CB750A project

[TheNoobzBiker]

First time post, hoping to get some answers.

I recently had gotten a 1978 Honda CB750A bike that needs quite a bit of work (carb rebuild, to restore the rusted inside of the fuel tank, new exhaust pipes, oil pan and filter gaskets needing replaced, and more) and i could use some help (im more of a computer IT guy than a mechanic). I just finished rebuilding the carb and noticed the throttle opener has a hose connected to it going straight to the carb. It's my understanding that its supposed to be connected to a solenoid valve then from the valve to the carb. Is it a bypass of some sort? Am i going to need to obtain the solenoid valve some how, and if so where?

 

[dirtdigger]

That is the idle kicker that kicks the idle up when it is put into gear. There is supposed to be a vacuum solenoid and a check valve that activates it. You can make it work without the kicker but you might need to bump the base idle speed up a little bit so it doesnt die when it goes into gear. Otherwise if you cant find it anywhere on the bike you will have to probably look for a used one off ebay. There should be a electical connector for the solenoid on the harness, maybe it is still there just disconnected.

 

[TheNoobzBiker]

That is the idle kicker that kicks the idle up when it is put into gear. There is supposed to be a vacuum solenoid and a check valve that activates it. You can make it work without the kicker but you might need to bump the base idle speed up a little bit so it doesnt die when it goes into gear. Otherwise if you cant find it anywhere on the bike you will have to probably look for a used one off eBay. There should be a electrical connector for the solenoid on the harness, maybe it is still there just disconnected.

Might be a dumb question but, any recommendations on how to adjust or bump the idle speed if i wanted to make it work without the kicker? (wont be able to do so until I get the bike to run, so it'll be a future reference)

Would the bike still run IF i don't bump the idle speed or find the solenoid? (not to drive it, just to get it to start for the first time in 3 years and to bump idle speed)

 

[dirtdigger]

There is an idle speed adjustment screw on the carbs. Yes the bike will run if you dont use the kicker, it is only there so the bike doesnt die when you put it into gear. Just like automotive automatic transmission when you put them in drive the transmission load brings the engine speed down. This kicker just bumps the idle speed up so it doesnt kill the engine when you put it into drive.

 

[dirtdigger]

I hope you have a manual and some tools as you will need to sync the carbs after rebuild and should go through the valve clearances and adjust timing chain as well as clean and adjust the points and set the timing. Would be a good thing for you to do a compression test before spending too much money on it in case the engine is worn out or bad.

 

[TheNoobzBiker]

I hope you have a manual and some tools as you will need to sync the carbs after rebuild and should go through the valve clearances and adjust timing chain as well as clean and adjust the points and set the timing. Would be a good thing for you to do a compression test before spending too much money on it in case the engine is worn out or bad.

I've got the shop manual I found on this forum, and I've got some tools. I'm dreading doing the carb sync lol

As for the compression test, if my understanding is right I just have to push the electric start button and feel for compression in the engine. If so, all 4 cylinders have compression and all 4 feel the same. After replacing the battery and jerry-rigging the fuse box (previous guy cut wires for the main fuse, i just added the new fuse) the bike cranks over and all lights work.

 

[dirtdigger]

You need to get a compression gauge. Just feeling it blow out doesnt tell you anything. It can blow are out of the spark plug holes but not be making enough compression to run. You need minimum 100psi for it to actually run. A healthy 750 will produce 150 to 175ish psi.

The carb sync you need to do with a 4 gauge set while running. You can get them close when they are on the bench but they need to be done when the engine is running because each cylinder does not run identical and require slightly different settings.

 

[TheNoobzBiker]

So back to the solenoid. Is there a reason you could guess, or assume, as to why it wasn't installed and that green hose is connected instead?

 

[dirtdigger]

The last person to work on it had no idea what they were doing.

 

[AutoMac]

When I bought my Japanese market CB 750 A it had already had the gubbins removed from the carbs . To get around this I did a full service on the bike including new points , re set the timing and points gap, new plugs, re set the valves and timing chain, carbs ultrasonically cleaned with new carb rebuild kits from Honda Japan and synchronisation using a home made manometer ( see YouTube how to for more details )

Fitted a new air cleaner and then adjusted the fast / slow idle using the large knurled knob on the carbs ( same as fitted to the later Honda PC 800 V twin ) .

It took a little trial and error to get it just right but now it doesn’t lurch when drive is engaged or creep when in gear. The only time it left me embarrassed was once at traffic lights.

I selected high gear ( being lazy ) and gave it a handful of throttle for a quick get away and was rewarded by the engine stalling .
Since then when I get to the lights I select low gear for quick get aways and rev it out up to 50 MPH before changing up to high gear.

But just setting off at a normal speed high gear pulls without a problem just open the throttle gently but progressively and it is fine it is something I have learned to “ride around” and to me it is a quirk of the bike and not something I need to repair. It also always fires up easily hot or cold but needs to be warmed up well at very cold temperatures.