Weird sound at high rpm

#1
Anyone have an idea what this sound is? I just replaced the cylinder, head and piston/rings on a Sachs 50 motor on my 1971 Arctic Cat Prowler. It made this sound when I had a bad / broken ring on the old piston. It r7ns much better and has much better compression but makes this sound at high rpms. I assume it is the engine as I can’t tell what else can be causing it.

 
#2
Anyone have an idea what this sound is? I just replaced the cylinder, head and piston/rings on a Sachs 50 motor on my 1971 Arctic Cat Prowler. It made this sound when I had a bad / broken ring on the old piston. It r7ns much better and has much better compression but makes this sound at high rpms. I assume it is the engine as I can’t tell what else can be causing it.

Kinda sounds to me like mild detonation. Does it only do that when it's hot?
 

Joe H

New Member
#3
I think detno too. How does the plug look?
That will break a ring or burn a piston fast if it is.
I would fatten up the jetting a bunch then see if it stops
 
#4
What does it mean to flatten up the jetting? I’m new to 2 cycle engines like this.

It seems to happen when hot. Here is a pic of the plug.

C9AE8A19-0978-461D-BC2B-81CFB94F674A.jpeg

I have also had the spark plug gap bridged after about 3-5 minutes of riding.

BDAE3738-0B11-4931-8544-4893EAEC704D.jpeg
 

Joe H

New Member
#5
Fatten up means richer. Larger main jet.

What carb does it have??

Problem is it could be rich down low rpm part throttle and lean on top high rpm full throttle.
When you back off the throttle does the noise go away ??

If that is detno it’s pretty bad if you can hear it on video.
The biggest problem with the older 2 stroke stuff is that the combustion chamber shape is poor and it leads to detonation with today’s fuels.
Here is a good article obviously your not doing anything that crazy but it’s good info to know.
https://www.klemmvintage.com/deto.htm
 
#6
Joe, it seems to only happen WOT and goes away when I back off the throttle. It’s a 12mm bing carb...will have to check what model and jets are in it.
 

Joe H

New Member
#8
Lots of good info on this site
https://www.mopedarmy.com/wiki/Bing

I’m currently having some problems sorting out a Dellorto 16.16 , I’m gettin close. I’m used to jetting Mikunis

My advice to you don’t run it at those rpms untill you figure it out you will blow it up.

Also check for air leaks , spray carb cleaner at the base gasket, head gasket , carb mangold gasket if the rpm rises you have to fix that first.
Did you replace crank seals?? If there old it could cause this also but it would most likely be hard to start to
 
#11
My main / needle jet says 1517, but any jets I see out there for replacement are 2 digit like 40-90.....not a 4digit number like mine. Did this numbering change? I see the Sachs manual references a 1717 jet but mine has a 1517. Not sure the diff or where I can get a 1717 to test (or comparable jet).
 

Joe H

New Member
#12
Plug gap is .024 per manual.
Oh sorry I meant is the partical at the gap where it bridged hard like aluminum.

So the sequence in my mind would go like this if it was mine
Timing : is it spot on factory spec, 2 stroke timing is differnt than 4 stroke. Running it to advanced even by a few degrees can cause problems on some 2 stroke motors.
Air leak: check make sure there are no obvious ones.
Carb: perfectly clean factory spec jetting to start out.
Fuel: it can make a difference along with the mix ratio. Maybe see if you can get some 100 octane or better but that can be tricky.

Wish I knew bing carbs I don’t....
Old 2 strokes on modern fuel often have to run pretty rich to avoid detno.

Fixing that is about squish band, timing and jetting.
Not sure if I’m helping :confused:
 

wjustice

Well-Known Member
#13
What kind of gas are you using? My CCW290 started acting up(detonation) and I found I couldnt use anything but preminum gas. Old 2 strokes have issue partly because old reg gas was 91 octane and is now 87. My 290 called for preminum that used to be as high as 95-97 and that was with TeL that was more effective but more harmful. Try a mix using Sunoco Ultra and see if it gets better.
 

Joe H

New Member
#14
Just an FYI, because Im a bit weird I run Sunoco 110 in my Bennelli and my CR60 , both are hard to find parts for and I figure its cheap insurance
 

Joe H

New Member
#16
Just keep in mind there are times when you can't hear detno and it can still cause catastrophic failure ......dont ask how I know :oops:
So listen carefully and check the plug for tiny spots.
The best way is to keep going richer on the main jet until it 4 strokes (runs bad but no noise) then go back down a couple sizes until it comes into line. Its a bunch of work but way less expensive and time consuming as a piston, rings or bearings.
 
#17
The problem is I don’t know what keys to get as mine is labeled 1517 but all the ones I find are usually two digit numbers. According to parts list for this bike, it should have had a 1717 but don’t know if that is bigger / smaller than the 1517 currently in it. I can’t seem to find any detail on it.

I did change the needle clip position to make it richer and seems to help....engine doesn’t get as hot and doesn’t make as much of the metal rattle / ping sound.
 

Joe H

New Member
#18
Keep going untill it starts to run bad then back it down
You want it to be just a touch rich (safest)
I tried looking up those jets too, can you post a pic of the jets, I’m interested it’s a bit unusual
 

Joe H

New Member
#20
Hmmm... well I would contact Bing directly, there still around.
It sure would help your situation to find jets.
Here is a link that has helped me identify weird stuff on small engines but I went through it and found nothing but maybe closer study will be fruitful.
http://www.myronsmopeds.com/category/carburetor/bing/

Just an FYI it isnt too hard to modify squish band to help with the detonation there are a couple things that need to be done to do it right but it does cost $$ and budget is ALWAYS the limiting factor. It does make a huge improvement IMHO. I have done it to all the 2 strokes I have if the squish area is above 1.5mm and on most older 2 stroke engines more like 2 to 4 mm which is WAY to big for todays fuels. But again you can jet and/or retrard the timing your way out of it too.
You will read alot of stuff on squish band and it makes it all mysterious and such, but its not, tighter squish better burn, detno happens at the edges of the combustion process so speed up the edges of the combustion gasses by limiting the space they have avalible to them. Biggest problem is correcting the CR ratio so as not to raise it to high. All the stuff about burn rate and which is better for high rpm or better for low rpm is a bit of hype, most seroius 2 stroke tuners want it as tight as they can get and always have from way back inthe 60's. The factorys left them loose because it was easier to manufacture and the fuels then where leaded high octane and they worked fine with it.
Sometimes its as simple as running no head gasket and sealing the surfaces with silicone if they are flat enough. You can check them on a piece of thick glass, if there flat seal it with good high temp silicone then let it set, then check the CR and hope for nothing above 145-150psi, if its to high go back to the gasket. If its fine run it and for the first few heat cycles make sure you retorque the head.
 
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