Why do 4s start easier when hot in neutral?

[Casper250]

In my race yesterday I lowsided and fell. Bike stalled and it took me about 20 seconds to get started again. For the life of me I couldn't get the motor to fire while in gear. Tried with the hot start, w/o hot start and nothing. I put the bike in neutral and it fired right up.

I know that 4s are a PITA to start when hot but what does being in gear have to do with it? Does the drag of clutch effect starting that much? I can start the bike in neutral with the clutch engaged which means it's spinning the gears in the tranny. Does the ignition timing change that much?

Also, do 4s not start when hot because they are getting flooded or not enough fuel? The hot start button acts like a choke so I was thinking that maybe a couple twists of the throttle might help when hot.

 

[CaptainObvious]

I know that 4s are a PITA to start when hot but what does being in gear have to do with it? Does the drag of clutch effect starting that much? I can start the bike in neutral with the clutch engaged which means it's spinning the gears in the tranny.

You answered your own question there. Clutch drag is significant, even on a warm motor.

Also, do 4s not start when hot because they are getting flooded or not enough fuel? The hot start button acts like a choke so I was thinking that maybe a couple twists of the throttle might help when hot.

They flood because when the bike tips over, fuel is vented from the carb. This causes the float to drop a little and fuel begins to run, and run, and run...

The hot-start lever actually changes the cam timing. Don't give the throttle a twist. You have a pumper carb so you are making an already flooded engine worse.

 

[Casper250]

The hot-start lever actually changes the cam timing.

Are you sure about that? On my CRF, the cable goes right to the carb, it's not a decompression lever.

 

[Rich Rohrich]

The hot-start lever actually changes the cam timing..

The hot start lever just opens a passage in the intake that leaks air directly into the intake tract to lean it out. It has nothing to do with the cam timing, and it's not a choke. :cool:

 

[CaptainObvious]

Opps, I was typing without thinking (must have been thinking about my '81 GN400). I believe that the decompression circuit is automatic and the lever does affect the carburetor air/fuel mixture.

EDIT: and now I see that I've been corrected by the master. How embarrassing :(

 

[Casper250]

So if its the engine flooding, why does is effect a 4s more then a 2s? Same with the clutch drag, why does is effect a 4s more then a 2s? On my 125 I could start it gear.

 

[Rich Rohrich]

So if its the engine flooding, why does is effect a 4s more then a 2s?

Four-strokes have a short intake tract that leads directly into the cylinder when the intake valve is open. Two-stokes only have their reeds open when the engine is running, so when it's stalled the reeds are shut effectively closing off the conduit between the carb and the engine. You also have the carb connected to the crankcase rather than directly to the cylinder so it will be mpore tolerant of excess fuel.

Same with the clutch drag, why does is effect a 4s more then a 2s? On my 125 I could start it gear.

You have more static drag in a four-stroke. You are turning the cam chain, which has to overcome the valve spring pressure and the additional mass of the crank . Add in the drag from the clutch and you have a lot to turn. Getting it to spin at a high enough speed to light with the kickstarter is tough unless you reduce some of the drag and putting it in neutral is the easiest way.

 

[Casper250]

You know what would be a good idea, when you pull the hot start lever, somehow it would lock out first gear so you could click down on the shifter till you cant go any further and end up in neutral. Neutral is so much harder to find when you are in a rush too. It would be like killing two birds with one stone.

 

[YZ450Rider]

I put mine in neutral every time I need to crank it and it fires right up. If it's in gear, not so easy... '03 YZ450

 

[Rich Rohrich]

You know what would be a good idea, when you pull the hot start lever, somehow it would lock out first gear so you could click down on the shifter till you cant go any further and end up in neutral. Neutral is so much harder to find when you are in a rush too. It would be like killing two birds with one stone.

The transmission has enough points of failure as it is. Adding another one seems like a bad idea. :yikes:

Just get a Rekluse clutch and you'll never have to worry about stalling it again. :cool:

 

[Vic]

Could something as simple as a reed valve assembly in the intake tract of a four stroke help solve the "crashy, no starty" problem?

 

[Rich Rohrich]

Could something as simple as a reed valve assembly in the intake tract of a four stroke help solve the "crashy, no starty" problem?

Possibly. Maybe one of the guys who owned a reed valve equipped Honda XR-500 can weigh in. :cool:

 

[ironworker]

If you notice when watching SX or MX pro's drop their bikes either the bikes remain lit or they fire up on the first kick about 80% of the time, this is probably because they are using some sort of auto clutch system. Or maybe i don't know sh*t which is probably the case.
I wonder if a Hydraulic clutch would help you in this area?

 

[Vic]

fire up on the first kick about 80% of the time

I don't know if it's 80%, but when they fire right up it's because the rider followed the proper starting procedure, or something close to it. :clue:

 

[Jaybird]

My 444 is amazing for staying running whilst I roll in the dirt. I think this is due to the flywheel weight.

If my timing chain ever gets here, I will mount up my new decompression cam and see how it works after I'm exhausted and covered in dirt.
I would already had a Rekluse on the beast, but they aren't gonna, won't, can't...(choose one) make one for my bike. (bummer)

 

[Vic]

If the hot start lever were used to de-activate the accelerator pump, the rider could just hold the throttle wide open when hot starting. I think this would be easier for them than remembering to not twist the throttle at all.

 

[Rich Rohrich]

Good idea Vic, lets make the bikes MORE complicated because some stooge can't keep his hand off the throttle when starting :)

 

[Casper250]

Do any of the electric start 4s have this problem starting or does the starter motor spin the engine fast enough and so many times that it's not an issue?

 

[Vic]

lets make the bikes MORE complicated because some stooge can't keep his hand off the throttle when starting

Hey, if that's what it takes.

If increased complexity were a concern, we'd all still be riding two strokes. :)

Besides, I don't think it would be that complicated.

Aren't you the guy that doesn't like auto-decompressors either? :ohmy: