Popped through the intake.

[Scott Danforth]

If spraying with water causes it to run bad, start with dielectric grease in the plug wire boots.

Water mist shouldn't cause it to run bad

 

[Fiberform71]

Reacting on water is indeed not good. I would start to check the spark cables for a good connection. Especially the one from the coil. Or replacing the coil. The rattling should be gone. You where also talking about a leak in the intake gasket. When a engine is warm the gap can extend. If there is air coming in through that gap the mixture will change (lean) this also can cause back firing/popping in the carb.

 

[Scott06]

OF course I couldn't duplicate on the trailer in the dark... So no go there. I lightly misted down the coil with water from a spray bottle... Caused it to run like sh!t and stall several times. Couldn't see any visible arcing. Monday is another day, got 3 days of work for the time being.

Replace the wire from coil to cap the archng is loosing spark

 

[Pmt133]

If spraying with water causes it to run bad, start with dielectric grease in the plug wire boots.

Water mist shouldn't cause it to run bad

I did add a little when I replaced the plug wires with the cap last week, but I can repack and check again. The coil had a heavy residual from the old wire I cleaned out. It was such a small mist too that it caught me off guard... I could see if it got hosed down but I'm talking like 2-3 mists from a water spray bottle.

Reacting on water is indeed not good. I would start to check the spark cables for a good connection. Especially the one from the coil. Or replacing the coil. The rattling should be gone. You where also talking about a leak in the intake gasket. When a engine is warm the gap can extend. If there is air coming in through that gap the mixture will change (lean) this also can cause back firing/popping in the carb.

I double checked all the wires last night after it started acting up. I plan to repack the ends with some dielectric when I get the chance. I am hesitant to replace the coil only because I can count of two instances where the coil was actually the problem... for the most part when I've delt with them, they either work or they don't and I'd rather not just start throwing parts at it. It needed plugs/wires/cap and rotor regardless so that doesn't hurt but I'd rather take the time and actually diagnose the problem and or just fix things that are problems as I find them... see below.

as far as the intake, yes I am suspecting a leak however I am no longer exhibiting sound signs of going lean... IE popping/back fire... I am just losing power, which still can be a lean condition I know. I will be replacing the intake gasket and packing the wire ends with grease. Then I will go from there. I have a suspicion that there are a couple things in play here.

Replace the wire from coil to cap the archng is loosing spark

It followed it from the old wire set to the new one.

 

[Scott06]

It followed it from the old wire set to the new one.

This is odd - is the top of the coil cracked? Only things I could think of is its coming out of the coil top, the wiring harness is too close and its jumping from HT wire, or you dont have a great cap to rotor connection and the spark has no where to go- only time I saw this was disconnected by coil wire off cap to spin engine without ignition and didnt have the cap end grounded. Made similar arcing to what you are seeing. was arching under boot on coil to one of the coil terminals...

 

[04fxdwgi25]

Have seen coils get a "carbon track" on them blow the spark right out from under the boot.

 

[Pmt133]

I'm hoping to get better eyes on it another night. Take it back out on a night run, duplicate the problem, see if I can find where it is arcing from. The fact I both stopped hearing and seeing it is suspect.

 

[tank1949]

Problem is different enough that rather than cluttering up my carb thread I figured it best to start a new thread.

So what happened: Ran the boat today after rebuilding the carb. Ran for about 2.5 hours with an hour break in the middle. So out on the water for 3.5 hours total. Upon nearing the ramp I started pushing it a little... nothing crazy, 3600 rpms for maybe a minute or two. Got a single pop through the carb under the spark arrestor. So I backed off to 2800 to investigate. It didn't sound quite right, but wasn't missing. Dad cracked open the dog house and all of a sudden it popped several times like pop corn. I dumped the throttle and idled it the 1/4 mile to the ramp. Seemed fine but I wasn't going to try pushing it any harder.

So when I got home I checked a couple things, pulled the plugs to make sure nothing looked scary, and they're good. No detonation, timing mark on the strap is good, fuel mixture seems about right. Nothing to write home about.

So next I ran a compression test, saw about 160 across the board, so again, nothing abnormal.

Finally I pulled the cap and rotor. Seems like a little scale build up inside and there were these points, not corrosion but a little slag hanging off the tip of one of the electrodes. I plan to pick up a cap and rotor tomorrow as well as plug wires.

I think that is a good place to start. I also have had issues in trucks in the past with the vortec style intake having vacuum leaks... but I wouldn't think it would run so well for so long then all of a sudden mess up. This happened a couple weeks ago too at nearly the same point in the trip... So there is that. I'm not denying that I messed up the carb but... I don't think it would run as well as it did if there were something wrong with what I did... but I am also an idiot and have been wrong many times before.

I'm just looking for input on if this is a good direction to move in for now. Appreciate it.

-Paul

My guess is timing off when running cheap gas.

 

[Pmt133]

So I went back out tonight, was able to more or less duplicate the problem. However I wasn't able to get the arcing from wherever tonight... Same symptoms, running maybe 30 minutes, gets a little backfire. I came off the throttle brought it back up to cruise (~3000 rpms) and about 5 minutes later it just started breaking up.

Intake gaskets are my next point of contact. Are there any specific tests I can run on a thunderbolt 5 aside from ohming out the coil?

 

[Fiberform71]

The ticking noise is now gone? If you're sure of a leaking gasket then change it. If the intake is leaking it can also cause back firing. When it warms up the gap can get bigger and if the choke is working it's getting more fuel when the engine is cold. When the engine heats up choke is coming of(more air) and it leans out. Are you sure it's getting enough fuel when the symptoms are coming up? If it's possible;try to close the choke when the problems are there. When it runs fine you know it's leaning out.

