Improving Carburetor Air Flow

[Vintage Engine Repairs]

I was watching an interesting video on jet ski’s and modifications they do to the carbs.

One is to eliminate the choke shaft and plate entirely, machine some plugs for where the shaft once was and then bore that and install a prime line to give it a shot of fuel for starting.

I’d be curious to know how much this improves air flow?

Also using a weaker / lighter metering spring for increased part throttle fuel supply.

 

[hacskaroly]

Also using a weaker / lighter metering spring for increased part throttle fuel supply.

I am curious too. Would there be enough pressure from below the needle to push it up and keep it open and overcome a weaker spring trying to keep it closed?

 

[Vintage Engine Repairs]

I am curious too. Would there be enough pressure from below the needle to push it up and keep it open and overcome a weaker spring trying to keep it closed?

Yeah, it must be enough to overcome pop off pressure, but less pressure than stock.

 

[farminkarman]

I was watching an interesting video on jet ski’s and modifications they do to the carbs.

One is to eliminate the choke shaft and plate entirely, machine some plugs for where the shaft once was and then bore that and install a prime line to give it a shot of fuel for starting.

I’d be curious to know how much this improves air flow?

Also using a weaker / lighter metering spring for increased part throttle fuel supply.

What carbs did the Jet ski have? Mikuni style carbs that I'm used to don't have an actual choke. Instead they either have a primer circuit or an idle circuit fuel enrichment that uses moveable plunger attached to a "choke" lever. The early 3120xp saws had a carb without a choke, and the throttle bore was larger. Not sure how they compared to newer ones in stock form.

 

[FederalQ]

The Dolmar ps6000i with a Tillotson carburetor that I recently acquired has no choke plate and uses fuel injection to prime/start. It starts this time of year without even having to manually set the high idle as per the staring instructions.

 

[Vintage Engine Repairs]

What carbs did the Jet ski have? Mikuni style carbs that I'm used to don't have an actual choke. Instead they either have a primer circuit or an idle circuit fuel enrichment that uses moveable plunger attached to a "choke" lever. The early 3120xp saws had a carb without a choke, and the throttle bore was larger. Not sure how they compared to newer ones in stock form.

Dual 38 Mikuni

 

[Vintage Engine Repairs]

The Dolmar ps6000i with a Tillotson carburetor that I recently acquired has no choke plate and uses fuel injection to prime/start. It starts this time of year without even having to manually set the high idle as per the staring instructions.

I presume the injector solenoid is controlled by a simple low voltage source from a stator and vacuum pulls fuel up? What causes the voltage to stop when the saw is running?

 

[huskihl]

Husky tried the primer/choke concoction in the 3120 but it didn’t last long. Many people have tried in race saws and it’s hard to get a proper tune because the air flow through the Venturi pulls fuel from the primer line and gives a false tune setting.

 

[FederalQ]

I presume the injector solenoid is controlled by a simple low voltage source from a stator and vacuum pulls fuel up? What causes the voltage to stop when the saw is running?

I believe it‘s mechanical. The choke lever swings circular 180 degrees when you activate and I can hear it injecting fuel.

 

[Woodslasher]

I was watching an interesting video on jet ski’s and modifications they do to the carbs.

One is to eliminate the choke shaft and plate entirely, machine some plugs for where the shaft once was and then bore that and install a prime line to give it a shot of fuel for starting.

I’d be curious to know how much this improves air flow?

Also using a weaker / lighter metering spring for increased part throttle fuel supply.

I had an 056 without a choke, the throttle plate was replaced with the larger choke plate, the bore was polished, etc. I asked a buddy who builds competition race saws if the carb was really worth it and the short story is no. Unless you have a full bore race saw, that little bit of extra air is going to be so minimal that in day to day use you'd never notice the difference.

 

[Vintage Engine Repairs]

Husky tried the primer/choke concoction in the 3120 but it didn’t last long. Many people have tried in race saws and it’s hard to get a proper tune because the air flow through the Venturi pulls fuel from the primer line and gives a false tune setting.

Interesting, though sounds like a different setup to what I had in mind. No fuel should be pulled because of the check valve?

 

[Vintage Engine Repairs]

I believe it‘s mechanical. The choke lever swings circular 180 degrees when you activate and I can hear it injecting fuel.

Oh interesting! Similar to an accelerator pump?

 

[Vintage Engine Repairs]

I had an 056 without a choke, the throttle plate was replaced with the larger choke plate, the bore was polished, etc. I asked a buddy who builds competition race saws if the carb was really worth it and the short story is no. Unless you have a full bore race saw, that little bit of extra air is going to be so minimal that in day to day use you'd never notice the difference.

Good to hear! I wonder why they bother with JetSki’s.

 

[drf256]

Husky tried the primer/choke concoction in the 3120 but it didn’t last long. Many people have tried in race saws and it’s hard to get a proper tune because the air flow through the Venturi pulls fuel from the primer line and gives a false tune setting.

I’ve been thinking about this for years Kev. Would require a primer bulb setup with a valve in the center of it. When you push the primer bulb, you’d mechanically push on the valve stem and open it. That way, fuel couldn’t be continually pulled through the circuit.

Not sure if anyone makes one like that.

 

[FederalQ]

https://patents.justia.com/patent/5241931 This is the Sachs-Dolmar patent.

 

[huskihl]

I’ve been thinking about this for years Kev. Would require a primer bulb setup with a valve in the center of it. When you push the primer bulb, you’d mechanically push on the valve stem and open it. That way, fuel couldn’t be continually pulled through the circuit.

Not sure if anyone makes one like that.

Or a shutoff valve right at the carb

 

[huskihl]

Interesting, though sounds like a different setup to what I had in mind. No fuel should be pulled because of the check valve?

The check valve obviously didn’t work that way if several guys from the racing world have tried it with no luck

 

[Vintage Engine Repairs]

The check valve obviously didn’t work that way if several guys from the racing world have tried it with no luck

Not sure, clearly works for the JetSki crowd.

 

[Vintage Engine Repairs]

I’ve been thinking about this for years Kev. Would require a primer bulb setup with a valve in the center of it. When you push the primer bulb, you’d mechanically push on the valve stem and open it. That way, fuel couldn’t be continually pulled through the circuit.

Not sure if anyone makes one like that.

Yep exactly.

 

[huskihl]

Not sure, clearly works for the JetSki crowd.

Possibly because the engine is 10 times the size and doesn’t notice a few extra small droplets of fuel