1980 CB125S Rough Idle

[MountainManDave]

384 original miles, garage kept in overall great shape minus some frame rust. 100% stock with the exception of an in line fuel filter and a maintenance free battery. The inside of the tank is visibly perfect, no rust at all. It obviously sat for a long time two owners prior as they were a collector. Guy I bought it from claims it ran but had not ridden it in at least a year (although he did specifically state he drained the gas tank prior to letting it sit).

It will not start without holding the throttle really high, at about ¼ of a turn regardless of idle screw position. When it does start, it idles very rough, and dies as soon as I release the throttle. I stayed on the throttle and took it around the block to open it up just to make sure it didn’t need a “wake up” and it backfired a few times at high RPMs under load.

Additionally, the turn signals when turned on stay solid in the rear, and do not light up in the front.

So far I have tried:
1) Because of the turn signals I initially started by charging the battery (which looks newish but who knows) all the way up just to make sure it had enough juice.
2) I checked the spark plug, condition, it appears to be in pretty good shape.
3) Air filter appears to be clean and I did not observe any obstructions between air intake and carburetor.
4) I removed the carb and cleaned it out even though it was in pretty good shape already. I can tell by the marks that someone has been inside the carb before. I adjusted the float to 12.5mm as I read on another forum (this was only a minor adjustment to the original position as best I could tell). I set the the air/fuel mixture screw to shop manual recommended (screw in all the way, back out one and one half turns). Drained all old fuel and replace with new gas. No change in behavior. I went back after downloading the shop manual from this site, and it says the float level should be 22mm, adjusted, put it back together, now it does not even start, and if I give it gas to start like previous I can't kick start, it gets stuck, presumably from too much fuel in the cylinder. It also visibly looks wrong with this high a float level. So I obviously have conflicting sources of information on float height, and I’m not 100% on how I am measuring. All the diagrams I see with measure examples have round floats where you measure base of carb to top of float which is in the center since it's a circle. I have the flat bottom floats and I’m assuming I measure in the middle of the float edge, because the measurement would be higher or lower if I measured to inside or outside.

So first things first, can anyone confirm the float height, and that I am measuring correctly? The first picture below shows how silly the 22mm adjustment looks (to me anyway). The second shows where I am measuring to, you can see the smudged line from my dot which is in the middle of the float and my point of measurement.

I intend to check points next but want to rule out the fuel system first, and ensure I’m setting it back up correctly.

I am using the Honda Shop Manual as primary reference.

I also found this which matches up to the manual sometimes, but not always http://oldmanhonda.com/MC/DataTable.html

Thanks for reading my novel.

 

[MountainManDave]

Still doing research when I have time and the more I look, the less I find. I think the carb is a a Keihin PD22 (PD series? 22mm?). The Keihin website has several series, no PD, and no others that resemble the one I have. Web and forum searches are coming up dry, and the most matches I get on image searches are for used or knock offs on eBay. If anyone has some insight here it would be greatly appreciated. Thank you!

 

[crazypj]

Pretty sure that style float should be around 12.5~16mm, the 22mm is for round brass float on earlier models
try setting base parallel to carb body. You need to tilt carb a few degrees further than in your first picture
I'll see if I can find anything on it, think I have carb specs somewhere 'safe'

 

[MountainManDave]

Yeah, you're right, I think they only used these float setups from maybe 1980-1982, and the shop manual is from 71 or something forward. A bunch of the stuff matches up but maybe they changed the carb for a couple years. I'm going to try setting parallel like you suggested, I know it's not exact but should get me close enough to rule out the carburetor I hope. Thank you!

 

[crazypj]

Set it parallel and see what number you come up with, if it's in the 12.5 or above it should be OK

 

[MountainManDave]

I went parallel, took my time, it came out to about 14-15mm. Put it back together, still won't start. Pulled the air box and tried again for good measure, same result. Checked the plug gap again, it's @ almost 0.029, everywhere read is 0.025-0.028 so maybe a little big but nothing crazy. I'm a little mystified, now wondering if it's getting any spark at all. Will look up how to test this next.

