compression test-above spec?

[EricHa]

I recently picked up a XS1100SF from niterider and just did a compression test using a Lisle tester. The bike shows 8200 miles and has all original equipment, though it's fairly rough, nickname Rusty.

I removed the plugs, and held the throttle wide open and spun the starter until compression ceased to rise (4-5 seconds). Numbers were 173, 173, 165, and 174psi. Engine cold, 600 ft. altitutde. Spec says a max of 156psi. I'm thinking that the bores are in good shape, but why such high numbers?

Thanks to all for such a great forum!

Eric

[bikerphil]

Might be carboned up?

[BA80]

The bike shows 8200 miles

8200 or 82000?

[EricHa]

8200, and yes, it could quite possibly be full of carbon. Plugs looked fairly dark with a little tan. What's the best way to clean it out? Is it time to hit the search?

[BA80]

That engine ain't even broke in yet. Ride it a while and recheck.

[EricHa]

She's running pretty rough down low, missing on at least one hole at startup, but cleans up somewhat when warm,, and scoots after 3 grand. Carbs really need to be gone through. OEM mufflers are rusted out and I picked up a Jardine 4 into 1 so it's time I guess. Thinking 142.5 mains and one bigger on the pilot jet?

[BA80]

She's running pretty rough down low, missing on at least one hole at startup, but cleans up somewhat when warm,, and scoots after 3 grand. Carbs really need to be gone through. OEM mufflers are rusted out and I picked up a Jardine 4 into 1 so it's time I guess. Thinking 142.5 mains and one bigger on the pilot jet?

Try it stock jets first. It shouldn't make a differrence unless you want to go 130MPH.

The early engines don't have much oomf under 3 grand anyway. You just gotta keep the RPMs up when riding.

Clean up the carbs and give it a try.

[crazy steve]

...I removed the plugs, and held the throttle wide open and spun the starter until compression ceased to rise (4-5 seconds). Numbers were 173, 173, 165, and 174psi. Engine cold, 600 ft. altitutde. Spec says a max of 156psi. I'm thinking that the bores are in good shape, but why such high numbers?...

I don't think those are high IMO... I suspect that the 'spec' numbers are low (to give Yamaha some 'cheat' room on warranty work?), as the only ones I've run into with numbers in the 140-150 range (or less) had mechanical issues or high miles. The SG I just finished putting together checked at 180-190 (plugs out, carbs off) and runs just fine so far. I've got a '79 motor that ran between 190 and 205 (same test procedure). Neither have more than normal amounts of carbon from what I can see.

[TADracer]

I agree with Steve. My last comp check in May was , 172, 172, 165, 168. But you should check that with the motor at operating temp and WOT.

On the jet thing, I would make that the last thing you try to clear up sooty plugs. Its a mistake, IMHO, that many people get into because before you start changing jetting you really need a good, established baseline to tune from otherwise you will be doing alot of head scratching. Try to get the thing running close to factory specs before you start changing up jetting and then ride it awhile to feel it out and see if it gets back on its legs before you dive into tuning changes. Just my $.02

[EricHa]

Then I'm going to assume my numbers are normal. I did think that Yamaha's 142-156psi was fairly low.

I'll try the header with stock jetting as a base line after I clean the carbs. Thanks for the advise.

[psycoreefer]

I'll just jump in and 3rd the oppinion that the published numbers are low, I just put SWMBO's engine back together and with a light hone and old rings I was at 130-ish across, after riding for a week that jumped up to 180+ on all 4. I cleaned out the chambers while I had it apart and I promise that it didn't build up that much carbon in a week.