pre ignition -detonation

[topdawg487]

New at this. I have a 83 140 about to go to the shop for a rebuild i have 2 piston that sufferd what the machanic said was pre ignition. This will be the second time he replaces theese pistons. He assumed that a looose wire in the stator had caused the pre ignition.Well turns out he ws wrong read in the forums about timming. He told me he had set it 2 past??? when i got it on the water ran very slugish, when i went to take of the cover got electricuted pretty bad. so i was shock right through the cover. Went home notice spark jumping from plug wire to the head. Changed the plug wires still jumped i moved the wires away form the block as best i could didnt see the spark but could tell from sound of the motor it was sparkig someware. I would assume that the two cylinders are reciving tomuch power to jump through the insulation of the wire. But why? my ? is what are some genral causes of pre ignition? and does my problem ring a bell with anyone? thanks

 

[Walker]

Re: pre ignition -detonation

Electricity of any kind takes the easiest route to ground. The fact that yours is jumping from the wires to the block tells me that either your wire insulation is bad, the sparkplug boot is bad, or your sparkplugs are bad. My guess would be the boot. Some or those older V4's were notorious for the hood clamps damaging the boots on the lower plugs. Thats why you got shocked. Now this problem by itself has nothing to do with your damaged pistons.

 

[Terry H]

Re: pre ignition -detonation

On that motor topdawg the lowest spark plugs are very close to the latches, and removing and replacing the cover can damage the ends of the wires and spark thru to the case...so if you have new wires that's prolly what's happening...change the ends. Hoped u used marine style wire and connectors. Chief

 

[pine island fred]

Re: pre ignition -detonation

wow, sitting here with a beer and stiring a bunch of cob webs. experience goes back to wright 3350 turbo compounds and prat 4360s air cooled radials, but gas mixtures are the same in all cylinders. pre ignition is caused by something setting off the charge before the spark. hot carbon in the cyl, to lean a mixture combined with high eng temp. low octain ( out of our control). this will result in a loss of power<br /> the big bad cookie is detonation, fuel/air mixture is set off by the plug, starts to burn at an even rate but then explodes. results in the ping, valve knock , knocking ( lots of different discriptions ). the fix is to enrichen the mixture, retard the timing, lower cyl. head temp or reduce manifold pressure. detonation is what burned your pistons. the fact that the mechanic retarded the timing helped but it did affect engine performance ( sluggish) sounds like your mixture is to lean. check for air leaks down stream of the carbs, float level to low, trash in carbs main jets. ever wonder why a 2 stroke runs best just before it runs out of gas ? they are set to run rich, insuring a cooler burn in the cylinder. as for the spark jumping Walker has it right. hope i remembered everything. regards fred.

 

[pine island fred]

Re: pre ignition -detonation

just remembered to add. to agressive a pitch on the prop will increase a high manifold pressure which will cause detonation. check size and pitch of the prop. did it allow full rpm at open throtle? fred

 

[topdawg487]

Re: pre ignition -detonation

Ok now what about double firing into the piston i belive that is when the plug fires twice once when the piston is at compression and once when it is in the exust phase. I think this is were my problem is my pistons look pitted from the spark when they are dry does this sound right and if so what causes this? thanks

 

[rodbolt]

Re: pre ignition -detonation

hello<br /> over the years I have learned to check ign timing on both banks of the dual pack v4-v6 engines. seems the blocking diode can cause advanced timing on certain cases. I have seen timing as far as 5* different from one bank to the other. if this is ongoing ask your tech to index the flywheel and check both banks ign timing. seems a crossfire can do the same thing. <br /> good luck but ya have to find the cause or it does it again.

 

[Paul Moir]

Re: pre ignition -detonation

I'm surprised no one has asked but what plugs have you been using? The wrong once will cause pre-ignition.

 

[pine island fred]

Re: pre ignition -detonation

Just cruising thru here this morning. Introducing a spark on during the exhaust phase should have no effect as the fuel/air mixture has been burned or detonated. As RODBOLT says, you could have advanced timing as he has seen in his experiences,which would lead to very sluggish performance. Paul Moir also brings up a very good point as to the plugs. Old trick was to start with a champion j-8, run for awhile , read the plug, if to hot, back down to a cooler j-7 or 6, if to cold bump up to a j-9. however you could be burning more pistons in the process. as rodbolt says, you have to find the cause or it will happen again. When the motor is running again, carefully operate it under load with the cowling off and listen for pinging. If it knocks,idle back to the dock and investgate further. Could the motor be running entirely to hot? <br /> Detonation is a real killer. It will put undue load on the wrist pins and rod bearings as well as tear up pistons. manufacturers went so far as to use ADI ,anti detonation injection, at takeoff power settings. Was straigh water. Early jets used water or water alchol, but thats another story. Good luck. keep posting. fred