Diaphragm Type Carbs?

I know some people have run diaphragm type carbs (Tillotson, Tecumeseh, etc.), but I haven't been able to find much info about them. As as I can tell, here are the deets:

Pros:

-Cheap. Like, really cheap.

-No jets, so they're super easy to adjust.

-No float bowl, so you can mount them in whatever position you want.

-Readily available parts.

-Super small.

-Really lightweight

-Not gravity fed, so you can get creative with your mounting position

Cons:

-You need to drill your case to accept a vacuum nipple.

-Possibly not quite as tunable? (only "low speed" and "hi speed" settings, as opposed to the multiple circuits on the nicer Mikunis and Dellortos)

-Apparently kinda weird throttle cable setup.

-No one seems to know much about them, so good luck getting help tuning it.

-???

So who's used one? How do they compare to the usual Mikuni's, Dellortos, OKO's, etc. in terms of performance and ease of tuning?

My assumption would be, since they typically seem to be used on engines intended to run steady state (small aircraft, lawn mowers, weed whackers, stationary engines, etc.), their transient throttle response might not be fantastic. They also seem to be mostly used in a narrow RPM band, so I would guess they are difficult to tune for a wide range of engine speeds. That said, I know they're pretty popular among kart racing dudes because the lateral G's don't cause problems like they do in float carbs. So, what's the deal? If I could swap in one of these bad boys and never have to deal with jets again, I would be a happy man.

Re: Diaphragm Type Carbs?

Moby Dick /

Actually, I take that back about them being cheap. Turns out I was looking at some of the really teeny chainsaw ones. They're comparable to the carbs we know and love/hate.

Re: Diaphragm Type Carbs?

A BIG advantage whit pumper carbs is that you can make a really short dead straight intake no matter what your engine orientation is - should make more hp than the long bends most people are fitting.

I've never used one and have no experience in tuning them though...

Re: Diaphragm Type Carbs?

Your assumptions mentioned above are well founded based on the experiences the local boys have had with these carbs around here. One discovery was that they are more tunable on reed valve set ups because the pull through reeders is more consistent during the piston stroke rather than the big sudden gasp of a piston porter. If you rigged two of these carbs you could adjust one for the lower rpm and the 2nd for the upper rpm to increase range. Before final tuning there is a blow off adjustment that must be performed that you can study up on

Re: Diaphragm Type Carbs?

Moby Dick /

I'm kinda considering getting one of these just to play around with:

http://vkakarting.com/?p=2486

The price is certainly right. I'm thinking these would be a pretty good fit for variated bikes, since they really only have to run within a small RPM band and the transient throttle performance doesn't matter much.

Re: Diaphragm Type Carbs?

There is a cool way to make housed clutch cable provide for a handle bar mounted 'jet on the fly' arrangement

Re: Diaphragm Type Carbs?

Interesting, don't some scooters use diaphragm carbs? Probably wouldn't be hard to make one work on a ped and it would make mounting and tuning much easier since it runs in any position and doesnt need jets

Re: Diaphragm Type Carbs?

Some e50's have a nice little port in the crank section of the bottom case that works good to hook up these carbs.

Re: Diaphragm Type Carbs?

Moby Dick /

Yeah, that's kinda what I'm thinking. I think I'm going to buy one of those slightly dubious Tillotsons just to see what's up.

Does anyone have more information on tuning these things? I found a bit, but most of it is pretty vague and very poorly written.

Re: Diaphragm Type Carbs?

I was thinking you meant diaphragm carb's like the ones on all the 4t modern carbureted bikes & scooters, where throttle operates a butterfly valve and the slide's drawn up by vacuum. those are kinda cool i guess on 4 strokes, for smooth operators. That's what grimm is thinking of

the pump/diaphragm carbs you mean are mostly used on things which don't have a specific orientation, like model airplanes and stuff because there's no consistent direction of gravity. Its also used in karts because the g-forces when cornering shift gravity away from the vertical.

However on motorcycles/mopeds, as you turn you're leaning to counter the lateral g-force, and the gravity vector always remains parallel to the bike

So basically the carb turns with the bike, there's always gravity feed, and that works better for motorcycles.

Re: Diaphragm Type Carbs?

kart.jpg
bike.jpg

maybe this help? basically the fuel level is always perpendicular to g-forces

g force on kart when turning pushes fuel to sides of carb

but with bikes, the carb leans with bike, and fuel's always properly in the float bowl.

that's why they use diaphragm carbs in karting, or like chainsaws n RC planes where you're moving the thing around alot.

not that you can't use a diaphragm on a ped tho. I had a murray that came with a tilotson rigged onto it. it worked but not as well as a proper carb... (edited)

Re: Diaphragm Type Carbs?

Moby Dick /

Yeah, I know the advantage of a pump/pulse pump/membrane/diaphragm/whatever you want to call it carbs is that they aren't really effected by g-forces, and that bikes don't really need that due to the whole leaning thing. The main reason I'm interested is the whole lack of jets thing. How cool would it be to be able to adjust your mixture without even having to stop the bike?

