Rich fuel mixture or oil in cylinders or both?

Hi guys,

So, my bike has started running great, it has good compression (140-155psi over the 4 cylinders), and pulls like crazy except for a dip at 3000rpm and at 5.500rpm approximately.

At those rev ranges, it surges, and seems to hesitate, before it kicks off again. I also get white/grey smoke at around 5.500rpm and up, not below those rev ranges.

Spark plugs have a dry, black look to them which is similar to, but not as bad as: http://www.dansmc.com/spcarbon.jpg

I have changed to hotter spark plugs, I have switched ignitors, I have a stock airbox set up at the moment and original jets. I don't want to do a top-end rebuild (at least yet), so really hoping you guys don't suspect oil leak into cylinders. Any ideas what it mau be?

 

Hi Andnil,
You say your bike has started running great - How was it running before and what has been done to get it to this condition?

When is the last time the valve clearances were checked?
When is the last time the carbs were cleaned and synched?

Those would be my first guesses.

 

Hotter plugs will not get your bike into spec, the correct ones will. Check the manual for your model and use what is recommended. Also valve clearances should be checked and corrected and carbs cleaned. The Church of Clean and Information Overload threads have all the info to get you there.

 

Have you still got the stock exhaust?
And what air filter stock or?? And is it clean?

 

Black-sooty plugs = incomplete combustion. That can happen either because of a lean condition or a rich condition. The smoke is a telltale of incomplete combustion also. You got dirty carbs, or carbs that aren't set up right.

If you had an oil problem you'd have wet-black plugs and blue smoke.

 

Hi guys,

Thank you for replies!

When I got the bike, it ran pod filters (hence why it runs great now compared). I tried rejetting the carbs and still could not get it to run properly. I got a hold of an original airbox and bought a new original airfilter to fit the bike.I also bought a rebuild kit and cleaned the carbs prior to assembling the new airbox. The exhaust is not stock, and it came with a 4 to 1 system of unknown origin, and an un-muffled reverse cone exhaust. This, however, shouldn't give power drops at specific RPM?

My carbs are now clean and I run 40 pilot jet and 110 main jet which is according to Haynes manual, correct for my model (Xj650, 1981, 4k0).

I ran NGK BP7ES previously as those are the standard heat range for the bike, and switched to NGK BP6ES which is next step hotter. I did this from recommendation from my mechanic.

What baffles me is that it is VERY specific rev ranges it drops power. If it is at exactly 3000 revs, I cannot accelerate unless I switch to either a lower or higher gear. Other than 3000rpm or 5500rpm it really pulls awesome.

I will check valve clearances next. Anything else I should do?

 

After valve clearances I would recommend resynching your carbs.
I don't know enough about spark plugs to say if what you did helps or hurts.

 

My carbs were resynced after the last rebuild, so they are all in sync. I will try to check valve clearances first and go from there. Thank you

 

Were the carbs cleaned, or were they fully disassembled and rebuilt?
IN THE CHURCH OF CLEAN

 

Not disassembled from the bracket, but opened and cleaned and all ports checked that fluid runs through

 

So they aren't really clean, or rebuilt.
I'm guessing that you didn't remove the emulsion tubes, and clogged ports in those will cause your symptoms.
That link I posted above; the steps therin are MUST DO.

 

A 4k0 is a seca? Sounds a bit lean in the jetting for that bike with a 4-1 exhaust unless your at higher altitude, or in very high temps. IMHO pilot is too lean and she falls on her face when transition from pilot to needle and main jet is a bit light as well.
And I have never seen the need to run a hotter plug in an XJ motor unless your driving in 5 C weather.

 

My understanding is if you adjust your valve clearances (which should be checked every 5k miles) that you also need to resynch your carbs.

Like Kmoe said; You need to go to Church. These are not lawnmower carbs, they must be torn down and have the 30+ year old rubber bits replaced

 

Yes and no. What North America recieved as the 650 Seca, the rest of the world got as the XJ650. N.A. is the only market that got the Maxim, hence the seperate designations here.

 

Ok guys, so list to do so far:

1. Clean carburettors again, this time disassemble the bracket. I have read that disassembling the bracket also means that reassembly should be done with care to maintain alignment. Is this correct? Does anyone in Australia know of a good carb cleaner readily available here?
2. Rejet the pilot jet, possibly try a 41 jet first and see how that goes?
3. Check valve clearances

 

The carbs almost self-align when you put them back on the brackets. A truly flat surface can be used to ensure perfect alignment (glass pate, thick plexiglass, table saw bed, etc..