Kzreelrods
Cadet
- Joined
- Oct 8, 2014
- Messages
- 14
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.
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.