matthewg

20 HP Briggs Diamond I/C model 460707 2278-E1 Opposed Twin engine. Ok, this thing has me completely stumped. It was given to me with a thrown rod which has been repaired along with a new oil-slinger. Engine is getting plenty of gas, has great compression, new plugs, new coil, and the timing has been set per Briggs Repair Manual but it still will not start. It will turn over fine, at least until the battery wears down , but will not attempt to fire even when starting fluid is sprayed into the carb. All kills have been disconnected from the engine as well. For reference : I timed the engine per specs with #1 cylinder TDC turned the camshaft to proper position then slid the timing gear on the crankshaft. Everything mates fine. The strange thing is, the camshaft has 3 line marks on it about 120 degrees apart but only one has the dimple next to it, this is the one I lined the crank with. What am I missing? I am at wit's end and ready to apply gas and a match What could be wrong with this engine? Pleas someone, HELP!!

manic

Pull a plug, hold it against the motor with something that will not conduct
electricity. Pull the plug and leave the wire on it. See if you get spark.
No spark, bad coil. A light spark, timing is off. If your priming it with gas
you should get some reaction. Your sure you know which cylinder is
#1? 50 50 shot,,,lol I hate it when you get multiple timing marks.
Does the manual refer to the mark with the dimple, or does it just say
line up the mark?
Are you sure your at tdc?

matthewg

Ok, the coil and plugs are brand new. This was happening with the old coil as well. I thought that maybe the coil was weak so I replaced it but there was no change. On the "opposed twin" TDC is pretty much the same for #1 & #2 cylinders , however, I was actually timing it with the #2. I have the motor back apart, again, and will go with the #1 as soon as it stops raining as I have no room in the shop to work on it. The Briggs manual only shows a dimple/circle mark on the camshaft gear and not the other lines/marks I spoke of. This is a line drawing, which I hate, so it really is misleading. A simple photograph would cause much less headache. Honestly, I am tired of the headache . Thanks for the input as it seems nobody else wants to take a shot. Will let you know the outcome.

manic

Does the crankshaft gear come off? If so its keyed. When you are at
tdc what position is the key in, straight up, or straight down. Im just
thinking out loud, but does that manual give you any indication as to
where that key should be pointed at tdc? Might make all the difference.
Ive never worked on that motor before. But if the motor/crank has to
come around again to get the key pointing in the right direction prior
to putting the gear on, and its not, you will be 180degrees off timing..
Just something more to confuse you...

matthewg

Ok, it rained until dark-thirty here, so a no-go today. To answer your questions/suggestions : Yes, the crankshaft gear ( timing gear ) comes off and yes it's keyed. At TDC the position of the key, according to the Briggs drawing, is about 2 o'clock. This is the position at which I have it timed along with the camshaft mark/dimple and crankshaft gear mark/dimple mating. I am not really that confused, it's just that I cannot figure this thing out. Actually, it doesn't make sense at all. I wish someone who is familiar with this motor would see my post. I've never had a motor of any kind stump me like this. My grass is knee-high and geting taller by the minute. Thanks for the help.

k2skier

The quickest and easiest way to check timing is to pull one of the cylinder heads and turn the engine over to TDC. On one of the TDC's both valves will be closed, and on the other both valves will be open and either way you turn the engine one valve or the other will start to close, this second check, where the valves are open, is the critical check.

matthewg

So, you're saying that the timing marks should align when the piston is coming up to TDC or after TDC ? & that both the intake and exhaust valves are slightly open/completely open? Correct me if I am wrong here but I always thought that the intake valve closes just before TDC. If that's the case, something is definitely wrong because the intake is opening after TDC with the timing marks/dimples aligned. By the way, I now have both heads off so seeing TDC is not difficult at all. This being the first opposed twin I have ever worked on, I am just not familiar with the differences that may be present. This may be the last opposed twin as well I am definitely missing something, what I do not know. Thanks and please reply back.

k2skier

You must have the sump off, correct? The inspection I mentioned can be done with only the cylinder head removed.

On compression stroke the intake valve closes when the piston is coming to TDC, you are correct there. The exhaust to intake stroke (the off compression TDC) is where you see the valves "rocking" when the piston is turned either way. The piston comes to TDC on the exhaust stroke just as the exhaust valve closes, when the piston goes past the intake valve opens on the intake stroke.

Move the piston to TDC (not compression firing TDC) and rock the piston back and forth, moving clockwise in running rotation the intake valve opens, turn the piston backward and the exhaust valve opens, at this TDC is when the marks should align.

DELLC640

I hope I'm doing this right, my first post... newayz.... Back to did you have spark?

If you don't have spark at the plug to case you can;t troubleshoot any further. How old is the machine? Check the magnet in the flywheel (assuming yours isn't elec timing) and be sure it is strong at all poles. (the magnets are multiple pole and cause the flux to build, reverse direction then discharge for hotter spark, more energetic collapse of field) I have changed coils on old mowers, got no good spark and had to swap a new flywheel/magnet (most aren't removeable) and then I got a good spark.

Also the electric starts sometimes aren't fast enough to tolerate a weak coil/magnet but a pull of the cord (if equippped) can get it running fast enough to get the spark hot enough to get it running.

And did you check the gap of coil with respect to flywheel? just as critical for good spark.

matthewg

TO k2skier

I am following you & I appreciate the clarification. It has rained nearly all day and I've no place inside to work with it so it will be tomorrow before I can work with it again. Thanks again for your advice. I'll let you know how things go.

TO DELLC640

I am not sure on the age of the machine although it is not an antique by any means. The coil is brand new. I was actually getting spark on the old coil but changed it to be on the safe side. I am almost positive the electric start is not a problem because I don't see me being able to pull-start it that fast Yes, the coil is gapped per Briggs specs. Just to let you know - I think your first post was fine and I appreciate your input. Anymore suggestions, please pass them along. I'l be at it again as soon as the sun shines