99, 5.0L Mercruiser, Carbed- misfiring and backfiring

[johno270583]

Hi Again

Boat had been running well apart from some hesitation issues from idle on acceleration, until a couple weeks ago.

This was even worse on last outing, I put a 1' spacer under carb to lift away from motor and try get some more low end torque. Motor coughed and backfired to start off, then eventually started running better. However hesitation at idle was much worse, and had to double pump throttle to help get it going.

Pulled plugs the next day and they were BLACK. Cleaned, regapped and reinstalled.

Fired up in the driveway, however was pushing alot of black smoke, and leaving a film of black muck on the drive, floating on the water. Not sure what this is, but thinking/hoping unburnt fuel? Also was backfiring occasionally when revved in neutral.

At idle, motor is noticeably missing. I have hooked up timing light to check, and the light would only flash occasionally, or for a short while, which at first I put down to my gun not working properly, but the more I think of it, maybe because the lead isnt actually getting a spark.


Have 12V at the coil, only 11 at the Red/White lead on the ignition sensor/distributor. Also hooked an ohmmeter up between red/white and green/white leads on Ignition sensor, and there is no reading, manual says should be 100 ohms.

If the Ignition Sensor is bad, will the boat run at all? With the above test of no reading between the 2 leads, its about all I have left in my inventory to check.


Im just about out of ideas on what this could be, Distributor Cap, rotor, sensor wheel, ignition sensor, leads are less than a year old. Holley carb which I have had off, cleaned and checked. Timing set to mark on crankshaft, which I assume is correct because there is only the one mark. I have this week pulled off the intake manifold and resealed with new gaskets and sealant, and also the carb, so am now assuming any air leaks would have been taken care of.



Any ideas would be greatly appreciated.


Also, anyone want to but a 1999, 1900SR in Australia??!!!!


Thanks

Andrew

[ss3964spd]

Andrew,

First question is - did it start running badly immediately after you installed the carb spacer?

As you suspect, black plugs, and black smoke from the exhaust, indicates the engine is running very, very rich (too much gas). This could be from the carb itself, or from the ignition system. However, if the ignition system isn't firing the plugs - thus not lighting the fuel mixture, you generally won't have black smoke - just raw gas smell.

Nevertheless, since you also have some indication that the ignition system is suspect you need to verify/sort that. I think your ignition system is pretty much self contained within the distributor. Yes, the modules can and do go bad, sometimes they become weak and other times they fail completely. I don't think they are horribly expensive to replace. Coils, too, go bad, get weak, etc. You can measure the coil's resistance on both the primary and secondary sides to make sure it's in spec. Timing; at this point it sounds like you have the timing set at 0 with the engine at idle. I'm pretty certain that is incorrect as it should be 2 or 4 or 6 or more degrees before top dead center (BTDC). You'll have to look up the spec for your engine. Having it set at 0 could easily cause the back firing.

The carb; because of the black plugs and smoke it's easily possible the float in the carb is, well, not floating - or not floating enough. If it isn't adjusted correctly it will allow too much gas into the carb and basically overflow into the engine. There is a specification for the float level. If the Holley has fuel bowl sight glasses (little circular windows) the fuel level needs to be set near the bottom of the sight glass. That's also a quick way to verify that the float and needle and seat are working. Holleys are also prone to getting junk stuck in the needle and seat assembly, thus not allowing the needle to stop the flow of gas into the carb. You might also be able to look into the top of the carb while the engine is idling; if you see raw gas dripping then the carb is flooding.

Dan

[pascavone]

i would agree on a float, pin, needle valve issue. If your getting black smoke its to rich, and the whole brass needle valve seat could have backed out of the alumium carborator housing. the float hinge is basically a short rod, like a paper clip.

If your getting lose of power or the "poping" in the mainfolds this would indicate to lean of a mixure or you new spaces has a air leak, which screws up your intake pressure. remember, your carborator draws fuel buy air pressure, the fuel pump pressue is held back by the needle valve.

I don't think you have an electrical problem, your engine miss is from wet plugs.

Not sue of where the "low end torch" and 1" spaces theory comes from. Your engine torch is based off of the engine RPM and torch curve, 4600 rpm max. More gas dose not give you more rpm, just burns more fuel.

