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mercruiser backfire

harley142

New member
I have a 1987 Mercruiser 350 in a Malibu Ski boat
not everytime I take it out but it has happened several times this summer
after about 40-60 minutes of running great and pulling skiers and tubes the engine starts to backfire badly.
it never stops running but not alot of power and alot of backfire. backfire on power up and while trying to accelerate.
after about 40-60 minutes of tooling around at low speeds it starts running great again and no further problems for the rest of the day. PLEASE HELP !!
 
update I checked the cap and rotor and they look great, it has been converted from points to electronic pick up. wires look new and I have gone through several tanks of gas. just bought the boat at the beginning of the summer.
 
Backfire is usually caused by a "lean sneeze" (lean fuel condition), or slightly retarded timing. If it's random, I would go over the complete fuel system, and filters.
 
Due to you not knowing the history of this boat your issue can be a tough one from the internet.

1. As Mikdee said back fire is typically a lean condition. So a lack of fuel could be the issue. What could cause this?
a. fuel pump issue, although it would be more consistant.
b. possible vent line clog, if the vent clogs a vacuum will be created in the fuel tank so gas cant be pumped to motor.
c. possible pick up tube issue in gas tank. (this sounds more likely) something may be in the tank and once in a while it ends up at the pick up tube in the gas tank and stope fuel flow until you slow down or stop and the crap moves out of the way.
d. As a 1987 you should NOT have a point converted ignition. You should have the Thunderbolt IV mercruiser electronic ignition. (I have a 1987 boat and have the thunderbolt IV). Assuming this is the case, there should be a timing control module mounted either on the exhaust manifold or somewhere else near the distibutor. If this module fails it can cause a condition as you describe. Typically it is a works or not works device but one never knows.


So the next time it happens you have to do some diagnostics.

To see if it is a fuel issue, when the motor starts to pop or run like crap, put it in neutral, pull the neutral start button on shift control out (used when cold starting) and push the throttle lever back and forth kind of fast (like pump a gas peddle quickly) to see if the motor responds and runs better.

If it does this may indicate a fuel issue.

As far as the ignition goes and seeing as you dont know the history, Spend the money as a precautionary measure and buy a new cap, sensor and rotor assembly. It may cost a few hundred dollars but should not need replacement for many years.

I have replaced my cap rotor and sensor on mine several times over the years not because it needed it but rather because I did not want to face a failure while using the boat (precautionary).

One last thought, the coil. It is now considered an older boat. The coil if original is 27 years old.......coils are cheap!!

If you need parts or at least part numbers and an idea of cost use this web site. Links above will take you to your motor.
Just get the serial number.
 
I tend to agree with the guys here. Give your complete fuel system a good look-over.
Use the good ole P of E... process of elimination...... one item at a time.

it has been converted from points to electronic pick up.
I agree with kghost. A Merc 1987 ignition system should be Merc's TB system. These are EST.... (electronic spark timing) with no mechanical flyweight advancing system.

If this had been a mechanical system that has been converted to electronic, is most likely now a Pertronix kit.

If so, and unless the flyweight system is working near perfectly, you may have an advance issue.
Check the progressive advance and where the TA (total advance) is, and post your results.

If you determine that the ignition is the issue, I'd loose that Hall Effect Pertronix kit.
Nor would I waist a single dime on replacing any Pertronix parts.
I'd go with a real Electronic system.

Merc's TB is photo eye EST. Requires Merc's ignition module.
Delco Voyager is photo eye EST.
Mallory's YLM is Variable Reluctor (VR) with mechanical advance.
Mallory's YLU is Photo Eye with mechanical advance.

You'll spend a few $$$, but you'll have a much better system.

.
 
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Before you get too far into your fuel system, Check this first since this "does not happen every time you take it out". Remove the distributor cap and see if you have any sideways movement in the distributor shaft. There should be None.
 
Wow have to first of all say thanks to all for the help!! You guys are great!
ok as far as the ignition I assumed it used to be points I did not know that way back in 1987 they had electronic ignition so it is probably original. I will go through and start to check the things suggested, first I will check for play in the distributor shaft, then I will check the fuel filter and water separator. I will attempt to see if there is anything in the tank for possible blockage. And when this problem happens if I put it in neutral and give it gas it does not backfire at all. So may be leaning toward the fuel.. .. Again thanks for all of the info and if anyone has any more knowledge please pass it along.
 
