low compression

[Bigwig99]

I have recently purchased my first Honda cb750, It is a 81 custom and has been sitting a few years.  I have done some maintenance to try and get her road worthy again but have run into a few problems that I hope someone with more experience can help.  First I have done a dry compression test on the cylinders and 1,2,4 all were about 110 which I know isnt great but can work with.  #3 was only about 40psi which is a big red flag.  The bike runs but I did a temp test on the headers and #3 gets hot but not nearly as much as the others.  I have checked my valve clearance also and my numbers indicate that almost all my clearances are tight around .002 even have a couple including #3 exhasut that were less then .002.  I am in the process of ordering almost all new shims to get my clearnces as close to .005 as I can.  What I was wondering is could that lack of clearance be causing my compression problem or is it more likely a valve or ring problem which Im really hoping it isnt.  Also when Honda sold these bikes was there a stock shim size (thickness) that the valves had in them, can I assume all the shims that are currently in there are the same thickness assuming the po never messed with them?

[shinyribs]

To test your valves:
Remove valve cover
Rotate crank til one matched pair of valves are closed
Remove Schrader/check valve from compression tester
Hook airhose to tester
Listen for leaks

On the phone. Sorry for extra brief.

[Beekeeper]

As far as a standard shim size, I don't think so.  Your going to need to measure each shim.  Plugging all those numbers, along with your clearances, into one of the spread sheets will tell you which sizes you need to order, and where to switch the shims you have.  Make sure you save a copy of the results saved under a different name.  Then you will know the shim sizes you start with at the next clearance check

[Bigwig99]

Ok I did a leak down compression test and my #3 cylinder exhaust is just pissing out air.  This is not what I wanted at all.  I'm gonna take the engine apart and check my valves.  Now the big question is can or can I not replace and lap a new valve myself.  I have seen people do it and make it look easy but I've also heard that you have to have a machine shop do it. If that's the case I will be junking the bike and calling it a loss.  Is there anybody who has successfully lapped and reseated there valves and it turned out ok?

[Re-run]

I am on the fence about lapping valves on these engine. Many times it is better to just cut the angle you need for the new valve.
http://www.dansmc.com/valve.htm

[TOOLS1]

A valve job is not that expensive. If the bike is decent, and has a title, I definitely would not junk it over the cost of a valve job.
TOOLS

[Re-run]

Especially with a dohc when you don't have to remove the engine from the frame to pull the head.

[shinyribs]

A leaky valve could be a few different things.

Rusty seat and/or stem from sitting. Carbon build up causing it to stick or, like you said before, just out of spec. Also could be bent, in which case the cause needs to be found.

But...since you only have one valve out of 16 that's leaking I'm betting its just sticking from sitting. Slather that puppy with PB Blaster and get her running and hot. Rev it up and see if it loosens. Works sometimes.

[shultz_1978]

I have to say Shiny is right. With on valve leaking as long as it is not hitting the piston or stuck. Lube her up and get it fired up. I have done two valve jobs on this bikes. One on sohc and one on dohc. If you can get the head off, All you need is a valve compressor and lapping compound. You tube....can help

[Bigwig99]

Thanks for the suggestions guys!  I really don't wanna junk the bike I might have just been a little frustrated at the time lol.  I will get some pb blaster and try to loosen up the valve if it is stuck.  Would you suggest I take off the header and spray into the exhaust port?  Or where would you suggest would be the best way?  If that doesn't work I will take the engine out and take her apart and have a look.  

[shultz_1978]

Do you know if its leaking around intake or the exhaust valves. If its exhaust, It sound like you are on the right track. You should be able to take off the valve cover and check to with valve may be sticking.

Been a while but I think it is like 8 bolts.  I would wheel it over by hand and see if one of the rockers on  number three cylinder gets more play then the others.

Never know if it is not leaking by to bad. It may just need to be ran.