750 won't red line - Valve?

It's about 4 years since I was on these forums. A big hello to anyon ewho remember me.. Bill Carr from Norway. Unfortunately had to sell my 1981 KZ750 in 2004. See here:

www.sekkepipenorge.com/kz1.jpg

The bike has mostly sat unused in the last 4 years and now the current owner (second after me) has some problems with it.

Please bare with me. I did a lot of work on the motor when I owned it. New rings. Valve grind. Valve clearances. I also jetted up the carbs and put a sports air filter on and a new 4 into 2 exhaust. New high performance coils and plug leads too. I also synchronized the carbs as best I could with some sort of vacuum gauge and the workshop manual.

After all this the bike pulled real well up to (I can't remember exactly) about 5000? but then would not go any faster. About 70 MPH is all she would do.

I suggested the new owner (who sold it on the the current owner) take it to the local bike mechanic and have it dynotuned. She never did.

Now the guy who bought it a few days ago is jumping up and down and claiming all sorts of things are wrong with it. He is convinced there is a burned valve but has not done a compression test yet. He will do that today.

My questions are: What are the symptoms of a burned valve? The bike starts first hit on the starter and runs very smoothly. It just won't go to red line under load or even near it.

When I put the new exhaust on it was pretty loud and so to quieten it down a put some extra baffle material into each exhaust pot. It's like very course and open weave steel wool. Could that have an effect on the motor? Maybe it can't breath out properly.. so to speak.

Anyway... It's not my responsibility to fix these things now but I did promise the guy I would ask around for tips and advice.


Any comments are very welcome.

Cheers

Bill

Well, to me that sounds like more of a carb issue. the reason being, if you did in fact have a burned valve, it in no way would idle "smooth" and pull well up to 5k. I hate to ask, but did you rejet it incorrectly? I had a suzuki with the same problem a while back, turned out it was improperly rejetted causing a backfire into the carb, which in turn screwed with the pilot jet.

Thanks for the reply. He just did a test and found litte or no compression in number 2. I don't get it? Anyway he is bringing it over tonight... He got a refund from the seller. I'll probably check it out a bit myself even though I should know better than to get involved in this.

He insisted it's a burned valve even though he has not taken the head off but only done a comp test. I'm thinking it could be a poor shim adjustment. I did a complete valve grind and adjusted the shims/valve clearance before I sold the bike 4 years ago.

Can someone explain how a motor would perform with too little and with too much valve clearance. Could the symtoms in my original post be a sign?

Bill

also make sure #3 spigot mount clamp is not interfering with the throttle pulley.

too little clearance could cause valve to hang open/low compression .too much clearance would be noisy depending on how far out it is,could possibly burn valve since its not coming off seat id say that would be more of a problem with exhuast valve since they run hotter.after 4 years i wouldnt take responsibility for the problem since youhave no control over how its been treated

Long time ago I burned an exhaust valve on a CB250. I got lots of backfiring in the exhaust. I would think that with 3 out of 4 cyclinders running one should be able to push a 750 beyond 70mph. Strange. I would be thinking carb related also and I wouldnt take responsibility. It's been 4 years. You should charge the guy a consultancy fee

The only warrantee I ever expected or gave on a used bike was that it go to the end of the street.

PS I remember you. piperbill. no "2".

Yes.. Piperbill. I lost my password and I have a different email addy now so I had to register again.

I've not given any warranty nor am I taking any responsibility. This bike was my baby and I spend almost a year rebuilding it. The couple who bought it from me did not have it fine tuned and adjusted like I said they should.. Heck they didn't even clean/oil the air filter in 4 years!

I made a promise when I sold it that if they ever wanted to sell it then I would help out.. since I knew the bike inside out. So one day they just showed up at my door with it!

I've done my bit and sold it on for them.. If it were me I wouldn't even give the new owner a refund. He signed a contract stating he had inspected the bike and bought it "As is". He claims it has a burned valve but has no proof.

I could fix the problem if I were so inclined and with help from this forum. However I am pretty much done with bikes and riding nowadays. I don't have the nerves for it anymore.


Cheers

Bill

If the engine runs smooth at idle it's probably not got a burnt valve. I would make sure there is no obstruction to intake air flow and no exhaust restriction. Also as was mentioned earlier will the throttle open to "WOT". If no problem found I would then look at carbs. Maybe also look at fuel flow from tank. Could also be valve timing.

Short video here.




Bill

what does it run like once its warmed up,what does #3 plug look like ?

try pulling the aircleaner off and see if it runs better/worse

Try removing the header and look for oil in the exhaust port. When my Saturn had a burnt exhaust valve, we could tell which cylinder wasn't firing by removing the exhaust manifold and looking for oil in one of the ports. Sure enough, the cylinder with the burnt valve was as oily as could be. I'm not sure this is a 100% sure way to check for a burnt valve, but its easy to do.