I am struggling with getting the 1988 150hp XR4 Merc to run right, as I accelerate out of the hole, the motor will begin to take off (rev up) but then dies. If I choke it until it starts to lope, basically over fueling it, the motor will pick up and take off. Thought I?d summarize the various things I?ve either measured or repaired since I last asked for help on various forums and hopefully will get some help/advice along the way.
Here?s what I?ve got:
Compression in all 6 cylinders range between 125 and 130 psi.
DVA readings ranged at the 6 different coils, running in neutral at 1000 rpm and 2000 rpm range between 210 and 230 DVA, when rpm?s increased to 3,000 and 4,000, the readings increased between 250 and 260 DVA. No voltage drop seen on any of the coils through entire rpm range.
Spark plug wise, I put a rinky dinky tester on the coils and found that the spark will jump ? inch (2/0) on all 6 coils, I didn?t look to see what the max gap they would jump. I did replace the spark plugs.
Replaced the advance/idle stabilizer module
I have the timing set, when cranking 7* BTDC, and 22* AFTDC for max timing advance. When running through the rpm?s with the boat in the water, in neutral, not in gear, the timing at 1,000 was 9*, at 2000 it was 3*, at 3,000 it was TDC, at 4,000 it was 8 ATDC. I ran through the timing with it in neutral but I think it shows the mechanical timing advance is working.
Tried my best to have the roller cam mark aligned with the center of the throttle roller, seems to be by the book. Played with moving the roller away from the cam by .01 but it didn?t change the condition/operation of the motor significantly
I put a diaphragm kit into the fuel pump and replaced the in-line fuel filter.
The motor has the WH-41 or WH-41A carburetor?s, I?m guessing that it is probably the 41 since it had the .052 in the idle bleeder jets. I re-jetted the carburetor for the elevation of the lake I?m on, 6,000 ft, reducing the size of the jets for the Main Fuel and Vent by .004, and increased the Idle Bleeder jet by .004, from the sea level sizes per the service manual. After test running it, thought it would be better to fatten (richen) up the mix and split the differences for the Main Fuel and Vent Jets so that they were only reduced by .002 from sea level sizing. (Sea Level, Original Jetting: Main => .074, Vent=>.076, Idle=>.052, Current Jets: Main=>.072, Vent=>.074, Idle=>.056)
I?m aligned with the fact that my problem has to be carburetion; I was hoping that maybe someone could help me understand what role the various jets play in the carburetor. My guess is the main fuel jet supplies the fuel, the vent jet provides atmospheric relief to the carburetor bowl and the idle bleed jet provide the air for the fuel mixture? Am I even close?
As I change the Idle Bleeder jet smaller the mixture gets richer, as I increase the Main fuel jet larger the mixture would get richer, but how does the vent jet play into the mix?
Tomorrow I was going to change the idle bleeder jet back to .052 and see if making it richer will cure my acceleration issue. If it doesn?t should I be looking to go to .048? I think that size is stock in the WH-41A carbs for sea level? I was also going to change the main fuel jet to .070 and the vent jet to .072, as called for in the manual for this elevation to see the impacts. I was thinking that this would set up the carbs for 6,000 ft elevation, as if they were the 41A models and be a richer mix than the 41 models.
Any advice would be greatly appreciated.
Steve
Here?s what I?ve got:
Compression in all 6 cylinders range between 125 and 130 psi.
DVA readings ranged at the 6 different coils, running in neutral at 1000 rpm and 2000 rpm range between 210 and 230 DVA, when rpm?s increased to 3,000 and 4,000, the readings increased between 250 and 260 DVA. No voltage drop seen on any of the coils through entire rpm range.
Spark plug wise, I put a rinky dinky tester on the coils and found that the spark will jump ? inch (2/0) on all 6 coils, I didn?t look to see what the max gap they would jump. I did replace the spark plugs.
Replaced the advance/idle stabilizer module
I have the timing set, when cranking 7* BTDC, and 22* AFTDC for max timing advance. When running through the rpm?s with the boat in the water, in neutral, not in gear, the timing at 1,000 was 9*, at 2000 it was 3*, at 3,000 it was TDC, at 4,000 it was 8 ATDC. I ran through the timing with it in neutral but I think it shows the mechanical timing advance is working.
Tried my best to have the roller cam mark aligned with the center of the throttle roller, seems to be by the book. Played with moving the roller away from the cam by .01 but it didn?t change the condition/operation of the motor significantly
I put a diaphragm kit into the fuel pump and replaced the in-line fuel filter.
The motor has the WH-41 or WH-41A carburetor?s, I?m guessing that it is probably the 41 since it had the .052 in the idle bleeder jets. I re-jetted the carburetor for the elevation of the lake I?m on, 6,000 ft, reducing the size of the jets for the Main Fuel and Vent by .004, and increased the Idle Bleeder jet by .004, from the sea level sizes per the service manual. After test running it, thought it would be better to fatten (richen) up the mix and split the differences for the Main Fuel and Vent Jets so that they were only reduced by .002 from sea level sizing. (Sea Level, Original Jetting: Main => .074, Vent=>.076, Idle=>.052, Current Jets: Main=>.072, Vent=>.074, Idle=>.056)
I?m aligned with the fact that my problem has to be carburetion; I was hoping that maybe someone could help me understand what role the various jets play in the carburetor. My guess is the main fuel jet supplies the fuel, the vent jet provides atmospheric relief to the carburetor bowl and the idle bleed jet provide the air for the fuel mixture? Am I even close?
As I change the Idle Bleeder jet smaller the mixture gets richer, as I increase the Main fuel jet larger the mixture would get richer, but how does the vent jet play into the mix?
Tomorrow I was going to change the idle bleeder jet back to .052 and see if making it richer will cure my acceleration issue. If it doesn?t should I be looking to go to .048? I think that size is stock in the WH-41A carbs for sea level? I was also going to change the main fuel jet to .070 and the vent jet to .072, as called for in the manual for this elevation to see the impacts. I was thinking that this would set up the carbs for 6,000 ft elevation, as if they were the 41A models and be a richer mix than the 41 models.
Any advice would be greatly appreciated.
Steve
