clear (dry or glazed) plug fouling??? Stumped....
 

Twin 515 cu. in. NA right at 600hp. 850 Holleys with Dart intake, iron heads. MSD distributor with 6M MSD boxes. My issue is with fouling plugs, and they come out looking clean. Plugs are NGK R5674-7. They do not gradually foul at idle, but no idle is the result after a run.

I run 5 - 10 miles down the lake, 3500-4000 rpm come to a stop and both engines die. Very hard to restart, cough, spudder and barely run given some throttle. No way to put it in gear with out dying. Check over everything and all seems right. Carbs not flooding or dripping. Floats are set properly. Put in new plugs and all is well again.

I did install O2 sensors over the winter and have found that I need to fatten up the mid range, I am at 13 at 3500, and need to get that to 12 before I hurt something.

Is it possible to lean foul plugs? Strangest thing is that it is both engines at the same time. I don't run the boat the same every time out, but is seems about every 3rd time out this happens. 3 sets of plugs already this year.

My plan is to fatten the mid range and go with colder plugs.

Any suggestions or experience in solving this problem?

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Tighten the gap up some I don't think its the spark plugs. They look clean enough to run. I would be leaning towards ignition. Do the plugs come out wet?

fatten up your carbs. Are cooking fuel off your pistons when you slow down to a idle after a run? After the pistons cool some it runs ok again.

How much timing are you running and what compression are the motors?

At first my gaps were at 0.030, I changed to 0.035 on the last two sets. Shouldn't that be tight enough? I was actually thinking of opening it up a little more.

Some of the plugs come out slightly wet, my assumption is from not firing and me hard on the throttle (acc pump) trying to get it going again.

I won't say that it isn't ignition, but after changing out this last set of plugs in the middle of the lake, I went on another 5 miles, had lunch and then back home (15 miles) stopping once on the way. No problems whatsoever. Will not leave the dock again without a spare set of plugs on board though.

Thank you for the reply!

34 degrees 9:1 compression.

Post some close up pictures of the plugs

https://c1.staticflickr.com/3/2198/1...c8a_z.jpg?zz=1Chinese Spark Plug Factory by abarndweller, on Flickr

https://c1.staticflickr.com/3/2385/1....jpg?zz=1Chinese Spark Plug Factory by abarndweller, on Flickr

These are from the port engine. The pic in first post is the stbd.

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My money is that its not the plugs they look ok. Put a set of the old ones back in after it stalls again see if it starts.

I'll put down $5 that it's the ignition. Generally speaking, electronics can work fine until strong continuous current warms them up exposing a fault, and MSD hasn't had the best track record here.

I'd open the hatch and wait the amt of time it takes you to replace the plugs. No shorter. And then try to restart and run.

plugs look dark enough. I don't know

I have read about the bad rap on MSD. Not saying anything good or bad, other than both acting up in the same way at the exact same time doesn't make sense to me, and more than one occasion.

I basically did this. After changing plugs in one engine, I started it to make sure that was the problem. Then I tried to start the other and still had the problem.

I can certainly try this. May try putting the plugs back in one engine just for the hell of it and see what happens.

Did the carbs get rebuilt lately? I did a dominator once put the wrong gasket in and when I shut it off it would siphon the carb dry. changed igniting on it drove me crazy.

One carb is new only apart to change jets, the other was on the boat/engine when I got it.

Thanks for the input!

coil weak or mounted on hot motor , check ignition with fine tooth comb

Ignitions have common power source and ground? There may be something going on there. The po of my boat ran a 20g wire to the coil when it required a 12g and it was at idle that noticed a significant difference.

Have you tried changing out the brand of beer that you consume when yelling at it? I've found that to be as effective as anything else troubleshooting boat problems.

Have you tried a different brand of plug? ( Champion RV9YC )

it,s hard to tell but it looks like the center electrode is burnt off of # 1&2.also reversion will cause the exact symptoms you described.

The other side of the V in the electrode is shining to the camera. I thought you had something. Then I blew up the photo.

Bill, what heads are on this 600hp 515ci? Are you sure the heads don't call for a .750 reach plug, and not a .460 reach like you have there? If they are stock merc iron heads, than disregard. But most aftermarket heads call for a .750 reach plug.

