Suzuki 2 Strokes

Hello folks. I finally made an account! I have been using this as a resource for most of my project and finally joined up because I need some advice!

I just tore apart and rebuilt a '74 L and aside from putting the pistons in the wrong holes it has been going pretty good!

After I corrected that dumb mistake she rose from her 30 year sleep!

Although she doesn't run very well with the choke off. It idles great and has good power (with a small bog down low) with it on.

Now take it off and it barely runs. If you feather it you can keep it going and riding it there is absolutely no power with it off.

These carbs (CV) had gas in them the entire time it sat and it was a mess. I had to buy a ultrasonic cleaner just to get the junk out of there. After hours of poking and soaking all the jets are free and clear of debree.

Replaced: Two floats & set height, Seat & Float needles, Main needle Jets, Choke & Bowl Gaskets, Choke Plungers, and O-rings

Timing is also dead on (With the marks anyways, don't have a dial gauge)

Where would you folks start? Thanks in advance, great forum here!

I don't have a lot of experience but I'd start by checking the timing. I used a auto timing light on my 550 and it worked perfect.

The timing is dead on the marks with a light. I wish that was the problem! Thanks.

Are the main jets genuine Mikuni small round jets, if not, replace them with Genuine parts. What slow (pilot) jets are in there now and where did they come from? There are two types of jets and VM mikuni types don't work in those carbs.

You mentioned replacing the "Main needle jets". Needle jets are not available AFAIK, so that probably relates to the main jets.

When you cleaned the carbs, did you test the air passage from the bellmouth through the middle of teh carb? Those tend to clog up and screw things up.

http://pinkpossum.com/GT750/carb/BS40carbs.htm" onclick="window.open(this.href);return false; for a more complete story.

Further to TZ's comments it does sound like you still have some blockages - given your description of how badly the carbs were gummed up, my own experience has been it can take several tries to get everything clean enough to work properly. The passageways are quite small for the pilot air jet and also the main air jet, and getting the needle jet out of the carb body and cleaning the annulus can be a pain. In addition to TZ's PinkPossum site (which you should bookmark), if you haven't done this before there are a couple of videos over on the Kawasaki board that may be worth watching - the carbs are different to what you have, but the principals are the same. I made up tools something like what are talked about in the second of the two videos some years ago and still use them.

The direct link to the page is:

http://kawtriple.com/mraxl/carb/carbclean.htm" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

The videos are listed at the top of the page.

Good luck!

tz375 wrote:Are the main jets genuine Mikuni small round jets, if not, replace them with Genuine parts. What slow (pilot) jets are in there now and where did they come from? There are two types of jets and VM mikuni types don't work in those carbs.

I replaced the main jet (bottom of bowl) with this ebay'd rebuild kit.

tz375 wrote:You mentioned replacing the "Main needle jets". Needle jets are not available AFAIK, so that probably relates to the main jets.

Jet Needle, with the clip is what I meant. Same thing as above, what came with the kit.

tz375 wrote:When you cleaned the carbs, did you test the air passage from the bellmouth through the middle of teh carb? Those tend to clog up and screw things up.

I used fishing line and heavily sprayed carb cleaner and air through the main air jet and slow jet on the mouth until they had good flow. I also ensured everything was free to the holes atop of the butterfly valve.

Honestly I did not replace the jet that is on the top of the bowl (kit came with one). They are really in there and I nicked up the slots pretty bad. I found out through a video on youtube I should have been using a flat blade screwdriver which I cannot find anywhere for the life of me. Not sure how to get them out now.

I am confused on all this termonology, sorry. Thanks TZ!

oldjapanesebikes wrote:it does sound like you still have some blockages - given your description of how badly the carbs were gummed up, my own experience has been it can take several tries to get everything clean enough to work properly. The passageways are quite small for the pilot air jet and also the main air jet, and getting the needle jet out of the carb body and cleaning the annulus can be a pain.

I soaked the bodys in gasoline for three days, then ran them all through quite a heavy amount of ultrasonic cleaning with simple green & water. By the way, Simple green discolors the aluminum very dark if you leave them sit in it!

I am just not sure where things are plugged up at. Maybe it's because I didn't replace the jet that screws into the top of the float bowl. I am beginning to think maybe I have a leak somewhere? I will test with some ether tomorrow.

Thanks guys!

