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I went out to try and find some of the old style Yamaha carb cleaner for my project bike. While out my 81 started acting up.

It was like it wasn't running on all four cylinders..hard to get above 40 MPH, like I ran out of gas but I had 1/2 a tank. So I stopped Filled up and took off it seemed to help but then it started again..putting along, hesitating etc.

As I tried to make my way home I realized that if I kept it under 4,000 rpm it seemed to run fairly well so I shifted up and buzzed along still keeping it 40 MPH and no higher..every time I down shifted and the RPM'S went above 4,000 it started missing..it only backfire twice out of the 2/4 side. Sitting at idle it purrs like a kitten. It will start right up again every time I shut her off. So any suggestions as to where to start. It seems higher fuel demand causes the problem.
 

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It would appear you have answered your own question it sounds very much like fuel starvation.

Make sure the fuel cocks are clean aswell, many have a little gauze filter which blocks just as easy as the jets in the carb.

thats my 2 bobs worth...:coollep:
 

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Looking at your spark plugs might tell a good story.

I'd also check the timing advance hose. Does it do this when you're sitting still, over 4k? Or only when "under load"?

Then see if your fuel filter is stopping up. Check the fuel pump output volume. (I note you were out getting carb cleaner... Could be a float valve?)


That should get things started.
 

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Just for grins try my old stand by trick which helps if its a float hanging up.

Turn the fuel off and let it run completely out of gas til she dies, turn the gas back on and give 'er a try. This has helped a few times for me.
Then if not, try tapping the bowls with the handle of a screw driver.
Both worth a try and dont cost anything.
 

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I looked at all the plugs all look great light tan ..except #2 it's white/gray running a little hot..IS?? the solution to turn IN the air mixture screw 1/2 turn at a time so it runs a little richer? If so any trick to getting in there to get at the screw (my limiters are off)

I will try the things above starting with the easiest..check adv. tube for leaks, tapping bowls check, then run bowls dry..and refill, last check fuel filter, if all that fails anything else I can try before the dreaded pulling the carbs routine????

Add fuel cap off..check that out too.
 

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maybe try running her with the fuel cap loose to check that the tank isnt vacuuming itself,if the fuel cap vent gets blocked it causes a vacuum in the tank and stops the fuel being pulled through properly

You will be able to tell if its vacuuming because it will hiss loudly when you undo the filler cap after a run (as the air gets sucked into the tank)

Hope this helps TG
 

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RB wrote:
..IS?? the solution to turn IN the air mixture screw 1/2 turn at a time so it runs a little richer?
To richen the mixture, turn the pilot screw out, not in.
 

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Ok you guys won't believe this. So I go out to start checking stuff, adv line ok, so I am going to run the bowls dry I take her out to run up and down the street..she runs great !!! Well for about two miles then she starts to sputter..so back home I shut her off and then try to start her again..nothing not lights etc..so I go to the main fuse sure enough it's cracked..this fuse is only 2 months old.

So I guess it could have been the fuse losing contact at the break at certain speeds..so I am going back out to run her up and down the street again to see if she "fixed". By the way I bought two extras when I brought the fuses two months ago..all they had in stock..for a$1 I figured what the hell. Of course this just could be consequence and now I have a fuse issue too!!!

I'll be back later to fill you in.....
 

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replacing the fuse seemed to have fix it..I do some more test riding later today when it's cooler and my wife's home to rescue me if I get stranded. But a very weird result if in fact it was only a cracked/busted main fuse.

It sure seemed like a fuel starvation issue of some sort. Well I hope all it was was a cracked fuse..it wasn't burnt or discolored in any way just cracked in two and only two months old!!! I guess at certain engine speeds/vibrations the crack opened up more causing the crappy running and a little backfiring. Glad it broke completely before I started tearing the carbs apart!!! Let's hope that was really it!!!

Thanks guys for the fast responses.

Andrew out for richer..how does that work?
Not saying that's wrong just it seems if one screws it in it plugs the hole tighter I guess that makes sense if air actually be sucked in around the screw past the washers and o rings. Is that how the it works? But, I thought the air mixture screw as it went in opened something up inside to let more air in..wrong I guess???? I couldn't find any detail picture to show me how it actually works.
 

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I may, Just not big on soldering and I don't want to do a major rewiring, electronics are not my thing. But I'll go get a blade fuse box and try and solder up some eyes on the leads and screw them in place. While I am at it I may try and solder my yellow wires I have cut then and spliced them together for the time being. I know I know not much better that the connector..
 

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Well, I have two old soldering irons which may actually be old wood burning irons but they metal solder. I if don't get it done by then that's a definite possibility..:cool:

Thanks for the offer..
 

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Endwell_Tim wrote:
Then see if your fuel filter is stopping up.
Nice call Endwell_Tim! Its' got my vote..................................

~teacher~~~
 

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RB wrote:
I may, Just not big on soldering and I don't want to do a major rewiring, electronics are not my thing. But I'll go get a blade fuse box and try and solder up some eyes on the leads and screw them in place. While I am at it I may try and solder my yellow wires I have cut then and spliced them together for the time being. I know I know not much better that the connector..
You really don't have to solder anything, just attach the wires of the blade fuse holder to the two screws that used to hold the dog bone link. Make sure the fuse holder is rated for 30A, some are rated 15A and will eventually melt.

As Teacher said, you might want to look at your fuel filter.
 
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