That said, it seems like there might be a few other advantages too.

On a single variated bike, the engine moves around so the float bowl is always changing orientations. It's not a huge change, but it's not insignificant either. Maybe it doesn't make a difference, but it seems like it would. Also, the engine wouldn't necessarily die if you dumped it. That's not really an advantage on a street bike, but it would be really handy on a race bike.

What exactly was the issue with the Tillotson you had?

Re: Diaphragm Type Carbs?

Check them out. http://www.tillotson-racing.com/

We deal with them all the time on small engines.

Re: Diaphragm Type Carbs?

Check with moped folks in Brazil - they use diaphragm type carbs

Re: Diaphragm Type Carbs?

Giovanni Giarratana /

I currently run them on all my peds check out their site for proper CC sizing, I run an HL 141 carb from tillotson, runs so smooth when you have it tuned right, which isn't hard, just doesn't seem to have the torque like a vm18 has, but it could be because the carb was a little to big and maybe coupled with a smaller intake manifold like 18mm intake it would gain back some of the low end torque, thing spools up so fast,

**Also you don't need to tap into the case for signal, if your carb has a nipple type signal feed, then you can weld a nipple to your intake and connect a small rubber fuel line tubing

Some have a signal hole on the flange where you would bolt up to the intake, again you only need signal so if you grind a little pathway from the signal hole to the flow of the intake then that is enough, I'd post pics but never took any, I can this week if anyone is interested let me know

Look at the little hole on the attachment, all you need on a 1/4 inch plate flange is a little path to this hole to recieve signal/vacuum to power the pump

Re: Diaphragm Type Carbs?

#CrazyWayne™ rocks. #CrazyWayne /

You jet the mid with spring pop off.

Re: Diaphragm Type Carbs?

John Joedicke /

Tillotsons were used on practically all early 2 T snowmobiles without issues. Simple to work on and very rarely failed with harsh conditions. Why wouldnt they work on a simple moped?

Re: Diaphragm Type Carbs?

Moby Dick /

I'm sure they would work, I'm just wondering if they would perform as well as a float carb.

Re: Diaphragm Type Carbs?

Local experience is that if the rpm range you are using is not too wide you are OK. Float carbs themselves could use a fourth jet for the very highest rpms and expansion pipe effect

Re: Diaphragm Type Carbs?

Flagrant - Wrote:

-------------------------------------------------------

> Local experience is that if the rpm range you are

> using is not too wide you are OK. Float carbs

> themselves could use a fourth jet for the very

> highest rpms and expansion pipe effect

I represent to you; The Powerjet!

Even better with a solenoid in the line - shut it off past peak to maintain the correct afr when the pipe starts working against you and the transfer port time-area is getting insufficient. (edited)

Re: Diaphragm Type Carbs?

Giovanni Giarratana /

What carb is that and how much? Looks sweet, I'm pretty sure with the tillys they just keep pumpin,

I used tillys in go carts, 4 stroke engines, and it was like going from

Carbureted to fuel injected

Re: Diaphragm Type Carbs?

Keihin pwk, or oko/polini/mk/stage6++ copy.

I think you can get a "tilly" with powerjet to, maybe...

I've seen pumpers with a very thin slide instead of the butterfly valve to, looks sweet!

Re: Diaphragm Type Carbs?

You can make your own power jet and adding it to a tilly might be sweet

Re: Diaphragm Type Carbs?

Moby Dick /

I'm thinking these might be pretty damn awesome on a single variated bike, particularly for racing applications. The engine runs pretty much full blast or idle all the time, with very little in between. That and the RPM should ideally remain within a fairly narrow window.

Re: Diaphragm Type Carbs?

my biggest issue with the tilo i had on that murray was it was really awkward in size shape and mounting and was sortof a giant mess. It idled OK, ran OK, but the thing ran a bunch better with a 14 bing. however a big part of the weaker performance may have been the retarded shitshow of an intake the guy'd fabricated to mount it.

As far as tuning on the go, some older excellent performing downdraft carbs like the dellorto UA19 were tuned by a screw rather than replacing jets.

Re: Diaphragm Type Carbs?

Jonathan Eaton /

I've got a rotary valve 100cc PCR engine in a racing kart I recently got on craigslist for a hundred bucks, they really are brilliantly simple and run just fine. Look pretty cool too, mine has a nice bright red velocity stack on it. Don't see why you couldn't run one on a moped.

Re: Diaphragm Type Carbs?

Post some picks of that engine!

Re: Diaphragm Type Carbs?

Jonathan Eaton /

Here she is

image.jpg

Re: Diaphragm Type Carbs?

Jonathan Eaton /

I'll get a better pic of the carb when I get home.

Re: Diaphragm Type Carbs?

Moby Dick /

Holy cooling fins...

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