Spoke to soon

I just made a post earlier today about my headers. In said post I was bragging up how good it has been; starts, idles and runs awesome. Got on it to ride home from work tonight. It started first kick, stalled once (my fault on the throttle as I don't use choke once its running) started right back up and then settled in. I spent a couple minutes putting my gear on and rode to the first traffic light, where it stalled when I pulled in the clutch to downshift into 1st. Luckily the light was red so I had time to find neutral and get her kicked back to life (2 kicks). Went through the intersection and pulled to the side, it would have died if I didn't stay on the throttle. My first thought was fuel taps as I've been reading a lot about how the rubber turns inside, blah blah blah. Also my fuel was quite low. All said and done, it wasn't a fuel delivery issue. I finally decided I wasn't doing any good just sitting there revving it, so I got on the highway and she ran right up to 65 with no problems and was fine the whole way (why I ruled out the fuel delivery). Pulled into a gas station to top her up at my exit and it almost stalled. Filled up and went 200 yards down the street to my apartment where it stalled as I coasted into the driveway with the clutch in. Ran perfectly at anything over idle.

Here's what I've done to cause this. I rode just a few miles yesterday after changing rear brake shoes. When I got back I decided to "play with the air mixture screw". I turned it out until it was idling as fast as it would, then turned it back in a bit. It was very close to were I started so I didn't even have to change the idle. '74 850 with single Mikuni. This is the only thing I've done that would change the way it runs.

Obviously I'll put the air mixture screw back to 1 1/2 turns out, where it was before I got the bright idea to mess with it. But are there any other suggestions??

Thanks,
Ben

You want your idle mixture to be rich; when you find the opitmum air screw setting (highest revs) set the revs down with the slide screw and tune again, when you find the optimum mixture at the revs you like for an idle turn the airscrew in 1/4 to 1/2 turn, raise the slide if the revs drop off, just a bit to compensate.

If you get the right rich idle mixture your take off will be amazingly smooth. Keep in mind that when you start twising the throttle off idle air can enter faster than fuel due to less inertial resistance, if you start with a rich idle mixture you actually hit a point where the mixture becomes ideal (14.1:1, or there abouts) just before you get into needle territory, that's the goal of setting a proper idle mixture.

If you achieve that and end up with an air air screw setting below 1 turn out get a bigger idle jet, if you end up with an air screw setting above 2.75 get a smaller idle jet.

RS

With a Mikuni VM, the manual specifically states to allow 10 seconds pause between air mixture settings. That is presumably to allow it to settle down before making another change.

Mick

You can also diddle the float up some to enrichen so when ya back off the air screw from highest rpm it lands darn close to 1.5 out which is base line all the rest are designed around.

RoadScholar said:

If you achieve that and end up with an air air screw setting below 1 turn out get a bigger idle jet, if you end up with an air screw setting above 2.75 get a smaller idle jet.

RS

Click to expand...

If you mean by idle jet, the pilot jet; wouldn't it be the other way around?

Edit: Mea culpa. My bad, you're right.

Make sure that the pilot jet is clear too. A clogged pilot jet was my first issue with a single Mik and it wouldn't idle (but ran fine at speed).

Hmm I had a problem similar to this where the bike did not like to run at low revs. The story goes like this, bought the bike second hand and had trouble on my first long ride. The bike managed to get me far enough away from home to be a pain. The moron before me put in a new regulator, only he put in a negative earth regulator with the battery still positive earth ( took me a while to trace this one, thought I had a run of faulty batteries). So once the battery had had enough the bike would stall at low revs, if I kept the revs up it ran like a dream. I also think he dicked around with the dymano as he left another treasure hunt for me as well. On the next big ride the bike got me several hundred miles from home this time before you guessed it the battery went flat. The dynamo was polling and starting to fail, I was getting some intermittent charge and by the time I got home the battery was 7v. Thank god for standard points as the Boyer would have given up ages ago. I have now installed a battery charge monitor and running Negative Earth, and yes I swapped the coil leads over for what it was worth, you know the spark leaping in the correct direction etc etc.

I guess what I'm saying is it might be worth checking your battery and charge volts too incase the problem is not carby.

Cheers from OZ

Ugh a loose charge wire connection stranded Wes and me 800 miles form home. It would start and run fine for about 1min. then misfire so bad it blew hot rust and carbon particles into my face following, till impossible to ride so finally just let fate bring a farmer with PU to take him to motel and mid night tear down to find the tiny show stopper.

Another thing that can cause 5000 rpm power limit with analog Boyer is if the timing chain gets loose, such as from a flattened cam lobe or two.

In this instant case I'd sure suspect resin crud in carb jets and internal engine surfaces, ugh.

Thanks for the replies, I really appreciate it.

I'll check the carb I guess. I cleaned the carb, put an inline fuel filter in and brand new petcocks with screens before ever starting it so I would hope nothing got to the carb to plug a jet. I think its more a mixture problem because it will start and idle with no problems at all. It seems that the bike has to be slightly warm to cause the issue, and then it only happens when coming to a light or stop sign where I'm not on the throttle and I clutch to downshift. I went for a ride last night and when I got back, I put it in neutral and it sat there and idled with no problems. It seems to be an intermittent problem. Frustrating.

Could it be the clutch is dragging a bit? It would be able to start and idle fine in neutral, but then once on the road, and I clutch to downshift, if the clutch drags it could stall, correct? I always free the clutch before starting, and it always seems to be quite stuck. Dang, really need to get a belt drive, that would at least eliminate sources.

Suggestions or comments?

Thanks again,
Ben

PS - I'm not overlooking all of your awesome carb suggestions. I can't work on the bike right now, so I might as well consider all options.