Topic: Anyone else on the board have this happen to them?

[Eydugstr]

My '72 CB750 (4 into 1 exhaust, stock carbs, stock ignition) runs okay around town, but if I take it out on the highway it will run only so fast and then it seems like the power stops, like it doesn't want to go faster.  It happens inbetween 5-6k on the tach, and usually a bit more after it's been ran for awhile.

Any thoughts or suggestions?  Searching the forum I ran across a thread that mentioned semi-plugged emulsion tubes causing a similar condition.  When I installed the 4 into 1 exhaust (I believe it's a BUB, if that makes a difference), I didn't re-jet the carbs to a different size jet (was told the early cb750's were jetted plenty rich to begin with).

[CB750Ken8]

I believe its just power torque curves hitting the flat spot. Do you happen to be in 5th gear when it hit that flat spot in in the 5-6k range? My '78 750 hit a flat spot in the 5-7K range if I get to 5th gear too soon and try to get to top speed only manage to get up to about 85MPH, but in 3rd I can get up to 95MPH and probably shift in to 4th and keep going and so on.

[Old Scrambler]

Most likely the emulsifier tubes but could be scale buildup in the mains...........or low fuel levels in the bowls caused by partially plugged fuel delivery system...........or a dirty air-cleaner...........or a faulty ignition advance.........or the header may require larger mains...........or???  After you check the air-filter and timing, I would get into the carbs. If you replace the jets be sure to clean the emulsifier tubes. I use gas-welder tip cleaners....some use guitar wire. Sea Foam or injector cleaner in the tank may also help........after a long drive.

[Old Scrambler]

Ken8..............check your timing..............no flat spots are allowed, even with pumper carbs.

[MCRider]

I believe its just power torque curves hitting the flat spot. Do you happen to be in 5th gear when it hit that flat spot in in the 5-6k range? My '78 750 hit a flat spot in the 5-7K range if I get to 5th gear too soon and try to get to top speed only manage to get up to about 85MPH, but in 3rd I can get up to 95MPH and probably shift in to 4th and keep going and so on.

You should be able to achieve the fastest speed the bike can achieve, starting from a cruise in 5th gear, at a modest speed, and simply accelerating on up.

If not, you've got something wrong. Running up thru the gears will get you there faster, but top speed should still come from 5th gear, its not an overdrive, it should produce the highest speed without complaint.

[TwoTired]

Propably should tell us what your spark plug tips say about combustion conditions.

Probably should tell us when it has it's last 3000 mile tuneup interval.

Probably should tell us how old the air filter is.

You might even check the head pipes for relatively even heating.  Running on three vs four makes a rather big difference in top end.

Cheers,

[Eydugstr]

Propably should tell us what your spark plug tips say about combustion conditions.

Probably should tell us when it has it's last 3000 mile tuneup interval.

Probably should tell us how old the air filter is.

You might even check the head pipes for relatively even heating.  Running on three vs four makes a rather big difference in top end.

Cheers,

Two Tired - Thanks. 

Spark plugs - Will get them out and post a pic.
3K service - Done about 500 miles ago - Adj. Valve & points gap.
Air filter - The air filter was replaced about 3k miles ago.

It's been a little while since I've check the pipe temp, but will do so again.

Most likely the emulsifier tubes but could be scale buildup in the mains...........or low fuel levels in the bowls caused by partially plugged fuel delivery system...........or a dirty air-cleaner...........or a faulty ignition advance.........or the header may require larger mains...........or???  After you check the air-filter and timing, I would get into the carbs. If you replace the jets be sure to clean the emulsifier tubes. I use gas-welder tip cleaners....some use guitar wire. Sea Foam or injector cleaner in the tank may also help........after a long drive.

Old Scrambler - TY.  Is there any good way to test the ignition advance?

Thanks again everyone.