Cub has spark & gas, but won't start!!

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Bus Driver
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Re: Cub has spark & gas, but won't start!!

Postby Bus Driver » Sat Feb 21, 2009 8:26 pm

Take a wooden or plastic handled screwdriver and using that handle, tap the carburetor in the area of the fuel line inlet. The float needle may be stuck in the closed position. And yes, a few drops of gasoline in the spark plug hole is OK. But try tapping the carburetor first.
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Re: Cub has spark & gas, but won't start!!

Postby Eikel » Sat Feb 21, 2009 11:02 pm

Is there a possiblility it could have jumped time? A quick timing check maybe in order, #1 TDC against the timing mark and rotor position.
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Re: Cub has spark & gas, but won't start!!

Postby Eugene » Sat Feb 21, 2009 11:20 pm

We got spark, fuel dripping out the carb, valves working, suction and thus compression. Engine won't even cough. Plugs dry. Tried starting fluid.

Check for an ignition timing problem. See if someone didn't play a trick on you by relocating the plug wires on the distributor.

Also a carburation problem - sounds like sticking needle valve or a bit of trash under the needle valve.

Another thought. Do you have a magneto for an igntion system or a battery powered ignition system? If magneto do you hear the impulse click when cranking the engine over?
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Re: Cub has spark & gas, but won't start!!

Postby Clark Thompson » Sat Feb 21, 2009 11:22 pm

If it is a stuck valve there are 2 ways that you can unstick it without taking the head off. #1 is to remove the valve tappet cover. Turn the engine to see which valve is stuck open, you will be able to tell by the one stuck wont move.By using a small pry bar and lube you may be able to get the valve unstuck. #2 is to remove the sparkplugs. With the hood off take a flashlight and look into the engine. Just under the sparkplug hole are the valves. If you look at the proper angel you can see the valves. Also you will be able to see which valve is not moving. Sprey it with PB blaster or equivilent then take a screwdriver on a angel and tap the valve down.Sometimes this works and saves a lot of time and some $$$
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Re: Cub has spark & gas, but won't start!!

Postby cubguy's dad » Sun Feb 22, 2009 9:35 am

What is good spark?

Testing for spark buy using the spark plugs will only tell you that enough voltage (1000 volts or less) is present to jump the .023 spark plug gap under atmospheric pressure, not cylinder pressure.

You need app. 5000-10000 volts to jump the same gap under 60-80 PSI cylinder pressure, even more voltage is required when the engine is under load.

A good cub ignition will produce about 25,000 volts or be able to jump a 5/16- 3/8" inch gap.

A good low buck tester can be made from an old clean spark plug. Open the gap to .250" or as much as you, then wrap a wire around the threads for a ground wire add a aligator clip and you are good to go.

If your ignition system can produce enough voltage to jump the .250 gap at the spark plug wire ends, then you have good spark. I suspect you have spark, not good spark.

Check the how to forum for testing for good spark.

Hope this helps

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Re: Cub has spark & gas, but won't start!!

Postby farmermike » Thu Feb 26, 2009 11:42 am

Thanks again for all the great advice. I've checked the valves (they seem moveable and unstuck), I've got spark to all 4 cylinders,and I've got decent compression. Even with a jet of starter fluid, I'm getting no start, not even a cough or back fire. If this is all true, it should start, shouldn't it? I blew compressed air through the detached exhaust/intake manifold and it seemed air was moving freely. All I can think to do now is put it all back together and try again. Any other suggestions out there? Would blown gaskets account for any of this? Is it possible to have a weak spark that wouldn't light fuel? (When I test this a blue spark is clearly visible at the plug electrode). Thanks.

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Re: Cub has spark & gas, but won't start!!

Postby Bigdog » Thu Feb 26, 2009 12:01 pm

As Mark points out - you should be able to produce a bright blue spark at least 1/4" to 5/16" or better in open air. Spark is reduced under compression so it needs to be really healthy in open air to work under compression.
Last edited by Bigdog on Sat Feb 28, 2009 8:46 am, edited 1 time in total.
Reason: corrected typo
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Re: Cub has spark & gas, but won't start!!