In my experience, the jets and needles in those kits are not made to the same degree of accuracy as genuine Mikuni parts. I use the gaskets and choke plungers and that's about it. The needles I measured were thinner than stock which will make the bike run slightly richer at part throttle.

In your case, the part that appears to be causing the problem are the pilot jets aka slow jets and they are the ones in the top of the float bowls. The calibrated orifice is less than 20 thou in diameter and it clogs easily.

I would recommend removing the bowls and getting those jets out and replacing them with genuine Mikuni BS style jets. They can stick in there and I have heard of people using EZ outs and drilling them out. You need a good screwdriver bit though. I found one designed for snap in application with the usual hex base that has a 1" long shank and 1/8" wide tip - perfect for small jets.

TZ, . After much swearing, cursing, and all around feeling sorry for myself I manged to get all three mangled slow jets out and replaced them with what was in the kit (marked with a 47.5, same as what came out)

It runs worlds better with the choke off! Purrs like a kitten. There is still a bog down low when riding followed by a strong pull but I am beginning to think it is because I used single pod filters. Right now the Jet needle clip is in the middle position, where I found them. Do you think trying to round up the old airbox would correct this?

Also, she is smoking quite a bit on heavy throttle.

Thanks a million!

After looking through the replies I'm sure both Richard and Ian will agree that one more ultrasonic cleaning session would not be a bad thing, especially the slow circuits in the bowls
Its probably not what you want to hear but considering the stuff you had in the carbs, I'm sure that every time you clean them some more debris seems to appear.
The stock BS carbs don't react that well with pods, so you are best to keep the stock airbox and filter fitted if the engine is going to remain in stock trim.
In regard to the smoking, on the engine at the lower front are the three SRIS check valves, take them out and see how much old oil residue drains from the crankcase.
Then clean the valves by soaking them in some solvent. Its a one way check valve that gets gummed up with old oil. When cleaning them what ever you do, do NOT blow compressed air through them.
Clean them and check the one way action with a hose fitted using your mouth to actuate it.
If you have not changed the oil in the CCI tank and don't know what it is or how long its been in there. You should drain it out and replace it with a modern two stroke oil.

I am really at my wits end with these!

I have pulled them apart two more times and ensured every passage is open using an entire can of Carb Cleaner and a few more ultrasonic baths.

It still has a slight bog (under 2500rpm) without the choke on. I am positive the entire slow circuit is totally clean. Good flow through all passages.

I also did track down all the air cleaner parts and this did make the bog less noticeable.

When cold the left cylinder exhaust does not seem heat up nearly as fast as the other two (although there is some heat), as soon as I turn the choke on it heats up instantly with the rest. So this would suggest a problem with the slow circuit for the left cyl. I do remember when putting the jugs on that I bunged up the left lower ring so in a bind I used an old ring. Threw it together and checked compression and it was 110, assuming it was O.K I continued on. The old ring was not 1st over like the rest. I know this is a "nono" but intended to get new pistons, rings and hone when I get some more cash to throw at the project. Maybe this lower ring is not creating enough negative pressure to pull fuel through the slow circuit but enough for the choke circuit?

Another thing I noticed while cleaning was that the center bowl is slightly deeper than the others. I remember reading somewhere that this is normal as the center cylinder is slightly larger displacement. On the drain plugs on the bottom of the bowl one doesn't seem to match, It is longer and has a passage through it, Looking at pictures of others I don't see this. I matched it up with the center bowl as the cavity is deeper.

I am getting pretty grumpy and running out of beer over this and I am beginning to think forget it, just run it with the choke on...

Another question I have is how do these things advance the timing?

Any more thoughts would be greatly appreciated!

Thank you!

Old bikes can be frustrating at times. Your symptoms could be anything from a spec of dirt in one passageway, a tiny air leak, or something as simple as the mixture screws being out of adjustment or the carbs out of synch.

Try the simple things first. check how many turns they are from fully seated but don't go too tight on them - just lightly tighten them and then open them back up. Start at 3/4 turn and try that. then 1 turn and the then 1 1/4 turns and see if that cures it.

Next, remove the air box or the connectors so you can get a finger into the carb mouths and lift each slide with a finger and left it drop back. They should all move relatively slowly. If one moves much easier than the others, the diaphragm isn't seated properly or has a leak.

You can also run the bike without filters and blip the throttle. The slides should rise and fall in unison.

Timing is fixed - no advance or retard, and that should be checked again just to be sure.

Compression at 110 with the throttle open on a fresh motor sounds a little low to me, but I have never built a completely stock motor so I can't swear to that.

