Super Jet Engine Issues

S

Soup

Location
Cache Valley
Hey guys recently I did some work on my 701 Superjet. Last year while riding I lost compression in a cylinder so during the off season it got rebuilt. Having read about porting for a few years I decided to give her a go and used the freestyle porting templates from Yamanube. Porting went good and had the cylinder bored out to 82mm. When it came time for reassembly I noticed the crank was locking up so long story short bearing was shot and now I have a new crank. Last night after reassembly we turned her over and it started right up and then continued to roll. Every time it starts it jumps to WOT and wont drop. It stops with the switch and if you block air to the carbs it wont run. We guessed with the porting its now drawing a ton more fuel and took the low speed and high speed needles in to see if it would change. After no change we screwed them in all the way and still it jumps to WOT. How is all this gas still dumping in? Any ideas on what going on here. and why it jumps to WOT. Should I have changed Jets with porting?

Info on the current build
701
Freestyle Porting
SBN 44s
B Pipe
MSD Enhancer

Any help would be appreciated

Thank you
 
smoofers

smoofers

Rockin' the SQUARE!!!!
Site Supporter
Location
Granbury, TX
First things first. Did you pressure test your motor? For this you block off the intake manifold and exhaust manifold, headpipe or exhaust ports on the head (rubber plugs, sheet rubber or specially made block off plates). Pressurize the case to 10 psi. It should hold for 10 minutes, a loss of 1 psi or more in that time is considered a failing test. This test is crucial on a 2-stroke motor because the crankcase pressure integrity is integral to proper tuning and function. A leaky crank case will cause a lean fuel mix and lead to a burned up motor. A leaky crankcase is also one of the main causes of runaways like yours.

Second, with porting, you've now reduced the initial velocity of air through your carbs due to the increased volume in the crankcase. What you have now done is lean out your fuel mix which can cause a run away situation like you're seeing. You need to add MORE fuel by way of larger jets or screw adjustments.
 
jonolove

jonolove

Location
oakland
When seals go bad it will become rich not lean unless it's leaking between the intake and carbs this is a misconception a lot of people have
 
S

Soup

Location
Cache Valley
I will do a pressure test again here in a day or 2 just to be sure. When I put it back together I used new seals. I pulled the carbs and the manifold and redid all of the gaskets there and still it runs away. Am I wrong in believing that shutting the low and high speed all the way stops gas flow through the carbs or does it still run in? Even with both closed it still runs away.
 
smoofers

smoofers

Rockin' the SQUARE!!!!
Site Supporter
Location
Granbury, TX
The adjusters you turned in are only for fine adjustments to the jetting that is already in place. By turning them all the way in you have made your fuel/air mixture leaner. There is still fuel coming from the carb, even if the adjusters are closed. You need to probably go up a Size on your jetting to be in the right spot.

On my SJ that is not ported but has a b-pipe, head, enhancer and stock 38 carbs, I am running 75 low jets and 140 highs. You are probably running something similar. You'll probably need to bump up your jets a size or so with your porting, but maybe not. You need to address whether or not you have an air leak first.

I'd turn your low screws out 3 turns from closed (max setting) and try from there.
 
Th3 VVay

Th3 VVay

Where did you get those jets, are they from a 650? Stock 701 jetting specs are below. Your jets are too small for a stock motor let alone modified.

115 Pilot
135 Main
95g spring
1.5 needle and seat
Low speed 1 7/8 +- 1/4
High speed 1 5/8 +- 1/4
 
Th3 VVay

Th3 VVay

Yep, that would explain it. I would call jetworks or another reputable shop of your choice, tell them your mods and they'll send you the proper jet kit. They should get you very close and then you'll have to do the fine tuning of the carb screws from there.
 
smoofers

smoofers

Rockin' the SQUARE!!!!
Site Supporter
Location
Granbury, TX
Th3 VVay said:
Where did you get those jets, are they from a 650? Stock 701 jetting specs are below. Your jets are too small for a stock motor let alone modified.

115 Pilot
135 Main
95g spring
1.5 needle and seat
Low speed 1 7/8 +- 1/4
High speed 1 5/8 +- 1/4
Click to expand...

I assume on a SINGLE carb? OP is running dual 44s
 
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