Losing power on one cylinder under higher RPMs

[ebond]

Hi guys, I'm having an issue with losing power in my left cylinder when the RPMs get anywhere above 4000 or so. It's a 1978 400E.

It will idle nicely right around 1200 rpm. The exhaust pressure feels identical on both sides at idle, however the left pipe feels cooler. If I give it some throttle in neutral, I can feel the right side getting stronger and hotter, while the left side exhaust sputters intermittently, getting worse as the revs get higher.

When actually riding the bike, I can feel the left cylinder drop out significantly by feel and sound around 4000 rpm. This happens to be about 25mph riding around town in 3rd gear. So, in order to go any faster, I'm needing to shift up high enough to stay within a lower rpm range. I've heard these bikes are meant to be run much higher than 3000rpm, so I'd like to get this sorted out before it gets too gunked up.

The left side (weak side) plug is carbon fouled, right side looks nice and tan. Could this indicate the issue? Or is it fouled because that side isn't getting hot enough to clean it up?

Any ideas on where to start with this issue? Thanks in advance.

 

[16VGTIDave]

Start with the easy stuff. I'd suggest replacing the plugs to eliminate them as a possible problem. Also check the points and timing. If that doesn't fix the problem, do a compression check. If both cylinders are within spec, remove the carbs and check the float in the left carb to see if it has sunk or is adjusted too high.

Let us know what you find!

 

[ebond]

Thanks for the advice!

I found an unexpected solution. I went in to re-check and set the points, timing, and valve clearance. I opened up the exhaust tappet on the left (weak) side to find that the adjustment screw and nut were missing. I pulled off the top of the case to have a look around and found both the screw and nut sitting there, completely undamaged and intact. I know that they were set and adjusted properly before, because I did it myself when I first bought the bike a couple months ago. Could vibrations have caused the screw and nut to work their way out of the rocker arm? Anyone else ever had this happen? Seems unusual but plausible.

Anyway, I expected that fixing it would remedy the weak exhaust pressure on the left cylinder...and it did! I also reset the timing and points, cleaned and re-gapped the plugs, and synced the carbs and now its running like new.

 

[arfstrom]

I havent heard of it happening, but with the number of revelotions per Minute, and everything else our bikes go though, I suspect your right, Perhaps one of the secure nuts wasn't as tight as spec specified. I am glad its not overly damaged by the tappets!