Engine Trouble Pre ignition / detonation at WOT

[Vikingimike01]

Hey guys, I've been having a problem with my 66cc 2 stroke motorized bicycle for some time now, wich I have figured out is detonation from this video:

I only have it at WOT for some time, at around 40-42km/h, then stops for a bit when accelerating, then starts again at 45-48 untill like 50. Then it stops completely. I thought it's 4 stroking, and bought a 65 jet for my stock NT carb (No filter) and when I tested, it made the sound a bit less. 70 jet is more of that sound. I thought I need to go down more. But that would be pretty stupid! (I was using banana pipe) So I guess it's detonation. I'm using 95 gas and unleaded. B7HS plug, sanded cylinder about 1mm. Should I go up to 100 premium gas? It's almost the same price here. I'm also installing a scooter exhaust so we'll see what happens after that.

 

[Lukesky36]

i get a lot of spark knock on my motor mostly from climbing hills been running like that forever some higher octane will help or a more rich fuel mix or get a low compression cnc head i been running a high compression they are great for low end but they just run pretty rough at real low rpm and they eat your rings and jug up pretty quick

 

[Vikingimike01]

Hmm alright. I did the sanding for some higher compression at lower end tho. So yeah, I'd prefer just getting the 100 octane gas and trying that.

 

[Lukesky36]

I just got a head for my new bike i'm building it has a 5cc combustion chamber with a 2cc squish band

 

[Vikingimike01]

Okay. I don't really have the money for those kinds of things right now
It does look good and they have very good cooling. And did I say that they look good?

 

[JerboaJohn]

a thicker head gasket might help if the compression is too high. but it's easier to try better gas.
I think bicyclemotorworks has various thickness copper gaskets. but if those options aren't possible, n you know it's too high compression, you could mod the piston.
there's even other free things you can do to lower compression,,,

 

[gary55]

Hey guys, I've been having a problem with my 66cc 2 stroke motorized bicycle for some time now, wich I have figured out is detonation from this video:

I only have it at WOT for some time, at around 40-42km/h, then stops for a bit when accelerating, then starts again at 45-48 untill like 50. Then it stops completely. I thought it's 4 stroking, and bought a 65 jet for my stock NT carb (No filter) and when I tested, it made the sound a bit less. 70 jet is more of that sound. I thought I need to go down more. But that would be pretty stupid! (I was using banana pipe) So I guess it's detonation. I'm using 95 gas and unleaded. B7HS plug, sanded cylinder about 1mm. Should I go up to 100 premium gas? It's almost the same price here. I'm also installing a scooter exhaust so we'll see what happens after that.

The best solution for detonation in the long run is the after market 7cc head like Lukesky is talking about. I don't know where you get 100 oct. gas, but it's gotta be expensive and not common at least around these parts. The squish band is designed to deter detonation by keeping the outer portion of the piston and cyl. wall cool enough to not allow it. It's additional cooling will also aid in the longevity of your engine.

A 65 is pretty small for someone near sea level, and could be aiding in the conditions for detonation. I would thing no lower than a 68, but that won't be able to be tested till you put the scooter pipe on. Have you done a compression test or squish clearance test to determine the squish gap? I'm also curious what the top of the piston looks like.

 

[Vikingimike01]

Actually here in Europe we have premium 100 gas for just about the same price.

Top of the piston is basically just a bit charred. If I get that off it's fully clear. Just a bit of carbon I guess.

 

[Spare_Parts]

Try an extra base gasket. It will change your squish by the thickness of the gasket.

 

[Vikingimike01]

Okay I'll do that after the gas. But again the next response will be in like a week till i get someone to weld the exhaust pipe lol. I don't have a normal welder just a stick welder and yeah