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

[Doug L.]

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

You are correct. it has only just now been determined the distributor has been over advancing and is the cause of the detonation.

 

[alldodge]

Outside of needing a better gauge (readings are high)

Previous

31 to 35 total advance

Total advance should only be 21 to 23 degrees

How are you measuring total
Start at 6 BTDC - 35 = 29 BTDC ??

 

[Doug L.]

Total advance should only be 21 to 23 degrees

How are you measuring total
Start at 6 BTDC - 35 = 29 BTDC ??

The 31 to 35 INCLUDES my initial timing of 6 degrees BTDC. I apologize for not noting that. So according to my Service manual #10, total advance should be 25 to 29 degrees BTDC not including the initial 6 degrees. Add 6 to 25 and 29 and that's where I was getting 31 and 35.

 

[alldodge]

Yes your correct, and I should have added and not subtracted

If your compression is that high (manual list 140 psi) then it could be your getting detonation because of high compression

Might have to go to air fuel ration O2 sensors to get the tuning correct

 

[kenny nunez]

Cometic makes custom head gaskets that will lower the compression. You might get away with that instead of going through the engine again.

 

[Doug L.]

UPDATE:

I decided to go ahead and pull the head and was pleasantly surprised to find my PISTONS AND CYLINDERS ARE PERFECTLY FINE. No damage at all. There is a thick layer of carbon build up on cylinders 1, 2, and 4 which has a pitted textured that made it appear the pistons themselves were pitted. Removing the head was not in vain. Cylinder 3 has oil leaking around the valve guides which can now be addressed.

PICS: https://photos.app.goo.gl/Q6fLWDLN6M4KxvLr6

Is the high compression due to the cylinders being a little too tight? The head gasket I had installed came from Fel-Pro's marine kit part # 17200. Maybe it is thinner than OEM? NOTE: The gauge I was using is a cheap one I bought on eBay several years ago so alldodge might be right about checking with a better gauge.

Cometic makes custom head gaskets that will lower the compression. You might get away with that instead of going through the engine again.

Hi Kenny, I will definitely go this route before pulling the engine again. Sounds like it could work.

Thanks,
Doug

 

[alldodge]

Is the high compression due to the cylinders being a little too tight?

No, compression might be from a bad gauge. Check it with a known pressure like from an air compressor

The quench of the motor should be close to 40, so choose a gasket thickness based on it
https://www.hotrod.com/articles/ideal-quench-height/

 

[Doug L.]

The quench of the motor should be close to 40, so choose a gasket thickness based on it

40 as in close to .040"? The compressed thickness of my current head gasket (felpro 17000) from the manufacturer is rated as .047"

I verified my compression gauge on my compressor with a regulator and it is pretty darn spot on.

 

[alldodge]

Yes, .040 for quench
The stock motor is not set up very well. Everything is fat, and this is why doing a few small things during rebuilds helps. Many zero deck the block and use a .040 head gasket which puts the quench spot on. There are other things to consider like combustion chamber volume and type of piston.

You have way to much compression

 

[Doug L.]

I really appreciate your insight alldodge. I do have a question though. If I shoot for a quench of .040, wouldn't that increase my compression more than it is since my current quench is already over .047 with my stock style head gasket from which I obtained the above compression readings? Or is the higher compression ok as long as the quench height is ok?

Thanks

 

[alldodge]

Yes, need to find out whay your compression is so high. I would not change the thickness of the gasket until you know what is causing the high numbers. In most cases there is liquid in the cylinder (water, oil)

 

[Doug L.]

alldodge,

I measured the distance from the top of my #1 piston at TDC to the deck and got .034" which would mean I would need a .006" thick head gasket (or shim) to reach the desired quench. As of currently, I sit with a .081" quench with my stock gasket which seems like a lot (I used a dial indicator for these measurements and verified my dial's accuracy).

I removed excessive carbon build-up from the pistons and at the top of the cylinders (it was thick enough to catch my nail on them). Cylinder 3 was higher because of oil leaking from the valve guides so I understand that reading. Does reinstalling the head and running the engine at idle until operating temp then shutting off and performing a DRY comp test with an alternate gauge sound like a good idea to verify the compression before resorting to a custom head gasket?

