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  1. Keri

    Keri Leader

    It's been a great couple of years!

    Comrade Trabi has been tirelessly patrolling the midwest for well over two years and 49,000km with no major issues after my 2nd hand parts motor overhaul in May-June 2012.

    However…. Lately, over the last couple of thousand kilometers, I've noticed….. Spark Knock! Particularly at high speeds of over 90km/h.
    Also known as "pinging", "pinking" "pre-ignition" and "detonation", this phenomenon can murder your motor quickly.

    At first I thought: "aha! those iridium sparkplugs must be too hot!" But no, re-installing the NGK BR6HS had no effect other than less throttle response.

    Remembering that if 2-cycle motors had a fault, which they don't, it would be the tendency to build up carbon in the combustion areas.
    So, I concluded that there must be carbon buildup. So, I started using high-octane fuel until I could get the heads off. This only reduced the tendency to ping.

    So, 3 days ago, at 49,500km since overhaul, I pulled off the heads and guess what? Both heads and pistons had considerable carbon buildup! And, the carbon had a hard, partially melted texture with many sharp, raised points due to the pre-ignition which would tend to encourage even more pre-ignition.

    Cleaning the carbon off revealed no damage to the underlying aluminum.
    A set of new head gaskets, torquing the bolts to 42NM and we're back on the road with no more spark knock! Probably good for another 45,000+ km!

    Some pointers:

    Spark Knock, which sounds like a light, rapid rattle that varies with throttle application, can damage your motor in the following ways:
    1- It can damage your cylinder heads, gaskets, spark plugs and piston crowns, tearing out small chunks of aluminum and breaking spark plug ceramic.
    2- It can cause very rapid wear to piston rings
    3- It can damage your connecting rod bearings at both ends.

    It can be caused by the following:
    1- low fuel octane for the motor's compression ratio
    2- overheating
    3- too lean of fuel/air ratio
    4- incorrect spark timing
    5- Too "hot" of heat range spark plugs
    6- carbon build-up, which increases compression, insulates the combustion area from the cooling system and might also provide sharp points that can glow at high load

    When experiencing spark knock, it is best to:
    1- reduce load immediately so that the pinging stops.
    2- High-octane fuel should be used to reduce the pinging until the cause can be determined and corrected.

    When decarbonizing a Trabi motor, best practice is:
    1- have a new set of head gaskets on hand
    2- Do one cylinder at a time, to prevent debris from one cylinder entering the other
    3- Clean the piston crown at TDC so that a minimum of debris gets past the piston. A wire cup brush on a drill works well. Avoid gouging the piston surface with scrapers or excess pressure on the brush. Also avoid solvents on the piston, which will get past the rings, washing off oil and bringing debris downward.
    4- Clean the heads in the least abrasive manner possible. Blasting with walnut-shell media is quick, effective and safe on aluminum. Solvents might also work. Avoid scraping if possible.

    Cylinder head bolt torque is 41.2 Nm or about 31 ft-lbs. it is best done criss-cross, in at least two stages.
    That's it! don't over-tighten the bolts, as the threads can easily strip out and over-crushing the gaskets will cause leaks, not cure them.

    Happy Motoring…..

    PS> Comrade Trabi accumulated enough carbon to cause light detonation after 49,000km using a "low ash" synthetic oil at 50:1 fuel/oil mix, or 2% oil with mostly highway driving.
    Using non-synthetic oils or a higher oil percentage in the fuel will result in faster carbon build up!
    Last edited: Aug 11, 2014
    Justin likes this.
  2. turbofiat124

    turbofiat124 Premium Member Forum Donor

    This one guy I work with says Seafoam will fix anything! Including decarbonizing the heads and pistons.

    Keri, have you ever seen a run away 2 stroke? When I was on breaker points, I was trying to set the timing and had the engine reved up past 1500.

    I must have had the timing advanced too far. I shut the engine off and it continued to run and the battery light on was. Even with the key out of the ignition! I had to pull the hose off the carb and smoother the engine with the palm of my hand to kill the motor.

