Flat Head Jet Size

MAS

Premium User
Im curious to know what size jet I should be running in my Flathead on menthanol fuel.
 

MAS

Premium User
Ive never touched the jet before (just got inro karting a month ago). It had a 51 in it
 
PM me your address I'll mail you some smaller ones. Or if that sounds creepy, could send them to your post office c/o general delivery. Then you pick them up
 

MAS

Premium User
Thanks for the offer i really appreciate it but i just bought a set of them so I wasnt sure which size i should use exactly. I have some buddies that run oxygen additives in methonal in their racing lawn mowers and they said jets make a huge difference for them so wasnt sure
 

alvin l nunley

Premium User
Here's a spreadsheet of jet sizes starting at .044". This gives you the jet size, jet diameter, jet area, percent increase from the 1st jet, and the percentage difference between each jet and and the previous jet. Maybe it'll help you. It does give you a good idea of how much flow difference there is between the 1st and the 2nd, or the 1st and the 4th jet.
jet size 2.jpg
 
If you have a loop pipe or a straight/muffled pipe will make a difference as well.
Assuming this is a WKA legal (built) engine: with a loop pipe, you're likely on a 48 jet - with the straight pipe a 50.
Throttle shaft offset, metering hole sizes, short stem diameter and length, venturi location, ignition timing, cam, cam timing, exhaust, air flow through the intake, (and more) all dictate what is the "correct" jet for your engine.
Having it properly tuned on a dyno would be best, but you can close in on your tuning at the track if you are methodic in your testing.


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Brian Carlson
Carlson Racing Engines
Vector Cutz
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bcarlson@CarlsonMotorsports.com
 
Here's a spreadsheet of jet sizes starting at .044". This gives you the jet size, jet diameter, jet area, percent increase from the 1st jet, and the percentage difference between each jet and and the previous jet. Maybe it'll help you. It does give you a good idea of how much flow difference there is between the 1st and the 2nd, or the 1st and the 4th jet.View attachment 6473

Al, this is great information to share.
Thank you.
Question: Is this using actual Briggs flathead jets, or Hilborn/Kinsler/Holley style jets?
I know there is a huge difference in actual flow between Hilborn and Kinsler jet of the same size due to the radius machined into one and not the other. Flathead jets flow terrible in comparison to these style jets, due to their screwdriver slot and sharp edges that cause cavitation.
I never built a flow chart for the flathead jets like the chart you have here, btu I did spend some time on a Waterman fuel flow bench testing jet sizes when we were machining our 1/2 thou. sized jets. The reason we did this is a result of exactly what your chart reveals -- too large of jumps between single pill sizes that were commercially available.
 

alvin l nunley

Premium User
Notice I have no actual "flow" numbers, only comparisons, which I'm sure would hold true when comparing any style jet, i.e. 2 jets of the same style with different size holes. Only actual testing, of course, would show the "quantity" of the flow. In my experience, working for rocket engine designer Bob Truax, we did actual flow testing of 200 jets and found that there was a wide range of "actual" flow difference between them. These jets were custom-made on precision CNC lathes. I understand there are carburetor builders in Europe that mark their jets with the actual "tested" flow numbers.
 
^ That's right.
That was my original intent, but after figuring the time invested in flow matching jets on the wet flow bench, the retail price would need to be higher than the market would allow.
The flow numbers that we recorded were very inconsistent (for instance a .050" jet having more flow than a .052" jet) with commercially available jets. This was due in large part (I think) to the jets at that time being drilled rather than reamed. We saw much better consistency with our own jets machined with a straight flute reamer than those that were drilled or even done with spiral flute reamers.
While the market never panned out for flow matched jets, we were able to offer .0005" increment jet sizes and continue to offer them today.
Although tuning to half thou. may not be for everyone, it is certainly an advantage for the experienced tuners.
 

kartracer_3

Member
I have seen 46-52 jet sizes in them and like Brian stated there are a lot of variables that would dictate the correct size. I have found that the weather does not play a huge role like you think it would , I just add tape on the flywheel housing to increase temps on the very cool nights however to answer your question YES , having the wrong size in could make a huge difference. It could go from completely shutting off in the corners( too rich ) and running like crap to running like a scaled dog with only one jet size change.
 

alvin l nunley

Premium User
Many times I have done the very same thing, my good sense says yes, go for it, my wallet says no, it "probably" makes no difference. When everybody else (a lot of people) is telling you it makes no difference, it makes you wonder. Trouble is, it's some times the wallet making the decision. lol.
Precision flowed jets, expensive, wallet says no, your experiences, if you have any, say yes. It's a conundrum.
 
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