454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

[tcgravy]

I am having performance issues with a mercruiser 454 magnum (1989 engine, 365 hp)....Rochester quadrajet carb, outdrive is Bravo I, has transom exhaust. Boat for this engine is a 1989 Baja Sunsport 210 (21', about 2800 lbs dry, shallow vee hull, open bow). Per previous owner, engine should rev to 4800 rpms at WOT (spec is 4600-5000 for this motor).

The current running condition/problems for the engine are listed below. Engine conditions have persisted since I purchased the boat (approximately a year ago).
- on slow take off and slow throttle advance, the boat will take off fine and accelerate as you would expect
- boat idles great at about 600 rpms
- idle to WOT the engine will backfire, hesitate, and usually stall
- running at 3000 rpms, upon hard throttle advance (to WOT) the engine stutters and backfires, nearly stalling
- at WOT, the engine will sporadically stutter at about 4200-4400 rpms (acts like it is running out of fuel, can't exceed this rpm)

To date, I have considered both fuel and ignition related issues. The items changed/checked are listed below:
- New spark plugs, plug wires, distributor cap (also checked coil resistance, which is within spec)
- checked timing (right at 8 degrees BTDC where it should be)
- cleaned and rebuilt carburetor (fixed float level too, also checked all jets/rods/etc., which are all 100% stock for this engine)
- Completed fuel system test (posted on this site, tests fuel delivery before carb), found air bubbles in fuel lines due to overly high vacuum before fuel pump....replaced anti-siphon valve AND fuel line (this helped smooth out idle and raised WOT engine rpms, but obviously didn't fix all problems). All other components of this test checked out good.
- replaced fuel filter in carburetor
- replaced fuel/water separator
- compression test (checked out good, 145-150 psi in every cylinder)
- tested accelerator pump (full streams of fuel into both primary barrels of carburetor upon throttle advance)


At this point, I'm stumped and open to any/all suggestions regarding what needs to be tested next or ideas to fix the engine issues above. Please provide any and all input regarding possible issues or items that I've missed in my diagnosis process. Thanks in advance.....

 

[Gettinther]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

I am having performance issues with a mercruiser 454 magnum (1989 engine, 365 hp)....Rochester quadrajet carb, outdrive is Bravo I, has transom exhaust. Boat for this engine is a 1989 Baja Sunsport 210 (21', about 2800 lbs dry, shallow vee hull, open bow). Per previous owner, engine should rev to 4800 rpms at WOT (spec is 4600-5000 for this motor).

The current running condition/problems for the engine are listed below. Engine conditions have persisted since I purchased the boat (approximately a year ago).
- on slow take off and slow throttle advance, the boat will take off fine and accelerate as you would expect
- boat idles great at about 600 rpms
- idle to WOT the engine will backfire, hesitate, and usually stall
- running at 3000 rpms, upon hard throttle advance (to WOT) the engine stutters and backfires, nearly stalling
- at WOT, the engine will sporadically stutter at about 4200-4400 rpms (acts like it is running out of fuel, can't exceed this rpm)

To date, I have considered both fuel and ignition related issues. The items changed/checked are listed below:
- New spark plugs, plug wires, distributor cap (also checked coil resistance, which is within spec)
- checked timing (right at 8 degrees BTDC where it should be)
- cleaned and rebuilt carburetor (fixed float level too, also checked all jets/rods/etc., which are all 100% stock for this engine)
- Completed fuel system test (posted on this site, tests fuel delivery before carb), found air bubbles in fuel lines due to overly high vacuum before fuel pump....replaced anti-siphon valve AND fuel line (this helped smooth out idle and raised WOT engine rpms, but obviously didn't fix all problems). All other components of this test checked out good.
- replaced fuel filter in carburetor
- replaced fuel/water separator
- compression test (checked out good, 145-150 psi in every cylinder)
- tested accelerator pump (full streams of fuel into both primary barrels of carburetor upon throttle advance)


At this point, I'm stumped and open to any/all suggestions regarding what needs to be tested next or ideas to fix the engine issues above. Please provide any and all input regarding possible issues or items that I've missed in my diagnosis process. Thanks in advance.....

