Question about the FCR 39 Carb...

[frogburner]

I am building my project 440.. Okay'' what would you jet your FCR 39 at?? right now its jetted for a 406.. Do you think Colby would be pretty close at C&D?? I live in MO if you were wondering about sea-level.. Thanks..

[cals400ex]

i would start around a keihin 175 and work your way down. i have a feeling around a 170 may be perfect depending on if you run the air box lid or not.

[speedy400]

I've got a 165 in my 416, open box, Uni, fcr39,hotcamstg2,ported'n'polished, and a full system, so yeah I'd take a guess 170 would be just peachy.-darren

[frogburner]

I think right now I have a 170 with a 52 pilot,, with 2 turns out on screw adjustment.. And I have the Accelerator pump as high as it goes.. Now that was on a 406, 11:1, 2 stage hotcam, yoshi pipes,.. It will soon be 440 with 12.5:1.... everything else will stay like it is.. Oh and Also I run with a K&N with the lid off...

[MIKE400EX]

Being that you will have more cubes with more compression and no other changes, all being fed by the same size carb....the mixture will likely be a little richer than it was with the 406. That because the carb will see a stronger vacuum signal to pull fuel out. I would just leave the carb alone if the 406 was running right (it may be a little rich but that is on the safe side), then test and rejet after the 440 is run in.

[Kilabanshee]

Mike, Ive been having some troubles with my carb running right. What should I set my airfuel screw at I'm running a 160 main currently?? I have the 416,cam,porting,valves, yoshi, k&n any sugjestions?? Also are you free this weekend? I may be heading up to freeland friday let me know!

[frogburner]

Well Mike I guess I will wait and see then,, I appreciate the reply.. Hopefully it will be dialed in pretty close... FTZ says I can use my stock headbolts with the high compression, what would you do??? heavy duty head bolts or stay with new stockers...

[Bad Habit]

With a 12.5:1 piston, I would go ahead and put in the heavy head studs. It might be ok with the factory studs, but I wouldn't take the chance of having to tear down the motor again.

Back on your FCR, one thing you might want to try if you find the jetting is a little fat, don't just start throwing different mains at it. Don't be affraid to move the clip on the needle and change the pilot. I was having some trouble getting my jetting perfect all the way through the throttle range. Then, I ran my bike on a dyno with a sniffer.

170m 55p was slightly fat up top and was way fat at 3/4. I thought it was a little off, but the sniffer showed it was extremely rich, it was literally off the chart at 3/4!

165m 55p was good up top but was still fat at 3/4.

165m 52p was good up top but was slightly lean at 3/4.

165m 55p with the clip raised one notch from middle was almost there.

165m 52p clip up one from middle had me doing nothing but :D :D :D :macho :D

I recommend to everyone to put your bike on a dyno with a sniffer if you get the opportunity. It makes jetting so much easier, it's almost like cheating.

[MIKE400EX]

Brent, I would think that a 160 may be a little lean, but Freeland is at 2000 ft. so up there it may be right on. Start with the screw at about 1 3/4 turns out, then fine tune a quarter turn at a time from there to get the best throttle response. When you get it at the best point tune it a little closer using 1/8 turn increments. You'll find that the best setting for the idle mixture screw will be different in Freeland than at your house. Not by much though, but it will make a difference. Can't do this weekend, have audits at work next week. Wish we could!

Frog, with 12.5:1 I think it would be a wise choice to spring for some better studs, or at least a beefier front one and the rest new OEM's. You WILL have to run "straight" race gas as any detonation/pre-ignition will pull the studs. That's if it is really 12.5:1.

[frogburner]

Well I was wondering about the headstuds because, C & D said that they normally have a larger exhaust stud then the rest being stock, they also said that Sparks Racing uses the larger exhaust one on all their Hi- Comp motors... Do you think I would be safe installin a larger exhaust stud then the rest being new stockers.. And I heard a couple people say that cosmetic head gaskets blow easy... Would a stock head gasket be better off than the cosmetic... Thats the only thing I am really worried about in this motor,, if the head gasket is going to blow or not...

[Bad Habit]

Yes, Cometic PAPER gaskets suck. You'd be better off using the cardboard packaging they come in. However, if you're getting your components from C & D, they also have Cometic STEEL gaskets. Although I've heard many people use the standard bore gasket on over-bores, Cometic makes them in 85mm, 86mm, 87mm, 88mm and 89mm. I use the correct size gasket for the bore in the cylinder.