Just scored my "Dream" bike

[Brown81]

Bike is cute as hell!

Thanks for sharing and for the education. You make it seem easy.

 

[GB500guy]

Carburetion problem, finally wrestled to the ground

The little bike came with an aftermarket Keihin 25mm race carb, way bigger than the stock 16mm carb and even given the big bore kit's displacement increase from 49cc to 89cc, was still way too big to run right. I did lots of jetting changes with only minor improvement, then stepped down to the 22mm carb from my CB125 with good improvement but still had an annoying wide open throttle missfire. And with only 89cc you spend a lot of time at WOT and need the engine to give its all! So again I did jetting changes up and down the scale along with changes to intake tract length and air box openings, always having the feeling that the real problem was intake dynamics - air velocity through the carb too low and allowing reverse pressure waves from valve timing overlap to mess up carburetion. That was my working theory anyway.

So while waiting for a genuine Keihin PC20 20mm carb to arrive I tested an el cheapo Chinese 20mm carb which actually almost cured the WOT missfire but added other running problems. Finally the PC20 arrived and the bike now runs like, well, like a dream! So in celebration I took time to fab up a proper manifold and air box connection. The gods must like me since the ID of the engine side of the carb matched the ID of the cyl. head inlet port, which happens to match the ID of 3/4" EMT conduit. So needing a slightly curved manifold tube I first used a conduit bender to bend a section of tubing, then used a hole saw to make a 1" hole in steel plate for a flange at the cyl. head end.

The plate cut to shape and the conduit test-fitted for correct angle to align with air box-

The conduit brazed to the plate-

The completed manifold bead blasted, cut to length and a Delrin bushing machined on the lathe to accommodate the step in the the rubber carb adapter.

Carb and manifold installed-

To connect the carb to the airbox the gods again smiled as the ID of a cloth-covered rubber hose I had on hand exactly matched the ID of the A/C end of the carb. I just needed to ream out a recess on the carb end to fit over the carb OD. A hole saw with depth stop did the job nicely for a seamless ID match between hose and carb.

Not only that, but the OD of the rubber hose perfectly matched the ID of 1.5" Sch 40 PVC that I had already fitted to the air box. All hooked up, the intake tract is now a smooth transition from filter through the carb to intake port, with the right sized carb, and the bike takes wide open throttle with gusto.

This was one of the toughest missfire problems I've ever had to deal with, but that makes success all the sweeter. And it's all been a really fun shop project. Life is good.

-Bill

 

[firstbuell]

dude, that's called, "gettin' it done"

"Congrats !" on your completed[?] Dream

 

[SFMCjohn]

... yet another stray motorcycle finds
health and happiness in
GB500guy’s good home ... aWeSoMe!

 

[spdt509]

OP,mad respect for your lathe skills......you should buy this one,then you have a bike for even days and odd days.....

http://bayarearidersforum.com/forums/showthread.php?t=529987

 

[GB500guy]

Thanks all for the comments. Aside from some minor cosmetics the bike is basically done and will be one of my five riders. So my zippy little bike will probably pop up next in Moto Photos now & then in ride reports. Hope to see you there!

-Bill

 

[bcj]

Thanks. To you as well.

 

[andrei3k]

Hi Bill,

You definitely have a very nice looking bike, and did an excellent job refurbishing the parts. It seems just about anything from Japan has some level of corrosion on it.
I also have a 1997 CB50V that I recently picked up, and although I havent ridden it much I noticed I keep topping out at about 50mph.
I instantly suspected the carb being the issue as the bike has a Takegawa 89cc kit and its easy to oversize the carb/jetting expecting to get a ton of power. (right now i can take it to redline in first gears but in higher gear cant go over 8K rpm and after a couple of passes it seems to loose power almost as if lean and overheating)
I will have to double check my carb diameter, but i do know its a keihin running a 95 main jet. Do you mind outlining your carb specs to help me get a confirmed baseline? My bike came with a cast manifold and a pod filter ( Airbox is long gone ). Thanks for any help and hopefully we get to meet one day. I've never seen another in person.

p.s. couldnt attach pictures, too large i suppose
but this is the one, with factory seat and black side covers and clip-ons instead of full bars now http://bayarearidersforum.com/forums/showthread.php?t=529987

 

[GB500guy]

Hi Bill,

You definitely have a very nice looking bike, and did an excellent job refurbishing the parts. It seems just about anything from Japan has some level of corrosion on it.
I also have a 1997 CB50V that I recently picked up, and although I havent ridden it much I noticed I keep topping out at about 50mph.
I instantly suspected the carb being the issue as the bike has a Takegawa 89cc kit and its easy to oversize the carb/jetting expecting to get a ton of power. (right now i can take it to redline in first gears but in higher gear cant go over 8K rpm and after a couple of passes it seems to loose power almost as if lean and overheating)
I will have to double check my carb diameter, but i do know its a keihin running a 95 main jet. Do you mind outlining your carb specs to help me get a confirmed baseline? My bike came with a cast manifold and a pod filter ( Airbox is long gone ). Thanks for any help and hopefully we get to meet one day. I've never seen another in person.

p.s. couldnt attach pictures, too large i suppose
but this is the one, with factory seat and black side covers and clip-ons instead of full bars now http://bayarearidersforum.com/forums/showthread.php?t=529987

Hi there, My bike had a Keihin 24mm carb which is a common modification along with the overbore kit. I tried everything but it just ran like crap much as you describe. I pulled a 22mm off my CB125, big improvement but still not right. Finally got hold of a Keihin PC20 carb and it now runs great. Can't remember the exact main jet size but it's close to 95. But I have reconstructed the air box and installed a large pleated paper filter inside and also added additional air box inlets. So it would be hard to compare jetting with yours lacking an air box. And I have heard from another guy who runs the stock PC16 carb with an 89cc kit and his apparently runs just fine. My bike turned out to have an 84cc kit, not 89cc as I first thought.

