Why are 4-Strokes Hard to Start??

First of all, I'm a newbie and I'd like to say Hi to everyone on the board. I've been reading this board for a long time now and figured it's finally time to register. Being an original 2-stroke guy, I'm not that educated on the 4-stroke technology, but my next bike WILL be a 4-stroke, and I want to learn about them before I make my purchase.

Anyway, on to the question.. Why are the 4-stroke engines so hard to start? And why are they even harder to start when they're hot? I understand they have very high compression, but figured that would only cause trouble in kicking them over, not how they actually start.

it's mostly high compression but most of the modern 4-strokes have decompression things.

If I choose to kick it mine starts in 1 or 2 kicks hot or cold

Why is it so hard for it to catch though? And why is it harder when the engine is hot? Anyone have a technical explanation for this?

its not that hard after you ride the bike for a while and get used to what it needs to start it...like mine when its hot i don't give it any gas and 1 or 2 kicks gets it going, and for a cold engine i just hold the throttle 1/2 open and kick it with no choke and that takes about 2 or 3 kicks. but when i start someone else's bike it takes a while to find out what it needs. i hope that helped

First of all WELCOME SHIFT23. A four stroke has much higher compresstion which doesn't allow for brisk kick throughs. Its kind of like a ritual that becomes easy if you know what to do. Every four stroke old and new that I have started require that the rider do the following.

1. do regular stuff (turn gas on, choke it Ect)

2. slowly turn kickstarter a couple times until you get a hard resistance, this is when the piston is at top dead center.

3 When the piston is at tdc bring the kickstarter all the way back to the top and give one hell of a mighty kick. Most four strokes will fire right up

Hope you have a good time here on thumpertalk!!

welcome shift!!! just like evo rider has already said, you need to get to that hard point, and then kick all the way through.... nice and strong... use your wieght to your advantage!!

They aren't hard to start. Get a better leg. No, scratch that, my 450 is easier to start than my YZ250 was.

The reason you see a lot of people having a hard time getting them going (especially 125 riders) is that they try to start them like a 125, with little sissy jabs at the lever. You don't have to kick a thumper hard, or fast, you just have to kick it ALL THE WAY THROUGH. Kick it properly once, or 100 times while looking like a dork - you decide. Its one of those "slow down to go faster" things.

They are harder to start when hot because their engines run a lot hotter than 2 strokes. That's what the hot start trigger does - it opens an air passage to the carb that allows cooler, denser air into the carb to help the engine start.

They aren't hard to start. Get a better leg. No, scratch that, my 450 is easier to start than my YZ250 was.

.

He is correct. It is all about technique. If you have always ridden a 2 smoke you are used to its routine. You just have to learn a new routine. One HUGE no no on a 4 stroke is you DO NOT give it ANY gas when you try to start it hot. It will immediately flood. Why do they flood so easily?... because they have accelerator pumps, just like a carburetor on a car.

Most all modern four-strokes are easy to start, and if they're not its because of improper valve clearances. Back in '98 and through '02 four-strokes (YZF's) were pretty difficult to start if you didn't know how to. You had to follow a certain procedure to get them to fire because they had manual compression release.

With the release of the '03 models (YZF...the CRF450 has always had an auto-decomp cam), Yamaha put in an auto-decompression cam that did all the work for you to allow easier starting. All you had to do was bring the bike to TDC, and give it a full kick. It is possible to put an '03 YZF cam in older models...this is a popular modification. When they're hot, you use the "hot start" trigger to help it fire as easily as when its cold. When you pull in the hot start, it allows cold air to leak into the intake tract. This is a big plus on a hot four-stroke engine because it takes oxygen to ignite the fuel, and a hot combustion chamber is almost completely devoid of oxygen.

And why is it harder when the engine is hot?

