[Almoral]

Hi All,

I have a Active 3G which has run 32000 KMs till now. It works fine and this is my primary mode of transport for now. I noticed something curious today and wanted to pick the brains of more experienced forum members on that.

I use the choke for cold start on the mornings and then close it after 15/20 seconds. I started the same way today. However, unlike other days I was a bit distracted and forgot to close the choke.

After about 1-1.5 KMs I noticed that my speed is not increasing beyond 30kmph irrespective of throttle. The engine was also not revving as high as it should with that much throttle.

My first guess was one of the tyres was punctured. So, I stopped and checked the tyres. They seem to be ok. I drove on slowly thinking what is wrong.

In between at a certain section of the road where it was slopping downwards, the speed did increase beyond 30kmph and once that barrier was crossed it seemed to run fine for a few seconds. As soon as the road levelled off, the issue was back. This prompted me to think that might be there is an issue with the transmission which is causing this.

I decided to take it to the nearest mechanic shop and midway I noticed that the choke was still on. I turned it off and voila, everything was back to normal.

So, I was curious what happened here. As far as I know choke is used to increase the fuel ratio in the air-fuel mixture which helps in cold starts. How does it result in the vehicle not going above a certain speed? And why did the vehicle could move normally on the slope and not on flat road?

[Mortis]

I'm guessing with the choke pulled and throttle applied the carb us getting flooded and fuel wasn't getting burnt efficiently

[ashwinprakas]

Originally Posted by Almoral

As far as I know choke is used to increase the fuel ratio in the air-fuel mixture which helps in cold starts.

There are different ways different carburetors skew the AFR ratio towards the richer side, normally in the CV and VM carburetors you have an enricher circuit. Meaning when you activate it there'd be more fuel flowing into the combustion chamber.

Now coming to the Activa, Splendor, CT100 etc they use a PB series carburetor that has a Butterfly Choke Plate, that literally Chokes the intake limiting air flow.





So I guess at this point you'd realize why your motorcycle felt restricted.

Hint: Cause it literally was restricted!

P.S. Excessive use of the Choke or running on Richer AFR for long can be detrimental to engine life, hence use the Choke sparingly and only if you absolutely have to.

Regards,
A.P.

[Almoral]

Originally Posted by ashwinprakas

So I guess at this point you'd realize why your motorcycle felt restricted.

Hint: Cause it literally was restricted!

P.S. Excessive use of the Choke or running on Richer AFR for long can be detrimental to engine life, hence use the Choke sparingly and only if you absolutely have to.

Regards,
A.P.

Thanks. Yes, that makes sense. And I am guessing when I was on the slope, the vehicle could move faster under it's own weight + whatever push it was getting from the engine.

I do know that using choke for a longer is not good. Just missed carrying out the usual routine today. Need to be more vigilant.

[srini1785]

Originally Posted by Almoral

So, I was curious what happened here. As far as I know choke is used to increase the fuel ratio in the air-fuel mixture which helps in cold starts. How does it result in the vehicle not going above a certain speed? And why did the vehicle could move normally on the slope and not on flat road?

Check your plugs, an enriched AFR results in plug damage. If its sooty black then your plugs are shot.

Back to the problem, IMO, a richer AFR due to closed choke, limits the power due to two factors.

1. Higher spark gap resistance, resulting in improper burn. When fume mix is not right, the spark gap resistance is increased resulting in improper burn and soot deposit on the plugs.
2. Not enough volume of mix to burn. With choke off, essentially there is only fuel in chamber and the spread of explosion is limited due to lack of volume. So the power is also limited.

[Scarlet_Rider]

If the scooter is being used regularly, then there is no need to use the choke every morning. We have a 2009 Suzuki Access 125 that is used almost daily. Don't recollect the last time I used the choke. Hence as Ashwin mentioned, use the choke only when required. For example if the scooter has not been used for many days and is not starting immediately.

[a4anurag]

AFAIK, choke is ONLY used for cold starts since the engine struggles and that 'extra' petrol (rich mixture) will help the engine start in cold conditions faster.

In my Karizma R and Activa 125, all first starts of the day were with choke and letting the engine run 10 seconds with choke ON and before setting off, I close the choke lever.

Activa had an inherent problem of not starting easily but somehow after a lot of 'kicks' it used to.

[Almoral]

Originally Posted by Scarlet_Rider

If the scooter is being used regularly, then there is no need to use the choke every morning. We have a 2009 Suzuki Access 125 that is used almost daily. Don't recollect the last time I used the choke. Hence as Ashwin mentioned, use the choke only when required. For example if the scooter has not been used for many days and is not starting immediately.

Yes, you are right. Earlier when the 'Work From Office' system was in place; I took the Activa daily and never had to use the choke. Now, the running is not that regular so some days it starts without any fuss and some days I need to use the choke.

Originally Posted by srini1785

Check your plugs, an enriched AFR results in plug damage. If its sooty black then your plugs are shot.

Back to the problem, IMO, a richer AFR due to closed choke, limits the power due to two factors.

1. Higher spark gap resistance, resulting in improper burn. When fume mix is not right, the spark gap resistance is increased resulting in improper burn and soot deposit on the plugs.
2. Not enough volume of mix to burn. With choke off, essentially there is only fuel in chamber and the spread of explosion is limited due to lack of volume. So the power is also limited.

Thanks. I will get this checked (had planned anyway for engine oil change, so will do this along with that).

[Sebring]

We have a 2012 Activa that has clocked 70,000 Kms. I recommend STP Carburetor Treatment once in few months. This eliminates cold start issues, and you feel the scooter perk up when you give throttle. I've been using for 10 years now, (just ensure its an original product from USA)

Originally Posted by Scarlet_Rider

If the scooter is being used regularly, then there is no need to use the choke every morning. We have a 2009 Suzuki Access 125 that is used almost daily. Don't recollect the last time I used the choke.

[condor]

Choke makes the air-fuel mixture rich - i.e, air reduced. So as long as the choke was on, your scooter engine was not able to burn the fuel properly, and hence didnt move freely.

Ashwinprakash ?

[ashwinprakas]

Originally Posted by condor

Choke makes the air-fuel mixture rich - i.e, air reduced. So as long as the choke was on, your scooter engine was not able to burn the fuel properly, and hence didnt move freely.

Ashwinprakash ?

This phenomenon is called "Hitting the Brick Wall"

If your AFR is too rich then the motorcycle will not rev past a point and would feel as though you've hit maximum output when in fact you have a lot more revs left.

Though in the case of Choke's if its Enricher choke when on the move its not that noticeable though when hot your motorcycle will fail to idle and would shut down.

If its the Choke-Plate type then you have the same response at Idle, though when running you can really feel the hesitation due to the plate actually blocking your intake.

Regards,
A.P.

[ManneAtWheelz]

Originally Posted by Sebring

I recommend STP Carburetor Treatment once in few months. This eliminates cold start issues, and you feel the scooter perk up when you give throttle.

This should come to my scooter cold start issues rescue. My scooter is hardly used like 2 kms per day roughly and I used to pull the choke lever for the first start of the day always.