Rickracer(non-O/B)

[Delta]

Hi Rick:
I figger you're the only one I know that may have a simple answer for this non-outboard question:
1985 Seaswirl. 3.8 OMC v6.
Runs fine at idle,and thru rpms,but when left at cruising speed(3500-4000 rpms) for 15 min or longer,it will suddenly loose power,backfire thru the carb with very fast 'pops'.Kinda sounds like popcorn,really.
It seems to get progressively worse thru the season,but the symptoms may only happen 1 or 2 times per hour.
New cap,points,plugs,fuel filter/separator.
Does anything obvious come to mind?
Is this enough info? It's my buddy's,so much of this info is second hand.
I thought it might be a fuel filter at the inlet of the carb that is clogged,if it even has one there. The backfiring has me stumped.
I will vacuum test it this w/e, but it does it while at a constant rpm.
Thanks for any thoughts you might have,Delta

[Rickracer]

You said the points are new, but what about the condenser? I have had even brand new ones drive me crazy before. Usually they will show up immediately though. I would tend to think it's fuel related, possibly tank not venting, building a vacuum, or possibly a fuel pump problem. After checking the ignition, I believe I would check the tank venting system, then pressure and flow test the pump. It should make at least 6 lbs of pressure on the outlet, and pull at least 20" of vacuum on the inlet side. That carb shouldn't have a filter at the inlet, but it might, and if so, that could be clogged as well. As a little side note, I picked up 3 mph on a 18' bowrider with a 140hp I/O 4 banger one time, by adding an MSD 6A ignition unit and Blaster coil. The owner kept leaving the key on and burning up points, so I recommended going electronic, and the speed gain was the icing on the cake.

[Steve Reist]

I had a 4.3 do the same thing on a friends Rinker. Had a chunk
of gasket caught in a main jet so it was running on 3 cyls under
power. (Every other cyl) Good luck, Steve

[Delta]

Thanks guys.
I will test what you suggested this w/e,Rick.

Doesn't the rapid backfiring thru the carb seem significant?
The only time my hot rods did that was due to a vacuum leak,or timing way out of whack.But those times,the backfiring was much stronger,usually accompanied by flames!
Thanks,Delta

[optimax135]

Keep in mind that when you change points....your also changing the timing. I would set the timing at 4 degrees and see if that doesn't take care of your problem. You might want to check if your distributor is advancing also. I think it's somewhere around 28 degrees. Good-luck!

[Rickracer]

:cool:

[Rickracer]

I see you didn't replace the wires. I would definitely check them at the very least. GM specifies 2000~2500 ohms per foot max, Ford specifies 5000 ohms per foot max. I think both specs are a bit too much. If your wires are any thing over 2000, replace them. I would use spiral core wires if it was mine. As you know, electricity follows the path of least resistance. The resistance in the secondary ignition system is not a fixed value, it will rise and fall with cylinder pressure, fuel mixture, and a few other variables. Whenever resistance in the secondary system ( the path from the coil to the ground strap on the plug) rises above resistance between the plug wires and any other ground, it will short out there. This can occur inside the cap (usually causing carbon tracking), between the firing cylinder and another with less resistance (crossfire), or between the wire and any grounded metal surface it may be near. You may have seen GM HEI rotors that have a hole burned in them underneath the metal strap that contacts the coil terminal inside the cap (usually carbon or brass), this is a classic case of spark going to ground through the path of least resistance. I also see ecore coils all the time that short out to their metal frame (Ford, GM and Mopar now use these on most applications, HEIs have been like that since the early 70s). I hope this helps.

[Delta]

We got a late start,and had limited time,so I didn't get to test everything that was mentioned. There was an inlet filter in the carb,but it was pristine.fuel pump was OK.Plugs looked perfect.
The condensor had been replaced.
The air 'filter'(metal) was full of carbon from the backfiring.
This is what baffles the heck out of me!
Why backfiring?
How is this for a theory:

The mechanical advance is messed up,causing the plug to spark a little too soon,sending fire up the intake valve to the carb?
See what I mean,though?
The only way to backfire thru the carb is to send a flame up the intake valve,right?

Can you price me a set of wires(I'll have my buddy test his),and also what ever you would recommend to upgrade the ignition system.Whether to just get an aftermarket electronic ignition,or actually replace the distributor with an HEI. I suppose he will need different wires for the hotter ignition anyway,right?

Thanks Rick! Mark

[Rickracer]

I would just go to www.summitracing.com or www.jegs.com and order up a set of MSD spiral core wires, and either an MSD 5 or 6A ignition system. They do have marine units, but they are a bit more pricey. Either one will work, but I'd prefer the 6A. When you add either system, you retain the points, but they no longer carry any current, they only act as a switch, so they only need service when the rubbing block wears out, not likely in a marine application. You can buy this stuff just as cheaply as I can, so save yourselves a few bucks. :cool:

[Delta]

Thanks Rick!!!!