How can you tell if you have a burnt valve without comp test

This Maxim I am working on sounds like it has a skip. When it spits it makes a wierd sound like photon shot from star wars. When I adjusted the valves, some of them had no gap. Is it possible a valve is burned? The bike will not idle at 1000 unless the choke is on a little and even then it is not steady. I have adjusted the valves, rebuilt the carbs, silconed the boots, all the usual suspects with the help from this forum's write ups and parts from Chacal. I never vacuum synched the carbs only on the bench with a business card. Also, at what year do you have the use a YICs tool? This bike is an '80. I don't see any YICs port, unless I dont know where to look.

Any ideas?

Thanks
Pat

 

regarding the YICS port, you don't have it the '80. Yamaha introduced it in all 1982-up XJ650 Maxim and Turbo models,, as well as other model bikes.

regarding the valve issue; no gap? meaning zero (0) clearance? Yeah that could be an issue. How do the spark plugs look?

have you thought about doing a compression check? Not sure if that would mean anything or not...

I'm not experienced in something of this nature but willing to hear what others have to say. Its an interesting situation because as my signature says, I have severe problems with my 650.

 

1980 would be pre-YICS; if your motor doesn't have YICS cast into the valve cover, it's not a YICS motor.

It's very possible a valve is burnt; if you don't want to invest in a compression tester, rent/borrow one from the Auto Parts store.

You'll need to do a vac sync; but I would get a compression test first.

Did you accurately, individually, WET-SET the float levels? Also important.

 

Re: How can you tell if you have a burnt valve without comp

Thanks for the replies. Yes I did everything with the carbs by the book. Meaning I took the writeups from here a did it step by step. Also set the valves from the info here. The bike has get up and go except for the skip now and then and having to have the choke engaged a little to get it to idle. I guess I will have to borrow a comp tester. Glad I dont have the YICs sounds like that causes some issues in tuning. What should the comp read on one of these?

 

If you have to keep the enricher on, then you're carbs are still not right. If you haven't balanced them, that can be a big factor. Did you pull the mixture screws out and service them? Did you make sure that the pick-up tube is clear AND the jet at the bottom of the well is not plugged?

If you can't do a compression test, can you do a leakdown test? They're actually easier.......screw in the adapter, open the air valve, and listen. If you hear air rushing at the carbs, you have an intake valve open. If you hear air rushing at the exhaust side, you have an exhaust valve open. If you hear nothing but the pressure is still dropping then open the oil filler and see if you hear air rushing there which means rings aren't sealing. If your pressure stays up, and you don't hear any rushing, you're good, and now it's time to check the next cyl.

If you think you might not know whether you'd hear the air or not.......think about this--you know what air sounds like rushing out of a tire? That's about 40psi. You're gonna be using air at about 100psi. If you have a valve leaking, it'll sound something like: PSSSSSSSSSSSSHHHHHHHH!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

not:

pssssss.

I learned about this from a local sled shop that builds racing sleds. Current sled is a 4-stroke Yammy engine that currently is about 700 HP, if I remember correctly. They're aiming to up that next time around...........

Dave F

 

Re: How can you tell if you have a burnt valve without comp

Compression test is the most fundamental diagnostic tool. Get hold of a tester and find out what you've got.

 

still sounds like carbs to me . This will give you an idea put your finger over the spark plug hole on every cyl they should all blow your finger off with about the same amount of force. I believe autozone lends them if i am not mistaken. if you have access to a set of gauges to sync the carbs if a valve is burnt the vac will jump like crazy on that cyl

 

fingers on holes doesn't tell you numbers....jsut tells you if you have some compression or not. I do that on mowers and stuff just to do a compression check --as in, does it have any compression. For accuracy, range, etc...... use a real gauge. The thumb gauge that I have is only accurate enough to tell if there's any compression vs. no compression.

Dave F

 

You DON'T "Not" want to do a Compression Test.

If you bring a Bike into a good Shop for the first time; the Tech often has to write the results of a Compression Test on the Work Order.

Buy a Compression Tester and use it ... BEFORE ... buying the Bike!