Power Issues Until Bike is Warm

[phillyc7787]

Hi there, everybody! New member here. I want to say in advance that I appreciate any help anyone can offer on this little puzzle I have on my hands. I recently got my 1973 R75/6 running again (had been sitting for two years) with the help of an older neighbor with experience working on bikes. It needed new points, new battery, a new rear tire, a new front brake master cylinder, and that's about it. I've had the bike back for a few days now and it's been super great riding it around again. That's all of the good stuff...now for the puzzle...

I'm having an issue that when I first fire the bike up and start driving, I reach about 35 mph or so, and it starts to sputter when I try to increase speed. On an uphill incline it's even worse. Sometimes it gets to the point where the bike almost totally dies - just a total loss of power over 30 mph or so. That said, after about 30 minutes of driving it, it seems to get better and I can easily reach speeds up to 55-65 mph without the sputtering issue. Obviously this is a problem, and dangerous, when you start the bike up and pull onto a main road where the other traffic is traveling 55 mph +.

Has anyone ever experienced anything like this, or have any idea what might be going on? Thanks.

[Kurt in S.A.]

Welcome! Doesn't sound like you did anything to the carbs. Sitting for so long, they'll probably need an overhaul. Once that is done and you can warm the bike up for a short ride, you will need to balance/synch the carbs. You should be sure that the choke mechanism is being actuated properly and turned off as needed. Be sure it truly turns off.

Another thing...did you check the valve clearances? They could be pretty tight which is not good for the engine long term.

Kurt

[SteveD]

Needs a carb cleanout and a good tuneup I think.
Initially I was thinking a main jet blockage but not if it came good. Then I thought diaphragm issue but once again, not if it came good.
Then I thought maybe a slide was stuck? Could be that it unstuck after some use?

Have you repeated the problem from cold again?

[Rob Frankham]

It's quite probable that a good tune up of the carbration will imptove matters but I would want to check for minor air leaks at... well... just about everywhere that air can get into the carb and inlet tract, That includes the enrichener (choke) casting gasket, 'O' rings on the throttle spindle, 'O' rings on the mixture screws, 'O' rings on the enrichener spindle, dome to body joint and the hoses between the carb and the head. The reasoning behind this is that a small air leak can make a difference to engine performance when the engine is cool (and requires a slightly richer mixture) but this may be less pronounced when the engine is thoroughly hot and can run leaner. If you're going to strip and clean the carbs anyway, it is probably a good idea to change all of the gaskets and 'O' rings as part of the process.

It would also be good if you could establish whether the problem is on the right cylinder, the left cylinder or both. If it's both, then you might want to look at the ignition system. Carburation problems seldom affect both sides.

Another thought... have you checked the ignition timing. Again, this is less critical when the egine is warmed through.

Final thought (I think)... have you tried applying a liitle 'choke' when the engine balks. If this makes a difference, it indicates that the problem is down to a weak mixture cause either by bad tuning or an air leak.

Rob

[melville]

Check your float levels. If they're too low, under power you can drain the bowl faster than fuel can get past the needle.