Coke!

Dear All,

Here is my today's question: looking at my combustion chambers and pistons tops, they look black and coated with coke (ie "calamine") in french despite an engine recently (about 3000 miles), fully and carefully rebuilt (ie "not by me").

I cannot distinguish wether it comes from unburnt fuel or from oil entering the combustion chamber, and hence I wonder wether I should:

a) set the carburation a bit leaner and /or use "hotter" spark plugs (currently NGK B7ES) so that the mixture is fullt burnt. The spark plugs are grey/brown, so do not look too rich

b) look for oil coming up through the pistons rings (due to wet-sumping) or down from valve guides and fix the problem

Many Thks for any advice (1st one: how to know wether it is oil or fuel residus? )

Kind regards,

Laurent

Previously wrote:

Dear All,

 

Here is my today's question: looking at my combustion chambers and pistons tops, they look black and coated with coke (ie "calamine") in french despite an engine recently (about 3000 miles), fully and carefully rebuilt (ie "not by me").

I cannot distinguish wether it comes from unburnt fuel or from oil entering the combustion chamber, and hence I wonder wether I should:

a) set the carburation a bit leaner and /or use "hotter" spark plugs (currently NGK B7ES) so that the mixture is fullt burnt. The spark plugs are grey/brown, so do not look too rich

b) look for oil coming up through the pistons rings (due to wet-sumping) or down from valve guides and fix the problem

 

Many Thks for any advice (1st one: how to know wether it is oil or fuel residus? )

 

Kind regards,

 

Laurent            

 

Hello there Frist thing you have not said which model your Machine is , or the type of ignition you weather  you have a magneto or coil ignition  but you could try Bosch W7DTC , there heat range is same as Champion N5/N6Y but they give you a cleaner exhaust , and there self shelding , well its a starting point anyway ? hope this helps   Anna J Dixon

Hi, it does not matter a bit what model you have, or even if it is a car engine!(so long as it is spark ignition, not diesel). If the carbon is dry, and sooty, then it is likely to be a fuel problem.  If it feels oily, then there could be any of a number of problems, but, after only 3,000 miles from rebuild, it is unlikely to be oil blowing up past the pistons (even if it was, it would not be due to "wet sumping", which would not occur while the engine was running), but more likely worn piston rings.   A more likely cause would be worn valve guides, allowing oil to seep down the valve stems.  Again, there should not be this much wear after 3, 000 miles, unless there is a restricted oil feed to the rocker box.   Unless the build up is really heavy, I would not worry too much about it.    In the "Good old days" it was quite usual to de-coke a 'bike engine every 2,000 miles.  Then, like now, you didn't know what additives the oil companies put in their petrol!

Maybe not a lot of help, but, if the bike runs o.k. otherwise, the saying goes "If it ain't broke, don't fix it!".

All the best,  John. 

What oil are you using? I would hope a 'Classic' 20-50 or a straight 40. If you use a modern multigrade after a rebuild, the rings don't bed in and oil will get past into the combustion chamber. Fine to use a modern multigrade once an engine is run in, but the engine does need to bed in first. I use Morris's classic 20-50 or straight 40 depending on the machine and the temperature. It seems to work for me.

The carbon may have formed during the running in period, does the exhaust smoke when all warmed up? white smoke is oil, are the plug rims dirty? ,a worn needle jet will coke a motor up fast.

Just to back up Robert's post. A worn throttle needle & needle jet will result in richness and they do wear.  Sometimes a ridge can be felt on the needle.  A plug test - runnning at full bore and 'chopping' the motor will only show if the main jet is too big/small.

When you blip the throttle (once warm) does the engine smoke at all? White smoke = oil, black smoke = rich mixture. A problem with oil (rings or guides) is usually very noticeable as clouds of white smoke when you shut the throttle off when riding the bike.

 A faulty float needle and/or seating can cause richness too - does the carb drip if the bike is left standing with the petrol on?

