99, 5.0L Mercruiser, Carbed- misfiring and backfiring

[johno270583]

Canuck- choke is off all the way

1RR- it is running rich, black plugs, smell of fuel, as though they are only being ignited occasionally or partly.

Boat is going into the shop tomorow so they can look at it.

I'll give a progress report. Hopefully they can solve it without it costin me an arm and a leg! Not likely

[johno270583]

UPdate

Shop didnt find anything wrong, spark apparently strong and regular to all plugs, though Im a little unsure if they actually checked this properly.

Timing checked and was ok.

All they did was lean off carb a bit, and said it was running better. Plugs still black, and a 'bit' of black smoke still coming out at idle, but runs fine otherwise. Told me to take it for a test run and see how it goes.

Wont get a chance until tomorrow, but my best bet is it will be running exactly the same as before.

If that is the case, it seems to be pointing back to the carb, so am going to get a new power valve today to install after first run, and see if any improvement.

That failing, Im back to square 1, not sure where to go from there, have to think about it some more.

Wish me luck

[ss3964spd]

It shouldn't be producing black smoke at idle Andrew, something is still amiss. Timing generally won't contribute too much to black plugs/smoke unless it is way off. If you're confidant the shop knew what they were doing when they verified the timing and spark then all that's left is the carb - and likely the power valve.

You'd mentioned your vacuum reading at idle - which seemed low to me. Were you able to verify what it's supposed to be? Idle vacumm drives the correct power valve selection.

Dan

[johno270583]

Dan- According to manual, vacuum should be 15-21. Lower suggests a vacuum leak or incorrect timing.

Shop verified that ignition system is fine, whether I believe their diagnosis is another thing!

My thoughts are that too much fuel is being delivered into the carb at idle (most likely from a busted Power Valve, which is dumping fuel. The A/F mixture screws can be turned all the way in, and the motor will actually rev up/run better. 2nd guess is float needle not seating properly). Also due to so much fuel at idle, idle set screw may have been adjusted so that the throttle blades are open more than necessary. This in turn would decrease the manifold vacuum, as there is air being freely sucked in. Do you think I am on the right track with this?

Failing the above, I think Im back to the drawing board.

Ill give an update later after I go for a test run

[johno270583]

Another thing I should ask is about the power valve.

In cars, it is recommended to use a power valve half the idle manifold vacuum, and is usually close enough.

However, the marine Carbs use a much lower number power valve. Stock in mine was 2.5".

I am replacing current one with the same size power valve, but have wondered what the purpose of such a low vacuum opening is? Obviously to do with the conditions in which boat motors run, but have wondered if it is worth experimenting with higher number power valves, which would open at a higher vacuum (once I fix the current problem of course!)

[johno270583]

Ok, took for a run, boat ran smooth and definitely didnt put out as much black smoke on idle. Checked vacuum, 18", which would be due to shop turning AF mix screws to only 1/2 turn out, and increasing idle RPM slightly.

BOat lacked any acceleration on the water, not sure what RPMs it got to, at a guess around 2000-2500, then would sit and plow through the water for a good minute or more before finally getting on plane and eventually picking up speed and RPMs. Very very sluggish.

Tried this a few times, with similar result, popping and backfiring.

Then stopped, removed primary fuel bowl, replaced power valve, and made sure float needle was seating properly (it was).

Restarted, vacuum the same, ran exactly the same. On acceleration bogged down completely again.

Checked timing with light, was steady on the mark, all ok.

However, I left timing light attached, and had someone start driving the boat forward, as we would while trying to ski (from idle to 3/4 throttle). Timing advance hardly moved, I would say at a guess it advanced around 15 degrees, and just sat pegged there, while the boat struggled along.

Tried again with boat in neutral, and hitting the throttle, with same result. To me it seemed spark is not advancing anywhere near enough, and may the cause of the sluggish performance.

Anyone else had a similar problem, and is this likely to be the Ignition sensor, ICM, or something else?

[ss3964spd]

I'm not real good with Holleys Andrew, but did find this: http://www.holley.com/forums/showthr...alve-selection

Since the PV is meant to provide enrichment at WOT I don't see why picking the right one would be any different for a boat application.

Plain and simple, based on your comment above about turning the mixture screws in all the way resulted in a faster idle speed, the engine is getting too much gas.

Timing advance: I don't know what the specs are, but 15 degrees doesn't sound like much. If your base timing is 10* BTDC, and then there is 15* of additional, you're at 25* total. Since your engine is carbed your distributor probably has a mechanical (centrifigal) advance system. Remove the cap and rotor and you'll see a couple of weights and springs. Make sure those operate freely. The centrifigal advance is mechanically limited so at some point in the RPM range it will stop advancing the timing regardless of how much faster you rev the engine. You have to check the specifications for the advance. If you have a "dial back" timing light you can verify the base timing yourself.

I still think you need to sort out the carb first.

Dan

[johno270583]

Timing Curve.jpg

Dan- Motor has Thunderbolt 5 ignition, so has sensor wheel and ignition sensor in distributor, not weights and springs.

I agree that I am getting too much fuel at idle, but not sure what to check next. I have checked that primary bowl is shutting off, it is as far as I can tell (cant blow through it when float is pushed up, when bowl is removed). Next may be to check secondary side, as far as I have read, a small amount of fuel is delivered from secondary bowl so fuel will not go stale. Apart from that, not sure what else to check or if anything can be changed.

I think I have attached the timing curve for a TB 5 ignition, going off this I am getting nowhere near enough advance, which I beleive is why I struggle to get on the plane. Also not sure what can be done to troubleshoot this further, unless I start replacing the ignition sensor, then wheel, then rotor, then dist. cap, and see which makes a difference?

[ss3964spd]

Ah, that explains that. Advance is handled by the control module. Remember the tid bit about needing to adjust idle mixture with the ICM set in it's base timing mode, or you'll just piss off the computer.

Regarding that graph - is it showing crankshaft or camshaft degrees? I'm assuming crankshaft. Regardless, if the ICM isn't giving enough advance it could be the ICM itself or the knock sensor.

[johno270583]

I assume the graph shows Crankshaft degrees (degrees advance).

Is there any way to test the ICM, or am I doomed to buy another one and hope for the best?

I am pretty sure there is no knock sensor on my motor. There is just the ICM. If there is a knock sensor, Im buggered if I can find it, it is definitely not near the ICM as it is supposed to be, and nothing else attaching to the block, apart from Oil Pressure sensor.