61X Engine Build — big bore, porting, crank, jetting

B

BupBup

Location
Washington
I’ve got my 95’ blaster torn apart right now and trying to get it ready for the summer. I got a few questions about boring the engine and re-jetting, etc. I’ll let you know what I’m running and the whole setup I have so you have a better idea of what I’m working with.
-61X bottom and top end
-ADA girdled head
-riva red pipe
-single 44mm carb with pro k filter
-lightened stock flywheel
-zeeltronic ignition with an msd coil
-9/15 skat impeller

Bore and dome size??
Right now I’m trying to decide which size to bore the engine out to. Can’t decide between 83mm and 84mm. The Ada head has 37cc 701 domes at the moment and I’m curious what I’d want to run for the 83 or 84mm bore. I normally ride river mouths and tidal areas with sloughs, as well as lakes. I like the bottom end quickness but I need a setup that’s reliable enough to go full throttle for a decent amount of time. Pump gas mainly but occasionally running av gas if I’m not traveling.

Porting?
I‘ve never ported an engine before but I’m pretty good with a dremel and/or die grinder and I’m sure I could figure it out myself if I have a template. Is it worth it to port the engine for what I’m looking for if I bore the sleeves?

Crank?
Should I look into a different crank when boring out the cylinder or will my stock crank be fine? I have about 3 years of riding on a stock crank right now.

Jetting?
Say I go with 84mm bore and 37cc 760 domes, no porting, stock crank, and running pump gas. What are the jetting specs that you‘d recommend starting with?

Thanks
 
H

Half flip95

Formerly pondracer95
Location
Lake Havasu City AZ
hone it, re-ring it, seal it up and run it with what you have. You're probably in the ball park of 140-150, 120-130, 2.0 95g. Make small refinements over time. 37cc on a 701 will run on 91 pump gas everyday as long as you don't get too goofy with the zeeltronic.

If you want to experiment with a grinder buy a junkyard dog and go to town.
 
Myself

Myself

manic mechanic
Location
Twin Lakes AR
Porting.......at least clean up the horrible sleeve to cylinder match-up, widen the exhaust port a little, grind the sleeve skirt away from the port openings, and thumbnail notch the intake side at the boost port. Easy to do, free extra power, no loss of reliability.
 
M

Motoman399

Location
California
Myself said:
Porting.......at least clean up the horrible sleeve to cylinder match-up, widen the exhaust port a little, grind the sleeve skirt away from the port openings, and thumbnail notch the intake side at the boost port. Easy to do, free extra power, no loss of reliability.
Click to expand...
Can you elaborate on this thumbnail notch? Are you talking about in the case to transition the reed area into the boost port easier, or? Thanks!
 
kevbo

kevbo

Location
dfw
Myself said:
Porting.......at least clean up the horrible sleeve to cylinder match-up, widen the exhaust port a little, grind the sleeve skirt away from the port openings, and thumbnail notch the intake side at the boost port. Easy to do, free extra power, no loss of reliability.
Click to expand...
Widen the exhaust a “little”??? How much is that??? How about a turbo and some nitro to go along with that porting! That will get him going in the right direction.
 
yamanube

yamanube

This Is The Way
Staff member
Location
Mandalor
Don't go crazy on your cases, gasket match all the intake pieces (found these to be pretty poor mismatched) and soften the edges between the reeds and the bottom of the transfers. Widen your exhaust port and match the boost port up is easy work and good gains.
Assuming your skat impeller is an oldschool straight blade style, swapping to a modern, radial edge impeller will make a big difference.

Porting templates can be found here:

Power to the People

7 Years ago, I started making porting templates to fill a gap that was left when Sport Port stopped making their templates. Since then, I have sold literally hundreds of porting templates. I want to thank everyone who purchased them from me over the years and now I am going to my best to pay it...
www.x-h2o.com
 
Last edited:
Myself

Myself

manic mechanic
Location
Twin Lakes AR
Motoman399 said:
Can you elaborate on this thumbnail notch? Are you talking about in the case to transition the reed area into the boost port easier, or? Thanks!
Click to expand...
In the cylinder sleeve that hangs down into the case. Take a look at the bottom of this Blowsion sleeve--->https://www.blowsion.com/yamaha-785-big-bore-sleeves-blowsion You can see a thumbnail sized notch out of the intake side. That lines up directly with the boost port. The Yamaha 800 and 1200 powervalve engines have this feature in stock form. Even a 1/2"x1/2" notch helps out. Then if you look at the boost port in the Yamaha cylinder you wil see they are usually BADLY mismatched. I like to open that up (along with the transfers) then use a straightedge placed inside the port. You want that aiming right towards the spark plug.
 
Last edited:
Myself

Myself

manic mechanic
Location
Twin Lakes AR
kevbo said:
Widen the exhaust a “little”??? How much is that??? How about a turbo and some nitro to go along with that porting!
Click to expand...
That sounds like a great idea...How much nitro??? What size and shape turbo wheel???? Maybe give us a little more info.



Just messin with ya.

Exhaust on those can be widened 3mm on each side no problem. If you want a good all around gain. Taper and blend it into the exhaust port going out.
 
B

BupBup

Location
Washington
yamanube said:
Don't go crazy on your cases, gasket match all the intake pieces (found these to be pretty poor mismatched) and soften the edges between the reeds and the bottom of the transfers. Widen your exhaust port and match the boost port up is easy work and good gains.
Assuming your skat impeller is an oldschool straight blade style, swapping to a modern, radial edge impeller will make a big
Click to expand...
Wasn’t planning on doing anything crazy but will definitely shave down the sleeve to match the angle of the intake, as well as soften those edges above the reeds like you said. As far as the impeller goes, it’s a 3 year old impeller from blowsion. C+75 Mag Swirl 9/15
 
B

BupBup

Location
Washington
Thanks for all the input! I’m sure the info has already been posted but it’s always hard to find the answers for my specific circumstances and riding style without making a post. Keep it rolling
 
fastgtfairlane

fastgtfairlane

Location
Charlotte, NC
Jhucke said:
Couple hundred bucks and you can get your cylinders ported professionally by a reputable builder if you don't want to 'just do whatever' hoping for the best.
Click to expand...
Couldn't agree more. Unless you have spare cylinders to mess with and the proper tools to port. Otherwise it can be an exhausting nightmare then to find out all that work you did netted little to no gains at the end of the day.
 
You must log in or register to reply here.
Top Bottom