When you've been out riding and the engine is fully warmed up, or even to the point where it is hot...your four-stroke will STILL be easy to start if you maintain your bike regularly. All modern four-strokes have a "hot start" that leaks air into the intake tract to allow for easier starting. How? Because you need oxygen to ignite the fuel, and when your bike is hot...the combustion chamber is really hot, and almost completely devoid of oxygen.

The hot start allows oxygen into the combustion chamber to help it start easily.

They aren't hard to start. Get a better leg. No, scratch that, my 450 is easier to start than my YZ250 was.

they really arent hard to start... as long as you know how to... I was able to start my dads 250 when I was 14... even the guy at the track couldn't do it... but I could :D

Yup. Usually they don't wish to start from compression. People do not get full, strong kicks.

The old YZF's were nightmares, as they, as was stated, had no automatic decompressor. KTM has had an auto-decompression cam since 2000. Honda, when they came out with the CRF450, put one in theirs. It took Yamaha a LONG time to do something about an automatic decompression cam.

Into some of the working of the cam, essentially, at low (far less than normal idle) and no RPM's (dead engine), a weight on the cam (in KTM's case, it was, probably still is) at the cam gear. This added some thickness to one of the exhaust cams, which, when rotated, would not allow the one valve to close fully. Therefore, air would be able to pass through the valve and allow a clean easy kick. When the engine started, centrifugal force threw the weight out, and lifted the "extra cam" out of the way to allow normal engine operation.

My 520, I could start with my hand. It didn't even have a hot start. I removed the manual decompression lever just for a cleaner look....I never needed it.

My 450 can be a bear - but when the jetting is on, it will start first or second kick, hot or cold.

Jetting, as well as "technique" is very important to firing a four stroke.

it's all about knowing your bike, i used to ride a XR-600 with a 630 bore kit and a huge 42mm flat slide carb, when i first bought it i was haveing alot of trouble getting it going...10 - 15 kicks and on a 600 thats alot especially if your on a hill..

but after i understood what it likes and doesn't like in the starting proceedure i was starting it first kick 2 kicks max, hot or cold...the only bikes ive had trouble with was the old wr 400,s 426's when they were hot, before they come out with a hot start button, but my mate just ran a hotter plug and it started alot better.

if u ride and get used to a bike and how it starts they arn't hard to start at all.

i just got a 06 ktm 525, so i got the magic button, and startin g is not an issue anymore..yay

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06 ktm 525 exc

99 gsxr 600

z- 50

xr 600 ....going soon

When you pull in the hot start, it allows cold air to leak into the intake tract. This is a big plus on a hot four-stroke engine because it takes oxygen to ignite the fuel, and a hot combustion chamber is almost completely devoid of oxygen.

That's the reply I was looking for!!

Thanks for your help guys!

No problem. After all, this is the place to learn about four-strokes. :D

Except that you bogied that directly out of MXA.

All modern four-strokes have a "hot start" that leaks air into the intake tract to allow for easier starting. How? Because you need oxygen to ignite the fuel, and when your bike is hot...the combustion chamber is really hot, and almost completely devoid of oxygen.

The hot start allows oxygen into the combustion chamber to help it start easily.

I thought the carb supplied oxygen to the engine, along with fuel. :D The hot start bypasses the carb leaning the mixture. Most usefull when the bike has been dropped and flooded. Not all modern 4 strokes have them.

Nobody has yet hit on the 2 big things. A 2-stroke fires every revolution. A 4-stroke fires every other revolution. That is why you need to use a strong full kick to make sure that it makes it through a firing stroke. Another big thing is 4-strokes have accelerator pumps in the carbs. Any twisting of the throttle will shoot gas into the engine, which definitely will flood a hot engine, this can be a problem for someone used to a 2-stroke where you normally hold the throttle open to start them.

Thanks

Mike

I thought the carb supplied oxygen to the engine, along with fuel. :D The hot start bypasses the carb leaning the mixture. Most usefull when the bike has been dropped and flooded. Not all modern 4 strokes have them.

Please name one modern four-stroke motocross bike that doesn't have some kind of hot-start.

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