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Top end scratching noise at WOT

sam000lee

sam000lee

Forum Mentor
When I cruising in 4th or 5th gear on my GS750 and whack the throttle open (not necessarily fully wide open) I get a scratching noise coming from the top end. It sounds kind of like scratching a piece of paper with your fingernail, but with the speed of the engine. It only happens at relatively low RPMs so if I really wind out each gear and hit 5th going 75+mph I don't get it in that gear. It's more if I'm cruising at 60-65 in 5th and then try to accelerate without downshifting.,

Normally when this happens, I just back off the throttle until it the noise goes away. In this manner, if I don't downshift, I can slowly accelerate and keep opening the throttle slowly and, as the engine speed increase, I can open the throttle slightly more without hearing it.

It's been happening for a while (at least 5,000 miles) or perhaps even since I got the bike (~10,000 miles ago) and has not gotten worse.

Any idea what it could be?
 
Sounds like pre-ignition, fill up with premium and see if it goes away. Ignition timing may be advanced too much.
 
BadBillyB said:
Sounds like pre-ignition, fill up with premium and see if it goes away. Ignition timing may be advanced too much.
Click to expand...

Thanks, that had come to mind I'll give it a shot. I've been meaning to upgrade to electronic ignition so maybe this is a good excuse...
 
Filled up with premium and the noise is still there. I had adjusted my points about two weeks ago, too. It happens when I give it throttle and the RPMS aren't haven't caught up yet. Sounds like it's coming from the top end since I can block the noise a little if I put my hand in the area between the tank and plugs. Spark plugs look fine and were changed when I did the points. I have no reason to think the CCT isn't working but I'll take it off and check anyway.

Any other ideas?
 
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when you adjust the points, you then have to check/set the ignition timing. Did you check it?........Billy
 
Well...I went to take the cam chain tensioner and one of the bolts snapped. Got it out with some vice grips and it's back on. Whoever last installed it backed the set screw out a little more than I think they should have when the reinstalled it so I reset it. It's about to rain so I'll have to wait to give it a ride. Also, picking up a Dyna ignition from EdBot in about 30 minutes (if you read this thanks Ed!) so I'll be able to eliminate the timing possibility once thats in.
 
Ok, cool. But you still have to set the timing for cylinders 1&4 and 2&3 individually. Also check the mechanical advance assembly for sticking. Rotate the cam clockwise then let go. Should snap back smoothly. You are going to need a timing light.....Billy
 
It was the CCT. The PO had the set screw turned quite far out. I reset it and rode about 30 miles. For the most part it's gone but I am still able to get the noise if I whacked the throttle open in 5th gear going 50mph - not a situation I am really ever in as I normally shift to 5th around 60/65mph and even then am only at about 1/3 throttle. Any one have an explanation?

At any rate I also put the Dyna in this weekend and went for 250 miles to the Berkshires. It was really easy to set up and the bike feels great!
 
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