...................
ok as far as the ignition I assumed it used to be points I did not know that way back in 1987 they had electronic ignition so it is probably original.
If this is a Merc TB system, you should be able to see the ignition module unit.

Also, with the distributor cap removed, you should not be able to see any rotation movement of the rotor, since there is no mechanicl advancing mechanism in this style ignition system (that you'd otherwise see with a conventional system).

I will go through and start to check the things suggested, first I will check for play in the distributor shaft, then I will check the fuel filter and water separator. I will attempt to see if there is anything in the tank for possible blockage.
There will be a dip-tube or pick-up tube just beneath the fuel tank's outlet fitting.
You can remove this, and take a look at the bottom of this tube.
You may see a screen. These screens can become corroded and very restrictive.
Remove the screen if need be. Your main filter will catch anything of substance.

There should also be an Anti-Siphon valve at the fuel tank outelt fitting.
This valve is a spring-backed ball-check valve. It's job is to prevent an un-wanted fuel siphon in the event of a severed fuel line that would be below the fuel level. It will allow for fuel flow in one direction only, but only under fuel pump suction.
These can go bad.

If bad, replace it, as these are a USCG requirement.


And when this problem happens if I put it in neutral and give it gas it does not backfire at all.
Increasing RPM while in neutral (no load) is not going to tell you much.
 
I checked the distributor shaft for side to side movement and don't have any. . , I do have some rotational movement? Does the slight rotational movement mean I have a worn distributor? Or other internal issues?
 
Update. . . I found a manufacture tag on the motor that list points information, dwell, 26-31. . . Point gap 016. . , so now I am wondering if it has been converted? My motor serial number does not come up in any search when I try to look up parts. , serial B696439. I don't have any type of mechanical advance in the distributor just a rotor. , , motor model is 260 SKI . Could the distributor have been changed out and not converted? I don't see a module anywhere? Help!
 

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Ok

Looked at the service manual

YES you could have and do have a Mallory point ignition/distributor

If no points then it has been converted.

Under the point plate where the new electronic parts are mounted there are two counter weights controlled by springs....which determine the total advance and when it comes on.


The serial number does not come up on this site..........But if you look at the 1987 260 (only one listed for 1987) in the (all engine and drive models) entire list your serial number is within the correct range for 1987................

look here http://www.marineengine.com/parts/mercruiser-sterndrive-parts/260-gm-350-v8-1987

The manual for this does show the ski motor specs for different hP ratings.......
 
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That is indeed an old Mallory ignition distributor. The cap hold-down clamps are a dead-give-a-way.

If a red and black wire exit the housing, it's been converted to a Pertronix Hall Effect kit.
If a red, brown and green wire exit the housing, it's using Mallory's electronic triggering unit. Could be photo-eye, or could be VR.

Pull the cap and take a photo for us.


As per what kghost said earlier.... underneath the breaker plate you'll have the advancing cam and flyweight system.
The flyweights function centrifugally with returns springs to control the rate of advance as per engine RPM.
If either the flyweights or springs have become compromised, the advance will not function correctly.

.
 
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Can't thank you enough for all of the help. If you ever need some advice on your heating or air conditioning system I have 25 years in the field. [email protected]
 

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I did pull the breaker plate and inspect the weights and springs. , everything seems to move freely. . Could weak springs cause my intermittent problem? Can I replace the weights and springs? How hard is it to change to a different conversion brand as some think the pert ronin brand is sub par.
 
I did pull the breaker plate and inspect the weights and springs. , everything seems to move freely. . Could weak springs cause my intermittent problem? Can I replace the weights and springs? How hard is it to change to a different conversion brand as some think the pertronix brand is sub par.
 
That is indeed a Pertronix Hall Effect kit.

The Ignitor that you see, is the sensor or pick-up unit.
The black ring that fits over the old contact point cam, is the magnetic wheel.
This plastic ring is embedded with 8 magnets that pass the sensor.
As these pass the sensor, the igntion event is triggered.
This is what's called a Hall Effect.
Hall Effect is great.... but not how Pertronix does it, IMO.

