123petey
XS650 Enthusiast
Hi - I’m hoping to bounce some of recent experience/issues off some or all of you to see what I can gain. I know there are tons of forums and threads covering lots, if not all, of the issues I’m having plus numerous other guides and resources and I feel like I’ve read them all. Anyway, to the problem - finished bringing a 1981 XS650 from the grave and have the whole 2 into 1 vm36 carb setup. Headers look OEM although someone poorly cut and welded what looks to be sportster or similar looking mufflers to it. Loud as hell. Annoying. Anyway... took a bit to dial in the carbs. Stator, reg/rec look original or at least old enough to be. Bought a tci box. Using a battery but deleted the starter. Kick only. New caps but coil is aftermarket but looks good. So took a while but finally dialed in the carb. Vm36 with 22.5 pilot, 190 main, p8159 needle jet with 6f9 needle middle clip, 2.5 slide. Valves and chain were set. Only bench testing was done so plugs always looked a little rich. Compression was 130-140 in both so decent enough. Left cylinder runs like a top but right seemed to always run intermittently like a harley. Chalked that up to the slightly longer manifold run to that side. Finally took it out and it ran great. On the way home from the last ride it gave one backfire at a light so I thought to raise the main to 200. Couldn’t get the bike to start. Should have left well enough alone. Had to adjust idle and air screw. Backfiring awfully and ran terribly. Put back the 190 and dialed it back in. Idled great. Took it out for a ride and sluggish and backfiring like crazy on decel and dropping rpm at stops. Plugs wet. Took of the air filter and the carb is saturated. Take off the carb and the manifold is pooled with raw fuel. Clean the plugs, dry everything out, let it go for a day. Try kicking the next day and it kicks back twice and blew the carb off the manifold. Take apart carb, clean everything, and verify float level since everything else is good. Way off. Adjust to 18mm. Put bike back together and it ran beautifully - both cylinders firing uniformly - no harley loping from the right. For about 30 seconds or so and then bogged out and died. Everything flooded again.
Sorry for the novel. So my question is the bike always seemed to be running rich despite the the slightly leaner jets. Figured the shorter pipes would want a richer jet but ran much better with lean settings. The rich float height I discovered explains maybe why I was always getting rich plugs. The signal to the cylinders through the one carb is so strong I even think it’s somehow drawing fuel through the main circuit which I why I had such a change in startup and idle when I swapped to the 200. At this point I’m at a loss - why are the cylinders flooding, manifold pooling, getting backfire through the carb, etc. Figure it has to be the electrical system then since the carbs were working great. Coil is fine. Battery was low so charged it and dried and cleaned plugs, carb, etc. Hoping maybe a weak spark. That was yesterday.
What are your thoughts, advice, opinions?
Sorry for the novel. So my question is the bike always seemed to be running rich despite the the slightly leaner jets. Figured the shorter pipes would want a richer jet but ran much better with lean settings. The rich float height I discovered explains maybe why I was always getting rich plugs. The signal to the cylinders through the one carb is so strong I even think it’s somehow drawing fuel through the main circuit which I why I had such a change in startup and idle when I swapped to the 200. At this point I’m at a loss - why are the cylinders flooding, manifold pooling, getting backfire through the carb, etc. Figure it has to be the electrical system then since the carbs were working great. Coil is fine. Battery was low so charged it and dried and cleaned plugs, carb, etc. Hoping maybe a weak spark. That was yesterday.
What are your thoughts, advice, opinions?
Last edited:
