Intermittent ignition failure at above idle RPM

[jakwi]

Hi all, I'm hoping that someone can point me in the right direction.

I have a mercruiser repower 357 alpha 4v. Which is just a standard carbed 350 with the thunderbolt iv ignition.

Before today it had been running great. On the way back to the ramp today I was cruising along on plane at about 3500 rpm. All of a sudden it died out of no where. I restarted a couple of time and it would idle fine but as soon as I gave it any power it would cut out.

I opened the engine cover and the bilge was pretty full of water, maybe up to the bottom of the oil pan and spraying out of the starboard side exhaust manifold feed hose. Apparently the hose clamp broke. I stole a clamp from somewhere else and fixed that issue. The bilge pump did its job and I thought we would be back in business.

However I found that if I ran anything over 1200rpm it would eventually cut out. 1400 rpm it took maybe 7 or 8 min. 3000 rpm maybe a min. 4800 rpm 10 or 15 seconds.

I tried opening the gas cap, possible vent issue, no go, pulled the spark arrestor in case it was plugged no go, chegood bat voltage, 13.9, 14.5 at 1400 rpm. No issue there.

So we idled back to the ramp, which sucked, but ok.

Back at the house I checked for fuel. Ran it til it cut out and then after it died "revved" the throttle, plenty of gas visible in the carb, so fuel isn't the issue.

Pulled the distributor cleaned the contacts. All good there.

New plugs, properly gapped were installed about a month ago. No more than 10 running hours ago.

I followed the troubleshooting guide achris, posted a while back.

Everything looks good, because the issue always happens at high rpm. It always restarts and idles perfectly.

So in my mind I'm down to cool or ignition sensor. The coil is a Sierra pn I replaced last year. Or possibly the module, I hope not.

Is there anyway to know whether it's the coil or the sensor? Which is the most likely failure? I hate to throw parts at it if I'm not sure.

Bear in mind the only issue was a lot of spraying water on the starboard side near the spark plugs. Before that it was running great. The coil, distributor, etc never got wet at all.

thanks for the input guys!

 

[achris]

Salt water? You need to hose off the engine, completely, dry it out and try again. Salt's nasty stuff for electrickery. I know, I put 4,500v over a mile down.....

And make sure you hose off the spark plugs and leads especially well. It sounds very much like a loss of spark at the plugs (or cap) when the demand (and hence the voltage required to jump the gap) is highest...

Chris.......

 

[jakwi]

Actually fresh, St John's river and Lake George in Florida. The last two times I've been out have been fresh. Although normally I boat in salt water.

Just the same I will give it a fresh water rinse with the hose and then dry it, and try again before buying any parts.

Thanks for the guidance Chris

 

[Scott Danforth]

Its almost never the coil. And because you start and run, it cant be the coil

The St Johns is somewhat brackish

Clean all the connections in the boat, starting at the batter and starter.

Water and boat electrical do not mix

 

[jakwi]

Good Morning guys,

I went out to the boat this morning, and I pulled the plug wires and sprayed everything down with a vinegar solution, since acid dissolves salts and minerals I figured this would be a good start. then I rinsed with fresh water I dried the plug wires individually. I then put a little marine grease on the ceramic part of the plugs to seal out any potential water incursion and reconnected.(this has been my standard procedure, nothing more than a light film) Everything looked pretty dry this morning, but it is very humid in FL, but I made sure it was dry with paper towels before putting it back together.

Anyway I started it up. I let it warm up fully, temp and oil pressure look normal, and then I tried again. Essentially it immediately starts failing I pushed the throttle to 4000rpm, and held it steady at that point, but it immediately starts dropping. If I try to compensate with more throttle it drops quicker and will eventually die. If I push it that hard, where it dies, but then immediately back it off to idle, it will recover and idle normally.

https://youtu.be/_Zz7lryuVHc

Certainly the St. Johns could be a little brackish, but where I was at is way in the middle of the state. The water is not salty enough to taste salty. That being said though we normally are in the Halifax river, which is quite salty, and while I've never had this much water in the bilge, I always have a little. So I take your comments seriously and have tried to address them.

https://goo.gl/maps/pSSicQBQPnBWWL9ZA

At this point though I'm not sure where to go. I'm back to coil or Ignition sensor, unless you guys have another idea. My problem with testing those is that it's not a hard failure, so when I follow the troubleshooting guide everything looks good.
The guide I followed is the one posted by Chris, which is identical to my manual.

https://forums.iboats.com/forum/engi...n-systems-work

Thanks again for the input.

 

[achris]

.... Essentially it immediately starts failing I pushed the throttle to 4000rpm, and held it steady at that point, but it immediately starts dropping. If I try to compensate with more throttle it drops quicker and will eventually die. If I push it that hard, where it dies, but then immediately back it off to idle, it will recover and idle normally.....

