Pinging issue

[1998RM250]

Thank you Rich, that was what I was wondering about ethanol fuels. The last question is if its only pinging at WOT that means richen the main jet correct?
Hopefully all these questions I keep asking will help future two stroke riders running into the same jetting problems on ethanol based fuels.

 

[Rich Rohrich]

The last question is if its only pinging at WOT that means richen the main jet correct?

The shape of the end of the needle can influence the last 20% or so of the fuel curve so maybe a richer mainjet will fix it, maybe a needle change will fix it and maybe neither will. Keep in mind what Dave said. If the engine is already running on the rich side and it's knocking then adding more fuel isn't likely to change things.

Chances are good that unless your engine was specifically modified to run on pump gas , it will need overly rich jetting and retarded ignition timing to run without knock on 93 octane premium. You could look for ways to lower the engines octane requirement and make it a bit more fuel tolerant.

A simple list on ways to lower the octane requirement would include:

Lower the combustion chamber temperature
- Overly rich mixtures will tend to do this to a point
- Efficient cooling systems will help this

Lower the cylinder pressure
- lower mechanical (static) compression ratio (CR)
- advancing the exhaust timing will lower the dynamic (CR) and bleed off some cylinder pressure

Speed up the combustion process to outrun the temperature and pressure rise
- correct squish band design will help here
- higher density charge from increased cylinder filling or increasing trapped charge purity through proper pipe tuning
- higher rpm speeds up the process


Decrease the amount of oil in the pre-mix. Whether or not this works will depend on the oil


Decrease the amount of time available to heat the charge
- retarding the ignition timing will decrease the time available to raise the temperature
- higher rpm speeds limits the time available to overheat the charge

It's important to remember that the air/fuel ratio that produces the best power also tends to have the best chance of knocking. Running on the rich side of best power on a two-stroke tends to cool the piston crown and minimize knock.


Your best bet might be to start with some basic investigation and tuning to determine a baseline to work from. Based on your comments in earlier posts it sounds like you have a lot of unknown variables in your engine that could be complicating things. Here are a few things you can start with:

1) Look at the carbon pattern on the piston crown as well as the underside of the piston along with the cylinder head to get an idea how well the engine is scavenging , how well the cylinder is filling, and how hot the engine runs on average. If you can keep a higher density charge in the chamber you'll speed the combustion process, which will go a long way towards lowering the octane requirement.

2) Get an accurate squish clearance, squish angle, piston crown angle, and squish area measurement. Often times it's as simple as accounting for production tolerances, other times the factory just gets the design wrong for your type of riding.

3) Document the specific patterns that cause the engine to ping, and the specific brands of pump gas and oil that cause the most problems. Working around a detonation problem can be as simple as switching brands of gas, or buying from a different location. Not all brands of 93 octane pump fuel are the same, plus the 93 octane rating at the pump is not always going to be
what comes flowing out of the ground tank. This is one of the toughest parts of the exercise. You have a random variable in the equation that is impossible to predict, so you have to err on the ultra conservative side, which leads to BAD performance. This random variable will also have a significant effect on jetting, especially on-off throttle transitions , which tends to be the major culprit in these types of cases.

4) Determine the running water temperature to verify cooling system efficiency.

5) Check silencer efficiency to minimize back pressure.

6) Measure static compression ratio from powervalve(PV) full open position,and determine PV open rate as compared to throttle position, to see if there is a correlation between part throttle pinging and PV position. Measure blowdown timing (exhaust to transfer open).

Hopefully this will provide some help.

 

[whenfoxforks-ruled]

Rich ever leaves chaos will take over, not unlike the kdx forum. I really do not read anything that backs up detonation, or a really lean condition. Any silver specks on the spark plug porcelain, or any parts missing, look into the exhaust port at the edge of the piston, is it still there? A ping under load could be a bad bearing or top end! The gas, I would believe other things that run on the same gas will have issues also? Try some decent race fuel by itself and see? If all that got modded was the head, just replace it for a hundred bucks or so. As long as they did not cut the top or bottom of the cylinder that is. Yes Chris, I hate used dirt bikes for the same reason, getting through all the bs that someone else did. The knurled piston skirt, crank vanes, ported and polished guys bike have to be handed down to someone sooner or later.

 

[ellandoh]

dont mind me....... I'm just cuttin' and pastin' all RR's posts into my next best seller, looking for the one he gives out permission to plagiarize :)

 

[1998RM250]

Awesome info! No specs on the spark plug thankgod, and when the head was off i checked for any up and down movement on the crankshaft and there was none, just slight side to side. I also yanked on theflywheel back and forth and no play, also turned it over by hand while listening for any binding or noises. I am never buying someone's old race bike again, even though its exactly what I've always wanted.

 

[whenfoxforks-ruled]

Now do not go to the other extreme. I believe I have a trail guys old bike. The spokes, suspension parts and brakes are beyond fixing. It can cost more than getting a abused/neglected engine back in order! I believe that my 480 has beeen cursed with 82 shocks, being just a tad shorter loses 4 inches of seat height. My 97's are just lucky to have 2 1/2 bikes to make 1, and still need to drop 4 to 600 on the suspenders. My newest does not have a liner slammed in it! Boxes of carbs Chris, or drop 250 for a new one? Back on thread, if the normal jetting procedure starts seeming odd, it is and its something else, related to jetting. And for us, there are a LOT more variables than normal. Once sorted out though, look out! Now to get out and ride more!