 

[MountainManDave]

Only other thing I noticed was that the carb butterfly sits open a bit by itself, like as much as the first level of choke. Only at 2nd and 3rd levels of choke does it forcibly open any further. I know it's not supposed to be completely closed, and I figured maybe the gap closes when air pressure is present?

First and second picture show choke all the way open, second picture is 2nd level as you can see by the bearing in the selector.

 

[crazypj]

First pic is all the way closed, lever up, choke on
It should close completely and be horizontal when open. Is the return spring fitted properly?

You have pulled the pilot jet out to check the cross drilling's and removed the fuel screw to check passageways are clear?

 

[MountainManDave]

Ah ha, so someone probably left it in the 2nd or 3rd position for a number of years and killed the return spring. It will not close all the way unless I apply pressure to it, and then it returns back to the position in the first picture when pressure is released. This spring housing looks pressed/sealed, not sure if I can actually perform any maintenance on it?

I checked for spark last night and confirmed it's lighting up.

I did pull the pilot jet out, all cross drilling holes are clear, same for fuel screw and all other passageways. I mean the inside is really clean even before I started pulling it apart. I'm beginning to suspect the previous owner was not completely up front with me, and that his mechanic cleaned the heck out of it but couldn't fix it and that's why it sat with him for the last 2 years.

 

[crazypj]

Opening the picture and enlarging it, looks like the return spring is in the wrong place. It should be on the metal arm, not behind the plastic one
You'll probably have to finagle it into position so metal arm is always in contact with the operating arm (or remove choke flap screws, remove shaft and re-fit spring in correct place)
It should be a real light spring so intake vacuum can open flap, could just have rusted and fallen off?

 

[MountainManDave]

Opening the picture and enlarging it, looks like the return spring is in the wrong place. It should be on the metal arm, not behind the plastic one
You'll probably have to finagle it into position so metal arm is always in contact with the operating arm (or remove choke flap screws, remove shaft and re-fit spring in correct place)
It should be a real light spring so intake vacuum can open flap, could just have rusted and fallen off?

There is no return spring visible, but there is 'some' tension in the choke, just not enough to close it all the way so I think the spring is internal. I'll have to look around at some other pictures but the return spring is not an available part in any of the diagrams I have seen. I'll probably try taking it apart as if to remove the butterfly, but like I said it looks pressed on and not designed for maintenance so wish me luck. *reaches for pliers*

 

[crazypj]

When you remove the screws the butterfly shaft will slide out.
The plastic lever should only have the tiny ball bearing and spring to locate it,
I thought I saw something but it was difficult to see in pic. so I took a guess ;D
Carb parts are not available, you have to buy complete carb for spares
Whatever you do, DON'T lose the spring and bearing

 

[MountainManDave]

I removed the butterfly shaft, the return spring is housed inside the sealed end opposite the choke. I simply flipped the shaft over so the front was facing back, set the slotted end of the butterfly shaft back on the spring, and rotated it forwarded, giving it tension on the spring, and re-assembled. It now stays closed, and choke settings work like they're supposed to.

I put it back together, and after about 6 kicks and a little throttle it started up, and NOW IDLES! Well mostly, the carb needs some tweaking as it dies after about 10 seconds of idle (low floats?). Whomever put this thing back together last placed the butterfly shaft in without spring tension, so the butterfly was never closed all the way. That explains why it would not idle at low RPMs, and run like crap at high RPMs.

Since I changed the float levels during my troubleshooting, I still need the specifications on float height to set it correctly.
I also forgot to bur the butterfly screws when I put it back together, so another to do when I go back in and correct float heights.

Appreciate your help thus far crazypj, she's almost street worthy now.

 

[crazypj]

Sounds like that was the problem. 8)
Someone has had it apart and didn't pre-load the spring.
It's such a long time since I've done one I forgot where it was (Kei-Hin swapped them around to make carbs cheaper)
The spec I have for a PD20, PD21A, PD30, PD31A,and PD32A is 12.5mm. Can't find any others at present, you'll probably have it fixed by the time they turn up ;D