An ODD Engine valve question

[jwsimmons]

I am working on an old Yamaha dirtbike and know this site has some great versatile mechanics on it. SO.... I figured someone may be able to give me a general answer based on internal combustion engines. THE QUESTION: I have a valve clearance GREATER than the recomended width on both INTAKE valves. Does anyone know what having a greater clearance would make an engine do. Mine is cutting out and dying over 5,000 rpm's and backfiring when I let off the throttle. Would this be the valve clearance? or maybe the carborator, timing or any other ideas?

Thank you

 

[wavrider]

Re: An ODD Engine valve question

Well I hope I do not make any of the members upset by replying as this is a boat forum but ??

Anyway it sems you have a 4 stroke you are working on, if that is the case you should have a cam, if you are getting to much clearance is it possible the wrong or after market cam has been installed? If this is the case then yes it sounds like a timing issue. If the engine starts and runs, but has a power problem at high rpm and backfires I would look at timing or getting the correct cam, also look at the cam and the head where the cam sits at, the cam seat persay,this may be worn also and causing excessive play with the cam.
Those old four strokes were alot of fun to ride back in my younger days. Now it is a whole lot easier on the lower back to sit in a chair and run the old Johnson up and down the river. Good luck with the engine repair.

 

[mattandmelissa]

Re: An ODD Engine valve question

Couple of questions? What year yamaha? What model? What cc? if it is old enough it may have points which could need attention Or it could be something in the carb or carbs.

 

[Silvertip]

Re: An ODD Engine valve question

It's a four stroke so why not adjust the valve clearance to spec? A worn cam, worn cam follower, worn valve stem can all cause the clearance to open up. If engine wear is very extensive, valve adjustment alone will not correct the problem. Valve adjustment is routine maintenance on a four stroke whether dirt bike, four wheeler or outboard (unless it has hydraulic lifters). You don't say how much out of spec the clearance is. That would be nice to know but backfiring is generally a cam timing, ignition timing or fuel issue (lean carburetion due gum and varnish or improper low speed mixture adjustment). In any case, you need to get over to a bike forum and to save everyone lots of time, be a lot more specific about what you have, what you've measured, what maintenance you have or have not done, and as a friendly suggestion, buy a service manual for your bike if you intend to perform extensive maintenance. It will answer most of your questions and can help prevent catastrophic failures due to improper adjustments or maintenance procedures.

 

[gregr1971]

Re: An ODD Engine valve question

it is backfiring on deceleration??

 

[dolluper]

Re: An ODD Engine valve question

On deceleration,unburnt fuel being burn't in the exhaust Valve lash to great causes valves to open later,less lash =sooner ...with more lash you introducing fuel later in the cylinder and if you have a timming issue also KABOOM

 

[rwise]

Re: An ODD Engine valve question

To much valve lash (by its self) will only make it run poorly.
This engine should not be points, even the 2 strockers before it were cdi, now if it were a 2 strock,,,,,