88' yam 90 hp does not want to idle consistantly

todd0872

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I will try to keep this short but give as much info as possible. I purchased a boat with this motor on it but it was not rigged just bolted to transom. Owner had taken motor off boat and had carbs rebuilt, waterpump, and trim motor replaced. On the way home from mechanic, the owner dropped motor out of the back of his truck and broke the pan which has since been replaced. Unfortunatly, this man as passed away and this is why the boat was sold.

I hired the mech that did the work to rig the motor for me and get motor to run. We were able to get motor to start but it did not idle very smooth. I changed the plugs and water tested. Found motor would idle just good enough to stay running but ran great at mid rpm. At top end motor would start to surge then want to die unless you pulled off throttle to middle. Problem was fixed by replacing cracked cap on fuel filter and also found top carb was missing one of two mounting nuts. Motor had no issues at that time after water test.

Last weekend we ran about 25 nm in the boat with no problems. When we got back to the launch it started having trouble with idle and staying running. During flushout motor was chugging. I adjusted carb screws byabout 1/2 turn and motor smoothed out.

Yesterday he'd boat out for same trip. Saw fish feeding so stopped for a minute. Had a really hard time getting motor to even refire. When it did, it seemed to have been flooded but it did not put out blue smoke. Motor will not idle now for very long at idle speed before stalling out. When this happens it shuts down very hard with the motor jumping. Not smoothat all.

Sorry for long post but I know details are important.

Mixed new batch of fuel last night, no change. Smoke when motor does fire is white. Still seems to run pretty good at mid rpm. Keep stabil in tank. Have had boat running and using it for about 4 weeks now. Thanks in advance for any imput!
 

todd0872

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Re: 88' yam 90 hp does not want to idle consistantly

After reading more threads, could this be fuel pump? Maybe ignition?
 

99yam40

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Re: 88' yam 90 hp does not want to idle consistantly

could be fuel pump easy to check but sounds like trash in carbs if you had to adjust
 

todd0872

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Re: 88' yam 90 hp does not want to idle consistantly

Thanks for your input! I thought I saw a thread earlier but can't find it now with a easy way to check the pump. Any ideas? I think it might have been "Rodbolt" but not sure. It also seems to be worse when the motor gets warm. As for the carbs, you are thinking like debris possible in carbs. Do you think this will mean another rebuild or is there someway to do a "simple" clean?
 

todd0872

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Re: 88' yam 90 hp does not want to idle consistantly

It now appears that the top cylinder is not firing. It seems to run the same with the plug wire disconnected. I don't know. It is getting frustrating.
 

99yam40

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Re: 88' yam 90 hp does not want to idle consistantly

Is it sparking but not firing?
running a cylinder lean will eat a piston, so I always make sure compression and spark are good and then look at fuel.

Remove the pump from block, but leave hoses hooked up. pump the primer bulb and see if fuel leaks out of the port that normally is attached to the block

You need a 10 micron filter/water separator to keep stuff out of carbs installed if you do not have one already.

Pulling carbs, disassemble, and blowing out may do the trick. Just make sure to do the link and sync after put back on.
Yamaha manual will give you the info needed to do this
 

todd0872

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Re: 88' yam 90 hp does not want to idle consistantly

I will start with compression tomorrow. Thanks for the help!
 

todd0872

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May 19, 2010
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Re: 88' yam 90 hp does not want to idle consistantly

Ok so I did a compression test on motor today from top to bottom was 115,100,115. Top cylinder is the one I believe to be the problem. I grounded plugs and they all seem to spark the same. When I remove the plug wire I do not hear any difference in the way the motor runs. Bottom two have obvious sound difference. Ithink I have done all I can at my level so unless anyone has any ideas it will be off to the dealer to check. Thanks
 
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