Author Topic: 78 AS backfiring under load  (Read 8512 times)

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Offline KEVN

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78 AS backfiring under load
« on: July 19, 2015, 05:07:57 PM »
I have a 78 AS that had the engine rebuilt some years ago and also added electronic ignition. I got the boat a little over 2 years ago and it always started backfiring under load after the engine is warmed up and possibly after idling for a few minutes. I have replaced the Holley carb this year and this is the 2nd time I've had it out. I was pulling someone for a while and then started backfiring again after idling waiting for them to get ready again. After letting the engine cool down it works fine again. I am wondering if it could be caused by possibly the electronic ignition or something other than the carb. Posted is also a pic of the engine setup. This has been frustrating but not sure where to go from here.

Offline shyskier

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Re: 78 AS backfiring under load
« Reply #1 on: July 21, 2015, 07:42:23 AM »
Engine timing is a most common cause of a back fire. make sure firing order, dist. position correct, ck. timing. Other poss. cause is cap, rotor, plug wire. It looks like they are pretty new, but wires right next to coil wire could cause a spark leak across wires, try to reposition.
Does it backfire thru carb or exhaust, poss. valve issue but that would run rough all the time.
If it backfires only on accel.,& hesitates would be lack of fuel.
hope this helps, good luck.

Offline KEVN

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Re: 78 AS backfiring under load
« Reply #2 on: July 21, 2015, 09:24:42 AM »
Thanks for the reply. The plugs, wires, and distributor cap maybe have 10 hours on them, but didn't seem to change anything. It is backfiring through the carb after idling for a bit and mostly on accel. If I slowly give it throttle, it will hesitate or run ok at a low rpm with little hesitation. I have been wondering about fuel as well. A buddy of mine who is more knowledgeable about engines than I am has wondering about lack of fuel. I will also try and re-position the wires near the coil as well.

I have most been putting 87 non ethanol gas in it and wondering if moving to a higher octane gas would help.

Offline KEVN

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Re: 78 AS backfiring under load
« Reply #3 on: July 21, 2015, 09:27:27 AM »
I will also add that it has a new fuel filter and the fuel lines have been flushed in the last month. Also may have some old gas in there as it is usually always filled back up at 1/4 to 1/2 tank or higher.

Offline backfoot100

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Re: 78 AS backfiring under load
« Reply #4 on: July 22, 2015, 09:35:11 AM »
Is that a marine carb that you replaced it with? It doesn't look like it.

Backfire is getting the spark lighting up the fuel mixture at the wrong time or in the wrong order, hence, timing related. It has nothing to do with the carb or any carb settings. What's the timing initial advance set at? Was it timed after the EI conversion was put in?
When people run down to the lake to see what is making that noise, you've succeeded.

Offline KEVN

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Re: 78 AS backfiring under load
« Reply #5 on: July 22, 2015, 06:11:22 PM »
I had the carb put on by my boat mechanic that built the engine Probably 4 years ago. The EI was already installed.

Where can I find out what the timing should be so I can check it?

Offline backfoot100

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Re: 78 AS backfiring under load
« Reply #6 on: July 23, 2015, 05:58:20 AM »
So if I'm reading your post right, this has always had the backfiring since you you've owned it or since the engine was rebuilt or both? It's kinda hard to decipher by your post.

Timing for a Ford is normally around 8 degrees BTDC. They don't like much more advance like a Chevy.
When people run down to the lake to see what is making that noise, you've succeeded.

Offline shyskier

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Re: 78 AS backfiring under load
« Reply #7 on: July 23, 2015, 06:48:28 AM »
Backfire thru carb will cause power valve diaphram to break. Don"t bother to replace till backfire fixed.

Offline backfoot100

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Re: 78 AS backfiring under load
« Reply #8 on: July 23, 2015, 08:19:41 AM »
Backfire thru carb will cause power valve diaphram to break. Don"t bother to replace till backfire fixed.

Not true. Holley fixed that in '92 so any Holley newer than that has blowout protection. The OP said it's new carb so it isn't an issue.
When people run down to the lake to see what is making that noise, you've succeeded.

Offline Midskier

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Re: 78 AS backfiring under load
« Reply #9 on: July 23, 2015, 11:08:14 AM »
aaahh the old commanders ............ with electronic ignition kit.......... the EI requires min +8 volts to operate
thanks for the pic - as it appears the positive lead of the ei goes to the + terminal of the coil (red wire) and will assume the negative lead goes to the - terminal on the coil (black wire) - that is how 99.9% are installed and is per the instructions.

assuming all ignition components are in good operation condition - you need to ensure you are getting @ least +8v  @ the positive terminal of the coil when in the key is in the "I" (ignition / run ) position, typically you'll have +12 in the start position - because...........
the start position typically bypasses the ballast resistor giving full battery voltage to the the coil for reliable starting, when in the run position the circuit goes through a ballast resistor reducing the voltage to the coil 2-4 volts from battery voltage - as the ballast resistor increases in temperature - the resistance increases - which decreases the voltage output........... and may cause the EI malfunction.
PCM and Indmar use a ceramic ballast resistor it is easily recognized and bypassed, the commander uses a ballast resistor wire - a wire wadded up, wrapped in tape and hidden in the wire harness - find it and either replace it with regular wire, cut it in half (half the length = half the resistance) or leave it alone and just remove the red EI wire from the positive coil post and find a suitable switched +12v source (switched = ONLY when ignition is ON)  splice it with the purple wire @ the carb electric choke If this is too much to swallow - bring to me :)

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Offline KEVN

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Re: 78 AS backfiring under load
« Reply #10 on: July 27, 2015, 05:30:29 PM »
Thanks for all of the replies. I will check the wiring this week and let you know what I find. It looks like moving the positive wire from the coil my be the easiest option to start with.

Offline Hellfighter334

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Re: 78 AS backfiring under load
« Reply #11 on: March 09, 2016, 04:49:42 PM »
Dan, what color is this resistance wire?  I have the same problem.  And is it in the main harness between the dash plug and engine plug.  I'm currently rewiring my 84' and having issues.