I have not, but worth a try

They are standard iron heads. 088 I believe.

Plugs look fine. I think you have something else going on. Odd that it's both engines but don't rule out another issue

Go back to mr43t. Personally I've never had good luck with champion plugs for the very same reason.

Good ref. Btw ck the ground straps on all your plugs. There difficult to see in pics. I think I would have to agree with others here also. Throw a HEI in for a quick test if doubts. Quick simple test.

Plugs look lean. Sounds like you have water in fuel. Dump water separators in a clear jug. Let sample sit for awhile and look for signs of water. Try running motors off an auxiliary tank with fresh fuel. 10 gallon outboard tanks work well.

Are the R43T the same heat range as the 8's in NGK? I'll have to look it up.

I don't have another distributor laying around, though I plan to thoroughly inspect all aspects of my current distributors, power and ground feeds and all connections.

The engines are lean at the cruising speed and will be fattened up before any further testing. I will check the separators. Obviously water in fuel is bad, but wouldn't it show up immediately or at least after the next run even with new plugs? I have not popping or back firing that I would expect with water.

Again, thanks to you guys and everyone for the input!

I run the MR43T in my blown 454s at 5psi with no problems. Try champion in one motor and ac in the other motor.

I know who to call when I need my plugs changed...............Bill I may have a merc dist. if you want to try something like that.

Since he changed one engine'splugs and it fired right up, and the other engine didn't until he did change those, I would go with that 1st and just try another set.

I believe his NGK's cross to a 42 Heat Range Delco. So, MR42T.

Difference would be the NGK's he has are not a resistor plug and the Delco's are.

What kills me here, is that is the main reason of using a MSD box. Enough power to fire off crappy condition plugs. Almost makes you wonder how the grounds are ? Do you knowhow many engines I find with the heads threaded holes that are used for grounds full of paint ? People love there paint !

I typically go one step further, and run a ground from the other cylinder head to that stud also, ust to make sure everythings grounded well. Clean those grounds ! Especially the threaded hole that you are grounding too. This goes for alternator brackets and their bolts and threaded attachment points.

that's why the grounding studs are on the bellhousings..............speaking of grounding, how important is it to ground engine to engine ?? I ask because when I pulled my 502's years ago they were tied together with a big black cable.

Just as important as the positives ^

Just some really good advice here (for all), whether or not this is your issue. Bad grounds are almost always the last thing considered, yet easy and cheap to rule out. The easiest way to learn this is to own an old Fiat at some point in your life

Showing my ignorance here, but why would a resistor plug be any better? Other than help with stereo interference? I have been searching for the heat range differences. Higher number NGK is colder, higher AC is hotter. I think the -8 probably would be a 42.

I will definitely pull the grounds and check the mating surfaces and double check tightness.

What - I'm not on a car forum ? Ha ! :braindead:

I see the bellhousing stud threads (in block) - thank you - all painted up too.


What would someone say if a boat i worked on a few years ago had no issues other than an extended amt of time of cranking over condition because of a leaking quadraet main plug. which I repaired and all was well after. Fired right up. No draining issues.

So....this same boat next spring is due for cap/rotor/plugs and wires...while routing the wires and touching the big ground wire, the stud moved. I was like, what ? The fuker wasn't even threaded into the block. In dismay, I moved stud further from block/bellhousing and the motor cranked and fired right up and all worked with no issue. WTF ? Is this magic ? Only one ground cable from battery (2nd battery grounded to main battery) to block. All the little grounds ran to this stud. I spent a few hrs looking for another way this engine was grounded and could find none. I still scratch my head on that one as the amt of issues I still see to this day from seemingly good grounds, but apparently not good enough, causing minor to maor issues. Engine was a basic small block with quadrajet and TBV.

I was just noting the only difference in specs. Was not getting into the difference resistor vs non resistor can make.

extended crank I get this now..........you mean it was hard to start. Ive heard of an extended crank before but don't own one.....lol

LOL. I edited it to avoid confusion. Also note my 'reason for editing'note. HAH !