It 490 Rebuild

[brightsparks]

Hi guys, I'm rebuilding my IT 490 for a road project and want to extract more power from it. Would anyone be able to shed some light on porting specs , headwork , exhaust ?

[Kane]

Some of the best advice going....................

EricGorr
SFO, I still do work for DGY! Back in the 80s we did tons of work on 465/490s. At the amateur nationals we consistently spanked the factory prepped Yamahas. Assuming rideblue is looking for a low cost solution, and considering its a teenage year old motorcycle, the pinging may have more to do with maintenance and wear.
Cylinder head: The head is prone to warpage. Look for black gas trails across the copper gasket. You can lapp the top of the cylinder and head to remove the warpage. I like to countersink the stud holes to prevent "mushrooming" of the stud hole that can cause a leak. Regarding the head dimensions, if you have access to a lathe with a 3-jaw chuck, you can use a spark plug mandrel available for $25 from Goodson, a large aftermarket automotive tool supplier. Then narrow the width of the squishban by adding a blend angle to the combustion chamber. The bland angle should be 30 degrees and start 8mm from the edge of the squishband.
Car jetting: That carb was a mess. We used these baseline settings for super-unleaded premium (no ehtanhol!) at 80 degrees Fahrenheit, and 40:1 with Spectro. 40 pilet, 3.5 slide, R-0 needle jet, needle in the middle clip, and a 440 main. The pilet jet, slide, and main jet are leaner than stock but the needle jet is much richer. Use stock reeds because Boyesen dual stage make the bike surge at low rpm and mimic a lean condition. Make sure that the hole in the slide is uncovered by the cable retainer and that the rubber seal in the top of the cap is in good shape. Replace the needle if it has any ridges down the length, those are wear marks caused by vibration. The slide can be filed to a 3.5mm cutaway height so you'll need to buy the R-0 needle jet, pilet and main. That will cost you under $25. DGY probably has the parts in stock Tel. (630) 971-2602
Air leaks: All the fancy head mods in the world won't help it if the engine has air leaks. Check the intake manifold for dry rotting, the left crank seal for leaks (pump gas with ethanhol causes swelling of the seal and leaks), cylinder base gasket leaks, and seal the front of the exhaust pipe with RTV high temp silicone.

Good luck

Uhhh, do not have exact specs. for the head modifications. But if you can wait a few days or so I will have some more specific information for you, gonna do another head for a spare engine.
I can tell you that I got the general info from a book I finally bought called "Performance Tuning Two Stroke Engines". By general info I mean squish band size in relation to bore as well as modifying combustion chamber. I increased the diameter of chamber but kept stock depth, so the squish band diameter had to increase, reducing its total band width.
In addition I reduced squish band height to 1.0mm. It started out at a little over 1.5mm which is way too much allowing flame travel to squeeze thru to the barrel, thus creating too much heat and causing detonation. By reducing squish band height contains combustion in the chamber. 1.0mm does not allow flame travel to the outer wall of the cylinder.
Like I said, I am getting ready to do another and will be glad to forward dimensions to you. I thought I was only gonna do one-off but kiddo wants a spare. Please bear in mind that it is still a 'mutha' to start. Gotta bring slightly over top dead center and then kick it like your life depends on it.
These mods to the head do result in a small decrease in power, actually we cannot notice it. But the detonation and overheating are gone. We ride this thing in the sand hills here in west Texas(something akin to living next door to hell in the summer) with ambient temps well over 100 degrees and have had not problems.
Texican
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Do not add another gasket!!!!! It only increases squish band by another mm or so and will make the overheating problem worse as well as most likely will not seal resulting in probably a burnt piston.
Do you have a 6 or 7 bolt head? I happen to have an extra 7 bolt head that I could machine for you. I am currently using the earlier 6 bolt but plan to someday go to the later cylinder and head so would eventually need it replaced when you could find one. I have my own lathe and do my own machine work.
The most important dimensions I would need would be deck height as well as your compressed head gasket thickness. The stock copper head gasket we have ended up 1.0mm after torqueing.
We too had to shorten the kick starter by nearly 2 inches so I know exactly what you are going thru.

