416 w/ FCR 39mm carb - what main jet

[Crowdog]

I have a 416 (10.8 compression) with a Hotcam Stage I and a FCR 39mm carb, slip on and K&N.

Right now, I am running a 172 main jet at 4000 ft.

What are the rest of you 39mm carb guys running for a main jet?

I know there have been tons of jetting posts. Searched and didn't see much that compared, so......

172 seems a bit high to me, but it seems to run OK.

Thoughts?

[Colby@C&DRacing]

Ya that is a little big for 4000 feet. I would drop it to a 165 or 168. :)

[Crowdog]

Forgot to mention that I run 50-50 race gas & premium. :D

[zephead400ex]

Originally posted by Crowdog
Forgot to mention that I run 50-50 race gas & premium. :D

octane junkie:devil:

[Crowdog]

The biggest reason for asking is my friend has a 465 with a FCR 39mm and he is running a 158 main jet. I figured that I would/should have been lower than him. That got me thinking that I was running too rich.

158 seems real lean for his setup, but it seems to run fine.

[atl_josh]

How much did you pay for your carb and from where?

[Crowdog]

I don't remember where I got the 39mm carb. Had it for over a year. Wasn't real impressed with the improvement in power though.

[cals400ex]

Originally posted by Crowdog
I don't remember where I got the 39mm carb. Had it for over a year. Wasn't real impressed with the improvement in power though.


i agree. it feels like you gained more than what you do. the throttle response is torquey but in a drag race i only gained about 1 bike length up to fifth gear on my mildly built motor.

[skemp]

I would run a 155. At 4,000 feet you don't need as much fuel. I run a 162 in my fully built 416 at sea level. I agree that FCR's don't add a ton of power, but it is truly a better carb and the throttle response and range of tuning is worth it.

[Crowdog]

Originally posted by skemp
I would run a 155.

Wow! I am that rich?! Sure doesn't seem like it.

[cals400ex]

Originally posted by Crowdog
Wow! I am that rich?! Sure doesn't seem like it.


yeah, it is hard telling. the accel pump really masks the mainjet jetting. it shows the plug black a lot because it squirts fuel for so long. this can cause the low end bog of these carbs. i reduced my squirt to about .5 to 1.0 seconds and the bog is gone. however, my plug looks a lot leaner now. i am currently running a 162 mainjet and sparks told me to run a 165. so, who knows.

[Crowdog]

Originally posted by cals400ex
yeah, it is hard telling. the accel pump really masks the mainjet jetting. it shows the plug black a lot because it squirts fuel for so long. this can cause the low end bog of these carbs. i reduced my squirt to about .5 to 1.0 seconds and the bog is gone. however, my plug looks a lot leaner now. i am currently running a 162 mainjet and sparks told me to run a 165. so, who knows.

Are you running a similar setup as me?

[cals400ex]

Originally posted by Crowdog
Are you running a similar setup as me?


my setup:

10.8:1 406, stage 2, x-6, modded airbox lid, uni filter, fcr 39.