a matter of ping

[tomale]

A friend of mine and I have been having this dialog about the use of race fuel and why we should or should not use it. I stated that I mix race fuel with Pump fuel to raise the octane up to Manufactors recommondations and because I have had trouble with Pinging. It addition to that all my bikes all recommmended using leaded gas, something that is not available at the local gas station. He suggest that the new premium gas is a better gas than the gas of 3 decades ago and it is not needed. If I am having issues with Pinging then maybe the best thing to do is change the jetting to accomadate the new fuel. This does sound reasonable and is probably the best long term fix but what do you guys think. Is the new gas better even though it is not as high an octane. Is it a better idea to alter the jetting to accomadate the newer fuel.
Why is it that it seems that the warmer it is the more likely I am to have trouble with Pinging...
This is not about who is right but what really is the truth and what is the best way to both preserve the integrity of the motor without sacrificing Horsepower. I figure that this is something others might be interested in this topic as well...

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
75' GS400 (project bike)
72'sixday (project bike)

[TGTech]

Tom,

   First off, you need to know, that KTM always, went after horsepower with compression, and that is a problem. Whenever Carl Cranke tuned up KTM engines, he would lower the compression along with his porting and pipe modifications. That's where his horsepower came from.

   High compression demands high octane to supress detonation. The head design is also critical. If you don't have the inner edge of the squish band, as the closest point to the piston and set to the specification that KTM recommended, then the detonation can get out of control.

   The "lead" itself, has nothing to do with supressing the detonation. The lead in the gas, does. When gasoline was first formulated, it was unleaded, just like today. The automotive engineers found that the valve stems and seats, needed something to protect them from heat and wear. Somebody found out that tetraethyl lead would do the trick. A side benefit of adding the "lead" to the gas, was that it raised the octane level of the fuel. The fuel companies found that it was cheaper to make a lower octane gasoline and add the tetraethyl lead to raise the octane to the levels that they needed. Remember when the EPA began to require that lead levels be lowered and the price of fuel went up? That was because that the fuel companies had to go back to making a higher octane fuel, without the lead in it.

   Unless your pushing your machine as hard as your "cousin" Ricky, you don't need the lead in your engine. Using leaded race fuel won't hurt anything and if you use a higher octane fuel, it will supress the detonation. The mixing fuel thing, won't exactly do what you think it will, even though you're technically adding "lead" to unleaded fuel. Again, it won't hurt anything but your wallet.

   I hope this has filled in some of the blanks for you. If you need more info, let me know and I'll see what I can do for you.

Dane

[tomale]

Thanks very much, I was looking through the spec sheet and found that the compression ratio for the at least some of the KTM motors  is 11:1 a Maico is 12:1.... wow that is a bunch..
I have been told that Harry Taylor said that lowering the octane would actually increase the horsepower. but in order to do that it would mean lowering a few other things too... Would fattening up the jets in the carb, help to keep the pinging under control, what about changing the timing and if so which way would you go, toward top dead center or away. I never did understand what is meant by retarding the spark. I think it means that you move it closer to TDC, thus delaying the timing until later in the rotation of the crank. I would think though that by moving the timing of the spark farther from TDC you would give the charge of fuel and air more time to burn and would lower the actual compression at the time of combustion....both I think would help in getting pinging under control... This is pure speculation, and is probably the ramblings of an insane dirt head.
I don't like spending money that I don't have to and would rather if possible make some changes that will keep the Pinging under control...Sounds like a bit of a tall order....
Again thanks for the help.

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
75' GS400 (project bike)
72'sixday (project bike)

[wfopete]

Tom;

Get the basic tuning right first. Find out where you have your timing set.  Retarding it a pinch could solve the whole issue and give you the performance you desire.  But that is not necessarily the complete or correct answer but it is a good place to start. Jetting is a factor too as is your riding style to some degree.  It may take some time but getting it right is worth the trouble.

I personally don't want a motor that needs race fuel to survive and my PVL equipped Jackpiner went through the last weekend in Arkansas with flying colors even though the timing was advanced to the PVL spec limit. I think that was 94 thousands BTC. I was using premium pump gas (91-93 octane) mixed with Maxima Castor 927 @ 40:1 ratio.  I had a happy 'Piner all weekend.[]

If your in doubt of what to do get with someone who knows the bike and can take you through the steps. Finally, here is a good rule of thumb: Only make ONE tuning change at a time!

Pete Petrick


'01 KTM EXC 520, '74 YZ 250, '86 Honda XR 250, '71 Hodaka 100, '74 SUZUKI TM 400, '73 Penton Jackpiner

[Big Mac]

Dane, You said above...head design is also critical. If you don't have the inner edge of the squish band, as the closest point to the piston and set to the specification that KTM recommended, then the detonation can get out of control....
 
I had been of the opinion that the compression ratio could be tamed a bit by thicker base gaskets, leaving more room in the squish area. But from your comment here, would that open up a can of worms with the inner edge(?) being further from the piston? Or am I just lost because I have little idea what the inner edge of my squish band is?

