Topic: '78 K model won't run off choke

[fredmalito4]

Good evening, all. First post.

My son and I are trying to revive a 1978 CB750K. We have gone through the carbs, spun in some new plugs, discovered my feeler gauges were missing so had to stop prior to any adjustments except the cam chain.

We've sonic cleaned the carb's covers and small bits, carb cleaned and piano wired the passageways, put in new gaskets and accelerator pump, cranked on new boots, put the idle mixture screws to 1.5 turns,  and reassembled. The bike immediately starts and idles, but won't run once the choke is pushed in more than just a hair.

With a severe lean condition in mind, I pulled out my little torch and propane the intake area searching for a leak. Nada. Searched all over the carb bodies. Nothing. confirmed propane is still flammable by passing it in front of the carbs. Yup, still burns.

So, tomorrow we pull the carbs back off. Where do we start looking? It has no low speed jets, so I'm kinda at a loss. Any ideas?

ALSO, when you look at the pump carburetor's float bowl, there is a square brass plug under an o-ring in one of the pump's passages. Should that move? What is it? it accidently got media blasted.

Thanks all!

Fred

[ekpent]

 I think gas just moves past the square plug,no movement. If your test running the bike without the airbox that could create a lean condition but yours sounds a little more severe.

[DaveBarbier]

Have you verified that the choke operates properly? This is a cable driven choke, right? I'm not too familiar with the 750. Stock intake/exhaust setup? Jets are all stock sizes?

[Progas]

It does have pilot jets, they are pressed into the carb bodies. Get some needle nose vise grips and grab them with a twist and they will pull out. They are not available from Honda, they are #35 and you can get them at Jets-R-Us if you can't clean them.

[robvangulik]

Progas was just a bit faster than i.
What do you mean : no slow speed jets?
If there are only holes there someone has pulled them and not replaced.....

[fredmalito4]

 Good morning, everyone.

I learned about that pressed pilot jet at about 1:30 last night, and decided we would be pulling the carbs back off to investigate that. Thanks for the heads up on where to buy them if they are crap.

It is going to be completely stock, we found an emissions air box on eBay for it and will be installing that when the new boots arrive and I make a new gasket for it. Stock exhaust, points for now though I will probably change that to electronic ignition due to my hatred of points.

The choke is operating correctly. This one is  a cable driven choke like the later model DOHC bikes.

 Any other ideas are definitely welcome. Thanks!

[evinrude7]

i'm unfamiliar with carbs that have an accelerator pump but since you re pulling the carbs can you bench sync them while out and then reinstall? 

[Mr. Mike]

Keeping in mind Fred, from my experience on the K8, a good warm up is needed for those pd carbs to idle decently. Mine used to take at least 10 minutes of ride warm up before I could fully push the choke in and not be concerned of a stall. Once warmed up, it ran ok.
I got tired of it and had ‘75 round top carbs put on...what a difference.

Regarding the accelerator pump, with your air box off, you should see those little stand off brass tubes sticking up right inside the intake. Cracking the throttle you should see a good stream of gas squirt in. Check all four. If you see it, your good there. And as evirude said, carb syncing is necessary.

[fredmalito4]

Thanks for the heads up on the warm-up time. We allowed this thing plenty of time to warm up, to the point that I started reaching for a fan, and it would stall as soon as the choke cable started to get pushed  in.

I’ll double check the accelerator pump today before I pull it down. It is a new pump.  Bench synchronizing is not an issue, and I do have synchronize engages watch this is all done.

If I can’t get these things right I think I’ll order to carburetor conversion kit for my chopper and put it’s carburetors on this bike. That would make my life much easier.  We are trying to keep this stock as possible, though.

[madmtnmotors]

The accelerator pump will not affect the engine's ability to idle. Could be a number of things:

>Incorrect float levels
>Idle jets and/or idle circuits clogged
>Vacuum leaks
>Fuel flow restriction. Have you replaced the in-tank fuel filter? These are often overlooked and remain in the tank when the petcock is removed. Most of the ones I've encountered appear to be original and require a deliberate effort to remove. This is the easiest to check since you don't have to remove the carbs.

[fredmalito4]

I noticed there was no filter on my petcock and figured it was lost long ago, but I will check that. Fuel flow seems to be very good, though. I’ll diuble check to see if it’s stuck in the tank. Thanks for that heads up!

Float height is spot-on, I think I did a pretty good job on the vaccum leak department.

[Bankerdanny]

I thought only the F3 model had the accelerator pump equipped carbs.

[fredmalito4]

These are the same carbs as he F model from 78. They shared. A model was a pumper, but different size throat if I recall.

[madmtnmotors]

Pretty sure 77/78 K/F/A all had PD "keyhole" pumper carbs. Shine a flashlight through the gas cap opening (not a lighter!) towards the petcock and you should be able to see the filter if it is still in place. From underneath (through the petcock opening) it just looks like a nylon sleeve inside the petcock bung.

[Airborne 82nd]

I thought only the F3 model had the accelerator pump equipped carbs.

77 &78 750 k both have the pump F models had the same carb body but different size jets no differences in throat size. If you do not get a filter on you will be taking the carbs off every Sat.

