95 rmx detonation/pinging

[argclh6670]

hey people, i just put a new topend on my RMX. have maybe 4 hours on it tops. Also, swapped out the PJ for a PWK jetted the same as PJ. I noticed it pinging on my last ride. Never done it before. It just started during the middle of the ride.

New fuel and I always run 92 oct.

Hey guys, last weekend i took the rmx out.....The weather is now 30* warmer When it was cooler I'd notice it would ping very little and just under certain loads. Now my bike was pinging badly. . Last summer it didn't ping at all! The only thing different with bike is a new topend and I swapped out the PJ for a pwk, I'm running same jetting. Any suggestions?

 

[argclh6670]

took it out today, checked the plug, nice mocha to dark mocha. before first ride, swapped out the shaved slide to non shaved #7, no difference. pulled card. raised the needle 1 position still pinging. back to truck, raised needle to 4th slot still pinging, thou not quite as bad. went back to truck, put in 1 size larger MJ, better accel. but the ping is still there. Frustrated, I guess i'll put the ole PJ back on and see if its any difference. Unless you guys can throw some more suggestions? Edited by argclh66

 

[IndyMX]

Pinging isn't caused by bad jetting, it's caused by too low octane.

Run a high enough octane to eliminate the ping. Obviously 92 isn't enough for your engine.

 

[argclh6670]

Pinging isn't caused by bad jetting, it's caused by too low octane.

Run a high enough octane to eliminate the ping. Obviously 92 isn't enough for your engine.

It used to run fine with NO ping before the topend. Is it possible that the new topend added enough compression to cause the detonation? I guess so?

 

[Rich Rohrich]

It used to run fine with NO ping before the topend. Is it possible that the new topend added enough compression to cause the detonation?

Of course. You raised the combustion chamber working pressure and the working temperature along with it.

Jetting can have an influence on knock. 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.

A couple of other things to consider:

- it will be harder to jet for good throttle response with alcohol based fuel because ethanol has a fixed boiling point around 173 degrees f and a very high latent heat of vaporization, along with requiring a different A/F ratio then the rest of the fuel. In short it acts very different then pure gasoline and screws up the fuels distillation curve which can cause jetting issues.

- the octane distribution of ethanol based fuels will be very different than pure gasoline. As a result the knock sensitivity can be very erratic from load to load of fuel. It's best to tune on the safe side till you get used to running with ethanol. Two-stroke riders who switch to ethanol based fuels without other changes very often run into knock and detonation issues they never had before.

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


Your best bet might be to start with some basic investigation and tuning to determine a baseline to work from.

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.

 

[argclh6670]

Of course. You raised the combustion chamber working pressure and the working temperature along with it.

Jetting can have an influence on knock. 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.

A couple of other things to consider:

- it will be harder to jet for good throttle response with alcohol based fuel because ethanol has a fixed boiling point around 173 degrees f and a very high latent heat of vaporization, along with requiring a different A/F ratio then the rest of the fuel. In short it acts very different then pure gasoline and screws up the fuels distillation curve which can cause jetting issues.

- the octane distribution of ethanol based fuels will be very different than pure gasoline. As a result the knock sensitivity can be very erratic from load to load of fuel. It's best to tune on the safe side till you get used to running with ethanol. Two-stroke riders who switch to ethanol based fuels without other changes very often run into knock and detonation issues they never had before.

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


Your best bet might be to start with some basic investigation and tuning to determine a baseline to work from.

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.

Uhh.....yeah.
Dang Rich,
my head was hurting before reading ur post now it feeels as if its gunna EXPLODE! lol

thank you for taking the time to write that up and post. so, long of the short of it:

1. find a pure gas store or,
2. have head reworked to run pump or,
3. mix race and alcohol gas to achieve better fuel octane.


i actually found a gas store close by that will be selling non-ethanol but they were going thru some EPA bs last time i stopped in. Hopefully they are up and running again. im heading out today to ride (day off) :cool: i'll report back tonight.

Do you have any input on the causes of a hanging idle?

Thanks again.

 

[dirt bike dave]

Rich also mentioned your cooling system, silencer packing and retarding the timing.

If your cooling system is not working well, that will absolutely contribute to pinging.

Retarding timing is free to do, if you have a flywheel puller. Many 2 strokes will see an increase in top end power with retarded timing from stock settings.

Hanging idle, IMO, is likely due to a frayed throttle cable (cheap to replace) or a worn out slide and carb body (expensive).

 

[argclh6670]

Rich also mentioned your cooling system, silencer packing and retarding the timing.