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I did pull the breaker plate and inspect the weights and springs. , everything seems to move freely. .
1.... Could weak springs cause my intermittent problem?

2.... Can I replace the weights and springs?

3..... How hard is it to change to a different conversion brand as some think the pert ronin brand is sub par.
1... Even weak springs will be rather consistant.
The issues is with rust or corrosion that may have compromised their value.

2... No need to replace the flyweights. You can lubricate the flyweight system, and free them up if need be.


3.... The Pertronix is definitely sub par. No question about it, but I won't go into detail.
Unfortunately, there aren't many choices for a conversion with this housing.
Mallory's VR or Photo-Eye triggering system would be much better, but these will not fit into this housing.


All said..... when it comes to the flyweight return springs, you'll want to know how sensitive these are. The slightest change to their value, can have a large affect on the progressive advance curve.
Gasoline Marine Engine ignition advance is extremely critical that it be near dead on as per OEM specs.

If you were to change these springs, it would behoove you to have a Pro do this for you while using a Sun, Allen or King distributor machine. He will be able to calibrate it for you, but he'll need your OEM Marine Engine advance curve specs.

Fist consider the cost of doing this.
If budget allows, you'd be better off replacing the entire system, IMO.


.
 
No, the springs and counter weights only come into play under acceleration in order to advance the timing when under load.

Seeing as it is a aftermarket ignition I am not sure if it is your issue or not..............

As Chris suggested if the distributor shaft has excessive "wobble" it can cause the air gap between the sensor and pick up in the distributor to possibly loose its signal.....

Could be the pick up (part mounted to plate) is failing after time maybe a heat issue???

WHat is your engine temp?

this may be a tough one............hard to determine form the internet world

There should be a tag on the distributor it will have a part number and possibly a some other id info.

Here is a replacement made by Mallory for Mallory

look here, only a suggestion, not an answer to you issue.

Main page for choices. http://www.jegs.com/p/Mallory/Mallory-Unilite-Conversion-Kits/748645/10002/-1

Page for the one that may be for your distributor. http://www.jegs.com/i/Mallory/650/558/10002/-1?parentProductId=748645#moreDetails
 
I still like the kghost suggestion of a clogged vent pipe. If you are still running the boat, the next time this backfire happens you might try loosening (and then retightening) the fuel cap to relieve any vacuum. If the problem goes away then you know the vent is the problem. That is easier than me to believe than a timing issue that heals after 40-60 minutes of tooling around at low speeds. But then again, with boats the odd occurrence seems to be the norm. Also, if the problem only occurs when you start with a full tank of gas, that would also support the clogged vent suggestion.
 
If I want to pull and check the dip tube or outlet check valve/elbow does the rand need to be empty?
where is the vent tube located for me to check it also
 
Harley, the Mallory ignition distributor conversion kit that you've been linked to is the YLU...... Unilite.
These use Photo-Eye triggering.

The Mallory YLM is VR..... VR uses Magnetic triggering.

IMO, the VR will be the better of the two should you choose.


.
 
If I want to pull and check the dip tube or outlet check valve/elbow does the rand need to be empty?
where is the vent tube located for me to check it also



No dip tube??????? The "outlet" is a NPT fitting with a NPT to barbed fitting with a spring loaded check valve (ball) inside it.

On the end of the PICK UP TUBE as said there may be a screen............

The Vent.............

Walk around your boat, get to know your boat, SNIFF your boat, learn to love your boat. Learn what every dimple, bulge, ZIT on your boat is.


Once you have accomplished this then come back and ask that question over agian..................


My opinion.

If you dont know where the gas goes in and where the vent for that under floor gas tank is............Huston we have a problem............

Never mind the rest..........
 
Get to know your boat, kghost is Right on! You, & your family's safety depends on it!

As kghost mentioned, take off the FUEL TANK PICKUP TUBE counterclockwise with a wrench, check the screen on the bottom for debri. It does not matter if there's fuel in the tank , or not, it's on the top. Just don't smoke or make any sparks when doing this ;)
Then check the barbed anti-siphon valve by blowing into it, both sides, one side should sound like a whistle, the other side should be closed, it is a one way flow valve. When done check out the rest of your fuel system, all the lines, & filters. If it has a Quadrajet carb there is also a filter built into the carb to check?
 
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