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Webster IC Engine build log.
dbvandy:
--- Quote from: craynerd on May 03, 2011, 10:37:07 AM ---
Doug, interesting post you made! Have you got any advice regarding the timing? I`ve been reading over the build notes and I`m trying to make sense of it!
Chris
--- End quote ---
There are two timings to consider:
The exhaust cam has to be timed to open about 20 degrees before bottom dead center on the power stroke and then close just after top dead center on the end of the exhaust stroke (to prevent blowback in to the carb from pressure in the cylinder). You can adjust this duration by moving teeth on the cam gear and by adjusting the clearance between the exhaust valve and the tappet on the rocker arm. It is not an exact science (a little different for all engines) and does not have to be PREFECT to get it to run, but once you have it running, you can fine tune it with small adjustments to get it to run smoother and idle better and run at higher RPM's. The plans for the webster have what worked for his engine, but mine are a few degrees different due to manufacturing tolerances. After weeks of fine tuning and tweaking I got mine to idle at about 600-700 consistently run at close to 5000 RPM wide open, but I do not recommend that because it could all go terribly wrong very quickly with parts flying at you at the speed of sound... :zap:
The other timing is the ignition timing. This to does not have to be exact to get it to run, but you will find that if you fire the spark about 10 degrees before top dead center on the compression stroke, it will run throughout the entire range of RPM's very well. On the Webster, I use ignition timing to control RPM in addition to the mixture on the tank. It is like a 1903 Harley with 15 different adjustment on the handle bars to keep it running perfectly.
With your electronic ignition (very envious of this by the way...) make sure you have fine adjustment as a degree or two will make a difference in how well it idles and revs up. also, the electronic ignition should allow you to idle it down to 200-300 RPM once it is running and tuned in. It looks like your magnet is on a cylinder that has some set screws so you can move the magnet to the exact right place, so this should not be a problem. You can set the initial timing by eye by just looking at the position of the connecting rod big bearing. When the connecting rod is parallel to the base of the engine, you are at bottom dead center and top dead center respectively. If you have a strobe timing light for a car, this will make timing easier. There is a video of timing the Otto in the build log, the webster is done the same way.
http://madmodder.net/index.php?topic=4425.msg49931#msg49931
I would recommend getting a one way bearing from an RC starter (most are 6 mm) and use a drill to turn the engine over when tuning it... if not, don't get discouraged if you have to wind the string over 100 times before you hear the first pop and even longer to get it to run perfectly. The physics are there and the design is very forgiving, so it WILL run...
http://www.amazon.com/One-Way-Bearing-Recoil-Starter-G-27CX/dp/B000X4Q2GY
I am very excited for you!
Doug
NickG:
I went to start my modern lawnmower with briggs and stratton engine the other day and honestly thought it wasn't going to! Took about 20 or more pulls of the starter! As doug said, if you've got those few vital ingredients it should run but they can still be tempremental so keep trying if it doesn't go first time!!
raynerd:
Well no such luck as yet :palm: :palm: :palm: :palm: :bang: :bang: :bang:
I have had a few "pops" so clearly fuel is getting into the cylinder but I can`t see the valve being pulled down due to the negative pressure of the cylinder. On models I have seen, you can visibly see the valve twitching down. Now before you see it, I went to an amazing place after work today just on my door step - Pennine Springs, thousands and thousands all hand made. Anyway, after about 30 minutes of searching I got a selection - I currently have the softest on, which is incredibly loose. It is enough to seal the valve but should definately be week enough to allow the intake valve to be sucked open - at present I just can`t see it happening.
I`ll keep it at tomorrow. Just before I had to leave the workshop because it was getting late, I could hear the spark plug randomly sparking. This had NOT been happening through the evening so I don`t know what is causing this and hope it stops tomorrow.
I think I need a drill mechanism as recommended to get it going. Winding the string isn`t helping and I think it needs a longer more consistent start. I honestly think it went for a few cycles a couple of times .... at least it isn`t totally dead!
Chris
dbvandy:
SWEET!!!!!!!!!!!!!!!!!! way to go.. you are just about there!
A few things...
You can't really see the intake valve moving until it gets going good.
when you have fresh fuel in the tank, it has to to wide open with air bypass (depending on how big the hole is.)
set the ignition timing at top dead center as it will be harder to start advanced.
check compression by turning it over by hand. on the compression stroke it should be very difficult to get past the compression stroke.
did you use the oring? it will help if you didn't...
double check the exhaust valve timing... make sure it is closing fully before the intake stroke starts.
Doug
metalmad:
Hi chris
I try to make sure the exhaust valve opens just before bottom dead center and is closed before top dead center
if your ignition is set at about top dead center then try to start with the fuel needle only open half a turn,but u will need to change this even if it does start.
Pete :beer:
opps different carb lol
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