Hard starting when hot

[jkv45]

Our 1990 Ninja 250 beater is having an odd problem.

When it's hot it doesn't like to start with the starter, but it bump-starts fine.

Gas is fresh. Carbs are clean and adjusted - idles fine hot or cold. Valves recently adjusted. Battery cranks strong (for a while) but it doesn't want to start. Tried opening the throttle slightly, doesn't help.

It will bump-start immediately even if you've cranked a bit and the battery is wearing down.

Idles and runs fine after it starts.

Ideas?

[csmith12]

Air filter status? Any debris in the airbox? If those are good, you most likely have a pressure/vacuum problem. Check your carb's/tank's vent tube to ensure it's not clogged. Carbs/tanks need to vent or hot temps can do strange crap to the vacuum pressures inside. In turn, makes for hard hot starts. Had an old honda that was terrible about this.

[jkv45]

Originally Posted by csmith12

Air filter status? Any debris in the airbox? If those are good, you most likely have a pressure/vacuum problem. Check your carb's/tank's vent tube to ensure it's not clogged. Carbs/tanks need to vent or hot temps can do strange crap to the vacuum pressures inside. In turn, makes for hard hot starts. Had an old honda that was terrible about this.

Thanks Chris, I'll have my son check into it. It had pods on it, and they have been cleaned recently. The carb have been off and cleaned not long ago and it runs great otherwise. My son it all over it mechanically, and has gone through everything short of pulling the head. Spends most of its life near redline. Looks like crap, but runs like a top except for this issue.

We did just pull the tank and clean/coat it inside. It had been doing this before that though.

We leaned the idle mixture, but it didn't fix the problem. Pilot is stock, 2.5 turns out (or very close) on the screws. Pretty sure the carbs have been synced lately, but I should check.

It almost seems like it could be an electrical or spark issue because it doesn't even sputter when you hit the button but it lights-up the second you let the clutch out at walking speed.

Any ideas about that angle?

[csmith12]

Well sure, there are 3 safety switches on the 250's. Clutch, neutral and sidestand. Check the clutch one first obviously, then neutral. Bypass each one separately until you find your issue and then correct it. Electrical contacts getting hot and not working is kinda common or a simply cleaning may do just fine. So you might have just found your issue, I am crossing my fingers for you.

[Lazarus]

Starter? Or maybe the solenoid? Maybe there is some sort of short/faulty wire between the battery-switch-solenoid-starter? (Not sure if that is the correct order for current flow)

It doesn't crank at all when using the starter? Not even a sputter?

[csmith12]

Originally Posted by Lazarus

Starter? Or maybe the solenoid? Maybe there is some sort of short/faulty wire between the battery-switch-solenoid-starter? (Not sure if that is the correct order for current flow)

It doesn't crank at all when using the starter? Not even a sputter?

I thought of that too. When I was dirt poor, we bought ourselves some time by tapping the starter solenoid with a hammer to get it to fire. But, by his wording I assumed the engine was turning over when he hits the starter button. Sometimes people get confused by wording when "cranking" (only starter turning) vs "engine turning over" is used. I don't think that is the issue here. Could be, good call. The safety switches would be in that "flow" you mention.

[Lazarus]

^welp there goes my helpful input haha back to my drawing bored

[choneofakind]

What kind of voltage is the battery showing while cranking?

Does it start better with a little throttle?

Any vacuum hoses that might be suspect? What about intake runners?


General question: do ignition coils tend to get weaker as they get hot? Spitballing here.

[csmith12]

What do you mean Rj? Hot start issues are kinda a pain to diag, but really boil down to a few things.

12v Current/Spark problems
Fuel flow
Vacuum issues
Valve issues

[csmith12]

Originally Posted by choneofakind

What kind of voltage is the battery showing while cranking?

Does it start better with a little throttle?

Any vacuum hoses that might be suspect? What about intake runners?


General question: do ignition coils tend to get weaker as they get hot? Spitballing here.

Nice add Chris! A hot battery not performing as it should. I haven't had this myself, but others have. I guess you could add a hot ecu to that as well.

[Lazarus]

O I'm just messing around, just stating that I'm going back to the drawing board to try and find another avenue that may help (if it turns out that the safety switches/starter/solenoid/ etc isn't the problem.)

Although, these bikes have been around so long that the issues have been well documented. Not that a new one couldn't arise.

[juliusmichaelhonrada]

i thought a very rich condition will crap out an engine when hot but starts so easy when cold , check your choke circuit condition.

i have a 1987 ninja 250 it wont start without choke even though the pilot size #48 the reason is cylinder #2 has very low compression and the starter motor can only spin the engine but not enough for cylinder 1 combustion to keep the engine from stalling , so with choke the rpm will rise because of the leftover fuel runaway effect , my idle is 4000 rpm. it looks like a carb problem but its a mechanical issue

[Motofool]

Originally Posted by jkv45

.......... Ideas?

