A year after the 650-750 swap

specialK · 03-10-2010, 01:41 AM

Well, a year after the swap, here's what she looks like. While everything was out of the engine hole, I took a power washer to the inside of the hull and cleaned her up a bit.

Ran her all last summer, but a few things are in order. The 38mm Keihin that I had bought for the 650, is sitting on this thing. it runs ok as far as throttle response, and idles fine when not in the water, but in the water, it runs for about 30 seconds and dies.

The engine vibrates a lot at idle. Things seem to be aligned ok, but how can you really tell. Compression: There was about 5 pounds difference between the front and rear cylinder. The engine temperature is where you can touch it, but you can't leave your hand on it too long.

I tried tuning the carb, but nothing seemed to change the idle roughness, or speed too much.

Should I take the head off and inspect inside before starting her up for this year?
I guess if I did take off the head, I should replace the head gasket?

I plan to do a dual cooling mod this spring, and try to find a 44 sbn carb.

Running 40:1 mix. Smokes some at idle which is normal, and not as noticeable at higher rpms.

Oh, I'm running the stock 1987 650 electronics and flywheel. From what I understand this has a 6200 rpm limiter? Should I look at replacing the electronics?

Thos1000 · 03-10-2010, 02:33 AM

Hey I was wondering what your compression ratio is? And what you thought of the diffrence in performance by doing the 750 swap. I've got one sitting here that's running 145 psi. My intake came in today to put the single 46 on that's on my 650 now. It's just that the 650 is running so well with the compression at 180 and the 46, I don't really want to mess with it but it's driving me crazy. Was there a big diffrence going up the 750? Just curious

betrezra · 03-10-2010, 04:29 AM

750 swap: there is some good details on a proven combo for you out there already......

750 sm pin
SBN 46 (jetting spring screw settings already spec'd out)
Factory Pipe with 650 chamber or blaster chamber
Water Box - You MUST use either a noisy aftermarket box + inline resonator OR a drilled stock water box...... if you're running the stock water box with no mods I found it killed performance and made a 750 swap seem doggy and perform worse or same as a good running 650.
Also water control is necessary (at least it was for me)....... the water screws on the factory pipe help..... and I run a jetworks water valve to the stinger to keep the water down at idle/low rpm and then allow water in the stinger tip at higher rpm......

It's all in the tune and combo........

Shawn and JeffX2SX both know the detailed SBN46 carb specs for your combo.

I'd say right off the bat your carb is the issue. (jetting / pop off / carb size).

Then when you get all that sorted out its' time to address the pump / prop....... some are running a cut back SXR prop (impros can help you there)..... and if you get nuts you can get a mag pump setup for the x2...... wife has one in her ski and it hooks nice.

good luck

specialK · 03-10-2010, 09:50 AM

Quote:

Originally Posted by Thos1000

Hey I was wondering what your compression ratio is? And what you thought of the diffrence in performance by doing the 750 swap. I've got one sitting here that's running 145 psi. My intake came in today to put the single 46 on that's on my 650 now. It's just that the 650 is running so well with the compression at 180 and the 46, I don't really want to mess with it but it's driving me crazy. Was there a big diffrence going up the 750? Just curious

It's a little hard to tell since I had to go over the entire winter between riding each one. I do know the 750 pulls stronger than my stock 650, which I don't know what the compression was on it. I didn't get a compression tester until after it blew a rod bearing. 180psi sounds really good.

I'm just wondering if the electronics are limiting the top end (besides the waterbox).
I'll do the waterbox mod, but the problem I'm having right now is with idle.

I'll measure the compression on the 750 again, I can't remember if it was 132/137 or 142/147.

specialK · 03-10-2010, 10:16 AM

As far as water control goes. Going from the 650 to the 750, I notice a lot less water coming out of the pisser at idle, but rev her up, and she starts shooting out like it should. Maybe the rough idle is keeping it from pumping smoothly.

kawix2er · 03-10-2010, 02:21 PM

What is your carb set at,try this 150 top jet every thing else stock.1 1/2 turn on top and 1 1/8 turn bottom and get rebuild kit.also check your check valve to make sure it only blows one way.

486x · 03-10-2010, 09:21 PM

check for steam coming out of the cylinder (with the plugs removed) after it's been run to see if the head gasket is blown

Twinturbostang · 03-10-2010, 11:34 PM

That carb and pipe are killing ya
What prop are you using?

specialK · 03-11-2010, 12:08 AM

prop is skat trak 9/17

I understand the carb is weak for this for the high end, but at idle?
The carb was new. I think I got it from Atlantic Jet Sports???
... but the westcoast pipe is killing it as well?

Also, I got boyensen reeds with stuffers.

Twinturbostang · 03-11-2010, 12:21 AM

The factory pipes head pipe has a much beter transfer to it and it's bigger all around and the chamber is much bigger. I'm a dual carb guy, more air in and more air out. That's how you make power.