Mixture screw headache...

[Adam Savage]

Okay, so I have a 1978 (model year '77) xs 750 triple. I had it up and running a few months back only to find that once it got going it leaked oil from the head-gasket by the cup. Albeit, it didn't run the best, but i could at least (if it weren't for the awful leak) ride it and troubleshoot that way. So it got a complete top end rebuild (by a local shop, not myself). I got it back, ran more or less the same but the carbs were really leaky. Went through and cut my own gaskets for everything because I loathe taking carbs off over and over. (I had rebuilt each of the carbs as well, needle, seat, jets, etc)

So, present day.
The choke is not assisting the cold start at all (i found that this probably meant that the mixture was lean), and there begins my problems. Out of frustration, anger and giving up, i have forgotten where they were when they worked well enough to be a starting point, not that those settings were at all correct. Currently the bike does not like to start easily (with some starter fluid and some gassing the throttle it will rev up), once it starts once and warms up a bit it will kick start no problem The choke does not assist the start up what so ever, if anything it keeps it from starting. And when the bike is finally warmed up, the opening of the choke will kill it.

The bike has Mark 2 carbs, airbox is as sealed as i can physically get it, boots are good, spark is good, compression is good.

Any help or assistance will be greatly appreciated, i really just need a ballpark of where to set the screws to begin fine tuning. I feel like i am just chasing my tail with every day i work on it feeling like ive solved the problem, just to start again with a cold engine the next day.

Thanks

 

[grizld1]

If the choke doesn't assist cold start it's probably got nothing to do with the mixture screws; the starter jet in the choke reservoir in each float bowl is probably blocked, the choke fuel pickup pipes may be blocked as well. Click the Tech button and find the link to the XS650 Garage USA Carb Guide for instructions on clearing. To set the fuel screws, turn them in (leaner) or out (richer) until you find the setting that yields highest idle, then go for a ride until the motor warms to full operating temperature. If idle climbs by more than 300 rpm or so from first steady idle after startup and/or the exhaust "pops" under engine braking, the setting is too lean; richen 1/8 turn and go again. If symptoms recur, richen an other 1/8 turn.

 

[angus67]

Griz...i cant hold a candle to your blow torch of knowledge, but ive found that if the enrichner circiut dosnt help start up, and floods the motor after warm up, doesnt that mean the pilot is too rich(big)? And in the case of bs38's, being overly rich with adequate pilots would mean th emulsion tune o ring is trash?

 

[gggGary]

Adam you are braver than I to cut your own float bowl gaskets. I've made more than a few gaskets in my day but that's some fussy work getting those just right. (that might be a hint) Some of the aftermarket gaskets aren't so great either.
Float bowl fuel levels checked? Did you do a quick skim of the float bowls on some fine paper on a flat surface? Hows the engine compression? Have you checked since the overhaul? Is this still running on the points ignition? Getting three sets of points set right isn't easy. angus made a good call out on that emulsion tube o-ring also

 

[grizld1]

Angus, the OP has to use starter fluid to fire the motor, and that doesn't indicate extreme richness to me. A warm motor usually dies or at least gets very unhappy with the choke actuated.

I judge PJ selection on BS series carbs by fuel screw setting and motor performance. If final fuel screw setting is less than 1/2 turn from closed, it indicates that the PJ is too large, or that fuel level is too high due to bad float or float valve (common problem), or that the enricher (choke) plunger isn't sealing the starter jet (rare problem). Another rich symptom is idle speed falling at full warmup. Lean pilot symptoms include excessive rise in idle speed at full warmup, hesitation in off-idle throttle transition, and exhaust popping under engine braking.

 

[bighairyralph]

Adam, I have nothing to offer. When I got my XS650 I had exactly the same problem. I went through those carbs a dozen times and reached out to see if any one had any thoughts. In the end I threw on another set of carbs (with exactly the same jetting and adjustments) and never looked back. Way harder to do with the 750 carbs. I'll keep my fingers crossed for you, good luck!

If you do find the problem, please post the solution here. I'm really interested.