VMAX 600 running like a 300?? Any Help?

Y

YamahaGuy-1

New member
Joined
Feb 5, 2007
Messages
43
Location
Pa
Was riding with my neighbor last night and his 94 vmax 600 starting running like it had fouled a plug. We stopped and he put in new plugs and it still was running funny. It will idle fine, but when he gets on it it sounds funny like it is running on one cylinder, but both jugs are hot and both sides of Y-pipe appear to fell the same temp? The thing is smoking like it is running on 1 cylinder and it was going thru the gas like there was no tomorrow. I thought maybe water frozen in a fuel line or the pump but he says he has put lots of drygas thru it this year, not that it couldn't still be that but I am getting stumped on this one. He still rode with us almost 40 miles last night so it is not like it has no power, but he says its got about half or less than normal and will not go over 50mph since it started this. Any ideas on things to check???
 
sounds like a weak cyl. Could also be a bad plug boot or something simple not letting it fire when the pressure starts building. Check the compression first & go from there.
 
If you don't have a compression tester just slowly pull on the recoil with key off. Both cylinders should feel about the same. If one cylinder is weak feeling you may have had a ring let go. If that is the case you will be able to tell.
 
You highly likely detonated a piston, took the ring land out and that cylinder is now low on compression. Common on the 600 twins and the 6000 RPM lean bog they have in the midrange.
If you have a bend-a-lite, look down in the spark plug hole and at the exhaust port side of the piston crown, look for errosion or melted away at the edge.
Easy way to check compression, hold your thumb over the spark plug hole and pull the recoil over (cool and with the kill switch off, of course). You'll know the low cylinder because it won't blow your thumb off like the good cylinder will.
 
Called the neighbor last nite and told him to bring it down and we would take a look at it. First thing we did was check compression and both sides were real close so that was out so we pulled carbs and low and behold the carb slide on the clutch side would not move with the throttle. Took the cover off the top of the carb and the screw had come out of the shaft making it so the shaft turned but the carb did nothing. I was kind of amazed b/c I figured you guys were right and we were in for a tear down which would have been a shame with just a little over 2000 miles.

Question though???
What should a guy put on those screws so they can never back out again? We just tightened it as much as we dared, but I would think we should put something on it but couldn't think of what I had that might work. I knew a little silicone on the screw threads would work for a bit but gas eats silicone, Loc-tite problably isn't good because those are some pretty precise things to try and break Loc-tite apart in later and once again did not know if it would stand up to the gas. Any suggestions on what will hold them in place and not get eaten away by the gas?

Thanks for all the help.
 
Holy cow, you got lucky. Good find.
Unless the manual indicates to apply a threadlocker, I wouldn't put anything on it other then make sure it's tight by hand. That is all I have ever done with Yamaha rack carbs. Chances are it wasn't tightened sufficiently when it was sync'd at some point previously in it's lifetime, it didn't rattle loose in one ride.
 
Had mine come loose a couple years ago not with the same symptom's as your neighbor's,the idle was just really erratic. Just tightened it up good no problem's since.
 
YamahaGuy said:
First thing we did was check compression and both sides were real close so that was out so we pulled carbs and low and behold the carb slide on the clutch side would not move with the throttle.
Click to expand...


I'm surprised your compression results were close. Normally if you forget to hold the throttle wide open compression is bad, substantially lower than a true WOT test. (which is pretty much what was happening here... Maybe it only opened partially?)
 
There may have been a TSB on this back in the day, I can recall a couple of my buddies sleds being checked out for this, one of them did in fact have the screws come out.
 
You must log in or register to reply here.


Back
Top