Need help Tuning Carburetor

[1975TS185]

Hello all,

I recently got my 1973 Suzuki TS185 over to my main residence in suburban Cincinnati. The bike is an old 2stroke enduro, 183cc to be exact. I am going to restore the bike and return it to street legal form, however before spending a lot of money tracking down parts for a 39 year old japanese motorcycle, i need to properly tune the carburetor (Mikuni VM36 round slide) as the bike doesnt run that well right now. Back in the day when i first got the bike it would die if i let the throttle go below 3k rpm, i ordered a carburetor rebuild kit for my specific carb and replaced all the parts that it came with, the bike now idles smooth and is ridable, but there are spots throughout the throttle that either sputter bad or start to bog out. I understand basic carburetor theory and the different jobs of different jets throughout the throttle range. for some reason i think my bike runs lean, there is barely any smoke coming from the exhaust and it just "sounds" like there is a little too much air in the mixture. Before Ordering any jets or different sized needles, I am going to try moving the jet needle clip up and down to see if i can notice an improvement.

I have noticed that this bike doesnt have very much power, but i feel like it should have more than it does now, although my speedometer is broken, i am positive that this bike will not exceed 50mph on a flat surface, which doesnt seem right.

CONCERN-

this bike and carburetor are 39 years old, so i am very nervous about tracking down different jet sizes for this specific carb, it will be very difficult and probably very expensive to find parts for this carb.


I will update the forum as much as i can throughout the process, i probably wont even get around to messing with the needle for a couple of days or so.

If there is anyone here on the forum that has experience with old 2strokes and tuning them, i would greatly appreciate some insight and advice


be safe and happy riding

[z9nine]

http://www.iwt.com.au/mikunicarb.htm

This may help you out. I just kinda skimmed over it and it goes over how they work, and what different parts do, and how to adjust.

A more indepth google search will prolly turn up tuning for your specific platform.



BE WARNED THIS OPENS A DL OF A PDf FILE FOR MIKUNI CARBS open at own risk, I opened it and had no issues, but you are warned

[LWRider]

Sudco for jets.

[1975TS185]

Okay so i pin-pointed exactly where my bike runs poorly

in 1st gear- The bike is very sputtery in all throttle positions but low throttle and very high throttle, the bike has almost no power in 1st gear

2nd and 3rd gears- at about half throttle the bike starts to sputter and kind of bog, as you give it more throttle it keeps on sputtering

TOP GEARS (4th and 5th)- at about half throttle the bike starts to bog out, there is a real loss of power and if i try to give it more throttle than half, it bogs pretty bad and it feels like the engine wants to die.

so i am pretty sure i need to mess around with whichever jets control 1/2 throttle


the bike also has a bad oil leak (coming from a damaged oil seal behind the front sprocket), this leak has been getting worse and worse the more i ride the bike, and it has gotten to the point where sitting over night it has leaked out about 1/4 of the engine oil, i am out of engine oil and will no longer be riding. I want to fix the oil leak but that would require splitting the engine cases and i dont have a shop/repair manual to aid me putting it back together,

whenever i get around to taking the carb off i will make a new gasket as the current one leaks (right on top of my engine!, talk about sketchy), and after that i will move the clip on the needle down one (recording its prior and current position) and see if the bike runs any better.

JETTING-

i can predict that messing with the jet needle will not aid the problem of sputtering and what not at half throttle, so i will most definetely need an assortment of different sized jets.


My carburetor is a genuine Mikuni VM36 round slide type, so upon ordering jets, do i need a specific type or will any mikuni vm series jets work? also, should i get an assortment of different type jets as well (like different sized pilot jets, different sized main jets, etc..)?


thanks for any insight

[z9nine]

Carburetor troubleshooting is simple once the basic principles are known. The first step is to find where the engine is running poorly, fig 7. It must be remembered that carburetor jetting is determined by the throttle position, not engine speed. If the engine is having troubles at low rpm (idle to 1/4 throttle), the pilot system or slide valve is the likely problem. If the engine has problems between 1/4 and 3/4 throttle, the jet needle and needle jet (most likely the jet needle) is likely the problem. If the engine is running poorly at 3/4 to full throttle, the main jet is the likely problem.

This is from the first link I posted.

[LWRider]

I am not familiar with that particular carb but, for instance, with my Keihin CVs there is a "nozzle" under the jets that has very tiny holes in their sides. You can have a squeaky clean "jet" but if that nozzle is blocked you will still have problems.

Good luck,

Mike

[Bede5man]

Emulsion tubes.

[markk53]

Originally Posted by 1975TS185

Okay so i pin-pointed exactly where my bike runs poorly

in 1st gear- The bike is very sputtery in all throttle positions but low throttle and very high throttle, the bike has almost no power in 1st gear

2nd and 3rd gears- at about half throttle the bike starts to sputter and kind of bog, as you give it more throttle it keeps on sputtering

TOP GEARS (4th and 5th)- at about half throttle the bike starts to bog out, there is a real loss of power and if i try to give it more throttle than half, it bogs pretty bad and it feels like the engine wants to die.

so i am pretty sure i need to mess around with whichever jets control 1/2 throttle


the bike also has a bad oil leak (coming from a damaged oil seal behind the front sprocket), this leak has been getting worse and worse the more i ride the bike, and it has gotten to the point where sitting over night it has leaked out about 1/4 of the engine oil, i am out of engine oil and will no longer be riding. I want to fix the oil leak but that would require splitting the engine cases and i dont have a shop/repair manual to aid me putting it back together,

whenever i get around to taking the carb off i will make a new gasket as the current one leaks (right on top of my engine!, talk about sketchy), and after that i will move the clip on the needle down one (recording its prior and current position) and see if the bike runs any better.

JETTING-

i can predict that messing with the jet needle will not aid the problem of sputtering and what not at half throttle, so i will most definetely need an assortment of different sized jets.


My carburetor is a genuine Mikuni VM36 round slide type, so upon ordering jets, do i need a specific type or will any mikuni vm series jets work? also, should i get an assortment of different type jets as well (like different sized pilot jets, different sized main jets, etc..)?


thanks for any insight

First off, that seal may be able to be pulled out (picked out) without splitting the cases. Seals usually pressed in from the outside in those areas. Check to see if that is the case. There also may be a bushing missing that rides on the shaft against the seal, with an O-ring on the shaft, but I'm guessing on that since I am not that familiar with the Suzuki. Just saying it because it's worth checking into it.

On the VM36, that's really BIG for a 185, the stock carb on a TM400 motocrosser was only a 34mm. My guess is that your bike had a 28mm or 30mm at the biggest originally. I ran a 32mm VM from a TM250 MX on my 175 Bultaco short track bike. Just sayin' the 36 is quite large and may be a headache to jet in. Jets for the VM will be simple to get, most common out there. Sudco and others have great schematics of carbs showing what circuit effects what. But for a rule of thumb generality here it is:

up to 1/8 is the pilot jet and pilot circuit
1/8-1/4 pilot and slide cut away
1/4-1/2 slide cut away and needle jet/jet needle (needle jet is the tube the needle goes in, they are changable, but seldom are)
1/2-3/4 is needle jet/jet needle and main jet
3/4 to full is main jet

You are likely to do okay with needle adjustments.

You may be able to get help from Sudco on the jetting too. Contact them.