Mercruiser 140 backfires and pops randomly when accelerating and when above 3000 rpms. Pease Help!

[Doug L.]

Mercruiser 140 backfires and pops randomly when accelerating and when above 3000 rpms. Pease Help!

I have a freshly rebuilt 1986 Mercruiser 140 4 cylinder engine (new bearings, pistons, bored cylinders, valve job). The mercarb carburetor has been professionally rebuilt by National Carb. I have the 3 ohm flamethrower coil with a jumper wire running directly from the battery to the (+) terminal of the coil. This is paired with a Pertronix Ignitor installed in my oem Delco distributor.

Now here is the extremely frustrating issue I am having. The engine idles beautifully at around 750 rpms in gear. However, when I accelerate, I get a lot of popping/backfiring from the carb but can move the throttle up and down quickly to get past it but other times it accelerates with zero hesitation and no popping. Then, once I get to around 3k-4k rpms it will randomly pop/backfiring through the carb again and will also experience the rpms going up and down like sudden hits of loss power. Sometimes bringing it back to idle when it pops then going back to 3k-4k it will run smoothly then the popping and power loss comes back.

This has been very upsetting as the engine has been completely gone through. Valve issue? I set them according to TM #10 for the "non-running" scenario. My timing is set a 6* BTDC at 700rpms. I have seen that maybe the shift interrupter could be an issue? Also, I tested the fuel pressure between the pump and carb and got about 4.7-4.8 PSI. Is this too low? Normally popping is due to a lean condition but my spark plugs are black with a dry black powder coating that makes me think it's rich or possibly a weak spark?

I'm really stumped and don't know what to do from here. I don't want to be replacing good parts with good parts. Any help or advice will be greatly appreciated!

Doug

 

[alldodge]

If your fuel pressure does not get any lower as the rpms increase then fuel pressure should be good

The backfire is classic lean condition, and the carb may also get to much fuel at times.

To see if its the shift interrupter, once on the water and clear of things, disconnect one side of the switch. Will need to shut the motor down to get back in neutral

 

[kenny nunez]

How much can you move the rotor shaft in the distributor side to side and did the engine rebuilder replace the distributor bushing above the oil pump?
With the boat in forward turn off the engine and advance the throttle all the way and look at the shift interrupter to check the position of the roller in the “V” notch. It should be in the center.
Did you have this problem before the rebuild?

 

[havoc_squad]

Here are the possibilities that need to be checked:

Wrong ignition timing
Carburetor set too lean
Faulty accelerator pump
Vacuum leak
Valve adjustment
Valve timing
Burned or stuck valve


If your timing range is spot on, sounds a lot like a fuel lean issue and not valves, especially since it acts up from idle to mid range rpm, then from mid range RPM to WOT.

A valve issue would probably manifest itself throughout the RPM range, the issue increasing as the RPM's get higher.

Also, what were the steps taken in the engine break in process?

 

[Doug L.]

Thank y'all for the thoughtful replies. I will verify tonight if the fuel pressure drops at higher rpms. I'll also verify the position of the shift interrupter. This problem was not present before the rebuild. Currently, I only have about 3 hours on the engine. First hour spent below 2k rpms on the water then below 3k for the second hour and now below 4k for the next 3 hours (I am varying rpms throughout this whole process).

I'll check the distributor and if it's advancing smoothly. My rebuilder did not replace a bushing above the oil pump (I had no idea one was even there).

When I first cranked her up, she would idle erratically then found out I had only 10 volts running to the coil and ignitor so I added a temporary 12 volt jumper wire from the battery and it idled perfectly after that.

As of right now, the air/fuel mixture screw is 1 1/4 turns out.

thanks,

Doug

 

[kenny nunez]

To tighten up the distributor it has to be taken apart and the upper bushing replaced and another added to the bottom. Any machine shop can do that. This fix will compensate for the bushing the rebuilder did not replace. Even if your distributor is not worn too much try to have the bushings replaced. If your distributor is worn it is constantly changing the distance between the reluctor and pick up coil which affects timing. If you still had points it would be even worse.
Depending on which Petronics kit you installed some require a resistor wire and the newer ones need 12 volts, do you still have the instructions for the one you installed?

 

[Doug L.]

Depending on which Petronics kit you installed some require a resistor wire and the newer ones need 12 volts, do you still have the instructions for the one you installed?

