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eight stroking #119121
07/02/07 11:29 pm
07/02/07 11:29 pm
Joined: Dec 2006
Posts: 10
New Jersey
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grommet Offline OP
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New Jersey
I have heard mysterious references to a phenomena refered to as eight stroking. confused Would anyone like to chime in with a description of how it happens and how to recognise it. I have to imagine that it must be a failure of ignition every other cycle. Or on triumph twins is it one cylinder failing to fire? As soon as I find a suitable place to work on one I plan to get a triumph twin. All these archaic technologies seem like a lot of fun. Compaired to the modern vehicles that I am used to working on. smile

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Re: eight stroking #119122
07/03/07 1:17 am
07/03/07 1:17 am
Joined: Mar 2005
Posts: 10,221
Boston, Massachusetts
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John Healy Offline

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Boston, Massachusetts
It refers to the firing of a cylinder that is so rich that it takes an addition firing cycle to get enough air into the cylinder to fire. On a two stroke it is called four stroking.

So for each time there is a combustable mixture it took two complete engine cycles to get it.

So for a four stroke motor with four strokes to make one engine cycle: intake, compression, power and exhaust strokes the motor has to do this twice, or two engine cycles, before there is any combustion. This is how I would describe the term eight stroking. Make any sense?

Like to experience it? Remove a main jet from your Mikuni or AMAL. Just above 1/2 throttle the bike will begin to eight stroke. Just be sure that you have a new plug available as you are apt to foul the one in teh motorcycle.


Re: eight stroking #119123
07/03/07 4:14 pm
07/03/07 4:14 pm
Joined: Dec 2006
Posts: 10
New Jersey
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grommet Offline OP
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New Jersey
Makes sense to me.

I will venture the guess that I have never heard of such a thing previously because as a rule people who tamper with the fueling of their vehicles don't buy Chevy's, excepting a fraction of Corvette owners who are a breed apart.

Are AMAL carbs apt to go out of tune with a bias towards going rich? What I am getting at is, are Triumphs any more likely to suffer from such a complaint than any other carburated machine? Reason being I have heard/read Triumphs being characterised that way. I'm thinking that is not correct.

Re: eight stroking #119124
07/03/07 4:31 pm
07/03/07 4:31 pm
Joined: Mar 2005
Posts: 10,221
Boston, Massachusetts
J
John Healy Offline

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Boston, Massachusetts
If we are talking about the AMAL MKI Concentric as the slide wears they get lean at the bottom and as the needle jet wears they get rich up through mid throttle. At lower throttle openings they tend to balance each other out, but as the throttle opens a bit the richness of the worn needle takes precident. Replacing the needle jet is a great way of getting back your gas mileage.

Given that the bike has an air cleaner, and the air cleaner and needle jet are routinely maintained (replaced) there isn't any reason that an AMAL fitted to a Triumph would go rich, and thus 8 stroke, any more than any other carburetor that has the needle hanging in the needle jet.


Re: eight stroking #119125
07/03/07 4:35 pm
07/03/07 4:35 pm
Joined: Nov 2005
Posts: 706
Tennessee
Fisherman Offline
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Tennessee
Just got to love this place! I've been around bikes for 25 years, heard of 2 strokes 'four stroking', but never 4 strokes '8 stroking'.

Prof Healy not only knows about it, but knows how to reproduce it....I don't think we have to ask how he got this knowledge.

If someone brought me a bike without a main jet installed, I just have to wonder how long it would take me to discover THAT problem...

I can hear it now..."oh yeah, I forgot to tell you...it started happening right after I reassembled the carb...."

Bernie


'Never argue with an idiot, they'll bring you down to their level, and beat you with experience'

'72 TR6
'12 Hinckley Scrambler
'95 FLHTC Road Sofa
Re: eight stroking #119126
07/03/07 4:42 pm
07/03/07 4:42 pm
Joined: Nov 2006
Posts: 176
Dillsburg, PA - USA
mattstriumph Offline
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During my first startup of my first and only Triumph, I got the bike running on two kicks after a complete rebuild. I was very proud.

