v1_0
Chief Petty Officer
- Joined
- Aug 27, 2007
- Messages
- 575
1989 Mercruiser 4.3L w/Alpha one.
I've been having some backfire issues - posted about it previously, this is a follow up.
The symptoms: boat ran fine at low to mid rpms. When I would go to WOT, the boat would run a bit (a few minutes) then loose power and backfire. It would continue to run badly until I went to a lower speed (1000-1500 rpm) then everything would clear up and I could go back to running at mid rpm (2500-3000).
So, I read a lot of posts and asked some questions that pegged the problem as the fuel system. Replaced the fuel filter - no luck. Sprayed carb cleaner in the carb - no luck. Cleaned the anti-siphon valve and made sure that the vent in the gas tank worked - no luck. Drained the gas tank, cleaned it out. Got a bunch of 'flakes' of something (previous bad gas?), used carb cleaner and lots of 'swishing' , waiting, and draining to clear it out. Put the tank back on, put in brand new gas, went to the lake - the backfiring was now worse!
So I bought a fuel pressure gauge and geared up for some on the water debugging of the fuel system. However, I was having a nagging concern about the spark plugs.
When I got the boat, not knowing anything differently, I went by the Clymer manual. It said to use a RS12YC plug with a 0.045 inch gap. I had since learned that the recommended plug was an MR43T with a 0.035 gap.
I was worried that the gap may be the issue - so I decided to check a few plugs. The first plug I pulled was FRIED (picture attached). The center electrode was 'burned' to below the insulator, the other electrode was burned to almost being a straight wire. I guess the thunderbolt ignition certainly spewed lightning on that plug. This was the middle right side (standing in front of boat looking back) cylinder. One other plug was baked like this - the left back one. The other plugs actually looked sooty as if the engine was running rich.
I replaced the plugs with the correct ones - gapped to the 0.035 inch specification - and the backfiring went away. I also did a compression test on the mid right cylinder to make sure there wasn't a hole in the piston. Now it runs great, no backfire!
Here's my analysis of what happened:
The 'flakes' in the tank were the initial problem. They were just the right size to clog the strainer in the intake tube instead of passing through it. They were heavy enough to fall away from the tube when there was a 'lower' suction from the intake tube - such as when the boat was running at lower rpm.
Basically they would get sucked up in a couple of minutes when I was running at WOT and "lean out" the mixture, then they would stay on the intake screen until the engine wasn't pulling as hard for gas - and they would go back to floating.
Meanwhile: spark plug gap was larger than it was supposed to be, which increased the voltage needed to make an arc, which increased the temperature/power of that arc. I suppose that the ocasional lean mixture (I would go to WOT once per trip to check if I had fixed the issue) would have the plugs running at a hotter temperature too. The higher energy/higher temp combination started the errosion of the plugs - which just got worse over time as the gap got larger due to the errosion. It got self sustaining in the two cylinders - I'm guessing that they were gapped a wee bit more than the others (human variance during the gapping procedure).
Does this sound about right? The engine seems to run pretty good now - but should I be worried about anything? I just did the initial winterization (got the water out), and will be pulling the outdrive later for maintenance. I might catch a nice day to go back out on the water during the next month or so. After that, I'll go into maintenance mode (pull outdrive, really winterize, work on boat seats, etc). Should I take a look at the valves/cylinders too?
Thanks,
-V
I've been having some backfire issues - posted about it previously, this is a follow up.
The symptoms: boat ran fine at low to mid rpms. When I would go to WOT, the boat would run a bit (a few minutes) then loose power and backfire. It would continue to run badly until I went to a lower speed (1000-1500 rpm) then everything would clear up and I could go back to running at mid rpm (2500-3000).
So, I read a lot of posts and asked some questions that pegged the problem as the fuel system. Replaced the fuel filter - no luck. Sprayed carb cleaner in the carb - no luck. Cleaned the anti-siphon valve and made sure that the vent in the gas tank worked - no luck. Drained the gas tank, cleaned it out. Got a bunch of 'flakes' of something (previous bad gas?), used carb cleaner and lots of 'swishing' , waiting, and draining to clear it out. Put the tank back on, put in brand new gas, went to the lake - the backfiring was now worse!
So I bought a fuel pressure gauge and geared up for some on the water debugging of the fuel system. However, I was having a nagging concern about the spark plugs.
When I got the boat, not knowing anything differently, I went by the Clymer manual. It said to use a RS12YC plug with a 0.045 inch gap. I had since learned that the recommended plug was an MR43T with a 0.035 gap.
I was worried that the gap may be the issue - so I decided to check a few plugs. The first plug I pulled was FRIED (picture attached). The center electrode was 'burned' to below the insulator, the other electrode was burned to almost being a straight wire. I guess the thunderbolt ignition certainly spewed lightning on that plug. This was the middle right side (standing in front of boat looking back) cylinder. One other plug was baked like this - the left back one. The other plugs actually looked sooty as if the engine was running rich.
I replaced the plugs with the correct ones - gapped to the 0.035 inch specification - and the backfiring went away. I also did a compression test on the mid right cylinder to make sure there wasn't a hole in the piston. Now it runs great, no backfire!
Here's my analysis of what happened:
The 'flakes' in the tank were the initial problem. They were just the right size to clog the strainer in the intake tube instead of passing through it. They were heavy enough to fall away from the tube when there was a 'lower' suction from the intake tube - such as when the boat was running at lower rpm.
Basically they would get sucked up in a couple of minutes when I was running at WOT and "lean out" the mixture, then they would stay on the intake screen until the engine wasn't pulling as hard for gas - and they would go back to floating.
Meanwhile: spark plug gap was larger than it was supposed to be, which increased the voltage needed to make an arc, which increased the temperature/power of that arc. I suppose that the ocasional lean mixture (I would go to WOT once per trip to check if I had fixed the issue) would have the plugs running at a hotter temperature too. The higher energy/higher temp combination started the errosion of the plugs - which just got worse over time as the gap got larger due to the errosion. It got self sustaining in the two cylinders - I'm guessing that they were gapped a wee bit more than the others (human variance during the gapping procedure).
Does this sound about right? The engine seems to run pretty good now - but should I be worried about anything? I just did the initial winterization (got the water out), and will be pulling the outdrive later for maintenance. I might catch a nice day to go back out on the water during the next month or so. After that, I'll go into maintenance mode (pull outdrive, really winterize, work on boat seats, etc). Should I take a look at the valves/cylinders too?
Thanks,
-V
