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Decoke? #719193
12/18/17 4:45 am
12/18/17 4:45 am
Joined: May 2010
Posts: 2,584
Illinois, USA
TR6Ray Offline OP

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TR6Ray  Offline OP

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Illinois, USA
Decoking or decarbonizing an engine -- is it really necessary? This came up on another forum and got me wondering (yet again). I would appreciate your opinions.

Here's what the Triumph Service_Manual says:

  • It is not normally advisable to remove the carbon deposits from the combustion chamber and exhaust ports until symptoms indicate that decarbonising is necessary. Such symptoms as falling off in power, loss of compression, noisy operation and difficult starting are all indications that decarbonising may be necessary.
  • If special decarbonising equipment is not available then a blunt aluminum scraper of a piece of lead solder flattened at one end, should be used to remove the carbon deposits. Do not use a screwdriver or steel implement of any kind on an aluminum surface
  • When removing the deposits from the piston crown, a ring of carbon should be left round the periphery of the pistons to maintain the seal. Also the carbon ring round the top of the cylinder bore should not be disturbed. To facilitate this, an old piston ring should be placed on top of the piston, level with the top surface of the cylinder block.


I have cleaned pistons when I have had an engine apart for some other reason, but I have never lifted a head specifically to "decoke" the engine. Do any of you do this? Does it make any difference, or does it just make you feel good because you were spending quality time in your fettling shed?

Will the combustion chamber not become re-carboned in a very short time after it's back in service?

The big question -- why should we leave a ring of carbon around the periphery of the piston and at the top of the bore? That is not present on a new engine or one that has been bored and honed, so why is it an advantage for a run-in engine?

Finally, what is "special decarbonising equipment"? Did such a thing ever exist? If so, what did it look like?

Thanks in advance for your opinions.

Ray


'64 TR6R Plus some Twins from other countries (U.S., Germany, Japan)
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Re: Decoke? [Re: TR6Ray] #719200
12/18/17 8:12 am
12/18/17 8:12 am
Joined: Aug 2014
Posts: 1,537
OZ
Triless Online content
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OZ
Whenever I have a head off, for a valve or headgasket check ( I won't mention pushrod seals !), and if I think carbon removal from piston crowns may be necessary, I have the shop vacuum cleaner in operation to suck up the loose carbon.

Re: Decoke? [Re: TR6Ray] #719204
12/18/17 9:22 am
12/18/17 9:22 am
Joined: Aug 2015
Posts: 1,160
Bolton Lancs UK
A
Andy Higham Offline
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Andy Higham  Offline
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A
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Bolton Lancs UK
Modern fuels and oils produce less carbon build up. Manufacturers used to specify decoke intervals of 10,000 miles, they don't any more
My Audi (A6 diesel) has 215,000miles and the heads have never been lifted
My Toyota van (Hiace diesel) has 260,000miles and the head has never been lifted
My previous Lexus (LS400 Petrol) had over 300,000miles and the heads had never been lifted


1955 BSA B31 500cc "Stargazer"
1962 Greeves 200cc "Blue Meanie"
1962/67 Greeves 350
1967 Greeves 360 Challenger
1984 Suzuki GSX1100 EFE "Sorcerers Apprentice"
GM500cc sprint/LSR bike "Deofol"
Jawa 500cc "Llareggub"
2003 Aprilia RSV Mille "Lo Stregone"
Re: Decoke? [Re: TR6Ray] #719223
12/18/17 12:54 pm
12/18/17 12:54 pm
Joined: Feb 2008
Posts: 4,359
New Jersey USA
Tridentman Online content

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"In the Day" it was what was called an Italian decoke.
Go to the nearest stretch of straight road with little traffic and go flat out for as long as the road permitted.
Burned off the surplus carbon and was fun as well!
Seriously--- the way a lot of Brit bikes are ridden these days---poodling along country lanes or a leisurely ride to the local show---a good thrashing does them a power of good.
In pre war days a regular decoke was a Saturday morning job for the bike owner in UK --making sure it was ready to take him to work on Monday morning.
With better (?) fuels and oils this is no longer necessary.
I would never dream of dismantling an engine solely to decoke it.
Just my two centsworth of course.

