650/X-2 Questions about dual 38s, milling head and other mods on a 650sx

[928Engineer]

Hey guys, I apologize if this has been discussed many times before, I tried searching but couldn't find what I was looking for...

I currently have a bone stock 650sx that I'm doing a few mods to and wanted to get your input on how I should set it up. Back when I lived in AZ I had a 650sx with a Group K sleeper kit that I loved so I wanted to build another one that at least matched that. I am going to be picking up a dual 38 carb setup with WC manifold for a price I couldn't pass up. I figure if I run into too many problems with tuning, I can just run one of the 38s and port match the stock mani. To give a little background, I am looking to set this up as a flat water freestyle-ish ski to toy around with here in MI. I know I'm wasting my time with a Kawi, but I have a soft spot for these and it was given to me by a friend who bought three, we used parts from this one to get the other two running and for my trouble he gave me the ski for free. My plan is to get it running for the rest of the summer and then install foot holds, chop the rear, d-cut the stock ride plate and install a PLD aluminum pole this winter.

The first question I have is for those of you running the dual setup, how is yours setup with regards to jetting and N/S setup? Can I run the stock jetting/ needle & seat or should I re-jet? I'm not sure how it is currently setup, I'm hoping whoever had this before me had it running well, but in the likely case that isn't true I'd like to hear what has worked for you.

I was going to send the head out to get milled as well so I wanted to know how much to have taken off? I read in another post that .035 was a good number. I read Group K's site and couldn't find any numbers regarding this...not surprised that they don't give away much. Any input from you?

Also, what other mods will I need to do to run this setup effectively? Will I be ok with porting the stock exhaust manifold and running the stock pipe with cooling mod and 3" extension in the expansion chamber or am I wasting my time? Also, I hear the stock reeds are durable but if running Boysen or another manufacturer will be more beneficial I am open to that. Also, what impeller would be best for this setup?

Any input with what you have had success with is greatly appreciated. If you have some parts you want to get rid of that would help me out, feel free to pm or post here as well.

 

[j0hnny]

dual 38's seem like way overkill without supporting mods. if u do a head (go aftermarket not shaved) exhaust and waterbox then maybe.

my 650 we shaved the head and the compression is great but power seems lacking. we switche to an aftermarket head and i picked up all sorts of power and the compression is actually lower than the shaved head. squish and bowl design make a huge impact.

my recommendation is a single 38, head, and do the expansion chamber mod. just did this to an x2 and it made the think so much more fun.

 

[928Engineer]

Good to know, thanks! I forgot that this one actually has the A/M West Coast water box, although I've read these don't do much for performance and the noise is pretty obnoxious. Although after racing off-road for several years I think my ears are used to it by now.

For A/M heads, is there any noticeable difference between them? I've seen tons of Mariner heads for sale so I'm sure I could find one for a good price but would another option be better?

 

[Fatman]

I have never found a good answer on dual carb jetting for a 650. 65ish low and 115ish high with a low pop off is a OK place to start. I'd put my money in the following : A head with 29 or 30cc domes; a Westcoast, Coffman, JetPro, or Factory pipe with a diverter style exhaust manifold; 15.5 or similar impeller.

 

[j0hnny]

i like where fatman is at.

im with u on noise. but my ultra4 truck is so much more annoying it doesnt bug me too much lol

 

[newbee]

I'm in the same boat just got a pjs dual manifold. Would like to know a starting point on the carbs .

 

[928Engineer]

I'm in the same boat just got a pjs dual manifold. Would like to know a starting point on the carbs .

So the carbs I picked up ended up being the old Mikuni BN38 with the non-removable jets. I'm not willing to drill out the main jet to run them stand alone so I'm going to give it a shot and see what happens. They came with a 90 main jet so I'm hoping this works after I install a different head. The only other thing I need to work out is the throttle cable...with the manifolds that I got with them, it turns the carbs 90° so I'll have to get crafty with the throttle cable or replace it with one that doesn't have the 90° bend at the end and make a new mounting plate. Figures I would have to do some modifications but I couldn't pass up on both carbs, westcoast intake, and flame arrestors for $100.