Why do 2 strokes require more compression than 4 stroke...Head gasket, crankcase gkst

[Rustytinhorn]

Ok, here it goes.

I heard that 2 strokes require more compression than 4 strokes to run. Why us this so?? Please give me a detailed and technical answer if you have one.

Also when my compression is this low should it make my bike run crappy or just with less power. My reasoning behind this is that my Atc200 is at 120psi, doesn't have a lot of power, but it runs smooth, never misses a beat. On the other hand, my 250r (also running at 120psi) it runs really blotchy, pulsing, sputtering. (I am still working on my jetting as it is running a touch rich) Will this low of a compression in a two stroke make it run crappy, or should it still run smooth, just with less power??

Disclaimer-before anyone says anything, please just answer the question at hand and not try and diagnose my bike, as I have already gone through the proper checklist of things. i.e. air filter, exhaust, reeds, plug chops, carburator, petcock, fuel, oil, etc... This is not intended to be mean as I do appreciatte any help, it is just meant to get everyone up to speed and on the same page so we can aviod the "beginners engine diagnosise 101" stuff.

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From all the reading i've done i've decided I want to replace my crankcase gaskets, or at least the left side to be safe. This is the seal or that is in the crankcase assembly itself right next to the bearing, correct? Is there any easyway to do that in a day, or will I have to remove the engine, split the case etc...If some one could give me some insight on this it would be greatly appreciated.

Thanks for the help

[84honda200s]

ok i will try my best here. a 2 stroke needs more compression because it requires more fuel to keep the rpm's up. a 2 stroke relies on its high rpm range to get up and go.

the compression compresses the fuel and air. if you have poor compression (bad rings are one of the biggest issues on a pinger) and when this happens the fuel leaks either threw the head gasket, or pass the rings with the compressed air. when this happens the engine cant make its high rpm range to get you going.


im not to good with words but im hoping that will help a little bit. basically it comes down to a 2 stroke needs more fuel and air to run at its high Rs. to make the high air and fuel burn the way it should it needs to be compressed more and faster than a 4 stroke.

[Billy Golightly]

They don't require more compression then a 4 stroke. Do they make more power with more compression? Yes, for short periods of time until the heat generated by the extra compression catches up to it and then it drops of big time. Thats why flat track, desert racers, etc anyone thats running high speeds for long times lowers compression. Drag racers on the other hand might bump the compression up considerably because the distance they are going isn't really long enough for the heat to take effect under normal conditions.

Theres limits both directions, up and down. My flat tracker has got about 130lbs or so, and it runs perfectly as long as the carb jetting is correct and that there are no airleaks in the cylinder/crankcase.

[Dracus124]

Because 2 strokes pull gas upwards

4 strokes pull the gas down wards

[Dirtcrasher]

Your left crankseal is a piece of cake to replace. You'll just need a flywheel puller. I don't see any major differences in compression from a 2 stroke to a fourstroke. They both do the same job, they compress a mixture of atomized fuel and air and add a spark to force the piston downwards.

It's not a diesel..... 4 strokes is just another way of turning a crank, thats it.

[deathman53]

2 strokes don't need more compression to run, the reason why 4 strokes seam to have less compression is because they have compression releases in the cam shaft(that only work when starting up). By the way, most of the time, you need ~170 and up compression for a motor to run right, 120 is low for almost any motor.

[toocheaptosmoke]

on a side note, what kind of compression tester are you using? I have seen huge differences in numbers from guage to guage. I have a cheap tester with the long flexible hose, it has never read over 125, and usually reads 90-110 psi. I think it's just a design flaw with the hose or something that gives low readings. I've heard of people rebuilding because of perceived low compression only to find out the guage was crap

[Tri-ZNate]

there isnt a difference in compression really. Hey if you want to see it from this perspective...

1983 Honda 250r - 6.7:1 compression raatio
1985 250sx - 9:1 compression ratio

[SWIGIN]

theres a difference in cyl pressure and compression


hes asking about a cly thats at 120psi (cly pressure) wich has nothing to do with the compression of that motor. for all we know it could be a worn out 12 to 1 or something.

i think hes a little confused

[Rustytinhorn]

on a side note, what kind of compression tester are you using? I have seen huge differences in numbers from guage to guage. I have a cheap tester with the long flexible hose, it has never read over 125, and usually reads 90-110 psi. I think it's just a design flaw with the hose or something that gives low readings. I've heard of people rebuilding because of perceived low compression only to find out the guage was crap

Thanks for all the advice so far guys....

I am using an Equus Innova brand compression tester I got from checkers or autozone for $25 or $30. It has a bunch of adapters and a 15inch rubber hose along with the guage.
I know it operates up to 150 psi, as I have used it on my BW. My '85 250r is only reading at 120psi. This is the second time I checked it several weeks apart with WOT and a hot engine and whatnot. I added about 4 Tbls of 30w oil to the cylinder and tookl another reading and it only went up to about 135psi. I think I have a blown headgasket. It appears to be the original. A bad headgasket would also explain why everytime I fill my resevior tank up with coolant, the bike will spit it all out the overflow after about 10 minutes of riding. So I have a new gasket kit coming from rockymountain. So I will install that and see where that gets me.

Also I can see where you are coming from on the "long hose" problem. It seems that most likely attatching a 15" hose would lower your compression unless the manufacturers compensated for this by slightly lowering the numbers on the gauge. When the hose is attatched to your cylinder, your displacement is increased due to the fact that the given amount of air that you can normally compress in your cylinder is now also required to fill the void of the hose. Although I don't know how much of a difference it actually would make.

As far as the lefthand crankcase seal, do I have to split the case to get to it or...????
Thanks again

[Rustytinhorn]

on a side note, what kind of compression tester are you using? I have seen huge differences in numbers from guage to guage. I have a cheap tester with the long flexible hose, it has never read over 125, and usually reads 90-110 psi. I think it's just a design flaw with the hose or something that gives low readings. I've heard of people rebuilding because of perceived low compression only to find out the guage was crap

Thanks for all the advice so far guys....

I am using an Equus Innova brand compression tester I got from checkers or autozone for $25 or $30. It has a bunch of adapters and a 15inch rubber hose along with the guage.
I know it operates up to 150 psi, as I have used it on my BW. My '85 250r is only reading at 120psi. This is the second time I checked it several weeks apart with WOT and a hot engine and whatnot. I added about 4 Tbls of 30w oil to the cylinder and tookl another reading and it only went up to about 135psi. I think I have a blown headgasket. It appears to be the original. A bad headgasket would also explain why everytime I fill my resevior tank up with coolant, the bike will spit it all out the overflow after about 10 minutes of riding. So I have a new gasket kit coming from rockymountain. So I will install that and see where that gets me.

Also I can see where you are coming from on the "long hose" problem. It seems that most likely attatching a 15" hose would lower your compression unless the manufacturers compensated for this by slightly lowering the numbers on the gauge. When the hose is attatched to your cylinder, your displacement is increased due to the fact that the given amount of air that you can normally compress in your cylinder is now also required to fill the void of the hose. Although I don't know how much of a difference it actually would make.

As far as the lefthand crankcase seal, do I have to split the case to get to it or...????
Thanks again

[BigGreenMachine]

Do not ride with a leaking headgasket. Antifreeze will seize your motor because it washes the oil off the cylinder wall until the piston seizes or the rings score the cylinder wall.