Can't get it to run more than a few seconds - help adjusting carb?

[motorpsycho]

so from what it sounds like you may have a "speed" or "cns" style carb with the chole plunger thing on top.
I think that it's more of an air bleed than a choke, but it kind of works the same as a choke,
when you do take the top of the carb off, there will be a couple of springs in there...don't worry the choke and throttle cable run through them so they won't go flying.
if you do have this cns style carb, i'm pretty sure that the slide will have those slots in it and the pin inside the carb to line the slide up with. make sure that the slide moves up & down freely. when you pull the cable out to pull the slide out it should slide up & out very smoothly and easily and not be sloppy or binding up.
this problem is just a gues on my part, but by what you're describing, it sure sounds like this is the problem.
what name is on the side of your carb anyway?

 

[FylingScot]

Hi Mighty Goat:

Classic SNAFU with the carburettor.

I have a similar setup but I am on a lightweight steel road frame with the rear tire close to the seat tube - much tighter than a mountain bike. This forced me to put in spacers on the U-clamps to move the engine and jackshaft mount forward so that the jackshaft would be tight behind the seat tube and clear that tight rear tire.

The side benefit was creating a little more space for the air filter forward of the seat tube. I made the spacer by sawing up some aluminum pieces that were part of the clamping system for the engine - they don't get used because you have the jack shaft.

I think trying to space things might be a lot more successful than rolling your own intake so bear it in mind as an option. I would also look to mess around on the air intake side of the carb before going anywhere near the engine intake.

I'll post a photo when I get home.

Good luck.

 

[FylingScot]

Sorry Mighty Goat,
For some reason I thought you had the jackshaft kit. However, my advice still stands, better to keep the standard intake and try to move the engine a little to make room for the carburettor and air filter. I see you have a big aluminum plate under your engine. Can't quite see why you have that and I wonder if that isn't part of the space problem.

 

[TheMightyGoat]

Sorry Mighty Goat,
For some reason I thought you had the jackshaft kit. However, my advice still stands, better to keep the standard intake and try to move the engine a little to make room for the carburettor and air filter. I see you have a big aluminum plate under your engine. Can't quite see why you have that and I wonder if that isn't part of the space problem.

My frame is too big for the supplied clamps. The upright portion of the frame was only slightly too big, so I was able to make the rear engine mount fit it by removing some material from the clamp. The lower mount however, was nowhere near close to fitting the frame - the supplied studs weren't even long enough to reach the other side. I could have replaced the studs with longer bolts, but I couldn't find any 6mm bolts that long. I could have used threaded rod, but I didn't want to attempt tapping the engine to a different thread, not being sure of the integrity of the metal. So, I used the stock holes in the frame where a water bottle holder had been attached, and mounted some aluminum I had laying around, and in the aluminum I drilled two holes to accept the studs from the engine (with some rubber in between). This way it is fully adjustable without modifying the engine itself.

The aluminum does lose me 1/4" of space... I suppose that might be enough for the carburetor to clear the frame in the correct position, but I think the tube I've modified is acceptable.