Excessive fuel dump on deceleration

[DylanS]

This one has me stumped
Running a set of dual 48 full specs on my 976
Ski ran all week awesome had no issues besides a minor rich condition up top
Pop off is set at 23psi exact on both 1.8 N/S
No leaks at seat
Doubled up on check valves on side feed kidney blocks
No clogged jets, have been through carbs
Suddenly about halfway through a ride the ski starts acting up around half throttle
Fine at idle and still clears up at WOT but has a horrible (what feels and sounds like) rich condition at 1/2-3/4 throttle
So as I’m going through these carbs and not finding anything I reassemble etc..
running the ski on the stand and after I blip the throttle there is a crazy excessive ton of fuel being pissed onto my butterflies
This issue did not exist at all prior and nothing has changed with the carbs/return jet/etc.
it also explains the horrible bawble that clears up at WOT I was experiencing.
what could possibly cause this?? I figured I had a bad n/s or my pop off got thrown off but I swear everything is proper. Return jet is not clogged either.
Not sure how but could this be a external fuel pump issue??

 

[kevbo]

Big engines shake a lot. I would install new N/S and use a gold spring. Larger pilot jets may be required.

 

[catpower]

Check for air leaks on throttle shaft. Unwanted air drawn in will pull fuel. AKA runaway engine on stand or high throttle on deceleration.

 

[DylanS]

No leaks or runaways but I could try to play around with my pop off pressure and jetting to see what clears it up
the N/s are almost new, probably around 10 hours on them but that would make the most sense.
I’m going to check my fuel pressure first to see what I’m running at
Thanks everyone

 

[Sparkplug]

Can someone, anyone, explain the logic behind pop off in the 20s? Why do people lower popoff on engines that have 2 or 3x the signal of a stock engine? No one ever explains why or what would be the side affect of say going from 22 psi to 40.

People wonder why needles dont seat. Venturis dribble. Engines fill with fuel.

In my experience low popoff is super temperamental. I run higher than anyone.

Yamaha also made a check valve that is almost 3x the thickness. Probably works better than doubling them. It has its own pn# can't remember off hand.

Sent from my SM-G970U using Tapatalk

 

[DylanS]

Can someone, anyone, explain the logic behind pop off in the 20s? Why do people lower popoff on engines that have 2 or 3x the signal of a stock engine? No one ever explains why or what would be the side affect of say going from 22 psi to 40.

People wonder why needles dont seat. Venturis dribble. Engines fill with fuel.

In my experience low popoff is super temperamental. I run higher than anyone.

Yamaha also made a check valve that is almost 3x the thickness. Probably works better than doubling them. It has its own pn# can't remember off hand.

Sent from my SM-G970U

That’s a really good question.
I had always assumed everyone went lower on pop off as they went to less restrictive air cleaners and lowered their manifold pressure. I guess you could compensate for high pop off with a bigger low speed jet but I only seem to be able to clear up the infamous “muddy spot” on my carbs around these numbers.
I guess I can go higher on pop off and fatter on my pilots until I can’t tune the lean spot away? Never tried it. There was a real cool dude on the forums that had sent me thicker plastic check valves that seemed to work well too but I lost those on the ski I sank LOL.

 

[Sparkplug]

The needle and seat. The diaphragm. And the check valve/anti siphon valve, are bar none, the 3 most important parts of a mikuni style carburetor. Lowering popoff makes the check valve/anti siphon, less effective.

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[DylanS]

The needle and seat. The diaphragm. And the check valve/anti siphon valve, are bar none, the 3 most important parts of a mikuni style carburetor. Lowering popoff makes the check valve/anti siphon, less effective.

Sent from my SM-G970U using Tapatalk

I’ll try raising my pop off and compensating with a larger low speed jet
The only reason I haven’t was because the ski was running awesome for the longest time before acting up on BOTH carbs
Never had this issue, nothings changed

 

[Sparkplug]

If you had it good with low popoff. And your at the same altitude and general temperature, I would find out what broke, before changing anything else.

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[Sparkplug]

I just get bored at work and reply to things I see people say on here, with statements. Very few on here are actual certified mechanics. Take it with a grain of sand, in the middle east

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[Desk_jockey]

Leave your setup as is because that is a known working setup. Check your fuel pressure. I know you say that your return jet is not clogged but the symptom you are describing is the excess fuel not being able to get back to the tank. Your return pressure at just off idle should be 2 to 4 psi depending on jet. Could also try removing the jet just to see if it stops dumping fuel.

 

[DylanS]

Leave your setup as is because that is a known working setup. Check your fuel pressure. I know you say that your return jet is not clogged but the symptom you are describing is the excess fuel not being able to get back to the tank. Your return pressure at just off idle should be 2 to 4 psi depending on jet. Could also try removing the jet just to see if it stops dumping fuel.

This is what I was assuming too, something fuel pressure related. I’ve checked every fitting on the tank and the return jet is unobstructed.
I swear I never seen that much fuel (coming out of the Venturi) prior but I guess I could be remembering wrong and maybe have a whole other issue.
even on the stand now it sounds terrible.
something changed mid ride that is causing a mid range sputter and nothing to my knowledge has changed in the carbs. Ive also had this same unfixable issue on a separate set of 66e cylinders that ended up being a water ingestion issue. But I see no water wash or signs of intrusion.
hoping this isn’t a electrical issue on a brand new zeel system.
thank you everyone for the info, I will also look into the plug suggestion.
On second thought though it has to be a fuel issue, it is a pretty insane ammount of raw fuel being dumped.