Re: Decoke? [Re: TR6Ray] #719238
12/18/17 2:35 pm
12/18/17 2:35 pm
Joined: May 2010
Posts: 2,584
Illinois, USA
TR6Ray Offline OP

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TR6Ray  Offline OP

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Posts: 2,584
Illinois, USA
O.K., so in modern times with modern fuel and oil, we don't need to lift the head for a weekly decoke. If you have the top end off for a hone and some fresh rings, how much do you trouble yourself to clean the combustion chamber? I try to scrub off all the carbon before reassembly, but I wonder if that is a waste of time? Carbon will be back soon anyway, and why did the book tell us to leave the carbon around the outer edge of the piston?


'64 TR6R Plus some Twins from other countries (U.S., Germany, Japan)
Re: Decoke? [Re: TR6Ray] #719259
12/18/17 5:07 pm
12/18/17 5:07 pm
Joined: Aug 2005
Posts: 1,402
melbourne florida
B
bodine031 Offline
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Posts: 1,402
melbourne florida
Do not take apart a fine running motorbike!!!!! Chevron Techtron dump a bottle in a full tank of fuel and go ride till you need more fuel.

Re: Decoke? [Re: bodine031] #719261
12/18/17 5:35 pm
12/18/17 5:35 pm
Joined: Jul 2017
Posts: 29
Hangtown, CA>
B
barncobob Offline
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Hangtown, CA>
Im old school and the EYETALIAN TUNEUP works great,,some cleaner in gas and drive the pasta out of it:}

Re: Decoke? [Re: TR6Ray] #719290
12/18/17 9:02 pm
12/18/17 9:02 pm
Joined: May 2014
Posts: 350
New Jersey
F
Fetch Offline
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Fetch  Offline
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F
Joined: May 2014
Posts: 350
New Jersey
Not to take the joy out of a good run at open throttle. Pro & Cons about products like Seafoam? Says to run the engine up to operating temp, spray into the air intake until the engine and stalls and let the chemical do it's work. A few minutes later kick it over and give it a good rev and some amount of the once deposited carbon is supposed to exit via the exhaust.

I tried this once. It did produce a momentary black cloud.

Re: Decoke? [Re: TR6Ray] #719311
12/18/17 11:23 pm
12/18/17 11:23 pm
Joined: May 2013
Posts: 4,847
Rotherham - S. Yorkshire
Allan Gill Offline

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Rotherham - S. Yorkshire
You could run a leaner mixture, or least until your about 14.7:1 and that will cut carbon deposits down by a lot, these bikes would have been tuned for power (and to help keep the engine cooler) so if you were to measure it you’ll be around 12:1. However just ride it and not worry, cokeing with modern fuels isn’t as bad as the fuels that were used when the manual was written (as already mentioned)


beerchug
Re: Decoke? [Re: TR6Ray] #720026
12/26/17 10:09 am
12/26/17 10:09 am
Joined: Apr 2015
Posts: 114
Wolverhampton, U.K
S
Simon Ratcliff Offline
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Wolverhampton, U.K
Originally Posted by TR6Ray
Decoking or decarbonizing an engine -- is it really necessary? This came up on another forum and got me wondering (yet again). I would appreciate your opinions.

Will the combustion chamber not become re-carboned in a very short time after it's back in service?

The big question -- why should we leave a ring of carbon around the periphery of the piston and at the top of the bore? That is not present on a new engine or one that has been bored and honed, so why is it an advantage for a run-in engine?


It becomes necessary to decarbonise an engine when the air: fuel ratio is fuel rich and/or engine oil is getting into the combustion chamber via piston ring leakage and/or valve guide leakage. If oil consumption is in the hundreds of miles/pint then you need to check bore ovality, surface finish and piston ring gap. Valve guides should be sized after fitting to manufacturers spec. Kibblewhite recommend valve stem clearance between 0.0008" to 0.0015" and oil seals fitted if possible. Oil can also leak between guide and head where the cylinder head has been scored due to inappropriate guide removal/installation. Exhaust tail pipe should be clean and not sooty, wipe your finger inside.