If it is running rich then a carb rebuild might be the answer (new slide, throttle needle and needle jet - as opposed to the main jet) then follow the Amal tuning procedure in the manual. Incidentally, both my Dommi & ES2 run main jets one size less than recommended in the original manual - modern fuels???

Cheers.

Ian. 

Many thanks to you all.

It clarifies my first question and tells me that it's fuel-coke, not oil-coke since she does not smoke (except for a white smoke for 1-2 minutes when starting after wet-sumping), rings-pistons-valve guides look fully OK and coke is not oily but sooty

It probably comes from a too rich mixture / too cold spark plug. Easy to adjust, hence 1st thing to try.

Re: Gordon's comment, I used a low-price oil for the first 300 miles after rebuild and then switched to good quality Motul. Looking at the bores, the run-in and rings bed-in seem to be OK.

My only remaining question is : why does she smoke white when starting after wet-sumping? how does the oil manage to enter the combustion chamber if rings / valves and guides are OK ?

 Cheers,

Laurent

It may be a bit of oil drain down from the head while standing,these motors don't have seals on the valves.

Hi Robert,

Mine has seals on Inlet valves & guides, but you're probably right anyway: oil may flow down from the exhaust guides into the combustion chamber .

Many thanks for your reply! 

Previously wrote:

Hi Robert,

 

Mine has seals on Inlet valves & guides, but you're probably right anyway: oil may flow down from the exhaust guides into the combustion chamber .

 

Many thanks for your reply! 

If the smoke stops when warm then its not likely to be pistons.

It is generally best to run a Dominator engine on a slightly rich mixture, especially if using old Monobloc carbs. Attempting to lean out the settings, with these carbs, is very difficult and you could end up with seized pistons if an error is made.  Concentric carbs, for whatever reason, do seem to be a little easier to set up.

The downside of the rich carb setting is the pistons quickly coke up.

However.......I suspect that your problem may be the result of other maladies which could need checking out.

Firstly - I am dubious that your rings have bedded-in after only 300 miles of running. Of course this depends on the quality of the bore, pistons and rings but in my experience, I would not expect good compression until around 500 miles shows on the clock. New pistons much further.

The choice of oils when bedding-in the pistons and rings is very important, as another contributor pointed out. Modern oils are just too good for a rebuilt engine and actually hinder the bedding-in process. Possibly, you changed to a higher quality oil a little too early and some of it is now slipping passed a glazed bore.

Next, some people rebuild their engines with a mix of parts from early and later models. For example they add a double speed oil pump to the bottom end or go for a pressurised oil feed to the rockers. The result often leading to too much oil in the wrong parts of the engine.

Lastly. Replacement Norton pistons tend to come with rings that require careful fitting. ie. the rings are usually marked 'TOP' (but not all) and must go on the pistons as marked. Some rebuilders get this wrong or worse than that, the new pistons arrive with the rings incorrectly fitted and nobody notices until the engine is started.    

Many thanks for your clever input Phil.

I think that coke is produced by too rich a mixture because of reasons explained hereabove: no white smoke except in case of wet-sumping, no need for adding oil between 2 oil drains, sooty coke.

I will first try and adjust the mixture cautiously (on my Amal MK2).

Kind regards,

L.

hello  to all you who read my threads , Well my Great uncle Freddie Dixon , tuned his engine without exhaust on , So he could see the flames coming out of the exhaust port and what colour they where. he get the engine to run with a nice blue flame. then he knew it was just right ?  yours   Anna J Dixon  

My Dad put his Electra together in the basement when I was a wee lad (about 8 years old), he started up the bike with no pipes just to see if it would run. I was standing there with my best friend and start it did, I thought it was the coolest sound ever & loved the big blue flames shooting out the front of the exhaust ports!!!!!! My mom came downstairs & was not pleased at all! I looked next to me to see what my friend thought, seems he ran off right after the Electra started, seems he had to run home to change his pants

Good times!!!!!!!

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