Can you post a pic or two of the bike and carbs. I doubt there is anything that we'll spot, but you never know.

And please order a set of genuine jets to replace those jet kit jets that are in there at the moment. You can get them from PSEP. JetsRus, Sudco or your local dealer. MikesXS sell them in pairs, so you could get 2 pairs from him. But try your dealer first.

http://www.mikesxs.net/product/48-0608. ... -0608.html" onclick="window.open(this.href);return false;

tz375 wrote:Your symptoms could be anything from a spec of dirt in one passageway, a tiny air leak, or something as simple as the mixture screws being out of adjustment or the carbs out of synch.

All the passage ways are clear, tore them down and ultrasonically cleaned them for the 4th time using simple green and warm water, verified with carb cleaner. I also made sure the carbs are synced, the butterfly valve is only exposing the first pilot hole at rest on all three. Also no air leaks.

tz375 wrote:Try the simple things first. check how many turns they are from fully seated but don't go too tight on them - just lightly tighten them and then open them back up. Start at 3/4 turn and try that. then 1 turn and the then 1 1/4 turns and see if that cures it.

Mixture screw has no affect on the poor cylinder.

tz375 wrote:Next, remove the air box or the connectors so you can get a finger into the carb mouths and lift each slide with a finger and left it drop back. They should all move relatively slowly. If one moves much easier than the others, the diaphragm isn't seated properly or has a leak. You can also run the bike without filters and blip the throttle. The slides should rise and fall in unison.

The diaphragms and slides are all seated and seem to be operating properly.

tz375 wrote:And please order a set of genuine jets to replace those jet kit jets that are in there at the moment. You can get them from PSEP. JetsRus, Sudco or your local dealer. MikesXS sell them in pairs, so you could get 2 pairs from him. But try your dealer first.

Genuine Jets did not solve the issue


I am now convinced it is because I used an old ring in a pinch because the poor cylinder (L) is the one with the old lower ring. Although it has matching compression I feel that it has something to do with it. Now that cylinder is barely even running with the choke.

When you ride it, it's like the poor cylinder only comes alive at more than 50% throttle. You can audibly hear it.
Changing the timing also has no affect on this cylinder. Pulling the plug wire at idle doesn't make any audible changes. Although pulling the carbs off and spraying starting fluid in the poor cylinder it will run and burn it off.

I bought these jugs off of ebay because they were in considerably better shape then mine they also came with a matching piston and ring set. He said they were first over but I am not sure, the old ring (which I am positive is stock) seemed to match identical to the new ones.

I think I am going to have to pull the heads for the 3rd time, measure them and get a proper piston/ring set + a hone.

Does anyone have any experience with these sets? http://www.ebay.com/itm/Suzuki-GT750-Pi ... bf&vxp=mtr" onclick="window.open(this.href);return false;

Not sure if this might help....

It does sound like a slow circuit fuel issue based on your description but it appears youve been thorough and gone through all that. Just a small point that you referred to the pistons and rings as 1st over but the picture on ebay is 1.0mm over. The pistons are sized in .5mm increments so if your pistons are truly 71mm, they are actually 2nd over. Every piston ring has a marking on the tang (opening). Its very hard to see but it will verify if you have the correct rings to match the pistons. The motor can actually run fine without the lower ring installed. I wouldnt recommend that for a stock street motor but some race motors were setup to run without the lower ring to avoid snagging on the intake roof where the blocks were raised up above 3mm. Anyway, if you are going to take the block back off, check the bore and makes sure your tolerances are correct for the pistons installed. Make sure the rings are correct for the pistons. You might also want to check and verify that the L coil is functioning properly and providing a strong spark when you kick it over. 110 compression is low for a stock motor but enough that it should run properly from idle all the way up. If its not A/F or compression, it might be ignition (coils).

If you have a spare coil set, I would try that to rule out a weak L coil.

Problem Solved!



Runs on that cylinder now. I cannot possibly imagine cleaning anymore.. I have swapped everything around (jets, slides, needle). I have no clue what's the matter with this carb is but I know its not blocked up.

I am going to a salvage tomorrow to try to find a different body.

Have you tried to spray carb cleaner through each drilling in that carb and then repeat on the carb next door? Do it one hole at a time and compare them. My guess is that there's still a partial blockage in the float bowl or in the slow jet passage in the main body.

that's the only way to know for sure that all the tiny holes are clear.