Misfire

[Johan van ZantenParticipant @johanvanzanten]

> Hi all,

I am currently constructing a 1910 Fairbanks-Morse RN engine from scratch. I make use of the plans of Pacific Model Design but I build in a smaller scale ( 1: 10 ) My engine is a 3 cylinder in line engine with a bore of 30mm and a stroke of 40mm and has valves in head. It has, like the prototype, 3 carburettors with 4 fuel needles each end low tension make and brake ignition.

Fairbanks-Morse

Old carburettors

When I was ready to test the engine it started almost immediately but it proved to bee impossible to get a proper carb. setting. It was also impossible to restart the engine when cooled down due sticky and malfunctioning igniters. Cleaning up the axels and contacts was the only way. I (temporary) changed the ignition to the proved high tension ignition system and used Rimfire plugs in the houses of the igniters, witch are dummies now. The breaker points and distributor are placed in the engine base and driven by a gear train from the crankshaft. These modifications cured the (re)start problems. Next I made a new intake manifold with only 1 carburettor and 1 throttle valve. The carburettor used is an auxiliary air valve type witch gave good service on my earlier Holt 75 engine.
At last the engine came to life! Starting is excellent now and under load it runs beautiful at 500-600 rpm. But when I release the load it starts to misfire and sounds like a 3 cylinder 2 stroke running idle.
Slowly making the mixture richer results in galloping and black smoke, while leaning out results in opening the throttle by the governor to maintain a constant speed. Only with a (partly) open throttle the engine runs ”decent” and sounds like an old fashioned caterpillar ( Beautiful ).
Is there anybody out there to shine a light on my problem?

Johan van Zanten, The Netherlands. >

Edited By Johan van Zanten on 12/09/2010 11:25:35

Edited By Johan van Zanten on 12/09/2010 11:28:34

[Johan van ZantenParticipant @johanvanzanten]

[Johan van ZantenParticipant @johanvanzanten]

Posted by Johan van Zanten on 12/09/2010 11:17:25:

> Hi all,

I am currently constructing a 1910 Fairbanks-Morse RN engine from scratch. I make use of the plans of Pacific Model Design but I build in a smaller scale ( 1: 10 ) My engine is a 3 cylinder in line engine with a bore of 30mm and a stroke of 40mm and has valves in head. It has, like the prototype, 3 carburettors with 4 fuel needles each end low tension make and brake ignition.

Fairbanks-Morse

Old carburettors

When I was ready to test the engine it started almost immediately but it proved to bee impossible to get a proper carb. setting. It was also impossible to restart the engine when cooled down due sticky and malfunctioning igniters. Cleaning up the axels and contacts was the only way. I (temporary) changed the ignition to the proved high tension ignition system and used Rimfire plugs in the houses of the igniters, witch are dummies now. The breaker points and distributor are placed in the engine base and driven by a gear train from the crankshaft. These modifications cured the (re)start problems.

 

New ignition

Next I made a new intake manifold with only 1 carburettor and 1 throttle valve. The carburettor used is an auxiliary air valve type witch gave good service on my earlier Holt 75 engine.

New carburettor

 

At last the engine came to life! Starting is excellent now and under load it runs beautiful at 500-600 rpm. But when I release the load it starts to misfire and sounds like a 3 cylinder 2 stroke running idle.
Slowly making the mixture richer results in galloping and black smoke, while leaning out results in opening the throttle by the governor to maintain a constant speed. Only with a (partly) open throttle the engine runs ”decent” and sounds like an old fashioned caterpillar ( Beautiful ).

 
Is there anybody out there to shine a light on my problem?

Johan van Zanten, The Netherlands. >

Edited By Johan van Zanten on 12/09/2010 11:25:35

Edited By Johan van Zanten on 12/09/2010 11:28:34

 

Edited By Johan van Zanten on 12/09/2010 13:34:45

Edited By Johan van Zanten on 12/09/2010 13:39:32

[Ian S CParticipant @iansc]

Johan, sorry can’t help this time, but even though its not quite running right it looks very good. Once you get it firing right you’ll be able to get on with the generator. Best wishes Ian S C

[MichaelRParticipant @michaelr]

I am no expert and no way could I build a masterpiece like your engine, but would a advance and retard mechanism do anything for the misfire.