Put your bike on a dyno to get your air:fuel ratio sorted.

No need to leave carbon rings in the engine if the above is done properly and the engine will not need decarbonising for tens of thousands of miles.

Carbon is very hard and if left to build up will start to produce small particles which glow red hot, break away and end up going through the exhaust port some of which get embedded into the valve and seat faces and cause loss of compression due to valve seat leakage.

Thrashing the engine to decarbonise it has next to no effect and is a bar room myth. Only way to decarbonising is by physically cleaning (scraping, beadblasting or ultrasonically) or possibly chemical.













Last edited by Simon Ratcliff; 12/26/17 10:14 am.

Norton Mk3 Commando.
Re: Decoke? [Re: TR6Ray] #720043
12/26/17 6:47 pm
12/26/17 6:47 pm
Joined: Dec 2013
Posts: 4,002
ohio, usa
kevin roberts Offline
fefsa
kevin roberts  Offline
fefsa
Joined: Dec 2013
Posts: 4,002
ohio, usa
my experience is different.

my aged aunt owned a 1965 pontiac starchief-- 389 cubic inches, 10.25 to 1 compression. as she was in a wheelchair, the car got about 100 miles of slow running per month in a small town, as she was driven about by her elderly sister. probably never saw more than 30 mph.

when i would visit, the carbon buildup was so thick that the engine would detonate violently just pulling away gently from a stop sign. i would take it out to the highway, pull into a straight stretch and floor it. the engine would rattle a moment, and then generate an opaque cloud of carbon out the pipes for half a mile. once it was cleaned out, the car would pull to the ton effortlessly with no smoke and no detonation.

nice car. i bought it from her, wore it out, rebuilt it and wore it out again. took it across wyoming once and don't remember it ever dropping below 90 mph across the entire state.


i have no idea what i'm doing.
Re: Decoke? [Re: TR6Ray] #720055
12/26/17 9:04 pm
12/26/17 9:04 pm
Joined: Feb 2008
Posts: 4,359
New Jersey USA
Tridentman Online content

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+1 with Kevin.
Not theory but hard experience.
From motorcycles in UK in the 1960s to bikes and cars here in US in the 2010s.
After slow running give the engine an Italian decoke, watch the black smoke out of the exhaust and after a few miles it runs sweeter and idles faster.

Incidentally, Simon, I too attended Coventry Polytechnic for my first degree.
however it those days it was known as the Lanchester Polytechnic after the famous automotive engineer.
One thing that I have learnt over the years is that engineering theory and engineering practice are normally the same.
However if they differ then believe the latter.
I learnt that in the University of Life!

Re: Decoke? [Re: TR6Ray] #720084
12/27/17 12:28 am
12/27/17 12:28 am
Joined: Aug 2015
Posts: 1,160
Bolton Lancs UK
A
Andy Higham Offline
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Andy Higham  Offline
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A
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Posts: 1,160
Bolton Lancs UK
I've noticed, especially in diesel engines, that after a good thrashing they feel more responsive. Whether this has anything to do with carbon build up or other factors I cannot say


1955 BSA B31 500cc "Stargazer"
1962 Greeves 200cc "Blue Meanie"
1962/67 Greeves 350
1967 Greeves 360 Challenger
1984 Suzuki GSX1100 EFE "Sorcerers Apprentice"
GM500cc sprint/LSR bike "Deofol"
Jawa 500cc "Llareggub"
2003 Aprilia RSV Mille "Lo Stregone"
Re: Decoke? [Re: TR6Ray] #720087
12/27/17 12:54 am
12/27/17 12:54 am
Joined: Mar 2006
Posts: 1,304
Magnolia, TX
htown Online content
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Magnolia, TX
Good story, Kevin. My dad was in the car business in the 50's and 60's. He would every so often take a car in trade, often from an elderly person, that didn't run right. He would do exactly what you described, the black smoke would roll. He called it "blowing the cobs out". Then it would go on the lot without having to have a mechanic touch it. Since the technology of our bikes is about the same level as those cars its a good idea to give them a little hard running every so often.


