Ran into some problems :( HELP!!

[Gishman91]

Just recently bought a 1980 cb750c Custom. Carbs were all gummed up so i did the following
I drained all bad gas, and cleaned tank, and filled half full with Fresh Fuel.
It would start and go up to about 2k RPMs, and then I got it up to 5k RPMs but it just drops and dies..
Cleaned air filter, and added some fuel treatment. It now starts and goes up to 3k RPMs but dies instantly after that. If I juice the throttle it will stay running but eventually bogs out?

Any help and or Suggestions will be very much appreciated

[Hellbilly429]

Sounds like the pilot jets are plugged, gotta pull the carbs apart and clean them boss!

http://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revD.pdf

This is a great link to help you tear down and rebuild your carbs

[sgtslag]

Agreed:  remove the carbs, disassemble, and clean, manually.  After that, use SeaFoam once a month, to keep any water out of the fuel system, as well as to dissolve any varnish deposits before they get to be a problem (once the jets get plugged, SeaFoam won't do the job).  SeaFoam will preserve gasoline up to 12 months, as well.  Cheers!

[Gishman91]

Thanks guys that was the plan, but wasnt sure if there was something i could do prior.
Carbs are off, and tomorrow between classes and work ill start tearing em down.
Again Thanks guys

[Gishman91]

Alright guys I had some between life and class and got to tear down my carbs. they weren't too bad but had some gunk to them. So now all cleaned up and re-installed Im still having the same problem, fires up and will run full choked for a few minutes until it floods out. but wont run a lick with choke off. Syncing issue?? or what?

Like ive mentioned before im knew to the carb game and right now its still carbs 2 - Gishman91 0

Any help will be fantastic. Thanks guys

[Lucky 1]

Idle jets are clogged up.
You have to take them out and clean the gunk out of them.
When fuel sits for 60 days with ethanol in it it gets all gummy like varnish.

You have to take the idle jet OUT of the carb and be able to see daylight through it.

[shinyribs]

Yep,probably still the idle jets. Fins a small piece of wire to run thru them to clean them. Guitar strings,torch tip cleaner and trash bag twist ties( stripped of the coating,of course) work good for this. When you can blow thru them they are clean.

[Gishman91]

Thats exactly what i did.  I soaked them over night in the cleaner recommended on the carb clean thread and i made sure everything was spotless, and i could see daylight through them. reassembled everything according to the way i took it apart and the way the thread explained. didnt miss a beat. But ill tear em down again. just was hoping for a different suggestion lol  

[Hellbilly429]

If these carbs aren't synced and the idle isn't set just perfect they will make the bike run like crap, very finaccy carbs. My bike was doing funny things like revving way up when warm and turning my pipes glowing red and it ended up being just that the idle screw wasn't set just perfect.

[shinyribs]

If you are confident that you cleaned your carbs well then there is no reason to pull them off again. Like you said,time to look elsewhere.

[Gishman91]

Yep I cleaned em good, guitar strings through the jets, and scrubbed everything with a brush. I have spark in all cylinders. and just started the idle adjustments. I dont know where else to turn. :/ hopefully something will look up. today wasnt a successful day.

I guess i did get my clubman bars on!!! :D

[Gishman91]

Pulled my exhaust pipes for shits and gigs. I have A LOT of unburned fuel passing through. But I have spark. Could this be the coil not allowing it to run?

[shinyribs]

Possibly. What makes you think you may have unburnt fuel in the pipes? Are they wet inside? Often people mistake carbon build up as unburnt fuel from lack of ignition,but in reality, if it was never burnt then it would not be able to leave carbon behind. Carbin is burnt fuel,just not completely burnt fuel. Are all four header pipes getting equally hot? Perhaps you are just running way rich allowing the carbon build up you see. Just a thought.

But Hellbilly is right on about the carb sync needing to be done. If you have not had the carbs synced since you had them apart that is the next thing to do. The bike will never run right until they are synced. It's not a hard job at all,but one of the most important things to have a properly running bike

[Gishman91]

Ya when I took the pipes off the fuel must have puddled and it ran out. There is carbon build up but there was also fuel fuel. And alright looking into syncing em up now. Again thanks guys for all the help

[Lucky 1]

"guitar strings through the jets" exact words.

No ..it has to be the right size electric guitar string. A #10 . guitar string.

And you have to hold it up to the light and make sure you can see daylight through the jet.
Carb cleaner may not work if it is clogged.

Scrubbing everything with a brush"???  i

Just the things you say do not make sense to me.

If you said I put A guitar string through the jet all the way
or I used a tooth brush to wash some of the parts would make more sense.
The hole in a idle jet may only be .013 thousandths!

BTW..I have never seen an exhaust that did not have carbon in it.

[Gishman91]

im just trying to keep it as common sense as possible. Just trying to get some help.

And yes i did all of that. #10 string and checked to see light through them. lol

and i cleaned all the bowls and all the parts with a toothbrush and another scrub brush one more course than the other.

My exhaust had carbon yes, but it had liquid fuel as well. it just trickled out as i tipped them up.  

[seestheday]

Lucky is a little more crotchety than most people on this forum, but he is a smart guy and gives good advice.  Even if we have some disagreements on pods .

For your problem, did you do a decent bench sync?  I found that I was able to get my 81 K carbs (same as yours) sync'd damn near perfect on the bench (verified when the carbtune pro arrived in the mail).

Regardless, a good bench sync should allow it to run long enough so you can sync it properly and not have it die after 3K.

[Lucky 1]

If you put the choke on and it still will not run then I would check the float heights.
Check them by measuring them with a ruler or calipers like the book says.

Also when you do that check the idle jets again to see if they got clogged up again.

If you have a fuel filter get rid of it and get a new one.
 There could be water mixed with the fuel or bad gas .

The carbs are the last part of a tune up.

[Gishman91]

Alright guys I synched the carbs how the right up told me too, and it screamed. But it still wont stay running. It jumped up to 5k rpms and dropped off. But if i spray carb cleaner or starting fluid into the carbs as im firing it it will run, rough but itll run. So is this a coil problem or is it something else?

Thanks for all the help this far


Was super excited to hear it bark like it did. First sign of life that I've seen it had.
   

[shinyribs]

LOL. They do have an awesome bark. If you thought 5 k was nice wait til it starts banging out 10 k!! .

I know you don't wanna hear it. But what you.describe sounds like lack of fuel. Both at idle and upper rpms.  Upper rpms fall on it's face could also be low battery voltage / charging.