Rebuild Struggles

[Mayor118]

20 plus year ago I spent a lot of weekends on square nose and round nose Superjets. Moving away from the sport when selling 785cc superjet with full spectrum carbs, MSD and as much carbon fiber as Chris at 247 could add to it. Miss that boat for sure. Fast forward and my 14 year old wants to get into riding. I purchased a well cared for 2000 round nose and the new 4 stroke which works out nice as my son and I switch back and forth as both skis have different personalities.

Last season we had a piston get hot and melt the rings (rear seal). The engine was sent the the machine shop along with the pistons and rings. After getting the back I had a few friends and fellow riders help me get all the parts ordered and the build began. Everything was touched including the stock carbs being rebuilt.

The reason for my tread is I keep getting a backfire as the ski is struggling to start. I’m super confident the timing is right but I think the carb build might have been done wrong. I took my time but as mentioned I have not had my hands on these in years. Before I start taking the boat apart again I was hoping for some trouble shooting advice.

The ski is a 2000 with a one over bore, Ada head for pump gas, compression strong, stock duel carbs, hot shot flame arresters, stock ignition, primer, fuel shut off still factory, all new seals, new motor mounts, new ada gaskets, new carb rebuild kits, new starter and Bendix. She is getting fuel and has spark but wants to backfire. I mean gunshot loud.

So in my head it’s two things…

1. Timing that I took my time with but maybe moved.

2. Guy who built the carbs sucks (me)

3. Anything else I should be looking at?

My now 17 yer old and (better rider the me ) and I look forward to the father son days on the lake together before he disappears to college and really would like to have this longer then expected project finished. I’m struggling here and hope you have some suggestions.

 

[Mayor118]

I can try and get a video if that makes more sense.

 

[mostexcellentdude]

You could rule out/bypass the carbs by dribbling premix straight down the spark plug holes and firing it up. Any chance the main carb diaphragms are in backwards? That would be an easy mistake to make.

Sounds like it could be a timing issue to me if its backfiring. You can pop the flywheel off with the motor in the ski and check that the stator mark is lined up with the engine case mark.

 

[WFO Speedracer]

Run me through the exact procedure it takes to get the ski started . It won't take much of an air leak to cause this issue, I chased a similar issue on a 650SX for a bit turns out the cap on the oil injection fitting was leaking just enough to make it pop and be hard to start , removed the cap and the fitting, plugged it wit JBweld and it was good to go.

I have also seen this with using aftermarket carb kits and with the needle lever being set too low on the carbs .

 

[Big Kahuna]

1. Have you checked the flywheel key? If sheared it can cause this.
2. If the stator plate is loose or moved and in wrong position can cause this.

 

[Mobile Chernobyl]

Reeds are worth checking too. If you're going through the trouble of checking the flywheel and key - pull the intake mani off and check the reeds while you're in there.

IMHO - just pull the engine, throw it on a bench and get to work troubleshooting what's already been mentioned. The engine can be pulled in under 20 minutes if you're determined (We've timed ourselves).

 

[joeschuit]

Worth checking for consistent spark and carbs that aren't leaking into the cylinder. I had bad sparkplug boots that appeared over the winter. Cranking with no/bad spark put raw fuel in the exhaust manifold and lit off a couple times. Not a backfire like the timing is off, but might as well be. Depending on where the pistons are at, one will always have the exhaust port closed and the other open, putting pressure on the reeds. In my case only one set of cylinder reeds had creases from the backpressure.

 

[WFO Speedracer]

Tons of stuff it could be , that is why I asked about the starting procedure, to get a little more insight into the issue at hand .

 

[Mayor118]

Sorry work keeps getting in the way. Well before it was two pumps of the primer and she fired up.

Now I have the flame arresters off and doing the same. Wants to start…..stumbles…. Then pop.

Should also mention a factory pipe is onboard. The thought I was building for the old limited class.

 

[Mayor118]

Thanks for the ideas I will start and make a list to work through. Here is what I’m working with. Don’t think it helps to show them but maybe it might bring up other thoughts.

Will try and spend some time with it tomorrow.

 

[Mayor118]

Worth checking for consistent spark and carbs that aren't leaking into the cylinder. I had bad sparkplug boots that appeared over the winter. Cranking with no/bad spark put raw fuel in the exhaust manifold and lit off a couple times. Not a backfire like the timing is off, but might as well be. Depending on where the pistons are at, one will always have the exhaust port closed and the other open, putting pressure on the reeds. In my case only one set of cylinder reeds had creases from the backpressure.

Reads were just replaced with carbon ones.

 

[Mayor118]

Reads were just replaced with carbon ones.

Reads were just replaced with carbon ones.

Sorry Reeds

 

[Mayor118]

Reeds are worth checking too. If you're going through the trouble of checking the flywheel and key - pull the intake mani off and check the reeds while you're in there.

IMHO - just pull the engine, throw it on a bench and get to work troubleshooting what's already been mentioned. The engine can be pulled in under 20 minutes if you're determined (We've timed ourselves).

I just got it back in….lol. Man hopefully I can check the timing and carbs first.

 

[Mayor118]

1. Have you checked the flywheel key? If sheared it can cause this.
2. If the stator plate is loose or moved and in wrong position can cause this.

Thats my next stop, along with the timing.

 

[WFO Speedracer]

I know you will not want to hear this but here goes anyway , if it was cranking properly and you replaced the reeds and now it isn't cranking properly it could be the reeds , this is the approach I take to diagnosing stuff , what changed , backtrack from there .

The two things that stand out TO ME right now are the reeds and the flywheel key since you had the flywheel off , maybe it didn't get tightened down enough or the flywheel is not properly seated on the taper letting it slip and shearing the key in the process .

 

[Mayor118]

I know you will not want to hear this but here goes anyway , if it was cranking properly and you replaced the reeds and now it isn't cranking properly it could be the reeds , this is the approach I take to diagnosing stuff , what changed , backtrack from there .

The two things that stand out TO ME right now are the reeds and the flywheel key since you had the flywheel off , maybe it didn't get tightened down enough or the flywheel is not properly seated on the taper letting it slip and shearing the key in the process .

I will check the flywheel then dig into the carbs. Thank you for the time!

 

[Mobile Chernobyl]

Also for what it's worth, I've installed those carbon tech reeds in a bunch of skis and occasionally (a coupla beers scenario) I find the reeds have a big gap. I flip them and reinstall the other way and they seal without any gap. I didn't think they had an orientation but some certainly do! Hate to make you rip the beast apart again but while your in there best to check.

 

[subnsxr]

Fuel lines on correct? return on top, supply on bottom?

 

[fastgtfairlane]

you said you rebuilt the engine correct? did you happen to do an air leak after assembly? if not, defintely do an air leak test to verify the engine is sealed up tight. compression test too just to verify both cylinders make even compression. i do both of these on every new engine i assemble as insurance and peace of mind. also check your popoff and jetting in the carbs. also something i do everytime i go through a carb is verify correct popoff.

 

[Mayor118]

Work has been crazy so I’m just digging into this now. Pulled the battery, tank and factory pipe. Then got in the cover and pulled the flywheel. When I pulled it off the key was stuck to it. Also inside the flywheel there are some groves. I don’t recall seeing that but again it’s been sometime since I pulled it last time. I think it should be smooth. The key also has some black around it and when removed it was clean underneath. Thinking this might be the issue.

Stopped here…. Should I keep digging into the reeds and carbs our put it back together and try it. No metal shavings on the flywheel or stater. Maybe it’s old groves since the boat is a 2000 and I’m the third owner.