79cc Bog coming off idle @ WOT

Topher

Member
PZ19 mm knock off carb w/95 main, stock idle circuit.

Motor runs and idles just fine. Steady and rapid throttle advancement sees strong motor response and good strong pull.

The issue is trying to slam straight to WOT. The motor stumbles and will stall if you don’t lift, fuel spits out the back of the carb as if there is no pull from the intake.

Don’t really wanna settle for “just don’t punch it off of idle”.

I’ve been all across the board with the idle air screw. Nothing helps
 
To add, anywhere else in the RPM range, WOT sees good motor response, no surging heard or felt, no backfire, good plug readings after 10+ laps.

(Kid kart racing)
 
Drill the idle jet or change the slide .
You could cut 1/16th off the bottom but , there's no turning back if it didn't fix it .
The slide cutaway affects off idle .
Your sure there are no air leaks .
 
Drill the idle jet or change the slide .
You could cut 1/16th off the bottom but , there's no turning back if it didn't fix it .
The slide cutaway affects off idle .
Your sure there are no air leaks

Help me with my understanding of carb adjustments.

I was under the impression that 1/8-1/4 throttle is where slide cutaway has an affect. In this range the motor responds well. Filing the slide will change that area of operation and would have no affect on WOT.

The idle jet is something I had considered, and I have a spare one to drill out. This was my initial thought thinking I had a lean condition coming off idle but after taking the filter off I see that the motor doesn’t start pulling fuel through the carb until it gets higher in the rpm range. Slamming to WOT off idle the fuel spits back out the inlet side of the carb. How would the idle jet affect this condition knowing that at WOT the slide is completely open and should bypass the small port of the idle air circuit coming off the bell face of the carb?

Also, about vacuum leaks, I did a check by running the motor at idle and spraying starter fluid liberally around any seals on the intake side. (At the head, at the carb, the carb itself taking care not to spray the air filter). Found nothing with this process. Thoughts on other methods?

Where is your clutch engagement at?
Max Torque SS clutch with black spring (3000-3100)
 
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maybe i mis understood the problem .
still fuel has to come from some where .
as you state it wont drw fuel at full open till the air volume /rpm is up too speed .
a smaller cut away could help with starting the fuel draw . a larger idle jet would also allow fuel to flow more easily in the negative pressure availble .
raising the float level might help , the lo206 guys tune that way
there is also the chance the carb is just too big .
shoot a shot of fuel in at the same time you open the throttle . if it catchs right up you know it needs an accelerator pump or some way too get more fuel at low vacum, low air flow.
 
why is fuel spiting out the back? that seems like a valve seal issue .
maybe a baffle direcly in front of the carb throat .
on the 4 cylinder jap bike when you put on a high flow after market air filter it had to be baffeled off or they would not run right.
without major jet changes.
 
why is fuel spiting out the back? that seems like a valve seal issue .
maybe a baffle direcly in front of the carb throat .
on the 4 cylinder jap bike when you put on a high flow after market air filter it had to be baffeled off or they would not run right.
without major jet changes.

I thought the same about the valve seal, just lapped these valves for the first time and from appearances it lapped well but now I’m questioning it.

Lash is set to .006 and holding well.

Now with the valve seal issue, I would think the problem would persist throughout the rpm range and not just at idle. Like, breakup, backfire through the intake, not rev to peak rpm. But the motor runs well everywhere. The condition only appears at WOT coming off idle ONLY.
 
I tend too agree . One thing I see though here at home my lawn tractor recently won't start . I pull the air filter it's blowing back through the carb every revolution . Once it starts it runs fine .
Reversion typically happens at a higher rpm . Fuel standoff is another name .
Still I think you need more fuel .
Any cam twisting or lobe modifications .
 
I tend too agree . One thing I see though here at home my lawn tractor recently won't start . I pull the air filter it's blowing back through the carb every revolution . Once it starts it runs fine .
Reversion typically happens at a higher rpm . Fuel standoff is another name .
Still I think you need more fuel .
Any cam twisting or lobe modifications .
Reversion happens often at low rpm/idle too. It’s something I did not think about.

I recently changed my exhaust setup, adding a bend right out the head to accommodate tight clearances. I wonder if the added backpressure with the cam and higher comp (decked head) is causing this.
 
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My understanding of the valve seal is that it keeps oil from running down the stem into the cylinder . That's all it's supposed to do. Fire up an engine with bad seal (or none) and it puffs smoke for a moment, thats it. I never use the seals except as a spring shim.
 
My understanding of the valve seal is that it keeps oil from running down the stem into the cylinder . That's all it's supposed to do. Fire up an engine with bad seal (or none) and it puffs smoke for a moment, thats it. I never use the seals except as a spring shim.
Ive both used it and removed it. I’ve gotten predators that don’t even have it out of the box.
Long intake tracts create issues also .
Nice looking set up .
Thank you. Bought the kart with the Predator and had to make a lot of changes to keep the kid competitive but we’ll be going to a blue printed comer for next season. This motor is heavy and is proving to be just as expensive and finicky as people argue that the 2 strokes are.

I knew the intake tract length isn’t optimal but it was necessary to fit the larger carb. Even the stock carb bored out is small in comparison to the ones being used by the 2 strokes in class.

Motor ran good yesterday in practice just have to be smooth coming off idle. I think I’ll be testing the back pressure problem by running open header on a stand and see how it responds. Just need this thing running good for one last race this season and we’ll be making the change to 2 stroke. I’d like the kid to be more focused on driving and having fun, giving the throttle all the beans instead of worrying about driving against the motor.
 
I haven’t made any hard changes yet since posting. Only played with the air fuel mixture screw at the track yesterday. Screw is downstream of the throttle slide so I believe backing the screw out increases fuel.

I slowly turned the screw in and saw the idle rpm start to fall so I backed it out as far as I was comfortable with and set the idle speed high st 2500 to give it the best chance of not stalling with fast throttle changes. As I mentioned, it did well but I’m gonna spend some time with a drill set Saturday morning and slowly open up the spare idle jet that I have and see how it responds.

Between that and the idea of added back pressure I’m hopeful it’ll solve this inconvenience.

Appreciate the help and suggestions and I’ll Post on how it goes, good or bad for anyone else’s future trouble shooting.
 
Late update

Pinned until I found that the stock pilot jet is .35mm for PZ19.

Slowly worked my way up, #79, 1/64, and settled on .4mm.

Took some idle adjustment and tuning of the air/fuel mixture screw and wound up with responsive WOT from idle.

A tiny bit of bog was left but it’s just for a split second. May have been able to use a #78 drill but to give it a touch more fuel but didn’t have another pilot jet in case it turned out to be too much and didn’t wanna leave the kid with too fat of a condition off idle which would’ve been just as bad.

Settled for that and a 2nd place for the weekend.

On to some 2 stroke Comer races to end the year.

Thanks for all the help.
 
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