Very general question

[Stoyle]

So, I bought a 77 cb750 Four, I don't know anything about the history of the bike and don't know much about working on motorcycles. The bike starts with the choke fully closed, if the choke is opened at all however, the engine shuts right down. The bike will rev but sometimes when I give it a little throttle, shuts down. The guy I got it from told me he just completely redid the carbs, cleaned, jetted, what have you, and he said he believed a little patience and a few adjustments would get this thing going. I'm not sure if I actually believe that and was wondering where you all would start? I can read a manual and don't need specifics, just a general rundown of the order I should go about doing things would be greatly appreciated.

Thanks!

[sgtslag]

I would guess the carbs are not configured properly.  Then, did he put pod air filters on it?  Pods can be made to work, but they're, quite frankly, a pain in the ass to re-jet for.  A K&N air filter, or better yet, a UNI, oiled foam filter (much less $$$...) will open up the lungs on the carbs, without the challenges of re-jetting.

I would have the carbs looked at (cleaned), balanced, and a full tune-up.  It's all in the Factory Service Manual (FSM), found on the Home page of this site, free to download.  If you don't feel up to doing it yourself, then take it to a mechanic you trust.

I'd also recommend adding SeaFoam to the crankcase (1.5 oz. per quart of oil), run it on the center stand, idling(!), shifting through all five gears, to flush out the transmission, as well as the engine; allow it to run for five (5) minutes, no fan needed, as the engine will only reach around 180 F.  This is just warm enough to suspend the crud within the oil, to be captured by the filter.  Then change the oil, and the filter.

Add SeaFoam to the gas tank, after you get everything cleaned, and adjusted:  1 oz. per gallon.  This will dissolve varnish deposits within the gas tank, and it will also ensure the entire fuel system is clean.  After that, I would change out all of the fluids in it:  oil and filter (done); fork fluid (relatively easy, refill with Dexron III+ ATF fluid, much cheaper than Fork Oil); flush, and bleed, the brake lines.  Cheers!

[shinyribs]

Sarge is right on track,but i will add: You having to be choked to run issue really sounds like plugged idle jets. I'd give the carbs a good cleaning and go from there. Clean carbs are the most important thing on these bikes,it seems. Good luck with it!

[Re-run]

Pull the plugs and see what color they are. Something is definitely wrong here. So part of figuring this out is finding if the carbs are running too much or too little fuel through.

[Stoyle]

Thanks guys. The bike does have pod filters, I also have the stock airbox. Between the filters you recommended and the airbox what would you recommend? Also, the seller told me he cleaned the carbs and I took the pods off and took them apart and they do appear to be very clean but the stock jets are still in there. Also, the spark plugs are a bit black, I read that this indicates a rich mixture. I've tried to play with the mixture screws underneath the bowls but it doesn't seem to make too much of a difference. Those screws are also really hard to get to! Is there a trick? could the problem be that the carbs are just really out of synch?

[Re-run]

I think you are going to want to pull of the carbs and pop off the float bowls. You need to check what you got for jets in there.
Now, do you have the F model or the K model? They had different jets in them.
Black plugs would indicate too much fuel but it could also be a weak spark not firing things right.
You should check that while you are at it.
Actually, just to make sure you have a good baseline, you should run through the standard tune up procedures.

[Stoyle]

I have the Four model.

[shultz_1978]

hi Stoyle
 It will be a help to know if your bike is a f or k model. But it really sounds like you need to get the carbs off and see what is going on in them. I think the guys are right it seems like you may have plugged idle jets or some thing. You my get the know the bike a little better.  You probably have #35  push in jets and can pull them out with pliers. Spray carb cleaner and compressed air work great for clean the jets.

P.S. The hole in the jets are very small, But you should be able to see light through them. Some times it helps to soak them in carb cleaner.

Good luck

[robs750]

shultz_1978 wrote

 But it really sounds like you need to get the crabs off and see what is going on in them.

Good idea to go to the pharmacy to get something for those crabs!

[shultz_1978]

Fat fingers  carbs crabs

[TOOLS1]

Richard Prior used after shave lotion, for crabs.
TOOLS

[shultz_1978]

TOOLS1 wrote

Richard Prior used after shave lotion, for crabs.
TOOLS

What????  lol

[shinyribs]

That sounds extremely painful...that is a sensitive area!