Agree with him on head warpage, hand lap head and cylinder after all other machine work.
I blended squish band w/30 degree but not as much as he does, our squish band is more than 8mm, more like 12 I think. I matched squish band angle in head with angle of wiseco piston crown, forgot angle now but squish band now at 1.0mm. The crushed head gasket thickness is 1.0mm, so piston deck height was set at 0.0mm and head had no step.
Jetting - we have 410 main, 40 pilot, clip at #3, needle and jet are stock 83 YZ490, forgot numbers. Boyesen dual stage reeds and do not have surging he described. Our altitude here is 2770' ASL, gets hotter n hell here in the summer time. Lots and lots of bottom end with massive hit when it comes on pipe. Oh, no airbox either, plumbed to large K&N with foam outer wrap. No fouling plugs either, just a mutha to start. Usually starts on 2nd or 3rd kick cold even in winter(course, our winters here are not really winter) and 1st or 2nd after warm.
I guess only complaint is she vibrates at high rpm, so we use that indicator as shift point, keeps her in mid range where she pulls like schitzoid godzilla.
----- Original Message -----

[Kane]

And part 2........................


Above are some of the questions I will try and answer in this technical article.

What is Piston Deck Height??

The Piston Deck Height is the distance the piston EDGE is in relation to the top of the cylinder deck. NOTE: This
distance can be negative (the piston edge is recessed in the cylinder deck) or positive (the piston edge is sitting above the cylinder deck). Please remember... we are talking about the relationship of the EDGE of the piston not the center. Of course, if the piston is of flat-top design then the edge and the center will be the same.


How do we determine Piston Deck Height??

1. Remove the cylinder head.

2. Grab your Dial Indicator and Magnetic Base (very accurate),Flat-Blade Depth Micrometer(accurate), Dial Caliper(less accurate), or straight edge and feeler gauges(much less accurate) OR your Step Micrometer (needed for pistons with a positive deck height .

3. Place piston at TRUE Top Dead Center (TDC) NOTE: Since every crank has dwell at TDC, determining TRUE TDC can be a bit tricky. Using a custom piston stop is the most accurate method for finding true TDC. Since some of you may not have a custom piston stop, finding true TDC can be determined using a dial indicator and a mag base. WHAT!! you don't have a spare mag base and dial indicator laying around??

Hmmmm... OK, Here is a fast way to determine true TDC without using a piston stop or dial indicator. This method, if done with finesse and time, will yield an accurate, or fairly accurate, measurement. This will require a very thin, but rigid, piece of material with a very fine point (if the piston is crowned).
a. Rotate the crank to what you believe is true TDC.

b. Now, take your rigid piece if material and place the sharp end in the cylinder to where it touches the piston.Be sure to hold this material in such a manner where it can be held in place without the backing of the piston.

c. Now, while holding the material on the piston, very carefully rotate the crank (by hand) forward and backward while keeping your material STATIONARY. If at any point the piston raises the material, even just a little bit, then you were not at true TDC. So... with this in mind continue this method until the piston no longer displaces the rigid material.
Once, this occurs, you will be very close to true TDC.

OK.. Now, you have found true TDC.. you are half way there!

Now.. you know when you are at true TDC, so, all you have to do is measure the distance from the piston edge to the cylinder deck.

How this measurement is done will be based on the measuring equipment you have chosen to use.

Please keep in mind... when measuring the deck height on a crowned piston, it is nearly impossible to get to the actual edge of the piston. This is because your measuring device does have mass and will hit the piston crown before it hits the actual edge. The finer the measuring device.. the more accurate the measurement. So.. if you are using the butt end of a dial caliper on a crowned piston, please keep in mind that this butt end is very wide and flat and will surely hit the crown way before the edge. Since the dial caliper is the instrument most commonly available for this measurement, I have determined an approximate offset factor for measurements obtained with the dial caliper. Of course, the accuracy of this offset will be determined by the accuracy of the initial measurement given. So, PLEASE take your time in measuring.

