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FCR41 jetting with porting, cams, 944HC motor

2K views 16 replies 5 participants last post by  BrianK 
#1 ·
My motor has finally started to go back together and I'm looking at dyno runs next month hopefully. So I need to order some jets for the FCR41 carbs. Can anyone, maybe with a similar engine, give me an idea what sort of a range i should have on hand? Main jets, pilots... Do i need different air jets? Slides?

The dyno is hours away from home so i would really like to get it dialled in one session.

heres what i have:
944cc HC motor with the V2 heads, slightly oversized valves, hand ported by J-precision. 900ie (8J) cams, long intake manifolds. Banked FCRs with pod filters.

I know there are a lot of variables here, but maybe with the collective experience i can at least narrow down what i should bring with me
 
#4 ·
i should add the one i did hasn't been dynod, customer wants to give it a good run in first. running an ignitech with tps too, 9 degrees advance at idle and some extra part throttle advance. cams were at 106 originally, i moved them back to 111 when i gave it a run in service and it seemed a little smoother and no less strong. typical of these, it doesn't feel that fast but builds speed quite impressively. and is silky smooth at low speed on and off the throttle. i was very happy with it.

sort of made me decide to fcr / ignitech with hall trigger and tps my early 900 frame with 659 engine project instead of injecting it. saved me cutting the frame too.
 
#5 ·
Interesting about the cam timing. So maybe I should start with a conservative value there as well. I had the stock cams at 109 degrees before this recent rebuild and that seemed to work well, although the biggest benefit probably came from having them properly synched compared to factory.

I have a TPS as well that i was planning to map once i have the base jetting sorted. All depends on the Dyno, i guess you need an Eddy current machine that can hold constant speed to accurately map part throttle advance
 
#6 ·
i more pulled the cams back to see if the owner would notice it. he thought it was a bit smoother in general. cam timing is about cylinder pressure, but if you've got enough with comp it's not such a big deal. no need to hammer the big ends.

i add up to 10 (maybe 12) degrees at part throttle when doing ecu mapping as a general rule, but it depends how much you can get as the leading edge of the lump defines the max with ignitech on std pick ups. i think i have the pick ups 4 degrees forward, so 40 is the most it'll give.
 
#7 ·
I have your old modified flywheel in this motor with 60 degree max advance. Had it up to 44 degrees at part throttle before the rebuild, but never dyno'd it. I'm thinking about rigging up some sort of a knock sensor microphone for testing. Well equipped Dyno shops are hard to come by here, the few that exist all specialize in HD
 
#8 ·
I found my old thread. I hope you don't mind me linking that instead of listing my results. I feel like you guys are having a technical discussion that's already head and shoulders above my abilities.. lol! Hope your project turns out awesome! I may try some jetting tweaks this summer after I put on new belts too.
Dyno graph feedback
 
#13 ·
This has already been helpful. So far i gather that i probably should bring a variety of larger main jets (i have 152s in there right now) and maybe a couple sets of slightly richer needles
 
#17 ·
Realize I'm jumping in late here but it took me a while to find my old post re my FCR settings.

FWIW, here are the jetting settings I ended up with on my FCRs after much experimentation - which was all seat-of-pants, no dyno or AFR measuring equipment used.

BTW, this is a 97 900SS CR with 11:1 JE pistons (ignition timing retarded 3 degrees), Dyno coils, and of course, 41 mm FCRs (linked, with big K&N pod filters). All parts from Sir Chris Kelley.

145 mains (were 155 originally).

Needle clips in position number 2 (second from top). I believe they started in position number 4 or 5.

Slow fuel screws out 1.0 turn (were 1/2 turn). (This was final step, to cure some popping on decel.)

Slow air screws 2 turns (were 1-1/2).
 
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