Not Real Familiar with the CB750s ...

[mespock]

Need some help from someone who may know more about the 750K ... I have a 1979 750K with a 1980 750 Custom's engine. I purchase the 79 because it was in great shape but engine was so so ... found the 80 because the engine seemed to be in good shape but the body .. yuck ... LOL .. So after the the build and having the carbs sync..ed ... I've been becoming familiar with the bike.

Youtube Video

On Monday I went a a 125 mile ride ... I took my GL1200 and I let a friend take my 750K.  He said he had a hard time keeping up with me and said the bike dogged up hills.  Plus at 100 miles he had to go on to reserve.

On Friday filled the tank and took it out to see if it was his riding skills or the bike.   After warming it up for about 5 miles I took it up to 60 mph and then found as the the bike struggling to go over 60 mph.  Then up a hill it would really struggle.  This was all in 5th gear.  Next I went to play more with running at about 8000 rpms before shifting.  I could get pretty good response up to 4th gear but then in 5th not so well.   I took it up to 90 mph twice on the second time I blew my muffler off my right side. Enough that they separated at the joint.

I replaced the muffler and then put on about 20 more miles today bringing my miles to 70 since I filled it up on Friday.  I blew air through my muffler to see if there was anything plugging it.   Bike ran OK but still seemed a little restrictive.  When finished I filled the bike and then measured the mileage.  only 27 mpgs.

So .. I'm only getting about 25 to 30 mpg on my DOHC 750.   I'm not really sure how it should perform.  I have been told that I should be getting somewhere from 45 to 50 mpgs, and I expected it to be more responsive.

Also how should is perform in 5th gear?  On flat surfaces it runs great up to 60 but then levels off and will slowly climb ... 4th gear is very responsive.   Going up a hill there it takes everything to hold at 60 mph in 5th gear.  In 4th it climbs fine.

So any suggestions?

[LukeM]

Welcome to our little playground.
You did a great job on the motor swap, and it's running.  That's the hard part.

In a good state of tune, your bike should be getting between 40 and 50mpg, depending on riding style.
Yours is about half that.  Have you tried revving the engine in neutral, to see if it gets near the redline?

You said you traded out one of the mufflers when it was blown off of the exhaust pipe.  You could also go for a short run with no mufflers, to see if it's an exhaust issue.  The bike probably won't run quite as well due to the lack of back pressure, but it should tell you if it's mufflers or not.

Some other things to check:
Battery up to snuff?
Carbs worked on recently?  Power jets plugged, maybe?
Ignition issues?  Plugs, wires, coils?
Sprockets and chain correct for the bike?

Best to grab the Factory Service Manual from the home page of our forum, and go through the Maintenance section.  Check it all.

I'm sure, with a little time, and some money, you'll get it going better.  Keep us in the loop, and we'll help if we can.

Luke M

[shinyribs]

It's possible that your sprockets have been altered far away from stock causing it to be a dog,but that should not kill the top end power, mainly just the bottom end.You should be able to cruise at 75mph+ with ease. You are correct in that your fuel mileage is way off. I dont know what your problem is,but I would guess they are related. Ignition timing is retarded,possibly??? Sorry,thats all i got.

Good luck and Welcome to the forum.

[Truck]

check your fuel lines and/or any fuel filters.  Sometimes they get gunky and that can kill performance and MPG

[mespock]

Thanks for the the advice ... Carbs were done this past May.  I'm working getting a local Motorcycle Guru who has spent time with the Honda 750's, but he's a hard one to track down.

I didn't swap the mufflers, I just reconnected the one that separated.  If on a flat surface I can get it to cruse at 75 and cruise.  It's on the inclines that take too much power away.

I will post my progress.

Oh by the way ... I will be selling off the rest of that 1980 Custom ...