minarelli spark knock at high rpms

[chrish97]

hi all. i recently decided to do some upgrades to one of my minarelli scoots after i had the top end burn out on it and had a hole melted in the piston from improper jetting i believe. I decided to put a 70cc big bore kit on it and changed the jet from a 80 to a 90. it runs great and has tons of torque but iv'e been having spark knock at high rpm's. i decided to check the plug and it is light brown and looks fine. i had an old aftermarket arrow exhaust on it that came off another minarelli and decided to switch back to the stock and now it has less spark knock but it is definitely still there. i have not checked the squish clearance yet but everything is set up stock and with the stock head, so i don't think that would be an issue. I have also tried multiple different plugs and still no luck. The only other thing i can think of is maybe it could be the timing, but the timing is still in the stock position on this bike i have never changed it. Anyone have any other suggestions? I think i'm going to go get a dial caliper to check the squish clearance and maybe pull the flywheel and look to see if the timing has ever been tampered with or maybe something is out of place. 

[repherence2]

I think you should check your squish clearance.

There are dangers in using a stock head on a bigger bore. I tried an NCY 47mm racing head on my 50mm bore minarelli because I didn't like how the stock head combustion chamber looked, it was not a squish band head. The bike ran like a champ for several weeks, I could hear pinging once in a while but I was a dumb noob to 2strokes at that time. I ended up blowing a hole in the piston.

[oldgeek]

A compression test may shed some light on the issue.

[gsx600racer]

Like others have said, squish and compression. Plus I would jet higher, a lot higher. Like 110-120. Why, more fuel will never hurt the engine, and sparky plugs are cheaper than a kit if you foul them.

My preferences, I like running 140-160 compression psi range. Squish, most will say 1mm(.035 inch), I like running a little bigger, .040-.050" .

Aslo running a higher octane fuel will help, but I prefer to dial it in to run on premium pump gas.

Not sure if you check the port timing numbers, but if your compression is high and squish is tight(lower than the #'s listed above, the easiest fix would be to add shims between the cylinder to bump the compression down and open up your squish #'s.

[SMALL CC TEK]

ALL of the above but check you plug gap also . Does it ping/knock under a load or at even idle or at what position on the throttle does it happen ? I think the squish sounds right on but you never know you might just have some garbage fuel !

You do have all the shrouds on the motor right ? Your not overheating the motor  are you ? Or just running a Lean lean fuel mixture   or   Lean as hell jetting ?  

[chrish97]

Jan 29, 2019 23:13:40 GMT -5 SMALL CC TEK said:

ALL of the above but check you plug gap also . Does it ping/knock under a load or at even idle or at what position on the throttle does it happen ? I think the squish sounds right on but you never know you might just have some garbage fuel !

You do have all the shrouds on the motor right ? Your not overheating the motor  are you ? Or just running a Lean lean fuel mixture   or   Lean as hell jetting ?  

ill be sure to check the plug gap. It has been knocking alot under heavy load at high rpms. I dont belive it is running lean because my plug checks are always light brown never white. I have also noticed that it is starting to have hestitation and bogging at mid throttle sometimes. I decided to remove the 50cc head and installed a 70cc head with 2 head gaskets and still had no change but noticed it seems to have alot more torque at low rpms. I have all of the factory engine shrouds in place and no i made sure i am not overheating it. 

[90GTVert]

Fuel octane would be the first move for me most of the time. If not running premium now, I would. If that doesn't stop it, then get into the more involved stuff. In this case, with a 50cc head on a 70cc, the squish should have been checked before it was ever started IMO. Should be done regardless, but def when mixing and matching.

Why did you swap from a 50cc head to a 70cc AND 2 gaskets? Sounds like it should be a big change. The squish has kinda 2 sides. If the squish is really tight, maybe that's telling you that compression is high. A compression test wouldn't be a terrible idea just to get an idea of where it is. On the other side, if squish clearance is wide then it's not conducive to a knock-free environment. Tighter squish (with proper compression spec) is preferred to reduce the likelihood of knock.

You noted swapping jets. You didn't mention any sort of tuning process aside from plug color. IMO it's best for most to ignore plug color during tuning as much as possible. Use it to see if you're way off, not to see if it's dialed in perfect. You'd need to read the mixture ring to be more effective if you want to use the plug as a tuning guide, but multiple test runs with different jets is how I go about it. The more stuff you can record (RPM, MPH, CHT...) the better. Sometimes a lean condition can be enough to put an otherwise OK setup over the edge and into detonation territory. You're only mentioning high load and high RPM when the engine is the hottest.

