Has anyone used Jet Ski carbs?

[esc]

I've been reading here about people using Mikuni carbs, having issues with floats, and various comments about fuel pumps and their use.

Coming from a 2-stroke Jet ski background, I'm just wondering if anyone used Jet ski Carbs on airplanes.  It seems to me that a Mikuni Jet ski carb would have a several of useful things going for it.

#1 Probably the most important reason: it will operate in any orientation.  It doesn't care if it is right side up, upside down or sideways.
#2 They have built in fuel pumps.   The fuel pumps are designed to be powered by the pressure/vacuum pulses from a 2 stroke crank case, but a 1/2 VW might have enough of a signal to operate the pump.  This could be very handy if you were up side down.
#3 They don't use floats or bowls (see #1 and #2) and all the issues those bring.  That isn't to say that they don't have their own issues, they are just different.  Also tuning them is a little different than tuning a "normal" carb, but not difficult once you get used to it.
#4 (for me anyway)  I have a few laying around and it might be nice to put them to work.

[Steve]

I've been reading here about people using Mikuni carbs, having issues with floats, and various comments about fuel pumps and their use.

Coming from a 2-stroke Jet ski background, I'm just wondering if anyone used Jet ski Carbs on airplanes.  It seems to me that a Mikuni Jet ski carb would have a several of useful things going for it.

#1 Probably the most important reason: it will operate in any orientation.  It doesn't care if it is right side up, upside down or sideways.
#2 They have built in fuel pumps.   The fuel pumps are designed to be powered by the pressure/vacuum pulses from a 2 stroke crank case, but a 1/2 VW might have enough of a signal to operate the pump.  This could be very handy if you were up side down.
#3 They don't use floats or bowls (see #1 and #2) and all the issues those bring.  That isn't to say that they don't have their own issues, they are just different.  Also tuning them is a little different than tuning a "normal" carb, but not difficult once you get used to it.
#4 (for me anyway)  I have a few laying around and it might be nice to put them to work.

The crankcase has oil vapor that will collect in your fuel pump and kill it... Might try getting a pulse signal off the carb intake tubes...

[esc]

You may be correct, I really don't know.  But the "fuel pump" is just a diaphragm that is pushed back and forth by the pressure/vacuum pulses of the fuel/oil mixture from a 2 strokes crank case.  It doesn't seem intuitive to me that the vapor in a 4 stroke crank case would be any harder to deal with.

But now that i think about it, having a carb that can run upside down may not actually be much of a benefit for a 4 stroke engine without a dry sump system.  Keeping the engine running while it is starved for oil may not be doing yourself any favors.  Which would be worse, flooding/stalling or keeping running for a minute or two until oil starvation blows it up?  I suppose that extra minute or two might be nice to have in an emergency and a blown up (or damaged) engine is better than a crash.

[esc]

There is one other possible advantage that I just thought of.
Many Jet Ski carbs have idle, mid range and top end mixture screws that can be adjusted while the engine is running.
Once properly secured, it was pretty easy and to run a jet ski at full throttle, under load and adjust for maximum RPM.

Not having to take anything apart in order to adjust your fuel mixture is a real convenience.

[Kamcoman77]

Regarding your fuel pump/crankcase pressure setup. A two-stroke has very defined crankcase pressure pulses because the crankcase is pressurized as part of the intake stroke. A four cylinder four-stroke has no such pulses. The crankcase air/oil movement is plus from a piston going down and that is relieved by another piston going up. In the 2-cylinder VW you might get a pressure pulse, but it would be nice to check with someone who is running a 2-cylinder to see if they can actually measure the pressure. My guess is, you will have to run a fuel pump.

[Dan_]

Walbro perhaps were first with the float-less diaphragm type carburetors you are talking about.  They have been used on chainsaws ever since there have been chainsaws... 


Also Walbro have a separate pulse pump designed for 4 stroke engines that is connected to the intake.  They add a spring to push the diaphragm back.  However under full throttle the signal is going to be low as the pressure differential falls to equal atmospheric, thus fuel flow will vary. 


They ask you contact them to discuss the application.  A header tank may be appropriate... 

[esc]

In the Mikuni design (what I am familiar with), the needle and seat that manages coarse fuel flow is moved by a diaphragm.  IIRC, the seat is closed by a small spring and opened when the diaphragm gets too close and pushes against the spring.  This maintains a low pressure volume of fuel for the mail/idle jets to draw from.  The suction that draws the fuel in, through the jets, also pulls on the diaphragm, opening the needle and seat as needed.  The strength of the spring must be suited to the engine and is an important tuning step.

The fuel pump feeds the needle and seat, but the fuel flow through the needle and seat is controlled by the main/idle jets.  If the jets have enough suction to draw the fuel in, they can control the needle and seat.

There are, of course, other factors, like maintaining the fuel pressure within the acceptable limits for the needle and seat, but there are similar issues with traditional float controlled carbs.

The "fuel pump" on the Mikuni's are just two chambers with a diaphragm between.  The pressure/vacuum pulses move the diaphragm back and forth from one side.  The other side of the diaphragm has two reed valves one to let fuel in and the second to send fuel out to the rest of the carb.  It is a very simple system and the only moving parts are the diaphragm and reed valves.

BTW, thanks for the link to the Walboro carb, I'll look it over as soon as I have a little time.

[stevejahr]

...
Also Walbro have a separate pulse pump designed for 4 stroke engines that is connected to the intake.  They add a spring to push the diaphragm back.  However under full throttle the signal is going to be low as the pressure differential falls to equal atmospheric, thus fuel flow will vary.  
...

Note what is making this work is not average pressure but changing pressure. Especially close to the intake valve the pressure in a 4-stroke will vary a great deal. Especially at higher power settings.

This variable pressure creates waves of positive/negative pressure which can be exploited by tuning the intake length just like on the exhaust side of the engine.

This is not the pressure that operates the diaphragm in the carbs though, that is triggered by the pressure drop of air passing through a venturi much like a constant velocity carb with a diaphragm and slide.

[Kamcoman77]

The Walbro pump that Dan linked to is the kind I have on my Briggs & Stratton 24-hp riding mower. I do not know if I would trust it on an airplane with 35 to 45 HP. I guess if you had some head pressure from a gravity-feed tank, the Walbro might add enough pressure to make it safe.

[Dan_]

I failed to mention Tillotson...  They claim the first diaphragm carburetors on their web site...

https://tillotson.ie/about-us/

[esc]

I just read the info provided by the Walbro link above.

It seems that the Walboro pumps work exactly like the pumps built into the Mikuni Jet Ski carburetors.  Except the ones in the Jet Ski Carbs are designed to support more HP.

[Theodore]

I'm looking into Snowmobile carbs, these have rich/lean adjustable(for high altitude use).