Advice On a 79 CB750 That Just Won't Start - UPDATE, the bike STARTED!

[fallawc]

Hey everyone,

I want to preface this post with open acknowledgement that I am very much a motorcycle beginner who is doing my best to learn. The 1979 CB750 (10th Anniversary) I purchased about a year and a half ago hasn't started in over 20 years. Since I purchased it, I've used the Honda Service Manual and completed the following:

• Changed the oil and replaced the oil filter.
• Changed the spark plugs and verified all four have spark (they do).
• Check and adjusted the valve clearances for all cylinders.
• Cleaned, disassembled and rebuilt the carburetors (twice) using a Randacks kit.
• Cleaned the gas tank.
• Replaced all 8 carburetor boots with new OEM boots.
• Replaced the air filter.
• Checked timing, which was in spec.
• Check compression, which was: 1 (159); 2 (149); 3 (165); and 4 (147).

So, after doing all of this, I attempted to start the bike for the first time. And, was disappointed. The bike simply will not start without starter fluid, and when it does start, its rev high (as if the throttle is wide open) and then immediately dies. At this point, I've exhausted my troubleshooting capabilities and seeking any and all advice as to what I should try to do next.

Since the spark plugs do have spark, I assume it is not an electrical issue, but more of a air or fuel issue. Most likely, a carburetor issue. If that is the issue, I can take them (or it) to a local shop who specializes in this type of motorcycle repair work, and have them sync the carburetors?

I also wanted to ask, in the attached photo, there is something on the handlebar that I'm wondering has impacted the revving that typically accompanies the bike starting. What is this, and should I remove it?

Anywho, I'm seeking suggestions on what I can/should attempt to check next? Any advice is always appreciated..

Cheers, Chip

 

[fallawc]

Following up this question with an exciting statement! After many months (18+) trying to get this bike started, today, for the first time, she started on choke. And, would restart without hesitation and run while they choke was engaged at about 80%. I didn’t keep her running for long (1-1.5 minutes), but she did start and it’s very promising and fingers crossed she keeps running!

Now, for what comes next. At the front side of the engine, near the exhaust ports, she is smoking slightly, although I wonder if this is simply because the engine is burning off old crud and oil that has been caked onto the engine over many many years. Keep in mind this bike hasn’t been started and run in 20+ years. I removed the spark plugs after and all looked fine, no issues or deposits/soot.

However, is there anything I should look (or listen) out for as I attempt to get the bike warmed up and to stay running without choke?

Attached is photos of the exhaust ports and exhaust itself.

Thanks to all for the support!

 

[Biebs]

Good start.. Do not worry about the exhaust. Now fill up the tires ( probably old tires 10 years or more). see if it will move / ride, around the neighborhood do not go over 15 to 20 mph on the old tires!!!

Next big step would be new tires so you can get some wind flowing over those cylinders.

 

[Davidson750]

That looks like some sort of locking cruise control on your bars... it will take a little bit of time for those rings to seat back in, that's probably where your burn off is coming from, or blow by.

 

[Bo82CB]

I would get two high flow fans (I am sure you can borrow few from your friends if you don’t have them) and start that thing in the garage on center stand. You will be able to check (CAREFULLY) clutch operation, go through the gears, rear brake operation… I would never go on the ride with the bike that has been sitting for 20 years, I can guarantee your brakes will suck bad or be nonexistent, wheel bearings may be rusted and turn into dust when you start rolling… you get the point.
Check basics first and work thoroughly to get that beauty safe and fun again.
Good luck!

 

[brettp]

If it runs with the choke out (on/engaged) then you're running lean. If all the rubber is new then make absolute sure the carbs are FULLY in the engine side stabilizers (boots). They can seem all the way in but are not. If you hear fast sips/whooshes of air, your heating a vacuum leak. Secondly, make sure you have stock jets in a stock set up, as well as each air/mixture screw set at 2 turns out before going through the process of setting those properly. Lastly, I would say it also sounds like your idle screw is push into the throttle body all the way. Make sure that isn't the case.

 

[Hondamaniac]

Make sure you’re working with a new battery. If it’s hold charge, great! If not, CheC3 your stator.

 

[evreddy55]

I appreciate all of the suggestions on the '79 750K. I just bought a '79 750K from the original owner. It has 6000mi and hasn't been started since 1994, so I am expecting a lot of headaches and challenges before putting down the road on the shiny dinosaur. All recommendations from the members are welcome.

 

[brettp]

I reread your original post. Do you have the stock vacuum operated fuel valve/diaphragm on the carb still?

 

[evreddy55]

I reread your original post. Do you have the stock vacuum operated fuel valve/diaphragm on the carb still?

I'll find out tomorrow when I pick up the bike. BUT, I suspect so because the owner had shown no interest in maintenance, mods, or preservation. You reckon that the diaphragms have disintegrated?

 

[brettp]

I'll find out tomorrow when I pick up the bike. BUT, I suspect so because the owner had shown no interest in maintenance, mods, or preservation. You reckon that the diaphragms have disintegra

That or the hoses have a leak. Either way, if you have that on the bike try feeding the carbs straight from the tank (make sure your petcock lets fuel flow, too.