Valve timing

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HMEM Supporting Member
Jan 4, 2011
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I have a question which I should know the answer to but I am not doing well. I am building a horizontal twin IC engine. My thinking is that the two cylinders should fire 180° apart. I have both cylinder set up for exhaust to open 40° BBDC and close 20° ATDC. Intake open 15° BTDC and close 45° ABDC with the other cylinder 180° after the first. I have good compression but I cannot get the engine to actually run or even fire well. The cylinders get warm but no actual firing. The engine turns over but the two cylinders seem to be fighting each other. The engine actually acts like it is backfiring. I have tried different ignition positions but I am not even close to having the engine running. Any idea what I am missing?
A silly question...are you rotating the engine in the right direction !?
Pretty straightforward.


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if it were mine and out of ideas, I'd get the overlap down to 0. Overlap is great on high speed engines but models in my experience are happy with very mild cams. Why keep the intake open 45 degrees after bdc?

Do you feel a pulse outward on the carb?

From the photos, your engine looks a lot like an Upshur twin. If you kept the same crank configuration with the crank pins 180 degrees opposed like the Upshur engine, I think it should fire every 360 crank degrees. What does your distributor look like?

Doesn't matter that he has that crank configuration, they're independant in all respects.
A few degrees of overlap between exhaust and induction is not going to hurt anything, can make the engine feel funny when rotated otherwise.

I'd like to see a photo a bit more top view showing the crank....something doesn't look right there with both inlets currently open
Shine a light down into the black depth................
Will it run on one cylinder? Remember it's a 4 stroke, I would have thought the other cylinder would fire on the stroke the other one doesn't, if you take my meaning. Anyway , I would suggest trying to get it running on one cylinder first.
Ok I have read every reply and here is my 2 pennies. I'm going to write this assuming zero knowledge. I realize this ain't your first rodeo but if someone just starting out is reading they may benefit from some extra details.

Overlap: Your fine. You only have 35 degrees of overlap. My Peewee and Demon have almost twice that at 60 degrees. The thing about overlap is, the lower the RPM the less vacuum created. That means fuel can't be pulled through the carb at very low rpm. What that means is I can't start the Peewee by hand like Bob Shutt can. I use an RC starter and just pop the trigger and it's running. It's not "harder to start" it just requires a higher starter speed. Ron Colonna had the same problem with his Novi V8 for years. Drill motor just didn't spin fast enough. He put a pull string on it and gave it a pull and it fired.

Another thing about overlap is once started the engine has less resistance from closed valves and will idle at a lower rpm than the same engine with no overlap. My V8 has an almost realistic idle. Hard to do in an engine that small. I just realized I'm giving away all my secrets! I guess that's what forums are for.

Also check your ignition timing. If your not waste firing check what cycle your firing on. Bring the engine to TDC and mark the crank position. Rotate the crank until both valves are open. Then rotate another 360 degrees to TDC again. This is where you should fire. My engines final setting was over 30 degrees advance. Don't be afraid to start out at 15 or 20 degrees advance and work from there. We don't have vacuum or mechanical advance so put some in there. I use a timing light to check mine.

So I guess verify, verify, verify everything which it sounds like you have and try spinning the engine faster. If your getting little pops your right there. Try spinning the motor with your starter and close the carb needle all the way and then slowly open it until you get some pops. Then work from there.
OK Thanks. I am not in the shop today but I have some things to try tomorrow. The engine is basically an upscaled version of the Upshur. I built the Upshur but was never able to get that to run so I decided to try a larger version. After building about 30 engines I have had very poor luck with the last few. Maybe I am getting too old and dim for this stuff.😒
Somehow I doubt that.
I built an Atkinson differential and could not get that to run for more than about one minute, then I built the Upshur twin and could not get that to run so I tried the upscaled twin and whatever my problem was with the Upshur followed me. In the meantime I built three Perkins for someone else and a Corliss and several other side projects so all of my projects have not been a disaster.
I also forgot to add that overlap reduces dynamic compression. The more overlap the less charge is inhaled at low RPM so you can compensate for that by upping your static compression by making a longer rod, taller or domed piston, or a smaller chamber in the head. But as long as you are higher than 4:1ish you should be able to run.