For pistons with a positive deck height, a step micrometer seems to be the most accurate tool for this measurement. It is nearly impossible to accurately measure positive deck without a step micrometer.

WITH ALL THAT SAID...MATH, along with measuring tools, can be used effectively to help determine deck height. If you know the piston's crown height, then this can be used to aid in determining deck height. For example.. if you can determine how far above the deck the center of the crown is at true TDC, you can subtract the known crown drop from this number to determine your deck height.

Why is it important when designing performance items for an engine??


Deck Height is a very important piece of information when determining engine parameters and designing combustion chambers.

Let's start with its role in determining engine parameters.
Many performance shops utilize software and / or math equations as an aid in determining engine parameters and upgrades. For example, it is important to know the position of the piston, in inches or millimeters, at different points along the stroke ie. exhaust opening, transfer opening etc.. This information can be calculated using software or by punching the numbers in the mathematical equation (software is much easier). TSR www.tsrsoftware.com has a nice program for this calculation.
OK, you may be asking yourself, "What does the deck height have to do with the piston position?" Well, not much from an initial design point, but for the person porting your engine, it can be very important. For example... many people send their cylinders out to be ported. Once the shop gets the cylinders they have the pleasure of determining how to modify it in order to yield performance gains. Many performance shops determine what to change based on the port timing of the engine. Now, if they only have your cylinders, not the entire engine, how can they determine what your port timing actually is??? Well, you may tell yourself that they have seen that engine before and know what the porting arrangements are and your engine will be exactly the same. Well, if you are convinced this is true then, you have nothing to worry about. But if you are like me, and KNOW that this is not always true, then you should be concerned if the shop you are sending your cylinders to does not ask for a deck height measurement or a base gasket thickness. There are often variances in cylinder castings, piston heights, connecting rod lengths, and base gasket thicknesses. ALL these things effect port timing and deck height. Knowing these parameters will assure that the shop has the necessary engine measurements to do the best job they can in modifying your cylinders.

OK, How about combustion chamber design? How does deck height effect the design of the combustion chamber?


Deck height is a VERY important measurement to consider when designing a combustion chamber.

Deck height is a major player in determining the squish clearance of an engine. While it is beyond the scope of this article to discuss squish band design (maybe later), let's just say the squish action within a combustion chamber is very important in the combustion process and power making process.

Squish clearance is the distance from the edge of the piston, at TDC, to the outer edge of the combustion chamber's squish band. So, one can easily see how the deck height effects the squish clearance measurement.

So, you may ask yourself..." How can one design a proper combustion chamber without knowing the deck height??" Well.... the answer is simple.. ONE CAN'T!! Sure, they can get close. All I am stating is that they can get a lot closer if they have a list of engine measurements, like the deck height.

It has already been determined that there are many factors that effect the deck height.. cylinder casting, piston casting (or forging), con rod length, and cylinder base gasket to name a few. So, with this in mind, how can a head designer properly modify your head, or design a new head, for your engine without knowing the deck height of your engine? THEY CAN'T!

You might be saying to yourself " So, what if I am off on my deck height measurement .008", how big of a deal can that be?"

Well... While I will not go into the difference .008" has on squish action within a combustion chamber, let's look at what .008" does to the compression ratio and volume of an engine.

Volume of a cylinder: PI * R^2 (radius = 1/2 bore) * H (Stroke)

So, let's take the 800 Rotax twin engine with an 82mm bore and a 76mm stroke. We will convert the .008" to mm so the numbers in the equation coincide. .008" = .2mm

What effect will .008" have on the compression ratio?

Let's do the math: 3.14 * (82mm/2)^2 * .2mm = 1.05cc of volume change. So, what does 1.05cc of volume increase or decrease on this particular engine? Well.. 1.05cc equates to a 0.4 change in untrapped compression ratio. OK.. 0.4 change in untrapped compression ratio will change a 12:1 engine to a 12.4:1 or a 11.6:1 engine. Well..may be that is not so bad. so, lets change the engine by .015" or .38mm

3.14 * (82mm/2)^2 * .38mm = 2.0cc of volume change. So, what does that do to the untrapped compression ratio? Well it changes the untrapped compression ratio on a 12:1 engine to 12.79:1 or to 11.21:1.