 

[Pmt133]

The ticking noise is now gone? If you're sure of a leaking gasket then change it. If the intake is leaking it can also cause back firing. When it warms up the gap can get bigger and if the choke is working it's getting more fuel when the engine is cold. When the engine heats up choke is coming of(more air) and it leans out. Are you sure it's getting enough fuel when the symptoms are coming up? If it's possible;try to close the choke when the problems are there. When it runs fine you know it's leaning out.

I've checked out the choke while running... it comes off completely within 30 seconds of starting... 95 degree ambient temps plus 85 degree water temps, doesn't come on very long. I had tried the choke before I rebuilt the carb and all it did was bog it out by going pig rich... black smoke and all. I could try again another night.

The arcing noise I was hearing last outing I could not duplicate. That is correct and the only thing that was present last time that wasn't the other times this happened.

I can't say for certain about fuel... but it was mirror flat last night, I ran it for about 30 minutes straight varying RPM from 3000 up to wide open and it ran fine under way. Heads, manifolds, risers, no problem holding my hand on them = nice and cool... well, hot, but not hot hot. Let her eat at wide open for about 2 minutes no symptoms either. If it were a fuel problem I would think it would show up sooner no? And even weirder was if it shut it down for 10 minutes it would run fine until everything got real hot again. I measured that needle and seat some 10 times, it's set right. When it starts acting up it just feels like I am flicking the key on and off... It's a clean drop out. It's maintaining fuel pressure best I can see. It's getting lean, no questions asked, but the fact it takes so long to start acting up makes me think it's beyond just the carburetor and the fact it was so exact on how long it took to occur... I've fought carbs in the past and it usually was apparent from the start. That's what is confusing me...

I am getting a lot of water between the intake and head... tasting it on the water, it is salty, so not condensation. When it happened on the trailer I thought it was a temperature difference causing condensation with this high humidity. Gaskets are definitely leaking. Going through some of the other work that was done by the PO/Mechanic on this engine... there is a lot to be desired with quality of workmanship for someone who was "certified". I've had the vortec intake gaskets throw me for loops on 350 trucks... so that is why I am going for it. We'll see. I have a suspicion this guy reused the gaskets at one point.

 

[Fiberform71]

I read that's a clean dropout. If it looks like you switching off the ignition (like a cut out) then most likely it's in your ignition system. You are also talking about the temperature. It only appears after warming up. When the engine is running move your cables a bit. Also the connection between engine and wiring harness. Also check the switch interrupter if it works(not loosen) Specially when temperature is involved I would search around the engine (hottest place) not possible to swap the ignition coil for an other one (another boater)?

 

[Pmt133]

I read that's a clean dropout. If it looks like you switching off the ignition (like a cut out) then most likely it's in your ignition system. You are also talking about the temperature. It only appears after warming up. When the engine is running move your cables a bit. Also the connection between engine and wiring harness. Also check the switch interrupter if it works(not loosen) Specially when temperature is involved I would search around the engine (hottest place) not possible to swap the ignition coil for an other one (another boater)?

Most of my friends have substantially newer boats with delco EST and MPI or very old stuff with points still. The points coil may work, but from my understanding the thunderbolt coils are a little different. I could ask my friend at the marina, they are gutting some late 80s small block boats, I may find a serviceable one there.

 

[Pmt133]

Well, this isn't pleasant.

 

[Fiberform71]

This one has leaked for a while... If it's possible try to check also the inside of the engine/pistons. That it was running...

 

[Scott06]

Well, this isn't pleasant.
View attachment 384923

you must have milkshake for oil. did you do a compression test before tearing it down? Wonder if the pop is a vlave hanging up and backfiring

 

[Pmt133]

This one has leaked for a while... If it's possible try to check also the inside of the engine/pistons. That it was running...

Alleged fresh rebuild from the guy I bought it from. I had the pan off right before I installed it, from the bottom, everything looked brand new... one of the lobes has pitting... I'm kind of disgusted right now.

you must have milkshake for oil. did you do a compression test before tearing it down? Wonder if the pop is a vlave hanging up and backfiring

Oil was clean both times I changed it... Ran maybe 9 hours before yesterdays outing which was the first after the change.... both look clean. Compression test when this started happening was 160 across the board.

 

[Pmt133]

Little bit of the cam lobe carnage.

 

[Lou C]

Is that all from the intake gaskets? Mine blew the head gaskets 2 years after an overheat but it didn't look that bad, even though it was in salt water. Judging by the rust in the intake ports it is possible an intake valve could be hanging open. When I pulled out all the lifters the cam looked like new. One thing I like better about the pre-Vortec engines, the intake gaskets rarely if ever leak.

 

[Pmt133]

Is that all from the intake gaskets? Mine blew the head gaskets 2 years after an overheat but it didn't look that bad, even though it was in salt water. Judging by the rust in the intake ports it is possible an intake valve could be hanging open. When I pulled out all the lifters the cam looked like new. One thing I like better about the pre-Vortec engines, the intake gaskets rarely if ever leak.View attachment 384963

I have no idea. It was a "fresh rebuilt, low hour and ready to go" engine I purchased early may. Obviously, based on the fact I've had all of 5 hours put on the thing, I don't believe that to be entirely correct information. I can say I know of someone who won't be getting my business again... but at this point it's a pull and rebuild as that metal from the cam went through everything and the lifters are all going to be junk. I can't tell where it's coming from... but I suspect it's more than just an intake gasket leak.

I can't comment on the V6 but I can't say in my experience in vehicles that the 350 gaskets were better one way or the other... both were well over 200k before developing leaks be it 12 or 8 bolt.