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Problem Starting

Tue Dec 16, 2003 11:25 pm

For the past year my '50 Cub has had a hard time starting. If I get it to start in the first few trys, all is good. If not, it won't start at all. As the weather has gotten colder, it's been worse! I decided to spend some time over the past few days and go through the basics (spark, fuel, air, timing, compression). I have strong spark from the magneto to the end of all plugs. I have plenty of fuel flow. Air intake seems good. I have near 100 lbs of compression in all cylinders and the valves are operating with ease and no sign of fowled plugs or carbon build-up in or around the valve seats (inspection through the plug sockets). I opened up the cap and reset the rotor arm to point at #1 while the #1 piston was at TDC. I thought that I may have a carb problem since I seem to have drippage out of the bottom of the body. I swapped the carb with one from my '49 Cub that purs....no difference, behaves the same. I get many pops from the engine like it wants to start, but won't. That made me look into the compression for real bad rings or a burnt valve. Dry compression check is fairly good (100, 95, 70, 95). #3 looks like it has some leak, but it shouldn't keep it from starting with 70 lbs. I am leaning towards my timing since my spark is real strong from the coil, through the cap, wires, and to the ends of all plugs. My only hesitation or concern is how do I check to see if I'm 180-degrees out? I know it needs to be at TDC on a compression stroke, but how can I best check that? Any other help would be greatly appreciated! My driveway has had to sit with this past snowfall for a week now and more is on its way!

Tue Dec 16, 2003 11:59 pm


Remove the #1 plug and hand crank the engine while you have your thumb over the plug hole. You will feel the pressure as it comes up on the compression stroke. TDC will be when the pointer comes to the notch in the pulley. If you have two notches, TDC will be the second one.

Wed Dec 17, 2003 9:04 am

Go to Rudi's web site and look at the blue ribbon manuals under Magneto and there are very good instructions on timing a magneto to the engine.

Your Mag may also need to be cleaned and oiled.

Wed Dec 17, 2003 11:29 pm

Oscar, I think that if it were to be a magneto problem, then I wouldn't be getting spark at all. I get great spark at all plug ends! I will take a look at the manual as you mention though....it can't hurt.

George, thanks for the advice! I checked to see if I was on a compression stroke and I was. I even changed the rotoe position to be 180-degrees out and tried that as well....not even a pop. So, I changed it back and I'm at the same point where it is trying to start by getting a small pop on each start attempt. Could I have a vacuum problem through the manifold? Maybe not enough fuel is getting to the cylinders? I see that #4 is quite wet while all other cylinders are not wet at all. Since I swapped the carb with one that works perfectly on my other Cub, I can rule that out. So, that is why I'm looking in the direction of the manifold or air vacuum leak. Any ideas of how I can test this part or what to try next? This has got me stumped!

Thu Dec 18, 2003 7:24 am

If you want to check for vacume leaks, one of the best ways that i have found is to get it running at an idle, and take a propane torch (unlit) and go around the manefold and carb If the engine speeds up, it is leaking and takeing on gass from the propane bottle. I do this right away as soon as the engine starts because some times as the engine warmes up the leeks stop from exspantion. My cub acts the same as yours, I think its flooding. My thought is that even though I have goood spark to the eye it is realy getting tired and poor fuel. "Jack"

Thu Dec 18, 2003 7:39 am

Sounds like a vacuum leak at either the intake manifold or between the manifold and the manifold and carburator. Also, make sure you have a good clean (no burrs) mating surfaces between the carburator and intake manifold and a nice new gasket. Hook up a vacuum gauge at the square plug to check out the manifold first.

Thu Dec 18, 2003 5:09 pm

Jack, I believe that you are right about the fuel. My #4 cylinder is wet with fuel, but not even the plug ends of #1,2, and 3 are wet. My problem: poor fuel to the cylinders. That is why I was looking in the direction of vacuum. The only thing that brings fuel vapor from the carb to the cylinders is vacuum. Of course, the alternative problem may be some carbon buildup on an intake valve making my vacuum little or non-existant. Has anyone else had this type of problem? In order to really check for good vacuum, I need a running engine! I think that my next step is to add a teaspoon of fuel to the #1 cylinder manually and see if I can get it to fire. Then, I can perform a vacuum test and maybe dive into my valves if need be. This is a very simple system and should not require this amount of thought and effort! But, sometimes the simplist things cause the most headaches!

Thu Dec 18, 2003 7:56 pm

I'm sory, I thought your cub was running. Well how about trying this. This is another reason I blame fuels . I'm lucky to burn ten Gallons a year. So if it has ben a long time sence the last run, I pull the plugs and dump in a couple caps of oil. I beleive after sitting a long time the cylinders are very dry and gass just runs by causing imeadit flooding. I have a petcock on the bottom of my carb instead of a plug, so when I shut it down, I can drain the carb. When starting if the cub seams to be flooded I shut the fuell off and drain carb again. And leave empty untill it eather starts or stops firing. This usualy works well for me . Good Luck "Jack" :wink:

Thu Dec 18, 2003 10:33 pm

Thanks for the advice Jack, but this tractor has been running well for the past few years regularly. I used it as recently as 3 weeks ago to plow snow. The gas I have in the system is new and is very clean without any buildup in the bowl nor at the carb inlet screen. I like the idea of the drain port in the bottom of the carb. But, I think if your needle doesn't stick and your seat is in good shape that the needle should stop the flow of fuel and not allow any fuel to drip from the carb. Of course that doesn't always happen, so your idea is a unique alternative. I have thought about adding a small amount of oil to my #3 cylinder and perform a wet compression test to see if I can get that 70 LB compression rating closer to the other cylinders of around 95-100 LBs. That will tell me if I have a ring problem or a burnt or carbon built-up valve in that cylinder. I will test some more in the AM. More snow is on its way!

