Rough Running Engine

[Rustkolector]

I have a newly completed 4 stroke, throttle governed engine (1"x 1.25" BxS) with an atmosheric intake valve. It is designed to operate at about 1000 RPM. It appears to have very good compression, and good spark, and the engine does run, but...it doesn't run well. It has a steady exhaust note, but it is sounds like 12 or 16 strokes between firing. I have tried releiving tension on the intake valve with no notable change. I substituted an RC carb for my carb with no change. The engine runs on propane. I moved the regulator and demand valve to a multicylinder engine of the same BxS, and that engine ran fine. Engine operation does respond to fuel needle valve adjustment, but from "too rich" to "too lean", the engine continues to skip at the same steady pace. About only every 12 or 16 strokes. I have checked for intake leaks. None found. The LED on the TIM-6c ignition module indicates the spark plug is firing as it should. Have these symptoms happened to anyone else? What did I overlook....?
Jeff

 

[Mike N]

Do you have enough spring pressure on the exhaust valve? Is the timing correct on the exhaust valve?

 

[Maryak]

As well as Mikes recommendations, is the ignition timing correct?

Best Regards
Bob

 

[Rustkolector]

The exhaust valve spring is firm, and should be ok . Exhaust opens about 20 degrees BBDC and closes very near TDC. I have moved the camshaft a tooth in either direction without notable change. The Hall sensor is mounted on a movable timing arm. The engine speed changes as it should from retarded to advanced, but the steady engine "skip" doesn not change. Only once did the engine run steadily as it should, but that was for only about 3 seconds. I have no clue why, as I changed nothing when it happened. The valve stems are smooth and don't stick. Dry or oiled makes on change.
Jeff

 

[TUE]

Any way of pulling the plug and getting a visual of the spark?

 

[Rustkolector]

The plug has a very good blue spark when grounded to the outside of the engine. Also a good audible "snap". The wire is 15kvdc so there are no shorts. I will try another plug tomorrow, just in case. That never occurred to me.
Jeff

 

[Stan]

Here is something right out of left field. If it truly repetitive at firing every 12 or 16 times, is it possible you are one tooth out on one of the timing gears?

 

[Rustkolector]

Stan,
The timing appears fine. I have tried going a tooth or two either way with no change in the engine skip rythum. I changed the spark plug with no change. I polished up the intake valve stem today with no change. It is free and smooth. The intake spring seems stiff enough to me, but I am going to stiffen it a bit more as Mike has suggested.
Jeff

 

[Stan]

Jeff: What I was getting at was that if you had one tooth more or less on one gear, you wouldn't have a two to one ratio and it would take X number of turns to get it to the right spot then repeat. Like I said, right out of left field.

 

[Kermit]

I would suggest checking to see if the timing is exactly 180 degrees out.

That would explain some things, like firing every third cycle. The exhaust would have to fill up with unburned propane for a cycle or two then when the fuel reaches the proper mix ratio, the engine pulls it in through the exhaust port and fires. Another two cycles are needed to refill the exhuast ports with enough fuel to fire again.


It's a thought,
Kermit

 

[artrans]

is this a home build or a hit and miss I would start with the basics here and just as the pistion comes to top dead center the plug should have a spark and the valves closed. If thats ok then its not timing then it would be a fuel problem but from what I hereing it sounds like a timing issue. any pictures would help the brain waves a little bit

 

[Rustkolector]

Update: Today I double checked valve and ignition timing. It was ok. I polished both valve stems and upped the spring pressure on the exhaust valve incase it was being sucked open on the intake stroke. I got slight indications of sporatic, but proper combustion cycles. It didn't last long. I switched to an RC carb that works on another engine with propane, but it didn't work any better. The exhaust valve spring pressure is a little much now as the valve action has gotten noiser. I will reduce it just a bit. The rocker/valve operation isn't as smooth as it should be. Because the carb adjustments don't seem to produce the expected operating result, I believe my problems might be in the exhaust valve operation and will focus on that when I have some time in a few days. Thanks for the ideas.
Jeff

 

[ghart3]

Jeff, couple things to try if you haven't done so already.
Try running the engine in poor lighting or dark and see if your spark is leaking off where it shouldn't be. Like the outside of spark plug or a close by valve spring or stem.

Restricting the air inflow to increase air velocity. Maybe put piece of tape over air intake with small hole and trying. Keep increasing the size of the hole to see if that helps.

 

[Mike N]

Stan,
The timing appears fine. I have tried going a tooth or two either way with no change in the engine skip rhythm. I changed the spark plug with no change. I polished up the intake valve stem today with no change. It is free and smooth. The intake spring seems stiff enough to me, but I am going to stiffen it a bit more as Mike has suggested.
Jeff

The exhaust spring needs to be at least twice as strong as the intake spring other wise the vacuum tries to open the exhaust valve instead of the intake valve. I had this happen on the Canfield Engine I built a few years ago. Do you have a buzz coil you could try? I think maybe your ignition system is probably the real problem.

 

[Rustkolector]

This is a slow speed engine so my carb throttle is almost closed when running at proper speed. I don't think a .030" drill would fit through the eliptical throttle opening.

The high tension plug wire is 15kv rated and isn't shorting. Checked that. I have had ignition problems, however. I failed 3 Hall sensors before I figured out not to run the high tension wires near the Hall sensor wires. Duhh...! Seems to be ok now. I have an LED on the transistor ignition module and it indicates the plug is firing many more times than I have combustion cycles. I don't have a model buzz coil, but I think the 1200-1500 RPM is a little fast for a buzz coil.
Jeff

 

[TUE]

Jeff: I have used a un-lit propand torch to check richness in full sized engines quite a few times. I suggest you trythis carefully with just a little turned on, this will give you an idea if its a fuel problem. also i spent quite a few years delivering propane and doing the repairs and inspections that go with that. If you use heavy gloves and shirt and be careful you will be safe. It takes about 10 percent propane to burn. less or more and it wont!

 

[Jack]

For what it's worth... I also play with old Model T Fords, all that is needed to make a engine run is Fuel, Compression and Spark. You don't need any fancy wires, resistors, LED's, gizmo or modern electronic stuff. You just need fuel, and spark and a engine that doesn't contain any broken parts.
Yes, this engine runs very well, starts with one one pull of the crank and a little choke, OH by the way it's a 1922 FORD Runabout that I have owned for about 10 years.

 

[cfellows]

Hmmmm, barbed wire for sparkplug wires?

Chuck

 

[tel]

Hmmmm, barbed wire for sparkplug wires?

Chuck

Love it! My sort of engineering.

 

[rake60]

I have played with the Model T buzz coils.
Who needs wires?
Get close enough to one of them and you will find yourself
screaming words that haven't been invented yet!
Rof}

Rick