hunting / surging / horse kicking on the overrun

[dietcoke]

Hi all,

I just want to check that I have my terminolgy correct. I have a mark 2 crm that pulls very well but when you close the throttle the " ding, ding, ding " part it very bumpy, the bike is jerky like a horse kicking. From searching the forum I understand that this is called hunting or surging.

Can anyone confirm that this is caused by a dirty carb jet ?? Should I just order a new mainjet and idle jet from leasure trail and then strip the carb / clean / put back together.

What causes this ?? is it a lack of fuel when the throttle is closed ??

The bike idles fine, but does take a while to go back to idle after high revs, it appears to sit at a double idle for 10 - 15 seconds when the throttle is closed whilist waiting at traffic lights. I have tried to fiddle with the idle screw but doesnt really help, a 1/4 turn up and it idles too high and over heats and a 1/4 turn down and it risks stalling.

I wannted to check with you guys before I start to strip the bike ( my moto is if its not broke dont try and fix it ). I presume i will need to take the tank off to get the carb out.

[fallenmikethebike]

Hi, yes, strip and clean the carb, sometimes, though not always, a new pilot jet is the only way to cure the problem.
Hopefully, all other things being in serviceable order, that should clear up the running defect.
Pop, a new, BR8ES, plug in for good measure, the standard pl,ugs are really cheap, and long lived.
Mike

[knackeredMk1]

Very briefy as you will find details of the following and what to do elsewhere -

The kind of symptoms you are talking about can be caused by -

Carb problems - gunk or partially blocked jets.
Throttle cable is frayed and isn't quite 'snapping shut' as it should.
Wear on the carbslide where the idle screw touches.
Bent idle screw.
Air leaks in the intake side. (loose clips, split hoses, non-functioning gaskets....).
Too lean jetting (engine getting too hot), particularly on the pilot jet, also too rich jetting on the pilot jet. (When coming down to idle the engine has been 'cleared out' it then quickly gets 'oiled up', so slows down).

[dietcoke]

Hi All,

Ok, I have cleaned the carb, put in a new main jet and idle jet and the bike is running much better, very good as you go up the rev range, but still getting surging / hunting at low revs and on the overrun. It appears to idle ok tho. So I thought I would check for air leaks. Sprayed WD40 around the carb and reed block whilist the engine was idling, engine did not Rev. I started to look at my air box and its been hacked about a bit.

Firstly the baffle plate has been removed but I understand this does not make a difference.

Secondly the rectifer has been relocated into the air box ( I dont think this is where its supposed to be ). If you lift the seat, a cable goes into the airbox entry point but instead of running down the straight someone has drillied down and left a 2 inch square hole going straight down into the airbox and the rectifer is bolted into the box oposite the filter.

Will this make a huge difference to my air intake causing the bike to run lean and thus causing the surging ??? The air intake hole is the standard size but the path that the air takes within the box has been changed. Also the rectifer may be helping to heat the air before it get sucked into the carb ???? Or am I going down the wrong path :-)

Could be a easy fix with a new secondhand air box. Where do people relocate their new big rectifers.

[dietcoke]

Also I have already checked the throttle cable, no fraying and it snaps shut with a good clunk at the carb.

I have a new spark plug, but havnt fitted it yet as I want to do one step at a time.

[SkidMark]

Mount it on the outside of the airbox to the rear ie in the wheel well. see:
viewtopic.php?f=6&t=4838&hilit=electrex

mine is also mounted here although my crud guard is just a plate with curled over edges that attached just with the reg/rect mounting screws.

Regards
SkidMark

[dietcoke]

Here are some photos of my current butchered airbox. I am awaiting delivery of a (hopefully) unmolested air box and will follow skidmarks advise of relocating the rectifer.

Do you think this butchered airbox is causing my problems ? The air intake hole is the standard size but the path that the air takes has been altered. Has anyone seen this before ?? Also the rectifer could contibute too heating the air within the airbox.

http://i855.photobucket.com/albums/ab11 ... AG0085.jpg

http://i855.photobucket.com/albums/ab11 ... AG0087.jpg

http://i855.photobucket.com/albums/ab11 ... AG0088.jpg

[knackeredMk1]

Changing air box and relocating reg/reg won't help at all.

If you think about it - the air intake side is at its lowest requirement/stress at idle. So beyond making an extreme restiction to air flow you won't make a change to the way the bike idles.

I did this to improve airflow a bit with no ill effects -

Inside:



From the top:

[dietcoke]

Thanks KnackeredMk1, maybe I wont go to the bother of swapping out the air box just yet.

So to confirm the problem I want to resolve. When I ride along and then shut the throttle whilist comming up to a junction say ( when the exhaust is at the "ding, ding, ding" part of its cycle ) the bike feels jerky, surging, horse kicking.

The bike starts first / second kick, smokes heavly when cold for a few minutes then clears.
It idles fine at standstill
It accelerates fine
Before the carb clean the bike also felt a bit jerky at constant revs when keeping to 40 miles per hour say. This has now stopped.

I have pulled the carb out and cleaned it, replacing the main jet and idle jet with new parts from lesuire trail.
The throttle cable is in working order with no fraying and when the throttle is closed the carb clanks.
I have sprayed wd40 around the carb and reed block at idle and no reving occured

Where do you think my investigation should go to next ???

[knackeredMk1]

Try dropping the needle one slot.

In the end you will have to cope with a little jerkiness on very little throttle under very little load. This is how my bike behaves but the only way to reduce this further is to reduce the pilot jet. Which I'm not prepared to do at the moment for risk of running a bit lean when it gets colder and I am already two sizes down from stock.

You may have the scope to change down one size. What is your current jetting .

I assume the 'takes a little time to slow down to idling speed after being reved' has gone .

[dietcoke]

I am currently running a Dep expansion chamber, and Dep end can ( I understand that no jetting changes are needed with this )

128 main jet
48 pilot jet
Middle setting on the needle ( if i remeber correctly )

[fallenmikethebike]

Hi, you could try a 45 pilot jet, running with the mixture screw turned in further[richer], then lean it out till it feels more ,user friendly.
Gearing also plays a part in the traits you mention.
But as others have said, it is something that 2 strokes are renowned for, a 2 stroke prefers to be working.
Mike

[dietcoke]

[quote="knackeredMk1"]
I assume the 'takes a little time to slow down to idling speed after being reved' has gone :?: .[/quote]

Yes, that part of the problem has been resolved. The guy I bought the bike from had taken off the hand guards as he didnt like them and replaced the bar ends with some motorax after market ones. Unfortunaely the one on the left hand side was too tight and pinching the accelerator grip, so it wasnt snapping shut cleanly.

Simple solution :-)

[knackeredMk1]

After market exhaust systems vary in the need to rejet and DEP are probably the ones that need least rejetting but there is a tendency for all systems to need the jetting to be a little leaner.

Try dropping the needle one slot and if that doesn't work try a 45 pilot jet with the needle returned to spec. Finally drop the needle again if you are still not satisfied. This is far as you should go without experienced help.

This will help I believe but please be aware that I have not heard or seen your bike and leaning it may cause you problems if what I am deducing is wrong. I.E It is at your own risk and I cannot be held responsible.

Remember that you are dealing with a 20 year old bike that wasn't perfect (compared to a modern bike) to begin with.

[knackeredMk1]

AS FMtB says gearing does effect this. By raising your gearing you will put the engine under more load for a given engine speed so the problem will diminish somewhat.