I have 3 KTM motors from '73 to '77, all with good Motoplats, checked and re-checked on timing to perfection, all with the cylinder deck height checked/set the same way. Two run great on straight premium at 40:1, better with 50/50 race gas. One pings like a madman under a heavy throttle, no matter it seems, how I fiddle with jetting to fatten it up, from pilot, to needle to main. Ping goes away w/ the 50/50 race gas setup.

My next step was to try increasing the deck height a hair to see if I might lower compression a bit...but now should I worry about this squish issue?  Too many bikes=too many sleepless nights.

Thanks for the advice!

[Chakka]

Dane,

I have seen Carl Cranke's pipe designs on the Yahoo group and the mapping for his porting specs are there as well. What did he do to lower compression on the KTM engine? Just add more base gaskets? What ratio did he try to achieve?
Thanks
Chris

[tomale]

Pete, my timing and deck height is set to stock, I am very carefull about such things. As far as jetting goes the jetting is bone stock as well, I do change it a bit from time to time but only as is necessary for the situation, then it goes back, I have been know to adjust the air mixture screw from time to time, generally I go in a quarter turn or so in the morning and out a quater turn in the afternoon as it warms up. Generally speaking it is set somewhere about a turn and a half out and the needle is set in the middle position. I replace them perodically because on Bings they do wear rather fast.
The other thing you brought up is the ignition, Having an older ignition might be part of the problem but my does have pretty good spark and it does start pretty easy, so I would think that it would not be that big of a deal.
It is not that I am wanting more horsepower or that I am having a pinging problem of any significance it is more that I do not want to loose any power and if I can learn to tune my bikes better then that is really the issue.
I think that Mac brings up an important issue, knowing Mac as I do I know when he says that he is careful about tuning his bikes then he has, so since that is true why would one bike ping and another not.. I could see it if it was two different brands but these bikes all use the same design and the specs from one to the other are very close. Why the Ping....
Also, if you change the deck height to decrease compression, then you also not only change the spark timing but also change port timing as well, The spark timing can be adjustted but not port timing, When you raise the cylinder you raise the port in relationship to the movement of the piston, if lowering the porting is important and so is lowering the compression then these two are anthetical to one another and there must be another way to accomplish both.... Am I corn fussed[8D]

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
75' GS400 (project bike)
72'sixday (project bike)

[john durrill]

Thom,
If you raise the cylinder in relation to the piston you would both raise the exhaust port and add to the area of the combustion chamber. That would lower the compression in 2 ways. First more empty space for the air/fuel to be compressed in and second less distance in the stroke from when the exhaust port close to TDC.( the real compression ratio on a 2 stroke). The second would both lower the compression and add some RPM to a stock engine while running a bit cooler.
 I don't think you would change the ignition timing any. From your first post it sounds like its a heat issue. The warmer it is the more likely it is too ping.
 I would rig a timing light up. Use a car battery to power it .Check your timing marks with a dial indicator. Mark the flywheel like you would a car and see what the timing on the motor really is as it runs. Adjust accordingly.
 What are you running for deck height and is the combustion chamber smooth or could there be some high spots or sharp edges from a broken ring or a spark plug thread in the head thats exposed?
 That should eliminate variables that can cause pinging.
 If every thing is in good shape with the engine and the exhaust is clean it should run well unless your fine tuned in the jetting to the edge. Most factory bikes come tuned for a bunch of different conditions. Temp spreads from 30 to 100 + F. That means you may lose a bit of power but the bike dosent need re jetting all the time.
 You can use 100 Oct. LL Av gas to get a race fuel just be care full which oil you use with it. Spectro works with LL and Kent can tell you what he used in the race engines they ran using LL.
The owners manual from KTM on the 77 models has a section on retuning the bikes for street use. Keeping the ping out is what it addresses.
Did I give you any ideas with this?
John D.

[TGTech]

Raising the cylinder with thicker base gaskets, will lower the compression, but it will also open up the piston to squish band clearance. It will also alter the port timing, and you don't want to do that either if you want to keep the basic horsepower.

The way that Carl and I lowered the compression, was to open up the actual compression pocket in the center of the head. I still have the tool today, that I used to do the job back in the day. Unfortunately, I don't remember exactly how much I opened up the pocket.

Recently, I did a head for ChicagoJerry, and he tells me that his is working OK now. He was having problems getting his to spin over easily enough to start.

The issue in the head, is all about the temperature, and there are a number of factors that govern what you have: compression, timing, and fuel are probably the three most important. If you have too much advance in the timing..... wait, let's discuss the semantics of timing first.

When you set your ignitions, you will always have advance. That is because in order for the engine to run properly, you always have to have it firing "in advance of top dead center". Now that the timing is set, now you can advance or retard the timimg from that point. To advance the timing, you would rotate the stator opposite the direction of the rotation of the engine, or further before top dead center. If you want to retard the timing, you rotate the stator in the same direction of the rotation of the engine.