Thu Dec 18, 2003 11:54 pm

I think your on the right track Nick. I recall a conversation on the board here about two years back as to the engine stoping with the #2&3 Cyl. valves open, causing a shorter life span for them valves. I had #3 valve stick on me from lack of use and good old mother nature. Also my carb float doesn't stick or leak, It's just that I think that if the engine is flooded, you can't clear it out unless the fuel soars is stoped. I believe that as long as you crank the engine it will continue to pull fuel, choked or not. So by shutting the fuel off and draining the fuel bowl in carb you just pull vapors and you have a better chance of starting a flooded engine. What ever works I always say. seams every Cub has it's tricks. Good luck Nick and I'll be watching for the out come post. "Jack" :wink:

Sat Dec 20, 2003 3:24 pm

Well, I finally solved the mystery! After everything else I tried I went back to the basics! Faulty Spark Plugs! The plugs worked fine when tested on the wires outside the engine. Under compression they failed! With some help from my Pop, he told me that years ago he had a box with a window to test plugs. You would screw the plug into the box, apply your spark source and be able to throttle the air flow into the box to increase compression. He said you could watch a strong spark out of the plug decrease to nothing when the compression was turn up! Another one for the books as far as I concerned! Now a days, most just toss out the plugs, cap, and rotor and buy new. There once was a time when things were cleaned, tested, and kept in service for average tune-ups. I think that those days are long gone, but it was nice to nabble into the memories with my Pop today! Thanks for everyones help and responses!

Sat Dec 20, 2003 4:29 pm

:) I remember them boxes Some if I recall corectly were (AC) & Champion. I'm sure all spark plug makers had them. Another good machine replaced by labor cost. "Jack" :)

Problem Starting .... Seem like the same as yours

Thu Dec 25, 2003 3:03 pm

Hi Nick

It seems like my problem is a replica of yours. It seemed like I wasn't getting spark, so I took off the mag yesterday, and have today replaced the coil. Condenser and points seemed like new! I also replaced the distributor cap and also the plug wires. I have new plugs.

Now I have really strong spark at the plugs. But it didn't start immediately. I check fuel flow at the carb again (took off the inlet line) and it flowed really good. I also took off the bottom plug of the plug, and let it drain.

I finally got it started. It ran good for about 10 mins, then I was checking idle, and she quit, and I couldn't get her restarted.

At several points in all of this, I had the plugs out. At one point they were dry. At another point they were really wet, and I took them out one by one, and dried them. I think after this was when I was able to get her going.

Have you solved your problem? I still have to check compression to see what that is like, but I assume OK, because when I get here running, she runs nice.

I am thinking now maybe a carb problem, or possibly vacuum. Did you check your vacuum using the propane method?

This is starting (no pun intended) to get frustrating!

Thu Dec 25, 2003 11:43 pm


I did not check for vacuum using the propane method. I had the air cleaner disconnected from the carb the entire time and had plenty of suction on the carb inlet. I did have allot of gas dripping out of the carb during this entire test time. I had my #4 plug really wet with gas the whole time and all other plugs slightly wet (I thought that they were dry at one point, but I think I was wrong). Seeing that I had pleanty of carb suction and plenty of gas to the cylinders, I re-checked my spark. I seemed to have really strong spark and then it looked like it became weaker the further I tested. I opened the mag and checked the points...they were quite burnt (and I didn't have an extra set to replace them with). So, I filed them down and re-gapped the points, readjusted the mag timing and checked spark again. I still had the same results. That's when I decided to try new plugs. After it was running, I realized that I actually had too much gas to the cylinders from too many start attempts. Gas was litterally pouring back out of the intake manifold to the carb from the engine sucking up so much gas and the engine not starting to burn this gas off. In my case, I was able to rule out the carb by swapping it with another carb on another Cub I have. I did at one point open the carb up and check the needle & seat which seemed to be fine (no sticking or worn grooves). So, my problem was clearly just a plug problem, but it was nice to go through and re-check all the basics....it keeps me on my toes and works my brain! Hope you get through this problem with your Cub soon .... it does get frustrating

Problem starting

Fri Dec 26, 2003 8:41 am


Thanks for the more detailed step-by-step.

I only just put in new plugs, but a friend of mine is suggesting to replace them again, because once they get soaked in gas, they don't work or don't work as effectively he said.

Where the plugs you originally had in there, old or new?

Today I am going to pull the carb, and re-build it. That way I have eliminated one more cause.

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