83 Suzuki SP100 Carb - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 07-06-2009, 09:58 AM Thread Starter
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83 Suzuki SP100 Carb

Hi Guys,
I'm new to the forum and have come seeking help for a 1983 Suzuki SP100 that I bought for my son. a few years ago.

The bike was in good overal condition except for the carb. The bike would not idle at all. It just reved right up when started.

I removed the carb for inspection and discovered that the pilot screw threads are stripped out of the carb body. I tried making a repair using JB weld and rethreading it. But it didnt hold so I went to a local Suzuki dealer and since the carb was no longer available they sold me a Mikuni VM-20 flange mount carb. The carb seems to be very similar in size to the OEM carb.

I have adapted the card to the bike and now the bike starts right up and idles fine. I thought I had it whipped unitl I rode the bike and found that the thing now surges teriibly. It will run fine unitl I start giving it throttle and then the engine kind of drops out, like it is starved for gas. Then it will come back and go fine for a few seconds and then fall off again. It just keeps doing this surging.

I compared the OEM carb needle with the new carb needle and the OEM was much thinner. I swapped the OEM needle into the new carb and it seemed to improve but the problem persists.

I have done a lot of looking on the internet and it seems to point to the carb needing to be rejetted. But this is where I get confused and am hoping someone with experience would be willing to advise me.

Is the surging behavior indicitive of a lean condition when under half to full throttle?

I am pretty handy mechanically but this is the first time I've worked with Mikuni VM type carb and am at a loss as what to do.

Any help or advice will be very much appreciated.

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post #2 of (permalink) Old 07-10-2009, 07:00 AM Thread Starter
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problem solved - went to Midwest Honda/Suzuki in Sioux City IA and talked to the parts guy about jets for the VM-20 carb. I took the carb with me.

He could not find any info on the VM-20 carb in his books so he had to make a phone call to a vendor tech support line.

We found out the jets for this carb are held in a brass screw that threads into the float bowl from the bottom outside. At first glance the brass screww looks like it would be a drain plug for the carb. Took the brass screw out and there mounted on the end of the screw was the main jet. The VM-20 uses the hex type Mikuni jets.

Since I thought the issue was a lean condition I picked up the next 2 larger jets (the carb had a 180 in it). I went home and installed the largest jet first and to my dismay the surging problem was worse. At least now I knew that I was on the right track.

I headed back to the suzuki dealer next day and got a 170 and 160 jet.

The 160 jet pretty much took care of the problem but there is still a barely noticiable hesitation so I am going to go to a 150.

the old bike runs much better now and is rideable!
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