Mercruiser 4.3LH backfiring & weird timing

[Kzreelrods]

What’s my next move?

Engine: 2000 Mercruiser 4.3LH 500+ hours thunderbolt 5 ignition. Weber 4 bbl carb.

Issue: mid summer the engine started bogging at higher rpms and while accelerating under load and would backfire through carb if accelerating too fast. Worse while engine was cool, but still present when fully warmed up. Was running strong during many outings prior to the trip where this started happening.

I checked the timing and it seemed way too advanced, so I brought it to 10 deg BTDC and it ran really bad… backfiring at the slightest bit of throttle. Used proper timing procedure / base timing mode etc.

Wound up timing it by ear till it seemed to run best and it got me through a fishing trip but still not full power, boggy under load, and an occasional backfire while accelerating. Idles fine… starts a little harder than normal.

It was 60+ degrees BTDC (past my dial on my timing light) at idle in this condition. Total head scratcher.

Here’s what I’ve done so far, testing and adjusting timing repeatedly throughout.
- cleaned / rebuilt the carb.
- changed fuel/water separator filter
- replaced fuel line from tank.
- confirmed breather and anti siphon valves ok.
- changed plugs and wires.
- verified spark from all cylinders.
- removed one plug wire at a time to see if certain cylinders were causing the problem… no differences noted.
- verified top dead center mark on balancer is correct (piston stop tool) and observing rotor position at the same time.
- Confirmed correct firing order / plug wire routing, etc.
- incidentally discovered one of my two batteries were bad so I changed the bad one (age 5 yrs).
- verified alternator voltage at 13.8 ish volts to battery when engine running.
- replaced rotor/cap and coil… no change…
- thinking it could be the ignition module I upgraded ignition to Delco EST ignition.

No change… it wants to run with the distributor very advanced and still bogs / backfires even when the distributor seems to be in the optimal position. Idles fine. Starts a little harder than normal.

What’s my next move?

- remove valve covers and look for stuck valve, bent push rod?
- clean and reseat wires on engine ground stud, etc.?

I’m all ears… thanks in advance for any sage advice.

 

[stresspoint]

i don't see any mention of a compression test.
a couple of mushroomed valves will cause a backfire and timing to need advancing to achieve proper acceleration.

lifters pumping up under acceleration or worn camshaft lobes can also make a engine run sloppy under load.

looks as if you have been thorough in diagnostics , time to start looking for a mechanical issue.

 

[Kzreelrods]

Thx. That makes sense. Compression test will be next.

 

[stresspoint]

have you done a quick stop test and pulled the plugs to check burn patterns , that can be done during comp testing if you choose.
turn the key off when running WOT or at the point its backfiring.

 

[Scott Danforth]

agree on compression numbers

generally if the timing appears to have moved that much, usually the outer ring of the damper is no longer bonded. use a cylinder stop and degree wheel to indicate TDC. if the damper ring has shifted, get a new damper.

 

[markhodges78]

Are we sure that we are on number 1 cylinder and also have we verified the balancer hasn't spun?

 

[markhodges78]

Lol hey guys look at the time stamps on the post

 

[Kzreelrods]

agree on compression numbers

generally if the timing appears to have moved that much, usually the outer ring of the damper is no longer bonded. use a cylinder stop and degree wheel to indicate TDC. if the damper ring has shifted, get a new damper.

Are we sure that we are on number 1 cylinder and also have we verified the balancer hasn't spun?

100% sure I was measuring #1 cylinder and honestly I was surprised to find that the mark is in fact at top dead center. I was convinced that the mark was off. I replaced the balancer about a year ago also.

 

[nola mike]

100% sure I was measuring #1 cylinder and honestly I was surprised to find that the mark is in fact at top dead center. I was convinced that the mark was off. I replaced the balancer about a year ago also.

Are you also sure that you're on the compression stroke?

 

[Kzreelrods]

Ok… compression test results (wish I would have done this sooner). Pretty clear where the problem lies.

Cylinder 1: 165
Cylinder 3: 20 (barely moved the needle)
Cylinder 5: 170
Cylinder 2: 20 (barely moved the needle)
Cylinder 4: 180
Cylinder 6: 180

What are the possibilities? Stuck exhaust valve?

 

[Scott06]

Ok… compression test results (wish I would have done this sooner). Pretty clear where the problem lies.

Cylinder 1: 165
Cylinder 2: 20 (barely moved the needle)
Cylinder 3 : 170
Cylinder 4 : 20 (barely moved the needle)
Cylinder 5: 180
Cylinder 6: 180

What are the possibilities? Stuck exhaust valve?

If its backfiring it is intake valve . can put air pressure in cylinder at tdc on compression stroke and see where it comes out most important to find out what happened and why is it running lean and tulipped valves detonation etc.

if your timing was too far advanced could be detonation. Pull heads and see what you have going on.

 

[Scott Danforth]

I'm going to go with.....

salt water boat and rusted valves from leaking manifold to riser gasket

Or

Fresh water boat and freeze busted manifolds

 

[Kzreelrods]

I'm going to go with.....

salt water boat and rusted valves from leaking manifold to riser gasket

Or

Fresh water boat and freeze busted manifolds

Not bad guesses…

Was a saltwater boat but has been in freshwater only since 2019. I had the port manifold off and didn’t see anything remarkable… gaskets were intact. I replaced the manifolds and risers several years ago.

It has never seen freezing temps (stored in heated shop).

The issue did come on suddenly… one day running strong, pulling kids on tube, wakeboarding, etc. A week later on a vacation and it starts bogging and popping.

 

[nola mike]

do you have a knock sensor?

 

[Scott Danforth]

Bet the manifold/riser gasket surface is rusted

You will find out as you pull the top end

 

[Kzreelrods]

Ok… I’m in deep now… got the starboard side pulled apart tonight. Will pull the valves on the problem cylinder tomorrow or this weekend.

Yesterday I repeated compression test on this cylinder (farthest right) after squirting some oil into the cylinder to see if it might be a piston/ring issue. Still no compression.

This is new territory for me, so what do you old pros see on initial reaction?

I see some kind of irregularity at 2:00 on the exhaust valve. Will see how it looks when I get it off. Also some sludge/buildup on the corresponding piston and maybe a spot of rust at 6:00?

 

[Scott Danforth]

Looks like there is a witness mark on the exhaust valve in the last pic

 

[Scott06]

Looks like there is a witness mark on the exhaust valve in the last pic

Yes and around the circumference of piston looks pitted ? Not sure if that is detonation marks?

center piston says 030 on it which normally would imply a 030 overbore on rebuild but other pistons have no such marks

 

[stresspoint]

it could be a combination of that dodgy area of the valve and or broken rings.
the piston looks to have a bit of detonation but its hard to say with the pic ,it could just be carbon build up ??. broken rings can show no comp when oil is put in the cylinder , but lets not get to that yet.
i would be interested in seeing the other side when you get it apart , and what the under side of that valve looks like when you get it out before commenting more.

 

[Kzreelrods]

Here’s that valve. Clearly it’s been flattened a bit on one side. Will pull the other side later this weekend or next week.