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

[2T]

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?

[pokey]

Wouldnt it be a great but rather boring world if we could diagnose  a problem with only one symptom and without the problem being in front of us. Even a doctor wont say your leg has been bitten off without taking your BP and Temperature.

Before any attempt at tuning the basics need to be addressed. is the air cleaner clean, is the pipe clean. How about that engine seals and compression? Is the carb float set correctly and is it clean, what about the needle wear and clip? Tested the CDI and coil and is the plug correct, timing okay? is the fuel correctly mixed and the tap and line free flowing? Reeds okay?
When you can answer a confident yes they are all good to all those then its time to think about tuning or your chasing the wrong end of the dog. And just to make it interesting jetting changes with elevation as the air pressure changes, even humidity has an effect.



You have probably seen these charts and they give you a headsup on where to look for carb jetting issues Vs RPM. Remember there is a crossover range  where jets share the job. grab a few jets and a box of spark plugs and your angle grinder as its time for plug chops. Flat out underload pull the clutch and press the kill switch at the same time. Remove the plug and check the polaroid it just gave you of what was happening at that time. When you get close with the jetting you can "chop" the thread off the plug and read the porcelain colouring with more accuracy.
Whats happening in the engine at that time? Have a look and the piston and plug will tell you.

[2T]

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)?

[John Orchard]

The issues that could arise ....

Fouled plug.
Blocked baffles in the exhaust.
Excessive carbon build-up behind rings.
Ring might get jammed in the grooves.

[skypig]

My guess:

It’s too rich to ignite/burn reliably. Possibly momentarily fouling the plug (bridging the gap with liquid)?
So the spluttering is a stroke where it didn’t light off?
Or potentially the mixture is burning more like a liquid than a gas?

[pokey]

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)?

Spluttering is an incomplete burn of the charge and this can occur at any point in the RPM range for various reasons as i had mentioned not just too rich on the main.
If as you state everything else is fine and your sure its a main jet issue the amount of fuel/Air ratio is too fuel rich which has an excessive cooling effect and the charge cannot complete its burn within its duty cycle. Its very similar to a flooded engine or running with the choke on. 3 things in a balance for a fire to burn, Fuel Heat and Oxygen, decrees one(Air) or two items(cooling) and no more fire its a simple as that .. As the boys have stated this results in oily carbons being produced which clog pipes gum ring landings foul plugs and crap performance. I hope this helps.

[2T]

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.

[pokey]

Pressure isnt actually an issue in the bottom end of a 2T engine as the charge isnt pushed its sucked, either by the pressure differential created in the combustion chamber or pipe scavenging. Maybe a whole 7PSI in the bottom end at most so if it holds a 10PSI (never any more or you will blow a gasket or seal) in a test for 10 mins its good to go. As for the gearbox leak my thoughts would be over full or a blocked breather.

Sounds like that engine needs a lot of love whatever it is so back to basics. Pull the clutch cover and clean the old gasket and crap off it and pull the locating pins then hone it flat with a sheet of fine wet and dry on a sheet of window glass so you know its flat. Then a new gasket or yamalube as per the specific factory requirements.

Sorry dont mean to be rude and basic but I gotta ask so please forgive me and remember we dont know what you have as you havnt said (WHAT bike do you have) and we dont know your experience or the bike history so again please forgive if i have over simplified and stated the obvious. The only way a bottom end can build pressure to blow a gearbox seal is if it is or close to hydrolocking. Bad float needles and bad fuel taps drip fuel while the bike is sitting and "SH#T" all runs down hill and you end up with an oily premix filling your bottom end halving the area. It will still run sort of after coaxing but be a Biatch to tune and smoke more on start up than a 13 year old with a hard on with his skills trying to impress a new girlfriend untill it starts to clear a bit and still run like shyte.

[2T]

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.

[Hoony]

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 

[2T]

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.

[pokey]

If you ever suspect a centre gasket or RH main seal you will get a smokey exhaust as gearbox oil will also seep into the crankcase ,The gearbox oil will be contaminated and look a bit milky and stink a bit.