GS550 Carb Rejet

[Anonymous]

I am trying to work out a rejetting problem I am having. The engine has .90 mains on now and has no power. I am thinking of going back to the original mains but I can't find them. Does anyone know what the stock mains were for a 78 GS550 w/mukuni carbs?

[Anonymous]

GS550 Carb Rejet

Sorry I don't know. I used the search feature at the top of this page and found that info for a gs550 with cv carbs so maybe after lots of searching you'll find what you need. Good luck and lots of patients to you.
Dave.

[KEITH KRAUSE]

Re: GS550 Carb Rejet

I am trying to work out a rejetting problem I am having. The engine has .90 mains on now and has no power. I am thinking of going back to the original mains but I can't find them. Does anyone know what the stock mains were for a 78 GS550 w/mukuni carbs?

I believe the stock main jet is #80. What re-jetting problem do you have? Any mod's? What have you done so far to try to fix this problem?

[Nerobro]

They only came with mikuni carbs. These are the VM carbs right? Are you using pods? A 4-1 exhaust?

[Anonymous]

The exhaust rusted out and I went to a V&H header. After the rejet, the bike will run but it doesn't have much power. Throttle response is sluggish, the engine needs time to work up to the rev. I suspect that the mixture is too rich, although the plugs don't look like they are getting sooty.

[KEITH KRAUSE]

Well, your problem isn't the larger main jets, unless they're clogged.
To allow for a V&H pipe, an 85 or 87.5 main would probably be perfect on an otherwise completely stock 550. A 90 may be a little rich, but you would only have a little less top end and maybe some bogging when rolling on from around 60 in 5th gear, not the sluggish throttle response and slow to rev' problems.
You have to tell us what throttle position you're at when the problem hits or seems the worst.
Also, what have you done so far to try to repair it? Carb clean? Carb rebuild? Do you have a fat blue spark at all plugs? Ignition timing good? If you still have the points, are they gapped and clean and not pitted? Air filter cleaned or replaced? What kind of filter? Is fuel flowing well from the petcock? Any rust in the tank? In line fuel filter?.....

[Anonymous]

Let's see if I can better describe the problem. I have just completed a carb rebuild and rejet. The engine seems to bog down in the lower revs (2000-2500). The throttle posisiton would best be described as just off of idle. Air filter is new, fuel is clean and good. I was gonig to start looking at spark. Points and plugs are probably not in the best shape. I will have a look this weekend. Thanks

[KEITH KRAUSE]

Let's see if I can better describe the problem. I have just completed a carb rebuild and rejet. The engine seems to bog down in the lower revs (2000-2500). The throttle posisiton would best be described as just off of idle. Air filter is new, fuel is clean and good. I was gonig to start looking at spark. Points and plugs are probably not in the best shape. I will have a look this weekend. Thanks

OK. Go ahead and check those things and the other checks I mentioned.
When you say you did a "re-jet", you're saying you only installed the larger #90 mains, correct? The jet needles were not touched, correct? Were the pilot fuel screws or side air screws moved at all ?
If your airbox is stock/not modified and still has the lid attached, then you should only have to install a little larger main jet (as you have) and then some minor adjustments at the pilot fuel screws (underneath).
But before accurately re-jetting, the spark quality and ignition system must be correct as I said. Also, the carbs must be vacuum synched.