Messages

1

Tech Talk / Re: Spluttering When Rich - What's Actually Happening in There?

« on: January 11, 2025, 09:07:56 pm »

WR500 (YZ490 leftovers sold as WR500) the old YZ490 suffered from an inefficient head combustion squish i believe, they were notorious detonators.
i think its well documented what mods needs to be done on the squish to achieve good combustion, i don't know the answer but i read it a few times over the years and forget....too many beers ago 

Yep, am currently using a variation of the common Eric Gorr style head mod.

2

Tech Talk / Re: Spluttering When Rich - What's Actually Happening in There?

« on: January 11, 2025, 01:33:26 pm »

Some good info there pokey, thanks.

Bike is a Yamaha WR500 (owner since 2004), set up for road use (supermoto/back road blasting) so have been asking it to do things it's not designed for.

Bike was (professionally) fully rebuilt 12 mths ago, including lapping all crankcase/cylinder/head join surfaces. Have had ongoing issues with detonation, tuning and high operating temps. This gearbox issue is just one more hard-to-explain event. Pretty sure the bike is cursed.


Re: bottom end charge sucked not pushed. Didn't know that. Thanks!

Re: g'box overfilled or breather blocked. Neither of those.

Re: hydrolocking. Do have a fair bit of oil around the crank but pretty sure it's not filling with fuel mixture and is not ultra smoky after start-up.

For the record, the bike is definitely not neglected in terms of maintenance and upkeep, quite the opposite. Having said that, I'm not a mechanic, but (probably because I've had so many dramas with this bike) I do now try and learn as much as I can about how these old 2 strokes work.

Also, have just had crankcases checked for microcracks and both are fine. Will get it back together and try again.

3

Tech Talk / Re: Spluttering When Rich - What's Actually Happening in There?

« on: January 10, 2025, 07:33:09 pm »

Thanks guys!

Are there any known immediate detrimental effects possible, like extra pressure in the crankcase?

I'm asking these questions as I'm trying to work out why I had gearbox oil forced out the clutch cover gasket and centre crankcase join immediately after I had been testing with overly rich main jet and spluttering (breather tube is not blocked).

Since then, a leak down test has shown no leaks, so it's a bit of a mystery at this stage. Next on the list is checking for cracks in the crankcase that may be opening up after reaching peak operating temperature.

Thanks again.

4

Tech Talk / Re: Spluttering When Rich - What's Actually Happening in There?

« on: January 10, 2025, 12:06:48 am »

I'm not quite sure why you posted the above, pokey. I may not have been clear with my question.

I'll try again in a different form:

If, when attempting to safely fine tune 2 stroke carb jetting (specifically the main jet), using the method of jetting much richer to the point of spluttering, before then gradually changing to leaner jets until suitable jetting is achieved, what activity is happening in the motor to cause the spluttering (assuming all else is functioning correctly)?

5

Tech Talk / Spluttering When Rich - What's Actually Happening in There?

« on: January 09, 2025, 05:55:20 pm »

Hi all,

Wondering what is actually happening inside the motor (2 stroke) when spluttering under load at higher RPM from too rich jetting?

Is it capable of causing any damage other than fouled plugs?

6

Tech Talk / Re: PVL Timing

« on: January 07, 2025, 06:10:21 pm »

I love this Forum. 

Me too, I don't know why more people don't contribute with either questions or answers.

It's a shame forums are not as popular as they once were. The good ones have been such a great source of accurate information that's not available elsewhere.

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Tech Talk / Re: PVL Timing

« on: January 05, 2025, 07:48:17 pm »

Thanks for the help pokey.

The bike gets a lot of WOT/high RPM and am a bit concerned about detonation and overheating if going higher than OEM setting.

8

Tech Talk / PVL Timing

« on: December 24, 2024, 06:07:15 pm »

Hi all,

I am about to replace my OEM ignition system with an analogue PVL system on an air-cooled 500cc motocrosser (Yamaha WR500).

PVL recommend timing start at 2.2-2.4mm BTDC for open class machines, however standard OEM setting for my bike is 1.7mm BTDC.

Should I set to OEM or PVL measurements?

9

General Discussion / Re: Yamaha IT forum not working anybody have any details.

« on: February 24, 2024, 04:01:58 pm »

It's back online now.