I have the pertronix 1146A unit and yes I still have the instructions. requires 12 volts with no resistor. How tight should the distributor shaft be? Like what would the side clearance need to be When I have my machinist make a bushing?

 

[Doug L.]

UPDATE:

I was looking around the carburetor and found the accelerator pump arm had pulled out halfway due to the set screw loosening. I took the carb apart and installed a new arm (had a spare as this has been an issue before) and set screw as the threads appeared pretty damaged. I used Loctite this time and am letting it dry overnight. Inside of carb bowl looked very clean. No contaminates or water present. I also marked where 31-35 degrees of total advance on the harmonic balancer to verify if the advance is working properly.

I noticed something kind of strange. There is a little bit of motor oil in the fuel pump sight tube going to the carb. Does this indicate the pump diaphragm is damaged or something? I have the Carter fuel pump (it's 4 years old).

Also checked distributor rotor shaft side play. there is some play but seems minimal. I will check it with a dial indicator if the problem persists.

Also, I looked at the shift interrupter and when I am in the full throttle position, the switch roller is in the center of the "V".

I will update again once I have carb back on.

 

[alldodge]

There is a little bit of motor oil in the fuel pump sight tube going to the carb. Does this indicate the pump diaphragm is damaged or something?

I don't see a problem, most likely oil vapor just collected there

 

[kenny nunez]

At this point you need to get 12 volts to the coil for the Petronics kit you have. Your distributor is probably still good and not the cause but it would be interesting how much wear it has.
Since you seem to have some spare carburetor parts check if you have a power valve spring.
I had a propeller shaft dynometer at my shop and had a 140 that was stumbling at 4400. After going over everything I noticed the engine leveled off if I just slightly closed the choke. It turned out that the power valve spring was the cause because at some time some water had rusted and weakened it. Replacing the spring cured the problem.
I realize that you cannot safely do this check with the boat wide open with someone closing the choke.

 

[Doug L.]

Good advice Kenny regarding the power valve spring! I hard wired (with soldered connections) a "keyed" wire from the ignition switch to take care of that today. I reinstalled the carburetor after adjusting the float drop and level, as well as the pump rod length. I attached a garden hose to the outdrive and ran it. idles very smoothly and easy start.

I checked the timing advance with a timing light and it appears very smooth and NOT erratic. At 2000 rpms it reaches its total advance which appears to be hitting around 39 degrees. According to my Service manual #10, it should be between 31-35 degrees. I marked where 31 and 35 would be on my damper using the arc length formula for a 6.875" diameter damper. It is definitely advancing further than spec. Is this adjustable or should the distributor be replaced?

Also, my fuel pump is hitting a solid 5 PSI at 1100 rpms but on hard acceleration, it drops to 4 PSI. Is this acceptable? When I slowly accelerate it stays at 5 PSI. Bad pump?

All of this was done with a flusher. Would being on water under load change these readings? There was no popping or backfiring but that occurs under load from what I've seen. Planning to take it on the water tomorrow.

 

[kenny nunez]

Take a look under the ignition mounting plate to inspect the centrifugal advance weights and springs. The advance is not affected with or without a load.

 

[Doug L.]

UPDATE:

I took the boat on the water again today and it started immediately from cold and allowed it to warm up. Idled with no issues. I accelerated and it was super smooth until I reached 1900-2000rpms where it would pop and backfire loudly. this was consistent throughout. I double checked total advance and it was hitting around 40-41 degrees BTDC. I was now convinced the advance mechanism was at fault here. Following Kenny's advice, I removed the ignition mounting plate and voila, the advance limit dowel and the hole surface it stops against were badly worn (see pics). This wear is allowing the lobe shaft to rotate too far and thus over advance. This would explain how my engine was destroyed before this rebuild (detonation destroyed the pistons before. Only 27 hours on the first build before this one).

PICS: https://photos.app.goo.gl/1Wk2k2EAzKnPx4wp9

Since I have so much invested in this engine, I plan to install a brand new distributor. Does anyone have any recommendations on a distributor? I've been looking at the Sierra 18-5486-2 from Summit: https://www.summitracing.com/parts/rra-18-5486-2

It's electronic and no points. Or should I source a good used Delco and transfer my Pertronix kit?

I really appreciate all the help y'all have provided. This has been a really big head-scratcher for me. Really glad to be finally getting it figured out!

