flashgordon

interesting,,,,,so was block the issue????
crossover in the intake manifold???
wrong head gasket???

distantthunder

Head gasket is correct, saga still continues...

distantthunder

Alrighty, so here we go....

I was able to modify the existing cooling system path to remove the bypasses. I completed this modification since I removed the mufflers from the tails and no back pressure in the water system bypass could be had, the water would come in and mainly flow through the bypass (path of least resistance).

The modified stock water system now routes: raw water pump > coolers > OEM circulating pump (with bypass to intake) > flow through block > thermostat housing with stat removed > exhaust manifolds > tails.

This change in water system routing netted even temperatures throughout the entire block and operating temperatures at a maximum of 140F while underway which is way more acceptable that the previous temperatures of 160-170. Since I removed the mufflers on the tails, it changed the water pressure in the system not allowing for even cooling in the system, now the water system is sufficient and effective.

However, this did not solve my issue. I went ahead and back flushed the entire engine and all the components when completing the modifications to the cooling system and also installed brand new circulating pumps. I felt that getting the water temperature in the proper range would allow me to further deduce the problem.

With the cooling system correct, we ran the boat with one size up on jetting (and timing turned down to 10* initial) at 3600 - 3800 RPM max for a period of 30 mins and completed a plug check every 10 mins. The odd cylinder plugs show little to no color. Prior to completing the 4th plug check at the 40 min mark, I opened up the boat briefly (30 Seconds) but noticed the port engine wouldn't spin up past 4400 RPM so I backed it down and felt it go down on a cylinder. Come to find out, number 7 cylinder had melted the electrode and the spark plug boot. The other plugs remained normal with little to no color change from new.

While at the restaurant, I went ahead and replaced number 7 plug and cable then upsized the jets two sizes all the way around. After we ate lunch, we got back underway. The boat fired up, but was hard to idle (assuming due to the oversized jets) and we head up the river for about 12 mins. Once we arrived at a small beach area, I checked the plugs once more. All of the odd cylinders still showed little to no color change. We departed the small beach and headed back to the dock. About 15 mins into the ride, a loud pop was head and the port engine blew out number 6 spark plug from the base (porcelain blew out). Upon shutdown, it was dieseling for a good 10 seconds. I located all of the pieces, reinstalled a new plug and wire, fired the engine back up and continued the slow ride to the dock.

In a fit of extreme frustration, on the last small straight section in the river before the docks, I said screw it and pinned the throttles. To my surprise, both engines spooled up and pulled hard to 5100 RPM and put the boat in the expected low 60's MPH (I believe I saw 63.1 before running out of room). This was the first time the engines ran as designed this season. They sounded healthy and had more power than I can recall. When we approached the no wake zone, we came off plane and my buddy looked at me and said "did they just actually run?" and I responded with "yes i think they did". To ensure this wasn't a mirage, we turned the boat back around and headed to the straight away. The boat did two passes back to back pulling hard through WOT, hitting the expected speeds and RPM ranges that they should. We got back to the no wake zone completely puzzled. At this point, the boat is now down to less than 1/4 tank of 2020 fuel which was my goal for the day to run it close to empty.

Now, I am not oblivious to the point of thinking the boat had fixed itself, but for a brief period it showed that it has the potential to run correctly. At this point, I am completely puzzled. In one last ditch effort before pulling the engines out for the season and getting a head start on next year, I plan to try the following:

1. R & R Intake Manifold with OEM gaskets in lieu of the Fel Pro set I have on there.
2. Replace ARP bolt set with OEM bolt set since I had some intake manifold bolt fitment issues
3. Fill up the tank with fresh fuel
4. Install new plugs

Some believe the ignition module may be causing my issue but I am not sure how I would check that. Many have recommended replacing for a different style but I have never had an issue with the TBIV system in the past.

I am at a loss here, I am not doing anything crazy to the engine to get more speed, just trying to repair it to OEM specifications. To recap, all I did in the offseason was replace the heads with a fresh set and remove the mufflers from the TRS tail pipes - these engines were running last year!

Now, here I am with two rebuilt carbs, two fuel pumps, two circulating pumps, head work twice, three sets of plug wires, five sets of spark plugs, two sets of intake gaskets, three sets of head gaskets, two sets of exhaust gaskets, and all kinds of new jet kits. What am I missing here??? Unfortunately, there is no one in my area that has performance boat experience to assist here, so I rely heavily on the power boating community here with ideas. I am really thinking something is wrong with the intake manifold fitment which would throw off the signal in the carb.

Someone please help before I end up trashing this entire boat in defeat....

BillK

As much as I hate to suggest this..... swap it with the other engine and see if the problem follows it ? See if somebody has a spare you can borrow and try ?

distantthunder

Thanks Bill, I reached out to Sick Stinger this afternoon and he is going to send me down one.

SB

iTrader: (1)

Bigger jets won't change your idlle. Something else caused that. Gasket crap from taking carb apart got in something ?

compedgemarine

stupid question but are the arp bolts in the intake? are you sure they are not bottoming out and not pulling the intake down tight causing a vacuum leak? have had that happen before on one we chased a lean condition on.

distantthunder

It may be possible, I will have to look into that a bit more. I am switching the ARP bolts back to OEM this week.

distantthunder

I am not sure, everything was cleaned by hand after the ultrasonic cleaner. I did notice a difference In the base gaskets that were used vs what came off the engine so I reordered the appropriate ones last night. I also ordered the factory Mercruiser intake gaskets.

distantthunder

Finally got her straightened out!

So, after many weeks of frustration, I was able to successfully run for a whole weekend without losing any spark plugs. The plugs are finally showing color and engine had all the power that it used to have.

I believe my issues were derived from the intake manifold gaskets, ARP bolt kit, and the carb base gaskets causing fueling issues. I was using Fel-Pro Marine gaskets and standard Holley base gaskets for the top end rebuild on the port engine. I went ahead and ordered the Mercruiser gaskets using the Part Numbers in the HP420 manual and when they arrived, there was a noticeable difference in the thickness, material, and fitment on these engines. The carb base gaskets were my fault for overlooking but the other components were ones that I have successfully used on other 454 engines in the past but for some reason couldn't come together correctly on these HP420's. Mercruiser used three types of carb base gaskets on the intake assembly, a spread bore - a square bore - and a 4 hole, in which I was previously using a square bore on all three levels. The intake gaskets were much thicker than the Felpro and were cut very closely to the proper fitment to the heads and intake. I also removed the ARP bolt kit from the intake due to fitment conflicts and replaced with the factory bolts.

When I reassembled the top end, everything fit up a lot cleaner and the engine fired without hesitation once the timing was set. There was also a stronger signal through the carb with smooth and consistent revving.

The final "phase" of repairs were:
1. Mercruiser Intake Gasket Set
2. Mercruiser Carb base Gasket Set
3. Factory intake Bolts
4. Replaced the TBIV V8HP module with a used unit for good measure

The boat ran great throughout the entire day and I couldn't be happier. This was the most troublesome repair I have done in my life of turning wrenches and I learned that the 420 engines seem to be a little different than most I have dealt with in the past. I am sure there are some that may question the gaskets causing the issues but I am confident it played a major role in the problems. Unfortunately, after running the entire day issue free, the Halon bottle in the engine bay exploded and the boat took some damage which I now have to fix. It never ends.....