Is the timing advance working in the dist ?
Is the timing chain getting to much slack from being stretched?
Possible secondary fuel delivery is restricted assuming backfire is coming up the carb?

General rule of thumb - backfire in exhaust =rich condition / backfire up carb = lean condition

 

[NHGuy]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

How is the fuel water separator? Get a new one, replace it and pour the contents of the old one into a container, check after it has settled.
Get the fuel level down low. Pull the sending unit and have a look, maybe there's crud in the tank bottom. Maybe you could shoot a little air down the pick up tube if there's no screen at the bottom. Might knock out a plug of crud.
Check timing at 3000 rpms, is it advanced to spec? How do the plugs look? Wires on the right plugs? Is the carb down snug, without leaks around the base, and is the intake manifold snug too? I found a couple of intake bolts I could turn when I bought my boat.
I hope it's some stupid thing like that.

 

[John_S]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

You covered my initial thoughts: accel pump, carb, timing, tuliped valve (comp test), and fuel delivery. Other things that came to mind, vacuum leak, colapsed lifter, sticking valve, worn cam lobe. Hook up a vacuum gauge and see if that tells you anything about your engines health.

 

[timberattler]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

I've got the exact same engine and outdrive in my 89 Concord LS7, unfortunately i just got the boat a few days ago and haven't even had it on the water yet. I am however an old drag racer, i'd make a couple of suggestions. It sounds like from what you've done you'e handy with a wrench and the other posts here make great recommendations. I'd lean towards 2 things - first - did you set your timing with the vacuum advance disconnected AND plugged? If not, disconnect the vacuum line to the distributor and plug the distrubutor side (can pick up vac caps at auto parts store - or cut a short piece of hose and plug end with a sheet metal screw...just so it doesn't leak.) Second suspicion is a vacuum leak - often under the carb, but could be many others around. I test for vac leaks when the engine is running by squirting a shot of ether at the suspected leak - if you hear a pulse in how the engine is running, that's your spot (make sure the ether isn't getting sucked into the air intake).

 

[John_S]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

I've got the exact same engine and outdrive in my 89 Concord LS7, unfortunately i just got the boat a few days ago and haven't even had it on the water yet. I am however an old drag racer, i'd make a couple of suggestions. It sounds like from what you've done you'e handy with a wrench and the other posts here make great recommendations. I'd lean towards 2 things - first - did you set your timing with the vacuum advance disconnected AND plugged? If not, disconnect the vacuum line to the distributor and plug the distrubutor side (can pick up vac caps at auto parts store - or cut a short piece of hose and plug end with a sheet metal screw...just so it doesn't leak.) Second suspicion is a vacuum leak - often under the carb, but could be many others around. I test for vac leaks when the engine is running by squirting a shot of ether at the suspected leak - if you hear a pulse in how the engine is running, that's your spot (make sure the ether isn't getting sucked into the air intake).

Welcome to iBoats, timberattler. One of first things you will start to learn about differences between auto and boat engines, is that there is no vacuum advance on a boat. Boat engines don't ever "cruise" like a car to take advatage of additional advance from vacuum. If there is one in a boat, it is an automotive distributer and needs to be removed asap, due to explosion hazard.

 

[tcgravy]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

Is the timing advance working in the dist ?
Is the timing chain getting to much slack from being stretched?
Possible secondary fuel delivery is restricted assuming backfire is coming up the carb?

General rule of thumb - backfire in exhaust =rich condition / backfire up carb = lean condition

Timing advance is working, yes, have checked
Not sure on the timing chain??

Regarding your "general rule of thumb", what is the best way to determine where backfire is coming from, exhaust or carb? If I had to guess (based on sound) it would be from the carb, just wondering if there is a better way to tell.

