Need an expert on timing...

[ScottyG]

Well, as some of you may recall, I had a timing problem on my Cub. The timing was retarded about 30 degrees. Because the distributor is more or less anchored in place due to a home made bracket for an add on 6V coil, I had to adjust it from the governor. When I originally reassembled the governor after restoration, I made sure I lined up the timing teeth properly so you can imagine my surprise when I found out I had a problem. Anyway, after eyeballing the governor position with that of the distributor, I managed to get my spark at pretty close to TDC. It might be 1 degree retarded according to the mark on the pulley.

The Cub is running as well as a brand new machine. I have plenty of power, I can now mow in 3rd gear if I want to (I won't though, just trying to make a point) and I am really very impressed by this vast improvement. The problem is that the machine is still stumbling and pretty badly. At high speed, it make a foop foop noise, kind of like a mini backfire.

As some of you may recall, I'm also having some minor carb problems and am wondering whether the exhaust noise could be due to a fuel problem, like a float setting or the fact that I still might not have my timing quite right yet. It is my understanding that I should have spark exactly at TDC correct? Also, how reliable is the pulley timing mark? Could I be slightly off and not know it without a timing light?

Please help.

Thanks.

[John *.?-!.* cub owner]

The mark is accurate, but depending on whetrher your cub had a mag originally, there may be two marks. fist one is 16 degress BTDC (running speed mark), and sedcond one is TDC (cranking speed spark). If only one mark it is TDC (cranking speed mark).,

[ScottyG]

John,

The Cub had a mag originally but I have only one mark. Basically, I played with the governor teeth until my secondary coil gave a spark to ground right at about where the mark is. Once I got close, I decided to leave well enough alone. Do you think the mini backfiring is from timing?

[John *.?-!.* cub owner]

If timing were slow, it would have low power, if it were very fast, it would tend to kick back when cranking. If you are getting a little backfireng in exhaust, I would check vavle adjustment,, which I think I remember you sayign you already have, air leaks around carb and manifold, and point adjustment, especially looking for side play in the shaft the points rub on.

[ScottyG]

Hi John,

At this point, I'm leaning to vacuum leaks at the carb. I've had that baby apart 5 times in the past week and I'll be you a tank of gas that there's some gremlins in those seals. I'll pull it apart tomorrow night to check her out once again. Valves are not out of the question either and that might be something I'll check next if the carb turns out to be a bust.

[John *.?-!.* cub owner]

Might want to check for leaks at the manifold to block junction too.

[Lurker Carl]

Retarded 30 degrees - it's a wonder it ran! The manifold gasket may be shot from the extra exhaust heat.

[Jim Becker]

A sticky valve could cause the symptom you now have too.

[George Willer]

A sticky valve could cause the symptom you now have too.

That's my thought too. I'm not much of a believer in better living through chemistry for our Cubs, but it may be time for a little MMO in the fuel and oil.

[ScottyG]

I'm surprised it ran too. Is it no wonder I was about to trash the whole thing? You can now imagine my happiness with how its running now!

George, can you explain to a novice what MMO is? Jim, because the engine was running so retarded for about a year, could this cause excessive fouling around the valves? I did a valve overhaul when I refurbed the tractor in May of 2004. And... yes Carl, you may be on to something since the thing overheated all the time.


Finally, John, what is the best way of checking for leaks at the manifold? I'm a pipe guy and work with soap solution or a gauge to check for pressure loss. I'm assuming that you can gauge the manifold at the opening just above the carb? If so, what pressure am I looking for?

[Bigdog]

Scotty, MMO is Marvel Mystery Oil. A few ounces in a tank of fuel helps keep things lubed up and will help prevent sticky valves.

As far as checking manifold leaks, carefully use an unlit propane torch. Whenever the tip is near a vacuum leak, the propane will be drawn into the manifold. There will be a noticeable audible difference in the way the engine is running. It should smooth right out. This will tell you where the extra air is being drawn into the system.
Go around all of the manifold attachment areas and the carb flange as well.

[Jim Becker]

With a valve job as recent as a year ago, I wouldn't expect a lot of build-up on the valves, regardless of how things were adjusted. However, it wouldn't take much unburned gas to keep the lubrication pretty well washed off the exhaust valves. That could be enough to cause some sticking. I think MMO or some other "quality commercial product" that is marketed to free sticky valves is worth a try at this point.

[ScottyG]

Thanks guys. Will let you know what I find out.