30 pict 3 blow through mods. Thunderjet anyone?

[Krochus]

Now that I have my dual Weber 2165 turbo bug running sweet I’m backing up and revising my little turbo single port 1600 in my rail

I’ve had this setup for awhile it was a gt15 drawing through a kehin CV40 and then later a S&S clone. I got it to run ok but it was always rich, not very responsive and had drivability issues.

So I am going to take a stab at blow through on this little motor. Part of my plan is to use a neat product that came with my bike carb a thunderjet. Basically it’s an add on high speed fuel circuit. My theory is this will let my 30 pict 3 retain fairly stock jetting thus behaving nicely just driving around.

The S&S g series thunderjet was amazingly easy to install after removing the choke mechanism. Drilled and tapped the shaft hole to 5/16 fine thread. As designed the thunderjet uses a collar between the carb body and jet holder that is drilled for air bleeds to the atmosphere. Well clearly this won’t work with boost so I added a port to the counterbored area under the choke shaft bushing that will let me bring a hose up to somewhere boost referenced.

Fuel will get taken up from the bowl where the original drain was that’s now drilled to 1/8npt thanks to a ham fisted previous owner.

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[Clonebug]

I ran a Brosol 34pict-3 for a couple seasons and found extending the float bowl vent into the boost pipe gave me stock jetting.
I didn't spend too much more time with it since I moved to EFI but I did run a consistent 13 lbs boost with it along with using a stock fuel pump.

[Krochus]

I ran a Brosol 34pict-3 for a couple seasons and found extending the float bowl vent into the boost pipe gave me stock jetting.
I didn't spend too much more time with it since I moved to EFI but I did run a consistent 13 lbs boost with it along with using a stock fuel pump.

I have read your postings on that where you also mentioned having a hard turn in your charge tube right before the carb.

I can’t help but think that was having a significant adverse effect on your overall carb function. I suspect that the tube you added had more of a mod ring effect creating a slightly low pressure area inside the carb throat simply by virtue of its size than it did by affecting pressure inside the float bowl.

I’m not limited vertically. I haven’t gotten the parts in for my charge tube to get you the exact length but preliminary it’s 1.5” and will have at least a 6” straight drop into the carb air horn.

Im also considering plugging my bowl vent and adding one straight off the top of the bowl score style and running that to a port right off the turbo volute where the pressure will be at its highest

[sideshow]

Not knowing how harleys work (isn't performance based on how well it parks?) I don't see how another jet circuit would help adjust for engine size...however in a blow through I could see how this might work.

I'll be watching with interest, this is far more interesting than the usual duals.

[Krochus]

I got everything buttoned up to an extent that I can drive the rail.

Rev testing the carb the thunderjet seems to do exactly what it’s supposed to do in a bike application. You can see the fuel level rise in the clear feed tube as you really whale on the throttle.

In general driving reinforces this. Now at this point I’m not sure if I even needed the extra fuel. Under boost at or near WOT the wide band shows full rich and the acceleration softens up some. But I haven’t lowered the fuel level yet in the carb bowl and I have a fairly large jet in the thing. After addressing these two factors I will report back

All in all I think it’s going to be a good option to get more fuel into a pict carb. Other methods work but this one can be tuned in seconds with jet changes

Now on the blow through 30 pict 3 single port setup overall I’m really pleased with how it runs. It’s responsive, makes boost instantly, accelerates well and is just genuinely enjoyable to drive with no flat spots or hesitations. I should have never wasted money on bike carb draw through setups!

[madmike]

That thing reminds me of the Alcohol carb I ran on my ATV

[Krochus]

So far so good. I definitely have too much fuel so I decided to block the pict’s power enrichment system. This was accomplished simply by placing a small square of electrical tape over the feed ports between the carb halves.

Unfortunately this is right where my poor oxygen sensor decided to give up so I don’t know exactly how much fuel I took out but in general an improvement was made as the buggy is much more lively at 6psi up around the redline than before.

Now I know what you are thinking. You added a jet you didn’t need. And well yes I did. However unlike the solex power enrichment system I can tune the thunderjet with jet changes in 30 seconds so all in all I should have a much more tunable carb.

[Krochus]

This is proving to be an outstanding setup.

Stock main jet, .70 drilled idle, 135 thunderjet, blocked solex power enrichment, 2x washers under the needle valve

Even with an 009 limited to 10 degrees total set at 24 all in throttle response and drivability is excellent the best I’ve ever had so far with this engine.

At light throttle low speed 15 to 1 or thereabouts
Higher speed where I’m fighting the wind 13 to 1
Power off boost 12 to 1
Boost 6-7 psi and WOT up around 5k rpm 11-1 ish

No lean holes no rich areas perfect seamless transition between all circuits. I should have simply blown through the stock carb to begin with and never jacked with draw through.

I did make one change. I had the air bleed hose for the thunderjet ran to the turbo discharge and I noticed some fuel getting pushed into the fuel pump hose probably when the throttle snapped close under boost. So I added another port above the carb float and now that’s where the air bleed breathes through eliminating the possibility of fuel being pushed somewhere it shouldn’t.

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[Jmcdo28]

Can you post some pics of the overall setup, carb, piping, turbo piping etc.

Thanks and you’re doing an outstanding job!

[Krochus]

Can you post some pics of the overall setup, carb, piping, turbo piping etc.

Thanks and you’re doing an outstanding job!

It’s a super simple setup that was originally configured to draw through a bike carb into the stock intake. So changing to blowing through the stock carb was as easy as re clocking the turbo and a mandrel bend for the charge pipe.

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[Jmcdo28]

I’m going to do this in my 66. I’m going to try to keep it under the hood with just standoffs.

[WickedWagens]

Interesting setup. Thanks for sharing!