T500 balks down anything over 1/4-1/2 throttle open

[rngdng]

Are you positive those are the older style carbs? 150 mains in a carb designed for 97.5s would act that way. I had the main jet fall out in an RMX250 once and it ran great until you opened the main jet. Then it completely died. It was quite rich with no jet......

Something else that happened to me, I got it in my head to use the short intakes from a T500 on my GT500. BIG MISTAKE, as the mains were no longer large enough due to lack of intake velocity, so I'm told. Running on the mains caused piston meltdown (sorry Dofin!).



Lane

[Coyote]

This is a puzzle. I'm not a 500 man so I am shooting in the dark. I have to assume the 150 jets were in the bike when you got it, so those are 'probably' correct. However there is no way of knowing what the PO's have done. Are the jets Mikuni or jets from some kit? If she smells rich and puff black smoke. the jets could be wrong.
Have you tried running with the fuel cap open? A clogged cap vent can cause that exact problem.
From there I would turn to this 'Power Dynamo" you installed. What exactly does that control? Remove it and go back stock and just maybe the problem will leave with it.
All that's left after that is crank seals. Who rebuilt the crank? Are you sure the seals are installed correctly / direction?. Like I said I'm not a 500 man, but my mind points directly at this Power Dynamo or a rapidly failing battery Good luck!

[keithwwalker]

I am gonna pull the carbs tonight and check the jet needles (I checked the floats, needles, main jets, pilot jets and and slides; they are stock for a 72' in both).

The left carb is original, and the right is not (ebay purchase). The right cylinder is smoking a bit more, so perhaps there is a difference.

The bike is a 1972 model.
Both carbs have the later floats (looking at the fiche for the early, there are four floats). I might of mixed and matched components of the right carb, not realizing the difference.

Again looking at the fiche, I to check the jet needle of the right carb to ensure that it is a correct match with the jet. It seems that the early carbs had a Q5 jet, rather than the later P4/P5 jets. And later T500's had the P4 jet needle on the right - this is exactly opposite of having a Q5 which would make it much richer with the stock needle. This would be 6 jets richer(!).

Question: There seems to be three different variations of carbs:
Early: T500 VM34 (barrel bowl with two floats)
Mid: T500III, R & T500J 150 main jet & 5FP8-3 needle
Late: T500's & GT500's had the 97.5 main jet & 5FP17-3 needle

How to spot the difference between the mid and late carbs?

Thanks for all the help. I rebuilt the engine, then my life had a major change and I moved from the east to west coast. So there have been years between working on the bike.

[ConnerVT]

Yes, Suzuki played around a bit with the carbs throughout the model years. For a bike where very little changed otherwise, this is the one thing that gets people mixed up quite a bit..

1968 carbs were 34mm bore, and rare as hens teeth. Even the parts manual most people find on line show the later VM32 as the replacement carb part number. I think Suzuki had no replacements, and had folks switch over to the 32mm carb if it needed replacing. Most of the parts are unique for that VM34 carb, and has jetting unlike any other model year.

1969 thru 1972 are VM32 carbs, which were "Homo pressure" What that means is that the fuel bowl was vented to the intake bell of the carb, instead of the more conventional venting to atmosphere (the outside world). You will note the lack of an external fuel overflow on the carb. It runs a #30 pilot jet, 150 Main jet, and P4 and/or P5 Needle Jet. For some reason, Suzuki changed the spec on these Needle Jets from year to year, with no change to the Jet Needle (5SP8). These engines have short intake boots, and a paper element air cleaner.

1973 thru the end of the line (GT500 included) have normally vented fuel bowls. Pilot is still #30, but the Main is 97.5. They all have P4 Needle Jets, and a slightly richer 5SP17 jet needle. The intake boots are longer, and the air cleaner is now a foam sock over a metal cage. Some say that there is a slight difference in the intake casting of the cylinders as well.

Having played with the jetting of my T500 (a '71 with a '73 engine, with '71 carbs and intake), I believe that the main reason for the difference in the Main Jets going from 150 to 97.5 is strictly due to the change for internally to externally vented fuel bowls.

[Coyote]

And to confuse things further, you have hex head jets, large round head jets and small round head jets The change from 150 to 97.5 is not really what it sounds like. It's probably a different type of jet all together. That's a difference of 21 jet sizes. That doesn't compute regardless of how it's vented.. What I believe is the numbering system changed right along in that era.
Just go to Sudco.com and download their carb catalog. Then go through the Mikuni section. You'll be a drinking man before it's over. The download is a PDF and it's free.

[ConnerVT]

All of the stock T500 carbs I have seen have been round jet carbs. As round and hex jet numbering don't match 1 to 1, the Suzuki documentation (Service manuals, part lists, service bulletins) would be all over this if Suzuki had put hex jets in T500 carbs at any point. If you find a hex jet in a T500 carb, someone other than Suzuki most likely put it there.

[Coyote]

I didn't mean to imply they were in a T500. I just meant that hex jets do exist to further confuse our lives. Personally I have never seen one in real life. Only pictures and illustrations. I have no idea what models used them, but I know some did. While were on the subject, I have never been inside a Kehin carburetor. Seen lots from the outside on Honda's. Are they vastly different than a Mikuni VM? Someday maybe I'll get to play with a set

[ConnerVT]

Sure, you can put just about anything into anything, if you set your mind to it. There is probably someone crazy enough to fuel inject a water buffalo motor, and stuff it into a Gixer frame.

I figure that the vast majority of the folks who find their way here (and aren't asking detailed, parts specific questions) have a mostly stock motorcycle, and parts bought from bike breakers on eBay, Suzuki dealers, or Paul Miller and the like. So the first line of answers should address what these bikes had as stock OEM parts.

As the OEM needle jet series is no longer produced, folks can switch to hex style needle jets (and appropriately sized hex main jets). but his photos show genuine Mikuni (square logo stamped on main jet) 150 round main jets. So the question is, are the appropriate Main Jets installed? If it has the original homo-pressure carb body, then yes, it is.

[GTandcbr]

I don't have an answer for you but have a read here
http://www.klemmvintage.com/mikuni-tuning.htm" onclick="window.open(this.href);return false;
Lots of useful info which may help you

[keithwwalker]

Guys, a quick update.

I got a chance over the weekend to ride the bike a longer distance to see if I could diagnose my rough running issue anymore.

I was able to do 100 miles with the bike (no melted pistons), but with the exception of a certain sweet spot with the throttle opening, it was rough to launch, and again with open throttle beyond 1/2 it just wouldn't run clean.

I pulled the plugs and the right side spark plug is showing signs of heavy oil build up. I am fearing that this is a failed center crank seal. The left spark plug was much cleaner. This coincides with other riders on the ride noting that my right cylinder was smoking.

I wondering if this is a particular problem with right cylinders, as this forum thread talks of the same failure, why does it affect the right cylinder and not the left?:
http://www.motorcycleforum.com/showpost ... ostcount=6" onclick="window.open(this.href);return false;

I will post photos of my carbs showing the bowls at a later time; but I need to know who can rebuild the cranks properly with new center seals.

I believe I have eliminated just about every other issue from consideration.
btw, here is a FB post on the ride, it was a 2 stroke ride in Portland - great bikes were in attendence, check it out and 'friend' me:
https://www.facebook.com/keith.walker.1 ... nref=story" onclick="window.open(this.href);return false;

Thanks again for all the help!