1978 Bonneville T140E
1974 Trident
1970 BSA Thunderbolt
1971 Norton Commando
1972 Norton Commando
1973 Norton Commando
1974 Norton Commando
2004 XL 1200R Sportster

Everything will be alright in the end. If its not alright, its not the end.
Re: Decoke? [Re: TR6Ray] #720109
12/27/17 6:42 am
12/27/17 6:42 am
Joined: May 2007
Posts: 2,023
Christchurch NZ
R Moulding Offline
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R Moulding  Offline
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Posts: 2,023
Christchurch NZ

Ray, when I did my time I was always told that you should never remove the carbon ring from the top of the bore as it would cause the engine to burn oil. I've disproved this theory several times since.

Rod


So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth;
And pray that there's intelligent life somewhere out in space,
'Cause there's bugger all down here on Earth!
Re: Decoke? [Re: R Moulding] #720112
12/27/17 7:15 am
12/27/17 7:15 am
Joined: Apr 2015
Posts: 114
Wolverhampton, U.K
S
Simon Ratcliff Offline
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Simon Ratcliff  Offline
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Wolverhampton, U.K
Originally Posted by R Moulding

Ray, when I did my time I was always told that you should never remove the carbon ring from the top of the bore as it would cause the engine to burn oil. I've disproved this theory several times since.

Rod


Totally agree, the piston rings provide the seal, as long as they (and the bore) are in good condition and have not badly worn by an incorrectly machined cylinder i.e surface finish.

Due to oil contamination and excess fuel I have in the past needed to remove carbon deposits from pistons, valves, exhaust ports and combustion chambers. A blow torch using propane burns at approximately 2,000 deg C, similar to combustion temperature's in a petrol engine. Applying the flame directly for several minutes to the carbon deposits removed only a tiny amount of particles. The carbon remained largely unchanged and is very hard (diamonds are made of similar stuff). Careful scraping and abrasive pads is one way of removing carbon. Running the engine at max revs is not.


Last edited by Simon Ratcliff; 12/27/17 7:17 am.

Norton Mk3 Commando.
Re: Decoke? [Re: Simon Ratcliff] #720114
12/27/17 8:50 am
12/27/17 8:50 am
Joined: Aug 2014
Posts: 1,537
OZ
Triless Online content
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Triless  Online Content
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OZ
Now, I'm no engineer ( yes, I know, so blast away you clever buggers !), but I think what is meant by the " mediterranean " decoke, is that a good rev out will clear t h e "fluffy stuff " after extended slow running, before it becomes " hard stuff ". Now, this blowing the s...t out prolongs the time before the " hard stuff " gets to the stage where it has to be physically removed.
So, basically, a good periodic thrash prolongs the inevitable ! Well, thats what I reckon !

Re: Decoke? [Re: TR6Ray] #720118
12/27/17 9:32 am
12/27/17 9:32 am
Joined: May 2013
Posts: 4,847
Rotherham - S. Yorkshire
Allan Gill Offline

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Rotherham - S. Yorkshire
I agree with triless, when I was a mechanic we would have cars come in with lights in the dashboard and running sluggish (usually diesels) I used to drive them up the bypass as fast as I could (which wasn’t very fast as they were choked up) then drop them into 3rd with no acceleration applied, let the revs bounce for a second and all will be restored.
However if it’s gone too far then the EGR gets blocked up and you need a new one of those too.


beerchug
Re: Decoke? [Re: TR6Ray] #720120
12/27/17 9:44 am
12/27/17 9:44 am
Joined: May 2007
Posts: 2,023
Christchurch NZ
R Moulding Offline
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Christchurch NZ

Allan, Ford released a TSB for the early EGR equipped Diesel engines in the Escorts and Mondeo. It basically stated that to cure excessive exhaust emissions a ball bearing should be jammed into the vacuum hose that operates the EGR.

