470 - backfire, cough and sputtering upon shut down

nwboater45

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From fixing one problem to the start of a new problem. I just got done replacing the exhaust shutters on a motor that probably hasn’t had them in years (see previous post). Anyways I have a 1989 470, 3.7L, 165hp motor that has decided now to backfire, cough and sputter upon shut down. I runs and idles fine while on muffs. I had rebuilt the carb last year along with installing electronic ignition. Before installing shutters I never had this problem.

Over the weekend I purchased and gapped new spark plugs, (wires, cap, and rotor are all fairly new). I located TDC using the finger test and set the rotor to the #1 plug. I also redid the settings on the carb (adjusting the idle screw to 1 ¼ turns past light seating and throttle screw to 2 turns past just touching). I started it up and ran it until it got warm. I put the timing light on it and adjust the timing to 4 degrees. I then put on my Acutron to adjust the rpm which I adjusted the throttle screw until I reach 600 rpms (which now puts the timing at about 8 degrees). It seems to idle fine and has no hesitation throttling it up to about 1,000 to 1,200 rpms. But when I turn the ignition key off it sputters and coughs about 4-5 times before shutting down. I have looked through the forums and could not find anyone else with this problem. Any suggestions on where to start? Is my timing out of wack? Could it be fuel still pumping into the carb?

Thanks.
 

Maclin

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May 27, 2007
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When you throttle up to 1200 rpm's does it go back to 600 rpm's crisply when you take it back to idle? Or do the rpm's float some even with the throttle at idle?

The timing should not change much at all when adjusting the idle, not sure what is going on there but could be advance weights not moving back to no advance. I would want to check the operation of the ignition advance in the distributor. With the dist cap off turn the rotor . It should only move one way then snap back, it will only move about 20 degrees or so, not much. If that snap back action is not crisp or it does not snap back or it cannot be moved then the advance weights may be staying open, that will keep the idle up and cause the run-on condition.
 

nwboater45

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First...is there weights with electronic ignition? They would have to be under the plate and I can check tonight. I recall it coming back to idle fairly quickly like it did before the shutter change. When I was setting the rotor to the number one plug I did turn it and it did snap back to the original position. I could turn it beyond that position but ever so slightly. I will check all that again tonight. I did notice that when it shut off I it did start back up again with the turn of the key (no gas via throttle) but it was at 400rpm +/- and after I gave it throttle then it would drop back to 600rpm. Which I though was weird. Could it be a bad distributor or the charging system not sending enough volts through?
 

Maclin

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Depending on the type of electronic ignition you installed, the electronic part is just the triggering, the mechanical advance is still there as you proved out with the rotor snap-back test. The other action when starting then throttling some then back to idle to get it to 600 is pretty normal on carbed engines, could be just a little rich on the idle air/fuel adjustment, no biggie.

If the timing is jumping around some then the bushings in the distributor may be worn and the top of the shaft wobbling some.
 

nwboater45

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Alright I check last night and the rotor snaps back into position. I checked all my spark plug and coil connections. The coil connections we a little iffy. I started it up and let it warm up for awhile and then set the timing back to 4 degrees. There was a big hesitation upon throttle. So I adjusted the throttle and air mixer and played with those setting for awhile. I shut it off periodically and the coughing and sputtering went away. But every time I restarted it, it seemed to come back in a different rpm (again using the meter and not the tach). So after spending about a half hour tweaking the idle and timing I finally got it to were it had very little hesitation and it was running at about 600 rpm and the timing mark was at about 5 degrees. Went to shut it off and had one big carb backfire. I was pissed and quick working on it. I am confused if I am doing something wrong or where to look for the problem. Any suggestions on where to start?
 

pro-crastinator

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I am sure that you are on the right track (easy for me to say) and apologies if I muddy the water - but -
Can we rule out fuel quality as the source of the problem?
Can you run on a remote fresh fuel source that results in a repeat of the coughing symptom?\
 

nwboater45

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No we cannot rule out bad gas. Filled it up last fall and due to various projects and problems with the boat I have not had it on the water. I did add marine Stabil to the fuel. I will run a line from a clean tank and see if that helps at all.
 
Joined
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Yeah, Stabil only works with fresh gas to make it last longer. It won't do anything for bad gas.... it even says so on the label. It could very well be lousy gas and a rebuilt carb last year could have bad gas sitting in it and cause gunk in the carb again...

What brand of plugs did you buy and the model number?
 

nwboater45

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Thanks Eric.......I am going to do the fresh gas on it hopefully tonight. The plugs are NGK's and I would have to confirm the number.
 
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NGK's are good stuff. I use them or the AC Delco's..

always stay away from Champion, have had some bad experiences with them.
 

UncleWillie

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...Anyways I have a 1989 470, 3.7L, 165hp motor that has decided now to backfire, cough and sputter upon shut down.... But when I turn the Ignition Key Off it sputters and coughs about 4-5 times before shutting down... Is my timing out of wack? Could it be fuel still pumping into the carb?
Thanks.

With the Key OFF, the ignition should be dead. Timing is no longer a factor.
The carburetor will still have fuel in it at shut down. This is to be expected.

I order for the engine to run it needs Air, Fuel and Spark. Turning Off the key removes the spark. No Spark, No Run!
If there are glowing carbon deposits in the cylinders, they can act as an ignition source.

The hot tips of the spark plugs can also cause the "Dieseling".
Some carburetors were designed with a solenoid that blocked the Idle Air passage when the key was OFF to prevent Dieseling. No Air, No Run!

When the key is OFF and the engine is Dieseling, if you Open the Throttle slightly, will it speed up, or die instantly?
 

pro-crastinator

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I gotta say that I have had bouts of similar sputtering on engine cut off.
My process is to run the tank near dry and get fresh fuel, either that or evacuate the fuel tank, using that questionable gas in the cars,
Going with a fresh batch of fuel for the boat always clears up the situation.
Usually happens to me after a period where the boat is on the lift - with a lot of air space in the tank, towards the end of the season. (Like now).
best O luck.

Regarding Spark Plugs, my life experience has seen 3 Champions fail and one NGK fail.
That said, I prefer NGK whenever possible.
The most dramatic failure was when the threaded metal portion of the plug became "disconnected" with the insulator - which resulted in a noisy, rough running engine blowing crap out thru the busted spark plug. An easy out pulled the culprit from the block.
 
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