Engine 4 Strokes at Cruise

Randall

Active Member
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May 23, 2016
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573
Location
Madras, Oregon
Can anyone help me with a 4 stroking issue. It accelerates just fine but when I turn the throttle back a bit to cruise it 4 strokes really bad. I have this carby: http://www.bikeberry.com/engine-kit-parts/fuel-components/66-80cc-high-performance-carburetor.html. An open pipe (for now). Thinking it might be the jet or the float. But I recently checked the float for damage and none found. It just started doing this after I put on the straight pipe. I put some copper scouring pad material in the pipe to tone it down and give it a little back pressure, but that didn't work.
 
That's sold as an RT carb some places. It can use the same main jets as the NT carbs.
Dellorto 5mm ones, if I remember right. SBP sells them in sets or singly for a few dollars each.
You can try one step smaller (leaner) main jet. I tried one of those carbs and it was never right on the top and the bottom at the same time, other guys like them just fine. It depends on your engine's set-up, I guess. I got mine for less than $30, that site might be a little high on something.
 
Can anyone help me with a 4 stroking issue. It accelerates just fine but when I turn the throttle back a bit to cruise it 4 strokes really bad. I have this carby: http://www.bikeberry.com/engine-kit-parts/fuel-components/66-80cc-high-performance-carburetor.html. An open pipe (for now). Thinking it might be the jet or the float. But I recently checked the float for damage and none found. It just started doing this after I put on the straight pipe. I put some copper scouring pad material in the pipe to tone it down and give it a little back pressure, but that didn't work.
What pipe did you use before? Did you figure this problem out in the last 3 weeks since you posted?
 
Can anyone help me with a 4 stroking issue.
It just started doing this after I put on the straight pipe.
You can't 'tune out' a mistake like exhaust...
You never want to run a straight pipe a on a 2-stroke, and why you are having issues.

2-strokes don't have valves.
You are literally spewing power and raw fuel out on the road when you ride!

Follow the green fuel in this vid.

With a strait pipe all that green gas/air/oil mix escaping the engine just goes out the end with a straight pipe and on the road!
An expansion chamber like the one shown has a back-force wave timed to shove the fuel mix back in just in time for firing for a bigger punch and more power, not to mention no more puddles of oily fuel on the ground wherever you park.



expansion_chamber.gif

The stock pipes do an OK job of producing back pressure to shove the unburnt fuel back in the cylinder, but a tuned expansion chamber exhaust does it much better, it works like a reverse turbo charger and gives you the best power boost and my #1 rated single upgrade there is for a 2-stroke engines.

That should set ya on the right track ;-}
 
I'm at 5200 ft. and have found these carbs. hard to tune to a sweet spot. When you lean them out enough to stop 4 stroking you have to set the idle high. You may have better luck at 2200 ft.. Try a #68 nt jet from SBP and keep an eye on your plug to make sure it's not running to lean.
 
I also agree with KC, and if your going to put on a expansion chamber you should wait till it is on before fine tuning your carb.
 
That's sold as an RT carb some places. It can use the same main jets as the NT carbs.
Dellorto 5mm ones, if I remember right. SBP sells them in sets or singly for a few dollars each.
You can try one step smaller (leaner) main jet. I tried one of those carbs and it was never right on the top and the bottom at the same time, other guys like them just fine. It depends on your engine's set-up, I guess. I got mine for less than $30, that site might be a little high on something.

Yeah I think running a smaller jet would help the situation.
 
You can't 'tune out' a mistake like exhaust...
You never want to run a straight pipe a on a 2-stroke, and why you are having issues.

2-strokes don't have valves.
You are literally spewing power and raw fuel out on the road when you ride!

Follow the green fuel in this vid.

With a strait pipe all that green gas/air/oil mix escaping the engine just goes out the end with a straight pipe and on the road!

An expansion chamber like the one shown has a back-force wave timed to shove the fuel mix back in just in time for firing for a bigger punch and more power, not to mention no more puddles of oily fuel on the ground wherever you park.

Okay, yeah, I understand, I think, about the back pressure pushing some of the exhaust/unburned fuel back into the combustion chamber to mix it with the next shot and all at least I think I explained it right. But my newest pipe that I welded up has two opposing 90 degree angles in it. Shouldn't that help a little? But what I understand about 4 stroking is that the engine is not scavenging all of the exhaust out of the combustion chamber for a new clean shot of fuel/air. Am I wrong? Sorry, still new at this.
And just so you know there are no puddles of fuel on the ground where my bike sits overnight. I even bring it inside the house overnight. Also, It seems to run a lot better after it has warmed up a bit. Still four strokes but not as often when it's cold.


expansion_chamber.gif

The stock pipes do an OK job of producing back pressure to shove the unburnt fuel back in the cylinder, but a tuned expansion chamber exhaust does it much better, it works like a reverse turbo charger and gives you the best power boost and my #1 rated single upgrade there is for a 2-stroke engines.

That should set ya on the right track ;-}
 
This is the current setup. The oil leak is coming from a bad welding job. My first weld in over 20 years. Getting better as the Motorized Bike Stand can attest.
IMG_20160928_171910337_HDR.jpg
LOL
 
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