Thumper--a 1 3/8" bore i.c. engine

[CFLBob]

.......If you don't have some over-riding control on the throttle, then as soon as your engine starts it will open the throttle wide open. The only way I have found to get around that is to make up my own throttle lever with a good heavy compression spring at the pivot point and a lever connecting it to the Traxxas throttle lever.

Is that the brass rod at the top right? If not, could you highlight it or do a picture of it?

If it's what I think, it looks a bit complicated - 3 or 4 parts.


Thanks!

 

[Brian Rupnow]

Close Bob---it's two parts. The brass is attached to the aluminum with j.b.weld.

 

[Brian Rupnow]

Quick question--What degree of ignition advance do you guys use on single cylinder four cycle engines? I have always set my ignitions up for 10 degrees advance. I am into fine tuning on this engine, and I'm curious. I tried to google it and got 4 billion answers and non of them really answered what I was trying to ask.

 

[a41capt]

Sorry I can’t help Brian, but my only experience is with my Ford Kitchen Sink engine, and that thing is unlike ANY other engine I’ve ever been exposed to. As I’m building a Webster right now, I’ll be watching for replies so I can sort out my own timing issues as they arise.

John

 

[minh-thanh]

Quick question--What degree of ignition advance do you guys use on single cylinder four cycle engines? I have always set my ignitions up for 10 degrees advance. I am into fine tuning on this engine, and I'm curious. I tried to google it and got 4 billion answers and non of them really answered what I was trying to ask.

Brian !
I usually set 15 degrees before TDC

 

[wthomas]

Hi Brian:
I bet the old dog's tail was tucked between his legs when that throttle
shifted wide open!! Ha! Ha! It must have really sounded out
as it rev'ed up.
I got out Wed. of this week!!!!
Bill Thomas
In Michigan USA Where people are dieing by the 100 every day!!

 

[Peter Twissell]

For a fixed ignition timing, the advance is a compromise between two factors:
1) The speed and load at which you want best performance. Ignition is advanced so that flame can propogate through the charge and combustion can proceed to a peak pressure just as the piston reaches TDC. At low throttle settings, the charge density is low and flame speed is also low, so more advance is required.
2) Starting - not so much of an issue if you're spinning the engine over with a power drill etc, but if you're hand cranking it can be a serious problem. My Panther motorcycle is quite capable of launching me over the bars if I tried to kick it over with ignition advanced.

So, I'd guess you are looking for a smooth engine at low RPM.
Best appraoch is probably to adjust timing with the engine running until you're happy.

 

[a41capt]

My Panther motorcycle is quite capable of launching me over the bars if I tried to kick it over with ignition advanced.

Wow, Phelon and Moore? I’m MORE than suitably impressed!!!!

John W

 

[Brian Rupnow]

Okay---I've conducted a poll. Art says 14 degrees. I thought it was 10 degrees. Three small engine shops I called answered 28 to 30 degrees, one guy was very certain that on single cylinder engines with no mechanical advance it was 23 degrees, and a third shop owner said he always set for 25 degrees before top dead center. Unless somebody can offer me up a better solution, i'm going to set my ignition timing up to fire 15 degrees before top dead center.

 

[awake]

Brian, I wonder if some of the diversity of opinion has to do with whether the measurement is on the cam or on the crank - it occurs to me that all of the answers above could be close to the same, if the 10, 14, 15 degrees are on the cam and the 25, 28, 30 degrees are on the crank.

I have played with the ignition timing on my Webster, from way advanced to slightly retarded, and I can't tell much difference - which makes me wonder what I'm doing wrong ... !

 

[Brian Rupnow]

Measurements are always taken on the crankshaft. For instance, ignition set to fire at 15 degrees before top dead center is talking about the crankshaft rotating and moving the piston towards top dead center. At 15 degrees before the piston reaches top dead center, the spark occurs. And you're right---it's a wide open target. I have always set up my engines to fire "Just before top dead center". I'm trying to be a bit more scientific about it this time. Incidentally, when the crankshaft goes from 15 degrees before top dead center up to top dead center, it only moves the piston 0.027" vertically.

 

[TonyM]

The old school method on a full size car engine was advance it till you get pre-ignition / knocking then retard until it stops knocking and that was about right. I don't think it helps with model engines as we can't generally adjust ignition timing with the engine running.

