500 2-stroke pinging

Johnny89superbike
I raced an 83 CR480 back in the day and tested mx bikes for an Australian publication during the 80's, back then l never really noticed the ping problem that l find with the vintage bikes today (could be the change to unleaded fuel?)

I have owned an 84 Honda CR500 for the last 12 months and am taking my time getting it race ready. Sure the press & forums go on about it being the 'ping king' (that and the YZ490).

So the only time l get it to ping is at high rpm/part throttle; poor combustion chamber design will cause the ping when under big throttle. So l get the head machined anyway, still pings its head off !

I find the stock PE38 carb (the only Keihin we're allowed to run in Pre 85) has a pilot jet that sits high in the float bowl; drawing fuel supply from the aerated fuel at the top of the fuel chamber. So l extend the pilot jet passage to the bottom of the bowl ........ well, nearly all the pinging is gone!

I also have a KX250A5 chassis with a KDX400 engine fitted that has a similar ping, although not as bad as the CR (probably because it does not vibrate as much). I extended the pilot jet passage to the bottom of the fuel bowl, well wadya know; no part throttle/high rpm ping.

Thought l'd share my find with you guys.

Regards, John.
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Brett Campbell 3/2/2017 4:52am
3/2/2017 4:52am Edited Date/Time 3/2/2017 4:53am
Great info John! Can you share how you extended the pilot jet passage? Did you make an extension tube?
RyanLester761
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RyanLester761 3/2/2017 8:03am
3/2/2017 8:03am
Brett Campbell wrote:
Great info John! Can you share how you extended the pilot jet passage? Did you make an extension tube?
Yes please share some pictures of this mod! Good info! Thanks!
Johnny89superbike 3/2/2017 4:20pm
3/2/2017 4:20pm
Thanks Guys :-)

The following pic is the PE38 with a piece of rubber fuel line pushed into the pilot jet recess, I cut a 'V' in the end of the tube to make sure the tube does not get blocked-off if pushed against the bottom of the fuel bowl.



The following link is to a thread on the ozvmx forum that goes into more detail on the subject.

http://forum.ozvmx.com/index.php?topic=36868.0

Regards

RyanLester761
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RyanLester761 3/2/2017 4:50pm
3/2/2017 4:50pm
I read the first couple pages of that thread. Very cool. And yes, surprising nobody has posted about this issue (and fix) before. Nice work and thanks so much for sharing with the world. Tongue

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chuckie108
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chuckie108 3/2/2017 5:27pm
3/2/2017 5:27pm
Good little piece of tuning info to have! (thumbs up)
450exc115
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450exc115 3/2/2017 7:38pm
3/2/2017 7:38pm
Honda also figured out in the 91 season to tee the carb vents and run a set up to a filter under the gas tank to over come a bog that occurred after a hard landing with PWK carburetors.

Good stuff thanks for sharing
mxrose3
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mxrose3 3/3/2017 7:12am
3/3/2017 7:12am
I always wondered why the pilot jet on most carbs is so high up? The Main jet is down at the very bottom of the bowl, yet the pilot is at the top, and the bowl would have to be full of fuel to be sucking fuel from it. Poor design, or is this on purpose?
Brett Campbell 3/3/2017 7:20am
3/3/2017 7:20am
I believe this may be the cause of the pinging on my CR480. Once at full operating temp, it pings when I close the throttle sometimes.
Johnny89superbike 3/3/2017 2:48pm
3/3/2017 2:48pm
Brett Campbell wrote:
I believe this may be the cause of the pinging on my CR480. Once at full operating temp, it pings when I close the throttle sometimes.
Yep l think you might be correct, let us know if it fixes it.
omalley
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omalley 3/3/2017 7:54pm
3/3/2017 7:54pm
450exc115 wrote:
Honda also figured out in the 91 season to tee the carb vents and run a set up to a filter under the gas tank to...
Honda also figured out in the 91 season to tee the carb vents and run a set up to a filter under the gas tank to over come a bog that occurred after a hard landing with PWK carburetors.

Good stuff thanks for sharing
As I recall they discovered that by accident with JMB. He was complaining of a bog on landings. After one of the many carb r&r's, they forgot to put the (old style single) hoses on and the bog was gone. Put them back on and bog re-appeared.
450exc115
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450exc115 3/3/2017 8:05pm
3/3/2017 8:05pm
450exc115 wrote:
Honda also figured out in the 91 season to tee the carb vents and run a set up to a filter under the gas tank to...
Honda also figured out in the 91 season to tee the carb vents and run a set up to a filter under the gas tank to over come a bog that occurred after a hard landing with PWK carburetors.

Good stuff thanks for sharing
omalley wrote:
As I recall they discovered that by accident with JMB. He was complaining of a bog on landings. After one of the many carb r&r's, they...
As I recall they discovered that by accident with JMB. He was complaining of a bog on landings. After one of the many carb r&r's, they forgot to put the (old style single) hoses on and the bog was gone. Put them back on and bog re-appeared.
Yup that's correct. I recently bought a 91 CR250R so I've been doing a bit of reading about it with the old rags
Johnny89superbike 3/3/2017 11:19pm
3/3/2017 11:19pm
Yep that was a problem back in the day, shrouding the main jet helped stop the hesitation when landing from jumps. I remember it much talked about then, but l have never heard about a lean part-throttle ping caused by vibration from a 500 two stroke; makes me think that all the bitching about pinging is guys riding around on the pilot-jet ;-)

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