235psi and 99 octane not cutting it...

[Micahdawg]

As the title says...I've got a 01 RM250 with standard bore, wiseco piston, approx 10 hours on rebuild. The head had been milled, but Eric Gorr re-milled some squish back into it. I've been using 99 octane gas plus Klotz at 32:1 (BR8ES) and the bike seems to still be detonating. You can tell at idle (surges) and at very low throttle (almost none) the bike jerks around.

I finally found some higher octane fuel around here...107 and 112 (VP maybe?) I'm wondering if anyone has a suggested MINIMUM octane requirement for 235 psi and can tell me if the bike will need to be re-jetted. I've dug around here with searches and couldn't find anything conclusive on the subject.

As is, i was going to go with straight 112 octane at 32:1 with Klotz and the same plug without touching the jetting. To memory I'm running 46 pilot, 162 main, 3rd clip and like 1.5 our on a/s.

Micah

 

[Faded]

What is the squish set at now?

 

[Micahdawg]

I don't know. I don't really know how to measure it. All I know is when I got the bike there was clear evidence that the head was milled. After talking to Rich he said they could machine the squish back into the head. I wanted it to be 93 octane friendly, and it's still not even that 99 octane friendly.

At this point I don't know if the cylinder was cut too? I came from a 97 RM250 that never was over 200psi and ran all the time on 93 octane.

Micah

 

[Faded]

I don't know. I don't really know how to measure it.

To measure the squish clearance you need to first find yourself a roll of 0.125" diameter solder (look at an electronics store like Radio Shack). Next bend a small length of solder about as long as 1/2 the diameter of the bore. Insert the solder through the spark plug hole making sure that the end of the solder contacts the side of the cylinder bore, preferably the left or right side of the bore (as opposed to the front or rear of the bore which may give a false reading due to the piston rocking on the wrist pin). While holding the solder against the side of the bore slowly cycle the kick starter so that the piston rotates near TDC, crushes the solder and descends enough to get the solder out. Be careful to only cycle the piston once so that you don't pinch the solder multiple times which could skew measurements. Remove the solder and measure the crushed thickness. This will be your squish clearance! Accuracy does count so try to use a measuring tool that will read down to 0.001".

All I know is when I got the bike there was clear evidence that the head was milled. After talking to Rich he said they could machine the squish back into the head. I wanted it to be 93 octane friendly, and it's still not even that 99 octane friendly.

At this point I don't know if the cylinder was cut too? I came from a 97 RM250 that never was over 200psi and ran all the time on 93 octane.

Micah,

It sounds like you have too much compression, but running a large squish clearance could make it prone to knocking as well. With so much unknown machining it's hard to say what exactly you have now, and your problem could possibly be two-fold. In order to figure out your problem you'll need to do some measuring to figure out where you stand and what your options are from there.

I would first measure the squish clearance with the head on. Next I would remove the head and measure the squish height (recess) in the head to help determine what the deck height is (height from top of cylinder to top of piston, can be + or - or even 0) in conjunction with you head gasket thickness. Your bike should have an O-ring head so measuring the head gasket thickness isn't required. Also I would measure the width of the squish band (the flat from the edge of combustion chamber to the opposing edge where it recesses into the combustion chamber).

After these measurements I would measure the volume of the head, otherwise known as "cc-ing" the head. Again, accuracy counts so being able to measure down to 0.1 ccs is to your benefit.

Since you're in there I would take two more measurements. The first would be the distance between the top of the cylinder and the bottom of the exhaust valve (with the exhaust valve closed) and the second from the top of the cylinder to the top of the exhaust port (with the exhaust valve open).

With all these measurements you can crunch the numbers and see exactly what kind of predicament you're in, including compression ratio(s) along with a better idea of what octane fuel you should need. This will give you a clue on what needs the work to meet your specifications. If you need the equations let me know.

 

[Micahdawg]

Wow, thanks for the reply. So the solder trick measures squish clearance or squish hieght? I'm hearing three terms, squish clearance, squish hieght and squish band.

Also is there some way that I can "cc" the head or do I have to take it to a shop. Do I do this with a spark plug fully installed?