That sounds like a carburetor problem.

Chris.......

 

[jakwi]

Really?, but why would spraying water in the bilge cause that. Or do you think it's just a completely separate issue?

I havent pulled the fuel filter yet to check for water in the gas.

 

[Scott Danforth]

Agreed, sounds like fuel system.

If it was ignition, it either runs or it doesnt.

 

[jakwi]

Alright, so I worked through a couple of more ideas I had about the ignition. I checked battery voltage to the coil while it is cutting out, but it was strong, and didn't vary at all. And I also noticed the Knock sensor is located right in the area where the spray of water was most intense, so I looked for water in the connector, and tried running without it, but it made no difference.

I know you guys said fuel, but these were the last couple of things I felt like I needed to work through to be sure about the ignition. The fuel idea is weird to me because I use this boat nearly every weekend, so it doesn't sit for long. Gummed up carbs are usually the result of sitting to much. I also use Non Ethanol fuel exclusively.

I pulled the fuel filter and dumped the gas into a glass jar, and found no water whatsoever. I put a new filter on, but it made no difference.

Anyway in order to confirm the fuel diagnosis I used a can of starter fluid. I removed the spark arrestor.

I increased the throttle until it started cutting out, and then I proceeded to spray starter fluid from about 1.5 ft away, so that I was essentially misting the carb with fluid. My thought was to increase the Fuel to Air ratio in the carb to see if I could get it to smooth out.

At first It dropped about 1000 rpm, but it wasn't cutting out any more, I adjusted the amount of spray coming out of the starter fluid can to be less, leaning the mixture, and it increased to the RPM I had set, 4000, and stabilized, ran smoothly. If I further reduced the flow it would start cutting out again. I did this twice just to be sure.

To me that says that confirms your fuel failure idea, and given the fact that it runs great at lower RPMs probably means that the issue is in the secondaries, or with fuel flow to the bowl.

At one point I ran it til it died and then looked down the throat of the carb while accelerating it and I can see the accelerator pump spray, so that makes me think it has enough fuel in the bowl, but to be honest I'm starting to get out of my area of comfort. I have rebuilt small single barrel carbs for motorcycles and lawnmowers, but never a four barrel.

So my question is, where do I go from here? Are there some simple things I can do without performing a full rebuild of the carb? Can I get to the secondaries to clean them without a full dissassembly?

If I need a rebuild is this something I can tackle? I have found the OEM overhaul kit for about $130 which seems like a lot. Or I have found an aftermarket rebuild kit for about $40
Or there is a place in Jacksonville that will sell me a reman carb for $250, National Carburetor.

I have an Edelbrock/Weber style marine carb that came with the engine, so it is about 8 years old now. The PO had about 50 hours on the motor and carb over the last 7 years. I've probably put another 50 hours on it in the last year.

When the motor died and I found the water spraying from the Exhaust manifold feed line I assumed they were connected, but it seems like these are unrelated issues.

Why would a carb suddenly fail like that will running along perfectly at 4000 rpm?

Thanks for the help guys.

 

[achris]

Running, even at 4,000rpm out of gear, no way the secondaries were open...

Those Webers are about the easiest carb in the world to rebuild. Just follow the service manual and you shouldn't have a problem... Pay attention to cleanliness, and make sure you blow all the drillings and passages out with dry compressed air (from a compressor, not a can of fart juice :facepalm

Chris......

 

[Scott Danforth]

I use fishing line most of the time to chase carb passages.

 

[jakwi]

Chris, Scott,

Thanks again for the guidance. So if the secondaries weren't open then the issue is in the primaries?

Is the standard Edelbrock overhaul kit good enough, or is there something special about the Mercruiser version of it. $40 vs $130

Sounds good on the Air compressor, and cleanliness, and fishing line. Like I said I've worked on smaller carbs before with no issues.

As an aside, I have to say running at 4k rpm on the trailer is pretty nerve wracking, I have a pretty high pressure hose feed, and I was diligent about watching my engine temp, but just the same, it isn't any fun. I really tried to minimize that as much as possible. It starts cutting out at a much lower RPM when in gear and in the water.

 

[achris]

...Is the standard Edelbrock overhaul kit good enough, or is there something special about the Mercruiser version of it. $40 vs $130....

Should be the same.

BTW, running at 4,000, even with high water flow. Not something you should be doing. Actually, even in the water 4,000 out of gear isn't wise. The MPI engines set a code is you go over 3,000 out of gear...

Chris...

 

[jakwi]

Good to know, it definitely didn't feel good, but I didn't have any other way to test. Would it be better in gear? I'm guessing it wouldn't matter because out of the water there isn't any load on the motor. It sort of makes me sick just thinking about it. I'm so petrified of making it worse, and it won't get fixed unless I do something. Unfortunately I am the warranty.

