Author Topic: Carbs or injection?  (Read 2078 times)

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Offline R51959

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Carbs or injection?
« on: September 14, 2011, 07:28:53 PM »
o.k. guys need help from the jet boat community. need your thoughts on this . i have a 1979 southwind tunnel dragster with a 454 and a jg Berkeley . 671 littlefield blower . have a 750 holley on top . will run a steady 68 to 70 all day long . 5200 rpms . everybody around here keeps telling me aw man you need to run dual carbs . not looking for much more as for as speed but i dont want to lean out the motor . i am running about 13% underdriven on the blower  . it has an A impeller pro loader and a place diverter. should i stay the way it is or do i need to put another carb on top?? live down south in the bayou state . accepting all suggestions .

Offline wolfie

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Re: Carbs or injection?
« Reply #1 on: September 14, 2011, 07:49:49 PM »
I would imagine that it's starving like a fat girl looking through the window of an all you can eat!!! You can tell if it's lean by the plugs, but you're definitely not feeding it what it wants. I'm running a 750 DP on a somewhat mild N/A 454 and it could use an 850.
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Offline David

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Re: Carbs or injection?
« Reply #2 on: September 14, 2011, 07:56:19 PM »
No pro here... I was told my little 460 with a mild cam could handle two 600 cfm carbs easily...  I currently have an 850cfm dp
I would have to think that two 750's on that setup would be good too...
There will be more feedback on this for sure...
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Offline R51959

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Re: Carbs or injection?
« Reply #3 on: September 14, 2011, 08:53:29 PM »
Forgot also to add it has a vertex magneto. used to run a hilborn injector hat .went back to carb because it was so much easier to start . that thing was a pain in the rear to keep adjusted. each outing was a different pill . but it would hookup when set right.checked the plugs they are slightly black but not wet. sure glad i found this site . i have learned more about jet boats from vjb than from any other source.

Offline David

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Re: Carbs or injection?
« Reply #4 on: September 14, 2011, 09:00:45 PM »
Sure glad i found this site . i have learned more about jet boats from vjb than from any other source.
ahhhhhhhhhhhh Music to my hard of hearing ears!!!!   :)
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Offline wolfie

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Re: Carbs or injection?
« Reply #5 on: September 14, 2011, 09:02:19 PM »
Glad you found this place as well. David's provided one heck of a place for us NUTTZ to hang!!!
Justsquirtin has a Placecraft with a pretty potent blown motor. I'm sure he could tell you a little about it!! Steve, the local pocket crew chief to us all, could give you more info than I could ever!!!
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Offline 78_Tahiti

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Re: Carbs or injection?
« Reply #6 on: September 20, 2011, 02:10:34 PM »
Welcome to VJB! This is a tough call from the amount of info so far. Being under driven, I dont know if your making any boost or not! You can effectively run what appears to be a badass force fed engine, when in reality it just looks that way. I need to know if the rotors in the blower have the teflon strips, a simple boost guage will tell you if your forcing air and fuel into the intake when you nail it or not. Mooneyham also makes a very popular street blower setup using magnesium rotors out of the old GM 671 that offer a softer curve of pressure, and can be under driven to allow a pretty docile setup. When you get time, All the engine info you can provide (carb model, jet sizes, cam specs, compression raitio of engine, heads, etc.) will help us try to give you the best advice we can. Also, what injection was it running? (brand and pill sizes it liked) If your running dry exhaust, checking it with O2 sensors will give you a much clearer picture of whats going on in the fuel department as well!
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Offline TIMINATOR

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Re: Carbs or injection?
« Reply #7 on: November 11, 2011, 12:42:37 AM »
Throw away the vertex mag and go to MSD 6AL, much better spark voltage so you can run a bigger spark gap. On alky, the mag works better because they have a longer duration spark, alky needs that, but they have less voltage and need a smaller plug gap. Gasoline responds better to a short, fast rise time, hot, high voltage spark. You want to run a .050 plug gap for best performance too,with gasoline. The dual carb thing is best because you really need the added airflow, and having 4 needle and seats and bowls it is much safer too. A pair of 850s or the chineese, blueprinted 920s will be fine. Carb size has nothing to do with mixture, or "leaning it out" UNLESS you don't have a big enough fuep pump or delivery system.We can set any of them up for a blower, or boost reference them too if you care about the fuel consumption.  TIMINATOR
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Offline justsquirtin

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Re: Carbs or injection?
« Reply #8 on: November 11, 2011, 05:02:43 PM »
my 81 swtd with a 500hp bbf with 850 demon on a single stealth intake ran 75 =something doesnt sound right 

Offline 78_Tahiti

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Re: Carbs or injection?
« Reply #9 on: November 13, 2011, 07:17:55 AM »
my 81 swtd with a 500hp bbf with 850 demon on a single stealth intake ran 75 =something doesnt sound right 
I was thinking that also Doug, so I talked to a friend who has been playing with Blown Jet Boats for many years! This is his take on it.

On Magnetos:   Steve you are correct in the fact that a magneto produces more output than any battery ignition system because it is a generator of electric  energy & the faster you spin it the more voltage (or spark) it produces! The battery powered ignition even with the best MSD box added to it LOSE'S energy the faster you spin the motor !! The reason for the .018 to .020 plug gap with a magneto is because it is a generator & at low speed (cranking RPM) it is not producing as much voltage as when the RPM's are up! Some people think that when the are watching the top fuel cars & they see those MSD units that are being run it's like there MSD ignition, but it is NOT the same!! Those are MAGNETO'S that are electronicaly controlled & cost $5,000.00 each !! At that cost if a regular MSD battery ignition system would work they would be using it!!!

On Blower Setup:  You are correct in that a small carb is in fact a restriction in a blower application, its ok to run for a very mild setup but you will never make any real power with it. I personaly like injection as I ran a set of Hilborn uprights for years on both gas & alcohol & never had a problem!! I even used to pull my kids water skieing & towed a lot of my buddies in that ran out of fuel or broke, with that set up, & if you didn't lean on it it was BETTER on fuel than the blown carburetor setups! As for performance there is NO comparison between the two! some people think it is cheaper to go with the carburetors over injection, but that is not true  because when you add up the cost of your carbs, adaptors linkage, scoop, big enough fuel pump, fuel lines ect. it is about the same as if you went with a good set of injectors!! Steve, if your friend that tried to run the Hilborns would like my tuneup that has worked flawlessly for over thirty years now, let me know & I will help you out. Thanks Bill!   :yesnod:
If It Floats, F@cks, or Flies, Its Cheaper to RENT IT!

 


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