Thunderbird 900 Mikuni carb problem

Ironstrike

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Hi all,

I have a problem with the carbs on my bike. The bike was standing for about 6 months and wouldnt start so I decided to split and clean carbs and fit pod filters.

I stripped the carbs down and did an ultrasonic clean on all parts, rejetted up one size and refitted with pod filters and fitted new coils. It still will not start. It is getting a good spark, it is getting fuel but just wont fire.

I took the pods off and tried it and it seems to have a lot of suction at the back of carb 1 but carb 2 and 3 have hardly any suction at all. I have checked the inlet rubbers and they seem fine and turning over the bike without the carbs seems to have good suction on all 3 cylinders.

It seems to be a vacuum leak somewhere, does anybody have and suggestions about where I should look first?

Any help would be much appreciated,

Thanks
 

Wire-Wheels

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Of course it is hard tell.from a distance what the issue is, but I would suspect you do not have the throttle butterflies set correctly. If they are not closed enough, you will not have suficcent vacuum to draw fuel. Once you get it.is started you can balance them, but you need a good bench setting so it will start. ...J.D.
 

Ironstrike

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Of course it is hard tell.from a distance what the issue is, but I would suspect you do not have the throttle butterflies set correctly. If they are not closed enough, you will not have suficcent vacuum to draw fuel. Once you get it.is started you can balance them, but you need a good bench setting so it will start. ...J.D.
Hi,

thanks for the advice, I did the bench synch with the carbs in the bike so it's probably not perfect. I will take them out and try again
 

Wire-Wheels

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Hi,

thanks for the advice, I did the bench synch with the carbs in the bike so it's probably not perfect. I will take them out and try again
I'd check the one that is working with a feeler gauge and set the rest the same. Also check the pilot screw settings. Bench set them at 2-2 1/2 turns clockwise from lightly seated.

Mikuni's are a bit different in that you can also adjust your needle height. Can't recommend a bench setting for that, but mine are at the 3rd. groove from the top. This adjustment is usually done AFTER you are sure what main jets you are going to end up with..Changing needle height on a Mikuni affects the transition from idle to main jet curcuit. It also affects mid-range mixture. You probably know most of this stuff, but someone else reading may not. Happy tinkering. ...J.D.
 

Ironstrike

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I'd check the one that is working with a feeler gauge and set the rest the same. Also check the pilot screw settings. Bench set them at 2-2 1/2 turns clockwise from lightly seated.

Mikuni's are a bit different in that you can also adjust your needle height. Can't recommend a bench setting for that, but mine are at the 3rd. groove from the top. This adjustment is usually done AFTER you are sure what main jets you are going to end up with..Changing needle height on a Mikuni affects the transition from idle to main jet curcuit. It also affects mid-range mixture. You probably know most of this stuff, but someone else reading may not. Happy tinkering. ...J.D.
All good, now, thanks for the advice, I bench synched again, raised the needle height, went up another set of jets and doctored (butchered) the pods because they were restricting air flow. running nicely now.
 

Rocky

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Good to hear TUP
 

CarlS

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Excellent!! Thanks for letting us know. TUP
 

Wire-Wheels

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Great to hear you are keeping another old machine running. Thanks for letting us know how it turned out. You might post a picture or two of the old gal [motorcycle that is ;) ] …J.D.
 

Ironstrike

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Great to hear you are keeping another old machine running. Thanks for letting us know how it turned out. You might post a picture or two of the old gal [motorcycle that is ;) ] …J.D.
Will do, just have a few little jobs to sort out on it first.
 
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