged jets. The #2 and #3 cylinder share the same coil and spark advance, since the issue is only in the #3 cylinder I think it is carb related.