I had put on an AMAL monbloc that I got on eBay which looked complete to me, however I did not realize that it was missing the needle jet. It idled OK, 8 stroked up to about 1/2 throttle, then ran good off the main jet after that.

If all I wanted to do was idle or go full throttle, everything would have been OK. Maybe when I was younger.



1965 Triumph T100SR
Re: eight stroking #119127
07/03/07 5:14 pm
07/03/07 5:14 pm
Joined: Aug 2005
Posts: 1,500
melbourne florida
B
bodine031 Offline
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Posts: 1,500
melbourne florida
hey fish; you never had the pleasure of 1 main jet falling into the carb bowel!! of an AMAL powered twin. its makes um run weird!!

Re: eight stroking #119128
07/03/07 5:22 pm
07/03/07 5:22 pm
Joined: Nov 2005
Posts: 706
Tennessee
Fisherman Offline
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Tennessee
Bodine, it took me years of breaking and stripping bolts and nuts to stop OVER tightening stuff....

(god bless loctite...)


'Never argue with an idiot, they'll bring you down to their level, and beat you with experience'

'72 TR6
'12 Hinckley Scrambler
'95 FLHTC Road Sofa
Re: eight stroking #119129
12/07/07 8:49 pm
12/07/07 8:49 pm
Joined: Oct 2007
Posts: 139
The Netherlands..
hein weijers Offline
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hein weijers  Offline
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The Netherlands..
Quote
Originally posted by mattstriumph:
During my first startup of my first and only Triumph, I got the bike running on two kicks after a complete rebuild. I was very proud.

I had put on an AMAL monbloc that I got on eBay which looked complete to me, however I did not realize that it was missing the needle jet. It idled OK, 8 stroked up to about 1/2 throttle, then ran good off the main jet after that.

If all I wanted to do was idle or go full throttle, everything would have been OK. Maybe when I was younger.
This sounds familiar to me!I have restored my T140V 1978 and made the first start yesterday.
It idles very nicely and roars like a lion on full throttle! But in between it's another story. It runs very bad and it got worse as the engine temp got higher. Pulling the plugs showed that they were black, so it seems that the mixture is too rich. The needle jets are present and the number is ok, but maybe they are worn. Can a worn needle yet cause this problem?
Do I also have to replace the needle and main jet?


Triumph Bonneville T140 1978
Yamaha XS 650 Heritage special 1982 (rephased, big bore)
Re: eight stroking #119130
12/07/07 10:43 pm
12/07/07 10:43 pm
Joined: Mar 2005
Posts: 10,221
Boston, Massachusetts
J
John Healy Offline

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Boston, Massachusetts
Can a worn needle yet cause this problem?
Yes!

Do I also have to replace the needle?
No, not unless it is bent or the wrong needle for the application.

and main jet?
Yes, although they do not wear as such, brass oxidizes and leaves "bloom" around the edge of the metering hole. Also modern gasoline isn't kind to these jets.
The leading edge of the main jet's orifice is VERY important. It is this edge that is burnished when they do the final sizing of the jet. A small burnishing tool is run around the edge until the jet flows the proper amount.

If the leading edge is oxidized from brass bloom or corrosion from old stale fuel it will flow less fuel that the size indicates. Although not a big problem on most street bikes, start running up the compression and turning up the wick and you can seize the motor.

When tuning a motor it is important not to use those old main jets that have been sitting in the bottom of the race box for 20 years. Check them with a magnifying glass before using them. The edge sgould look just like the day it was made or I suggest that you destroy it.