The main difference between starter and push is the rotational speed of the crankshaft.
Higher speed increases compression, if marginal due to leaking valve or worn/dirty rings.

A marginal spark when hot may be caused by:
- Ignition coils going bad or marginal wires/connections, as materials that are classed as conductors tend to increase their resistance with an increase in temperature.
- Insulator parts of spark plugs and high voltage wires/boots are liable to decrease their resistance with an increase in temperature.
- The gap between the generator exciter and of the pick up coil may change with temperature.

Fresh air that goes through hot carburetors expands, while fuel expands less dramatically.
For that reason, hot mix tends to be lean, which ignites and burns slower.
Opening the throttle even more than just cracked when starting hot may induce the proper mix for starting.

Just some ideas ......

[jkv45]

Originally Posted by choneofakind

What kind of voltage is the battery showing while cranking?

Does it start better with a little throttle?

Any vacuum hoses that might be suspect? What about intake runners?


General question: do ignition coils tend to get weaker as they get hot? Spitballing here.

We need to check a few things this weekend, like voltage, hoses, and leaks.

He has tried opening the throttle a little and a lot while cranking - didn't help. It starts right up when bumping with the throttle closed.

[jkv45]

Originally Posted by Lazarus

Starter? Or maybe the solenoid? Maybe there is some sort of short/faulty wire between the battery-switch-solenoid-starter? (Not sure if that is the correct order for current flow)

It doesn't crank at all when using the starter? Not even a sputter?

It does "crank over" (meaning the starter spins the engine) just fine, but it doesn't start or sound like it's trying to start using the starter.

Cold it starts right up with the starter.

[InvisiBill]

Originally Posted by csmith12

Well sure, there are 3 safety switches on the 250's. Clutch, neutral and sidestand. Check the clutch one first obviously, then neutral. Bypass each one separately until you find your issue and then correct it. Electrical contacts getting hot and not working is kinda common or a simply cleaning may do just fine. So you might have just found your issue, I am crossing my fingers for you.

FYI, the starter circuit relay is wired directly to the clutch switch (yellow/green wire at the junction box), which passes through to the neutral switch. If there's a problem with those, the starter button will be completely dead, as if the killswitch had been flipped.

The igniter is wired up to all three safety switches individually. If the starter is working fine through them, the igniter should as well. If there was a problem with the sidestand switch, it may die when you put it in gear and release the clutch.

[InvisiBill]

Originally Posted by Motofool

The main difference between starter and push is the rotational speed of the crankshaft.
Higher speed increases compression, if marginal due to leaking valve or worn/dirty rings.

A marginal spark when hot may be caused by:
- Ignition coils going bad or marginal wires/connections, as materials that are classed as conductors tend to increase their resistance with an increase in temperature.
- Insulator parts of spark plugs and high voltage wires/boots are liable to decrease their resistance with an increase in temperature.
- The gap between the generator exciter and of the pick up coil may change with temperature.

Fresh air that goes through hot carburetors expands, while fuel expands less dramatically.
For that reason, hot mix tends to be lean, which ignites and burns slower.
Opening the throttle even more than just cracked when starting hot may induce the proper mix for starting.

Just some ideas ......

I think this is a good analysis of the situation. If it still bump-starts fine when it's hot, then the issue must be with something that's specific to the starting system itself, as opposed to a general problem causing the engine to not run. If the starting system works fine when it's cold, then the problem seems to be heat-related. Combining those two logically, it would seem it has to be some heat-affected part of the starting system.

The comment about the difference in crankshaft rotation speed points out another difference, but it doesn't sound like the engine is "marginal" so it doesn't seem like that would be the cause of the issues. Maybe a combination of issues, like low battery voltage or poor connections resulting in the starter not spinning as fast as it should, on top of slightly bad compression? Basically multiple small barely-issues combining to be enough of a problem to become evident.

[juliusmichaelhonrada]

Originally Posted by InvisiBill

I think this is a good analysis of the situation. If it still bump-starts fine when it's hot, then the issue must be with something that's specific to the starting system itself, as opposed to a general problem causing the engine to not run. If the starting system works fine when it's cold, then the problem seems to be heat-related. Combining those two logically, it would seem it has to be some heat-affected part of the starting system.

The comment about the difference in crankshaft rotation speed points out another difference, but it doesn't sound like the engine is "marginal" so it doesn't seem like that would be the cause of the issues. Maybe a combination of issues, like low battery voltage or poor connections resulting in the starter not spinning as fast as it should, on top of slightly bad compression? Basically multiple small barely-issues combining to be enough of a problem to become evident.

Yep , to confirm a low compression issue , you can check the spark plug color a low compression cylinder will always make the spark plug black

[juliusmichaelhonrada]

hey jkv45 , are you the same dude on forum.ninja250.org lol