Topic: 1972 CB350 Twin, bike loses power at around 4-5k rpms.

[BenjaminCorey]

Yesterday I was running some errands, the bike was running excellently. I went into a store for about 15 minutes, and when I started riding again the bike is acting all gurpy and loses power at around 4k RPM. Extra throttle pushes it to around 5k RPM, but that seems to be the limit. The bike revs fine in neutral, or with the clutch pulled in, but hits that limit at the same RPM in every gear.

Is this a carb problem? or something else.

[mlinder]

Check for air leaks. Also, timing can go out fairly easily and quickly on the 350's. Be sure to check it.

[BenjaminCorey]

Okay, I just took it for a quick ride.

I pinned the throttle and pushed it through that dead spot, and hit a bit of a surge of power, but mostly it just sputters and runs really badly at higher RPM. After a little while riding like that, the idle stopped dropping and even stayed pinned at around 5k RPM at one point. I turned the gas kill switch and rode it for about 2 blocks before it returned to normal. Seemed to run a little better as the float bowls emptied, but that could've just been my imagination.

It seems strange that the timing could get so far off from one minute to the next, the bike did see a little bit of rain while I was in the shop, but it wasn't very much... just a handful of drops.

Any tips for when checking for air leaks? I'm still pretty new to this... I'm sure the clymer will help me to check the timing though.

[mlinder]

Once the timing slips, it slips. It may get worse, but it goes out pretty much right away once it slips.
Sounds more like air fuel mixture though.
Make sure the floats arent sticking, and clean the air filter.
You can check for air leaks by spraying carb cleaner or similar on the carb boots while its running. If RPM changes, you've got a leak.

[crazypj]

Either busted diaphragm's in carb's or the main jets fell out.
 Probably dropping a cylinder and running on 1
PJ

[tbpmusic]

Okay, I just took it for a quick ride.

I pinned the throttle and pushed it through that dead spot, and hit a bit of a surge of power, but mostly it just sputters and runs really badly at higher RPM. After a little while riding like that, the idle stopped dropping and even stayed pinned at around 5k RPM at one point. I turned the gas kill switch and rode it for about 2 blocks before it returned to normal. Seemed to run a little better as the float bowls emptied, but that could've just been my imagination.

It seems strange that the timing could get so far off from one minute to the next, the bike did see a little bit of rain while I was in the shop, but it wasn't very much... just a handful of drops.

Any tips for when checking for air leaks? I'm still pretty new to this... I'm sure the clymer will help me to check the timing though.

Could also be a sticky spark advancer - they can stick open or closed.
Could cause the symptoms you speak of - I've had this problem, it can be confusing, things just don't make sense at all. You end up chasing your tail for a long time.
One minute it runs fine, the next it's way off. You let it cool down so you can actually touch the advancer, and everything looks ok - but you check the timing and it's way off.

The Clymer is worthless - here's the Honda manual-

http://home.comcast.net/~tbpmusic3/CB_CL_SL_250_350.pdf

bill

[mlinder]

Good points I forgot to mention, guys.