83 Z50R Carb Problems

J

jaustinmd

New Member
I have an '83 Honda Z50R that was given to my brother by a friend who has since passed away, so I don't have any previous history on modifications, etc. I "think" it is a genuine Honda Z50R ... it has this label:
IMG_0079.jpg
However, it has a "Sheng Wey" carburetor which I understand is not original:
IMG_0076.jpg
I have cleaned the carb and tried mightily to tune it, but I can't seem to get it adjust predictably! When I turn the idle speed down and rev the engine, it seems to rev up and take forever to settle back down to idle, if it idles at all! I noticed it is bolted directly to the intake manifold without seals or O-rings, so I'm wondering if I have an intake leak. I'm also wondering if it is the correct replacement carb ... it's bolt pattern to the intake manifold is approx 48mm on-center:
IMG_0080.jpg
When I look for replacement carbs, seems most have 32 mm bolt patterns???? If that is correct, seems my carb and intake manifold are too big???
The engine block indicates it is 49 cc. Have I got a waaay too big carb and intake??? What is a good source and brand for a quality replacement carb (and intake if necessary???)
Thanks,
John
 
In your second photo, it looks like paint overspray on the engine case, indicating the cylinder may be aftermarket, and a bigger cc displacement than what it says. DRATV sold "cheater" kits with 75cc even though it said 49cc on the cylinder. Try a magnet on the cylinder to see if it sticks or not. Most big bore upgrades are 88cc and use an aluminum cylinder.
As far as the carb goes, there should be paper gaskets under the heatstop and under the manifold at the head, the carb has an o-ring at the other end of the manifold. Set the idle up enough for it to run, and then spray contact cleaner at the head/manifold area, and also at the manifold/carb area, and listen for a change in idle speed (up or down) for a few brief seconds, and then a return to steady idle speed. If that occurs, you have an air leak. A light coating of grease on the gaskets and the o-ring will help make a better seal.

The 20mm ShengWay carb is relatively easy to tune right out of the box. It comes with the #89 main and a #35 pilot jet, both of which you can buy the extra jets from TrailBikes or any TB sales rep. As a baseline setting, I use the #90 or #92 main jet, the #38 or #40 pilot jet, set the jet needle at the 3rd or 4th notch from the top, and the air screw at 1-1/2 to 1-3/4 turns OUT from seated. Make the necessary air screw adjustments IN to richen it a 1/4-turn at a time and see what the bike does. With some of the carbs, I've run it to within a 1/2-turn of being all the way IN to get rid of 98% of the off-idle hesitation. You might have to raise or lower the clip on the needle, depending upon your altitude, I'm at sea level.
 
A magnet sticks to the cylinder, so it's not aluminum.

I'm still confused about the carb nomenclature. You're saying the Sheng Wey is a "20 mm" carb - I suppose that's the actual bore through the carb? The bolt pattern is about 48 mm on-center. The "20 mm" is about right for the size relative to a 49cc engine? In reading all the aftermarket carb descriptions, I'm under the impression that are about 18-20mm and seems a lot have a "32 mm bolt pattern on-center." I just wondered if my intake manifold and carb are the right size relative to the engine? Any general information about what's correct would be appreciated.

Thanks for all the detailed information. I think I'll run down the checks for intake leaks and if no improvement, then look at rejetting, needle position, etc.

Thanks again,
John
 
20mm is enormous for a 50cc engine. the stock should be… 13, 14, 15 maybe?

the 32mm bolt pattern is distance between center of bolts/studs on the carb.

sounds like you have a vacuum leak. with the engine running, blast the surfaces with carb or brake cleaner from the outside, and see if the idle comes down. if it does, you found your leak. without an O-ring or gasket between that flange, i'd suspect it's leaking there.

your intake is not stock as well.

Make sure the bottom of the carb isn't hitting the cylinder or cases. i've seen that before, especially on the 50's. this holds the carb and manifold away from the head, creating a vacuum leak at the cylinder head.


you really need to figure out how big your engine is, and get the right carb. if it's a 50 and a stock cam, maybe go back to stock size, or even stock carb altogether. if it's up to 88 cc, i still think a '20' is pushing it, unless you're revving to the moon. i have a 110 cc, and my carb ID is 19mm or so.
 
Enginedoctor said:
20mm is enormous for a 50cc engine. the stock should be… 13, 14, 15 maybe?

the 32mm bolt pattern is distance between center of bolts/studs on the carb.

sounds like you have a vacuum leak. with the engine running, blast the surfaces with carb or brake cleaner from the outside, and see if the idle comes down. if it does, you found your leak. without an O-ring or gasket between that flange, i'd suspect it's leaking there.

your intake is not stock as well.

Make sure the bottom of the carb isn't hitting the cylinder or cases. i've seen that before, especially on the 50's. this holds the carb and manifold away from the head, creating a vacuum leak at the cylinder head.


you really need to figure out how big your engine is, and get the right carb. if it's a 50 and a stock cam, maybe go back to stock size, or even stock carb altogether. if it's up to 88 cc, i still think a '20' is pushing it, unless you're revving to the moon. i have a 110 cc, and my carb ID is 19mm or so.
Click to expand...

OK, so here's where I am with the problem:

I removed the carb and manifold, measured everything, and then reassembled with gasket grease (the paper gaskets and O-ring were dry.)

The cylinder is iron and is embossed "49cm3," so I assume it likely is 49cc?

The intake port on the head has a 45mm bolt pattern and the intake port measured 16.3mm:
IMG_0081.jpg
The plastic insulator had dry paper gaskets on both sides. The insulator's bore measured 21.4mm:
IMG_0083.jpg
The intake manifold had 45mm bolt patterns on both ends and the bore was about 20.7 mm on the insulator end and 23.8 on the carb end:
IMG_0084.jpg
The carb indeed has an o-ring and measured about 20mm i.d. on the manifold side and 28mm i.d. on the air cleaner side. The carb's bottom is clear of the engine. Here's some shots of each end after detachment:
IMG_0086.jpg
IMG_0087.jpg

After all this, I can't detect any intake leaks. The bike starts easily enough, but I can't get anything near a normal idle - seems it slows down to a certain point and then won't turn any slower. Am I way over-carbed and this is the problem? Doesn't make much sense to me that everything upstream of the intake port is way bigger in diameter. Would the proper intake and carb likely help?

Thanks,
John
 
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is the carb slide in the correct direction? this will cause a high (very high) idle all day. even with that big carb, you should still be able to cut down the airflow to the point of stalling.

have you blasted the assembled carb/head/manifold with carb cleaner while running yet? it's really your best bet...
 
Update!

The correct carb solved the problem! I purchased an aftermarket carb/air filter/intake from "KDF Sport" in Australia thru Ebay for $72 delivered to my doorstep! Took about 2 weeks to arrive. The carb was physically much smaller than the previous one on my Z50R. Bolted it on, attached the throttle cable, popped on the gas line, and it started after 2-3 kicks and settled right down into an idle!

I only had to tweak the idle about 1/8-1/4 turn to slow it down just a bit and it was perfect! The bike now usually starts on the first kick with a bit of choke and runs perfectly! The carb adjustment screws are on the left (from the rider's perspective) side of the carb, which makes for easy carb adjustments. The only negative is that the short choke lever is on the right side, so you have to reach around from the left to use it or risk getting burned by the exhaust on the right (although the engine is usually cold when using the choke, anyway!).

So, I think the analysis was correct that someone had previously installed an aftermarket carb/intake that was waaay too big for the apparently 49 cc engine!
 
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