exaust reversion problem solved

[Bt Doctur]

from 9/8/17
Not sure if this was posted last year but a customers 31 Formula W/ 6.2`s that was having trouble .
,

Hello boaters, a new member with a old problem. Trying to find the answer to this problem for the owner.Boat had BB motors are they were removed for 6.2 SB W/ 3 inch riser extentions. When first serviced for a bad starter we found the manifolds heavily rusted .Motor is very poor condition and it was pulled for a rebuild at the end of 2015. For 2016 no reported problems and was winterized. Fall of 2016 port motor pulled for rebuild, same conditions of the manifold. Heavily rusted but no water in the oil.

Fast forward to spring of 2017. New port motor installed and tested on land 2X. stbd motor started and it had some stuck valves.

??
Removed heads and they were just ugly. No issues found in the heads and motor was new so new heads installed.Boat put in service.
2 hours later port motor takes a dump. 3 valve hung wide open

?

Shop keeps saying water ingestion but oil remains clean so we then suspect exaust reversion. Cannot raise the risers or use dry headers,
cannot lower the exaust or swim platform so we did the next best thing , extended the riser outlet.Not knowing how long they should be I used 12 inch tubes hammered in 2 inches leaving 10 inches exposed.

The manifold and riser pics shown only have about 2 hours run time and were loaded with what looked like rocksalt debris. So being a test engine were guessing on the length. The tubes are 2 inch OD SS thin wall 12 inches long .Our "Think Tank" says it should work to prevent the reversion .If this is true I should find nothing but carbon in the manifolds and risers when I remove one for inspection at the end of this season.

Stay tuned for a update and pictures soon

An Update 4 hours run time at power

Burned a small hole thru the exaust hose but it was expected because I`m in uncharted waters with the length of the tubes. Remove the risers and shortened then 2 inches and repaired the hole for now. You could see where the tube was pushing down on the hose because it was shiny clean.

The good news is there was no sign of any moisture and the manifolds, the extention or the risers .There was a nice black coat of carbon deposits and no white salt residue.

Finally some sucess. Pics at the end of this season​

 

[Bt Doctur]

9/11/17
the motors were rebuilt with the same cam , nothing exotic. No history from the PO about the running of the BB`s, met the customer with the 6.2 already installed. Reversion was already present given the condition of the manifolds and risers, heavily rusted but no water in the oil or combustion chambers. Just heavily rusted exaust ports and exaust valves.

The salt in the pics was after a run time of approx 1.5 hours. on the new motor with brand new heads. It seems it`s involved with the "Silent Thunder" swim platform exaust system using straight thru hull exausts into the swim platform.

Inspection of the manifolds and risers on the port motor that burned a hole thru the wet exaust showed nothing buy nice carbon black, no salt crystals.
So, I`m on the right track, we just dont know how long the tube must be.It is a Formula with Silent Thunder swim platform exaust.
the height is 15 inches from the water level to the top of the riser, extention blocks are 3 inch, its the highest it can go due to the engine hatch. Using the extention tube was our only other option and it seems to have cured it.​

 

[Bt Doctur]

As promised the engines were inspected and nothing but nice black carbon deposits, port motor has 10 inch tubes and stbd motor still has the 12 inch tubes. Port motors tubes will be getting a 10 or 15 deg kick up and weld to center then in the 4 inch exaust tube. This is a pre shortened tube.you can see the salt deposits only after 1 hours run time.Once the tubes were installed the salt deposits were eliminated and never returned.

 

[Bt Doctur]

Post 3 is a pic from last year showing the port motor covered. This spring we noticed a miss an ,intermittent miss .Compression was 175 across except for #6. It had 90 psi. So off with the heads that had been redone last year. These are the original heads that suffered thru the ingestion issue since the customer bought the boat.Found a slight rust stain coming from the intake valve into the cylinder , not wet moisture, just a stain.I suspect that eroded the exaust valve because during the leakdown air was heard coming out the exaust tube.So now we have 2 fresh 6.2`s with new heads on both motors.
FINALLY

 

[Rick Stephens]

Pretty good way to spend ones excess money, swapping motors and figuring out the custom exhaust. Lots of art goes into fine tuning. Nice results though.

 

[Bt Doctur]

When I met this customer the reversion was already at work, a motor dies and when I started taking it apart we found rusted manifolds and found a manifold that was leaking, once the heads were off I could move the pistons side to side. Motor pulled for a rebuild and heads recond.Thought is was a good idea to replace the other motors iron and that was just as bad internally as the other motor, new manifolds and risers . ran it 2016. winterized and stored, Spring `17 stbd motor has some stuck valves, exaust ports in head and manifold rusted. Had head cleaned and checked , nothing found so suspecting a bad head we replaced stbd heads with new.
Ran 2017, customer reported miss. power loss toward end of season I winterized the motors but the port felt a bit funny.
Spring of 2018 port motor had a bad miss/dead cylinder sound. Comp check revealed a very low #6 , pulled heads and found a rust stain from the intake valve and I suspect the owner felt this happen toward the end of 2017 and the miss ws the exaust valve or valve seat going out. During the leakdown air was heard from the exaust tube indicating a leaking exaust valve.

 

[Lou C]

I recall reading an article that came out a while ago about this very issue. It turns out that some time back, Merc cast in a hook of sorts in the inner elbow water passage casting that would keep water from climbing back up that area to get into the exhaust passage. Then, when the parts were redesigned, it seems that this was left off. The article had pix of elbows cut in half and you could see it. The author called it a vacuum break. Kind of what Volvo did with the 4.3s that had this problem. They designed a 90* elbow adaptor that adapted the 4" hose coming off the elbow to the 3.5" Y pipe hose. The angle of this part was much steeper than those used on the V8 models. Then they added check valves in the elbow casting to prevent a vacuum from developing in the gas passage by allowing fresh air in, but no exhaust out.

I wonder if contemporary exhaust elbows are being cast with this hook or vacuum break, or not!

 

[Lou C]

http://www.michel-christen.com/ExhaustElbows.pdf

check it out!

 

[Bt Doctur]

I have that link , Dont know about the new dry joint ones but since the tubes were installed there have been no salt deposits in the manifolds or risers. There is some info that the swim platform exaust has something to do with it also