[seapuppy]

"remember, your carborator draws fuel buy air pressure"

actually it draws air by vacuum pressure.....sucks the fuel out of the bowl via the venturiis....

and Pasc is correct about changing torque range...the only way to really change torque range say from an upper range to a lower range is to change the way an engine breaths ...which is either by the cam or the head design...
ie..if you have a 68cc combustion chamber with say 192 intake valves and 185 exhaust...by changing the head to a 185 intake valve and a 165 exhaust...your increasing velocity thru the head chamber and your torque max range goes from say 3500 rpm down to about 3000 rpm..inversely if you change the intake to a 202 valve and exhaust back to 185...you'll move the range up to about 3800rpm.....

so...in reality the carb doesn't do anything even adding a spacer except 1) cause a delay in throttle response...2)remove carb from heat source (ie the manifold) thus preventing carb vapor locks....3)increase a chance of vacuum leaks......

to me after reading your post this morning it does sound like the float is either stuck open or sank in the bowl....either way I'd be pulling that carb apart to find out what is going on....

SP

[johno270583]

Thanks for the replies.

Dan- Boat is running, so I am suggesting cylinders are maybe getting a weak spark (and an intermittent strong spark). This is why I was wondering what symptoms of bad Ignition sensor are. Could well also be the coil as suggested, though it is fairly new.

Timing- Is set at the only mark on the crankshaft pulley, on the Port side of the front of the engine. There is a single arrow on there, which I have always assumed is the 10 BTDC mark that the timing is to be set to. Is it likely that this is actually the TDC mark? I havent seen any other marks on the pulley?

Carb Float- On the 4160 Marine Holley carb, Float is not adjustable, all sealed. Carb was apart yesterday, float is sitting level when the needle is seated, but the one thing I didnt do was blow through the thing to check if was sealing properly!! I will try do this again today. Also there is no dripping into the carb that I can see.

Pasc- Like I mentioned, the needle was in place when I pulled the carb apart, but I didnt check if was blocking off the fuel supply properly. If it was continuing to flow though, wouldnt the overflowing fuel be flowing out the J tubes?
I have also removed the spacer to try get motor back to running as it was previously. Manifold vaccuum was 14 1/2, which is the highest it has been in a while, Not sure if this is due to me reinstalling manifold again, or using Permatex No. 2 on carb gaskets. I should also mention that a stainless throttle bracket is mounted under the carb as well, so have manifold, gasket, bracket, gasket, then the carb. I am fairly sure there was a leak under this, particularly when the spacer was on, which is the reason I decided to use the Permatex, and seems to have made a difference. I should also mention that I had it running again in the drive yesterday, and with the few hits of the throttle I gave it, couldnt make it backfire, so maybe one problem solved? Will have to give it a river run to check for sure.

Looks like I will be pulling the bowl off the carb again to check the float.

I should also mention that I had suspected a blown power valve in the carb, due to the backfiring I had been getting. A test for this is to wind the Air Fuel Mix screws all the way in. If the engine dies, the power valve is ok. I did this, and the engine revved up, but when I pulled the carb apart, the power valve seemed to be ok, when vaccuum applied, the valve would close, and could blow air through it then stop the air flow. Im wondering if it is worth replacing power valve anyway, due to it being a cheap part? Any other suggestions as to what could cause this?

So whats the best way to check the coil and Ignition sensor?

Here is the description of the spacer I used, I got it because had read some posts on another website that mentioned they had better results by using a spacer between the carb and manifold. The theory is the carb is raised off the manifold and protected from heat, so air is cooler, can suck in more. Supposedly the 4 hole spacers give slightly better low end torque, and every bit helps in a boat!
Part#TCO-6213 - Torco insulated carb spacer 1 inch tall Holley 4 barrel square bore flange, 4 hole plenum. This carby spacer is made from high quality insulation phenolic material and can significantly reduce the amount of heat transfer from the intake manifold to the carburettor resulting in a denser intake charge to enable you to consistantly make good horsepower.

Thanks again for your wise words

Andrew

[ss3964spd]

Andrew,

Carb spacers can and do work - on some applications. Not so much because they help insulate the carb from heat but more because they can provide a stonger vacuum signal to the carb by increasing the plenum volumn of the manifold. Unfortunately they can also have a negative effect. Regardless, whatever they may add you are unlikely to preceive any positive impact - the numbers just arn't big enough. Less than 10 HP or pound feet of torque.

I don't know how to test the ignition sensor. The coil has a primary side and a secondary side, both of which can be ohm'd but you'd need to know what model coil to get it's specifications. If it's fairly new I'd look elsewhere. As far as the timing mark on the balancer goes usually that mark indicates "0" degrees (or TDC on Cyl #1 and 6). However, since I've not timed one of these engines I won't swear to that so you'll need to verify. I do know that timing has a profound effect on how the engine runs.

