Backfiring TO-20

[FergFord]

TO-20 is now backfiring and I can not determine why.

History.

Just finished a full engine and carb rebuild. Doing some tune-up work today after installing a new steel gasline. Valves were set correctly. Good fuel flow. Good spark.

Choking helps a little as does a slight increase in tension on the throttle rod. However no clear solution.

I will check compression later today to see if valve issue. Any other good suggestions?

Mark

 

[FergFord]

More info. Prior to the fuel line work this week I had her running smoothly for about 30 minutes at low idle after finishing the rebuild and valve adjust.

 

[Mark from the Fort]

Might be worth re checking rocker gap and re checking head and manifold torques, after a first good warm up, these ofter need to be re-torqued. Could be a slight air leak in intake manifold or carb gasket. Other thing that pops into mind is sloppy distributer? Is it possible the the new fuel line as disrupted the old carb settings, in other words, maybe carb needs re- set to accomidate new line?
Congrats on the rebuild. Mark

 

[Mark from the Fort]

not sure about causing a back fire, but did you re check timing after "gluin'" her back together?
MM

 

[Jerry/MT]

What's your definition of "good spark"?. Make sure that you have a FAT, BLUISH-WHITE SPARK.

Also make sure that your initial timing is correct and that that the distributor advance system is working as it should.

Check the distibutor for shaft/bushing wear (often the cause of weak spark) by making sure the points are closed and pushing radially (from the side) on the distributor shaft. The allowable radial runout is 0.002" and the points should show no motion when you perform this check.

 

[FergFord]

jerry,

thanks for the reply.

I have read your comment about spark many times. The spark is as expected. This does not appear to be a weak spark issue.

I have not touched the timing since she was running last week. It would seem odd if the timing/points/distributor had a failure just by sitting. I will check anyway. Maybe I have an intermittant miss.

With the engine warm 10-15 minutes low idle the presssure is better than 100psi for all cylinders so it does not look like a valve.

Mark

 

[Anonymous-0]

I would make sure you attached the spark wires in the correct order. It's supposed to be 1-3-4-2 and the rotor goes counter-clockwise. The backfiring is actually cylinder 3 and 2 firing on the exhaust stroke from the fuel not igniting on the compression stroke. Maybe...

 

[FergFord]

doubled checked this but it is correct. di have it backwards once this spring after changing the plugs. runs like hell with reversed wires.

Mark

 

[FergFord]

Mark,

I did not pull the distributor during the rebuild and she ran well after reassembly. so the timing was never messed with.

I will recheck the carb to manifold bolts maybe something is a little loose.

The new fuel line should make things better. The old fuel delivery was a mess of splices, hose and tubing. She clearly now flows like "a cow pissing on a flat rock" as is said around here.

Mark

 

[Anonymous-0]

my distributor was loose and i didn't know it,and when engine was running distributor would turn back and fourth causing it to run bad.

 

[Anonymous-0]

Hello, A bad condenser will cause severe backfire,If you have changed it recently, put the old one back in and try it. Regards-ed

 

[Jerry/MT]

Your compression is good mark I suspect soemthing is wrong with the advance mechanisn. What is your initial timing? It should 7oBTDC for a TO-20/Z120 engine. Your spark should advance to 21oBTDC@1000 rpm and to 31oBTDC@ 2000 rpm. Make sure that your fuel mixture is not too lean. The engine should accelerate smoothly from idle to max rpm. If it coughs or sputters, richen it by 1/8 turns on the power jet adjustment till it accelerates smoothly.

 

[Jerry/MT]

Your compression is good, Mark I suspect soemthing is wrong with the advance mechanisn. What is your initial timing? It should 7oBTDC for a TO-20/Z120 engine. Your spark should advance to 21oBTDC@1000 rpm and to 31oBTDC@ 2000 rpm. Make sure that your fuel mixture is not too lean. The engine should accelerate smoothly from idle to max rpm. If it coughs or sputters, richen it by 1/8 turns on the power jet adjustment till it accelerates smoothly.

 

[Andy D.]

You say it helps some to choke it? Is it posible that the final fuel filter or the main jet is partially clogged from some debris in the new line?