Without a manometer and a set of standards for the size jet you have, cleaning the edge will not work as you will have no idea how much fuel the jet will flow. You could be putting in a jet, that by size should flow more gasoline, but because of the compromised orrifice flow less than the one you are replacing.
John


Re: eight stroking #119131
12/08/07 8:53 am
12/08/07 8:53 am
Joined: Oct 2007
Posts: 139
The Netherlands..
hein weijers Offline
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hein weijers  Offline
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The Netherlands..
Thanks John. I will replace the jets (and the 29 years old needles) and have the magnifying glass ready!

Hein


Triumph Bonneville T140 1978
Yamaha XS 650 Heritage special 1982 (rephased, big bore)
Re: eight stroking #119132
12/08/07 6:46 pm
12/08/07 6:46 pm
Joined: Oct 2003
Posts: 3,695
Emsworth, sunny south of Engla...
D
dave jones Offline
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Emsworth, sunny south of Engla...
I have asked this question several times and got no answer. I have a .106T needle jet in my carb but the only spare I have is a .106 (has hole on side). Can I fit this and not notice any difference (1967 TR6R)?
Dave

Re: eight stroking #119133
12/08/07 9:21 pm
12/08/07 9:21 pm
Joined: Jun 2005
Posts: 626
Western Oregon
enigmaT120 Offline
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Western Oregon
Jets are cheap, Dave. Which one does your parts manual call for? I don't remember seeing a .106T, don't know what it is.


Ed
1970 Bonneville
Re: eight stroking #119134
12/08/07 9:59 pm
12/08/07 9:59 pm
Joined: Mar 2005
Posts: 10,221
Boston, Massachusetts
J
John Healy Offline

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Boston, Massachusetts
David: The "T" is a Monobloc needle jet without the bleed hole. I find no reference for this in the Concentric as AMAL use two different part numbers to identify the "hole" vs "no hole" versions.

Yes, if you fit the drilled one you will see a difference. The bike will run leaner in the circuit that is effected by the hole ( I would have to do some looking, but you could expect it to be in the 1/4 to 1/2 throttle range).

If you experienced any adverse effects from puting in teh new jet you could easily solder the hole in the new jet closed and you would have a "T" needle jet.
John


Re: eight stroking #119135
12/09/07 12:26 am
12/09/07 12:26 am
Joined: Oct 2003
Posts: 3,695
Emsworth, sunny south of Engla...
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dave jones Offline
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Emsworth, sunny south of Engla...
Thanks for that. Yes I have the standard 389 Monobloc and the jet must be very worn. I think I sent off for a .106T (without the hole) but they sent me this .106 and it has been sitting around for ages.

dave

Re: eight stroking [Re: grommet] #443320
07/05/12 5:58 pm
07/05/12 5:58 pm
Joined: Jul 2012
Posts: 2
Oklahoma, USA
Larry Lane Offline
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The "eight stroking" is caused by exhaust gas dilution of the air fuel mixture in the combustion chamber which is caused by a cam with a lot of duration which causes the intake and exhaust valves to be open at the same time (valve overlap). It's not caused by an over rich mixture. There would be black smoke coming out the exhaust in that case.

Re: eight stroking [Re: Larry Lane] #443323
07/05/12 6:21 pm
07/05/12 6:21 pm
Joined: Nov 2005
Posts: 7,492
Back on the mainland!
JubeePrince Offline

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Originally Posted by Larry Lane
The "eight stroking" is caused by exhaust gas dilution of the air fuel mixture in the combustion chamber which is caused by a cam with a lot of duration which causes the intake and exhaust valves to be open at the same time (valve overlap). It's not caused by an over rich mixture.


Hi Larry -

Eight-stroking is a condition of not having enough air and too much fuel in the combustion chamber. The crank needs an extra revolution(s) to gather enough air to fire. Whether you arrive at this condition due to valve overlap or over rich mixture, AIUI, it would still be considered eight-stroking.

Steve


'77 T140J
"Vintage Bike". What's in your garage?

"The paying customer is always right."

