Intake & Exhast Leaks - Next Steps?

[tcgravy]

Recently purchased boat in poorly running condition (would start and briefly rev up, but would not idle). Engine is 454 magnum w/ Bravo drive, 1989 model yr. Transom exhaust.

Have removed and thoroughly cleaned carb (and checked that it has all correct jets)...its a q-jet by the way; adjusted idle and rich/lean mixture screws...now idles great at 650-750 rpms.

It seems to have a major intake leak:
- very loud "hissing noise" at all engine speeds
- vacuum gauge is steady but low (10-11)
- hesitates or backfires under load/acceleration at around 1800-2500 rpms



Anyways, my first step was to replace carb to intake gasket (I had re-used old one, assumed it was bad from re-using multiple times so replaced)....that did NOT fix hissing noise / vacuum readings. Next step was to spray carb cleaner around intake....did NOT change engine rpm at all despite spraying generously around carb and intake to head spots. Cannot feel intake leak anywhere.

QUESTION - should I just assume it is an intake leak, replace gasket for intake to cylinder head, and re-diagnose? Or are there other "tests" I should do before doing this. Looking for input here......

It also has an exhaust leak on the starboard side exhaust manifold (I can visibly see this). Planning to replace head to exhuast manifold gasket and see if problem persists.

Comments and answers please...thanks...

 

[tcgravy]

Re: Intake & Exhast Leaks - Next Steps?

UPDATE:

Have re-torqued all intake manifold bolts. Found that most were only tightened to around 20-25 ft/lbs (Specs say torque should be 35 ft/lbs).

Would this small change be enough to fix intake leak?

I would test myself, but outdrive is off for another reason (gimbal bearing is shot) so I can't run the motor.

 

[Bondo]

Re: Intake & Exhast Leaks - Next Steps?

I would test myself, but outdrive is off for another reason (gimbal bearing is shot) so I can't run the motor.

Ayuh,... Sure you can, just by-pass the drive,+ feed water directly to the motor...

I doubt you Fixed it, but anything is Possible...

 

[tcgravy]

Re: Intake & Exhast Leaks - Next Steps?

UPDATE:

After some extensive searching around the carb to intake and intake to head mating surfaces, I found an intake leak where it attaches to the starboard side cylinder head.

Planning to replace intake gaskets - I'm assuming this simply involves slightly lifting the intake?? Does it require removing the distributor?? Just wondering how time consuming of a job this is....hoping to take boat out this weekend.....

 

[tcgravy]

Re: Intake & Exhast Leaks - Next Steps?

UPDATE:

Changed intake gaskets, but problem NOT fixed. Symptoms/issues/etc. are described below:

- Compression good in all cylinders (145-150)
- Engine idles fine at about 800 rpms, will idle lower but is inconsistent
- likes about 3 throttle pumps to start
- stumbles on acceleration (1800-2500 rpms)
- Timing is set to 8* btdc (tdc of piston #1 confirmed)
- Fuel pressure is good, ~5 psi (electric fuel pump)
- Carburetor was cleaned and rebuilt very recently
- fresh gas only being used

Vacuum reading at idle is consistently/steady around 11, although it will "hunt" or "surge" from 10-12.

If idle speed is increased to around 1100 rpms then vacuum will increase to ~16 inhg
If timing is increased to around 14* btdc then vacuum will also increase to ~16 inhg

Anybody have any input here?? I'm stumped.

 

[HT32BSX115]

Re: Intake & Exhast Leaks - Next Steps?

Howdy,

if you have a vacuum leak and you try to run the engine at high power settings, you could burn a valve or have a detonation event.

- Carburetor was cleaned and rebuilt very recently

By whom? Is it the original carburetor? Is it a "Marine" carburetor?

Is the automatic choke adjusted "out-of-the-picture"? (mine is disconnected permanently)


When you had the intake manifold off, what kind of shape was it in? Is it aluminum or cast iron? (original?)

What kind of gaskets did you use, did you seal them with some sort of sealant?


Regards,


Rick

 

[tcgravy]

Re: Intake & Exhast Leaks - Next Steps?

I rebuilt carburetor (soaked in carb cleaner for days, blew out passages w/ compressed air, confirmed all jets are stock for my motor, replaced all carb gaskets, accelerator pump, etc. w/ new components)

Carb is original marine yes, rochester quadrajet

Auto choke is hooked up, but once engine is warm it is definitely all the way open.

