Compression off by 10 PSI—Troublesome?

[Dirk_S]

I found that my Guzzi is once again experiencing a bit of a lag in power at higher speed. Last year, I’m fairly certain that this was due to a dirty fuel filter, as when I tipped the pump over, colored fueled poured out. Today I once again removed the fuel pump, and shot some carb cleaner through the various ports, making sure to remove and spray down the filter screen. This time I didn’t see much dirt, so my next step was to do a compression test.

Started the bike up and let it run around 4 minutes before hooking up the tool. Right side read ~180 psi, left side ~170 psi, keeping mind that the V7 II has a 10.5:1 ratio. I believe last year’s compression test—before rebuilding the engine—both cylinders read around ~185.

I’m curious if this 10psi is worth any concern. I think I may do a leakdown test anyway, and perhaps recheck my valves. BTW, I’m at 320 miles since the rebuild. Of course, there’s also the chance that the fuel pump clean job maybe loosened the right stuff up and took care of the issue; I’ll have to hop out onto an open road tomorrow to see.

[n3303j]

You've retorqued the heads then reset the valves a couple of times since the rebuild?

[Dirk_S]

You've retorqued the heads then reset the valves a couple of times since the rebuild?

I re-torqued the heads and reset the valves right after the initial heat cycle. Was going to do them again after 609 miles.

[n3303j]

Not sure what your head gasket is but when I re-ringed my V11 last season the heads moved a lot in the first 300 miles. It was almost enough to close up the rocker gaps (.006/.008).
If you replaced the rings 300 miles should pretty much have them seated.
I wouldn't be happy with a 10# differential on a newly topped engine.
I would check rocker clearances then retorque heads (loosen a nut, then smoothly bring it back to specs). Then finally reset rocker clearances again making a mental note of hiw much they closed up after re-torqueing.
I do the second time around at 250 miles.

[mechanicsavant]

IMHO the rings are probably not fully seated yet . Certainly do a leakage test  . If it is leaking past the ring just check in another 1k mi & record changes . Don’t get freaky,.

[moto-uno]

 I have to admit that after all my years doing m/c work, I'd be surprised if you would notice a 10psi difference .
 Others may disagree, but it's maybe a bit early to fret over small variations , and yeah YMMV :) . Peter

[Old Jock]

I don't know the V7 but I agree with moto-uno.

Far as I recall if you've less than 10% difference between the cylinders then that's considered acceptable and 170-180 psi is a pretty healthy compression

As said I doubt you'd notice any difference and balancing the injectors will have each cylinder doing the same amount of work

[Dirk_S]

I read somewhere that the small block 750s are supposedly known for taking a while to seat their rings, so I’ll chill my worries; I’ll hopefully do the leakdown and check valves this weekend.

[Dirk_S]

Checked left valve clearance—perfect.

Retorqued the left cylinder head.

Checked valve clearance—too tight!

Readjusted valves.

Checked compression—equal on both sides (!), albeit @ 170 psi—why/how did my right side lose 10 psi? User error?

Performed leak-down test—great on both sides.

Saw lots of oil in the airbox, but the filter was still decent (not beautiful though). Cleaned out the box.

With everything back together, I hopped onto the highway. I MIGHT have all my power back—kinda hard to tell sometimes, because in order to get above 70 mph when climbing long hills, I need to knock down to around 4th, sometimes 3rd (if I remember correctly), and the engine has to buzz between 6500-7500 rpm (oof) to maintain 75 mph. To reiterate, I won’t sustain that for long periods of time, I promise, but I do have to sometimes keep up with my 1150 GS oil head buddy, especially later this year when we reattempt Newfoundland and Labrador.

[Stretch]

Glad you're getting things sorted, Dirk!   

Oil in the airbox at this stage is no big deal, methinks.
There would be some blow-by before the rings are fully seated,
and that would pressurize the crankcase, which would force oil
into the airbox. If it continues, maybe there would be an issue,
but I wouldn't worry too much tight now.

Why only 170 psi? Different air/engine temps this time you measured?
Your aforementioned - albeit SLIM - possibility of human error?   
I wouldn't worry too much about the actual number right at the moment.

Ride it another 500 miles, get a few more heat cycles in it, give the rings
a little bit more of a chance to finish bedding in, re-torque the heads,and
check everything again.

                                                              -Stretch

[n3303j]

You did retorque the RH head too?
If one moved the other one is also loose.
Always should be done in pairs.

[Dirk_S]

You did retorque the RH head too?
If one moved the other one is also loose.
Always should be done in pairs.

…goddangit.

[Vagrant]

Seeing as you have a special situation with oil levels I'd hook a drain tube to the nipple on the left lower side of the air box. make a catch can up for it that you can access. It should save your air filter.

[Dirk_S]

Seeing as you have a special situation with oil levels I'd hook a drain tube to the nipple on the left lower side of the air box. make a catch can up for it that you can access. It should save your air filter.

Not a bad idea at all. I removed the evap canister, so there’s certainly room. Do catch cans on the small blocks provide any additional performance benefit beyond just making sure the intake stays cleaner?