Too much advance will make the engine run hot and will limit the RPM. Too much retard, will also make the engine run hot as well. If you're looking for more bottom end, then you want to advance the timing. If you're looking for more RPM, you want to retard the timing. In most cases, the porting, the pipe, the carburetor size, the head design, and the timing, will provide you with a power band that is ideal. By fooling with the various factors, you can make the engine run differently.

More exhaust port height, will make more power, but it will also narrow the power band of the engine.

Another thing that high compression does, is to limit the RPM of the engine. Yeah, it does help the bottom end, but you don't want to limit the engine's rev capability.
 
Those are the thoughts running through my head at the moment. If there are more questions, please feel free to ask.

Dane

[john durrill]

Dane,
 Thanks for the accurate info. Do you have the porting specs for a Carl 175 cylinder? The added .010 deck height must not have been enough on my sons 175 to noticeably change the engine power band. It did help some with the starting for him though.
 My 175 is a one off so we dont have a base line to comepare it with.
Your insight and help is a blessing.
John D.

[rob w]

Dane,
When you machined the combustion chamber in a head, did you do it on a lathe. ? Is this tool you still have, a holding/clamping fixture for the set up, or is it a cutting tool. ?

Thanks for the topic.

I like to tune my bikes so they start on the least amount of kicks. I've found they start a whole lot better when my timing is set right on the recommended specs.

Thom, The few times I've noticed some above-normal pinging with my 400, I attributed to old fuel.

Remember for a little while, the rage was to stretch strips of cut inner tubes, over and around your head fins, to quiet the engine noise some.

Bob W  

[TGTech]

John,

   Unfortunately, I don't have any of the tuning specs. for the 175. My personal work on all the engines that I've worked on, was to look at the port and passage configurations, and make them "flow" as well as possible. Basically, you have to look at the engines as air pumps, and the better that they move air, the better they will work.

Rob,

   I have a mandrel that screws into the spark plug hole. The tool that I have, is a form tool that is cut to the shape of the compression pocket, and I increase the diameter of the pocket.

Dane

[TGTech]

Tom,

   If the compression of the engine, is too high, there is another strange phenomenon that takes place. When the engine is under a load, if it is loaded for a long enough period of time, then the engine will "build heat". This is something that I learned when we were working with the Lectron people. We had some bikes on their dynamometer, and when we'd load them down to see what the metering rods needed to be, the engine's temperature would rise to a certain point and after a little while, it would start to rise to a danger point. At that point, we had to stop the load, otherwise, the engine would toast itself.

   Back in the air cooled days, when the engine was loaded heavily, and the compression was in a reasonable level, the exhaust temperature right at the exit of the cylinder, would run around 1150 degrees. If it reached 1200 degrees, then you could pretty much expect a seizure or a hole in the piston.

   The engines with compression that was too high, would run at 1100 to 1150 for a short period of time, and all of a sudden would start to climb to the danger point. And it didn't make any difference what metering rods were in the carb. Of course, you could make it richer and that would slow down the process, but then the carburation of the engine would not be good enough to be able to ride the bike easily.

   When the compression is lowered to a reasonable point, the heat building problem goes away.

   So to answer your question about just using larger jets, no, if the compression is too high, then just putting larger jets in the carb, won't cure the problem.

Dane

[Chakka]

Dane - Speaking of the tool for lowering compression, what I envision from your description is a half radius somewhat larger than the original "dome" depression located in the center of the head. As you lower the cutter into the head it enlarges the "dome area slightly. Did I picture this correctly? Almost lke a half round router bit. Somthing like that should be easy to make up.

Chris

[tomale]

Thanks to everyone, you have given me lots to think about. I am not sure just what to do just yet,  but it appears that slightly changing the deck height might be in order. And I think I am going to have to recheck my timing too. I do not ride the 250 much but the plan is to ride it in a cross country this next month and it will be warm... I am expecting temps. to be in the mid 80's.
As far as Gas goes, I run Chevron Premium and mixed at 30% race fuel. I have tried it at 50% race fuel and that does work better than at 30% or straight Premium but if I can get away from running race fuel altogether then that would be the best...By the way I have tried several different Premiums too, and have notice that the chevron works the best, It maybe because it is the only premium that is avaiable that does not have Ethanol in it. Ethanol is very bad for 2-strokes.... (Dirt Bike mag. April 94' Page 87)
I agree, the real problem might very well be a heat problem. If that is the case then it seems reasonable that I need to address that directly...
The 250 still has the stock piston in it and a year or so ago I put a new set of rings in it, first time for those too. So it is important to me to keep it running in top form...
Question, I was looking at my books and realized that Deck height when set up for Cross country is different than when set up for Motocross. Cross country requires a greater deck height, is this partly due to the need to minimize heat build up?
Also I am wondering about going to a different Plug heat range in the summer verse in the winter...

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
75' GS400 (project bike)
72'sixday (project bike)