Would a goofy flapper on a '99 3.0 cause tuning issues due to exhaust backpressure?

[insttech1]

I pulled my 3.0 today to replace a wobbling harmonic balancer.
(Guess those do go bad from time to time....)

Anyway, wanted to check my exhaust to see if I had the flapper or not. Heard they went away in '99.
I do have one, but it's 15% melted, still barely rotates on pivot pin, but is "sticky" and never stays in one spot.
I'm assuming these originally had some manner of spring, or was it just exhaust pressure that opened them up?

I've had problems setting idle and stumbling out of the hole w/ this boat ever since I bought it 3 years ago.
Done just about everything possible:
New 4-blade prop (helped getting out of the hole a LOT!!) WOT hits right at 4600 RPM with it.
Fuel filters
Complete carb rebuild w/ correct marine valve, 30cc pump, etc...
Correct setting of pump operating adjustment and checked other carb/choke settings--all OK.
Vacuum tuning of idle adjustment screws (about 16-17in hg; manual says 14 to 19 is "normal")
Compression test (one cylinder 18psi lower, but within volvo spec')
New plugs every spring
New rotor
New cap
Set timing (correctly, w/ jumper)
etc...etc....etc...
I'm currently ordering a new pump lever operating arm and cam just in case they're worn.
Adjusted throttle controls on boat and set some preload per Don S. instructions.

It seems like every other time I go on the water, I need to adjust the idle up or down 100 to 150 RPM to keep a consistent, warmed idle at 700RPM, in gear. Any lower sounds horrible, and stumbles horribly on throttle-up.

This is with or without the throttle from the boat disconnected.
I even called Holley two or three times. They told me I have a vacuum leak.
I sprayed carb cleaner all around the carb and lines when warmed up; RPM never moved at all. No vacuum leak.
They also told me if I back the idle screw out completely, the boat should shut off. It won't; it will only back the idle RPM down to about 600RPM, but it will still run with no primary idle screw contact.

So I will be pulling the remnants of the flapper out; I will leave the pin in to minimize the possibility of water leaks.
I'm just wondering if the flapper kept going to different positions and screwing up exhaust backpressure, and subsequently, making idle tuning a pain....

I'm also pulling the carb to try and find out why turning the main idle screw all the way out won't kill the engine.

And no, I've not had any heating issues; engine always warms up to same spot on gauge (which is horribly inaccurate, but I would guess due to the graduation marks that it always settles right around 150 degrees on the water, or on muffs).

Any thoughts???

 

[Maclin]

Re: Would a goofy flapper on a '99 3.0 cause tuning issues due to exhaust backpressur

Regarding Holley's advice, if you told them this...:

"I'm also pulling the carb to try and find out why turning the main idle screw all the way out won't kill the engine."

... then that is why they said vacuum leak as that is the classic symptom.


Regarding the flapper, I doubt it would make that much difference it idle unless it was dead closed and could stay that way. Usually flappers are shaped to have more area on one side and heavier on the other side, the pin being the dividing point. When closed they are at an angle with heavier side up and longer/lighter side down. Exhaust pressure works the lighter but more area side against the weighted side and opens the flapper. With no exhaust pressure the flapper should reurn to closed aided by gravity, but not tightly sealed. If water tries to come back up the exhaust that pressure will work the flapper to close even tighter blocking most of the backflowing water.

If the flapper allows any flow at all now even in the closed position then it is probably not the cause of the idle variations. Now, pieces if it (or previous flapper fail) can be stuck somewhere in the exhaust downstream of the flapper, so maybe a combination of sticky flapper and actual permanent blockage from a previous flapper breakup is at play.

 

[Maclin]

Re: Would a goofy flapper on a '99 3.0 cause tuning issues due to exhaust backpressur

BTW I would remove that pin and plug the holes with suitable JBWeld or other product. If it does come out and heads down the exhaust chute it could lodge and eventually cause corrosion action and ruin the exhaust pipe.

 

[insttech1]

Re: Would a goofy flapper on a '99 3.0 cause tuning issues due to exhaust backpressur

OK thanks much for the advice on total removal of the pin.

As far as the flapper position:
I think it was usually stuck in a 60% closed position.
The weighted (upper end) had significant melting. (About 1/2", of what would probably be the heaviest, denser material.)

I could move it by hand, but it took rather significant effort.
And once I did move it, that's where it would stay.
It did not move "by gravity" with no exhaust pressure.

As far as the vacuum leak:
I sprayed everything from the outside, and have performed on on-carb vacuum check from the base port; that was within spec'.

I "suspect" that the previous owner did something to the throttle body/carb base during a previous rebuild.
I say that because of a few things:
They "bent" the pump operating lever to tune the carb, and it does not sit squarely on the cam.
If I "bend" it back to sit squarely on the cam, it will rub against the bracket the cam is bolted to.
The cam and lever had been completely covered in thick, black volvo paint, which was peeling; I took all of it off.
I don't think my throttle body is closing completely w/ the idle screw backed all the way out, but I can't see why not--yet.
The "stop" for the throttle body closed on the other side of the carb has been bent so far as to not touch anything, at any position.

