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Discussion Forum
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TO-20 back Firing.

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Mose1974

03-29-2008 16:41:10




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Ok guys I have hit a brick wall, went to put some new fence in today and the TO-20 ran perfectly for about a half hour then all it would do is back fire through the exhaust and the carb. Took the carb into the shop sprayed it out with cleaner and compressed air nothing in it really, replaced the plugs with a spare set I had laying around, then guess what a half hour later it was back to back firing again. Can anyone tell me what I am missing.

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mose1974

03-30-2008 03:33:35




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 Re: TO-20 back Firing. in reply to Mose1974, 03-29-2008 16:41:10  
I forgot to say it is a 12 volt system and I forgot to say it will only run with the choke pulled. Haven"t changed anything that is why I am having problems diagnose it I have been using it all winter long for moving snow so it hasn"t just been sitting around. The coil being bad is a real possibility though haven"t changed it over to a 12 volt coil yet just running a resistor block before the coil.

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John(UK)

03-30-2008 03:47:41




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 Re: TO-20 back Firing. in reply to mose1974, 03-30-2008 03:33:35  
Running it with a resistor is OK. Having to run with the choke pulled seems to be a fuel problem. Have you check the little filter in the elbow where the fuel line attaches to the carburettor, it is very small and always gets forgotten. But you should be able to check the fuel flow by removing the drain plug from the Carb. bowl and allowing it to run for a few mins into a jug, you need to allow it to run for a while to see if the flow changes, it should be reasonably strong, not dribbling..You can check the coil by removing the centre HT lead from the Distributor and holding it close to the engine block, then turn the engine over with the ignition switched on, you should get a nice blue fat spark that gives a crack as it fires.That will give you some idea of what the coil is like but the only true way is to try another one..John

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mose1974

03-30-2008 04:02:52




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 Re: TO-20 back Firing. in reply to John(UK), 03-30-2008 03:47:41  
I had great fuel flow when the drain was removed after I removed the carb I will try it again later to see if I still have good fuel flow.



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john(UK)

03-30-2008 03:28:58




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 Re: TO-20 back Firing. in reply to Mose1974, 03-29-2008 16:41:10  
The favourite thing to cause your problem is the Condenser, you cant test them, just change it.( it is 2 microfarad capacity) They can break down without any warning at all.You could also check if there is any appreciable amount of wear in the bearings in the Distributor as that would cause the points opening irregularly. Turn the engine until the points are about to open and then push the distributor shaft slightly towards the cam on the points, if the points open then the bearings are worn and should be replaced. Make sure that the points are gapped accurately to (.015")
Check that the battery is wired Positive ground unless you have an Alternator. Move the Ground lead from the back of the Dash Panel if it is still attached there and attach it to one of the bolts close to the starter. Check that the coil feed wire from the switch is going to the correct terminal on the HT Coil, it will be marked SW or + or IGN..John

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Jerry/MT

03-29-2008 18:08:43




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 Re: TO-20 back Firing. in reply to Mose1974, 03-29-2008 16:41:10  
What changes have you made just before this hapened? Are you doing any thing (increasing the rpm, etc) besides running steady state when this occurs? Yhe more info you can provide, the better chance the folks on this forum can help you to diagnose diagnose your problem.

Is this a 6 V system or a 12 V system? If it"s 12V, do you have a true 12V coil like NAPA IC14-SB? If you have a 6V coil on a 12V system the coil can overheat in 15-30 minutes and short out or loose it"s capapbility and misfire. When you shut down and it cools off, it will be OK for about 15-30 minutes and repeat the failure until it burns out completely.

Check the timing and make sure that at idle(~400 rpm) it is at 7 oBTDC and when you rev it up it will advance to 21 oBTDC at 1000 rpm and 31 oBTDC at 2000 rpm.

Make sure the diring order is correct (1-3-4-2).

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Dennis(Ohio)

03-30-2008 07:19:22




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 Re: TO-20 back Firing. in reply to Jerry/MT, 03-29-2008 18:08:43  
Please consider the fact that the resistor can also go bad. This happened to me on a Chrysler product I once owned that utilized a ballast resistor.



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Jerry/MT

03-30-2008 19:37:32




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 Re: TO-20 back Firing. in reply to Dennis(Ohio), 03-30-2008 07:19:22  
My experience is that resistors either work or they don"t. When they don"t, you have an open circuit and the coil won"t fire.

I had a friend who had a Dodge that used a resistor in the run part of the ignition circuit but was not connecte when in the start mode so the coil got full current at starting. His Dodge would start but as soon as he let go of the key it would die. The resistor failed and he had an open circuit in the run position.

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