Stuttery acceleration once engine warms up

waywirk137

waywirk137

Location
Minneapolis
Over the spring I swapped a 701 61x/61x (from a 95 wetjet) into my 91 SN 650 (oh man does this 701 beat the hell out of that old 650 haha). Engine-wise, it's a stock 701, single carb, stock exhaust, stock 701 cdi, new coils. I did a carb kit and replaced all fuel lines. I also added a pisser line.

The issue I've run into is that once it's warmed up (after about 15min of riding) the acceleration becomes jerky at anything above 15% throttle. I've read through the forums (trust me I've checked) and I can't seem to find anyone with the same issue. It doesn't sound like cavitation. It behaves the same way when still warm right once I pull it out of the lake. The new fuel lines are a clear yellow and air bubbles are visible, maybe that's a factor? The pisser water feels hot to touch but not too hot to burn me. It won't die on me but just hesitates back and forth, if that makes sense. Has anyone run into this issue before?

Hopefully I successfully uploaded pics with this post, not that it'll necessarily help in a diagnosis. Thanks in advance!

PS: I'm changing the chinpad vinyl, it was the only vinyl I had at the time; I know it's obnoxious..
 

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Harbor

Harbor

Location
Stockton
You might try cutting back you ignition cables till you get solid wire then reinstall the plug boots. Had the wire core break inside and give me a studder. Also you migt spray around your wires with a water bottle while running and look for voltage leakage out the wire to ground, might be easier in the dark. Had that too good luck
 
waywirk137

waywirk137

Location
Minneapolis
Harbor said:
You might try cutting back you ignition cables till you get solid wire then reinstall the plug boots. Had the wire core break inside and give me a studder. Also you migt spray around your wires with a water bottle while running and look for voltage leakage out the wire to ground, might be easier in the dark. Had that too good luck
Click to expand...

The ignition wires only have about 4 hours on them since I replaced them last season but I'll consider it a potential issue. Strange that it only happens once the engine is warmed up.
 
waywirk137

waywirk137

Location
Minneapolis
Harbor said:
Oh I missed "new coil" .... Do have resistor spark plugs in it and tried disconnecting the temp sensor if it had one
Click to expand...

Haven't tried removing the temp sensor yet but I feel like if it was going into limp-mode it should have cut the power almost completely. Worth a shot I guess
 
Baumanator

Baumanator

Location
Washington
I just spent awhile trying to figure my similar issue out, anything above 1/3 throttle it wouldn't make power and seemed to run in limp mode (like it was hitting rev limiter very low) check one way valve on the tank, temp sensor, exhaust leak, my issue ended up being a bad CDI box. hope it helps at all
 
waywirk137

waywirk137

Location
Minneapolis
Baumanator said:
I just spent awhile trying to figure my similar issue out, anything above 1/3 throttle it wouldn't make power and seemed to run in limp mode (like it was hitting rev limiter very low) check one way valve on the tank, temp sensor, exhaust leak, my issue ended up being a bad CDI box. hope it helps at all
Click to expand...

Thanks for the response. I just received a new check valve in the mail yesterday but haven't put it on yet, as the old one wasn't working; hopefully that'll solve the issue. It could be that the broken check valve had something to do with the air bubbles in the lines. I also tightened the lines where they connect to the fuel pickups before I took it out last time and didn't seem to have an issue.

Also, this is the check valve I purchased for anyone else who comes across this post: MJ-1200XL-Ltd ' CHECK VALVE EU0-67767-31-00 for only $16. Compared to the OEM Superjet one they wanted $22 for. I think it's for a different ski but should work just the same.

Hoping it's not the cdi, but at least that would be chance to finally bite the bullet and buy an MSD Enhancer haha.

I'll report back to this post after I have a chance to test it with the new check valve installed.
 
DAG

DAG

Yes, my balls tickled from that landing
Location
Charlotte, NC
internal carb filter sounds like it could be clogged since it's a old carb but new to you.

