Jet Size on Stock Flathead

Kstrycharz07

Kstrycharz07

Member
Just purchased a Flathead off marketplace to use as a project engine to learn more about tuning/working inside small engines. Anyone know what size I need to drill the jet after I remove the governor. I have a header, other than that stock internals and carb. Thanks.
 
For an unmodified, unrestricted alcohol carb with a bore <=.695" you should be around .052" with a straight/muffled pipe.
With a loop header, you'll be @ .050."
Now, if the throttle shaft is aftermarket and offset heavily (ie .030" offset) and the carb's been worked on to be a good WKA legal carb, you will be down to a .047"-.048" jet.
.058" is huge for an unrestricted carb -- we would run that in a blue or purple restrictor plate application OR a fairly modified stock appearing carb with considerably larger bore (ie .735.")

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We put a 13.5" straight 2 stage unmuffled pipe on it.

I should be able to tweek on an .058 jet and make it run no?
 
It'll run, but it'll temporarily wet foul (wash out) the plug at peak rpm.
It'll feel like someone hit the kill switch for 1 second, then the engine will come back to life.
Keep going down on jet until that is cured, then you're spot on with fueling an alcohol flatty.
Likely you'll be on a .052" when it's all said and done.
 
On gas is it best to tune that kill spot out with the needle or just down like you are explaining here? I have it pretty well figured out I think. Half the fun is tuning the carb daily with the weather but sometimes I wonder if there is a better way. Btw the other day I discovered the low end torque loss I was experiencing was a very small piece of plastic from where I drilled out my pick up tube. I thought for sure I had flushed it all out but I found it in between my needle jet and the the needle. Almost transparent but now it is back to it's old awesomeness. Except the new style carb give me way more throttle. Last question. At wot where should the butterfly position be located. I have a thinner than legal shaft, filed screw with the driver slot perfectly flat front to back. When at wot I noticed it goes beyond center to roughly a 16th past pointing towards the metering side of carb. Is that ok or too much? Feels awesome but havent had it any other way. Thanks in advance. Long winded I know.
 
FlatheadsForever87 said:
On gas is it best to tune that kill spot out with the needle or just down like you are explaining here? I have it pretty well figured out I think. Half the fun is tuning the carb daily with the weather but sometimes I wonder if there is a better way. Btw the other day I discovered the low end torque loss I was experiencing was a very small piece of plastic from where I drilled out my pick up tube. I thought for sure I had flushed it all out but I found it in between my needle jet and the the needle. Almost transparent but now it is back to it's old awesomeness. Except the new style carb give me way more throttle. Last question. At wot where should the butterfly position be located. I have a thinner than legal shaft, filed screw with the driver slot perfectly flat front to back. When at wot I noticed it goes beyond center to roughly a 16th past pointing towards the metering side of carb. Is that ok or too much? Feels awesome but havent had it any other way. Thanks in advance. Long winded I know.
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Gas can be tuned with the needle -- alcohol, it won't do much because of the volume.
@ WOT, for max air flow, the butterfly should be straight. For stockers on alcohol, there's an advantage to have it just fore or aft center so that it pulls more fuel, especially on restrictor plate engines.
 
I have found so far it depends on the day whether it likes past straight or not. Kinda nice cause if it doesn't I just simply let off throttle a tad bit to get it straight
 
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