Noob needing 2-stroke advice

C

Compulsive Tinkerer

New Member
Hey everyone, this is my first post so please bear with me here. Alright so let me start by listing what I'm working with: (I underlined the most important stuff)

Elevation: Sea level
Terrain: All flat other than trails which rarely have hills of more that 6 feet gain in elevation at 40 degree angles
Current and recent temps: 70's to high 80's (F)
Me: 5'11" 155lbs
Bike: Old (steel frame?) men's hard-tail MTB 21-speed with straight handlebars & cloud 9 seat and 26" wheels with off-road tires.

Engine (kit): PK80 (66cc 2-Stroke Angle Fire Flying Horse engine with NT carb and stock exhaust) ordered from Bikeberry via Amazon. Engine is broken in. I've had it for about 3 months.

Mounting: Standard/Stock hardware with NO rubber or anything between the frame and the engine mounts

Fuel: BP Ethanol Free (I will check the octane later, sorry)
Oil: Promix 2-cycle low smoke, fuel stabilizing, synthetic blend
Mix ratio: 40:1

Sprocket size: 44t, 415 chain, stock chain tensioner, stock rag joint sprocket mount

General Modifications: Mousepad on clutch cover and magneto cover etc, Plastidip on inside of clutch cover with 3 clutch cover gaskets as spacers, Silicone sealant around magneto cover seam to block water from getting in, dual brake lever

Engine/Performance Modifications: NKG B6HS Plug, Dellorto #64 main jet from SBP, and finally a really ghetto but non-restrictive -(I tested it to make sure but I could be wrong) - air intake mod where I hooked a short 8 inch garden hose diameter piece of tube to the intake on the air filter and ran it up under the seat to quiet the intake noise (also to keep dirt from shooting up into and through the stock air filter while trail riding)

Primary uses: Trail riding and riding around town (I ride usually under 20 miles)

Alright so I probably forgot a bunch of stuff so I'll gladly add any missing info and pictures as needed.


OK! so here's my issue:

Everything was bone stock on the bike up until a month ago, and I was running it at a top speed on flat asphalt WOT and no wind at about 26 or 27mph. Minimal Vibrations. Some 4 stroking.

After adding that new NKG plug and air intake hose thing it ran the same. 26mph WOT Minimal Vibrations. Some 4 stroking.

About a week or two ago, I was reading on here that the stock NT jets are almost always too rich at about a #68 or #70 (correct me if I am wrong). I wanted to solder mine and re drill it myself but was short on free time so I bought a Dellorto #64 jet from SBP. So I switched the jet and here are the results:

I immediately noticed the exhaust smelled a little different (or maybe I just inhaled too much gas fumes haha)

The bike seemed to rev much higher as I tested the throttle with the clutch pulled in. The bike vibrates a ton more now and was enough to break my 2800 lumen headlight =( and cause my gas cap to blow off.
the handlebars rattle much more now.

When riding it: I seem to have lost about half of my low end torque but the bike seems to 4 stroke a little less or at least at different and less throttle positions.

Now keeping in mind all I did was swap the jet, the bike now does 35!!!

So now I being a noob am kinda just sitting here thinking what the ****...

My goal is to find the right balance where I still have my low end for trail riding but can get going a little quicker than 25. I plan to put an expansion chamber on it, port it, save up for a shift kit, and etc but I want to start off on the right foot so I know the best my little engine can offer (i.e. getting her running her best) before I go bolting on new parts.

So my question to yall is, what have I done and what should I do with the jet size?

Thanks a ton in advance
-Noob
 
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35 is a good speed & what I usually look for in a new build

with rough terrain and a tendency for a lot of vibration, I'd stick a piece of PVC between motor mount and frame just in case

#64 jet might be a bit lean at some throttle openings, so do a lot of testing of needle positions & raising & lowering float levels (with practice over time, you'll be able to tune pretty well all across the range)
 
Will do! Thanks for the advice, I was worried my post would get lost in the sea of threads and never seen haha. Could you by chance point me in the right direction regarding how to adjust the float and needle, or show me where to look to learn how? It would really save me a lot of hassle. Thanks again!
 
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I can't really advise you, Compulsive Tinkerer, since I'm pretty un-knowledeable about jets and fuel mixtures and such.

But I thought I'd chime in to say, "Great post!". You gave all sorts of useful info about your bike and engine. Give this thread a couple of days to simmer and you'll have answers. More, maybe, than you can digest. But that's kind of a good problem to have.
 
you'll find quite a bit of info if you search here for carb tuning - the needle can be set higher or lower to tune the first 2/3 or so of throttle opening, while the float level can be raised or lowered to tune across the whole throttle - the main jet has its greatest effect on the last 1/2 or so of throttle opening
 
Hey, sorry for the 2 day gap. I will update with my findings soon. I've just been really busy up until today and I haven't been able to find a guide regarding how to tune or adjust the nt carb/adjust needle and float positions. I have however found great advice regarding how to read my plug at different throttle positions/mixtures etc.

