XJ650RJ Main Jet Issue

OK - So I've been really enjoying my 550 and as it's been really running great I've started tinkering with my 650 as it isn't running quite as it should.

Here's the situation:

At WOT somewhere 7.5~8k rpm the bike acts like it's struggling - I initially thought it was running lean as it felt starved for fuel. I hadn't pushed the bike as hard last year but when I do it's really noticeable. So yes I've been pushing the bike harder this year....
Every other point between idle and almost WOT the bike runs great.

What I'm seeing:

Plugs look tan across all 4, I guess this isn't surprising as I don't run at WOT and at high rpm too much. Fuel economy seems to be between 38 - 42 mpg - not great.

I'm sure you will all ask me the following:

Valves - Yes they're in spec.
Carbs - Zestfully clean.
Throttle shaft seals - Replaced last year.
Clunk Test - Slides polished with 1500 wet/dry sandpaper.
Diaphrams - Pliable and no holes.
Carbs - Running vac sync this spring.
Pilot Screws - 3 turns out.
Float Height - Replaced the needles and seats this spring, wet set verified @3.5mm (per the FSM 3mm +/- 1mm). Yes they are a bit low but in spec.
Fuel Enrichment Circuit - Polished needles and seats as I did have the #1 carb FEC leaking fuel when closed (the #1 plug was sooty until this was solved).
All Stock - Exhaust, Air box, air filter etc...
Carb to engine boots - no leaks.

OK so now what?

So I grab the FSM and go through checking all of the jets and needle to verify size - Everything is the correct size EXCEPT the main jet!! Someone prior to me purchasing the bike put #120 size main jets in the carbs (all 4) and according to the FSM this bike needs #110 main jets.

So knowing that this is appears to be the one item that isn't right I'd like to get any expert advice before I send my order to Len.

My gut is telling me this must be it and even though I thought it was running lean at WOT, it's actually running rich!

PS my 650 has a bit over 16k miles on it and the only thing I haven't done is a compression test, so I do suppose I could have a weak cylinder, but...

 

I bet you're rich. Mine is a tad. Does it feel lumpy?

 

Lumpy - Yes, it gets to 7.5k and falters then surges a bit above 8k and falters again and surges again and generally repeats this as it struggles to pull closer to redline.

 

Rich, I'd say. Colortune is your best bet.

 

Hmmm - I agree it rich, but only at WOT and the top end of RPM. The bike runs great until I hit 7.5k+ and only at WOT where most of the fuel is being supplied by the main jets. It's not lumpy below 7.5kRPM.

Colortune only allows you get idle mixture precise at idle - at 7.5k+ rpm and WOT the percentage of fuel being supplied by the idle mixture oriface has little effect (of course not none, but very little).

I've been digging through the threads and came across this article: http://www.johnsmotorcycleparts.com/Kei ... g_tips.htm

It really helped me understand how the different jets in CV carbs supply fuel to the engine at differing loads and rpm.

 

I also just read this, particularly the part about running rich (now that I've realized that the bike is running rich instead of lean)....

http://www.xjbikes.com/Forums/viewtopic/t=44642.html

Scroll down to the part about "The engine "eight-strokes" as it loads up and skips combustion cycles. "

This is what I thought was running lean....

Well I sent Len a request for #110 sized jets so I guess I'll know when I get the correct ones installed - probably next weekend....

 

a bike running lean at wot will feel like it accelerates briefly before it slows if you let off 1/8 throttle quickly.

 

At the RPMs you run, a 40mpg average indicates a rich condition also.

I would say it's not too far on the rich side though, otherwise you would notice it on the plugs. They would be dark brown instead of tan.

Does it have the same symptoms at WOT in the mid rpm range?

 

Polock - I didn't notice the lean behavior that you describe. For sure it ran better when it was colder out this spring than it does now with the weather warmer.

Tskaz - the bike seems to run good in the mid rpm range and builds power quickly up 7.5k. It doesn't build power and run as smoothly as my 550 which is partly what I began to notice when riding both bikes. I agree that it only a bit rich and if I did a wot plug chop I'd imagine you would start seeing the plugs indicate rich.

So new jets have been ordered, I'll update this thread as soon as I get the bike running.

 

OK Update - Got the jets in and the carbs back on the bike late in the evening on Thursday, too late and tired to go for a test. Got out for a ride tonight and the bike runs night and day different! Seems a bit more responsive in the mid RPM range 4k to 6k. As revs build 7k+ just amazingly smooth with no hesitation whatsoever right to the redline. Very happy with the results! Anybody need a set of #120 Hitachi Jets.... Just PM me.