 

[awake]

I did set the Webster up to allow adjustment while running, but that's where I wasn't able to detect any difference, moving through the entire range of adjustment that I have, which is roughly 50° advance through 20° retard. Mind you, I've never adjusted timing on a car other than years ago with a timing light, so I'm not sure what to listen for. And of course, when I tried this I was still trying out different needle and throttle settings. All of which is to say, I have very little idea of what I'm doing!

 

[Brian Rupnow]

It's not often that I learn something new which impresses me, but it does happen. I have always set my engines ignition timing by ear and "about 10 degrees advance". This morning I was calling around to small engine repair shops to get a concensus on this timing. I never did get a clearly cut concensus on this, but ended up taking 15 degrees before top dead center as the best time for spark. However, I heard the following method of setting ignition advance, and it just blew me away. The first step is to use either cad or trigonometry to see how far up the piston moves between top dead center and 15 degrees before top dead center. In my case, with a crank throw of 0.687", the piston moves up 0.027". Pull the cylinder head off, and with the piston at top dead center, measure from the top of the cylinder down to the top of the piston. In my case that was 0.106". add the 0.027" to that, and you get 0.133". Make a disc that thickness, slightly smaller than the piston diameter. Set it on top of the piston, then bolt a bar across the top of the cylinder and then turn the crankshaft in the correct direction until the piston presses the disc against the underside of the bar. that is exactly where you want the spark to occur.

 

[Brian Rupnow]

Remove the flywheel on the side where the ignition points are, set the ignition points for a 0.020" gap, then loosen off the grub screws in the ignition cam. Hook up the battery and sparkplug and while still holding the piston tight against the underside of the bar, rotate the cam in the right direction until you get a good fat spark. Lock the cam grubscrews there, and the engine will be timed to exactly 15 degrees before top dead center. Remove the retaining bar and the disc and bolt the cylinder head and flywheel back in place.

 

[fabricator]

That seems like a foolproof way to get the desired number perfectly with zero seat of the pants (guess work} involved. I'm going to save that one.

 

[teeleevs]

Wow, great excitement, good for the pacemaker.

 

[minh-thanh]

Hi Brian !
Just one more discussion:
when I changed the fuel for the engine: gasoline (mixed w40) and nitro fuel (including 2-stroke and 4-stroke engines)
With gasoline ( mixed w40): I set the ignition to 15 degrees before TDC
Nitro fuel: I set the ignition less a few degrees , somewhere around 12 degrees
I don't know why, maybe I didn't set it right, or because the fuel was easier to burn, or because nitro fuel had more oil, so the compression increased, or ...something
So, for every fuel I use, I usually choose 15 degrees before TDC, and then adjust the ignition a little bit - until I feel the engine runs best and stop ,.

 

[wthomas]

Andy (awake)
Would you mind telling us what kind of carburetor and the engine you were setting the timing on?
Also, about how slow of RPM was you able to get it to Idle?
Thanks in advance for the information as it will answer my thoughts on how slow can you Idle an
engine.
Bill Thomas
Still locked down in Michigan USA

 

[dsage]

There is no magic wand on where to set the timing. Advance is totally dependent on the engine you're setting it on and the conditions it is running in. That's why it's one of those things that even the simplest engines that have a variable load have to manage. i.e. vacuum advance - centrifugal advance in order to get peak performance.
If you're going to just sit it on the bench and run it at a certain rpm then set it anywhere reasonable that it's happy. It won't care.
Your process is perfect to get it set exactly for what ever you've decided to set it at.
But I assumed all along that you were going to un-leash this beast on "the saw"

Do yourself a favour and enlist some of your wonderful design talents and make it at least manually adjustable. Or if you want to get fancy have the advance increase with throttle position (to a ratio). (a simple linkage)
In most cases a simple lever adjustment is just a matter of turning the plate the points are mounted on.
Once you have it adjustable you can set it depending on the running conditions (half the consensus reached here). You can retard it slightly for starting and in general the engine will run smoother and have more power if you advance it with rpm. If you're going to load it i.e. "the saw" then it will be necessary to be able to adjust it for best performance (ability to carry the load). You'll find a marked improvement in it abilities.

Looks really good.

Of course - all this coming from a guy who can't even get the Atkinson to pop