1980 Honda CB750F Starting Trouble

[Nwalker300]

I’m currently fixing up a 1980 Honda CB750F and can’t get it quite to fire. I’ve replaced starter clutch, starter solenoid, starter motor. It has spark and is getting fuel. I did a compression test on a cold engine but also didnt do it with the throttle wide open and all the cylinders are with in 10% of each other 115-130 psi so I’m guessing each one probably would be 15-20psi more or so which I know isn’t fantastic but should be enough to run. Only think I can think of is maybe I have a weak spark? I have a brand new aftermarket ignition coil kit I haven’t put on yet. Should I start with replacing the old ignition coil system? All suggestions are appreciated.

 

[Mysp1]

I’m currently fixing up a 1980 Honda CB750F and can’t get it quite to fire. I’ve replaced starter clutch, starter solenoid, starter motor. It has spark and is getting fuel. I did a compression test on a cold engine but also didnt do it with the throttle wide open and all the cylinders are with in 10% of each other 115-130 psi so I’m guessing each one probably would be 15-20psi more or so which I know isn’t fantastic but should be enough to run. Only think I can think of is maybe I have a weak spark? I have a brand new aftermarket ignition coil kit I haven’t put on yet. Should I start with replacing the old ignition coil system? All suggestions are appreciated.

I had the same problem with my 1981 750, decided to install a new ignition system. Dyna 2000 Digital Performance Ignition, that has cured the starting and the engine sounds so much smoother. Have a look on their website for more info - www.dynaonline.com

 

[Nwalker300]

I had the same problem with my 1981 750, decided to install a new ignition system. Dyna 2000 Digital Performance Ignition, that has cured the starting and the engine sounds so much smoother. Have a look on their website for more info - www.dynaonline.com

Oh really? And it fired right up after the new ignition system?

 

[Mysp1]

Yes, with a new set of plugs too, great test is to leave it standing for about 7 days then try starting her, historically this was a problem especially when the fuel had evaporated from carbs and vacuum pump had to pull new fuel. Now she starts on the second turn with full choke.

 

[Nwalker300]

Yes, with a new set of plugs too, great test is to leave it standing for about 7 days then try starting her, historically this was a problem especially when the fuel had evaporated from carbs and vacuum pump had to pull new fuel. Now she starts on the second turn with full choke.

Maybe I’ll have to try that then. Mine turns the starter over good but it doesn’t ever fully fire. My fuel is gravity fed. No vacuum. I haven’t had it started since I’ve bought it. Just slowly eliminating problems. My next thing was maybe checking valve clearance? I’ve had a lot of people suggest checking timing but I’m unfamiliar with it.

 

[Motogrady]

First off nice bike.

If you have good compression, which your numbers say you do, pistons, valves, rings and all that stuff is good. If valve clearance is off the compression on that cylinder will be off.

I dunno.
Off hand I’m betting clogged carb or carbs.

I’d first charge the battery real good.
The 750s always start and run with a strong battery.

Check for fuel at the carbs when you open the petcock. Shut the petcock off. Pull the fuel line where it connects to the carbs. Open the petcock. Gas should flow all over the place. If it doesn’t, clogged petcock. There is a small brass screen in the petcock. Sometimes they get clogged.
Some models around your year have a son of satan petcock. It’s a petcock with a second what looks like gas line, but isn’t. It’s a vacuum line. When you start the bike vacuum from the motor opens a diaphragm in the petcock. Or, is supposed to open. They are famous for not doing so.
Do what you gotta do to get fuel to the carbs
when you turn the petcock on.

If you have fuel to carbs, try pulling the float bowls, or if there is a drain screw in the float blows, pull them. See if all 4 of the carbs drain fuel when petcock gets turned open.
Yeah, I know, it’s tight in there.
You should have fuel there also.

Or, remove the left side cover, remove the air filter, squirt no more than 3 seconds of starting fluid directly into the air box. Make sure the ignition switch is on. Key on, hit the button with throttle maybe half open.

If it starts, and revs for a few seconds, carbs, petcock, something in the fuel system is not right. By squirting starting fluid directly into the air intake you bypass all of that stuff, carbs, petcocks, gas lines.
Which tells you that’s is where its problem is.
It’s a fuel issue.

Now listen very carefully.
You get into the fuel system, be very careful.
You are working with highly flammable, volatile things. Act that way.

If you do go with a squirt of starting fluid, and it runs for a few seconds, that’s it. Don’t be doing it over and over, all happy because it sounds so good. Once or twice. Then, suck it up. Do the petcock thing. If that’s working, fuel to the carbs, the carbs need a good cleaning or rebuilding.

A squirt of starting fluid, and it doesn’t start up and rev, then go to ignition.

 

[Nwalker300]

First off nice bike.

If you have good compression, which your numbers say you do, pistons, valves, rings and all that stuff is good. If valve clearance is off the compression on that cylinder will be off.

I dunno.
Off hand I’m betting clogged carb or carbs.

I’d first charge the battery real good.
The 750s always start and run with a strong battery.

Check for fuel at the carbs when you open the petcock. Shut the petcock off. Pull the fuel line where it connects to the carbs. Open the petcock. Gas should flow all over the place. If it doesn’t, clogged petcock. There is a small brass screen in the petcock. Sometimes they get clogged.
Some models around your year have a son of satan petcock. It’s a petcock with a second what looks like gas line, but isn’t. It’s a vacuum line. When you start the bike vacuum from the motor opens a diaphragm in the petcock. Or, is supposed to open. They are famous for not doing so.
Do what you gotta do to get fuel to the carbs
when you turn the petcock on.

If you have fuel to carbs, try pulling the float bowls, or if there is a drain screw in the float blows, pull them. See if all 4 of the carbs drain fuel when petcock gets turned open.
Yeah, I know, it’s tight in there.
You should have fuel there also.

Or, remove the left side cover, remove the air filter, squirt no more than 3 seconds of starting fluid directly into the air box. Make sure the ignition switch is on. Key on, hit the button with throttle maybe half open.

If it starts, and revs for a few seconds, carbs, petcock, something in the fuel system is not right. By squirting starting fluid directly into the air intake you bypass all of that stuff, carbs, petcocks, gas lines.
Which tells you that’s is where its problem is.
It’s a fuel issue.

Now listen very carefully.
You get into the fuel system, be very careful.
You are working with highly flammable, volatile things. Act that way.

If you do go with a squirt of starting fluid, and it runs for a few seconds, that’s it. Don’t be doing it over and over, all happy because it sounds so good. Once or twice. Then, suck it up. Do the petcock thing. If that’s working, fuel to the carbs, the carbs need a good cleaning or rebuilding.

A squirt of starting fluid, and it doesn’t start up and rev, then go to ignition.

I have done all that for fuel system and everything is in order. I’ve had several people tell me to check timing. But I haven’t found anything helpful to figure out what to do for that. Since it’s an electronic ignition system. Not even sure where to start to figure out the timing.

 

[Motogrady]

You might find something here.
It’s an old mostly abandoned site.
But there are still some factory service manuals there that can be downloaded for free.
About half way down the thread……1981 c model…..pull up the ignition link.

http://vintage-and-classic-hondas.8...e-ups-too-Updated-04-26-2014-td685918i60.html