Choke needed at high speed

[sbosbach141157]

I have a 1983 R80RT with about 27K on the odometer and a recent carb rebuild. The bike has had the emissions rebreather plumbing removed and vacuum nipples capped. Plugs and plug wires are new. Fuel petcocks have been cleaned and rebuilt.
I have an intermittent drop in power at speed, where RPM is limited, speed begins to drop and additional throttle does nothing. This happens after extended running above 4500 RPM and is rectified by introducing some choke. Suddenly power comes back and I can do hills again! Slowly, over 10 - 15 minutes of riding like this, the need for the choke decreases and the bike will go back to normal, (riding high speed without the choke). I've checked everything I can think of and still the problem persists. I have to assume it's fuel delivery related, as the choke has an immediate effect, but I might be missing something. Any suggestions would be appreciated.

Steve in Austin

[skiteach]

Could be nignition related- a rich mixture ignites more easily. I suspect it's not fuel delivery from tank: the choke enriches and if starvation was a problem it wouldn't help to turn choke on. I have seen automotive ignitions that failed in similar ways. My motorcyle expierence is all points and condenser. Which I've seen fail in a similar way as your bike. The best part of this forum is that someone has expierenced most of the problems(if not all) that we see.

[20774]

Maybe holed diaphrams. Were they part of the carb rebuild?

Doesn't the choke pull fuel from a different circuit in the carb than the main central stack? If the fuel in the bowls were low, then getting gas through the normal venturi would get grow weak after a while. But if the choke pulls gas through one of the inlets deeper in the bowl, like on one of the corners, then it would be below the fuel line and would deliver gas.

[osbornk]

I suspect it is fuel starvation. I would look at fuel tank venting, plugged up fuel filter/petcock and low float level.

[dduelin]

This a classic lean mixture example. A piece of trash in the the high speed (main jet)circuit intermittetly causes one carb to run too lean. Applying the choke richens the mixture to compensate. It can get worse to the point the engine needs partial or full choke to run at all.

[sbosbach141157]

Thanks for all the replies. Dave, you hit it on the head. Pulled the main jet, needle jet and atomizer and all were dirty - cruddy with an oily grit. I'll get it back together tomorrow and see if this was the issue, but you were probably correct. My next task is to find how this dirt got into the carb. I suspect air entry, rather than fuel, as I've got two extra inline filters (new). I need to find the entry pronto as I can't have this crap in my engine! Thanks again!

Steve in Austin

[lostboy]

That oily grit comes from the crankcase breather. Cleaning the jet carrier, needle jet and atomiser are standard maintenance.

[lkchris]

High speed problems are most often bad diaphragms.

Bing recommends either one- or two-year replacement intervals.