Postby Mag Man » Thu Feb 26, 2009 12:15 pm

How fast is she Cranking Over? Is It a mag or distributor? Sometimes on dist the starter steels power from the coil and it spins but will only fire when you let off on it.I am sorry I do not know the fix Can not remember. Is it turning over good and fast? Have you put your hand on the carb while cranking it? Sometimes the choke is just not enough. Mine has a small hex head or what ever bolt in the manifold above the carb and I take that out spray a little carb cleaner in to help start it is a lot better on the engine than either. Then put your hand over the carb nice to have someone there to either turn it over for you so you can put your hand on the carb and spray some carb cleaner in that hole if it has it while spinning it over she should fire up. Other wise if you do get her to fire but not start I would definitly try the wire in the carb trick mentioned earlyier I have to do that a few times a year with my 51 from sitting and it does not get any gas to the intake if its pluged?
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Re: Cub has spark & gas, but won't start!!

Postby Eugene » Thu Feb 26, 2009 12:58 pm

Eugene wrote:We got spark, fuel dripping out the carb, valves working, suction and thus compression. Engine won't even cough. Plugs dry. Tried starting fluid.

Check for an ignition timing problem. See if someone didn't play a trick on you by relocating the plug wires on the distributor.

Also a carburation problem - sounds like sticking needle valve or a bit of trash under the needle valve.

Another thought. Do you have a magneto for an igntion system or a battery powered ignition system? If magneto do you hear the impulse click when cranking the engine over?

Magneto or battery ignition system? You are gonna have to check and double check everything instead of guessing.
I have an excuse. CRS.

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Re: Cub has spark & gas, but won't start!!

Postby EZ » Thu Feb 26, 2009 2:24 pm

Eugene,

Try some new plugs.
This has happened to me many times. I switched to D21's.
I had been using NGK A6's but they don't seem to last.

Yes, they had spark, but tractor would not fire.
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Re: Cub has spark & gas, but won't start!!

Postby Eikel » Fri Feb 27, 2009 9:48 pm

Beware of spraying anything flammable in or toward the intake while turning the engine over - you could have flames coming back in your direction.
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Re: Cub has spark & gas, but won't start!!

Postby c172skyhawk2 » Fri Feb 27, 2009 10:20 pm

you could also remove the plugs and put your thumb over the hole and see what the compression is, if you dont have a gauge. but a gauge would tell you a whole lot more.
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Re: Cub has spark & gas, but won't start!!

Postby wilbur » Sat Feb 28, 2009 12:47 am

Mike, all this info is right on, but why dont you try just simpily pulling it? It cant hurt as long as you do it safely. If nothing more you get to drive it for awhile.Who knows ? you might get a pop then a studder than a rough idle.Nothing ventured,nothing lost.
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Re: Cub has spark & gas, but won't start!!

Postby Mag Man » Sat Feb 28, 2009 9:48 am

Thats a good idea but, I still think thatthe jet in the carb may be plugged and no matter what you do pull crank or what ever its not going to start untill you get it clean in there it does not take much on these little carbs to plug that jet. and you can have fuel dripping at the carb and everything but it does not get to the valves so really try the wire thing in that jet and maybe a little air from a can of pc cleaner or something I have been told too much air from a compressor can damage the float I dont know but really try that jet before you do anything else.
It happened to me twice last summer and I sat there scratching my head like you untill these guys told me the same thing I am saying.
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Re: Cub has spark & gas, but won't start!!

Postby RRRRyan » Mon Aug 21, 2017 8:42 am

Hi everyone, I know this is an old unresolved thread, but I have these symptoms on my 48. Leaning toward "bad spark". It looks like fireworks on these plugs and I've opened the carb twice now, no obstructions on either of the small holes in the jet or screw. She was running/ starting great and just got done mowing and she quit like I'd shut her off. I swore she just ran out of gas but after refill not so much as a cough.


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