New to me CB750 Project! Help needed.

[trosey]

Hello all,

Just got myself a 75 CB750 to fix up and ride for a little while. I'm not new to wrenching, but most of my knowledge and experience is in the car world. I've worked on some dirt bikes and smaller (less than 450cc) bikes, but I'm still very new to the motorcycle world and carbs. I could really use some knowledge from y'all and maybe some guidance on how to get this bike running. The bike has been sitting for a year+ in a garage, after the previous owner pulled the bike apart to clean the carbs. It ran according to him, but started having a problem where it would die. Anyway, I got the bike, changed fluids, hooked the carbs up, and tried to get it to idle. I got it to the point where it will idle, but only fully choked. Throttle response is non-existent and it will die if I get more air into it. I have both the stock airbox and pod filters, which I understand might need different jets to run properly. Is that correct? And if so, what jet would I need? I will also be doing a compression test when I change the plugs, but it all sounds healthy to my ear. Any advice or places I should start?

Thanks in advance! I'm very excited to get this bike running!

 

[dirtdigger]

Sounds like carbs have some blocked circuits or not getting enough fuel to the carbs. No one can tell what jet you need, too many things affect jetting. Elevation, temperature, exhaust system, air filter setup, engine mods, engine condition, fuel type and quality all affect jetting, there is no one size fits all.

 

[trosey]

Sounds like carbs have some blocked circuits or not getting enough fuel to the carbs. No one can tell what jet you need, too many things affect jetting. Elevation, temperature, exhaust system, air filter setup, engine mods, engine condition, fuel type and quality all affect jetting, there is no one size fits all.

I see. Thank you. I'll pull the carbs apart again and give them a good clean then. That makes sense. How would I figure out what jet I need? Is that something that I determine after it runs based on whether it's consistently rich or lean? Sorry, kind of new to this stuff.

 

[dirtdigger]

like i tell everyone, use the choke.....when you get it running decent take it for a ride. go wide open throttle see how it runs, if its got a problem start closing the choke while riding, if it starts to pick up then its still jetted lean if it bogs down worse then its already too rich. Do this at different throttle opening to see what needs to be done with the jetting.

When you have them apart look to see what jets you have in there so you have a starting point and know what jets to get.

 

[UK K6]

These 750Ks are cold blooded, even when set up well. They won't idle well until nicely warmed through.

I had blocked pilot jets, so the bike wouldn't run off choke. All came good with a thorough clean. Not just the jets, but also carb cleaner through the passages in the carb body, then blow through with compressed air.

Now, when cold, I start the bike on the choke, then straight off and keep it running on the throttle, to avoid sooting up the plugs. I take off straight away, but baby it. It will do a lumpy idle off the throttle at my 1st junction, about 800yds. It's warmed through after about 5 miles.

 

[trosey]

These 750Ks are cold blooded, even when set up well. They won't idle well until nicely warmed through.

I had blocked pilot jets, so the bike wouldn't run off choke. All came good with a thorough clean. Not just the jets, but also carb cleaner through the passages in the carb body, then blow through with compressed air.

Now, when cold, I start the bike on the choke, then straight off and keep it running on the throttle, to avoid sooting up the plugs. I take off straight away, but baby it. It will do a lumpy idle off the throttle at my 1st junction, about 800yds. It's warmed through after about 5 miles.

Awesome. That's really good to know. Thanks for the info!

 

[mitchi]

Those are open exhaust pipes aren't they?

If so, you'll have to go up a larger slow idle jet.

I had the same pipes, cut off like that, and I could never get it to idle. So I went with a set of stainless steel DelKevic 4-1.