 

[DylanS]

Today I got a fuel pressure gauge fitted to the ski and was running it on the stand (before taking it to the lake for a strapped down test). My rear cylinder is room temperature after idling on the stand for about 45 seconds and my front cylinder is warm/hot to the touch. Almost to the point that I would say the rear cylinder isn’t even firing but the ski was idling perfectly on both cylinders. Both low speed adjustment screws are set identical and there is a hot bright spark across both plugs. Compression is perfect across both cylinders. Has to be fuel related issues, what am I missing here? Rear plug was covered in fuel/oil when I removed it so it’s dumping fuel somewhere at idle but not visibly enough for me to tell. Idle i would say is around 900 rpm.
Thanks
Update
Idling ski, pull rear plug wire off, nothing changes.
not running on the rear cylinder. Plug is drenched.
I’m beginning to think I have a clogged return passage on my rear carb, there is almost no fuel coming back from the return side

 

[Harbor]

I had the same issue and it turned out the low speed adjusters were uneaguailly flowing fuel for the same turns out from closed. The low temp/wet plug cylinder needed the low screw set a little leaner till the 2 cylinders were equal temp and plug wetness was equal…

 

[DylanS]

I had the same issue and it turned out the low speed adjusters were uneaguailly flowing fuel for the same turns out from closed. The low temp/wet plug cylinder needed the low screw set a little leaner till the 2 cylinders were equal temp and plug wetness was equal…

Interesting, I was just playing with them earlier to try to get it to run but I’ll give it another shot..
If I start the ski and remove the rear plug boot it’ll idle off of the front cylinder
But if I take the front cylinder boot off the rear one will occasionally try to run a bit then die out again.
starting fluid helps it get going but I figure it’s because of how volatile it is even when it’s flooded.
checked the reeds, compression again and everything is fine.
I dry the cylinder/plug out and prime the carb with my hand and it’ll run half ass at 1/8 throttle but die instantly anything less than that.
I’ve always noticed a slight temperature variation between front and back when the ski was running good but not enough to concern me. What’s wild is that I’ve seen the ski idle and rode it with wayyyy more fuel dribbling from my Venturi and never had this issue..

 

[Harbor]

Yeah I had the dribble bad too from the rear carb, plug was wet and 10 degree cooler cylinder temp.. figured the dribble was the cause… so I swapped the carbs front to rear on the speed plate and the cylinder temp and wet plugged followed the carb, BUT the dribble did not follow the carb, rear cylinder still dribbled bad.. so these clues ment fuel was getting in from under the throttle blade, so I traced the fuel flow thru the carb via the Mikuni exploded fuel circuit diagram and found at this particular time it was even fuel flow for the same turns out of the low speed screw… every one says tune each cylinder independently, I guess this is what they were talking about…

try leaning the wet cylinders low screw out and see if you can even it out.. mine was less than 1/4 turn difference to get it correct… if it won’t then you got other issues..

I

 

[DylanS]

Yeah I had the dribble bad too from the rear carb, plug was wet and 10 degree cooler cylinder temp.. figured the dribble was the cause… so I swapped the carbs front to rear on the speed plate and the cylinder temp and wet plugged followed the carb, BUT the dribble did not follow the carb, rear cylinder still dribbled bad.. so these clues ment fuel was getting in from under the throttle blade, so I traced the fuel flow thru the carb via the Mikuni exploded fuel circuit diagram and found at this particular time it was even fuel flow for the same turns out of the low speed screw… every one says tune each cylinder independently, I guess this is what they were talking about…

try leaning the wet cylinders low screw out and see if you can even it out.. mine was less than 1/4 turn difference to get it correct… if it won’t then you got other issues..

I

Very good Info thanks a ton man
I have the carbs on the bench currently I’m going through everything one last time and I’ll give it another shot
Only thing weird is that none of the adjustments were touched from the time it ran fine to started acting up
I was actually out chasing wake and had tried and failed a few flips and on the setup for the next one I noticed the issue
Felt like I was out of fuel
Putted back fine and pulls hard up top but the mid is a horrible bawble out of the blue but I can’t find any issues with N/S
No clue

 

[Harbor]

Very good Info thanks a ton man
I have the carbs on the bench currently I’m going through everything one last time and I’ll give it another shot
Only thing weird is that none of the adjustments were touched from the time it ran fine to started acting up
I was actually out chasing wake and had tried and failed a few flips and on the setup for the next one I noticed the issue
Felt like I was out of fuel
Putted back fine and pulls hard up top but the mid is a horrible bawble out of the blue but I can’t find any issues with N/S
No clue

Agreed, that’s really odd…,you may have more than one thing going on. My cylinder temp difdenent issue didn’t equal a drivability issue performance wise..

 

[DylanS]

Agreed, that’s really odd…,you may have more than one thing going on. My cylinder temp difdenent issue didn’t equal a drivability issue performance wise..

Lol
So it turns out there was a hairline opening in my front butterfly that was just enough to let the front cylinder run and keep the back cylinder from getting air
When I bumped the idle up it came to life but was still super rich
I went from a 67.5 main restrictor (3.5psi at idle) to a 110 (2psi at idle) and for the first time in a week it sounds beautiful on the stand. I will have to water test to make sure there’s no gremlin yet.
Absolutely confused as to what changed (if that did clear my issue).
I rode no joke 5 days on that exact setup with that 67.5 jet with NO issues.
And you were 100% on the money with the low speed adjustments being off
I took the rear in 1/8 turn and there running at almost identical temps

 

[DylanS]

Turns out I was actually experiencing the beginning of a head gasket failure, when I got out on the lake yesterday i rode for about 2 minutes before steam started blowing out of one of my pissers.
Rear cylinder got so hot i could hear the water sizzling in the water jacket. Fortunately minimal damage to the plating but there was some deto on the exhaust side of the piston.