    What would have caused this? carbon the head getting too hot and acting as a glow plug? The engine had to be getting an ignition source from somewhere.
    Last edited: Jul 28, 2014
  3. Keri

    Keri Leader

    Yes, that is very likely the cause.
    Something was providing an ignition source and that was almost certainly a glowing chunk of carbon or perhaps aluminum. It could also be a glowing too-hot spark plug.

    Be careful about cleaning the piston crowns. You want a minimum of stuff getting past the piston top and into the rings.
    Any sort of liquid is a bad idea unless the pistons are out of the cylinders.
    This is why I used a 2-¼" (60mm) wire cup brush on a drill. the cup brush flings the debris outward rather than downward. With the piston at TDC, very little went down.
    The cup brush I used is also fairly gentle on aluminum and left the surface smooth.
  4. DDR

    DDR Loyal Comrade

    When I had a BSA Bantam (the two stroke motor is actually a DKW copy) I had to de-coke it every few thousand miles , I found a plastic scraper worked well to get the thick of it off and it left no scratches, to get the finer stuff off I used a brass brush, the small ones you get for suede shoes , took slightly longer than with a steel wire brush but left no marks. I agree with keri, any solvents are a bad idea .
  5. Matteo

    Matteo Premium Member Forum Donor

    What do you think guys about this?:

  6. JFB

    JFB Puttering Along

  7. DDR

    DDR Loyal Comrade

    Um, Is this a hoax? I would not attempt this especially on a 2 Stroke engine ! . How can water or steam shift carbon . If you have to scrape something hard for it to come off then I cannot see steam at relatively low pressure shifting all that . Besides, even if it could do that I would not want chunks of carbon getting on my valve faces or seats (on a 4 stroke ) . Hmmm seems abit dubious to me .
  8. mati0921

    mati0921 Loyal Comrade

    I have seen an engine treatment product here that they all say works (dont remember name currently) it said on the back of it it was 70% water + some other stuff to mix the water with the fuel in the tank. :)
    Put it back on the shelf. not sure if this is healthy at all for an engine. on a fourstroke it will probably get in your valves, and in a trabi if one of those chunks gets between the cylinder wall and a piston ring its toast... :)
    Unless it dissolves the carbon into such small bits it wont do harm. but just a theory. :D
  9. Matteo

    Matteo Premium Member Forum Donor

    Theory says at high temperature (sorry, I cannot say how high is high) water dissociate in hydrogen and oxygen in a quite violent, " explosive" way. This is how, pouring or rather better spraying a little of water in a hot, running engine you could possibly decarbonize the engine.
    PhilipM likes this.
  10. JFB

    JFB Puttering Along

    I tried the water trick on my first car that I had. I was a stupid teen learning mechanic and trying every single thing that heard!
    And I won't never try this on a 2 stroke, but on a 4 stroke engine, I tried on my '85 Honda Civic.
    It had a simple 2 barrel carb and as I asked a friend of mine to rev the car, I dropped a glass of water into the carb. He had to raise the RPM very high to not let the motor shut up, it made a weird noise, the ignition seemed to work like shit, but you had to see the black smoke coming from the exhaust! It was incredible!
    I tought it was all the carbon going out of the exhaust, today, I understand it was more likely an bad air/fuel mixture caused by the water that takes the volume of the air in the mix. So it was smoking like a carb that is totally out of tune.
    And at the end, I never destroyed anything on the motor, I kept the car for another 4 years adding more than 150 000 on the clock using it as a express messenger in the city.
    I never really noticed a difference either.

    However, do you know that they use that on airplanes to artificially boost the power at takeoff on radial and jet engines?
    They vaporize a mixture of water and alcohol I guess, into the carb inlet and that will expand the volume of the mixture and artificially increase the compression. That help to take off in very hot and dry places in the world, and it worked best on old low bypass jet engines and piston radials engines.
  11. turbofiat124

    turbofiat124 Premium Member Forum Donor

    I breifly ran a water/alcohol injection system on my Fiat Spider after installing the turbo until I could get my detonation under crontrol. It was more/less a canister with a windshield washer pump and a fine spray nozzle.

    Just to make a long story short, the water over time removed all traces of carbon from the top of the piston. This is a twin cam and I can actually see the top of the pistons with the spark plugs removed. Just like a Trabant engine.

    Eventually determined the alcohol was making up for a lean fuel mixture. I corrected that and sold the injection system.