 

[1998RM250]

Funny part is the guy raced this bike one season and it pretty much sat unused for many years, it's really clean and not even that ragged on that I know of. Oh well hopefully it's all worth it in the end, this bike seems to make some serious power even though it's still not running 100%.

 

[RM_guy]

...Hopefully this will provide some help.

Ya think??? :nod: Rich comes through yet again with some superb wisdom. :cool:

 

[jsantapau]

Funny part is the guy raced this bike one season and it pretty much sat unused for many years, it's really clean and not even that ragged on that I know of. Oh well hopefully it's all worth it in the end, this bike seems to make some serious power even though it's still not running 100%.

It will be fine once you get it sorted out , couple of points to ponder not that it is going to help with anything except for maybe understanding the learning curve of the new bike;

If the guy was a racer he might have been using a racing fuel, if at the least if he hasn't rode in a while he could have been using a non ethanol added fuel, If he was a good racer on a groomed track he could of been keeping the revs up while maintaining good momentum on a harder based track compared to starting out (lugging the motor) on a soft sand

again once you get the bike dialed in for you and where you ride you should be fine

 

[helio lucas]

one simple tip to check if your compression ratio is too great.
install a thicker or two base gaskets on the cylinder. this will lower the compression ratio and the overall pressure on the chamber.
of course this is only a test that is cheap, simple and fast, ain´t no final answer. it will change too many things including timing of all ports and squish velocity but will help to isolate your problem.

if you cannot buy a thicker base gasket or by any reason cannot get another for your bike just go to a auto parts store and ask for gasket paper. they usually have lots of thickness to choose and you can make your one.

 

[1998RM250]

Okay update on the bike, the guy came to pick it up and retune it last week. He installed a new a/f screw and said it was bogging bad again because the old one was not properly seated and was causing a lean condition. So far he said he's running a 165 or 168 main (which is larger then a stock 162, he had a 160 in it) and a 42 pilot which I think is pretty small honestly with the a/f screw two turns out. He said the bike absolutely rips, so we'll see but I think its weird how it's now running a way larger main jet then stock and such a small pilot! What do you guys think?!?

 

[ellandoh]

I think its weird how it's now running a way larger main jet then stock and such a small pilot! What do you guys think?!?

it is what it is.......if its running good

 

[1998RM250]

true story, I've just never heard of anyone running such a small pilot!

 

[whenfoxforks-ruled]

I hope you can ride it like he does?

 

[_JOE_]

I hope you can ride it like he does?

If not it's surely HIS fault!!! ;)

 

[1998RM250]

lol I'm almost always wot on that bike when I'm on the throttle, probably
cant ride as good as him though for sure. you guys don't think thats a small pilot though, all jokes aside?

 

[2strokerfun]

I don't think thats a small pilot. 42 is perfectly acceptable and used by many many people.

 

[ellandoh]

they sell em for a reason :ride:

 

[FNG]

true story, I've just never heard of anyone running such a small pilot!

I was running a 40 pilot in my '95 YZ250. Stock is a 48.

I absolutely hate bottom end blubber, or rich jetting of any kind for that matter and there is no reason why it can't be tuned out. Whatever it takes like others have said, each bike is unique to a point and with mods, then the rules can get thrown out the window.

Good to hear it is running right, hopefully it isn't a moody bitch for you. The closer we get to the limits, the less consistant they can become.

 

[RM_guy]

Here is how I have mine set up. the 42 pilot is fine. Keep in mind I had the Eric Gorr Mo Power Everywhere port job but he adjusted the compression for 93 octane which probably means he didn't change it from stock. I never bothered to ask.

In the carb I have a 158 main, a JD needle with the clip 2nd notch from the top, slide cut away an additional .05 mm, 42 pilot jet and the air screw out 1.25 turns (it changes with temp). I use 93 octane pump gas with 32:1 Maxima Super-M oil.

The JD neddle really clean up the low end blubber.

 

[1998RM250]

Thanks again guys, I still haven't got to ride it yet. I think hes running a 168 main now which is way bigger then the stock 162, so I ordered a 162 as well to test out myself (he didn't have one he said).

 

[jsantapau]

If you have an issue with pinging that you are trying to quell with jetting why would you go leaner? I know its late in the discussion but have you checked your timing?

 

[1998RM250]

There was a 160 main when I heard the pinging issue, the timing is lined up with the stock mark so it's not advanced or anything. I also heard powervalves can make some noise, especially when clean?

 

[helio lucas]

There was a 160 main when I heard the pinging issue, the timing is lined up with the stock mark so it's not advanced or anything. I also heard powervalves can make some noise, especially when clean?

i was amazed the first time i did a top end in a powervalved mx engine. old yz 125 pretty beat to death.
after the assembly the engine made a terrible noise and after several top end inpections found that the clean powervalve was guilty. some minutes riding and a lubbeb powervalve and the noise was back to normal.
BUT, made the knoking noise from low rpm to full rpm, not ramdomly.
you may want to check the chain and sprockets as well. seen it before, with loose chain and wear chain slider it made some noise when loaded. :cool:

 

[1998RM250]

I guess there's lot's of variables in the equation. I actually just replaced the chain and sprocket, but this new chain I ordered is binding up after a few rides! It was on the bike for quite a few months before I put any ride time on it, so I can't send it back either.