Finally got the Mack Mag built!

[Kpi890]

There's ignition/combustion things that could maybe be mistaken for four stroking.. Not throttle lag though. That one has to be lean condition.
Small jet, intake vacuum leak, foamed fuel, fuel inlet valve blockage, fuel line blockage, vacuum forming in tank.. That's the whole list as far as I can recall.
What jet sizes are you using now? And what brand?

Right now I’m running a 66 jet from a jet kit from Gasbike, it seems to be running fine at WOT now, slight 4 stroking but nothing major. When I pull the clutch while riding it takes quite a while for the engine to come down off the RPM’s. Throttle lag at lower and mid range rpms and sounds like it’s 4 stroking pretty bad when I give it throttle at the low end. Running without the air filter on the way to work this morning its still showing fuel shooting out of the carb. Could this be an exhaust issue? Blockage or something? The MZ 65 pipe comes in the mail today so I may see if that’ll fix the issue but I’m not too hopeful.

 

[FurryOnTheInside]

Right now I’m running a 66 jet from a jet kit from Gasbike, it seems to be running fine at WOT now, slight 4 stroking but nothing major. When I pull the clutch while riding it takes quite a while for the engine to come down off the RPM’s. Throttle lag at lower and mid range rpms and sounds like it’s 4 stroking pretty bad when I give it throttle at the low end. Running without the air filter on the way to work this morning its still showing fuel shooting out of the carb. Could this be an exhaust issue? Blockage or something? The MZ 65 pipe comes in the mail today so I may see if that’ll fix the issue but I’m not too hopeful.

Exhaust blockage doesn't cause throttle lag. I left a bit of rolled up tissue in my exhaust (for a drip stopper) by accident once and the engine just didn't want to start, until the "exhaust tampon" popped out after about thirty yards of pedalling.

 

[Kpi890]

Now this might not be correct but is a theory at the moment. From seeing how much fuel is coming back out of the carb on throttle it seems like the engine is running really rich on the low end and there’s just a lot of gas sitting in the carb. When I throttle the engine it will 4 stroke hats right away, bog for a second and then get up to rpm and the engine does fine once the throttle is opened and left open. I believe that each time I hit the throttle the engine is bogging down due to too rich a mixture or two much fuel in the carb and once that is cleared out the engine runs fine until I pull the clutch to come to a stop sign or something and accelerate and then it bogs on initial throttle and then catches up to the rpm.

 

[FurryOnTheInside]

Now this might not be correct but is a theory at the moment. From seeing how much fuel is coming back out of the carb on throttle it seems like the engine is running really rich on the low end and there’s just a lot of gas sitting in the carb. When I throttle the engine it will 4 stroke hats right away, bog for a second and then get up to rpm and the engine does fine once the throttle is opened and left open. I believe that each time I hit the throttle the engine is bogging down due to too rich a mixture or two much fuel in the carb and once that is cleared out the engine runs fine until I pull the clutch to come to a stop sign or something and accelerate and then it bogs on initial throttle and then catches up to the rpm.

Which notch is the c clip on, on the needle?
If you take the bowl off, turn the carb upside down and place the float on the tangs, is the bottom (now top) of the float approximately the same level as the jet? Or how far either way?

Gasbike jets maybe inaccurately sized, but I think you may be on the right track with the fuel level..
Be nice to hear some other people's ideas lol! I only really know what I remember reading here. The previous threads in this forum are where I learned everything I know.

 

[zippinaround]

sounds to me like a dirty jet or else possibly fuel overflow i.e. the tangs not shutting off fuel, possible bad float when running well not a lot of fuel should be coming out of carb,and yes check that the jet is what it says with a gauging pin quite often they are incorrectly marked.

 

[Kpi890]

Which notch is the c clip on, on the needle?
If you take the bowl off, turn the carb upside down and place the float on the tangs, is the bottom (now top) of the float approximately the same level as the jet? Or how far either way?

Gasbike jets maybe inaccurately sized, but I think you may be on the right track with the fuel level..
Be nice to hear some other people's ideas lol! I only really know what I remember reading here. The previous threads in this forum are where I learned everything I know.

I’ll check that when I get home and my c clip I believe is one notch from the top, so it’s on the second available notch. Just wondered why it does it when I swap over a brand new NT carb it does the same thing.

 

[Kpi890]

sounds to me like a dirty jet or else possibly fuel overflow i.e. the tangs not shutting off fuel, possible bad float when running well not a lot of fuel should be coming out of carb,and yes check that the jet is what it says with a gauging pin quite often they are incorrectly marked.

I’ll look into it when I get off work, only concerning thing is when I swap the current NT carb with a brand new stock spare I have it’s doing the same thing, getting too much fuel unless it’s at WOT.

 

[FurryOnTheInside]

I’ll check that when I get home and my c clip I believe is one notch from the top, so it’s on the second available notch. Just wondered why it does it when I swap over a brand new NT carb it does the same thing.

Yeah it is totally perplexing me. The only reason I can think for lean on the bottom rpm and rich on top, that makes sense to my understanding of how these engines work, is a small jet with a restrictive air filter, or large jet with a way low needle. So I'm pretty stumped. :/

 

[Kpi890]

Yeah it is totally perplexing me. The only reason I can think for lean on the bottom rpm and rich on top, that makes sense to my understanding of how these engines work, is a small jet with a restrictive air filter, or large jet with a way low needle. So I'm pretty stumped. :/

And by “way low needle” you mean the c clip being closer to the pointer end of the needle?

 

[FurryOnTheInside]

And by “way low needle” you mean the c clip being closer to the pointer end of the needle?

No, the other way, but the second notch from the top is the most commonly used position so that should be fine.. and even if they made the needle too long on your NT carb it would be too unlikely to find the same fault on both your NT carbs.