For what it's worth, I pulled the choke closed by hand....that did kill the engine.

So I'm suspecting I need to remove/inspect everything on the base plate of the carb, and see what is up.
Obviously there is some spacing issue, if the pump operating lever hits against a bracket.
On Holley's pump/cam video, it has about 3/32" space for free travel as it passes around/over the cam. Mine doesn't.
Maybe the PO took it completely apart and forgot a washer or spacer....??

Anyway, just throwing out some ideas, as the carb base and lines aren't leaking.....from what I can tell.
I had also previously cut all rubber tubes/hose and re-seated or clamped them because they were finger-loose.

And one last little hint:
If I disconnect the boat throttle (on the water), have everything to running temp, and idle at 700, then I can
manually throttle the engine (quickly), let it spring back, and it will idle back to 700.
However, I can also put medium finger pressure on the throttle bracket, pulling it towards the closed position, and the RPM will
drop by 100 to 150. I don't think that amount of "play" should be there, and it should come to a hard stop.

Holley says these carbs are designed to have another spring-assist for closing the throttle, like they do on a car.
I had a hard time explaining to the Holley guy that this was a boat carb, and they don't work that way.....

Thanks again for your time!

 

[insttech1]

Re: Would a goofy flapper on a '99 3.0 cause tuning issues due to exhaust backpressur

Oh and my drive is off now too--I will check for evidence of the melted pieces and/or previous flappers--thanks!!

 

[skydiveD30571]

Re: Would a goofy flapper on a '99 3.0 cause tuning issues due to exhaust backpressur

In case you haven't seen it, here is the Volvo service bulletin in regards to the exhaust flappers and how to remove them.

http://www.marinepartsexpress.com/jpg/Flapper%20Parts%20Bull.jpg

 

[insttech1]

Re: Would a goofy flapper on a '99 3.0 cause tuning issues due to exhaust backpressur

Yup got the bulletin--thanks!
Just had never taken the engine out/exhaust off until this year. (3rd year of ownership).
Previously I had only taken the drive off and done regular winterizing.

 

[Walt T]

Re: Would a goofy flapper on a '99 3.0 cause tuning issues due to exhaust backpressur

=======================================
Holley says these carbs are designed to have another spring-assist for closing the throttle, like they do on a car.
I had a hard time explaining to the Holley guy that this was a boat carb, and they don't work that way
========================================

Yes they do work that way. They are automotive carbs after all modified for marine use. That is probably why your idle isn't consistent. No spring or the one one the throttle shaft is weak. Just install a small return spring. No harm done. If you still have a bog accelerating after the rebuild you need another rebuilder. Preferably an old guy who used to do them in his sleep and mumbles the adjustment specifications for every carburetor manufactured in the last 50 years while he putters around looking for a wrench he misplaced in the 70's which somebody tossed in a different toolbox drawer as a joke.

 

[insttech1]

Re: Would a goofy flapper on a '99 3.0 cause tuning issues due to exhaust backpressur

Sounds like a plan, Walt!!!

I ordered a new operating lever and cam last night, and they're on the way.
I might also install a "window kit" from Holley in the float bowl so I can monitor the float/fuel level. (And they're only $6...)
And I'll take the shaft/spring apart and get any remaining black paint out of there while I'm at it. It was super thick....

With all my tweaking, I've been able to get 90% of the bogging to go away; there's only one spot in the throttle that will start to bog down....doesn't matter if I'm creeping up on the throttle or mashing it down. Ironically, when I am coming off the throttle slowly, the same spot is where RPM suddenly goes up 200 RPM, and then continues down the RPM curve. That's why I suspect the lever arm/cam are not going thru a smooth curve (plus it was previously bent....).

My 30cc pump and internal spring are new, and the adjustment to .015" clearance at WOT has been checked multiple times.

And I hope not to replace myself as the rebuilder....however my wife might be happy:facepalm:

 

[Maclin]

Re: Would a goofy flapper on a '99 3.0 cause tuning issues due to exhaust backpressur

My throttle setup has return springs, I am sure they are factory because they are VP red and just have the appearance of always having been there. I have kept from posting this fact because the consensus in most topics about it was that no return springs should be there.

 

[insttech1]

Re: Would a goofy flapper on a '99 3.0 cause tuning issues due to exhaust backpressur

Thanks for the info.
If my other fixes (operating lever, cam, inspection/re-assembly, etc....) don't prove anything out, then I'll fab something up for a return spring.

 

[insttech1]

Re: Would a goofy flapper on a '99 3.0 cause tuning issues due to exhaust backpressur

Pulled the carb tonight....first thing I noticed is that I can pull the main shaft thru the base of the carb back and forth an 1/8", and had a few drops of gas come out of the shaft hole where it meets the base as well....don't think that should be that way....but I now have lots of time to tear it apart.