If your cooling system is not working well, that will absolutely contribute to pinging.

Retarding timing is free to do, if you have a flywheel puller. Many 2 strokes will see an increase in top end power with retarded timing from stock settings.

Hanging idle, IMO, is likely due to a frayed throttle cable (cheap to replace) or a worn out slide and carb body (expensive).

Yeah, i totally agree with you on the cooling system.
other than visible leaks, is there a way to test the cooling system? I flushed and replaced coolant before last ride.
Also, have made sure with cap pulled to see coolant flowing while running.

To retard the timing, which way do i turn the stator? CW or CCW.

 

[dirt bike dave]

CCW to retard.

Not sure about testing the cooling system, but a radiator cap can go bad, fins can get bent and internal passages in the radiator can get clogged. Old bikes that were not well maintained are going to be most likely to have trouble. In extreme cases, hoses can get clogged and water pump impellers can get damaged.

Best bet is to closely inspect, and do the things you did (flush and replace coolant).

Your problem may have just been poor fuel and maybe a little lean on the jetting for all the new heat/power you are making with the additional compression. But good for you for using this as an opportunity to learn and do some tuning.

I'd probably rule out some simple things before changing he timing, but that's just me. FWIW, Another thing that could make your bike run hot is dragging brakes, but it sounds like your problem started after the top end work.

 

[argclh6670]

I wonder at what temperature the coolant runs at full operating temp.

 

[argclh6670]

So, in Rich's reply he notes that ADVANCING timing will lower cylinder pressure. But also says something about retarding timing. So I'm confused again. Sorry if I'm being a total newb.

 

[dirt bike dave]

Rich said advancing the exhaust timing will lower pressure... tech speak for changing the porting in the cylinder so the exhaust porting is open earlier.

Some bikes also have an adjustable power valve, where you can adjust the power valve to open earlier, which would be advancing the timing of the power valve.

For ignition, he was saying to retard to avoid ping and reduce pressure. Advancing the ignition timing will make pinging / detonation worse.

 

[arnego2]

which could be countered by better (?) fuel like race fuel, AV gas etc. :)

 

[2strokerfun]

Describe what you mean by "hanging idle" please. Do you mean the bike keeps running higher rpm after you back off the throttle? Do you mean an erratic idle (surging) when bike is warm? Or Do you mean idle hangs up and you have delayed acceleration when you give it throttle?

 

[argclh6670]

Describe what you mean by "hanging idle" please. Do you mean the bike keeps running higher rpm after you back off the throttle? Do you mean an erratic idle (surging) when bike is warm? Or Do you mean idle hangs up and you have delayed acceleration when you give it throttle?

Keeps running at a higher rpm after i back off the throttle.

 

[argclh6670]

Rich also mentioned your cooling system, silencer packing and retarding the timing.

If your cooling system is not working well, that will absolutely contribute to pinging.

Retarding timing is free to do, if you have a flywheel puller. Many 2 strokes will see an increase in top end power with retarded timing from stock settings.

Hanging idle, IMO, is likely due to a frayed throttle cable (cheap to replace) or a worn out slide and carb body (expensive).

I went out today after retarding the timing half the distance to full. I rode in all throttle ranges, from totally lugging trail style to fully pinned mx style on alittle track. I didn't notice it pinging at all. Not sure if the cool weather aided in the NO pinging.
On the trail, it seemed as if didn't have the normal lowend grunt before timing adjustment. However, when I was on the track it felt fine. In going to retarde timing half the distance back and test again. Thanks again for all the advice.

 

[argclh6670]

went out today, i readjusted timing closer back to stock. No pinging, power down low was fine. Also, last few times i went riding i used no left over fuel more than 5 days old, before i'd use up 2-3 week old fuel. maybe that helped too?

 

[High Lord Gomer]

Just looked at this...great information, Rich!

 

[sbest]

Wow, you got an awesome answer from Rich. Good stuff. Lots of good answers here.
So, for trouble shooting, good rules are to make changes one at a time and when you have a problem, go back to where you knew it was right. Put the old carb back on and try that first. It will answer a lot of questions for 20min work.

Did you forget the base gasket or use a different thickness basegasket?

Hanging idle? Hmmm, often caused by air leaks or mixture problems.
If not solved by the carb change, try leak testing the engine, where you seal it and pressurize it to 5-7psi to look for leaks.

Where the jetting colour is important is right down where the porcelain meets the steel at the insulator base.
The colour up by the electrode mostly tells you if the heat range is right for your riding.

Good luck, Steve