Home |
Search |
Today's Posts |
#1
posted to rec.boats
|
|||
|
|||
Engine stumble, backfire, boat won't plane...
Hi All,
This is a 4.3 V6 Merc newly rebuilt. EST, magnetic points. Boat is 19 ft B/R with 19P Laser II prop. Idles OK, goes to 2000 rpm and that's all. When trying to accelerate above that it backfires and stumbles, rpm drop to below 1500. You back the throttle off and it speeds up (but not above 2000 rpm). It won't plane. Pushing the throttle all the way will cause carb and exhaust backfire and stumble with rpm drop but it won't die (accelerates and decelerates on its own erratically). Wires are OEM Merc old but seem good, distr. cap had a tiny (!) crack (from constant manipulation) on the outside of no.1 collar but I covered it with lots of epoxy (no.1 fires OK). Timing 4-8 deg. before TDC (not much change). Idle at 550 is OK up to 2000 in neutral (never tried above unless in gear). Problem appears at higher rpm. Don't wont to kill freshly rebuilt engine... How to test all ignition components? Could this be fuel starvation (pump or fuel tank vent)? Any ideas? I also have slight clicking when turning prop dry (especially forward), less loud in reverse (sounds like lower unit ...) Much louder in the water though (knocking - ticking) when in gear. Could this be maladjusted linkage? Oil level was OK when checked last time (not recently). Any reason to be worried? |
#2
posted to rec.boats
|
|||
|
|||
Engine stumble, backfire, boat won't plane...
To start at the end, alphas click when the prop is turned and they are
in gear. That's the gears skipping. Alphas use the input torque from the engine to hold the gears in mesh. When you apply pressure from the prop the beveling on the gears works the opposite way and forces them apart. Completely normal. A crack in your cap means replace it. Not epoxy. If the wires are old replace them too. When you say magnetic points you mean it has the merc thunderbolt ignition? It does seem like an ignition problem. Does the ignition advance when you watch it with a timing light while someone revs the engine? You should be able to see the mark swing away with the light. Look down the carb throat with the engine off and make sure you are getting a good squirt of gas from the accelerator pump when someone moves the throttle. Paul wrote: Hi All, This is a 4.3 V6 Merc newly rebuilt. EST, magnetic points. Boat is 19 ft B/R with 19P Laser II prop. Idles OK, goes to 2000 rpm and that's all. When trying to accelerate above that it backfires and stumbles, rpm drop to below 1500. You back the throttle off and it speeds up (but not above 2000 rpm). It won't plane. Pushing the throttle all the way will cause carb and exhaust backfire and stumble with rpm drop but it won't die (accelerates and decelerates on its own erratically). Wires are OEM Merc old but seem good, distr. cap had a tiny (!) crack (from constant manipulation) on the outside of no.1 collar but I covered it with lots of epoxy (no.1 fires OK). Timing 4-8 deg. before TDC (not much change). Idle at 550 is OK up to 2000 in neutral (never tried above unless in gear). Problem appears at higher rpm. Don't wont to kill freshly rebuilt engine... How to test all ignition components? Could this be fuel starvation (pump or fuel tank vent)? Any ideas? I also have slight clicking when turning prop dry (especially forward), less loud in reverse (sounds like lower unit ...) Much louder in the water though (knocking - ticking) when in gear. Could this be maladjusted linkage? Oil level was OK when checked last time (not recently). Any reason to be worried? |
#3
posted to rec.boats
|
|||
|
|||
Engine stumble, backfire, boat won't plane...
Secondary ignition breakdown. Look for cracked plugs, arcing plug wires
or cracked/carbon tracked dist. cap/rotor. Also, possible open circuit in dist. pickup wires when advance plate moves with RPM. JR Paul wrote: Hi All, This is a 4.3 V6 Merc newly rebuilt. EST, magnetic points. Boat is 19 ft B/R with 19P Laser II prop. Idles OK, goes to 2000 rpm and that's all. When trying to accelerate above that it backfires and stumbles, rpm drop to below 1500. You back the throttle off and it speeds up (but not above 2000 rpm). It won't plane. Pushing the throttle all the way will cause carb and exhaust backfire and stumble with rpm drop but it won't die (accelerates and decelerates on its own erratically). Wires are OEM Merc old but seem good, distr. cap had a tiny (!) crack (from constant manipulation) on the outside of no.1 collar but I covered it with lots of epoxy (no.1 fires OK). Timing 4-8 deg. before TDC (not much change). Idle at 550 is OK up to 2000 in neutral (never tried above unless in gear). Problem appears at higher rpm. Don't wont to kill freshly rebuilt engine... How to test all ignition components? Could this be fuel starvation (pump or fuel tank vent)? -- -------------------------------------------------------------- Home Page: http://www.seanet.com/~jasonrnorth |
#4
posted to rec.boats
|
|||
|
|||
Engine stumble, backfire, boat won't plane...
