Carb settings for big bore piston

[Dean Southall]

I had to have the barrel of my d14 175 overbored and fit the Rex Caunt Suzuki piston. Since then the engine has sufferred a knock when it warms up. I have checked that the piston is not hitting the head or fly wheels. I found a dent had developed on top of the piston below the spark plug which I initially thought was due to contact. However increasing the distance to ensure clearance (extra washer on plug, thicker head gasket) had no effect.
A mate suggested it could be pinking and that I try a slide with a smaller cutaway.
This had reduced the knocking but not entirely so perhaps it is a carburation issue (I had not made any changes to the carb from the original setting.

Has anyone else had similar experience and can offer advice?

PS I am running the small bore exhaust pipe because the engine in in a D1 frame. At 175cc it was happy with this however

[rhyatt]

I will assume that you've got the ignition timing correctly set.
There are 1.5mm head gaskets available if needed, and are you using the correct plug ( not extended nose).
Does the carburettor have flat spots that warrant a new slide or is your current slide mechanically worn in the carb body

[neil1964]

Good suggestions above.
Did you try the plasticine test on the piston to head (and plug)?
That will tell you the clearances you might or might not have.
Developing a divot in the piston is a bit concerning.
Have you tried backing the ignition off by a degree or two for a bit - just to see?
A knock due to a carburation issue would be unusual.

[Dean Southall]

Thanks for your respnses guys. In answer

I have measured piston to head clearance (squish)several times (1.5mm)
I have tried gaskets both 1 and 1.5mm
Timing (electrex ign) tried from 19-16.5 deg btdc
Carb is nearly new (bike was running fine on it before rebore)
Plug is same as previous (I have tried hotter plug also)

I'll contact RCR, although Rex is no longer around

[rhyatt]

From memory, D14 should be 16.5 degrees btdc

[cdsdorset2]

I met someone that hed fitted a suzuki head to a bantam.( stud spacing the same so he thought why not) . Since fitting it he has never had the "dang,dang" knocking noise you normally get on a bantam on the overun!

[Lone Wolf]

Wotcha.

I'm running a "big bore" Bantam.  All I did was raise the needle one notch.