Rod


So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth;
And pray that there's intelligent life somewhere out in space,
'Cause there's bugger all down here on Earth!
Re: Decoke? [Re: Triless] #720144
12/27/17 4:29 pm
12/27/17 4:29 pm
Joined: Apr 2015
Posts: 114
Wolverhampton, U.K
S
Simon Ratcliff Offline
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Simon Ratcliff  Offline
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Joined: Apr 2015
Posts: 114
Wolverhampton, U.K
If your engine is producing carbon at low revs why not get your carburetion sorted out? How do you know it's not producing carbon at high revs.

P.S I'm quite happy revving my Commando 850 to 6,000 rpm through the gears on a regular basis not to decoke it but because it's fun.


Norton Mk3 Commando.
Re: Decoke? [Re: TR6Ray] #720149
12/27/17 4:52 pm
12/27/17 4:52 pm
Joined: Feb 2008
Posts: 4,359
New Jersey USA
Tridentman Online content

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New Jersey USA
Kill two birds with one stone!

Re: Decoke? [Re: TR6Ray] #720150
12/27/17 4:58 pm
12/27/17 4:58 pm
Joined: May 2010
Posts: 2,584
Illinois, USA
TR6Ray Offline OP

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TR6Ray  Offline OP

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Illinois, USA
Thanks for all the replies, several of which made me smile. I guess I should own up to the fact that I am about to pull the heads and jugs off another Bavarian Money Waster (Triless' terminology) to replace PRT seals. Anticipating cleaning the piston tops, valves, heads while it is apart, I was wondering how much effort is worthwhile and how much is wasted effort. I guess if I get things spotless in there, it makes room for the process to start over. Simon, I will be making my best effort to get the carburetion sorted at the same time. My TR6R seems to be doing fine, but I sought BritBike input anyway for my latest Brand X machine. I wish you all a happy and carbon free 2018! Keep the cobs blown out.


'64 TR6R Plus some Twins from other countries (U.S., Germany, Japan)
Re: Decoke? [Re: TR6Ray] #720151
12/27/17 5:46 pm
12/27/17 5:46 pm
Joined: Dec 2013
Posts: 4,002
ohio, usa
kevin roberts Offline
fefsa
kevin roberts  Offline
fefsa
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Posts: 4,002
ohio, usa
if you're taking the pistons out, a simple overnight soak in water and detergent will soften the carbon deposits enough that much can be removed with a hardwood stick.


i have no idea what i'm doing.
Re: Decoke? [Re: Simon Ratcliff] #720179
12/27/17 9:34 pm
12/27/17 9:34 pm
Joined: Aug 2014
Posts: 1,537
OZ
Triless Online content
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Posts: 1,537
OZ
With all due respect, Simon, I don't really plod my engines, but I always thought that on occasion if my T140 got a bit snotty it was due to valve guides. These were replaced quite a few years back in the only, to date, rebuild. I'm not riding it as much these days ( race project taking precedence ), but I'll get onto this. I actually thought my carburetion was OK, but, unfortunately, I never attended one of those poly things ! The things we learn !
Anyway, I was making a general comment, not specifically referring to my machines, other than in a previous post when I mentioned about having a head off for reasons other than a decoke.

Re: Decoke? [Re: Triless] #720230
12/28/17 7:49 am
12/28/17 7:49 am
Joined: Apr 2015
Posts: 114
Wolverhampton, U.K
S
Simon Ratcliff Offline
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Posts: 114
Wolverhampton, U.K
As most valve guides are an interference fit into the head the guide bore is no longer parallel once fitted and has an hour glass profile i.e tapers down into the middle from each end. This offers little support to the valve stem and guide to stem clearance is excessive which allows oil into the combustion chamber. This is typical of most available guides. However there are manufacturers such as Kibblewhite that supply guides which require reaming and honing to size when fitted. You end up with a parallel bore with controlled clearance so that oil loss is minimal and the guides offer good support for the valve stem and subsequently last much longer than commonly available guides which typically 'leak' oil from new. Valve to seat sealing is better and again lasts much longer.

Going to a technical college dosen't directly improve your bikes carburetion but putting your bike on a dyno does, or at least points you in the right direction.


Last edited by Simon Ratcliff; 12/28/17 8:07 am.

Norton Mk3 Commando.
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