Thanks

 

[alldodge]

Don't know Scott Danforth have any ideas why compression s so high

 

[Scott Danforth]

Top 3 reasons to have compression higher than 150 max on a 3.0

Bad gauge
way too much oil added to the cylinders for the wet test
Wrong pistons
or someone milled tge head too much

For a wet test literally a single 1 second blast of wd40 is all that is needed

Also, i believe the mecanical advance for a 3.0 is only 20 degrees, so base at 6 BTDC would be 26 total at 2700-2900 rpm

 

[Doug L.]

Here's the latest,

I reinstalled the head and warmed it up to operating temp then performed a dry compression test in the order of Cyl 1, 2, 3, 4 respectively:

DRY: 160, 158, 160, 152

Still a little high I verified my compression gauge is reading accurately. I installed a beautiful NOS Delco distributor brand new from the 80s and transferred my pertronix kit. TOTAL ADVANCE (including initial timing of 6*) is hitting about 30-31*, much better than 40*. Scott, according to my Service manual 10, it should be 25-29 degrees mechanical advance, not 20?

My spark plugs are still black and there is a small amount of blue/white smoke from the exhaust. The smoke I assume is from Cyl 3's valve guides but maybe all the valve guides are leaking a little oil making the readings higher? Either way, If one set is leaking oil, should probably have all of the guides checked.

The pistons I have installed are from Sealed Power H896CP.

I suppose right now I'll have the guides repaired (there's a noticeable amount of slop in them). Hopefully that will help lower my compression. If it does not, should I still consider a thinner head gasket? If so, is a .006" thick head gasket/shim unheard of? Just trying to avoid pulling the engine but may have no choice.. Thing is, the engine idles beautifully like a sewing machine. I just don't want to have it destroyed by detonation again.

Pics of the new distributor and current spark plug color: https://photos.app.goo.gl/7PDVeJrUik6fuuFn8

Thanks y'all

 

[alldodge]

I wouldn't go thinner on the head gasket

If all your plugs stay black after a run then this will help keeping down on the detonation
Could set timing closer to 0* to reduce the total advance. This will also reduce the possibility

 

[Doug L.]

Thanks alldodge.

I'll have all my valve guides checked and bushings installed as needed and go from there. I'll likely play with timing closer to 0 and just run it. I've spent a large part of the summer already with this engine and am ready to enjoy it.

 

[Doug L.]

UPDATE: PROBLEM SOLVED

Had to put the head repair on hold for a few weeks but finally got it done. Installed custom valve stem umbrella style seals at my machine shop to fix the oil leaking into the combustion chambers(also new head gasket, stock thickness). Engine started right up and brought to operating temp. I performed a DRY compression test immediately and got the following (lower) readings in the order of Cyl 1, 2, 3, 4 respectively:

DRY: 154, 155, 152, 147

Took the boat out to the lake and set the initial timing closer to zero and went with it. It Idled fine and would run smoothly around 3800-4000 rpms BUT it backfired HORRIBLY on acceleration and would randomly sputter between 2200-3500 rpms. Played with timing and no change. I was at an absolute loss on what to even do anymore.

I decided that maybe I was having a weak ignition issue as my plugs were still very black even with the idle screw nearly all the way in. Verified 12V were coming to the Pertronix unit. So I swapped out the pertronix unit with the original points and condenser assembly. Engine fired right up and idled EVEN SMOOTHER. The acceleration was extremely responsive throughout its entire range and ran better than it ever has in 4 years! My Pertronix 1146A unit was FAULTY!

I really appreciate y'alls help and advice with this issue. No single thing is to blame here though as the carburetor had issues, the mechanical advance was over advancing, and the pertronix unit suffered from sub 12V for an extended period of time leading to it malfunctioning.

I am very pleased to have a humming engine now and no backfiring or detonation. I'll still get some use out of it before it gets cold here in MS. I can finally complete the break-in period.

Cheers,
Doug

 

[alldodge]

Sounds good, never cared for the Pertronix, much prefer Delco EST for a conversion