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1979 CB750F Power Issue

woodlandsrider

CB750 Member
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Location
Austin, TX
Hey all,

So I was out riding the other day and noticed the bike had almost a non-existent idle, unless I was giving it a bit of gas it would die. Then when at a light it had died because I wasn't paying attention to giving it gas to keep idling (Which I know should not be the case on a normal basis). Then I went to start the bike back up and it had to sit for a couple minutes before it'd turn over and have enough juice to fire up. This is the second time this has happened. The bike runs great when keeping the RPM's up, super responsive to throttle, etc. The bike started right up after not being run for 4 months and without battery tending. Then after it died, I charged it back to 100% and again it starts right up, which leads me to believe that the charging system is to blame, just don't know where. Additionally, the bike had died on me after I was riding at fast speeds and repetitively high RPM revs, which also points to the charging system I feel like.

That all said, I've got plans to proceed with testing the alt, reg/rec, etc., but first wanted to check and see if anyone had any guidance or had been in this spot before.

Lemme know if anyone has any insight!

-Sam

_________________________
Stats

Year: 1979
Model: CB750F Super Sport
Battery: 423808 12v 14aH Shorai LFX Lithium-Ion Battery
Charging System: Factory
 
The dying issues you bring up could easily be lost compression as nobody ever sets the valves on 1st gen DOHCs and then they expect a 40 year old bike to still run perfect. Dying at idle or letting the throttle go to drop back to idle is a low compression hallmark. Clogged idle circuits in carbs come to mind too. Those are famous in ethanol Texas.

' Additionally, the bike had died on me after I was riding at fast speeds and repetitively high RPM revs, which also points to the charging system I feel like.'

Even brand new they'll do that if you are simply letting the throttle drop back to zero after a max rpm runout. BTDT on perfectly running ones that otherwise would idle all day long. The engines have very little flywheel weight and the rpm drop has enough impelling inertia to keep pulling them all the way down to zero.
 
Hey amc49,

Thanks for the reply! I performed a compression test and she's putting out 140-150PSI on each cylinder. I've rebuilt the mid-top end, set all the valve shims to their needed widths.

As far as the dying on me, it seems more so electrical as the bike was running great, super responsive, sounding great, then at a light I wasn't paying attention to the idle and it got too low and died. The oddity is that I went to start it back up and it was failing electrically, sluggish turnover on the engine, lights flicker, etc. I then waited about 2 minutes and then tried again, started right up and rode all the way back home (5 miles or so) with no issues. Then get it back and the battery was drained.

So I'm thinking it's one of the following:

1. Need to replace regulator/rectifier with one like Ricks Motorsports Electrics Lithium Ion Battery compatible Reg/Rec; https://ricksmotorsportelectrics.com/Lithium-Ion-Battery-Compatible-Rectifier-Regulator-14_103 (Since I'm running the Shorai Li-Ion battery, I think the standard reg/rec may be causing issues)

2. Review/possibly replace stator, if it tests bad

3. Battery may be defective or was cooked with older reg/rec

Thoughts?
 
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