i have a jc speciality motor built for stk hvy the guy i bought it from said its temp should be 415-450 i ran it last week for the first time and the hottest it got was 378 i checked the jet and there was no #'s on it so i dont kno where im at what jet should i run and where should i set the needle
jetting question
- Thread starter tmcmike
- Start date
Jon Brogden
Member
I like to put the needle at 1 1/2 turns out and leave it there, the needle taper was designed for gas and has little effect with alky, change the jet to get the heat where you need it , shoot for the low end of his range, 415 to 420 is plenty to assure good top end. this time of year a .048 or .049 would typically get it there but carb signals differ so be prepared to go a little lower or a little bigger if necessary. If your close on temp but not quite enough , tape the blower screen up some to bring the heat up if it is untaped A .046 to .050 a thou at a time will fit most situations unless your at really high altitude, like Denver or higher. Jon
alvin l nunley
Site Supporter
The first thing you need to know is what jet you have in the engine now. Going from gas to alky, the jet needs to be in the neighborhood of twice the "area" of the gas jet. Pi()RSq. is the formula for calculating the area of a jet. For instance, a .055" jet is 98.8% bigger than a .039" jet. If you don't have a set of numbered pin gauges, either buy a set off of eBay or find someone who does have them. They sell pin gauges in sets. The one you want is .011" to .060". (.011, .012, .013, etc. etc.) The Chinese sets are fine for your purpose.
Be aware that the difference in area, (flow) between a .049" and a .050" jet is 4.1%. Also be aware that the "air density" in your area, (it changes with the temperature and the barometric pressure.) changes daily, and from hour to hour.
From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory.(Al Nunley
Be aware that the difference in area, (flow) between a .049" and a .050" jet is 4.1%. Also be aware that the "air density" in your area, (it changes with the temperature and the barometric pressure.) changes daily, and from hour to hour.
From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory.(Al Nunley
alvin l nunley
Site Supporter
I have this idea that any performance gained by taping the blower screen has more to do with the amount of air the flywheel is pumping than the temperature under the spark plug. It would be interesting to see what affect taping the screen has on the EGT. The EGT tells you what is happening in the combustion chamber, and, it's been my experience, that that is much more important than what is going on under the spark plug.
From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory.(Al Nunley