On another note, the temperature here has just dropped about 15 degrees. Yesterday I ran the bike and noticed that it would actually bog whenever I fully opened the throttle. It barely bogs down but it's enought to notice.
 
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2-strokes can be funny in how they hide the power band in diff temps and on diff slight uphill/downhill grades - moving slightly in needle or float level, then running a lot should give you hints about what the motor needs, but it's a bit of guess work too
 
Thanks Bikeguy Joe and Crassius, any and all help is much appreciated. Without this forum my bike would be going nowhere fast. Bikeguy Joe, I will definitely move the clip down a notch. My only question is, what does that mean? Sorry, I feel like an idiot asking all these simple questions. I generally try to figure these things out digging around on the forum before breaking down and asking for help but I really haven't been able to find what I need yet regarding this specific thing. Thanks again in advance!
 
it's the "C" clip on the carb needle, it can be adjusted up or down to richen or lean out the motor.
unscrew the throttle connection from the carb, you will see it.
 
Awesome guys, that's exactly what I was looking for. I'll go inside the carb this evening and then do some testing and see if I can't get my low end torque and throttle response back. Really appreciate it! Next upgrade will be an expansion chamber; I just need to find out the best/most effective one for the money.
 
Hey guys, thanks for all the helpful information. Yesterday I moved the C clip on the needle up to the top notch (furthest position from the pointy end) and ran it up and down the street. It seems to have some of its lower-end acceleration/torque back so that's definitely an improvement. I do have a question though regarding the setup I have. Do you think that running a .64 main jet with the needle all the way down could be risking running it too lean? I like the speed gain but not if it's at the cost of running my engine too hot/too little oil.
 
I did the solder & redrill thing myself, also, if you want a decent amount of speed without the ridiculous amount of vibration, swap out the 44t for a 36t, you'll still have enough hill climbing power for most things (and aside from steep mountain roads in rural areas, nothing can't be overcome without a bit of pedal-assist). Some other stuff I'd recommend is to replace the 415 chain with a 420, the motor is too powerful and will eventually break a 415, the 420 just barely fits and is stronger. Also I would not recommend using a shift kit, the extra gears eat up some of the engine's power. Instead, make a good torque pipe. Also, the four stroking is fixed by redrilling smaller and smaller holes in your carb jet until it stops. Set the clip on leanest setting first, I'm at 200ft elevation, so close enough to you, and I've got mine on leanest setting and it was still four-stroking all the time, just set it to leanest and redrill the jet a bit smaller. You can get micro drill bit sets at harbor freight for like 3 bucks, but do it with a hand drill and take your time, a power drill will break the cheap harbor freight bits, also rough up the surface of the jet with a bit of sandpaper before you solder it so the solder sticks better. BTW I run my engine oil mix at 40:1 and it's been a year and it hasn't blown up in my face or anything.
 
Thanks, will definitely get the 420 chain and redrill my jet until I get the desired effect I want. Also, thanks for the heads up about the shift kit. I am currently swapping out the stock restrictive exhaust gasket with a port matched one for better flow and will look into how to make/choose the right kind of expansion chamber/torque pipe. I have also been looking into porting my engine and balancing it, which is wayy over my head right now haha.
 
My goal is to keep the speed right around where it is +-35mph but to really amp up the torque and reduce the vibrations.

My goal for this bike is to be a stunt/offroad high torque little dirt bike.

Next year I plan on building a second bike built for nothing but high speed and cruising on the street since everything is flat here. So in other words this 2nd bike will be a project to see just how fast I can possibly make one of these things go. I am aware that safety will be a major factor when building bike #2 haha.
 
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Oh, one more thing, you should replace the stock mounting screws and brackets with the strongest stuff you can find, maybe even to the point of redrilling for slightly larger threaded rods in the back (I just had a 1/4" tempered stainless steel rod shear off, found out that the surrounding metal is so soft that I can not manage to drill it out, and now am kind of screwed) I wish I had redrilled them for 3/8" rods. In terms of a torque pipe, I might be able to help you out with that, I am currently swapping out my 66cc 2 stroke for one of the 79cc harbor freight predator 4 stroke engines, but before I decided to do that, I had been in the middle of upgrading my exhaust system. I was making a pipe which another guy did the math on and designed specifically for the 66cc engine, and I have all the pieces cut out and ready to be welded together, but I no longer have a use for them. Send me a message if you're interested, I'd sell them to you for shipping costs. I might also be able to do the welding for you as well.
 
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