Stumbling / 'missing' at high rpm and or under load - 1980 JZ Stock 50cc

[Matchymarty]

Hi all,

Just for the information for all I thought I'd post the following.

I Have been having problems with my stock 50cc rebuild on my JZ.
OS piston, and head job, with 2 new valves, a good clean up, new gaskets, seals and bearings throughout was the build.
A brand new OEM 'JP' model carb was bought and used - supposedly 'suitable for the JZ'.

From the get go, the engine fired up nicely, idled well, and revved to half throttle cleanly, then began to stumble and kind of 'miss' like a rev limiter and die in performance. Particularily noticable under load when going up a small incline.

I’ve changed or done the following:

1. Crank assembly (which happened to be a long rod) – The original short rod crankshaft had bent threaded section at the end of the shaft so thought, maybe the shaft wasn’t running true- throwing out the timing at high revs.
   1. New barrel and piston to suit the long rod.
2. Played with jetting, needle setting etc on the New OEM JP Carb
3. Rebuilt the Stator
4. Checked valve adjustment many times – head was rebuilt with new valves only.
5. Changed valve timing 1 tooth either way
6. Swapped plugs
7. Checked for air leaks many times and changed o-rings for carb etc etc
8. Checked float levels
9. Swapped ignition coil for the From J1 to Jz – no change
10. Swapped flywheels – one better than the other – no change
11. Checked timing a million times with the ‘paper in the points trick’ (which is how I time my Old English bikes)
12. And finally, swapped the Carb from the J1 to the JZ, and… VOILA!

Yes, the standard old 1977 model J1 carb worked like it came out of the factory, even though I'm running a Take Z style exhaust.

So I found the original JZ old carb, junked the bowl, cleaned up the carb, and tried it. Was loads better than the New OEM JP carb, but, still stumbled a little high up. Put the J1 carb back on and VOILA again! Beautiful.

Basically, it seems that in MY experience, the new OEM JP carb did not work well. Looking at them, the slides seem different, but mostly, the J1 needle is a lot fatter and much more slowly tapered, than the new JP model. Perhaps, given a JP is a 12V newer model, perhaps the head and valves are different, necessitating different carburetion.

Moral of the story, make sure your Carb is set to exactly the year correct OEM specifications. I do suspect also, that my original JZ carb has an aftermarket needle in it, but, needles only work in 1/2-3/4 throttle if i'm not mistaken.....

I'm now on the hunt for another J1 carb.

[Classic Honda 50's]

I much prefer the process of elimination by part substitution just like you have done till you get the right result, have had people phone and say that they have been trying to jet a carby for days only to destroy their new motor from inexperience running their bikes way too rich or lean.

I have run the same Honda OEM carb on three of my own bikes (J1, 83R, 89R) without an issue and have sold 100’s of the 16100-165-924 carburettors over the past 12 years in business and have never had an issue before at all, we have quite a few businesses that buy them regularly as well and would have picked up if there was an issue with the carb by now, may just be a one off issue???

The carb needle will be matched to its own needle jet, if you would like to post the carb to me I will have a look over it with my 1000x magnifying set-up to see if I can see any issues and will run it on my own bike to see what it is doing.

Cheers

Jaimie

[Matchymarty]

Note, I'm NOT taking a shot at the OEM Honda JP Carb, I too know people that have used the carb with great success.

I"m more making mention of the fact that Carburettion is HIGHLY important and, can present as many other types of LIKE symtoms.

I've had English bikes with the exact same symtom, and, its nearly ALWAYS the Condensor or, something to do with the windings in the Magneto. Hence why, I went for everything else, before I changed out the carb entirely.

[Alfie]

If bike iddles I set my timing with a timing light, most accurate and once set you won’t need to look at it again.