4-stroking with plug pics...

[Lunatuck]

My 80cc (66cc) two stroke is 4 stroking at 3/4 throttle to WOT. Mostly at high revs, but it also seems to be randomly missing on any WOT acceleration. At slightly below 3/4 but consistant throttle, its fine.

I’m using the Runtong carb, and the only solution is to order new jets which I’m about to do. I presumed I was running rich and needed a smaller jet. But I pulled the plug and it seems normal to lean.

I just finished my 3rd tank of 16:1 and put 20:1 in and did a nice run until I knew the mix was getting richer, but the 4 stroking persisted. Does this sound like I should still go leaner? I’m assuming the jet is in the 70’s, and I’d need something in the mid 60’s.

 

[GreasyChris]

If its 4 stroking then it is running rich. You'll definately need to rejet to a smaller jet. Id also change to 32:1 fuel ratio

 

[PsychDoc]

I’m going through the same process- have a look

Runtong jets and upgrade

I’m breaking in my engine and getting ready to change the jet for a proper tune- the engine is the standard 66/80 cc China girl- (specifically ‘flying horse’ epa approved 🙄 with “upgraded high performance profile carburetor” ) so my questions/thoughts... It starts and runs easily- 4strokes quite...

motoredbikes.com

 

[extremeodd]

I just finished my 3rd tank of 16:1 and put 20:1 in

Well, there is part of your problem. You're running far too much oil!

The 16:1 recommendation from China is due to the available oil there typically being garbage quality so they must run a very heavy mixture. Even if your buying the cheapest crap from Walmart, it's far higher quality than what those recommendations are based on. The current rule of thumb is 1 tank of 30:1 with standard dino 2 stroke oil then switch over to 40:1 pure synthetic.

It might seem like more oil = more better but too much oil can be hard on the motor and make it run like crap. At the very least switch to a 30:1 mix, it'll make it run a bit better.

 

[Lunatuck]

Yeah. I thought I was following “the book”. I would love to jump right into 32:1. Will do that asap! Also just got word my exhaust will be here tomorrow and the jets are ordered.

I’m just surprised my plug didn’t look like dog turd from the rich mix. Everything but the plug says rich!

 

[extremeodd]

This is going to go against the grain but plug chops can be misleading as hell on these engines. I've had a tune where it literally couldn't NOT 4 stroke and after 2 miles (8 times longer than needed) of WOT at 7k rpm (yay 4 stroke rev limiting) the plug was reading as if the engine was running lean/barely had ANYTHING to get a reading of. Yet there was oil dripping from the muffler and the engine was 70f cooler than it should have been.

What I ended up doing to try and find the sweet spot was a combo of using the PHBG choke (it's just a 60 jet) to see if it would start to 4 stroke or run better when used after warmed up and a cheap infrared temp gun to check the rough head temp. With the stock head and Phoenix weather (110-115f) I've found a max head temp of 440-450f after atleast 45-60 seconds WOT to be acceptable. This dropped to the high 300s with a CNC head.

 

[Tyler6357]

Finish breaking the engine in, it will often times run leaner after you put some miles on it. I would finish off your 20 to 1 mix and try a 32 to 1 mix, you really don't need that much oil, too much oil will just clog up your exhaust pipe. Test it on a full tank at 32 to 1 and run it for awhile and see if the 4 stroking stops, if not, go down one size to a smaller jet.

 

[Lunatuck]

I picked up two jets from our local bike builder. He had a 68 and a 64. He’s pretty sure the stock jets are 70’s.

So I put the 68 in... no discernable difference.

Put the 64 in. A little power loss from takeoff, then waaaaaaaaahhhhhhhh... Right up to 30mph slight uphill no two stroking. Previous record was 24 on the same road. I may be done, but I’ll always wonder if the little low end weakness can he fixed.

Oh. And I burned my leg on the new pipe. It’s loud!

 

[PsychDoc]

That’s great! Hopefully you aren’t too lean. Word on the street is that if you are mixed correctly on the upper end the low end will suffer- don’t quote me on that.

 

[Lunatuck]

I am guessing I’m a bit lean on the low end. When still warming up, I get a bit of hesitation on low throttle low speed acceleration. After a few minutes, it gets a bit better. It feels like low end 4 stroking, but I’. Expect it to be lean not rich. Still not perfect, so I’m gonna try a 66 jet when it shows up. Still, this is a completely different bike.

I am no longer worried about the engine or speed. Now I’m worried about the brakes.