I'm going to be out of town for a couple of weeks, but I'll have the rebuild kit waiting for when I get back. Hopefully this fixes it.

 

[Scott Danforth]

mercruiser buys the parts from edelbrock and then put in mercruiser box, and then mark it up

test in the water under load

 

[jakwi]

Ok, thanks Scott

 

[jakwi]

Hey guys, I finally have everything together and am rebuilding my edelbrock 750 carb. 1910.

The manual calls for a 7/16 inch float adjustment, as it came out it is set at 5/16. Should I change it to 7/16? This will just result in a fuller bow . And it has never flooded, so I'm leaning towards the 5/16 setting. Both sides are the same.

Also the original floats look better than the new ones I bought. Should I stick with the old ones? Or do the fatigue and need replacement?

I'm really hoping this fixes it, but to be honest I haven't found any smoking gun. Basically my carb looks clean.

Thanks

 

[jakwi]

Things I have done today that haven't worked


I rebuild the carb. Disassembled, blew it out with carb cleaner, chased with monofilament then with compressed air, I didn't see anything obviously wrong or very dirty, but it is clean now.

It took about 20 seconds of cranking for it to fire from a dry carb, so that seems to indicate the pump is working fine.

As far as the floats, I used the old floats, they seemed fine. I split the difference and went with 6/16, but it was flooding out on the port primary side. So I disassembled and adjusted to 7/16, still flooding, same spot,

I pulled it apart again, I found a small piece of paint on the needle, at least I think that is what it was. Cleaned, readjusted to 7/16 using a drill bit, Reassembled and the flooding issue was gone.

as far as I can tell the pump is working well.

Unfortunately when I rev it to 4k it immediately starts cutting out, just like the video I sent. It only takes about 2 seconds to happen. Slightly longer than a throttle blip.The only improvement is that the choke works properly now, it didn't close before, but that didn't matter, because it always started fine.

At this point I'm out of ideas. I know you guys said it's not spark, but I decided to throw a coil at it. Besides west marine will let me return it, if I want, but no matter, there was no change. It isn't the coil.

I pulled all of the sparkplugs on the side where it got wet, just to see if one was cracked or something, but they all look great, they only have about 15 hours on them, so they are basically brand new.

I also pulled the tach wire just to make sure that didnt have anything to do with it, but no change

I'm at a loss. I can throw an ignition sensor at it I guess. But at this point I'm just guessing.

Summary

I have tried:

fuel filter
Carb rebuild with new needles and seats, gaskets etc
Coil
spark plug inspection
disconnecting tach wire from coil

​​​​​​​Thoughts?

 

[jakwi]

Just a couple of more notes from the carb rebuild for anyone that is interested.

The stock carb for the Mercruiser Report 357 Alpha 4v is an Edelbrock/Weber 1410 750 CFM carb
Primary Metering Jets - 392 - so .092"
Secondary Metering Jets - 398 - so .098"
Metering Rods - 6852 with a pink spring (7 "Hg)
the Accelerator pump jet housing has the number 40 stamped on it.
My Idle Air Screws were set 1 full turn out from seat
The float needles did not have the clips that attached to the floats.
The needle seats utilized screens, but the fuel inlet did not utilize the screen.

The odd thing about this is that it is setup more like a 650 CFM, infact these Jet sizes don't even fall in the rod/jet reference chart for the 750 cfm carb. In fact the primary jet is quite a bit smaller than the smaller than the smallest jet on the Rod/Jet reference chart, with the secondary jet only slightly larger, but the real tell is that the Pink spring is one strongest, requiring a ton of vacuum to open the secondaries.

This sort of makes sense given that if you look at the cfm calculator for a given engine size the 750 is to big for a 357ci engine. The only thing I can think is that they bought the carbs in some sort of bulk purchase and jetted them as needed.

All in all it's a pretty unique setup as far as I can tell. It so different from the Edelbrock recommended setups that I can't compare it to anything similar on the calibration Reference charts. That being said before this incident the motor was running great, and when I pulled the plugs yesterday they looked great, nice sandy color.

 

[jakwi]

If you guys still think it is fuel, I'm really open to ideas, but as of right now, the only other thing I can think to check on the fuel system is to figure out how to install a pressure gauge at the carb to see if I have the required 3 to 6 psi.

Chris, in my research I found something you posted about 4K being the magic point for the Thunderbolt ignition system. It is the point that the Acceleration Advance, and the MBT(Mean Base Timing) Advance stop, Between 4k and 5k the only controls left are the Knock Detection system and Over rev control.

I've already tried disconnecting the knock sensor, but maybe the Knock control module is falsely indicating knock at 4k, Maybe it is sending the signal to retard ignition when not needed..? Seems like I can't simply remove the module because it sends an 8 to 10 v signal to the ignition module when no knock is detected.