Landracing Forum
Tech Information => EFI Questions => Topic started by: Colesy on February 05, 2017, 06:15:40 AM
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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. :
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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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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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On blow through Carbs there is now a fuel bowl with dual inlets and float set ups.
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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)
(http://i1195.photobucket.com/albums/aa399/Blowncbr/PowerJet_zps631eda6e.jpg) (http://s1195.photobucket.com/user/Blowncbr/media/PowerJet_zps631eda6e.jpg.html)
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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If you have a flow problem, check out this needle/seat.
https://daytonaparts.com/daytona-carburetor-float-valve.html
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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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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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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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:cheers:
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One of the reasons the British used a 360 degree firing order on their twins was that they had a single carb. There was the same time interval between "gulps" from each cylinder. It was easier to use this with a single carb setup 'cause each carb got the same amount of fuel mixture.
The V-twin has an uneven firing order and one cylinder has less time to get its mixture out of the carb than the other. Also, it might be an advantage to run a richer mixture on the rear cylinder. Maybe the dual carb Mikuni setup might be the answer?
These are just ideas from an "outsider." I have never worked on an H-D.
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I always had good results from the CV carb. Less finicky than a mik.
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Before you tear apart your carb are you sure your petcock is not starving the system?
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Fuel tank breather?
Dave
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Sudco makes a needle and seat for pressure, used it on my Aerocharged Buell.