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Need HELP with '80 Yamaha DT100G Fuel System

Joined
Jul 13, 2007
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#1
I'm a new guy here. I did an intro and mentioned what was going on.

I've owned this '80 Yamaha DT100G for at least 20 years. It was running when I first bought it used that long ago. The fuel tank started to leak, so I parked it about 15 years ago.

I was invited to do some trail riding, so I pulled it outa' storage.

I repaired the fuel tank. Soldered numerous pin holes in the tank and cleaned the fuel shut-off valve. I also cleaned the tank inside. So that part should be OK.

I pulled off the carb and gave it an intense cleaning. The throttle valve was stuck and there was a bunch of sludge in the float bowl area and fuel passages.

Got the valve to slide, disassembled the carb, cleaned out all the fuel passages with Gumout, fine wire and compressed air.

The small fuel line hoses were cracked and brittle. I guessed where they were supposed to go.

It'll start and run for a short time and then die. Runs better with the choke valve pulled out.

So, here's what I need help with:

1) A need a spec for the float level setting. I'm sure it was disturbed during the cleaning process.

2) I need to know where the small fuel lines go. There's the main fuel line from the tank (no problem here), and three (3) small lines. One small line goes to the oil pump, but I don't know which fitting on the carb it goes to. Where do the other two (2) go? Maybe they're just vent tubes???

3) What adjustments does the carb have other than the idle speed? I assume the large knob on the carb does that by regulating the closure of the throttle valve. I don't see any other external adjustments.

4) Would the run then shut off be a symptom of an incorrect float level setting?

Any HELP, suggestions and comments are welcome. I tired of kicking this thing!!!!!!!!

I know just enough about this stuff to be dangerous, but I have no access to a Yamaha technical manual. All I have is the original owners manual and it tells ya' nothing about the carb except it's preset and take to a Yamaha dealer for adjustment.
 
Joined
Mar 24, 2006
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#2
hi, ill try to help, i had a 73 dt100 back when, i think they didnt change much between then and 1980.

1 on the float level, i cant help but these older bikes were far more forgiving about float level, so you could probably ball park it

2 does your auto lube pump have a bare hose end, a fitting that "plugs" into a hole on the carb/intake boot, or a tiny banjo bolt fitting? mine had the banjo.

3 the carb will have the bigger idle screw, and also a small flathead screw (most likely recessed into the carb body) for the "pilot air" adjustment

4 have you checked your fuel flow? pull the float bowl off the carb with the fuel still hooked up, open the gas and see how much flows out of the float valve . you say it runs then shuts off. how long does it stay running? when it is running, does it run right? also after it cuts out, can you restart it right away or do you have to wait a few seconds?
 
Joined
Jul 13, 2007
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#3
Thanks for a quick response to my HELP question!!!


1) I set the float so the parting line (mold line) on the float was horizontal with the carb top when it's turned upside down.

2) The auto-oil pump has two lines. A large oil line and a small fuel line. Neither have a banjo. They just slide on to a fitting.

3) I'll have to look for those disguised carb adjustments. I didn't see any.

4) It'll run for awhile then stop. When it is running, it sounds normal and has good throttle response. After it stops running I have to wait awhile before it'll restart. Like ya' suggest, I'll check the fuel flow with the float bowl off. Must be restricted.

I still need to know where these small fuel lines go. One goes to the auto-lube pump. I need to make sure I have it attached to the correct fitting on the carb, or else KABOOM!!!!

I'll play with the float level setting when I have the float bowl off.

Thanks again. This'll keep me busy for awhile!!!




ZOMBIE666 said:
hi, ill try to help, i had a 73 dt100 back when, i think they didnt change much between then and 1980.

1 on the float level, i cant help but these older bikes were far more forgiving about float level, so you could probably ball park it

2 does your auto lube pump have a bare hose end, a fitting that "plugs" into a hole on the carb/intake boot, or a tiny banjo bolt fitting? mine had the banjo.

3 the carb will have the bigger idle screw, and also a small flathead screw (most likely recessed into the carb body) for the "pilot air" adjustment

4 have you checked your fuel flow? pull the float bowl off the carb with the fuel still hooked up, open the gas and see how much flows out of the float valve . you say it runs then shuts off. how long does it stay running? when it is running, does it run right? also after it cuts out, can you restart it right away or do you have to wait a few seconds?
 
Joined
Mar 24, 2006
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#4
hi agin,

1 that should be good enough for now, once you get everything going you should go back and set it to spec but for now that should do ok

2 the big side definitly goes to the oil tank, small hose goes to the motor. if it has no banjo or connector of any type it mst go straight to the carb. are there any nipples that dump directly into the carb bore, post throttle slide? look in the end of the carb (the side that goes into the intake/reed boot) do you see any external hose nipples that just go from the nipple and open up on the inside of the carb bore?

of the 3 small hoses that go to your carb 1 will be the oil (assuming the oil inlet isnt molded into the boot or in the case of banjo style, into the cylinder casting) the other 2 are over flow tubes, probably 1 comming from the bottom of the float bowl and one from the upper portion of the carb main body

you dont have a pic of the carb do you? it would make it much esier
 
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Jul 13, 2007
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#5
OK, here's the update.

I looked in the bike owner's manual. It says I have a Mikuni VM22SS carb. The carb has a 3A3 60 stamped on it.

I disassembled the carb again and removed the brass emulsion tube that the main needle runs up and down in. It was plugged up deluxe. Cleaned it up and made sure all 5 small holes were clear. Recleaned and reassembled.

So, now it starts with a little help from ether and idles pretty good. BUT ........

I'll give it some throttle and it'll do OK, but then increases RPMs like crazy and stays there. This is with the throttle slide valve is still in the idle position!!!!!!!! The safety switch and ignition switch won't turn it off until I crack the throttle!!!!!!!!!!!

I still think the small hoses attached to the carb are messin' me up.

There's a brass fitting on the engine side of the throttle slide valve. I hooked the line from the Auto-Lube pump to it.

There's another brass fitting fon the float bowl. I attached tubing to this and ran it up straight.

Then there's a plastic fitting for another small line on the air inlet side of the carb just before the throttle slide valve. Where does this this small tubing go???

I'm referring to this carb, as, "The Carb From Hell"

I'd appreciate some additional technical assistance.

ZOMBIE666 said:
hi agin,

1 that should be good enough for now, once you get everything going you should go back and set it to spec but for now that should do ok

2 the big side definitly goes to the oil tank, small hose goes to the motor. if it has no banjo or connector of any type it mst go straight to the carb. are there any nipples that dump directly into the carb bore, post throttle slide? look in the end of the carb (the side that goes into the intake/reed boot) do you see any external hose nipples that just go from the nipple and open up on the inside of the carb bore?

of the 3 small hoses that go to your carb 1 will be the oil (assuming the oil inlet isnt molded into the boot or in the case of banjo style, into the cylinder casting) the other 2 are over flow tubes, probably 1 comming from the bottom of the float bowl and one from the upper portion of the carb main body

you dont have a pic of the carb do you? it would make it much esier