Dead spot at 3-4000 rpm

[caliph23]

Hello All. I have 1982 R65LS. I haven't had it for very long. Generally it runs pretty well, but there seems to be a dead spot in the 3-4k rpm area. It
is not massively pronounced, just down on power. I can power on through
in the first 3 gears, but if you are cruising the back roads at between 55-65mph and roll the power on not much happens and because of the way it is geared shifting down to fourth isn't much better. Above 70mph everything
is better, but that can be little fast for where I am riding.
 Is this a power band issue or a carb issue. Any advice would help. Thanks

Any other forum members in the Athens, Ga, area?

[Barry]

Sounds like you have a weak mixture in that 3000 to 4000 rev band.

3000 RPM is just where the needles have begun to lift so it's worth checking they are in the correct slot and perhaps try raising them one slot.

My needles are supposed to be in position 2 but set like that I get the same symptoms as you are experiencing. Not too bad when the engine is really warmed up but still a definite dead band where the engine is running weak. Curiously the period road tests from when the bike was new report the same issue so they were never set up properly or rather they were compromised to meet emission tests. 

Moving the needles up one slot is quite a big change but in the end that's what I did and it transformed the way the engine pulled  through that rev band.

You might reasonably say that above 4000rpm the mixture was already OK and raising the needles is going to make it excessively rich but the effect of raising the needles is not at all linear across the rev range. It makes a big difference for the initial few mm of needle lift but by the time the needles are on full lift the effect is minimal.

[montmil]

What Barry said. [smiley=thumbup.gif]

Some years ago, I did a complete overhaul of the 32mm Bings on my 1981 R65. If a part could be removed, it was replaced with new. On the initial test ride, I experienced the same symptoms at lower revs. After a quick alteration to the needles, a one-notch change, the engine has performed flawlessly. Easy experiment. Give it a try.