Rear Valve Clearance too Tight

[WaterMover77498]

After some help on the carb forum, the good folks of this group helped me see that I had a valve clearance issue. I have noticed a small but noticeable amount of compression going through the rear carb.

The intake valve clearances for the rear were 0.0025" and 0.0030". These were really tight compared to the specs I saw in the manual.

Any ideas why the clearance is so tight? As far as I know, the previous owner didn't do any valve work that might have left put too much shim on the lifter. I'm guessing the bike sat for two or three years before I have started working on it.

Thanks in advance! You guys have been great/

[QBS]

How many miles on the bike?

[fiat-doctor]

Yes, that is pretty tight but not for sure the problem....   what are the clearances on all of the valves? 
Is the rear cylinder a lot tighter than the front?

Yes, we want to know the miles....  as the valves and the seats wear together the clearances tend to tighten. 
A bent valve or one that is sticking or has a bit of carbon between valve and seat will have greater clearance than it should.

Measured as the OEM manual says so that the cam lobes are away from the lifters???

[WaterMover77498]

14,000 miles on the bike.

I'll measure the front clearances, hopefully tomorrow.  Rear exhaust valves had 0.0065" and 0.0075".

[Rikugun]

....see that I had a valve clearance issue. I have noticed a small but noticeable amount of compression going through the rear carb.

I am curious about this phenomenon and how you've determined this. Is this observed while running or did you also try introducing air into the cylinder (cold engine) at TDC compression and hear some escaping from the carb?

If so and if you have even slight valve lash when cold, I would think air shouldn't be leaking? Unless there is something keeping the valve from seating as fiat-doctor suggested. I suppose if the valve timing had been inadvertently altered on that cylinder you may also get air blowing from the carb. You can check valve timing while the valve covers are off and eliminate or confirm that as a possibility.

Did you confirm both cylinders run from a cold start and while warming up?

[jefferson]

 The intake valves would be the suspect if you are getting blowback through the carb. The timing as suggested should be checked.

[Walt_M.]

It could also be a bent valve, try a compression check.

[Rikugun]

A bent valve is a possibility (I think fiat_doctor suggested above) or something else keeping an intake open given it's blowing back through the carb.

Compression readings can be a bit ambiguous. I still use them occasionally but it's often after performing a poor man's leak down test. I've used a rubber-tipped blow nozzle as pictured. The tip has pipe thread so different elbows and lengths of nipple can be added to get the reach and angle required in difficult plug locations. I've even done this in remote locations (no air or electricity) using one of those cheap portable air tanks with the blow nozzle attached to the hose in lieu of a tire chuck.

With the engine at TDC compression it will want to turn one way or the other once air is applied through the spark plug hole. I've done it by myself but it's easier with someone holding the engine while you concentrate on getting a tight fit with the rubber tip. Listen at the oil fill, carb and exhaust for leaks. If there is one, it will be obvious.

[WaterMover77498]

Rikugun, the air can be felt coming out of the carb when the bike is running. I'll have to look for a couple more air fittings to be able to but compressed air in the spark plug hole.

Some other requested data:
Compression: front, 145 PSI rear, 100 PSI
Front valve lash: both intake: 0.005" both rear 0.007"

I've put a scope camera down the carb boot with the intake valves fully open to try to see the valve seat. I could see about the front 60 degrees of both valves. They looked good. Still lots of valve I can't see.

Other than head gaskets not being super available, is there any reason I should go ahead and pull the head now? Bent valves or bad seat conditions seem the most likely culprits. I'm not sure I can determine anything else at this point.

Thanks again for the help!

[The Prophet of Doom]

Are you sure you aren't detecting the crankcase vent which empties by the rear carb intake.  Very noticeable with the air box off.

[WaterMover77498]

ProphetofDoom, I'm sure it is coming out of the carb. I've made sure the breather hose is no where near the carb. I've also gingerly put a finger into the carb opening. It is coming from inside for sure.

[Rikugun]

Paul, as far as gasket availability is concerned there are options but I'm not a fan of what I consider major surgery if outpatient procedures may work. Also, before committing to engine disassembly, I prefer to be as sure of the problem's cause as one can be - and I'm not sure you are there yet. With the info given thus far I'm assuming it's a problem with an intake valve but is it bent, stuck or does it have debris holding it open? The test rig I outlined with the blow nozzle could help with the last BTW.

Before we go down that rabbit hole there is a possible cause that can be remedied without the expense and hassle of head removal that being confirmation of correct valve timing. If someone had the cams out for whatever reason it is not uncommon to get them back in wrong. If it is wrong and depending on how off it is, a valve could be *bent. However, I'd still go ahead and correct the timing then check to see if the cylinder held air and made compression before moving forward with disassembly.

*Since you have access to a scope camera, did you examine the piston crown for signs of valve contact? Also, with the carbs off and a light shining through the spark plug hole (when the valve should be closed) is light emanating from the intake port?

[QBS]

wm77, I'm in 77018.  Let me visit you and see what you are seeing.  Schedule is flexible.

[WaterMover77498]

QBS, I work in 77018 often. Can I take you to lunch or something? I know looking at the bike would be most productive, but maybe we could make connections that way sooner.

[motoracer8]

Valve seat recession is a issue on 35 year old motorcycles.