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Author Topic: Thunder Jet stops flowing  (Read 3615 times)
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Colesy
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« on: February 05, 2017, 06:15:40 AM »

I have installed a thunder jet on HSR 48mm Mikuni to ensure there is sufficient fuel when engine is at WOT to prevent a lean mixture. Running a 170 main jet and 100 thunder jet. On the dyno it was working fine i.e. fuel was flowing well but on a run we kept the throttle open was looking at about 40 seconds but at around 20 seconds with revs at 7000 the fuel stopped flowing through the thunder jet. We shut the engine down but on the salt this would of been a lean out causing major problems. Anyone have any ideas??? Engine is a 1987 HD Sportster making 120hp. Float bowl is extended with 1/2" spacer and fuel is supplied via electric fuel pump at 4psi. :
« Last Edit: February 05, 2017, 06:35:21 AM by Colesy » Logged
sofadriver
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« Reply #1 on: February 05, 2017, 08:19:56 AM »

You already know that a deeper float bowl will only delay fuel starvation. Did you extend the pick-up, too? What fuel are you running? Thicker fuels may require a bigger petcock, filter, fuel lines, float jet, pick-up tubes or fuel pump. 4psi doesn't guarantee you've got enough flow. Is your pump running in a total loss electric system (weak battery?). Replaced (not cleaned) all your fuel lines? Fuel lines getting too hot?
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Mike in Tacoma

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TheBaron
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« Reply #2 on: February 05, 2017, 09:23:49 AM »

you may need to enlarge the fuel float needle/seat in the carburetor to get the needed flow rate....test it.

Robert
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SPARKY
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« Reply #3 on: February 05, 2017, 10:55:39 AM »

On blow through Carbs there is now a fuel bowl with dual inlets and float set ups.
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« Reply #4 on: February 05, 2017, 02:58:37 PM »

I also have been down this road Colesy, I'm surprised you got the Mikuni float valve to seat with 4 psi ? I had no luck @ 3 psi (max)
  Have a look at Sumner's "formula" pages to work out your fuel supply needs (HP v's fuel required), then measure the flow out of the bowl cap orifice (after removing it) & see how the flow relates to your requirements 1st.
 I had to go to a 70 gph low (5-8) psi pump to get the flow I needed , then find a way to control it,,
Here's my solution,,,,(the HSR was way too big for the 250)

It's an old Holley float bowl tapped into the Mikuni bowl(floats & fuel valve removed from Mikuni), the setup safely handles up to 8 psi, and flows 2 liters/ minute (methanol)
Tiny
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Tiny (in OZ)
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Vinsky
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« Reply #5 on: February 05, 2017, 09:50:34 PM »

If you have a flow problem, check out this needle/seat.
https://daytonaparts.com/daytona-carburetor-float-valve.html
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John
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« Reply #6 on: February 11, 2017, 05:00:10 PM »

Anybody ever try a pump around carburetor setup at this level?  Common setup in karting to deal with
high lateral Gs but it could also serve consistent float bowl levels at high fuel flows.
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Colesy
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« Reply #7 on: February 11, 2017, 07:13:23 PM »

Thanks all for the advice. Some has already been done/checked. Daytona don't make a needle jet for the Mikuni. Flow rates versus what is required (HP) are more than enough working off the formula. Have placed another vent on carby. Have one more chance on the dyno this week to test it and also test the Thunder Jet to see if there is a difference in pressure at high RPM (theory) intereferring with vacuum in the Thunder Jet. If it loses fuel again then I am a spectator this year, no more time left. At least I will have 12 months to come up with something, like the extra float bowl at 8psi. (Good idea)
Cheers
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generatorshovel
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« Reply #8 on: March 30, 2017, 04:35:15 PM »

Colsey cured his problem in time for Speedweek, extra float bowl ventilation was needed apparently?
  I think he melted a piston though ? (after getting a good speed)
Tiny
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Tiny (in OZ)
I would prefer to make horsepower, rather than buy, or hya it, regardless of the difficulties involved , as it would then be MINE
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