Micah

 

[Faded]

Wow, thanks for the reply. So the solder trick measures squish clearance or squish hieght? I'm hearing three terms, squish clearance, squish hieght and squish band.

No problem, sorry for the confusion. The squish band has two phsical dimensions, the height and the width. The height is the distance between the gasket surface of the head to the top of the squish band.

I mentioned checking the height with the head off because this should give you an idea of head gasket thickness (not in your case) in conjuction with possible deck height (as I said before, distance from the top of the piston @ TDC to the top of the cylinder). This can be checked with a set of dial calipers after removing the head.

***Also with the head off (and the dial calipers) you can check the width of the squish band which will allow you to figure out the percentage of the bore diameter the squish band occupies.***

The height of the squish band in addition to deck height (which can be a + or - measurement) and head gasket thickness will give you your squish clearance, which is easily checked prior to taking the head off. This is why I recommend checking the squish clearance first and checking the height and width after removing the head. :cool:

Also is there some way that I can "cc" the head or do I have to take it to a shop. Do I do this with a spark plug fully installed?

If you can beg, borrow or steal an accurate measuring device (like a burette, syringe, etc.) that will allow you to meter down to 0.1 ccs and a small piece of plexiglass with a hole in it than you can do it yourself. Yes, you will need to fully install a spark plug, turn the head upside down and seal the gasket area of the head to the plexiglass with a small amount of grease. Use your measuring device to fill the head with the liquid of your choice (water works) until it's full and record the amount it took to fill the head.

This measurement will only be the volume of your head if you do not use a head gasket and run 0 deck height. If you have + or - deck height and/or a head gasket (and/or a domed piston!) this will have to be figured into the equation to come up with total head volume.

With your head specs you should be able to determine what the problem is and what course of action you should take to correct it.

Hope that makes sense. :coocoo:

P.S. I re-edited my first post to try and better reflect the use of the terminology.

 

[Micahdawg]

Thanks for the clarity. I'll take the following measurements this weekend.

For now, I'm having some trouble understanding what these measurements will tell me exactly...other than isolating that the high compression is definitely due to the head being altered. And if I take the measurements to the exhaust valve it should tell me exact compression ratio's at open/closed.

This is the first time I've heard that a large squish clearance can induce knock though. Unless the fuel is having a harder time reaching the spark with increased clearance.

Micah

 

[Faded]

This is the first time I've heard that a large squish clearance can induce knock though. Unless the fuel is having a harder time reaching the spark with increased clearance.

Most two-cycle engines have a squish band machined at the outer edge of the cylinder head, this band serves two purposes . The close proximity of the piston to the cylinder head helps to cool (or quench) the end gases that were heated by the pre-flame reactions, but more importantly the squish band helps to add motion to the burning charge, which helps speed up the burning and limits the amount of time available for pre-flame reactions to heat the end gases. A fast burning chamber will tend to be very resistant to detonation . This is part of the reason you've seen manufacturers switching from domed pistons, to flat top pistons and back again. They are always looking for that magic combination of scavenging efficiency and charge motion. Unfortunately, on most stock engines that squish band doesn't serve much purpose. In theory squish bands work very well, but production line tolerances leave squish clearances so great that the only thing being squished is horsepower. Cutting the cylinder head to bring the squish clearance into spec while still retaining the original cylinder head shape and volume is not the easiest thing to do, though it will definitely pay dividends.

Fuel For Thought by Rich Rohrich
http://www.eric-gorr.com/techarticles/Fuel_Basics.htm

Making sense?

The measurements are a starting point and will help you plan your next move, or you could just keep guessing and put the head on the lathe again. :yikes: J/K

Also, trying to equate pressure (PSI) to a certain compression ratio is haphazard at best. The only way to to find the actual compression ratio is to calculate it.

 

[Rich Rohrich]

For now, I'm having some trouble understanding what these measurements will tell me exactly...

There are a number of ways to modify the cylinder head to increase compression and/or alter the squish characteristics.

The squish characteristics can be altered while retaining the original compression ratio through reshaping of the bowl area. It's a balancing act based on what you are trying to accomplish.