 

[tcgravy]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

How is the fuel water separator? Get a new one, replace it and pour the contents of the old one into a container, check after it has settled.
Get the fuel level down low. Pull the sending unit and have a look, maybe there's crud in the tank bottom. Maybe you could shoot a little air down the pick up tube if there's no screen at the bottom. Might knock out a plug of crud.
Check timing at 3000 rpms, is it advanced to spec? How do the plugs look? Wires on the right plugs? Is the carb down snug, without leaks around the base, and is the intake manifold snug too? I found a couple of intake bolts I could turn when I bought my boat.
I hope it's some stupid thing like that.

Replaced fuel/water separator at beginning of season, did not appear to be water in it.
Through fuel system teste, checked pick-up tube, screen, etc. which was all clear
Have triple-checked that all plug wires are on correct plug hole
Have checked multiple times for air leaks around intake manifold and carburretor (using carb cleaner spray, ether, and even dropping engine oil there and looking for bubbles), no leaks found

 

[tcgravy]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

You covered my initial thoughts: accel pump, carb, timing, tuliped valve (comp test), and fuel delivery. Other things that came to mind, vacuum leak, colapsed lifter, sticking valve, worn cam lobe. Hook up a vacuum gauge and see if that tells you anything about your engines health.

Will hook up vacuum gauge this weekend and let you all know. Perhaps it'll tell us a thing or two....


Thanks for all the responses, keep em coming

 

[timberattler]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

Welcome to iBoats, timberattler. One of first things you will start to learn about differences between auto and boat engines, is that there is no vacuum advance on a boat. Boat engines don't ever "cruise" like a car to take advatage of additional advance from vacuum. If there is one in a boat, it is an automotive distributer and needs to be removed asap, due to explosion hazard.

Yeah - realized that when i got home last night and looked at mine - open mouth insert foot. Good thing you caught me soon - i was going to cite advance curves/springs next

Mulled over this some - your issue is definitely timing/firing.
Takes you one of 2 places
- either the module is dirty/bad
- or (and sorry for this) you have slack in your timing chain. this is most probable, but less desirable. You can check this by putting a timing lgiht on the engine an having a buddy gag it. If you notice a hesitation in the advance of the timing (position of white mark vs. flash of light) - chain stretch. If your chain is stretched enough to do this - it won't be long before it jumps a tooth and your valves are down when the piston is up - not good.

 

[tcgravy]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

You covered my initial thoughts: accel pump, carb, timing, tuliped valve (comp test), and fuel delivery. Other things that came to mind, vacuum leak, colapsed lifter, sticking valve, worn cam lobe. Hook up a vacuum gauge and see if that tells you anything about your engines health.

Hooked up the vacuum gauge today....see video via link below:
http://www.youtube.com/watch?v=M7qw1LNJKWM

Essentially the vacuum is at a rock solid 12 mm Hg at idle (about 800-850 rpms).

Any more ideas here?

 

[mpdive]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

My first reaction is Accelerator Pump. Check down the venturi and pump the carb. Has all the signs of going lean under load to me. That would explain it worsening when going to WOT when allready at cruising speed. That would be my first suspicion. Also makes sense why it doesn't do it when your easy on the throttle. Take a HARD look at the pump.

 

[tcgravy]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

My first reaction is Accelerator Pump. Check down the venturi and pump the carb. Has all the signs of going lean under load to me. That would explain it worsening when going to WOT when allready at cruising speed. That would be my first suspicion. Also makes sense why it doesn't do it when your easy on the throttle. Take a HARD look at the pump.

I agree that accelerator pump seems like the issue...however, I have tested it via pumping throttle and get 2 steady streams of fuel. I have also double-checked the spec for accelerator pump setting, which is spot on.

Is there an alternate way of testing this? like volume of fuel being squirted or something like that?

 

[mpdive]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

I agree that accelerator pump seems like the issue...however, I have tested it via pumping throttle and get 2 steady streams of fuel. I have also double-checked the spec for accelerator pump setting, which is spot on.

