Does a 4 stroke fire every crank rotation or every second rotation???? Please help

[steveo3318]

Ok I have a 85 atc70 with a 86 trx70 cdi motor. while I have the top end off getting ready to get the gaskets off I noticed my son was rotating the crank. Well I know I had the crank at TDC compression stroke before I removed it but I have no idea how many times he turned it or which way. So heres my question, Does the crank trigger fire every rotation, or every second rotation? If it fires every single rotation I know I can just put it on the T mark and get my cam O on the notch and I am good to go. but if it fires every second rotation its possible that I get the cam wrong even with the marks on. Please help

[jonolanracin]

4 strokes fire every other time the piston comes up . a 2 stroke fires every time

[steveo3318]

Thank you Jon but if this is true how do I know which rotation is the stroke that fires before I put the head back on? I see tons of conflicting information on the web saying it fires every rotation

[yaegerb]

Set your cam so both lobes are pointing down, then align the "O" on the notch to the T on the flywheel.

[steveo3318]

Thanks yall. I did finally find it is a wasted spark setup and it fires every revolution. Again thank y'all so much

[Scootertrash]

I'm
Thank you Jon but if this is true how do I know which rotation is the stroke that fires before I put the head back on? I see tons of conflicting information on the web saying it fires every rotation

Are you following the service manual as you reassemble the engine?

[DohcBikes]

I like turtles.

[steveo3318]

Yes I am following the service manual but it says nothing about if it fires every revolution or every second revolution like a typical four stroke. Thanks again.

P.S. I like turtles too

[DohcBikes]

Actually most 4 strokes use a wasted spark ignition. You're getting spark and explosion confused and quite frankly I find it irrelevant but entertaining.

[steveo3318]

Glad my ignorance can give you a smile this morning. I understand now the bottom end doesn't care as long as it's on the T mark. If you would like any more ignorance or smiles please just ask me a question I will give you the best wrong answer I know.

On a side note 4 out of 5 atv mechanics I know said it fires every other revolution. The 5th guy was just honest and said he wasn't sure. It's like the wasted spark ignition is a mythological beast.

Keep your turtles herded up sir lol

[yaegerb]

Ok I have a 85 atc70 with a 86 trx70 cdi motor. while I have the top end off getting ready to get the gaskets off I noticed my son was rotating the crank. Well I know I had the crank at TDC compression stroke before I removed it but I have no idea how many times he turned it or which way. So heres my question, Does the crank trigger fire every rotation, or every second rotation? If it fires every single rotation I know I can just put it on the T mark and get my cam O on the notch and I am good to go. but if it fires every second rotation its possible that I get the cam wrong even with the marks on. Please help

In case you are interested. 1-4 below, are the cycles of a four stroke. A four stroke motor will spark every two revolutions of the crank.

1. Intake:
This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing vacuum pressure into the cylinder through its downward motion.

2. Compression:
This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.

3. Combustion:
This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. (the end of the compression stroke) the compressed air-fuel mixture is ignited by a spark plug (in a gasoline engine) or by heat generated by high compression (diesel engines), forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft.

4. Exhaust:
During the exhaust stroke, the piston once again returns from B.D.C. to T.D.C. while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust valve.

[RIDE-RED 250r]

Many modern small 4-stroke engines fire every revolution and have a "wasted" spark at/near TDC of the exhaust stroke. Older 4-strokes that use the camshaft to time ignition, IE cam driven breaker point ignition setups like found on the old cast iron Kohlers and other similar setups will only fire at/near TDC of compression stroke. Other examples of 4-stroke engines that do not have a "wasted" spark are automotive engines that use distributors driven by the cam.

That said, many modern small 4-stroke engines use an ignition system that is timed by the crank via the flywheel. Many use a one piece ignition module like found on modern Briggs and Kohler lawn and garden tractors and such. Ignition is simple, the magnet on the flywheel sweeps by the externally mounted magneto/ignition module (whichever you prefer to call it) and ignition fires. No stators, pulse generators, or breaker points needed..all in one tidy package including the spark plug lead. This method is most common on modern lawn and garden 4-strokes and has been for at least a decade or so, maybe longer.

Basically what you want to look for to figure out if you have a "wasted" spark ignition or not is what controls the ignition timing...crank or cam.

[DohcBikes]

Glad my ignorance can give you a smile this morning. I understand now the bottom end doesn't care as long as it's on the T mark. If you would like any more ignorance or smiles please just ask me a question I will give you the best wrong answer I know.

On a side note 4 out of 5 atv mechanics I know said it fires every other revolution. The 5th guy was just honest and said he wasn't sure. It's like the wasted spark ignition is a mythological beast.

Keep your turtles herded up sir lolGreat reply :) Nah it's not ignorance you just didn't have the info and i'm busting your chops a little.

It's actually a really cool topic when you start talking about multi cylinder engines. Harley Davidsons sometimes use a specialized single fire ignition that requires 2 sets of pickups and two outputs for each coils as well as some magic stuff I don't understand yet in the coil itself. The theory is that because of the unusual timing of the two cylinders, that produces the trademark sound that I hate, a wasted spark could potentially ignite any mixture that might be lingering in the cylinder and create pressure in on the piston's up stroke, reducing overall output and efficiency. With the single fire coils, there's no chance of this happening.

4 out of 5 of those mechanics were stuck in the thought that the mixture is ignited every other revolution, but chances are if you asked them again if the spark plug sparks on every revolution they might change their answer. I hope. If not, start calling different mechanics. :lol:

The 2 guys above know their snizz and have provided good info as usual. At MMI they repeated intakecompressioncombustionexhaust so many times that if you woke me up from a dead sleep and asked me how an engine works it'd be the first thing I said without a question.

[steveo3318]

thanks guys and i truly appreciate all the help and answers. these little bikes are a ton of fun and i cant let them rot. i must save them. lol

[Dirtcrasher]

Thats the one thing people don't understand. The crank and cam don't know where they are. Which is why you couldn't shim a modern 4 stroke as fast. They want you to install the cam at T" and the cam chain/tensioner also and torque the bolts, take a measurement, change the shim, check it out and repeat.
The very 1st one I did I laughed at myself; I thought "You idiot, the crank has nothing to do with this, leave the damn chain off!".
So I just popped the cams in as my buddy watched in horror saying I didn't follow the manual!
If you understand it, you don't have to.... Any degree of dwell or "the time the cam doesn't do chit!!" is where you adjust the valves.
A two stroke is actually very more complex as to timing of the port windows and their height, etc etc etc.

I was afraid to shim a 4 stroke 12 years ago when they came out to compete with the 2 strokes. And I realized how easy it really is.....

[RIDE-RED 250r]

Simple in theory, yes... but when it comes to execution, shims are a PITA compared to good ol adjustable rockers..

[wonderboy]

To sum it up:

* If the ignition trigger is on the flywheel, you'll get a spark on every crank revolution (350X for example)
* If the ignition trigger is on the cam, you'll get a spark on every 2nd crank revolution (200S for example)

4-strokes only need a spark every other revolution (at the top of the compression stroke) but the 350x method (wasted spark) keeps things mechanically simple.