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Pop and bang at 2000rpm

G

Grizz1148

New member
I'll give details as best as I can. This is what it does, it being a 99 mercruiser 5.0 alpha 2bbl carb model. Pops and bangs at 2000 rpms (under load) and won't accelerate higher due to this. Idle is great, smooth acceleration up to 1900. Under no load it will rev fine all the way up if throttle is pushed moderate or slow, if pushed fast it will pop briefly. This is what has been done so far.
dist cap and rotor replaiced
coil replaced
plugs replaced
fuel pump replaced (electric)
ign sensor under dist cap replaced
noticed neither old or new fuel pump will fill empty fuel filter/water seperator, ran fuel direct to carb and gets fuel but same issue as above.
Stumped now, thanks in advance for any help
 
popping can be water in fuel, running lean, ignition timing problem if it not getting fuel check for:
clogged anti-syphon valve
internally clogged fuel lines
internally rotted fuel lines
loose fuel fittings sucking air.
 
Grizz1148 said:
Any way to diagnose timing module?
Click to expand...


There is no specific test. It is process of elimination.

You have eliminated everything else.

Fist double check firing order

if that is good then it may be the timing module. Typically when you loose acceleration around 2200 rpms it indicates a loss of timing advance and it also can back fire.

if you know someone with the same motor swap modules to see if it works...........then you will know
 
popping can be water in fuel, running lean, ignition timing problem if it not getting fuel check for:
clogged anti-syphon valve
internally clogged fuel lines
internally rotted fuel lines
loose fuel fittings sucking air.
Click to expand...

Ayuh,.... Sounds like a gummed up fuel system to me too,.....

Probably the Carb,....
 
bondo said:
Ayuh,.... Sounds like a gummed up fuel system to me too,.....

Probably the Carb,....
Click to expand...
carb has been checked out, seems good, no grit or grime, took main jet out, looks clear. As I said in first post, found it odd that neither pump would fill an empty filter so bypassed the filter/seperator and issue still exists. Hooked an external pump to fuel line coming from tank and fuel seemed to flow free.
 
You might have a worn camshaft lobe. You can tell by pulling the valve covers and rolling her over with the plugs out; ALL of the valves should open the same amount. If not, that's the problem.

Jeff

PS: I assume you made damn sure the ignition wires go to the correct plug?
 
Triple checked firing order. If it was a valve or engine issue wouldn't it show more prominent at idle or am I mistaken? Idles very well and smooth to 1900-1950 rpm.
 
You may not detect a slight miss when idling. A bad cam lobe would cause this. But under load the valve not opening all the way could cause such an issue as you are seeing. It may or may not happen with no load......depends.

Jeff does make a good point. Only way to be sure is to remove valve covers and watch each and every rocker arm for the same movement/distance up/down.

If one or more lobes are rounded you will see it immediately!!

Remember we are not there so we can only make a guess.

How many hours on the motor? That is a key piece of information that is missing.

You may have a burnt valve, bent push rod, a host of possible issues.

We don't know the history of your boat..........has it ever over heated and if so how bad and how often.

Things like that.

I have only seen it once but what you described was the timing module.
 
Don't know the history, bought it last fall, ran fine. Typically if it's icm there wouldn't be any spark at all? Reading these forums this seems to be a common issue with several solutions. I'm going to rebuild the carb next just to eliminate that. We shall see. Thanks for the input.
 
Grizz1148 said:
Don't know the history, bought it last fall, ran fine. Typically if it's icm there wouldn't be any spark at all? Reading these forums this seems to be a common issue with several solutions. I'm going to rebuild the carb next just to eliminate that. We shall see. Thanks for the input.
Click to expand...

Full rebuild on carb. Found 1 jet partially clogged, did not fix the issue though.
 
OK, time for some additional testing. If you have ruled out a lean carb and hooked up a remote tank directly to the fuel pump and the problem persists:
Remove the spark plugs checking for signs of water. red is rust,
Replace wire from coil tower to dist cap
Drain the manifolds and riser, remove the riser checking for wettness or rusting in the manifold, a sign of water
Last, remove the valve covers checking for a broken valve spring
 
This manifold was on a running motor believe it or not
DSCN3434_zpsngaoau97.jpg
 
Grizz1148 said:
Don't know the history, bought it last fall, ran fine. Typically if it's icm there wouldn't be any spark at all? Reading these forums this seems to be a common issue with several solutions. I'm going to rebuild the carb next just to eliminate that. We shall see. Thanks for the input.
Click to expand...

As I said I have only seen it once before but,

When everything is correct on a motor, initial timing, spark plug wiring, mechanics are good (valves and cam) and all ignition components have been replaced then there is only one thing left.

