carb tuning help

I picked up a '73 Suzuki GT250 from West Virginia last weekend, and with the weather finally turning to spring here, I've been messing with the bike a little to get her running better.

Tried to go for a ride yesterday afternoon. Bike started on the first kick, and settled into idle at about 2000rpm with the choke on (too low?). Let it run to warm up for a few minutes while I put my jacket, helmet, and gloves on, and it dies as I get on. berkeley... after much MUCH kicking I finally got it to restart, but its not happy idling at all. It will not hold an idle with the choke off, though it settles in at 1000rpm or so before cutting out.

I did get moving and rode around the neighborhood for a little bit. Twisting gradually from nothing to about half throttle, as I tip in the throttle, the bike falls on its face and feels like its going to die, then catches and runs like a raped ape. Once the throttle is open, response is fine but coming from a closed throttle it chokes. Pulled into my parking spot, let it idle off choke for about 10 seconds before it died.

Got it started again, tried messing with the air screws but that didn't seem to help (started at the factory 1.5 turns, moved up a half turn and down a half turn on each carb with no luck) and it still idles low on choke at 2000rpm steady and still won't idle with the choke off. Pulled the plugs, and they look ok but are coated with oil. There is also a puddle of oil under the tailpipes lol EPA friendly.

Here's the left plug, right cylinder's was very similar so I didn't take a picture.

What I have posted probably isn't enough to get an answer, but I'm hoping someone can point me in the right direction. I have a feeling I have two separate issues, am I in the ballpark? 1) oil pump over-oiling at idle, causing the plug to oil foul, and 2) wrong sized pilot jet or wrong needle clip position causing the stumble at tip in, but am I too lean or too rich?

Carbs are Mikuni VM26's, jetting is stock afaik and the motor is stock except for a pair of unifilters on the carbs.

I don't know much about that particular setup, but it seems to me that the oil build-up could easily be causing the stumble as you open the throttle. Gasoline acts as a solvent, perhaps cleaning the oil off the plugs, and away you go. I'd tackle the oiling problem first, then see if you've killed two birds with one stone. If not, then deal with the carb situation.

A few things come to mind. 1) Get the idle set. see below. 2) get rid of the uni filters and put the air box back in or you'll spend $$ tring to jet it proper nad it will change with the wither dramticly with out the box.

set your mixture screws to 1.5 turns out. warm motor up to temp USE a fan blowing over the front wheel to the motor. shut off pull left plug wire and refire (yes run on right cyl only) now turn right idle screw in untill it just idles. ( if you have timing light with tach on display read the rpms). realy slow idle. shut off. reconect left plug. pull right plug wire. Refire. turn left idle screw as in above to just idle. shut down. reconect. Refire it should idle just right after you rev it to clean both plugs. Next you need to check the cable sync. pull the air filters. stick fingers (one in each) in to carbs. Feel at the top of the slides and carb body now with other hand twist throttle just a wiggle do both slide move at same time? if not adjust the brass cable holder on top untill you feel they both move at same time.

Untill you do this theres no since in messing with jetting or oil ratio.

Best of luck 44dwarf

Well, I took the carbs off for cleaning and I think I found my issue.

Left one came off no problem, nothing funking in the bowls just a little sediment and a little varnish, so sprayed the hell out of it with carb cleaner, wiped the bowl out, sprayed again, then sprayed through the main and pilot jet for a few seconds each. Let dry, reassembled, reinstalled.

Right side, different story. As I pulled the slide out of the carb body, the spring went flying along with a long pin... Turns out this thingy, as shown from the left carb, wasn't pinned in place on the right. Assuming its the slide stop, as thats all I think it could be, that would explain my issue.

I think I have some small c-clips from karting or safety wire that will work at my dad's house that I can get tomorrow, hopefully this will fix it and I didn't screw anything else up lol.

IIRC a 73 GT250 is an oil injected 2-stroke (in the early 70's I had a 69 X-6 & a friend had a 71 GT250 or something similar).

You can never go wrong with cleaning the main & pilot jets & using a fresh set of plugs.

Good luck with it.

Right those pins are the idle screws. Normaly there on the sides but here there on the tops. saftey wire will work but a cotter pin would be best.

44

Cotter pin found, carbs back together, new plugs, let it sit on prime for a few seconds to fill the bowls, and it started on the third kick with the choke on.

New problem, after a little fiddling I did get it to idle at +/-3000rpm with the choke on, but I only left it running for about a minute because the right cylinder got REALLY hot really quickly. Turning the choke off was like hitting the kill switch. If I twisted the throttle to try to get the motor to rev, it struggled to gain rpm and the intake and exhaust note went blatty. I'm guessing I have an air leak somewhere on the right carb (if not both), no more time to keep fiddling today as I have to be somewhere around 5pm.