Ever had a carb that was too lean?

Agreen

Agreen

Member
It started with a weird condition at WOT. After riding at WOT for a minute or 2 it would kind of bog. I thought it was 4 stroking. Never had much experience tuning 2 strokes. On a car, lean will surge and buck, and rich will bog. Turns out, on a 2 stroke it will braap at you when rich, and bog when lean.

So I started reading up on how to tune the jet of this NT carb. Sounds simple enough. I have everything I need except some tiny drill bits, let's get to it!

I read all about how the carbs come tuned too fat out of the box, so I bring a spare jet with me. It came out of another NT carb that broke one of the little clamp tabs. I found that a 69 didn't fit, and a 71 did. The hobby shop I bought the bits from only sold individual bits, not a kit. Unfortunately, he didn't have a 70, so I guess that could be the stock size? Anyway, I get everything from a 69 to 74, thinking I would never need to go bigger, but I just grabbed the 69 bit anyway.

Now, I'm not sure if it was because of the "poopoo pipe" I slapped together in a rush, the transfer ports that I barely gasket matched, or the fact that I shaved my head about 0.005" because it was warped...

Really, none of those should matter much. When I started tuning the carb I went down to a 72 at first. Hmm, not sure why but it didn't feel right. It would straight bog down at anything over 3/4 throttle. Weird, but maybe it's still too rich?

So after getting worse and worse the leaner I went, I finally got the hint. So I pulled out the #69 bit and gave it a go. Instantly, I knew I had found it. Not a single bit of bogging (and definitely not 4 stroking). Although I will say that the idle suffered a bit after rejetting, so I leaned the needle down and fixed that.

Now I'm going to make a CDI to get some better spark. I'm kind of curious just what I can do with this little guy!
 
What you're describing is exactly how I tune. Starting with the stock jet, I solder it closed then start small and work up until I find that much-talked-about 'sweet spot' where it all seemes to come together. Yep, a lean run will most ceratinly bog. That's when I know it's time to head back to the work bench and redrill to the next size up.

Interesting discovery this summer too. I and others have always thought, and advised, that high elevations (above sea level) would impact jet size. I found it didn't hold true when I took a bike from home, 6000' above sea level down to central Illinois. I was prepared to change jetting for the lower elevation thinking I would be lean but in fact the bike ran the same, or maybe a little better at the lower altitude with my small jetting that has always seemed to perform well here in the Rockies. Go figure.

Tom
 
There's always the possibility to go lean with the stock jet after a considerably less restrictive exhaust is used along with some porting and a free flowing air filter, all these things will add to the amount of air that can go thru the engine so the fuel must be compensated accordingly. If someone goes out and buys a performance pipe and air filter this is going to make the air fuel ratio go from a rich condition to either where it needs to be or even to a lean position. With this added amount of air that can get into the cylinder comes added cylinder pressure... this is what makes the power, but with this added cylinder pressure if your plug gap is on the wide side it's possible the engine could start to miss under acceleration, and especially once the engine reaches it's power band. This can be mistaken for 4 stroking causing the rider to think they need to lean out their jetting instead of leaving it alone or richening it up to where it needs to be which could end up blowing the engine before the rider or tuner realizes the real issue. If you got an engine that ran great, then you added some upgrades like a high compression head, a better pipe, or free flowing air filter, or any combination of these, including doing some port work, then it starts to cut out at a certain rpm range, and may or may not clear up when it gets closer to its top speed, check the plug and try closing the gap a few thousandths or replace the plug with a new one that's been gapped .002" less than the one that's currently imstalled and see if the "4 stroking " clears up or clears up for the most part.
What's happening is that as the cylinder pressure increases it becomes harder and harder for the available voltage to jump the gap so as the engine aproaches it's power band during acceleration it can eventually reach a point where the spark will begin to miss intermittently due to the higher cylinder pressures as the engine gets closer to its peak power.
Have you ever wondered how a higher performance ignition can reduce or eliminate 4 stroking? This is why. It's the bigger coil that can produce a considerably higher voltage and jump the gap regardless of the cylinder pressure, it has nothing to do with the air/fuel mixture, but it could be why your high performance China doll is 4 stroking right where it should be making power. The ignition system didn't fix the 4 stroking, but it did stop the miss so the engine could rev out cleanly under load and thus the power goes up because the plug can fire every time instead of every other or every third time. The high performance ignition didn't add power, but it does allow the engine to make full power when it couldn't before.
Just one more thing to consider when your bike is 4 stroking relentlessly and it doesn't matter which way you go with your jetting.
 
Funny though, that all this talk about doing all these modifications, and I feel as though I've barely done anything! I matched the transfers as soon as i got the engine, so that one is possibly the biggest change. The poopoo pipe, well, that's the stock muffler plumbed further back. Probably more restrictive. No change to the air filter, and I lapped the curvature off the head. Literally.005" was taken off.

My new speedometer arrives today. I'd like to see what this little 48 is doing now :)
 
It can be amazing what just a longer pipe can do for these things... no expansion chamber, same restriction as before, but it runs better as well as stronger. The longer pipe will still have a scavange effect where at a certain length the first pulse can start enough momentum so it can pull out the following exhaust pulses. This length can be tuned to work at a certain rpm range but typically a properly tuned straight pipe would be longer than the bike it's on. One can still benefit by adding length, it won't be as big of a difference like an expansion chamber makes, but it will be noticeable
 
It did fall flat on its face when i put the pipe on without a restriction. It was a straight open pipe, 1/2" conduit (3/4" O.D.). Loud!

I then welded on a washer to the end of it. Power came back, but i had that E3 plug in. NGK plug livened it up more, and the carb tuning sealed the deal. I wonder if a better CDI would even help. The one i have has been working since April, so it's done well so far.
 
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