Super Jet Need some help Backfire.

mikidymac

mikidymac

Location
California
Ok guys, finally got my superjet back together and fired up. 62T/61X.

So full rebuild, JM crankshaft, new 0.25 over pro-x pistons, Yamanube porting with stock head, carbs, jetting and exhaust and CDI. Carbs were rebuilt using all new Genuine Mikuni parts and pop off and leak down were good. All carb jets and screws are set per the manual.

So ski fired right up and idles good. Throttle response seems rough and if you stab the throttle it backfires out the exhaust.

Any ideas?
 
mikidymac

mikidymac

Location
California
First thanks for the responses.
Flywheel went on perfectly, key and key way were good and torqued with some loctite on the taper.

OEM CDI had a connector pull off and the wires looked a little brown instead of perfectly shiny copper but I did use the correct connectors and really expensive correct crimpers. Probably saw water at some point.

History of this engine and electrical were from a twin carb 62T/61X B1 that I only ran to make sure it worked as it had a factory pipe and badly scuffed pistons.

I also pulled the plugs and they looked pretty wet for only running a few seconds.
b59f5b33ca5b718e19344fb77c28571e.jpg
 
mikidymac

mikidymac

Location
California
Okay gentlemen.....

I tried a 6M6 cid and no change. Tried a different coil and no change.

Pulled the flywheel cover and key looks aligned.
cea6ba5702df52af9988aad84e5266a8.jpg



So what’s next? Stator carbs?

It won’t take throttle and kind of bogs and if you try to force the rpm’s it tries to backfire out the carbs.
 
Harbor

Harbor

Location
Stockton
mikidymac said:
Okay gentlemen.....

I tried a 6M6 cid and no change. Tried a different coil and no change.

Pulled the flywheel cover and key looks aligned.
cea6ba5702df52af9988aad84e5266a8.jpg



So what’s next? Stator carbs?

It won’t take throttle and kind of bogs and if you try to force the rpm’s it tries to backfire out the carbs.
Click to expand...


You can’t check the flywheel key with flywheel still on....part will still appear in the grove like you have their, the sheared part is in the crank and rotates away..... not saying it’s your issue just letting you know it can’t be checked without flywheel removal
 
mikidymac

mikidymac

Location
California
Harbor said:
You can’t check the flywheel key with flywheel still on....part will still appear in the grove like you have their, the sheared part is in the crank and rotates away..... not saying it’s your issue just letting you know it can’t be checked without flywheel removal
Click to expand...

Removed it and key is perfect and in its slot.
 
fastgtfairlane

fastgtfairlane

Location
Charlotte, NC
Is there any type of water temp sensor there that's not properly hooked up? I had the same problem with a kawi 750. I realize yours is a Yamaha. In my situation, I had the 2 temp sensor wires connected which is a normally open circuit. Connected, the engine would pop and backfire exactly how you described. Mine actually blew the westcoast exhaust in 2 at the coupler and started a small fire inside the engine compartment. Accidentally spilled a little gas inside the engine bay while priming the carbs. The backfire ignited the fuel and it singed my eyebrows haha. Good times. My neighbor got a good laugh over that watching me run for the water hose
 
mikidymac

mikidymac

Location
California
fastgtfairlane said:
Is there any type of water temp sensor there that's not properly hooked up? I had the same problem with a kawi 750. I realize yours is a Yamaha. In my situation, I had the 2 temp sensor wires connected which is a normally open circuit. Connected, the engine would pop and backfire exactly how you described. Mine actually blew the westcoast exhaust in 2 at the coupler and started a small fire inside the engine compartment. Accidentally spilled a little gas inside the engine bay while priming the carbs. The backfire ignited the fuel and it singed my eyebrows haha. Good times. My neighbor got a good laugh over that watching me run for the water hose
Click to expand...
Yes, the yamaha does have an overheat sensor but mine is not connected anymore because I used those wires for the bilge pump.
The Pink wire from my CDI has been disconnected and heat shrinked over to prevent any arcing or false grounds.
Keep the suggestions coming guys, I am at a loss.

So at this point....
Brand new OEM start/stop switch and harness.
New OEM pigtail from start/stop to elec box.
Flywheel was aligned and key correct.
62T and 6M6 CDI's tried.
Two different coils with wires tried
Fresh engine with new SBT crank , PRO-X .25 over pistons fitted by Group K, OEM gaskets and seals and Boyesen reeds.
Engine pressure tested 8 psi for over an hour.
Carbs rebuilt with Genuine Mikuni parts, pop-off and pressure tested.

Not sure what is left, stator? Carbs?
 
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Harbor

Harbor

Location
Stockton
When you did the carbs did the coned filters on the pump get cleaned out ?

In your first post it was an exhaust backfire now your mentioning it kinda bogs and wants to caugh out the carb?

Made me wonder if it was a lean condition or lack of fuel in the idle circuit... if the cons are clean I guess I’d try enriching the idle circuit and see if anything changes....

So it starts and idles good you say.. that’s good.... usually a timing issue would show up there

If you have a known good flywheel and stator set on hand, at some point I’d try them.....

Is the spark weak ?

Enough to run it but can’t ignite fuel when loaded ?
 
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fastgtfairlane

fastgtfairlane

Location
Charlotte, NC
at this point try disconnecting the wires that were for the temp sensor that you are now using for the bilge. unless you already tried that. these issues can be a real pita to diagnose. im dealing with something similar in my superjet only i put a zeel ignition setup in it and im not getting spark now. gotta love jetskis haha
 
mikidymac

mikidymac

Location
California
Harbor said:
When you did the carbs did the coned filters on the pump get cleaned out ?

In your first post it was an exhaust backfire now your mentioning it kinda bogs and wants to caugh out the carb?

Made me wonder if it was a lean condition or lack of fuel in the idle circuit... if the cons are clean I guess I’d try enriching the idle circuit and see if anything changes....

So it starts and idles good you say.. that’s good.... usually a timing issue would show up there

If you have a known good flywheel and stator set on hand, at some point I’d try them.....

Is the spark weak ?

Enough to run it but can’t ignite fuel when loaded ?
Click to expand...
Not sure what cone filters you are talking about.
Fuel Pickup in the tank was checked and cleaned and a new rubber hose added for the RES pickup.
All new OEM fuel lines from JM, new OEM fuel selector, New OEM fuel filter/water separator. NEw OEM molded pulse line and carb connection hose.
New Mikuni fuel pump parts, internal screen finger filters and needles and seats.

If you stab the throttle you get a backfire out the exhaust but if you try to increase the rpm from idle slowly you get the cough out the intake.

My friend has his engine pulled for a new top end so I am going to try to get his carbs on Sunday and try them. From there I guess I will start pulling his stator and electrical but I really don't want to start butchering his and buying new gaskets for both of us if I can avoid it.
 
mikidymac

mikidymac

Location
California
fastgtfairlane said:
at this point try disconnecting the wires that were for the temp sensor that you are now using for the bilge. unless you already tried that. these issues can be a real pita to diagnose. im dealing with something similar in my superjet only i put a zeel ignition setup in it and im not getting spark now. gotta love jetskis haha
Click to expand...
Already tried disconnecting that to get it out of the equation and no change.
 
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