JR North wrote: Secondary ignition breakdown. Look for cracked plugs, arcing plug wires or cracked/carbon tracked dist. cap/rotor. Also, possible open circuit in dist. pickup wires when advance plate moves with RPM. JR Spark advance is controlled by the ignition amp... isn't it? No moving parts in the distributor only rotor. Replaced dist. cap and rotor, will test tomorrow. Still wondering about fuel delivery especially fuel pump. Spoke today with pump rebuilder and he says if the pump is good to 2000 rpm chances are it should also be good above that (no leaks etc.). Coil spark is inch long, clean and blue but if ran through the plug wires to the screwdriver becomes purple-blue with (yellow from the screwdriver to the ground). but still jumps over 1/4 inch. At the plug it is purple. Disconnected tach wire and will remove shut-off wire from the coil. Would be great to have ignition tester like Mallory or MSD to take ignition through paces without running the engine. Pretty annoying. Carb has been rebuilt 6 weeks ago, accelerator pump works OK. I was also asked about the prop pitch if not too high for this boat/engine but 19P Laser II should be OK, don't you think? |
#5
posted to rec.boats
|
|||
|
|||
Engine stumble, backfire, boat won't plane...
After browsing through the online resources I would have to slightly
correct myself. Proper term for what I experience beside stumble is probably "detonation" rather then backfiring. At WOT (measly 2000 rpm) the engine detonates like a cannon. In the water it's not too bad but on land, boy...sounds like a shooting range.. I'm using (as stated) BR6FS (couldn't get AC MR43T). The equivalents that I found in Merc manual are AC-MR43T = Champion RV15YC4 = NGK BR6FS... Is that your experience guys? |
#6
posted to rec.boats
|
|||
|
|||
Engine stumble, backfire, boat won't plane...
SORRY FOR MULTILE POSTINGS!
(BUT OTHERS MAY STILL BENEFIT FROM THAT AS WELL) This is what I found while researching ignition problems: http://www.outdrives.com/precisionparts/263.html: Quote: "Outdrives.com Ignition Modules Outdrives.com! stocks most of the Mercruiser electronic ignition modules". and then: "All modules must be properly installed or they will fail. Over tightening of the mounting screws is a common mistake. Most Mercruiser Distributors and Modules have a ground wire attached to the housing. Modules DO go bad from time to time. Symptoms can range from not getting over 2500 RPM's to the engine not being able to start at all." Could this be it? The description of the potential problem sounds unpleasantly familiar... |
#7
posted to rec.boats
|
|||
|
|||
Engine stumble, backfire, boat won't plane...
Detonation is a secondary ignition of the mixture during the
compression stroke. It usually sounds like marbles rattling in a can. It is usually caused by too high a compression, or too much advance. Pre-ignition is a similar problem where the mixture is simply detonating before the spark happens. Sometimes these are accompanied by "run on" where the engine will keep running after you kill the ignition. You still have not said exactly what ignition system you have. The merc thunderbolt electronic ignitions do have the advance built in electronically. As I mentioned before if you have set the timing with a timing light you should be able to rev the engine and watch the mark move away as the timing advances. There are other electronic ignitions that still use a mechanical advance in the bottom of the distributor. ( In truth the merc systems starts out retarded and remove the retard as the rpm increases. If you think about it you realize that the ignition cannot fire the spark before the pickup gets triggered.) Prop pitch will not cause your problem. The wrong plug can cause detonation but only after the engine has gotten hot. But it does not really sound to me like you have detonation. Fuel pump problems at high rpm typically initially accelerate and run normally for a half minute or so and then just die out or fall back as the fuel in the carb is exhausted and there is not enough being supplied by the pump. Since this is a rebuilt engine can we assume it has not run right yet? I would replace the cap, rotor, and wires. Recheck the wires to confirm they are all going to the correct cylinders. Check the timing again and watch for advance to occur as the engine revs. Run it at night and look at the wires for arcing. It is much easier to see in the dark. At this point if it's still acting up I would do a compression test. A stuck intake valve could cause this problem. As well as improperly adjusted valves. A compression test will find those problems. I've never heard of a problem cause by overtightening the flanges on the thunderbolt ignition module. Mine just look like plain old flanges. The thunderbolt module is pretty reliable if that is what you have. I think because it is a marine environment the engineers used some pretty overkill ratings on the components. You can always try one but once you get to swapping ignition moduels and carbs it starts getting pricey. That loud a noise as you describe it sounds to me like you are heaing the mixture fire off through the intake. Or possibly out the exhaust since it is a lot louder on land. I would see if I could tell if the noise is coming back through the intake or going out the exhaust. Paul wrote: SORRY FOR MULTILE POSTINGS! (BUT OTHERS MAY STILL BENEFIT FROM THAT AS WELL) This is what I found while researching ignition problems: http://www.outdrives.com/precisionparts/263.html: Quote: "Outdrives.com Ignition Modules Outdrives.com! stocks most of the Mercruiser electronic ignition modules". and then: "All modules must be properly installed or they will fail. Over tightening of the mounting screws is a common mistake. Most Mercruiser Distributors and Modules have a ground wire attached to the housing. Modules DO go bad from time to time. Symptoms can range from not getting over 2500 RPM's to the engine not being able to start at all." Could this be it? The description of the potential problem sounds unpleasantly familiar... |
#8
posted to rec.boats
|
|||
|
|||
Engine stumble, backfire, boat won't plane...
