Forum

[John]

Power Loss
A week ago,the bike has been suffering from a loss of power when accelerating through each of the gears, as if it has a dead area and the revs flatten out just before the point I'd generally change up gear. This happens even running on the level. It does it continuously when running up hill, yet if I pull in the clutch, revs increase, and immediately drop when I let the clutch out again. It's as if something is hanging on to the back end and holding it back. When I run the bike down the hill, the bike runs OK so long as don't accelerate over much. I re-set the timing, which is a major operation, thinking it was retarded, but this made no difference. When stationary, the tick over is fine, and blipping the throttle, the engine sounds great. The petrol taps are blockage free,and I have cleaned the carb. I've just ordered a couple of spark plugs, and shall see what happens when they arrive. In the meantime, has anyone any ideas.

[Mark M]

John, is the choke on or partly on? Look up the bellmouth, open the throttle all the way and make sure that the throttle slide and the choke slide disappear completely when fully open. Otherwise, have you recently filled up with cheap fuel from a supermarket, say? Is your advance retard lever operating through it's full arc, take the cover off the end of the magneto and check it rotates all the way round the slot. Hope it's something simple!
REgards, Mark

[Chris]

I have a WDCO and have re-built the engine a couple of times.

Did the problem happen suddenly? Is the exhaust valve burning out? (I use added lead in mine). Re-setting the timing does need some care, did you use a dial guage to measure 3/8" BTDC? Why would you suspect the timing, unless you haven't been running it regularly over a long period.

When you say clutch in and out. Is that when out of gear? If the engine drops with no load, is there some gearbox snag causing drag?

MArk M's tips are good, check the smallest, most likely things first.

[John]

Thanks Mark. The advance lever does travel all the way, and the action can be seen to do so when the cover is off the magdyno. I did fill up with regular grade petrol, as I usually do, at the Regent filling station, and I fill my Bullet 65 with the same with no ill effects. There is no miss firing, the effect being as if the revving has a limit, and literally slows down. On the level, it can go lack the clappers, then suddenly repeat the holding back, as if the fuel is being rationed. I shall strip the carb again tomorrow and give it another blow through with carb cleaner. If I discover the remedy by trial and error, I'll post my finding. In the meantime, the hints are useful.

[John]

Thanks Chris. I set the timing by lowering the piston 3/8". The clutch is pulled in to release the gears, and the engine speeds up again, and slows down immediately the clutch is released. When I'm revving up to the point when I consider changing from 2nd to 3rd gear, it holds back then. As I just mentioned before, I shall have another look at the carb. Because there's no miss firing, I feel it might be fuel starvation. I forgot to mention, it can fire back when kicking it over to start, something it wasn't doing before. I felt this was a build up of fuel in the combustion chamber, exploding on first spark.

[John]

Hi Mark, I shall also check the choke slide, but judging by the sound of the engine, the choke slide is withdrawn, however I did not check it visually with the throttle wide open. I shall do so before I remove the carb.

[John]

I have solved the problem. There was a partial blockage in the main jet on the carb, which I suspect was a fragment of fibre scraped off the large fibre washer used on the seal of the larger flange union next to the jet. This union is not easy to tighten, and I guess this is how the fragment came to come loose. I cleaned all the jets and passages with carb cleaner, and now the bike runs trouble free. Thank you for your hints Mark and Chris, and I hope my experience and result of my find, together with the useful suggestions has been valuable to anyone with a similar problem.

[Chris]

I'm glad that you have found the snag. That happened to my WDCO once too. Make sure you thread-lock the union and screws on the carb, it saves finding them loose one day!

On the timeing. You say you lowered the piston by 3/8", but due to back-lash in the magneto drive you need to rise the piston to 3/8" BTDC to eliminate the back-lash and set the timing then. Does that make sense?