I noticed there was no filter on my petcock and figured it was lost long ago, but I will check that. Fuel flow seems to be very good, though. I'll diuble check to see if it’s stuck in the tank. Thanks for that heads up!

This tells me you have not cleaned the tank correctly

  Bench synchronizing is not an issue. Explain what do you mean by this

i'm unfamiliar with carbs that have an accelerator pump but since you re pulling the carbs can you bench sync them while out and then reinstall? 

Yes you can. You can do everything you need to do on the bench other than a vac sync.
On my stock 78K if you pull the choke it will give the bike a shot of gas without turning the throttle that is what your fast idle cam is for. Have you set up the fast idle cam?

ALSO, when you look at the pump carburetor's float bowl, there is a square brass plug under an o-ring in one of the pump's passages That is a check valve. Do yourself a favor and make a carb stand for doing all your bench work  the 2x4 that the carb bank is on pivots so you can set your floats. Hear is one I made years ago out of scrap 2x4s Just c clamp it to your bench 

 

[Brian Barr]

+1 with airborne 82

Have you set your fast idle cam?
Been there. done that. More than once....

[fredmalito4]

 I’m afraid I don’t know what that is. Would you elaborate for me please? I’m new to these carbs.

[BobbyR]

+1 with airborne 82

Have you set your fast idle cam?
Been there. done that. More than once....

The fast idle cam on the carbs holds the throttle open some amount while the bike warms up. You will see a cam on the shaft. When you pull out the choke the cam keeps the carbs open. One look and you will see how it works and how it is adjustable. These are very nice carbs when set up properly.  I have a 78K and I struggled with the carbs.

As mentioned you can pull the idle jets out with a pliers and just a wiggle while pulling up.  Grip gently as they are brass and you do not want to make them oval.

You will see several holes in the sides, Rod them out with some copper wire or a high E guitar string. White deposits are from the MBTE that was in the gas. You have to scrub them off - they will not dissolve.

Running the wire up the center is key as that is the orifice. Copper wire is best since you will not enlarge the hole. Flush it out and work it a few times. Put the jets back on and make sure they are straight, tap the jets until they seat.

Synching can wait until you have the valves adjusted.

[Airborne 82nd]

One other thing do you know about the clear tube method for checking your fuel level in your carb bowls? It is the best way to check your float adjustments and they are easy with these carbs Build your self a jig Do it once and do it right.

[fredmalito4]

Clear tube method; kinda like sight glasses? attach tube to drain, curve it up, open drain and check how high it goes? (I mic'd the floats, should be groovy).

Fast idle cam; I see it, it moves, got an adjustment method? Didn't see that in the manual. Looks like it must be done prior to installation.

Carbs now bench synced. Idle jet fuel passages were completely plugged across the board, air mixture holes were fine. What a #$%* to get clean!   

[Brian Barr]

Fredmalito4 -  squat down on the right hand side of the bike, between carbs 3 & 4, there should a a round knob sticking out near the top end of the carbs. It points towards the engine. Turn it clockwise to increase the rpms. Once the bike is warmed up us this nob to adjust the rpm up or down to achieve the 900 to 1000 rpm range with the choke closed.

[evinrude7]

i haven't used this video so i can't vouch for how good it is.  https://www.youtube.com/watch?v=oVlLitzF38I  google "bench sync cb750"

lost my bench sync link as i had to refresh my computer which wiped it out. 

[DaveBarbier]

Fredmalito4 -  squat down on the right hand side of the bike, between carbs 3 & 4, there should a a round knob sticking out near the top end of the carbs. It points towards the engine. Turn it clockwise to increase the rpms. Once the bike is warmed up us this nob to adjust the rpm up or down to achieve the 900 to 1000 rpm range with the choke closed.

Sounds like you're describing the idle setting process. Don't you want/need to set the fast idle cam when the bike is cold? The fast idle cam doesn't have a knob but a 10 or 8mm screw with a spring tensioner on it. Usually marked with yellow paint. With the tank off it might be able to be set. The manual may have a spec for the cold idle rpm.

[Brian Barr]

True enough DavidBarbier.  On my 1978 cb750 k, there is a 8 or 10 mm screw with a spring on it which also has a circular knob that protrudes between the carbs. It must be screwed in enough to slightly lift the carb sliders to adjust the idle rpm. If fully backed off, the bike will not idle without full choke.
I have missed this adjustment more than once after reassembling the carbs. Lol
It is easier to make this adjustment with the tank off but it can done from the side with the tank on.

[fredmalito4]

Hey guys, I want to get back with you and let you know the solution to the problem.

It was the idle jets. We had to chuck a piece of piano wire cut at an angle into a drill to get them cleaned out. It was a royal pain. The bike now runs off choke, but is running quite rich even with the idle screws backed in quite a bit. I have a feeling we probably punched those jets out a little bit too much in an attempt to clean them. However, now that I know the bike runs in the engine sounds healthy, i’m willing to pull those carbs back off and drop in new jets. They shipped out today!

 It sure felt good to ride this thing, even if it was just up-and-down the street. Really brought back some memories from my first bike, a CB900 custom that my father-in-law gave me. May she rest in peace.

So far so good on this thing, I have a new post up about a valve adjustment tool if anybody wants to give me a hand on that. Thank you for all your help!