Too much squish clearance renders the squish band essentially useless and will tend to increase the octane requirement in most cases. Too little squish clearance can complicate things as well.

As Andrew pointed out, good measurements are the correct starting point to work from.

One other thing to consider is that not all 99 octane fuels are the same. A 99 octane fuel designed for high speed four-strokes could easily run into knock issues sooner than a 95 octane fuel specifically designed for use in a two-stroke. The 90% distillation temperature and the end point temperature are important considerations in choosing a proper fuel for a high compression two-stroke.

 

[Micahdawg]

Well...I've taken the measurements...so hopefully between you, rich, and some equations I'll have an arrow pointing me in the right direction.

Squish Clearance - I used 1/16 solder wire and got that to compress, so I took measurements. The very end, touching the cylinder was .047. Then moving in toward the middle of the cylinder about 3/16 of an inch showed .060. Dead middle of the solder wire was .051

Squish Height - .481

Squish Band - .308

Due to the fact that the head had been milled (25 is scribed on the outside of the head) and Eric remachined the bowl, there is now a step up cut into the head. It begins right at the edge of the cylinder bore. I thought it may be useful to note that measurement as .027 (cut higher into the head).

Deck Height was .000-.001+. Virtually 0.

Top of cylnder to top of exhaust port with PV closed was 1.936

Top of cylinder to top of exhaust port with PV open was 1.601

Head volume was approx. 15.3 cc's. Sorry, I used a syringe and this was probably the least accurate measurement. But at least to the tenth, I feel pretty confident about it.

Not having a clue what stock was supposed to be....I'll eagerly anticipate the next comments.

Micah

 

[Micahdawg]

Also, the piston was all there :) Last time the bike made low end noises like this I found a chunk of the piston gone. It was at the crown toward the exhaust port...looks like it popped off and flew out.

This time the piston was all there and looks good. Found absolutely no spotting on the head or piston...so it looks like there hasn't been any detonation on 99 octane. The piston was uniformly dark...looked carboned up a bit, but before shutting the bike down it was idling for a long period of time.

I could still read the numbers that are faintly stamped in the dome and even see all the little cnc marks in the piston.

Micah

 

[magneto]

Have you tried rejetting after the engine mods? My CR5 had a vicious low to midrange knock after a mo'better port job and reed cage change... going richer on the jetting eliminated the knock completely.

 

[Micahdawg]

Yeah, I've played with the jetting quite a bit. I got it used and the 1st owner made it a race bike (e.g. the milling). 2nd owner was a dumbazz and grenaded the bike quite handsomely. Paid a guy to rebuild it and then off'd it to me.

I ran the bike on 93 octane and tore it down after a few hours to investigate cause it didn't feel like it was running right at all. That's when I found the crown of the new piston gone. Got pretty lucky as the new nicasil lining was good and ring was still planted pretty good in the cylinder.

For jetting, the 2nd owner's mechanic tossed a #55 pilot in there with factory #162 main. I'm now running a #46 pilot with #162 main which I think is factory jetting. A/S is 1.5 turns out and float height was adjusted level.

I don't want to go any leaner than stock because I've got the pipe/silencer, uni filter and no air box lid (didn't come with it). So I think the factory "richer" jetting is probably pretty good. Especially considering the extra compression these days.

The plugs burn really clean and Rich helped me with the plug chops...he seemed to think it was a tad rich still based on what the carbon ring was showing.

Probably too much info, but long story short, yes...I've messed with jetting.

Micah

P.S. Also have carbon reeds with factory cage.