I assume you know these bikes had a rev limiter built into the CDI module which limits speed to about 60 kph (37 mph) but only in 4th and 5th gear. The easy fix for that is to jump between the three CDI plug wires shown below. That disables the rev limiter for 4th & 5th gears. Yours may have the Takegawa Hyper CDI which also overcomes the limiter. However from my testing with a timing light, the Hyper CDI also imposes a fixed timing advance matching the stock full advance. In other words it's just like stock timing except it does not retard at lower rpm. I found this to cause poor idle and off-idle performance as well as causing potentially damaging kick back during starting, so I would recommend ditching the Hyper CDI and using the jumper wires instead.

With the CDI speed limiter disabled, the speed limitation becomes wind resistance and engine breathing - with stock cams the HP peak is 10,500 rpm and the engine is really running out of breath by 11K. I've increased C/S sprocket size by one tooth, so for me 11K is 55 mph. I can get it up to 60 or more if really tucked in and maybe encounter a bit of tail wind.

Here's the CDI wiring mod-

Hope this helps, let me know how you get along.

-Bill

 

[andrei3k]

Thanks for the fast reply Bill,

It sounds like i'm maxing out the bike more than anything then. I am indeed limited somehow in 4/5th to around 9k. But i can get to 80kmh pretty fast.
Underneath that rpm the bike runs great and moves pretty quickly. I would say timing and or limiter might be a bigger probelm as my carb is closer to or smaller than 20. In 3rd gear i can get the bike to 13.5k without any issues.
I'll check today and update.

Thanks,
Andrei

 

[andrei3k]

Well, after some testing on a stand, it seems your rpm info was correct. 3rd gear goes to 13.5k rpm, 4th gear cuts out at 11.5k rpm and 5th gear cuts out at 9.5k rpm. This confirmed it is a speed calculation based limitation.
I had a KISS limit cut module in-line which did not function and once bypassed along with wires shorted it revved to 13.5k in all gears.

Thanks again for the help.

Andrei

 

[GB500guy]

Well, after some testing on a stand, it seems your rpm info was correct. 3rd gear goes to 13.5k rpm, 4th gear cuts out at 11.5k rpm and 5th gear cuts out at 9.5k rpm. This confirmed it is a speed calculation based limitation.
I had a KISS limit cut module in-line which did not function and once bypassed along with wires shorted it revved to 13.5k in all gears.

Thanks again for the help.

Andrei

-Bill

 

[andrei3k]

I managed a test run to 92.5km after the wire fix, and
Also noticed I have a pc10c carb on it. Still searching info on it.

 

[GB500guy]

I managed a test run to 92.5km after the wire fix, and
Also noticed I have a pc10c carb on it. Still searching info on it.

Great, that pretty well matches my bike's top speed. My aftermarket speedo read high. My quoted top speed of 55-60 mph is actual GPS measurement, not speedo reading.

I believe the PC10 carb has a 20mm venturi size, same as the PC20.

-Bill

 

[IvoryHammer]

Holy wow, I just want to say that is an amazing bike!

 

[bergmen]

Here are pictures of my restored CR110 that I sold in 1991 to an expert in southern California. It was the best thing I ever did since I had planned to restore it someday but did not have anywhere near the skills that this person did. My tachometer started at 5,000 rpm. Note that the cam drive is on the right side (gear train). No kick starter, bump start like International GP. Exhaust noise was absolutely ear splitting, loudest exhaust noise I ever heard in my life. Engine would barely run below 5,000 rpm, always had to blip the throttle.

Dan

 

[bergmen]

A few more pictures. Me racing the CR at Cotati in 1971 where I took second place behind the AFM champ (also on a CR110). Next photo is the collection of Honda CRs that the restoration expert has restored and owns. My CR is fourth from the left. Some extremely rare CRs here (including a CR72 which only maybe five were built).

 

[GB500guy]

A few more pictures. Me racing the CR at Cotati in 1971 where I took second place behind the AFM champ (also on a CR110). Next photo is the collection of Honda CRs that the restoration expert has restored and owns. My CR is fourth from the left. Some extremely rare CRs here (including a CR72 which only maybe five were built).

Dan, wow very cool stuff, thanks for posting. I watched a couple of AFM races at the old Cotati airport back in the day, may have even seen you race! Since my wife has developed dementia and requires constant care my little Dream 50 is mostly decoration in my shop other than a short ride every couple of weeks. Life can be so disappointing.

-Bill

 

[bergmen]

I'm so sorry Bill, my heart goes out to you. I have been there (my Mom) and I know what it is like.

Your Dream 50 looks fantastic!

Dan

 

[bergmen]

Brand new Dream 50 up for auction here:

https://www.mecum.com/lots/LV0121-435423/1999-honda-cb50j-dream/

Wonder what it will go for?

Dan