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Discussion Starter #1
Ive suddenly developed problems starting my engine in the mornings. It started after i completely didnt ride for a day. 1 of 3 things can happen:

1)Engine fires up ok but idling speed suddenly shoots up to 3k revs. Idle speed knob doesnt help but choke helps brings the revs down. Try blipping the throttle & the engine croaks (croaking sound). Persist with it & the engine dies.

2)Engine fires up ok. Try blipping the throttle & the engine croaks. Give it a bit of choke & the engine will rev quite nicely as it should be. Release the choke & try blipping & the engine croaks. Try maintaining that throttle level & the engine croaks out.

3)Today it took 6 kicks to get it to start & idling was super low & the engine died. Pull the choke & kick again, the engine starts with 3k idling revs & we're back to 1) above.

Even when the idling speed seems to hv stabilised, when i try to move off, the throttle input causes the engine to croak & i will immediately stall.

But once the engine is at a certain temperature, much warmer then when i usually start moving off anyway, all these symptoms disappear & the bike behaves normally.

Try starting the bike up at after work & you will get all these cold engine symptoms again.

How did my carb settings suddenly change? Time to give carb a wash? Putting the fuel tap to "off" for overnight parking doesnt help. So its not a flooded carb case.
 

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Swamp Rat Rider
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Definitely sounds Cold Natured which some Bikes naturally are, My Old Ironhead Sportster was an Olympic Event to Kick Start in the Morning if was below 50 degrees .. Possibly your carb may be on the lean side if have to use the choke the way you are describing ..
 

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American Legion Rider
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Could it be a loose carb boot connection that after it warms, it expands enough to seal it again?
 

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Discussion Starter #6 (Edited)
Thanks everyone for replies.

@gatorjoe: I did get the mech to tune down the carb for better fuel economy. No problems sacrificing a bit of power since i dont do track but mostly urban riding. Joyriding on the expressways maybe once or twice a week. Previously i wld do abt 150Km before reserve, less then 20km/l. Now I can do abt 200 - 210km before reserve. So abt 25-26km/l. So yes the mix is leaner. But no problems before. Only started recently.

@ninjarebel: I have suspicions of dirt/blockage. You might have read or heard abt the haze in south east asia due to large scale agricultural burning in Indonesia. The Singapore pollution index is quite high (unhealthy range) tho still not as bad as some places in Indonesia or Malaysia where visibility can be less than 50m. Since the haze became an environment issue recently, i theorised the polluted air might hv caused my bike to become 'sick'. The haze has caused the daily temperatures to drop by 3-4 deg C. So its a bit cooler albeit more stuffy.

@angel - Honda NSR 150. A 2 stroke sportbike.
 

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Gone
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In colder weather the engine has to run longer to get warm. Start it using the choke and let it warm up before turning the choke off. Don't mess with the throttle while the choke is on. You'll know when it is warm enough when you can add throttle with the choke off.
 

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ZAMM Fanatic
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Is this your first winter with this bike perhaps?

Some bikes are cold blooded. My Ninja750 had to warm up FOREVER before it ran properly. I'd have to adjust the choke several times to keep the idle from climbing to neighbor-alarm levels.

But let's discuss what's going on:


1)Engine fires up ok but idling speed suddenly shoots up to 3k revs. Idle speed knob doesn't help but choke helps brings the revs down.

An engine is THROTTLED by limiting the amount of air it can suck in. The idle screw may hold the main throttle plate open a tab, OR control some sort of limited air bleed AROUND the throttle.

FWIW, modern, fuel-injected cars ALL have a stepper-motor based, computer-controlled idle bypass valve because maintaining idle is A) so difficult under varying conditions of air temperature, altitude, fuel grade, etc., B) So important towards meeting emissions goals.

So your bike, when the idle skyrockets, is getting too much air. How does that happen. Well, your throttle cable could be sticking a little. Or your throttle shafts are a little worn. Or your intake boots are not sealing, they're cold and hard (they can be softened in an overnight bath of zylene and wintergreen oil). Once they warm up, the leak goes away, and everything operates normally. Or your intake manifold gasket has a leak. I'm sure I haven't listed EVERY possibility, but these are common.

You can often FIND an idle air leak by taking a can of aerosol brake cleaner, which is flammable, and spraying AROUND your boots, etc. If you get a sudden surge in rpm because of the additional "faux fuel" you've introduced you have found your leak.

Or you can take cigarette or vape smoke and see if if it gets sucked INTO your machine at one of these locations.

Lastly, in the auto world, we use a smoke machine to PRESSURIZE the intake manifold 1-2 psi with smoke and see where smoke leaks OUT.

Things like pleated (accordion-like) snorkels are notorious, in the auto world, for developing cracks over time. Molded vacuum lines, which get hard,and crack due to engine heat. etc.

>Try blipping the throttle & the engine croaks (croaking sound). Persist with it & the engine dies.

This indicates the engine isn't getting enough fuel to run. YOu provide it a big slug of cold air when you blip the throttle. Unless the choke is full on, it can't enrich the charge sufficiently. Note that on a cold (30-40F) morning your COLD engine may require FIVE TIMES as much fuel to run as it does when warm. Yeah, that's a LOT of enrichment!

2)Engine fires up ok. Try blipping the throttle & the engine croaks. Give it a bit of choke & the engine will rev quite nicely as it should be.

What I just said! :)

Release the choke & try blipping & the engine croaks. Try maintaining that throttle level & the engine croaks out.

3)Today it took 6 kicks to get it to start

Six kicks? Are you sure your choke is working properly? And what about your fuel bowl. Is it dripping, (bottom is wet, dirt on bottom indicating it has BEEN wet?) so you're trying to start a motor with no gas in the carburetor bowl, and having to "pump up" the fuel?