Two things you mentioned above warrent further investigation. 1) Manifold vacuum at 14 1/2". These engines have pretty mild camshafts. You should be seeing considerably more vacuum at idle - I'd guess something closer to 18". Again, you'd have to look up the spec. 2) When you screwed the idle mixture screws in (seated) the idle speed picked up (how many turns were they out?). That's obviously an issue and suggests to me that the engine is being fed too much gas; when you seated the screws you cut off the gas from them and the A/F mixture was more inline and increased the idle speed.

Timing, idle speed, and idle mixture all have an impact on idle vacuum. Timing - because as you dial in more ignition advance idle speed will increase. To slow down the idle speed you adjust the carb's idle speed screw. When you adjust the idle speed screw you are further closing the throttle blades. When you close the throttle blades vacuum goes up. When you get the idle mixture (A/F ratio) correct idle speed goes up and so too will vacuum.

At this point I think I would 1st verify the idle timing. If that isn't right you'll simply be running around in circles. It should be X degrees BTDC @ XXX RPM. Once you know the idle timing is right (if you have reason to suspect it isn't) then you can address the carb.

Dan

[oneradride]

This is interesting and may or may not apply? http://www.justanswer.com/boat/1zyqg...timing-10.html so is this.... http://www.chevelles.com/forums/showthread.php?t=232844

I just installed a brand new long block in my boat and had to drop the distributor after installing the intake...... although (the distributor drop) is not the same as a 5.7 automotive... Just about everything else is. If you are timed at "0" BTDC this may be a problem.

Mines a 95... looks like yours is a 99, pretty sure they are both 8/10 degrees BTDC for initial timing. I think it's 34 for total.... but don't worry about that right now...

I'm agreeing with SS396 make sure your initial timing is correct before jacking with anything else. I realize all you did was install a spacer but you have to also think there is only ONE 9/16 bolt holding that distributor in place.... and you very well have bumped it, if it's not tight.

[johno270583]

Thanks again for responses

I use either 91 or 95 octane fuel with no ethanol.

I have been setting the timing myself since I got the boat, and am sure that the mark I am setting it to is the 10 btdc that it is supposed to be. It is where it was set when i first got it checked out by a mechanic. It is also done while in Base timing mode, with the idle speed set to 650rpm an engine at operating temp. A/F mix screws were set out 1 and 1/2 turns, as manufacturer reccommendations, and was actually out of box at 2 turns.

Another thing I should mention on why I beleive this problem is related to ignition sensor or coil is that I managed to run the battery dead a few weeks ago after installing spacer and before taking it for a test run. Without me really thinking to much of the consequences, I pulled the battery out of my car and started boat with it, then switched battery cables quickly over to boat battery to charge it. No jumper cables were available. I think I may have done some damage to the sensor and maybe coil then.

Another thing is that the distributor has been respeared twice since I have owned it, once when installing new manifold and again last week when I put in the gaskets and RTV sealant on motor ends for peace of mind that there are no vacuum leaks. Both times I have been careful to mark up distributor carefully to ensure reinstalled as it come out. If it was reinstalled incorrectly, which I doubt, it would only be 1 tooth out. Is this going to make a difference? In my mind it's not, because the distributor will be rotated to fire at 10 btdc again anyway. Also it was respeared again after it started running rough, so wouldnt make sense.

I will say again for clarity, boat had run well on previous outing. Only adjustment made was installing spacer, slight adjustment of accelerator pump for better throttle response an battery going dead an being badly jump started. After this, the boat ran like a hairy dog.

Since then I have removed spacer, reinstalled intake manifold and used permatex 2 under carb to give a better seal. Timing reset to mark on crankshaft, where it has always been.

In my mind this problem is related to the ignition system (timing light only flashing periodically on checking) or fuel delivery (way too much fuel, which I thought was blown power valve, but check has shown ok, I might replace anyway. OR an air/vacuum leak?)

[cunuck]

I checked my sensor last year and had a reading of 0 no. 100ohms.Got a new sensor checked no reading 0 ohms but they both work just great.could it be a print error in the manual.I would try another timing light to be sure .the choke is off all the way??? good luck

[oneradride]

The fact that the timing light is intermittent on number "one" could be several things..... coil, distributor, rotor, cap, ICM (if you have one) plug wires or the light itself may be faulty....
I'd try it on plug wire 3 or 5 just to see if the flash is intermittent there as well....

If you are not getting spark, or getting it intermittently it's going to appear to be running rich because the fuel is not being ignited.

Make sure the battery is fully charged first then start eliminating variables.

[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]

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.