xj600 top end rebuild and it runs really strange

Hi peeps,

been a while on here but just thought I'd pick your brains for a mo.

rebuilt the top end of a UK '91 xj600.

carbs professionally rebuilt
head and barrel skimmed
valves seats recut and gapped
standard air box
yamaha gaskets used

the issues i have are;

Starts quickly with no choke and then picks up all the cylinders but have to use 1/4 throttle to do this
high RPM when warmed
won't rev over 6k and makes a funny noise if I push the issue

spark plugs suggest rich mix before carbs rebuilt, no air leaks

I'm just wondering if the skimming has affected the running ie too high compression or I've mistakenly didn't do the valve timing correctly (checked and double checked through during install)

anyone comes across this or give me some pointers please?

I'm off to IOM classic TT next week and would really like to take the bike as I've just had a repaint in Gulf colours

 

we will need pictures of both!

 

Have you done a running sync on the carbs? Also sound like one of the butterflys is binding.

 

Language is sometimes a little different from other countries
Let me try to understand.
When you say head and barrel "skimmed"
Do you mean head and cylinders machined or re-surfaced flat?
Also when you say valve seats recut and gapped
Do you mean you had the valve seats machined and valves resurfaced hence
"Valve Job"? Then Gapped means shims replaced for proper valve clearance?
Some machine shops will cut the end of the valve stem to square it up,and give it a new flat surface. No problem with that but I would wonder how he dealt with the shims. Doubtful he had a box of Yamaha shims sitting there , did he mill the shims to set the clearances? It all has to do with installed valve height for a base line so they can be shimmed properly.

Now if its true you had the head and cylinders re-surfaced, how much did the machinist remove?
Then when you removed the cams to install the head assuming the machinist installed the cams on his workbench and replaced the shims for the correct clearance. Did you re-check the clearances once you bolted the heads back on and installed the cams? Cam alignment with timing marks would be very critical, did everything line up properly?

Finally.
When you put the carbs back on did you check the float levels? And did you sync the carbs properly?

My thoughts right away would be check clearances, float levels, Sync
The noise over 6K would need to be investigated further once you get the things above checked.

 

a compression test is needed,

 

thanks guys.

the machinist is a trusted engineer and has done work for me before.
valve surfaces in the head were machined to fit each individual valve
skimming the head takes 0.001 of material away and he did that on the head and top of the barrel (jugs)
the carb has been professionally checked ,again by one of the top guys in the country.

I fitted the cams and shimmed the valves myself ( done this before)

I think I'll do a compression test, and recheck the valve timing.

do you think the TCI might be at fault? they can "go off" when it gets hot if faulty?

I'll get pikkies when I refit the plastic to the bike, here's a taster

 

For sure check compression.

I would not exactly trust anything till you put your own eyes on it.
Check the float levels for sure after the compression test.
Then pop the valve cover and double check your clearances.
Finaly sync those carbs in so it will balance out. One of your carbs could be out of sync and causing problems.

Let us know what you find.
Nice scheme on your plastics by the way!

 

can we be sure which bike we're talking about here.
you say 1991 uk xj600.
but the plastics would suggest a diversion model but I don't think that was introduced till 1992?
twin headlamp fairing looks to be fz600
or have you swopped plastics?
stu

 

This would be the first thing that I would do, as well as verifying the correct fuel levels in the bowls.

 

I'll be in my bunk....

 

I have always loved the Gulf colors. I call them Blorange. My brother painted his den in that color scheme, complete with number from one of the GT40 Fords that won
LeMans back in the 60s. Soo many great Porsches in that livery as well...

 

the engine is a '91 xj600 in an Fz600 chassis.

well I've finally sussed it. As usual it was a few things;

1) the exhaust cam was 1 tooth out
2) A few valve gaps were out of specs (poss due to the head bedding down)
3) re torqued the head down and it took another 1/2 turn to get the necessary torque

thanks for your input fellas!

top forum

 

It's always good to double check your work. Even though you think it's ok

 

here's the photos...

 

She looks awesome! Will she be ready for the Isle next week? One of the greatest races in the world! Try to catch some air for us! (But stay shiny!) Awesome, awesome!

 

thanks, yes she's ready just broke in on the motorway from Liverpool (checking out the ferry terminal)

few more tweeks.....