Fitting round pegs into square holes since 1961...
Re: eight stroking [Re: JubeePrince] #443331
07/05/12 8:07 pm
07/05/12 8:07 pm
Joined: Jul 2012
Posts: 2
Oklahoma, USA
Larry Lane Offline
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Not true!

When the cylinder is full of inert exhaust gas, there is not room in it for the air fuel mixture. It is not an over rich condition. There simply isn't enough air fuel charge in the cylinder to fire.I am sure you have heard of a "lean misfire". the condition you are describing is called "flooding" and there would be black smoke associated with it.

When there is valve overlap, the exhaust rushes back into or never completely leaves the combustion chamber. It is under pressure. The intake is at a negative pressure, so the flow stops and cannot enter the cylinder because it is already full of inert exhaust gas, so the cylinder misfires. Then on the next power stroke, since there was no ignition on the previous power stroke, the air fuel mixture is able to fill the cylinder, and it "fires" which starts the whole process all over again


Re: eight stroking [Re: John Healy] #443339
07/05/12 8:34 pm
07/05/12 8:34 pm
Joined: Aug 2001
Posts: 7,604
scotland
triton thrasher Offline
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scotland
Originally Posted by John Healy
David: The "T" is a Monobloc needle jet without the bleed hole. I find no reference for this in the Concentric as AMAL use two different part numbers to identify the "hole" vs "no hole" versions.

Yes, if you fit the drilled one you will see a difference. The bike will run leaner in the circuit that is effected by the hole ( I would have to do some looking, but you could expect it to be in the 1/4 to 1/2 throttle range).

If you experienced any adverse effects from puting in teh new jet you could easily solder the hole in the new jet closed and you would have a "T" needle jet.
John


For a temporary trial, you can superglue a piece of paper over the holes, or block them with the end of a cocktail stick or similar.


Amateur Loctite enthusiast.
Re: eight stroking [Re: Larry Lane] #443342
07/05/12 8:47 pm
07/05/12 8:47 pm
Joined: Jul 2009
Posts: 3,549
Vic. Australia
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Pete R - R.I.P. Offline
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Vic. Australia
Originally Posted by Larry Lane
Not true!

It is not an over rich condition.

When there is valve overlap, the exhaust rushes back into or never completely leaves the combustion chamber. It is under pressure. The intake is at a negative pressure, so the flow stops and cannot enter the cylinder because it is already full of inert exhaust gas, so the cylinder misfires.

It is a rich condition,and you can always get it if you richen the mixture enough.
Exhaust reversion during overlap can cause the richness too.A pressure wave returning to the exhaust valve during overlap will blow fuel/air back out the intake and it picks up more fuel at the carb.When it's drawn back into the engine,some of that fuel/air has passed the main jet 3 times and become richer.

If it's too rich to burn,it won't burn.You get no following exhaust wave,so the problem doesn't happen on the next cycle.

Re: eight stroking [Re: John Healy] #443353
07/05/12 9:17 pm
07/05/12 9:17 pm
Joined: May 2010
Posts: 2,587
Illinois, USA
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Well, this is an excellent thread, even if it started 5 yrs ago. Beyond the good discussion of 8-stroking, for all those who have wondered what John Healy means when he says (as he often does), "a jet isn't just a hole", they should bookmark this thread for future reference!


'64 TR6R Plus some Twins from other countries (U.S., Germany, Japan)
Re: eight stroking [Re: grommet] #443361
07/05/12 10:33 pm
07/05/12 10:33 pm
Joined: Mar 2005
Posts: 10,221
Boston, Massachusetts
J
John Healy Offline

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Boston, Massachusetts
Larry:
Are you talking two stroke? Either 4 or 8 stroking.

I believe it has to do with low speed scavenging as you described.