I am only into simple Hit and Miss engines, however I am sure you will get the problem sorted out

Maybe you have this fitted.

Edited By Stick on 12/09/2010 16:27:26

[Ian AbbottParticipant @ianabbott31222]

Making a guess here, but I don’t know what type of carburetter you have fitted.  

The problem could be something to do with the air flow through the carb.  Too small a venturi and the engine will get too little air, too big, and the airflow through the venturi will be too slow at idle to draw fuel and vaporize it properly.

From the symptoms, the engine running acceptably at a mid range speed, could indicate the latter, with the airflow then moving quickly enough to get a good mixture.

Is it possible to eliminate the fuel with the throttle closed and fit an auxiliary jet for idle? 

 

Ian 

[Sub MandrelParticipant @submandrel]

Johan,

 

I can’t give you any advice I’m afraid, but i’d like to congratulate you on a fantastic piece of work. I hope you solve what do sound like mixture problems.

 

Neil

[Johan van ZantenParticipant @johanvanzanten]

Problem solved!>>

> >

The problem of my misfiring engine is solved last week !>>

Preparing my engine for a show I dropt one of the sparkplugs and found the gap totally closed. By opening up the gap again the ground electrode broke. I had no spare plug at hand so I decided to use the plug and tried to start the engine. Much to my relief it started at once and seems to run even better. I opened up the gaps of the other plugs to about 1.5 mm and the engine was reborn. Next day, on the show, the engine ran for 6 hours without any misfire.>>

>>

Has anyone an explanation ?>>

> >

The plugs I use are Rimfire plugs ¼ x 32 VR2L >>

[Richard ParsonsParticipant @richardparsons61721]

Johan- Hi, First may I congratulate you on a fantastic model.

I have messed around with petrol engines for many years. Multiple carburettors are always a problem. Normally you start to set them up with a vacuum manometer gauge, but you always have to end up using a twin piped stethoscope and listen to the Hissing noises. If I had a few pennies for every twin carb engine I have spent a Saturday afternoon setting up (with the rain running down the back of my neck) I would be rich enough to eat ham every day for my tea. Your single carb set up is OK. It is not a ‘racing’ engine but one which will drive a dynamo and at constant speed.

Plug gap settings are often the cause of problems, but I have found (from 30cc chain saws to a 500cc singles –a Vincent Black Comet) that 25 thou ).025” – 0.635mm was a good setting. Where you are using a very powerful HT coil then settings of up to 2mm may well be right. The reason is that as the pressure in the cylinder raises the voltage needed to drive the spark also rises, so increasing the gap can retard the spark.
Good luck

Dick

[Clive HartlandParticipant @clivehartland94829]

While owning a Ford Escort I experienced misfiring, suspecting the plugs I installed new ones but it still happened. This was using the recommended settings of 25thou.

I had the use of a Spark Plug tester and was able to observe the plugs working upto 150Psi.

As the pressure was increased the plugs started to misfire and the spark became erratic, my first reaction was to open the points to 30thou. which took the plugs up to the maximum pressure with no misfiring evident.

I re-installed the plugs in the car and added another 2thou. to the gap making thirty two thou. in all.

No more problems and I also gained an increase in power and mileage.

 

I look at the carburreter induction tubes Johan, they are long and narrow. There is a chance that the petrol air mix is condensing in the tubes and not reaching the cylinders as vapour?

I would feel a carburreter at the same level as the cylinder head might be better with much shorter unduction.

 

Clive

[Ian S CParticipant @iansc]

Clive the problem was with the three carbs, see the second photo, I think tuning those would be harder than tuning a piano. The way motors were built, a three cylinder was virtually three single cylinder motors. Ian S C

[Richard ParsonsParticipant @richardparsons61721]

Carburettors can also be a source of problems. Vibrations can cause ‘frothing’ inside the carburettor. On some baby Triumphs (500cc) my old mate always joined the carb to the manifold by a rubber pipe and held it to the frame with ‘O’ rings. He also reckoned that slight roughness in the straight parts of the manifold increased turbulence and mixing. It also provided little reserves of mixture for sudden acceleration. The curves parts were smoothly curved and polished. He wanted to get 10,000 rpm usable.

A short induction passage can also be both a benefit and cause problems. The benefit can come from warming the mixture and cause problems by evaporating the petrol in the float chamber and supply.

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