"jamesgangnc" wrote in message oups.com... Detonation is a secondary ignition of the mixture during the compression stroke. It usually sounds like marbles rattling in a can. It is usually caused by too high a compression, or too much advance. Pre-ignition is a similar problem where the mixture is simply detonating before the spark happens. Sometimes these are accompanied by "run on" where the engine will keep running after you kill the ignition. You still have not said exactly what ignition system you have. The merc thunderbolt electronic ignitions do have the advance built in electronically. As I mentioned before if you have set the timing with a timing light you should be able to rev the engine and watch the mark move away as the timing advances. There are other electronic ignitions that still use a mechanical advance in the bottom of the distributor. ( In truth the merc systems starts out retarded and remove the retard as the rpm increases. If you think about it you realize that the ignition cannot fire the spark before the pickup gets triggered.) Prop pitch will not cause your problem. The wrong plug can cause detonation but only after the engine has gotten hot. But it does not really sound to me like you have detonation. Fuel pump problems at high rpm typically initially accelerate and run normally for a half minute or so and then just die out or fall back as the fuel in the carb is exhausted and there is not enough being supplied by the pump. Since this is a rebuilt engine can we assume it has not run right yet? I would replace the cap, rotor, and wires. Recheck the wires to confirm they are all going to the correct cylinders. Check the timing again and watch for advance to occur as the engine revs. Run it at night and look at the wires for arcing. It is much easier to see in the dark. At this point if it's still acting up I would do a compression test. A stuck intake valve could cause this problem. As well as improperly adjusted valves. A compression test will find those problems. I've never heard of a problem cause by overtightening the flanges on the thunderbolt ignition module. Mine just look like plain old flanges. The thunderbolt module is pretty reliable if that is what you have. I think because it is a marine environment the engineers used some pretty overkill ratings on the components. You can always try one but once you get to swapping ignition moduels and carbs it starts getting pricey. That loud a noise as you describe it sounds to me like you are heaing the mixture fire off through the intake. Or possibly out the exhaust since it is a lot louder on land. I would see if I could tell if the noise is coming back through the intake or going out the exhaust. Paul wrote: SORRY FOR MULTILE POSTINGS! (BUT OTHERS MAY STILL BENEFIT FROM THAT AS WELL) This is what I found while researching ignition problems: http://www.outdrives.com/precisionparts/263.html: Quote: "Outdrives.com Ignition Modules Outdrives.com! stocks most of the Mercruiser electronic ignition modules". and then: "All modules must be properly installed or they will fail. Over tightening of the mounting screws is a common mistake. Most Mercruiser Distributors and Modules have a ground wire attached to the housing. Modules DO go bad from time to time. Symptoms can range from not getting over 2500 RPM's to the engine not being able to start at all." Could this be it? The description of the potential problem sounds unpleasantly familiar... IMHO Shouldn't he look to see if the distributor is installed correctly ( not off by 1 tooth or 180 degrees?) especially if the engine is a fresh rebuild. Check #1 TDC and all that stuff? Fredo |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
rec.boats.paddle sea kayaking FAQ | General | |||
rec.boats.paddle sea kayaking FAQ | General | |||
Bought a Reinel 26' | ASA | |||
rec.boats.paddle sea kayaking FAQ | General | |||
power vs sail | Cruising |