YZF750R runability problem - Yamaha Forum : Your Yamaha Motor Products Community & Resource
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post #1 of 2 (permalink) Old 09-12-2019, 07:59 AM Thread Starter
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Join Date: Sep 2019
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YZF750R runability problem

Hello everyone. I am working on a friend's 97 750. The problem is that it really hesitates in low rpm. Just falls on it's face as soon as the clutch grabs. Runs quite well without a load.

Originally it didn't run. Fuel was stale. I cleaned the carbs although they did not look like they needed it. Took them apart, soaked them, put them back together and synced. Seemed to be lean at idle, I put 40 size pilot jets. It's better, but not good. Seems too rich. No matter what adjustments I make, it just isn't right. For fuel mixture screw adjustment, I ended up with (turns out, from cyl 1 to 4) - 4, 4.5, 1.5, 4. Plugs show rich condition.

I decided to check the valve clearances.

From cyl 1 to cyl 4, left to right, exhaust clearances are

.229 .229 | .203 .229 | .229 .203 | .229 .203

Intake valves are

.152 .076 .052 | .152 .051 .102 | .102 .102 .102 | .127 .127 .127

Do you think those clearances are enough to cause the problem?

A stretched timing chain can compound this problem? How does one test a timing chain wear?

The bike runs very well once the engine is up past 4k RPM.
GinoMachino is offline  
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post #2 of 2 (permalink) Old 12-08-2019, 10:23 PM
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Join Date: Nov 2018
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First off I would start checking for a vacuum leak especially on #3, your fuel screws should all be pretty close to one another.

Presuming valve clearance measurements are in mm, yes? Some of your clearances are pretty tight (intake anyway, some of the exhaust are tight but I don't think they would be tight enough to cause any real issues) and can definitely cause a runability issue once the engine is fully warmed up. Worn/stretched cam chain will usually cause issues related to valve timing being off but it would have to stretch a lot to cause the engine to not run well I would think. Usually you would need to pull the cam chain and measure a distance of links or # of pins to determine stretch, I looked in the manual and I saw no such measurement for cam stretch.

Both problems usually cause low compression runnability issues not necessarily a rich condition, but I would not rule it out entirely. Have you checked float height yet? Also, when you did your carb clean, did you replace the seals in the bottom of the float holder? The way those carbs are designed if those seals go bad it can pull fuel past the jet from inside the carb.

I just put my 750 back together after it sat for more than 5 years and am having sort of similar issues, bike doesn't respond to choke input and doesn't want to start on its own unless given external stimuli like spraying carb cleaner down its throat. Once it gets started and running it idles ok but doesn't respond well to throttle input in the lower rpm range. Once you get it above 4-5k it seams to be ok... checking the float height reviled it was way high.
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