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Valve job gone bad pt 2

  • Thread starter Thread starter max
  • Start date Start date
M

max

Guest
So after a dodgy valve adjust/carb job by a guy who I don't really trust anymore, my bike is worse than ever. Under 6000rpms is definately lacking in horsepower, altho it is drivable. Over 6000 is as normal. Also the idle is very erradic, it surges up to 4000 when it's warmed up and idling, altho when this happens if I choke it, it will go back down, sometimes die out. No backfiring anymore, at least that's good. To the guy's credit the engine is running a lot smoother, but I'd really like my hp's back. This sounds like carbs to me, but he did go in there and switch around the shims, put in new plugs, adjust my valves, and tinker aorund with the carbs. Now he won't return my calls and I'm kinda stuck.

Where should I start?

Thanks
 
I'm assuming your bike is stock.

1. recheck valve clearances. Easy.
2. Make sure carb is synchronized well.
3. Make sure airbox is sealed correctly.
4. Check for vacuum leaks.
5. Make sure float levels are correct on the carb
6. air/fuel mixture screws set correctly.
7. Petcock working correctly.
 
My guess is he did the valves and neglected to adjust anything afterwards. Go through the checklist Mark stated above.

Brad bt
 
I agree with the checklist, especially the valve adjustment. You did not mention how long ago this happened, but it sounds rather recent. Although there should not be any changes to the clearance, this is when I would expect it the most. The valves might not have been bedded fully in their seats when the clearance was checked. If there was just a bit of grit when they were checked, it has worked its way out by now, and clearance will be less, maybe much less.

It really sounds like low valve clearance because at low speed, the air is going slow enough to get blown back by the piston, if the valve is not quite closed. At higher engine speeds (you mentioned over 6000) the moving air has enough inertia to keep from blowing back on itself. This can happen because of severe overlap (usually found in racing cams) or a valve that is just plain stuck open. If you don't get that valve closing rather soon, it will start to burn, and you will be doing this all over again. :oops:

If you have run it a few miles since the rebuild, it's time for an oil (and filter) change, anyway. Whenever there is major work, like valves or rings, I like to run the engine about 10-20 miles and change the oil to get the first batch of junk out of the way. I will change the oil again at about 100 miles, then go back to the regular schedule.


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