Power loss, what's next?

[JTGS850GL]

I've been trying to chase down a problem with a loss in power pretty much across the RPM range and need a little help on what to do next.

I thought I may have a cylinder that's weak so I did a compression test and was surprised to find that all cylinders are coming in at between 145 to 150. When I purchased the bike it was more like 125 to 130 max. If I understand this correctly then I can rule out cylinder and valve issues. No smoke from exhaust at idle nor when revved up and back down.

Some history: All valves have been adjusted, carbs were rebuilt by a GSR member and new intake boot O-rings were installed all before the power loss and it ran great for a couple months, pulled very hard from just slight throttle opening all the way up to WOT, then bam... Sudden loss in power like a cylinder cut out. I replaced the coils and plug caps. Things improved but it still has a perceived lose in power. Idle is fairly good and steady though. Fairly new stock NGK plugs and they look clean with a slight grey tone. No black soot and not all white. Petcock is still original but I don't see gas in the vacuum line. Charging system working with about 13.8V at idle. Rises to about 14.2V when revved up. I did notice that, for a brief time, the #3 exhaust pipe didn't seem to get as hot as the others and then it came back.

I'm thinking to check the sync again but I'm afraid that I'll need to rebuild the carbs as well.

Any ideas on where I should go next?

[duaneage]

Air filter?
Fuel flowing properly>? Check it on PRI
Fuel filter? remove it and test.
Stuck diaphragms? Make sure they move freely.
Leaky carb boots on either side of the carbs?

[JTGS850GL]

Filter is a new K&N
No fuel filter

I'll check it on prime, but the loss is right off the line. Not much fuel flow at that point. Just doesn't seem to pull as hard off idle. I use to be able to take off in 2nd gear with no problem. Now I would have to really feather the clutch to take off smoothly. Exhaust note is a little different as well.

[chef1366]

Rust in tank?

[Suzuki_Don]

Slipping clutch.

[JTGS850GL]

No visible rust in tank. I can drain a float bowl and see what's collected. Doesn't seam like a clutch problem since the RPMs don't rise disproportionately.

[duaneage]

This sounds like a lean condition so I would focus on fuel delivery.
The pilot circuit could be dirty, the holes in the brass jets are small and can be easily clogged.
Check the carb boots and o rings for air leakage when the engine is running by spraying a little wd-40 around the boots.

[JTGS850GL]

Tomorrow I'll go back and check the carb sync and do the WD-40 check as well. I may just end up pulling the carbs and go back through them to make sure everything is still clean and clear.

The only strange thing I found when I first picked up the bike was that two of the shim buckets would not rotate when doing the valve adjustment. Both were exhaust valves on the #2 and #3 cylinders. I could not do anything to get them to rotate around, but they would go up and down with the cam. Just wondering if this is a common thing and what problems would this cause if any? I have a spare head and one of the buckets on it was hard to turn. The problem followed the shim bucket. If I replaced the shim bucket the problem would go away. During winter I'm thinking of pulling the exhaust cam and replacing the buckets that were tight with one of the spares I have laying around.

Is my reasoning correct that if compression is good (145-150) on all cylinders, then I'm not looking at a bad cylinder rings, bad valve or bad cam timing issue?

[JTGS850GL]

After taking it for a ride yesterday I've become convinced that the power loss is mainly in the beginning of throttle opening. Taking off from a stop just feels weak. Once underway I can roll on the throttle and get reasonable power. I tend to lug the bike more then I should though. I like cruising at 50mph in 5th rather then 4th or 3rd so that tends to amplify the problem.

Still sounds like a pilot circuit issue to me since idle and WOT seem less effected. Temps are dropping so now may be a good time to drop the carbs and see what's up.