Thanks,
Doug

 

[havoc_squad]

UPDATE:

I took the boat on the water again today and it started immediately from cold and allowed it to warm up. Idled with no issues. I accelerated and it was super smooth until I reached 1900-2000rpms where it would pop and backfire loudly. this was consistent throughout. I double checked total advance and it was hitting around 40-41 degrees BTDC. I was now convinced the advance mechanism was at fault here. Following Kenny's advice, I removed the ignition mounting plate and voila, the advance limit dowel and the hole surface it stops against were badly worn (see pics). This wear is allowing the lobe shaft to rotate too far and thus over advance. This would explain how my engine was destroyed before this rebuild (detonation destroyed the pistons before. Only 27 hours on the first build before this one).

PICS: https://photos.app.goo.gl/1Wk2k2EAzKnPx4wp9

Since I have so much invested in this engine, I plan to install a brand new distributor. Does anyone have any recommendations on a distributor? I've been looking at the Sierra 18-5486-2 from Summit: https://www.summitracing.com/parts/rra-18-5486-2

It's electronic and no points. Or should I source a good used Delco and transfer my Pertronix kit?

I really appreciate all the help y'all have provided. This has been a really big head-scratcher for me. Really glad to be finally getting it figured out!

Thanks,
Doug

Go with marine Delco EST kit, they are widely available in the aftermarket category and should be built the same as the Mercruiser Delco EST parts.

Additionally, that should allow you to use the Mercruiser #26 manual's troubleshooting steps for the Delco EST ignition system.

I cannot say which manufacturer of this type of kit is better or worse, you might have to do some more digging on that.

I do recommend that you do a compression check on the cylinders before going any further.

If you have detonation damage due to too many times/duration of excessive ignition timing, you want to deal with it now, not later.

 

[kenny nunez]

I wish I would have suggested that earlier. I once had a distributor that the flyweights almost cut through the inside of the distributor body. Since I closed my shop in 03 I forget some of the unusual things that kept me scratching my head.
Hopefully the engine is not hurt .

 

[Doug L.]

I do recommend that you do a compression check on the cylinders before going any further.

If you have detonation damage due to too many times/duration of excessive ignition timing, you want to deal with it now, not later.

I really hope it has not damaged the pistons. it took 27 hours to destroy it before. Hopefully 2 hours of light driving has not done the same. I performed a compression test prior to starting after this second engine rebuild so I'll have something to compare to. This evening I''ll perform a comp test and also verify the condition of the pistons with a borescope. I don't think I have it in me to rebuild a 3rd time within less than 12 months..

Thank you for the distributor advice. Gonna look into it.

I'll update.

 

[Bondo]

I don't think I have it in me to rebuild a 3rd time within less than 12 months..

Ayuh,...... Welcome Aboard,..... What killed the motor each time,..??..??

 

[Doug L.]

Ayuh,...... Welcome Aboard,..... What killed the motor each time,..??..??

Hi Bondo, the first rebuild was due to spinning the number 1 rod bearing but the engine was already worn out even then (first rebuild in 25 years). The second time was due to detonation that was suspected to be due to improper carburetion, week ignition due to not enough voltage supplied to coil and pertronix kit, and wrong size intake manifold gasket supplied by builder.

 

[Doug L.]

Today I inserted a borescope into the cylinders and.. I cannot believe my eyes. I am afraid I have destroyed my engine again. there is pitting on the outer edges of pistons 1, 2, and 4. Piston 3 looks ok as it was the only cylinder that had oil in it (leaking valve guide?) and the other three were dry. I have pictures in the link below of the pistons. What should I do from here? THERE ARE ONLY 2 HOURS ON THE ENGINE. Should I pull the head to inspect the damage? Is it possible the pitting won't be too bad to run with a new distributor installed correcting the over advance?

I performed a COLD compression test below today in the order of Cyl 1, 2, 3, 4 respectively:
DRY: 160, 160, 168, 156
WET: 235, 205, 208, 250

Below are the readings PRIOR to running it after the rebuild:
DRY: 145, 160, 155, 145
WET: 150, 190, 180, 183


PICS: https://photos.app.goo.gl/LwfjtCvS3GM8xQb86

 

[Bondo]

there is pitting on the outer edges of pistons 1, 2, and 4.

Ayuh,...... You obviously haven't fixed the detonation yet,......