Help! I can't figure it out.

[nzsailor]

I have a 79 XLS Sportster with electronic ignition which is driving me nuts. If you ride it sedately it’s fine. If you get on it in 1, 2 or 3 it’s fine. If you get on it in 4 at under 80 kph (50 mph) it’s fine. If you get on it over 110 kph (67 mph) it’s fine. BUT…………. If you crack the throttle wide open in 4 between 80 and 110 kph you completely loose one can. Sounds electrical. Complete cut out. If you close the throttle back down the can cuts back in. If you advance the throttle slowly it’s fine. If you don’t close the throttle back down the remaining 500 cc’s are still enough to make the bike accelerate slowly. When it finally gets above 110 the can cuts back in dramatically and you’d better be holding on.
I’ve cleaned the carb thoroughly, have good fuel flow, replaced the plugs and lead wires, tried plug gaps all the way from 0.025 to 0.60 in. The gap twixt the pickup module and the little cam thingy is correct at 0.004 in. It seems to be slightly better with the air cleaner removed but not markedly and that may be my imagination. The plugs show normal deposits after use. The bike has good compression and doesn’t burn or leak oil. The timing is set close to spec and I’ve played with it near 10 degrees both sides. Centrifugal advance sure looks to be operating freely.
Thanks for any help.
Sailor

[flathead45]

souldn't be like that at all , the coil fires both plugs at the same time so if it fires on one it sould fire on the other

sounds more like push rod addjustment to me , or maybe a leak in the intake manifold

kinda hard to tell without seeing or hearing it thou

[GRAYFEATHER]

Do you have a loose lead conection some were this happend to my 750 duke and I found that an earth conection vibrated loose at 140 ks when I slowed down the conection would remake contact,after a quick fix ths bike was fine.

Regards Grayfeather.

[Gone]

What kind of carb are you running

[nzsailor]

Thanks for the quick responces. The pushrods are adjusted correctly. I've checked for loose wires and bad connections but don't see how they could be sensitive to throttle position. I've wondered about an intake leak but can't understand how it could be affected by throttle position either nor how to check for a leak that only occurs twixt 80 and 110 k's. Carb is a Keihin stock OEM. It's been cleaned and the passages blown. With the throttle position factor I keep thinking carbureation. But how does that affect only one can. Other than this the bike is sweet as sugar.

[Gone]

lose the stock carb or get a bigger jet,your probably running the bowl dry and only have enough fuel for ,most likely,your front can.i have a 76 ironhead and i was running a bendix,same prob,went to a screaming eagle and it can handle everything i give her.

[nzsailor]

jester
how does it only go to one can
and how does it turn back on when i hit 110
been havin a lot of probs getting parts to fit
been lookin for a new carb
thx

[nzsailor]

im in new zealand

[Gone]

Sailor
I have a 1980 and I went to the S&S Super E. It does the job. I have seen Sportsters
fall on their faces with the stock carbs. It is usually a jet problem!!!!