How did you make sure you weren't overheating it? CHT?

Ignition timing is probably the least likely issue right now, but that doesn't mean it couldn't be the issue. At least make sure the woodruff key hasn't sheared off. I'd expect that you'd get knock elsewhere if timing were off by much though. Stuff like rolling on and off of the throttle slowly will usually make it rattle somewhere.

[harleyracer59]

I agree with 90GT. if it wasn't pinging as a 50 the same ignition usually wont do it on the bigger cylinder.
run premium if not, check fly wheel woodruf key, and compression squish and plug gap. maybe even go to a cooler plug. using the spark plug to tune a cylinder that you're breaking in and running extra oil in the mix, will give false readings. another thing to check would the carb circuits. you could be good lean on the needle and ok on the main. would run ok at idle and wot. but cruising neighborhood streets, on and off the throttle/ part throttle riding its gonna get hot and when you get on the throttle fully, its not gonna be happy and it wouldn't be the main jets fault.

[ThomasTPFL]

What kind of CDI are you running?

[190mech]

If ya keep running it you'll either hole/melt the piston or hammer the top end bearing to pieces..Gotta keep that from happening!!

[chrish97]

Jan 30, 2019 23:56:01 GMT -5 ThomasTPFL said:

What kind of CDI are you running?

Stock cdi

[chrish97]

Jan 31, 2019 1:21:39 GMT -5 190mech said:

If ya keep running it you'll either hole/melt the piston or hammer the top end bearing to pieces..Gotta keep that from happening!!

I have not ran it anymore since it started knocking. I have just tried different things to try to correct it and ran it about 30 seconds in between.

[chrish97]

Jan 30, 2019 12:11:59 GMT -5 90GTVert said:

Fuel octane would be the first move for me most of the time. If not running premium now, I would. If that doesn't stop it, then get into the more involved stuff. In this case, with a 50cc head on a 70cc, the squish should have been checked before it was ever started IMO. Should be done regardless, but def when mixing and matching.

Why did you swap from a 50cc head to a 70cc AND 2 gaskets? Sounds like it should be a big change. The squish has kinda 2 sides. If the squish is really tight, maybe that's telling you that compression is high. A compression test wouldn't be a terrible idea just to get an idea of where it is. On the other side, if squish clearance is wide then it's not conducive to a knock-free environment. Tighter squish (with proper compression spec) is preferred to reduce the likelihood of knock.

You noted swapping jets. You didn't mention any sort of tuning process aside from plug color. IMO it's best for most to ignore plug color during tuning as much as possible. Use it to see if you're way off, not to see if it's dialed in perfect. You'd need to read the mixture ring to be more effective if you want to use the plug as a tuning guide, but multiple test runs with different jets is how I go about it. The more stuff you can record (RPM, MPH, CHT...) the better. Sometimes a lean condition can be enough to put an otherwise OK setup over the edge and into detonation territory. You're only mentioning high load and high RPM when the engine is the hottest.

How did you make sure you weren't overheating it? CHT?

Ignition timing is probably the least likely issue right now, but that doesn't mean it couldn't be the issue. At least make sure the woodruff key hasn't sheared off. I'd expect that you'd get knock elsewhere if timing were off by much though. Stuff like rolling on and off of the throttle slowly will usually make it rattle somewhere.

I believe it has premium in it but I cant remember 100% so I'm going to try draining it and swapping it for fresh premium. I decided trying to run 2 head gaskets was a bad idea so I switched back to 1 gasket on the 70cc head. I'm going to run a compression check on it tomorrow and go get a dial caliper to check the squish. I ran it again today for a out 30 seconds or so after I swapped the head back and it now doesn't seem to be knocking as much at high rpm but is knocking when letting off the throttle. I dont believe it to be overheating as it starts knocking as soon as I start it. I'm going to buy a cylinder head temperature gauge and a tach online before I do start riding it. I tried a few different jets in it including an 85, 90, and 100 but I need to buy some more jets. The 85 seemed lean and the 100 seemed rich.

[chrish97]

Thank you all for the help and support so far I cant wait until I finally figure out this issue and get this bike on the road

[chrish97]

A link to a video I made of what the scoot is doing now: m.youtube.com/watch?v=vWjOcBVhmKA