Is this signs of knocking?

YesImLDS

YesImLDS

Member
Hi everyone, I have a peculiar problem. I had a pretty decent 66cc motor with a type b piston that I recently replaced with a brand new 66cc motor with a type a piston. I literally just transfered over my performance parts from the old motor to the new and broke it in with extra oil in the gas and moved onto 32:1, but since I have had this newer motor it has had a really loud noise at high rpm. It sounds like an exhaust leak, but only happens when the motor is warmed up and when it does happen the motor looses just a tiny bit of power, but is hellaciously loud

The parts that are on it:
Slightly milled down stock head for higher compression
SBP expansion pipe
NT carb with .66 jet
Lightened top end (piston with holes in areas that have no ports)
Ported intake and exhaust
36T sprocket

All the gaskets are not leaking and are tightened down properly. Exhaust isn't leaking as it was just rewelded to stop any leaking. I guess I am just not sure what is wrong. I cracked it open and took a couple pictures.

This one to me looks like flaked off carbon
3a6AG5t.jpg

Same angle but with flash
cuv2Uot.jpg

Picture of milled head
pQ5wI25.jpg


The head looks like the plug color is a light brown. Notice how the head has built up carbon in the area that doesn't have direct access to the spark. Due to it being cold I try not to ride it at this point, but this noise really bothers me. It doesn't sound like marbles on tin, but if you were to cut a slice in the exhaust. I'm going to re-inspect the exhaust just incase there might be a pin hole or something, but even before it was rewelded on my previous engine is was very tame and not loud at all. Any advice?
 
I might try putting a stock head on and see if that changes anything tomorrow. I did begin with a leaky head gasket and I thought that was the cause so I changed to a fresh one and recreated an exhaust gasket from some gasket maker I had left over and the noise still persists as well. I am just plain stumped!
 
different motors can be very different in how they run & sound - may be nothing wrong

things to try are higher octane gas to see if it changes, 2nd head gasket to see if too much compression with just one, richer main jet to see what changes

2 rules for these are 'don't get in a hurry' - 'don't get lazy'

this may fall under rule #2
 
crassius said:
different motors can be very different in how they run & sound - may be nothing wrong

things to try are higher octane gas to see if it changes, 2nd head gasket to see if too much compression with just one, richer main jet to see what changes

2 rules for these are 'don't get in a hurry' - 'don't get lazy'

this may fall under rule #2
Click to expand...

Well I believe I figured out the problem. With the head from the previous motor that was milled down got hit with part of a piston ring when the previous motor self destructed. I thought it was nothing and continued to use said head on the new motor. It worked perfectly fine until heated up. What I believe happened was that once warm a crack expanded in a weird way causing a slight loss in compression going straight out of the top of the head causing the noise. Switched the head and I have a little less compression because it's just a stock head, but it works flawlessly. I didn't think it was fuel due to the fuel I am currently using being 91 octane with no ethanol. Problem solved I suppose even though I froze my ass off figuring it out! Appreciate the help!
 
Something I did realize was with the loss in compression between the two heads the lower compression head had some leftover fuel in the combustion chamber. I'm no expert, but I guess that would be from the not as hot of a burn with the lower compression?
 
I guess I might just have too big of a jet in the carb! .66 is probably too big for the motor itself. Might need to change back to the stock .70 and test it out. With the lower compression head I was four stroking slightly at WOT
 
Last edited:
You must log in or register to reply here.
Back
Top