Exhaust leak?

When coils start going bad they do weird chit when they get hot aka higher rpms
 
waywirk137

waywirk137

Location
Minneapolis
DAG said:
internal carb filter sounds like it could be clogged since it's a old carb but new to you.

Exhaust leak?

When coils start going bad they do weird chit when they get hot aka higher rpms
Click to expand...
Coils are nearly brand new and cleaned the carb/new mikuni carb kit before I took it out this season. Exhaust leak is a possibility but I haven't seen any sign of one when I ran it without a hood once it started exhibiting the issue. The issue also began at anything above 15% throttle so not just the high rpms. It's like it would get "jerky" for a bit then eventually take off.
 
DAG

DAG

Yes, my balls tickled from that landing
Location
Charlotte, NC
Sounds like you got alot of it covered. Maybe it's the check valve but you should be able to run (air quote) "fine" with no valve at all.

Is the fuel line clear by chance? At a minimum don't used clear line for the pulse line as it can be sucked close under vacuum.

Stator plate loose screwing with timing?

I'm running out of ideas and runs me back into the carbs. Maybe something got put in back words or pinched at assembly.

Just reaching at straws now lol
 
Kpietro

Kpietro

Location
Kansas
Sounds like you have covered most of it.. Double check carbs again, Pop-offs and your jetting. Have heard of people cleaning their tanks and some left over debris getting into the carbs and causing issues. Better to check again and rule that problem out.

After that all I could say is maybe a bad cdi.
 
waywirk137

waywirk137

Location
Minneapolis
DAG said:
Sounds like you got alot of it covered. Maybe it's the check valve but you should be able to run (air quote) "fine" with no valve at all.

Is the fuel line clear by chance? At a minimum don't used clear line for the pulse line as it can be sucked close under vacuum.

Stator plate loose screwing with timing?

I'm running out of ideas and runs me back into the carbs. Maybe something got put in back words or pinched at assembly.

Just reaching at straws now lol
Click to expand...

Fuel lines were replaced with clear Tygon lines, which I've read to be some of the best fuel lines available so idt it's that. I cleaned out and reused the original pulse line so that's probably not it either. Stator plate was torqued down pretty well. I'm pretty confident with my carb rebuilding so I don't think I could have put it back together incorrectly. Sorry to shoot down all your ideas, I hope to not come across as a dick haha.

I've read these engines don't require the tank to be pressurized to run but I wonder if the lack of a functioning check valve is allowing fuel to drain back into the tank, possibly because of an incorrectly tightened ziptie on a fuel line (possibly on the fuel selector - which I should remove anyhow)

Looks like I'll be taking it out on the lake today or tomorrow, so I'll be able to install the check valve and see if that's the solution.

I really appreciate all the recommendations!
 
waywirk137

waywirk137

Location
Minneapolis
Figured I should go back and update this post if anyone having the same issue comes across it.

I'd replaced the entire stator with a brand new one and still seemed to have the same issue with the ski not accelerating properly once warmed up.
I finally got around to checking compression; one of the cylinders was reading lower than the other so I decided I'd just replace both pistons with pro-x ones from Jetmaniac.
I installed an ADA girdled head (w/ 35cc domes) at the same time.

The acceleration issue is finally gone! When installing the new pistons, I observed a fair amount of scoring on the cylinder walls of the lower compression cylinder, as well as a chip on the outer part of the piston; the scoring wasn't deep, but it was for sure visible. I made sure to hone the cylinders before installing the new pistons.

My ski seems to have a little less low-end than it originally had but I accredit that to the new rings not being worn in yet; I may also need to adjust the carb settings since I'm running ~180psi now (I can't find info on if I need to adjust this after the high compression head install-if anyone knows let me know!).

Next season I plan to install a b-pipe and msd enhancer, once the funds are available and the ice is gone.

Hope this info helps someone having the same issue I struggled with for so long.
 
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