So, you can see that the compression ratio is effected but what is also effected is the squish action within the head. Squish action is important in determining power characteristics of an engine. The squish band acts as a cooling layer to help cool the end gases as they are being rapidly compressed. By keeping these gases below their combustible temperature, one can prevent undesired combustion of these end gases in the squish band area. If these end gases are allowed to combust before the oncoming, spark initiated, flame front chooses to combust them, then you have the receipt for detonation and engine damage.
The squish action also creates turbulence within the combustion chamber. This turbulence has a direct effect on the flame front speed so ,in actuality, it effects ignition timing.

[Kane]

And part 3................

OK, One important measurement for the squish action is the Maximum Squish Velocity (MSV). In short, this is the max velocity of the end gases as they are be compressed. It is actually a lot more complicated than that, but I will leave it at that for now. It is measured in meters/sec (m/s).

Squish velocity has a very large effect on the heat release and rate of burning in a two stroke engine. Hence power output and engine reliability.

Software exists to give a close approximation of this velocity.

Let's take our above examples and see how squish velocity is changed by a small variance in squish clearance.

The first example showed a change in squish clearance of .2mm. Using 2 stroke software this .2mm change in total squish clearance will increase the squish velocity in a 13.5:1 head by 7.4m/s if this .2mm is removed from the total squish clearance.If this .2mm is added to the total squish clearance, then the squish velocity is decreased by 5m/s.

The 2nd example, showed a squish clearance difference of .38mm
This .38m change in total squish clearance will change the squish velocity by 18m/s when this .38mm is removed from the total squish clearance. If this .38mm is added to the total squish clearance, then the squish velocity is decreased by 8.4m/s.

NOTE: The above MSV calculations were taken from a specific head design. Since MSV has many determining factors, the changes in MSV could be much less or much greater than the ones listed above. The overall head design and cylinder port timing determines the magnitude of the MSV changes.

One can see that the relationship between adding and subtracting squish clearance is not linear and does have pronounced effects on squish action.

So, you can see how one needs to be careful when purchasing a new aftermarket head or modifying a stock head. Next time you are talking to a head maker or a shop that may be modifying your head, ask them what the piston deck height is for that engine. If they do not know, how can they design a head for that engine that will have acceptable squish velocity, squish clearance, and compression ratio? OR when speaking with these people, tell then that you have added an extra base gasket to your engine. Ask them how that will effect the "on the shelf" head they want to sell you. I have already showed you what a .008" and a .015" difference in deck height will do to compression ratio and maximum squish velocity.

So... the old adage of shaving off .015" of a stock head can get you into excessive squish velocities in a very big hurry. Excessive squish velocities can lead to piston breakage and severe engine damage.


Is there a benefit to having a smaller or larger Piston Deck Height??

There maybe some benefits to having a large or small deck height.

The one that comes to mind first is in the cooling effects of the engine. For example.. if the deck height is large in the cylinder, then there may be an argument for the end gases retaining more heat due to them being trapped in the cylinder vs. the head. One may argue that end gases trapped in the head portion of the squish band would be subject to the better cooling properties of the head. This would be a hard theory to prove, but it does have merit.

Can the Piston Deck Height be easily changed??

Yes, it can be easily changed.

Below are several methods of altering deck height, which, as I have shown, also alters MANY other operating factors.

1. Changing base gasket thickness
2. Decking cylinder base
3. Decking cylinder top
4. Changing piston
5. Changing crank
6. Changing rod length
7. Changing stroke
8. Altering piston crown

[DariusKTM]

Good stuff.

Do you guys ride that 490 at Kermit?

I have a KTM 550....should have a big bore field day over there one day!

[bryan raffa]

so could you put it in short terms for the head mods and squish band, to get rid of the pre det. problems. I have a 86 490 if you have the specs for that motor...... thanks I pm you kane