deceleration knock with 2 strokes


georgieboy

Member
Jan 2, 2001
416
0
Hi Pushin,
congrats on the bike, you have the same as me, only i own the european version ;)
I am still figuring it out, what cld be the cause. Despite the decel knocking the bike runs flawlessly. I have tried all with jetting, richening it up and leaning it down, advancing and retarding the timeing. All to no success. Leaves me to think it is, either a pipe thing, or it is a squish height thing, as i measured the squish and it comes with a 1.6mm high squish height. That is huge.
Looking at the cilindee base, i discover two base gaskets, so lowering the squish is easy. I feel that unburned fuel due to that high squish leaves the clinder into the pipe with decel.
Eveerybody feel free to express yr opinion, especially the brighter one, amoung us.
Maybe Eric can shine a light on this?
 

Rich Rohrich

Moderator / BioHazard
LIFETIME SPONSOR
Jul 27, 1999
22,838
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Chicago
If it is a re-fire in the pipe and not combustion knock, then you have to look in a different direction then the things that were discussed earlier.

A couple of things to look at :

- speed of combustion - If unburned fuel is being dumped into the pipe a re-fire is possible. This will be related to ignition timing, squish efficiency, mixture strength (rich and lean both burn slow), fuel quality (especially the 90% and end point temps of the fuel) as well as the the efficiency of the scavenging loop.

Fuel can be short circuited into the exhaust from a port mismatch or some other manufacturing flaw. Porting is the only way to fix this. Speeding up the combustion by correcting the squish clearance is an excellent way to fix this, and probably the simplest.

Unless you want to deal with race fuel, fuel quality is a hit or miss proposition.

It sounds like you are on the right track towards fixing it.
 

georgieboy

Member
Jan 2, 2001
416
0
Thanks for help, and yr encouraging words ;)
I don't want to deal with racefuels.
In europe fuel quality is no issue, we can both have 95ron and 98ron, of good quality. Sometimes you walk into some bad fuel with the smaller brands. But that rarely happen.

A mismatch in the porting... I really hope not.. as we are talking about a new 2008 200exc ktm here.
But when i lift the jug off, i know for sure.
By mismatch you probably mean a flow directly from the intake port into the exhaust port?

The strange thing is, that the whole back knocking thing is most present when on a downhill braking on the engine. The throttle is closed, and the engine is kicking and accelerating on every knock. When i pull in the clutch, it is vanishing.

My dilemma is, why is this knocking only on closed throttle, when decelerating to a idle. Shows a too high squish only on deceleration?
When the engine finally come back to its idle rpms, the knock is gone, and it idles just fine.
 

Rich Rohrich

Moderator / BioHazard
LIFETIME SPONSOR
Jul 27, 1999
22,838
16,902
Chicago
The engine tends to go rich on decel which dramatically slows down combustion speed and dumps raw fuel into the chamber. Scavenging, and combustion in general is already compromised at slower speeds, so decel under load would logically make things worse.
 

dirt bike dave

Sponsoring Member
May 3, 2000
5,349
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Engine braking is going to result in the highest vacuum through the intake of any condition. So not much air is passing through the closed carb, but the high vacuum is sucking in more fuel.
 

georgieboy

Member
Jan 2, 2001
416
0
DBDave, you suggest to go smaller on the pilot?
The bike starts rather easy, and uses hardly the choke on cold start.
I check out that pilot first, see if it is still a rich setting that creates the symptom.
And further, with closing the squish i give the engine a better chance to burn cleany at low rpm's and get rid of the raw fuel, under deceleration.
Probably the fuel-mileage will benefit too.
Now is the squish ex factory 1.6mm. That is huge.
The idea is to go for 1.0mm or is smaller even better?
 

dirt bike dave

Sponsoring Member
May 3, 2000
5,349
3
georgieboy said:
DBDave, you suggest to go smaller on the pilot?

No. My suggestion is to pull in the clutch when you are slowing down.

Fuel injection might solve the 'problem' in the future. Until then, it just seems that while engine braking, some two strokes are going to suck in more fuel than they can burn in the combustion chamber.
 

georgieboy

Member
Jan 2, 2001
416
0
Hee, that is a easy solution :)
But with technical downhills i like the feel of the motorbrake.
Because i am not used to this backknocking thingy, i feel insecure with the engine on technical parts.
On a mx-track there won't be any problem though.
In the end it is not a huge problem, but i just try to get rid of it.
And since the squish gap is soooo big, i hope that there is a win/win situation there. ;)
 

bloodhound51

Member
Oct 16, 2007
2
0
closed throttle whats that?
hahaha
my cr250 had a (backfire) as southeastern u.s. citizens call it
mine was due to the pipe being loose and allowing the engine to suck a little cold air in the exhaust creating ample (unburned) fuel air ratio to ignite in the pipe. i never thought a two-stroke could do that, but a new flange cured the problem.
check the sealing surface of the pipe and see what it does. running a two-stroke engine down a hill using compression to slow you down is bad news anyway.(high rpm's low fuel rates=low oiling. hope this helps
 

georgieboy

Member
Jan 2, 2001
416
0
bloodhound, although i see a little oil at the flange i don't consider it the problem, but will put extra attention to it.
Will put an extra o-ring to eliminate that part.
thx for the thoughts.
 

georgieboy

Member
Jan 2, 2001
416
0
Ok guys,
for all who helped, here is what i did to (almost) cure the deceleration knock.
I lowered the cilinder to establish a 1.1mm squish gap.
The factory set up the bike with a 1.65mm squish gap and i felt that this was the culprit with this decel knocking.
Remember that the decel knocking started directly with a cold engine, by just blipping the throttle.
When i first started the "new" engine with reduced squish i immediately noticed the change for the better. Firstly it didn't knock with blipping the throttle anymore, secondly the engine itself was better running and not so jerky on low throttle.
I was statisfied and happy, but offcourse needed the test ride to confirm things further.
The ride was good. Bike responded well to throttle changes and the knock was hardly noticed on deceleration anymore.
(btw i discovered yesterday, that my trialbike had also a slight of a decel knock with a cold engine. The decel knock went away after warming up the bike )
Before the worst knocking was produced on a downhill section. That was also hardly noticed anymore. So i am a happy 200cc rider now. Thnks for all the help.
Still need to do some jetting testing though, but that is the easy part. :)
 

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