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35 min starts backfiring

H

homerl

Regular Contributor
"1986 chris craft scorpion 187

"1986 chris craft scorpion 187.runs perfect for 35 mins at all speeds,than starts backfiring and no power.It is a305ci. with no points.Timing is bang on,new high voltage coil,rotor ,cap,wires,plugs.Temperature is 160 degrees.This motor when it starts to backfire will always run smooth in neutral at 650rpm,but as soon as you try and give her any fuel in gear or not,starts to backfire.My mechanic and I are frustrated.We were wondering if the mercury iv spark advance computer on the port side riser maybe causing this problem.I please hope someone can help me.Thanks dave"
 
"Dave:

Did the mechanic fin

"Dave:

Did the mechanic find a rusted optic sensor under the rotor? The rust will interfere with the optic signal or cause tiny cracks in the sensor circuit that will open under heat.

Next time it backfires be prepared to perform the tests below quickly.

Tests for Thunderbolt Ignition:
W/ignition key on AND BILGE WELL VENTILATED OF GAS FUMES!!!

#1 - Connect your voltmeter neg. lead to the engine ground and pos. lead to the white/red wire terminal at the dist. It should read 12 volts.

#2 - If 12 volts is present, remove the coil spark wire from the distributor and connect it to a spark gap tester to ground. Remove the white/green lead from the dist. terminal. Turn the ign. key to on and strike the white/green lead to ground. If there is spark, replace the ignition sensor in the dist.

#3 - If there is no spark, substitute a new coil and repeat test #2. Now if you get spark, install a new coil. NO SPARK, replace ign. amp.

#4 - In #2, if there is No voltage present, disconnect the white/red wire and check it again for 12 volts. If 12 volts is present, replace the ignition sensor inside the dist. cap. If no 12 volts present w/it disconnected--ignition amplifier is shot and must be replaced.

OTHER TESTS FOR THE MODULE ON THE RISER

The backfiring sounds like a HEAT related problem. There are two ways to check the module for it; apply heat when it is working or apply cold when it isn't working. Electronics does not like heat.

With the engine running at idle hold a hair drier on low fan and medium to high heat about 2" - 3" from the module and heat it (about 5 mins.) until it kills the engine or backfiring occurs. Replace the module if it .
dies or backfires.

If nothing happens after it gets HOT from the drier then it's time to apply COLD. You will need 2 cans of computer canned air. Run the engine until it begins to backfire and then immediately hold both cans 2" inches from the module and spray all over the module until the backfiring stops or the cans empty. If the backfiring stops replace the module."
 
"kevin,all I really know is th

"kevin,all I really know is that it is a 2 barrell carb.Thanks Guy for more ideas,I will forward these to the mechanic"
 
"I found another thunderbolt 1

"I found another thunderbolt 1V module.Installed and has run over an hour with no backfiring.Now off to the lake,hope all goes well.Will report back tomorrow."
 
"Guy, hit the water again,aft

"Guy, hit the water again,after 45mins,started backfiring under load.Where is the ignition amplifier?.Also where is the ignition sensor?This motor does not have points.A drawing,or dissasembly drawing would be most helpful

thanks dave"
 
"Thanks Guy,the way I see it,I

"Thanks Guy,the way I see it,I should not have to loosen the distributer,and effect the timing.Do you think this is a job I could do myself?.Changed everything else myself so far."
 
"Yes. But it may change the t

"Yes. But it may change the timing since a part of the ignition timing is being changed. Mine did. Check it if it won't idle or stay running under 1,000 RPMs."
 
Next time it begins acting up

Next time it begins acting up look at the voltmeter. If your voltage regulator is going south it may be letting the voltage go high enough (15-16 V)to cause cross-firing. Mine did. About drove me nuts til I spotted it. Replacing the regulator cleared it right up. Good luck. JW in Dixie
 
"Warren,where would be the vol

"Warren,where would be the voltage regulater on a 305 gm motor?My gauge seems to read between 12 and 14 volts,but I really havn't noticed that when backfiring."
 
"Dave, you got me there. My on

"Dave, you got me there. My only experience is with the MCM470 that I have. The alt may have a built-in regulator or there may be a free-standing one. Someone here will know for sure.

What my thinking was that after thirty or so minutes the battery voltage would have recovered from the starting drain and maybe the voltage at that point is going too high. That's what mine was doing. Anyway, good luck. JW in Dixie"
 
"Thanks Warren,but if anyone e

"Thanks Warren,but if anyone else can help me locate where the voltage regulator is on a 305ci gm motor,I would really appreciate it.


thanks dave"
 
It's an integral part of t

It's an integral part of the alternator. Just connect a digital voltmeter to the battery terminals while underway and you'll see if it is going over 14.8 volts.
 
"Wouldn't you think that i

"Wouldn't you think that if the voltage regulator was over charging[14.8],you would see that on the gauge right after the startup/bilge/blower were used,almost right away?Just a thought

thanks dave"
 
"Your problem happend to be se

"Your problem happend to be several times, both in the boat that uses points and my truck that has elect ign. All that was wrong is very light moisture getting inside the dist cap. So light I could not see it. That caused backfire and cross fire.
If it happens again, pull the cap and squirt some starter fluid or similar stuff inside the cap, wipe it dry and reinstall the cap and start the engine and see if the backfire stops.
If this fixes the backfire, you may be getting some crank case vapors from the dist shaft or between the cap and dist body.
Good luck
Bill"
 
"Thanks bill,but the cap has b

"Thanks bill,but the cap has been changed,I hate to think a new one would be defective"
 
Dave:

I wouldn't rule o

Dave:

I wouldn't rule out a moisture or oil vapor problem inside the distributor just yet. Make sure the cap vent is open and clean. Wipe the vent outlet w/a white paper towel and look for oil stain. Do the same for the cap inside but first inspect for signs of carbon tracking.

Your problem is a time/heat related problem. Did you change the sensor under the rotor? What did it look like?
 
Both my caps were new also and

Both my caps were new also and they were not defective but moisture was getting in from either the shaft or between the cap and dist body.
Would not hurt to try it..may save you some grief

Over and out.
 
"I have new ignition pickup se

"I have new ignition pickup sensor,ready to put in as soon as it stops raining here in barrie ont.Calling for 100% rain today and tomorrow.Will look for carbon,oil film under sensor and around inside the cap.

thanks dave"
 
"Was able to change ignition s

"Was able to change ignition sensor.Mine was rusted bad.Checked under cap,all nice and clean.Ran for over half hour in driveway,but can't take on lake due to thunderstorms.Old sensor was two threaded shafts with nuts,the new one has red green and ground wires directly out of sensor with external terminal nuts and bolts.Hope all will run well now.The voltage runs at a constant 13.5 volts"
 
"Good luck Dave. <[img]"http:

"Good luck Dave.
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"
 
"Finally out on the water toda

"Finally out on the water today,over 90 mins and no problems.Changed ignition pickup sensor and installed a new flame thrower II ignition coil.Just like to say thanks to everyone who helped with all my problems.Going out friday on a little longer run,hope all goes well.

thanks dave"
 
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