>& idling was super low

So today your intermittent vacuum leak wasn't there, and since you'd adjusted the idle for when it WAS there.... you got a super low idle.

& the engine died. Pull the choke & kick again, the engine starts with 3k idling revs & we're back to 1) above.

Are you starting to see why SOME of us will ONLY purchase fuel-injected bikes anymore? @#$#'ing around with chokes gets old. I'd simply give up and spritz a little starter fluid sometimes rather than break my leg kicking a bike over and over.

>Even when the idling speed seems to hv stabilised, when i try to move off, the throttle input causes the engine to croak & i will immediately stall.

Indicative of insufficient choke, OR...an engine that just hasn't warmed up enough to ride yet. Air cooled motors...yeah, motorcycles and the engines on them were designed to ride in RIDING weather from 65-95F degrees. Optimized for that. Cold blooded.

But once the engine is at a certain temperature, much warmer then when i usually start moving off anyway, all these symptoms disappear & the bike behaves normally.

Yep!

Ok, so now you know what's going on, the causes and cures, and can take appropriate action. Adjust & lube your throttle & choke cables & mechanisms, check for sticking throttle shafts, look for a drippy carb, cheat with starter fluid, make sure the choke DOES fully engage

Now a final word. Choke is a double-edged sword. It will permit an engine to start and run when cold.

But all that excess, LIQUID fuel, that doesn't vaporize and burn will scrub the oil off the cylinder and cause excessive wear of the rings and cylinder.

Fuel both gets ATOMIZED by a carburetor, that is, made into tiny drops through the emulsion tube/venturi action, etc, and VAPORIZED by the heat of the intake manifold. Cold fuel, cold engine, you end up mostly with LIQUID fuel which does NOT combust going into the engine. Remember, gasoline/air VAPORS burn, not LIQUID gasoline.


So you want to use choke as sparingly as possible, and turn it off as SOON as you can get the engine to hold a reasonable RPM. Idling for 20 minutes in the driveway with full choke is NOT a way to make a bike last 20 years.

The replacement of choke by software-controlled "cold start enrichment" in fuel-injected engines, which lasts 2 minutes or less, with major reductions after 30 seconds, is why most modern engines (car) will last more than 100,000 miles. Back in the "carb" days, an engine was typically worn out, due for rebuild and TRADED IN by 100K. Sheesh, lot of cars will go 250K today and it's NEVER the engine, rings, main bearings...it's always little @#$ that wears out.

Ok, ready now for the quiz?

1) The best way to deal with cold weather starting is

A) Move to Arizona for the Winter
B) Park the bike in a heated garage
C) Seal up any boot air leaks and use choke as needed
D) Buy a fuel-injected bike and spend all the time you save on cold mornings choking your chicken instead of your carburetor.

Addendum:

>tune down the carb for better fuel economy.

You can't get better fuel economy (miles per gallon, mpg) by leaning an engine out, UNLESS you're the designer of that engine and can adjust ignition advance and all sorts of other factors to prevent damage. I don't know what your mechanic did but running a 2-stroke lean is a sure-fire RECIPE for engine damage.

And 2-stroke fuel (with added oil) is EVEN harder to vaporize on a cold morning than normal "gas." One trick we did with chain saws was to take the plug out, warm it with a cigarette lighter, put it back in and THEN start the saw. The hot plug VAPORIZED just enough liquid fuel to get the saw to start.
 

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Discussion Starter #10 (Edited)
Wadenelson,

Thanks much for a very detailed reply. Took some time to digest & get acquainted with the stuff!.

Just to clarify, Singapore is an equatorial country. So no winter as such. Nov - Jan would be our monsoon season. So there will be cooler temps with lots of heavy rain. Max temp maybe abt 28 deg C during that time.

Yes, leaning out the engine too much can cause piston seizure as the fuel also plays a part of cooling agent. So if too little fuel => insufficient cooling of the cylinder, piston expands & seizes.

Strangest thing - Today the bike was completely ok. Dunno if it was due to my aggressive style riding on my way home the night before (thinking higher revs would help clean out any jet blockages in the carb), followed by clearer morning air (much reduced haze) the next morning.

Or does all this trouble have anything to do with filling up my fuel tank past the bar? (Theres this bar if you look down the tank opening which I suppose is to mark the max fuel level of the tank) So now that abt a litre or so of fuel has been consumed, whatever pressure, fuel weight or whatever it is that is exerting on the fuel feed or carbs etc is gone??

Ive noticed this 'vacuum leak' is an on -off occurrence. Maybe once in a mth or two kinda thing. As if the bike is PMSing or something...lol

Edit: For those who have never heard or seen my bike model before, here's a link featuring it with some specs. Mine is all red tho.

http://raresportbikesforsale.com/little-smoker-2001-honda-nsr150-sp-repsol-edition/
 

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It looks like it is a nice little reed valve low compression type 2 stroke according to the description in your link. Something we have not checked yet is the fuel tank vent. If the vent hole is directly on the bottom of the cap it might get blocked when your tank is too full. If you have trouble starting again, try momentarily loosening the fuel cap between kicks to let it breathe then tighten it again and try another kick. If it starts up right after that, your vent was being plugged and you released the vacuum.
 

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ZAMM Fanatic
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I don't know exactly how the fuel tank is vented on your model but in general filling a tank beyond where the nozzle clicks off is a mistake. In a car it can result in a check engine light, a carbon cannister filled with liquid fuel, and more than a few $ spent to get the emissions system back to working condition. There's just no benefit in overfilling.
 
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