72 T500 carb weirdness (long post, early vs late carbs)

[m in sc]

so, t500's are an interesting bike for sure. early ones ran 34mm carbs, later they ran 32s thru the rest of the run, but 2 types. they are unique to the t500 as far as I know, and are easily the heaviest 32mm round-slide I've ever come across. up to 72 they were internally vented, and that caused all sorts of weirdness.

My 72 had the last year of the internally vented carbs, and they have worked pretty well since 2017. all stock, but it always ran a bit 'rough' in midrange, to me, and I remember my OLD 74 was much smoother, or so i thought. its been 26 years since I rode that one.

look below, you can see the weird jetting and setup,. but in particular, look at the jet tubes. they are p4 and p5. that's not an 'option', that's literally one size for one carb, one for the other.      always seemed like an engineered 'bandaid' to me. I mean, the plugs always looked even, there's no differences in cyls porting, etc, so i never quite understood it. I've also never read a solid explanation either.





me being me, i actually happen to have a spare set of later carbs. all i know is they are 74 or older. the 73 and up are externally vented vs the earlier ones which are internally vented. ie: the airjet gets it air from that weird little channel and boss on top of the bellmouth on most old VMs, and feeds from the carb bellmouth itself, much in the way a 'smart carb' claims it self adjusts. the RD400s in stock form use a version of this as well with the air jets getting fed from the airbox. the later ones just have an exposed nipple off tot he side there and gets air from the atmosphere.

 keep in mind, internally vented carbs also have NO overflow tube as its suppose dot be a 'sealed' system. I don't like that one bit either. But again, the old carbs did work fine. 

last weekend i rode the 72 and the midrange in traffic was just chunky. so, i broke out the spare carbs ive had (longer than the bike actually) and cleaned them in an ultrasonic. they cleaned up well, all the jet tubes were in great shape as were the needles. (for the 73 and up model , externally vented carb)



then i was like : f it, im going to get my local guy to clean them a bit more since they are totally torn down already. (vapor blasting)

same carbs, 24 hrs later.




Sure enough, after i got them on and got it running (after a cable debacle I caused @ the splitter, but got sorted) they fred right up and the motor immediately sounded better. again, all stock jetting for 74 and up, stock airbox/pipes/ etc.

now, the bike does have a vape cdi and is set at 22.5 degrees btdc (stock is 24) but thats the only performance 'mod' mechanically. t500s have a terrible head design so this helps reduce detonation.



so, the general result, on my bike, i feel is, it seems to have lost a tiny bit of bottom end grunt (it still has PLENTY) right off the line, but tends to idle better, and the power is much smoother, especially at steady low speed setting but it buttery in the mods and up top. it revs higher as well. the pull feels about the same.

what's interesting is the jetting changes. it went from a 150 to a 97.5 , this tells me the signal across jet tube is actually stronger on the later ones, and would explain the smoother operation with the later carb. the needles between the 2 years are almost identical when they are measured  out, the earlier one just has the taper start at a 2 or 3 mm different height, but the diameters are the same.

the 72 seems to have been at a weird developmental crossroad for sure. Supposedly they claim this is the reason the later ones had 4 more hp (claimed) and the later ones were 'detuned' but, i just don't really get that impression that that was solely the carbs.

So, for now it seems the later carbs were the right choice for me, the whole riding experience is more refined. and thats the ONLY change ive done on this in a few years.

If you noticed, there also was a set of vm 34s in one of the pics  . if this didnt work, those were next to go on, but the airbox wont fit w those.


of course, test runs int he bougee neighborhood around the corner on the river      



 

[LMS]

Vapor blasting... After this is done, do you have to do anything else to the carbs? Do they clean the heck out of them after, or is best to do that yourself?

[m in sc]

you should for sure. I sprayed thru everything w brake clean as usual just to make sure. that's it. I didn't see any residue persay come out or off, but better to be safe than sorry.

[LMS]

You cannot beat the looks, makes them look better than new.

[m in sc]

fwiw, it also lays over the aluminum a bit, which sort of seals up the 'pores' of the metal, and it will actually keep it from corroding quite a bit vs just a cleaning.

the vm 34s in the background were done 5? years ago and run for 3 on premix on the lc till i put the tm32s on, they still look new inside, and i didn't drain them properly when i took them off. (was till some oil residue and fuel in there in the corners)

[msr]

This is all fascinating! Probably the Suzuki engineer who knows why there were two different jet tubes died in 2005.

[Kawtriplefreak]

Very cool!

[m in sc]

This is all fascinating! Probably the Suzuki engineer who knows why there were two different jet tubes died in 2005.

or he went to go work at smart carb.     

speaking of, I ran across this not too long ago. pretty interesting. he also has a smart carb ona 50 cub and seemed to like it in another video.

[m in sc]

as a follow up, rode the bike in today after it sat for just under 2 weeks or so, still so much better than it was. will be cleaning up the original early carbs and selling them soon i think. 

[mixedbikes]

Did you have a company or a hobbyist vapor blast? Reason I ask is I currently have my blast cabinet apart, and am converting to vapor blast. Have watched many videos on doing it, and finally bought stuff to do it. Your carbs look great!

[m in sc]

Thnaks! local dirtbike shop, same place that mounts all my tires. he got it to do cyl heads and cases for rebuilds primarily. one day im going to pull the top triple tree off have him do that as well.

[mixedbikes]

The company that sells vapor cabinets is somewhere in your area I think(carolinas). I got info from them, but for the price(they wanted 3 to 4K), i can convert my cabinet. I only have about $300 in my conversion, and my cabinet has long since paid for itself, so we will see how it goes.

[m in sc]

yeah, chuck paid like 5k back 5 or 6 yrs ago, he's getting a bigger one so he's going to use the smaller one for different stuff. he charged me 60 bucks to do the carbs, which to me was more than fair. if one ever doesn't become available, i'll look into one maybe. i literally drive by the shop 2x a day so its super convenient.
 

[mixedbikes]

That sounds like a very fair price for that quality of work. We have pretty much no bike shops (decent ones) around here anymore. really like the T500, such a clean well designed bike!

[edgefinder]

A banshee engine builder that did Pikes Peak hillclimb for years told me he jetted richer on the right side because of higher heat from gearbox oil on the right side. As if jetting wasn't already hard enough there.

When 2 strokes got really fast in the 60's it took a while for the engineers to learn how to design a gearbox that didn't generate too much heat. Some did and some didn't so they figured it out pretty quick. Kevin Camerons story of the first KR250 race bikes with compact cases around the gearbox that looked really cool but got so hot the paint came off comes to mind.