XJ600 3KM 1989 sluggish after installation of motad 4to1 exhaust

Hello,

I installed today 4 to 1 Motad exhaust on my Yamaha XJ600 3KM. Bike now runs sluggish, throttle response is bad until it reaches over 6k RPM. Also after it warms up and I rev engine revs stays at 2k for 6-10s to get back to 950 RPM.

Bike was running fine with stock exhaust so I think now I need new jets right?

 

No!
You have other issues and a co-incidence (even though I allways say I don't believe in coincidences, a motad exhaust will not cause a high idle, so why would it be the cause of your other issues?)
You know what the causes of most problems on these old bikes are don't you - four things really - carb 1, carb 2, carb 3, and carb 4.
Believe me, your exhaust is not your issue.

 

Carbs are fresh 2 month ago rebuild. Been running my bike everyday since then. In the morning I took it for a spin returned and on "hot" engine I swapped exhaust in 30minutes and symptoms are just as the post states.

I had same issue year ago when I first installed that exhaust but I just said f it and returned to stock to save some time. But now I want my motad exhaust xd.

I know it's fuel related as I can feel the bike is not running like it was in the morning with stock exhaust

 

Motad generally don't require re-jetting - that was one the selling points if I recall correctly. But given you're convinced its a fuel issue you will need to take the machine to a rolling road/dyno and track the power vs mixture to optimise teh stoichiometrics - this will cost in the ordr of 500 $s £s or euro things. Hopefully the operator will have access to the range of jets required. Failing that revert to standard jetting and experiment. 6k and above is main jet area so problems are lower down, pilot and and needle maybe. Hanging idle is indicative of an air leak on the intake .

 

Can you guys tell me what pilot jets does this bike take?

Is it mikuni n151.067 or bs30/96

I did starter fluid test on all parts that might be leaking air and revs didn't change. Also inspected boots, vacuum lines and throttle seals

 

I have a manual somewhere that should cover your model. Note that 'BS30' is a Mikuni CV carb size, XJ550s have BS28s, FZ600s and Radians BS30s, and pre-91, non diversion XJ600s had BS32s (in UK at least).

Although I'd be inclined to swap the exahaust back and see if the 'problem' goes away again.

 

Bs30/96 is a type of mikuni pilot jet

I unscrew my pilot set screw to 4 and 1/2 will see how it runs now - same did for my fj1200 before swaping from 40 to 42.5

UPDATE

OK so that was a bad idea , engine runs rough and has almost no power when starting in 1st gear, the original issue got worse
So it wasn't too lean, adleast not that lean to go to 42.5 jet. Will look into this tomorrow. Maybe it was too rich from begging. I hate not heaving O2 sensor

 

reading this may help..

https://xjbikes.com/forums/threads/fj600-jetting-dilemma.128374/page-2

 

Do you not know the difference in sound between rich and lean?
Worth practising.