Topic: 1970 CB750 question about speed and power

[TXmudman]

With a 230 pound rider on my bike, it seems that I'm really out of power when driving into a mild headwind while at about 80 mph in 5th gear (5000 rpm). Downshifting brings about a little more power, but I doubt that I could get to 100 mph if I tried.

My question is, does this seem about right for our bikes, or am I lacking in power?

Timing and dwell are correct, bike seems to run fine.

[STLrocker]

you are lacking for sure. a stock bike should be good for about 125-135. i have a 76 750. i only weigh 165, but it is never out of power and i dont even think of downshifting at 80mph. ive been well over 100 plenty of times. uphill, headwind, with a passenger, it dosent matter.

what is your history with the bike? if you recently got it, have you done a full tune up? any other running issues? there has to be a problem somewhere.

[GroovieGhoulie]

It should have more oomph than that. 

I weigh 185 and on my 400F, which is down 30hp (but also 100 pounds) from your 750, I can see 80 all day and the ton if I hug the tank like a cafe racer.  The 750 should easily carry a 230-pounder to the ton.

[TXmudman]

what is your history with the bike? if you recently got it, have you done a full tune up? any other running issues? there has to be a problem somewhere.

Thanks for the response.  My history with the bike is long.  My dad bought it 33 years ago.  I drove it a lot as a teenager, then it sat for 20 years.  I recently got it going again.  After that long, it's hard to remember exactly how it felt at speed.  It does seem to accelerate off the line about how I remember, so maybe I'm starving for fuel at speed?  Timing is good, advancing correcly.  Sounds great.  I'll re-check everything, including tank screen.  Thanks.

[STLrocker]

well, glad you got it back on the road after so long.

 if you didnt do it alredy, a full tune up is a must. new plugs and check the wires for corosion where the caps go on, adjust cam chain, valve adjustment, synch carbs, oil, air filter, timing (done), points, condenser. this sound obvious. maybe you've alreaady done it all.  if that is all done, it can really help to find other problems by eliminating a lot of common causes.


the petcock screen is a good idea to check. it would also be good to put an inline filter on if you dont have one.

[Bob Wessner]

The gearing sounds a bit tall, are the sprockets (front/rear) stock?

[Bob Wessner]

Are the sprockets (front/rear) stock? I ask because I think at 5,000 RPM I believe you should be doing 75 MPH, or at 80 you should be turning 5,500 RPM if they were stock.

[TXmudman]

About the sprockets, yes, they are stock, and you are right, the RPMs are more like 5300 rpm.  This is a sandcast, in the 5000 serial number range.  About 10,500 miles on it.  New paint, everything else original, except pipes.  I just put a new 530 chain on it.  It still had that original 400 series chain on it.

About the tuning and fuel filter idea, I have filters in the lines now.  I am going to remove them and see if they're causing a restriction.  I had some moisture problem when I put them in, but I think that I can do without them now.  Everything was rebuilt.  After sitting for 20 years, the wrinkle tank is a rusted mess inside, and the float bowls were corroded with the worst varnish I've ever seen.  The tank on it now is one I got off of ebay, everything else is original. 

I'll pull the filters, check the carbs, and post again after I take it for a ride.

[KB02]

I'm having the same issue with my bike and am also suspecting fuel issues. Keep us posted and let us know what you find.

[Bob Wessner]

Wow! Nice job, great looking bike. Hope you get it sorted out, sounds relatively minor.

[TXmudman]

I pulled the filters.  I think I was starving for fuel.  It went 90 in 5th gear without too much effort.  That is a major improvement. 

Thanks for all the input.  This is a great forum!