1981 KZ750 E2 will not accelerate past 70 mph

Cam timing was checked using method for 4 cylinder KZ 750.

Compression test today on warmed up engine revealed the following:

Cylinder #1 130 psi
Cylinder #2 90 psi
Cylinder #3 135 psi
Cylinder #4 125 psi

1 cc of 10W 40 engine oil injected into cylinder, rechecked compression:

Cylinder #2 95 psi

Leaking pistons rings or valves perhaps.

When valves cleanances were checked on 2/17/2011 the following clearances were recorded:

# 1 Inlet .003
# 1 Exhaust .005

#2 Inlet .003
#2 Exhaust .002

#3 Inlet .007
#3 Exhaust .002

#4 Inlet .003
#4 Exhaust .005

The Cylinder with the least amount of combined clearance was # 2.

It is possible that I have a valve that is sticking open slightly. Are there any magic snake oils that could possibly free up a stuck valve if it is carbon fouled. Seafoam, Stabil, Marvel Mystery Oil perhaps. It would not be a big deal to recheck valve clearances, however, it would be frustrating to go through all of the motions only to have the same problem persist.

If you do check the valves and find #2 has changed more than a little you may have isolated the problem. Another option is to do a leak down test to see if you can tell where the compression is going. Ed

Just took a lap on the bike, problem persist. Reinstalled the original pick-up coils along the the IC Igniter just to see if anything would change, it did not.

I may pull the valve cover next to check clearances. Will report back when I am able to measure the gaps.

This one is an enigma wrapped in a riddle. Mysterious indeed. Please keep us posted.

Hi, I had a 1980 750 LTD. I know you recently noticed #2 has lower compression, which maybe be the issue, but have you also checked you diaphragms for holes? If there are holes, they will not operate correctly.

As a side note, I had stock intake and exhaust on my bike. Never had any lean issues at all...She always fired right up. I then had to install some slip on mufflers because my stocker rusted out inside. They were louder and much more free flowing, but I still did not need to change the jetting/needle position, just richened up the pilot air/fuel screw a tad. Always pulled like a beast for me. Maybe try and throw the stock jets back in and see if that helps?

Oops wrong page sty it was late

A leak down test helps determine whether fully closed valves are leaking (for whatever reason, such as zero clearance, burnt/damaged valve or seat, blockage between valve and seat, bent valve, incorrect cam timing, weak/broken valve spring, etc.) And may also indicate loss of compression due to piston/ring/cylinder wear.
And can sometimes reveal a blown or leaking head gasket.

Can perform a poor man's leak down test by introducing compressed air into a spark plug hole and listening to where it escapes from the combustion clamber.

This may be done at TDC or any other cam positions where both valves are fully closed.
Top dead center (TDC) is when the piston is at its highest position.

An easy way to tell TDC is by alignment of the T mark with the case mark when viewed through the timing window.

May introduce a spurt of compressed air by using a rubber cone-tip blow gun held into the spark plug hole.
And of course keep holding it in position while listening for escaping air at other places.

Hearing escaping air at the carb intake indicates that the intake valve isn't completely sealing when closed (for whatever reason).

Hearing escaping air at the exhaust indicates that the exhaust valve isn't completely sealing when closed (for whatever reason).

Hearing escaping air at the crankcase vent indicates an issue (for whatever reason) with the piston, rings, and/or bore.

Hearing escaping air at some position along the edge of the head gasket indicates a leak in the head gasket.

A leaking valve may sometimes be resolved by adjusting the clearance to within specs.

A leaking head gasket may sometimes be resolved by torquing the head fasteners when the engine is stone cold (such as after sitting overnight).

Good Fortune!

Removed valve cover last night, measured valve clearances, values are list below:

#1 Inlet .004
#1 Exhaust.005

#2 Inlet .004
#2 Exhaust.000

#3 Inlet .007
#3 Exhaust.003

#4 Inlet .004
#4 Exhaust.005

I now suspect the zero clearance measured at position # 2 Exhaust is the cause of the low compression and perhaps the reason the bike is running poorly.

The exhaust cam was removed after checking cam timing. Cam timing had been set correctly.

This week as time permits the cam follower will be removed and the shim measured. If everything is with-in wear limits a thinner shim will be installed and clearances will be checked again. If all goes well the bike will be re-assembled and compression checked again.

Thanks to all for sharing ideas and insight to this problem.

The diaphragms were carefully inspected for damage to rubber and for any holes, none found.

I have recently discovered zero valve clearance on my # 2 Exhaust Valve. The cam has been removed, the shim will be pulled this week.

One cylinder with a little low compression will not keep the bike down to 70mph in my experience.

Pulled the cam follower and shim on # 2 exhaust valve. Measured it at 2.60 mm. Will install a .006" thinner shim this weekend. A 2.45 mm shim should do the trick. Also did an old aircraft mechanic trick and gave the exhaust valve a couple of good whacks via a wood dowel, hickory, no splinters or damage to metal occured. This technique is rumored to be approved to use on Aircraft engines per AC43-13-1a, hope it works on motorcycle engines. Plenty of clearance existed between valve on top of piston. Once reinstalled will do leak down check and compression check.

Thanks again to everyone for helping out on this one

SoDak52 wrote: Pulled the cam follower and shim on # 2 exhaust valve. Measured it at 2.60 mm. Will install a .006" thinner shim this weekend. A 2.45 mm shim should do the trick. Also did an old aircraft mechanic trick and gave the exhaust valve a couple of good whacks via a wood dowel, hickory, no splinters or damage to metal occured. This technique is rumored to be approved to use on Aircraft engines per AC43-13-1a, hope it works on motorcycle engines. Plenty of clearance existed between valve on top of piston. Once reinstalled will do leak down check and compression check.

Thanks again to everyone for helping out on this one

"Also did an old aircraft mechanic trick and gave the exhaust valve a couple of good whacks via a wood dowel, hickory,..."

What is the reason for doing this? Was the piston at BDC when you did this? Ed