I think you will find this is not the case with a four stroke engine that has forced induction. "Eight stoking" in a 4 stroke engine, as described here, can happen at any throttle opening, or engine speed due to a rich mixture. In two strokes it mostly happens at idle or just above. N.B. below

It will also happen as Pete described where due to the engine being "off the cam" the incoming charge is pushed back through the carburetor and drawn in again. It effectively has 3 times the normal amount of fuel. While you could describe it as "flooding," I don't find it very descriptive, you get the distinctive rhythmic miss fire most people refer to as 8 stroking. You can see this effect with a Gold Star with a GP carburetor as a cloud of fuel collects just outside the bell mouth and is drawn in as the engine rpm's are raised and the engine comes on the cam.

I could stand to be corrected, but what you described better fits the 4 or 8 stroke condition on a two stroke.

N.B. "Scavenging of small two-stroke engines relies on inertial scavenging through the Kadenacy effect. At low rpm and low gasflow velocities, this effect is reduced. Scavenging thus becomes less effective when idling, and so it is when idling (at either low rpm or low throttle) that four-stroking is most likely to become a problem.[1] Schnuerle or loop scavenging is considered to be less prone than the simpler cross-scavenging" WiKi

N.B. "The major advantage of the four stroke is that its power, as the throttle is initially opened, begins near zero and increases smoothly, especially in mildly tuned engines with short cam timing. In the two stroke this is not possible, because at low throttle, fresh charge is so diluted by exhaust gas left from the previous cycle that it cannot at first be ignited. When it fires, (two-stroke) it does so only after several cycles have delivered enough charge to be ignited. Torque therefore increases in chunks, as eight-stroking becomes four-stroking, and four-stroking jumps to steady firing-torque doubling at each step. Devices such as exhaust power-valves have helped, but this problem has never gone away. Therefore a rider on a four-stroke can safely send power to the rear tire sooner, and so begin acceleration earlier."

Kevin Cameron - Technical Editor Cycle World Magazine - February 2003


Re: eight stroking [Re: grommet] #443452
07/06/12 2:43 pm
07/06/12 2:43 pm
Joined: Jan 2006
Posts: 7,089
Laredo (South) Texas, USA
GrandPaul Offline
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This is the kind of thread that makes me miss Gordon Jennings.


GrandPaul (does not use emoticons)
Author of the book "Old Bikes"
Too many bikes to list, mostly Triumph & Norton, some BSA & European
"The Iron in your blood should be Vintage"
Re: eight stroking [Re: grommet] #443457
07/06/12 2:55 pm
07/06/12 2:55 pm
Joined: Mar 2005
Posts: 10,221
Boston, Massachusetts
J
John Healy Offline

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J

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Boston, Massachusetts
While the symptoms are the same: the engine fails to fire when it should and you miss a power stroke, the root cause is different.

- The use of the phrase "eight stroke" to describe a symptom caused by two different causes then can become a matter of semantics.

As Larry is describing you don't get combustion because of the two stroke's problem of clearing exhaust gases from the previous combustion cycle at idle speed. In the other, the four stroke engine fails to fire because the incoming mixture is so rich with gasoline that there isn't enough oxygen to support combustion. This can happen any any rpm. Both are the engine missing a power stroke, but for different reasons.

Either way, what we hear, and try to describe with words, is the engine missing one, or more, combustion cycles. The two stroke can 4 stroke or eight stroke in an effort to clear exhaust gas from the bottom end, while the four stroke tends to just eight stroke, or miss a power stroke, in an effort to get enough oxygen into the combustion chamber to make an gas that will burn.

Why worry about Gordon, when we have Kevin.


Re: eight stroking [Re: grommet] #443595
07/07/12 1:31 pm
07/07/12 1:31 pm
Joined: Oct 2003
Posts: 3,695
Emsworth, sunny south of Engla...
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dave jones Offline
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Emsworth, sunny south of Engla...
I covered my airfilter with tape to stop water getting in while I cleaned the bike once and forgot to take it off. I went for a ride and around town it ran surprisingly normally but out in the country if I opened the throttle more the engine sounded like it was running at half the speed which I guess is eight stroking from richness.

Dave

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