 

[Faded]

The manufacturer’s list two compression ratios, a high speed CR (one with the exhaust valve open) and a low speed CR (one with the exhaust valve closed). For comparison’s sake you can determine both compression ratios with your modified head and see where you stand. The formula for finding a compression ratio is:

CR = (V1 + V2) / V2

CR = Compression Ratio
V1 = Cylinder Volume
V2 = Combustion Chamber Volume


The difference between high and low speed CRs will be the V1 measurement due to the difference in effective stroke (the piston covering the exhaust port with the exhaust valve open vs. with it closed). The formula for finding V1 is here:

V1 = [ 3.1416 * D2 (squared) * ES ] / 4000

D = Bore Diameter
ES = Effective Stroke


The 2001 RM250 has a bore of 66.40mm. The effective stroke(s) you provided are 1.601” or 40.67mm open/high speed, and 1.936 or 49.17mm closed/low speed. With that you can plug in the numbers and come up with:

V1 open/high speed = 140.83ccs
V1 closed/low speed = 170.27ccs


You already know V2 is the volume of your head that you cc-ed, or 15.3ccs, and with that you can go back to the CR formula and find that:

High Speed Compression = 10.21
Low Speed Compression = 12.13

A stock 2001 RM250 is advertised as having a compression ratio of 9.0:1 (high speed) and 10.5:1 (low speed) for reference.



From what I can decipher from the measurements it sounds like your squish band is 7.8mm wide (0.308”), which is about 42% of the bore diameter, and is cut on a 4* angle. The actual clearance of 0.047”-0.060” sounds a little tight but may be okay; Rich and Eric would have a far better idea if this would tolerate pump gas or not. If it is okay then you’ll need to look at removing material from the bowl of the combustion chamber (about 2.2ccs) in order to get your compression ratio(s) back in check for your desired fuel.

Long story short, sounds like you have too much compression. :cool:

 

[Micahdawg]

Very enlightening. I wonder if the deck height that I measured was accurate for my bike...and that the jug was not altered.

Also, now that I see the compression ratios are definitely higher than stock, I'm assuming I have two routes to go. (A) either adjust my fuel for the modfied head or (B) adjust the head for which ever fuel I want to run.

It sounds like the bike would be faster if I brought the fuel requirement up.

And thanks big time for all the help. This is the most conclusive info I've yet to extract from this bike. Hopefully the days of cursing while wrenching on it will soon end.

Micah

 

[Faded]

...I'm assuming I have two routes to go. (A) either adjust my fuel for the modfied head or (B) adjust the head for which ever fuel I want to run...

Either way, good luck! :cool:

 

[Micahdawg]

Maybe Rich can suggest an octane then? Otherwise I'll just try 112.

Thanks guys for all your help.

Micah

 

[Faded]

Maybe Rich can suggest an octane then? Otherwise I'll just try 112.

There's more to it than just octane. :nod: If you're willing to pay the extra for high(er) octane 'race gas' it would be in your best interest to find something that will benefit you in more ways that just suppressing detonation. IMHO I would make a list of what's available to you and what you can afford and ask Rich for help with a suggestion from that list. :cool:

 

[Micahdawg]

I'm gonna take it one step at a time. Sounds like the VP112 is a good start. If I can just get the bike to run normal........then I will move toward optimizing it. Thank you everyone for your help.

Micah

 

[magneto]

A new head or a used one from a junkyard might be the easiest and quickest solution to the problem.

 

[skipn8r]

Cutting the cylinder head to bring the squish clearance into spec while still retaining the original cylinder head shape and volume is not the easiest thing to do, though it will definitely pay dividends.

Can someone define the "spec" squish clearance for an otherwise stock '01 CR250 engine? Others have recommended that something in the 0.035" - 0.040" range would be appropriate.

 

[Micahdawg]

Based upon what was measured on my 01 RM250 (0.047”-0.060”) the guys in this thread thought that was a little tight. And this is on a head that has definitely been milled. I can't understand how Honda would have a factory squish clearance that is even tighter than that...so I would say no, that is not representative of a stock CR250. But I'm just guessing.

Micah

 

[skipn8r]

I would say no, that is not representative of a stock CR250. But I'm just guessing.

Yeah, it's certainly not the stock squish clearance; mine was 0.060" bone stock. I'm looking for an optimum spec type of number. I realize this varies with application, but there has to be a generally recognized "good starting point".

 

[alaskamx]

hi comp motor

I came across a 1996 RM250 from a guy who had the motor done by FMF. Hi comp motor, required race gas 104 or better. I installed 2 base gaskets and have been running prem pump ever since.
Not the "best" solution, but my wallet liked it.