    Since this was basically a windshield washer system, I considered from time to time just disconnecting the tubing going to the spray nozzles and connecting it to the intake manifold and activating my washers under wide open throttle to blast off any carbon buildup.

    But like everyone else, I'd be concerned about doing this on a 2 stroke engine.

    I may try the Seafoam stuff.
    Last edited: Aug 7, 2014
  12. Matteo

    Matteo Premium Member Forum Donor

    In the past years I used to work as a professional car mechanic in a Citroen garage.
    I can assure that every time I pulled off a cylinder head due to head gasket failure, if water was going into one or more cylinders, the piston and the head of that cylinder always was spotlessly clean and free from any carbon built up.
  13. Keri

    Keri Leader

    The trouble with water ingestion:

    on 4-cycle motor:
    Water does not compress. Since it is not being vaporized in the carburetor, it is very easy to "slug" the motor , cause hydraulic lock and severe damage like bent connecting rods, blow head gaskets, or even a hole punched through the cylinder wall! This could work if a method is found to vaporize the water thoroughly and it is used for a long period of time.
    Pouring a liter or two down the intake under no-load conditions is more likely to cause great harm than any good.

    On 2-cycle motor:
    Severe and extreme damage to roller bearings is likely from either poor lubrication or corrosion. This is in addition to the hazards mentioned above.

    On a Trabant:
    In the time it takes to slowly pour water down the intake with the motor running, you could have had both heads off, physically cleaned and re-intalled.
    Why would you waste time on a procedure that might not work, might ruin your motor and will do nothing about any metal damage, as shown above?
    Justin and DDR like this.
  14. DDR

    DDR Loyal Comrade

    My thoughts exactly, I do not think there are short cuts to de-coking a motor. These silly products like seafoam in my own opinion will do more harm than good. Here in the U.K they sell a product called Wynn's Engine flush. Usually , it is used on 4 stroke engines to remove carbon build up on rings and valves and to de-sludge a sump . A roadside mechanic I know has been in the business for 40+ years and he has seen many a ruined engine as all the 'natural' build up on rings on worn engines has been stripped away causing ' Blow past' of the gasses which ruin rings ,pistons and bores. As Keri has pointed out before, if you cannot invest time and patience and do the job correctly then you should not own a fine classic car. I am old fashioned , I would rather take the time and carefully strip the carbon away manually ,this is an Ideal opportunity to check the condition of my bores ,pistons and the gap between the rings and bores. Why waste time on this silly idea and money on these "Snake-oil" remedies/wonder potions , another capitalist con. Buy this = Money.... use it and ruin your engine = MORE money . Yes, the water thing is free but I am sure if it was the be all and end all of decarbonizing then every garage would do it. If you are daft enough to do it then why not try bleach in your oil to get your engine nice and clean !( I am not being serious, please do not try this !) :D
    Justin likes this.
  15. turbofiat124

    turbofiat124 Premium Member Forum Donor

    Somewhat on the subject. If you are going to pull the heads to decarbonize the heads/pistons, would it be worth shaving 1mm off the heads to raise the compression ratio a bit? TnTrabbi said he heard you can knock off as much as 1mm and be in the safe zone, 2mm max if you dare but you run the risk of the spark plugs hitting the pistons. John Short said he tried somehting like this and it messed the piston skirts up. Then I've heard taking off 1mm will hurt the "squish" and cause detonation.

    Machine shops shave heads all the time to resurface them.

    I'm not sure how much taking 1mm off the heads would raise the C/R. Or even how much power it would increase.
  16. Justin

    Justin Founder Moderator

    I agree also with all the above..."Short cuts" usually cost you a lot more time, money, effort and troubles in the end. Do it correctly the first time and when needed. These cars get rarer and rarer every year....Why risk it?
  17. JFB

    JFB Puttering Along

    I just did the decarbonizing manually by taking of the heads and scrape off the carbon on too of the piston and on the heads. It takes about half an hour if you take your time to enjoy and shoot pictures, and probably less if you are in a rush.
    I used a plastic scraper to remove the carbon, and brushed off the rest with a scotch bright and a bit of varsol. No need any water or seafoam ...
    Especially on a Trabant!

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