1982 CB750F...Better Devil

[Jimbonaut]

1982 CB750F Resto-mod. At least that's the current plan.

So I shouldn’t loop one hose like I did? Both hoses should be connected to the carb and open at their other ends? Strange if not, because that hose is looped on my other CB


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[irk miller]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

 

[Jimbonaut]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

Man that’s really helpful - thanks mate. I’ll re-route those hoses and see what gives.

Really weird though. Guess the other CB threw me with its hoses and their routing. Could that routing be so different?


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[irk miller]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

I have no idea. Maybe a seal leaks on the other one, so it pulls air through the seal and seems routed right. Funny things happen. When I got my 1979 C10 from my dad, it started making a clicking sound while I drove the 2.5 hours home. It ran and drove fine, just made that sound. Turns out, my dad had the vacuum hoses routed wrong, so it was sucking air through the main seal. He only drove it around his property and occasionally took a load of metal a couple miles up the road to the scrap yard, so the motor never heated up enough to do that for him.

 

[Jimbonaut]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

Thing is Irk, that diagram shows four hoses but as it sits my carb has five -

My (limited) understand would suggest that -

1. is a fuel line from the auto fuel cock to the carb body
2. not sure but I'm pretty sure it's in the right place
3. is a fuel line from the gas tank to the auto fuel cock
4. is a hose from the carb body (in between carb 2 & 3) to somewhere
5. is a hose from the auto fuel cock to ??

So I've got five hoses, but the diagram only shows 4. Hose 4 and 5 are the mystery meat.

 

[irk miller]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

Thing is Irk, that diagram shows four hoses but as it sits my carb has five -

The diagram only shows hoses attached to the auto fuel valve. What you have marked 4 doesn't go to the fuel valve, so it's not in that diagram. I'm guessing that 4 is attached to a nipple on your #2 carb, which is the carb that holds the pumper. As such, I'm also guessing that your pumper runs directly off the tank, and not through the auto fuel valve. Just guessing by what I can see in the pics.

 

[Jimbonaut]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

Ah, good catch. So would that mean that line 4 would - along with line 3 - connect directly to the petcock on the gas tank?

 

[irk miller]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

Ah, good catch. So would that mean that line 4 would - along with line 3 - connect directly to the petcock on the gas tank?

Right

 

[Jimbonaut]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

If that’s the case then I think I smell a snafu. I can’t tell from the VB number on these carbs where they originally come from, but I don’t think they’re from an F model. And now I have a K model tank. The petcock threads on the tank seem to be different for each model and year, and the only petcock I can find that fits this K tank only has one outlet.




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[irk miller]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

If that’s the case then I think I smell a snafu. I can’t tell from the VB number on these carbs where they originally come from, but I don’t think they’re from an F model. And now I have a K model tank. The petcock threads on the tank seem to be different for each model and year, and the only petcock I can find that fits this K tank only has one outlet.




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It appears the K and F share a part number on the petcock, so we know that's at least correct. 16950-MA4-771

What I do in parts searches is borrow Western Hills' site and do an OEM part search:

https://www.westernhillshondayamaha.com/index.htm

 

[irk miller]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

3 and 7 are fuel.
4 is vacuum
5 is air vent

Now to figure out what your 5th hose is.

 

[Jimbonaut]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

Now to figure out what your 5th hose is.

Yup, that's the question mark alright.

 

[Jimbonaut]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

Ok, may be getting somewhere. Hose 4 is apparently a float bowl vent. Hose 5 is vent for the auto fuel valve. Stands to reason then that both these hoses should be open at the ends. If perhaps one (or both) of them is blocked then that could be causing the problem I guess.

 

[Jimbonaut]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

Trying, trying hard not to look at today as a total fail. There's a lesson in there somewhere - just need to dig it out. And I'm digging deep.

Pulled the carbs again, re-routed the hoses and double checked everything before putting them back in the bike. Hooked up the gas tank, turned on the the petcock and smelt gas - carb one was pissing gas out of the overflow tube. Thought maybe it was a stuck float valve - gave the side of the carb a whack with a rubber mallet but it still overflowed. Figured fuck it - I'll try starting the bike and see what gives, and that was when problem two arrived.

The bike wouldn't fire, instead I heard a rapid clicking from the starter relay/solenoid and then a rather disconcerting mechanical clank from somewhere. Killed it, thought maybe the 30A fuse had blown so switched it out with my other CB, to no avail. Then switched out the whole starter relay/solenoid assembly with the other CB - still just a rapid clicking, another clank and that was all she wrote.

The bike fired up fine before I pulled the carbs. Ran rough, but it fired. I'm sure the starter problem doesn't have anything to do with the carb work I did but have no idea what to do or check next.

Any pointers?

Edited to add - I'm pretty sure it's not the battery as it was charged, but I have the thing on trickle anyway and will try it again in a few hours. Could the clutch switch have something to do with it? The bike is in neutral, at least the neutral light is on.

 

[irk miller]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

What's battery voltage?

 

[Jimbonaut]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

Battery voltage was about 11.9V

 

[irk miller]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

Battery voltage was about 11.9V

That's basically discharged. Charge it up and try again. If it doesn't hold charge, get you a new battery.

 

[Jimbonaut]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

Cheers man, really appreciate the help as always. Had the battery on trickle and sure enough it fired up after dinner. I thought 11.9V sounded pretty much fully charged but I now know better.

Gas is still pissing out of carb 1. More rubber mallet whacks but no dice.


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[irk miller]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

Try blowing through the fuel line from the tank. If it's debris on the float, it may clean it off. Works more than it doesn't.

 

[Jimbonaut]

Re: 1982 CB750F Resto-mod. At least that's the current plan.

Disconnect the fuel line from the gas tank and blow through the thing? I'll give it a go mate.

When the bike did start it idled like absolute shite - the revs climbed and climbed until I was sure a disgruntled neighbour was going to launch a frying pan at my head. :-\

I'm starting tho think these carbs may be in need of more than just a rudimentary clean. So plan B will be to pull the carbs from the other CB and switch them out, just for long enough to get the bike through its safety exam.