Is there an alternate way of testing this? like volume of fuel being squirted or something like that?

Not that I can think of. I am still suspicious of the carb. Once your up and moving you are running on the main metering circuit. Should your float level be off even slightly, you may be starving the primary circuit. Personally for what a kit cost, I would be doing a rebuild. I love rebuiding quadrajets. Another poster suggested looking for a vacuum leak especially at the carb intake gasket. I use brake cleaner to check and watch for an idle discrepancy.
Keep us posted

 

[tcgravy]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

Not that I can think of. I am still suspicious of the carb. Once your up and moving you are running on the main metering circuit. Should your float level be off even slightly, you may be starving the primary circuit. Personally for what a kit cost, I would be doing a rebuild. I love rebuiding quadrajets. Another poster suggested looking for a vacuum leak especially at the carb intake gasket. I use brake cleaner to check and watch for an idle discrepancy.
Keep us posted

Yeah....carburetor was rebuilt about a year ago. Float level is spot on. Vacuum leak would likely show up in fluctuating vacuum gauge while running (which doesnt happen, test was rock solid with no fluctuation)....and I have also tested multiple times using carb cleaner and engine oil (no leak found).

It is certainly possible that this is an issue within the "secondary" circuit, however, to my knowledge the spring windup, secondary air valve setting, and all secondary jets/rods are set to specification. I will clarify/re-check.

 

[tcgravy]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

UPDATE:

Took boat out again today. After further adjusting the accelerator pump (richer) and the secondary air spring (tighter, to open "slower")....

Boat runs like a champ for the most part. WOT 4800 rpms on the top, just as it should be.

Only issues are as follows:
- From time to time, seems to be running out of fuel around 4300-4500 rpms (engine stutters and does not gain rpms)
- There is additional steam coming from crankcase ventilation hoses (there was always a little bit, but seems to be more now especially at idle).


Thoughts on the remaining issues?

 

[Scott Danforth]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

replace the carb would be my next step. I would look at a marine pro-jection setup (TBI) myself

 

[Volphin]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

I wonder if you are getting some valve float at higher rpms? Else, sounds like some richer jets or metering rods might help out.

V

 

[tcgravy]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

Took boat out again last weekend. Results were:

First hour or so of day, boat would run flawlessly on take off and up to 4800 rpms.

After that, boat would run out of fuel around 4300 rpms and kind of stutter there.

Totally stumped....any ideas?

 

[88Supra]

Re: 454 Mag Performance Issues - Hesitation/Backfire/Stumble on Takeoff and WOT

Took boat out again last weekend. Results were:

First hour or so of day, boat would run flawlessly on take off and up to 4800 rpms.

After that, boat would run out of fuel around 4300 rpms and kind of stutter there.

Totally stumped....any ideas?

Yeah Well...
I think you've all missed the point of what the Engine is trying to say
Spit, Spudder, Back fire up the Carb; "I'm starving down here"!
!FUEL PUMP! ...and possible water being passed by the filteration system into the Carberator
(by means of Alcohol in the fuel maybe) that 10% Alcohol fuel your getting from Arco; is not good.
!FINER WATER SEPERATOR FILTER! <<<<More than one and replace everytime you run until fixed.
Anytime I buy a used boat (and I've bought a few) I replace the fuel pump anyways. <<<Period
Ya know it's rather dangerous to have a faulty or why would you even trust an old fuel pump...
See that clear tube going up from the Fuel pump to the carb? ...That's for in the case the old pump diafram fails
the excess fuel SHOULD go to the carb, kill the engine and not in the Bilge (for fire fuel).
Some Mercruisers have a more sophisticated method which won't let ya start it again after the fact but, the above clear tube method is ment for the operator to see fuel in the line, not try and restart.

Not to mention, Should any of this happened, Saturated filter, Fuel pump failure, Fire??? You're not going no where!!!


...Post the good running feeling!!!