Intake manifold seemed to be fine - cleaned gasket surfaces and replaced w/ felpro gaskets placing rtv sealant around water passages. After installing I have extensively re-checked for leaks and found none.

 

[HT32BSX115]

Re: Intake & Exhast Leaks - Next Steps?

Sounds like you did everything the way most people would....


Other than a timing problem, with good compression, it still points to a vacuum leak and/or carburetor problem.

 

[tcgravy]

Re: Intake & Exhast Leaks - Next Steps?

Thanks for the input.

I agree that it points to a vacuum leak and/or carburetor problem, however, I don't know how/what/etc. to do anything except for whats already been done to try and remedy those issues.

Thats why I'm stumped.

Is there any way that something in the distributor/plug wires/spark plugs could cause the engine to appear that it is correctly timed, while in fact the timing is retarded? Again, timing is stock @ 8* btdc, just wondering if there is any way my timing light would show this but in all actuality the timing would be delayed/retarded?

 

[HT32BSX115]

Re: Intake & Exhast Leaks - Next Steps?

It's possible that your centrifugal advance is not advancing. That would absolutely cause power problems.

A timing light that has an graduated adjustment on it will display timing at different RPM.......especially at an RPM where the distributor should be fully advanced.

 

[tcgravy]

Re: Intake & Exhast Leaks - Next Steps?

Thunderbolt IV, electronic ignition and electronic advance......have not yet checked this, but even if was bad it would not explain low vacuum at idle (where I know timing is set to 8* btdc).


Other ideas????????????????????

 

[tcgravy]

Re: Intake & Exhast Leaks - Next Steps?

Hissing noise has reduced dramatically.....

now I can hear the air intake through the carb / flame arrestor, but it is much quieter and sounds as though its likely normal

 

[tcgravy]

Re: Intake & Exhast Leaks - Next Steps?

UPDATE:

Took boat out on lake today and found some new symptoms/problems to add to the mix:

- When running at 1300 rpms in gear, engine will randomly backfire and/or stall
- Boat will not exceed 4000 rpms - when 4000 is reached it begins to stutter slightly (max rpms should be 4600-5000, was told by previous owner that this setup revved to 4800 rpm)
- if timing is advanced to 12* btdc engine starts pinging at about 3500 rpms
- all other symptoms previously noted remain

 

[Icefire55]

Re: Intake & Exhast Leaks - Next Steps?

Probably a carb problem for the backfire/stall, had similar problem with mine that was rebuilted

For the vaccum leak, look if it's the right gasket, I used one in the rebuild kit and it made a major vaccum leak (was not made for my intake).

Also had a problem with plugs/wire as I changed both and not I can idle a 650 with no problem, would swing too much to idle unless at 800+

 

[Bondo]

Re: Intake & Exhast Leaks - Next Steps?

Ayuh,... Do a Compression Test,+ make SURE the plug wires are in the Right order...

 

[tcgravy]

Re: Intake & Exhast Leaks - Next Steps?

Compression Test - good in all 8 (145-150 psi in every cylinder)

Plug wire check - all routed to the correct cylinder

 

[Bt Doctur]

Re: Intake & Exhast Leaks - Next Steps?

I`m guessing the wrong carb base gasket,

 

[tcgravy]

Re: Intake & Exhast Leaks - Next Steps?

Have done some further testing and think this is likely a fuel system issue.

I performed the "HOW TO: Test your boats fuel delivery system" test on boat, and found a problem there......Installed vacuum gauge in between fuel/water separator and fuel pump. On muffs at around 1000 rpm, the vacuum gauge reads about 4.5 In.Hg. This is TOO HIGH (anything higher than 4 indicates a restriction in the fuel system BEFORE the fuel pump).

So, I've tried to test the common issues:
- checked fuel tank vent, clear as a bell
- anti-siphon valve is clean as a whistle, seems to work just fine
- fuel tank pick-up mesh filter is clean as a whistle too
- I bypassed the fuel/water separator and problem persisted, same symptoms, so this can be ruled out as well

The only other thing to check (I think) is the actual FUEL LINE from the tank to the fuel/water separator (and eventually the fuel pump). I can blow through this, but am not really sure how to test it for restrictions. Any ideas here? Any way to test this hose? or should I just replace?

Anything else it could be?

 

[airdvr1227]

Re: Intake & Exhast Leaks - Next Steps?

Old trick. Get some carb cleaner and, while the engine is running spray it around the base of the carb...the intake base...etc. If there is a vacuum leak the engine will idle up significantly at the leak site.