IGITION TIMING CONTROL MODULE

When a motor increases in RPM, the timing is also supposed to advance. If it does not typically you reach ~ 2000 to 2400 rpms and then it falls flat and with this module it can back fire/pop and not go past the 2000 -2400 rpm range.

Of course from the key board it is only a guess so understand that.

If you can find one to swap that would be best
 
kghost said:
As I said I have only seen it once before but,

When everything is correct on a motor, initial timing, spark plug wiring, mechanics are good (valves and cam) and all ignition components have been replaced then there is only one thing left.

IGITION TIMING CONTROL MODULE

When a motor increases in RPM, the timing is also supposed to advance. If it does not typically you reach ~ 2000 to 2400 rpms and then it falls flat and with this module it can back fire/pop and not go past the 2000 -2400 rpm range.

Of course from the key board it is only a guess so understand that.

If you can find one to swap that would be best
Click to expand...
We have considered the icm, can't find one to try. Most of what we read about the icm says if it's bad the engine won't fire at all, have you heard of these icms causing this kind of issue or typically do they not fire at all? Thanks
 
I have witnessed the issue first hand as described..........on a trailer not even under load..........When I worked in the business.


other things just came to mind.....

1.Have you removed the tachometer lead from the coil? Grey wire? If not try that. retest

2. disconnect the shift interrupter switch at the shift bracket, remove the ground or other connection from the terminal so there is no continuity to ground if the switch is marginal. retest

3. the large connector at the motor wire harness, remove, inspect pins/sockets. spread pins out (very little) and reseat connector. Should be a hose clamp on it if not put one on and tighten. Not real tight but enough to keep it from being able to be pulled off. retest........

also after rereading your original post it does sound more like a carb related issue even though you did work on it....Maybe you missed something.

If a main jet was clogged that would do it.....So maybe go back in and double check fuel circuits again!!...........
 
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kghost said:
I have witnessed the issue first hand as described..........on a trailer not even under load..........When I worked in the business.


other things just came to mind.....

1.Have you removed the tachometer lead from the coil? Grey wire? If not try that. retest

2. disconnect the shift interrupter switch at the shift bracket, remove the ground or other connection from the terminal so there is no continuity to ground if the switch is marginal. retest

3. the large connector at the motor wire harness, remove, inspect pins/sockets. spread pins out (very little) and reseat connector. Should be a hose clamp on it if not put one on and tighten. Not real tight but enough to keep it from being able to be pulled off. retest........

also after rereading your original post it does sound more like a carb related issue even though you did work on it....Maybe you missed something.

If a main jet was clogged that would do it.....So maybe go back in and double check fuel circuits again!!...........
Click to expand...

Today we fired it up and noticed one exhaust riser much warmer than the other and water much slower coming out on that warm side. removed exhaust from the warm side and it seemed to clear up. Can't put it in the water that way to see if it is ok under load but definite difference.
 
One side is typically 20 or so degrees hotter than the other.........can be even a bit more. NORMAL.

This makes no sense.........how did you determine one side slower than the other?
 
The exhaust is a two into one, manifolds and elbows flow into a Y pipe referred to as a rams horn.
this one pipe exits at the transom and thru the bellows thru the prop but before that it also exits at the transom assembly at 5 and 7 o'clock positions.
it is an open exhaust once reaching the "rams Horn" so there is no way to tell and I mean NO WAY to tell if there is an issue. Path of least resistance....


the water goes where it wants to...........


so this cannot be the case...

we get questions all the time about this water flow (one side vs the other) all the time.

This is 100% normal what you describe.
 
So the fact that removing that side and the engine seeming to run much better is just coincidence or does it give us a new place to look?
 
OMG

What was removed?

You cannot simply remove one side without completely disconnecting it and then the boat would fill with water on the inside...........and be very hot!!

Something is missing here..........you best talk with your "mechanics" and see what exactly they did to get so called better "results".

Not to get a bit upset "read my subtitle" but information is key when trying to answer questions from a KEYBOARD

NOTE: on second Vodka drink so be careful!!!!!!
 
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kghost said:
OMG

What was removed?

You cannot simply remove one side without completely disconnecting it and then the boat would fill with water on the inside...........and be very hot!!

Something is missing here..........you best talk with your "mechanics" and see what exactly they did to get so called better "results".

Not to get a bit upset "read my subtitle" but information is key when trying to answer questions from a KEYBOARD

NOTE: on second Vodka drink so be careful!!!!!!
Click to expand...
Exhaust was removed at the riser, popping and backfiring stopped immediately. Would this mean blocked exhaust or valve issues?
 
Oddly enough the exhaust was put back together and took it out on the water, running good now. Have to assume the carb rebuild fixed it.
 
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