Topic: CB750 K7 Engine Revival

[mkoski]

Hello all,

I'm new here to the forum but have been reading for quite a while.

So I recently picked up my bike (77 K model with 20000 miles) and rode her home (which was a 80 mile ride) and right off the bat I noticed a few things were very wrong. The bike did not like starting at all and would not idle whatsoever until warm and then it would only idle at 2000+ rpm. Next, the bike stopped producing power just before WOT. It seemed to bog down and I had to back off the throttle to maintain speed. Last, my largest concern, is that my top speed was around 60 mph in 5th gear at 7000 rpm. This, I feel, is the only problem that doesnt have anything to do with the carbs, which I plan on rebuilding right away. One thing to note is that the bike had pods installed and the PO didnt know if the bike was rejetted, so I assume this is causing many of the problems.

Anyone have any idea as to why my top speed is so very low and if I need to lean it or richen the mixture? Could it be sprocket changes or anything else? I have read the manual through and this is still something that doesnt make sense, as the bike goes much faster than this...


Thanks,

mkoski

[70CB750]

3000 miles maintenance is a good start.  High idle is often associated with dirty carburetors, my K0 will idle at solid 1000 when clean and 1500 when there is a dirt in the idle jets.

[calj737]

Almost every bit of what you've described points at the carbs. Might also check ignition and plugs, but that should be dealt with under the 3k service as mentioned.

[flybox1]

3000 miles maintenance is a good start. 

+100
download the shop manual here. http://www.honda4fun.com/materiale-documentazione-tecnica/shop-manual
scroll down to the 750 get all sections.
go thru everything in the Scheduled Maintenance section.

[mkoski]

Thanks for the replies guys! This weekend I'll do up that maintenence as per the manual and see what happens. I'm still curious about the top speed though, as I feel like that has nothing to do with engine performance. That high of rpm in 5th gives a speed that is way too slow I think.

[flybox1]

if the carbs are clogged, or youve got a ishtty air filter, you'll not have the proper air/fuel mixture at WOT.  you'll feel the bike bogging down.

 a few other things can decrease top speed, but the bike still revs easily....
improper sized rims and tires.
poor choice if chain/sprockets (if replaced by the PO)

[mkoski]

Alright, so I found the problem... I was running on three cylinders. Im impressed the bike did so well like that! I gotthe fourth going by adjusting the spark lead. The cylinder that wasnt run put out a decent amount of white/gray smoke but that tapered off and now she runs better. Much more power! I still need to pull and rebuild the carbs from how the bike is idleing, however. Thanks for all your advice! I bet she will run great after the 3k mile tune!