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1988 150hp XR4 - In need of advice

Cut2Short

Member
Still struggling with getting the engine to run right, as I accelerate out of the hole, the motor will begin to rev up but then dies. If I choke it, basically over fueling it, the motor will pick up and take off.

My first move for this condition was to rejetted the motor for my elevation, 6,000 ft, reducing the size of the jets for the Main Fuel, and Idle Bleed, by .004, and increased the vent jet by .004.

This improved the performance a little but not much. Fuel wise, I don’t ever seem to lose prime on the fuel system, and the unit runs well once up to higher rpm’s.

I’ve checked the voltage going to the coils, 220 DVA, with number 6 reading 210 DVA. Not sure in the voltage variance is significant or not.

Spark plug wise, I put a rinky dinky tester on the coils and found that the spark will jump ½ inch (2/0) on all coils, I didn’t look to see what the max gap they would jump, should I?

I did find that the advance module/idle stabilizer was bad and have one on order. I do have to keep the motor set for an idle at about 1,000 rpm and the timing set at 5* BTDC to keep it running, which I hope will change once I get the advance module installed.

Any thoughts on what I should be looking at to get the motor running right? The only other bit of information I could add is that the inline fuel filter seems to stay about 1/3 full when the motor is running, could this create a problem somehow?

Thanks for the help and your time
 
Since choking it helps it definately sounds like fuel starvation.

"After" you have replaced your module, if it's still hesitating I might be inclined to replace the fuel pump diaphram and gaskets (since you rejetted the carbs those should not be clogged, but if you did not rebuild them completely you could have an issue with the float valve(s)) - maybe one other area to investigate if the symptoms persist)...
 
Replacing the diaphram and gaskets sounds like some good advice, I am hoping to stay out of pulling the carbs apart but we'll have to see where it leads.

Thanks for the idea
Steve
 
Still in need of help, Please

Still in search of an idea to get my motor to stop dying when I hit the throttle. I’ve recently increased the main fuel jet and vent jet by .002, from the .004 reduction that was recommended for my elevation. I left the idle bleeder jet at the .004 increase, hoping that the additional air and increased fuel would help. It did as far as starting the motor, I also replaced the idle stabilizer that help significantly around the idling of the motor.
“However” the motor still bogs down and dies when I accelerate quickly. I was thinking that maybe increasing the maximum timing from 22* to 24* may help with addressing the stalling issue, any thought on this? Or am I headed in the wrong direction?
Suggestions are definitely needed.
 
When you punch it & it don,t go but ever thing works look to the carbs. I had that pro. i "riched out" the ideal screws & it shot. when back throught the carbs now alls good
 
The Saga Continues;

Over the weekend I took the following readings, compression in all cylinders was between 125 and 130 psi.

DVA readings ranged in the 1000 rpm and 2000 rpm 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 was seen on any of the coils.

I have the timing set, when cranking at idle 7* BTDC, and 22* AFTDC for max timing advance. When running through the rpm’s, 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.

I rebuilt the fuel pump, which was in pretty bad shape. I thought I had solved the puzzle because right after that I took it for a run and it ran very well but the next morning it was back to its old self. I’m wondering if I have a vacuum leak on the fuel pump, I didn’t have enough hose clamps with me so I had to use nylon ties to finish the connections.

Any ideas or input would be greatly appreciated.
Steve
 
Still Tryin'

I am still 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.


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 ½ 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
 
Tomorrow I was going to try to run it on an external tank and tie it directly to the fuel pumop and see what kind of results I get.

Thanks for the input
Steve
 
Still Tryin'

Put the .052's back in the idle bleeder jets and gave it a try, unfortunately, it didn't fix the problem. I also hooked up a pressure gauge to the carburetors as suggested, at idle it's running at about 5.5 psi and when accelerating it increased to 8 psi. The pressure stayed good until the motor died. There was no sudden loose of fuel pressure that I saw. I also tried to put the .076 back into the vent, it got worst so then I tried to go all of the way back to .70 in the Main Fuel Jet, and .072 in the vent jet, things were headed down hill , so I went back to where I started the day at, .052 in the idle bleeder jet, .072 in the Main Fuel Jets, and .074 in the Vent Jet.

I'm getting frustrated, any suggestions would be greatly appreciated.

Steve
 
Steve,

Does it give you a pop or bog on accel at all or just run till it dies, will it idle fine and then just dies or is this only in gear.

What do the plugs look like any fuel wash or even burn. Can you look at the piston domes and see if they have an even carbon pattern from intake to exhaust or is one or two of them clean.

Those 41's do not have an idle air adjustment screw do they? then that is what the idle air bleed jet is doing in place of the screw and yes metering air. I'd prefer the manual adjusment any day.

I get screwed up with V-6 carbs sometimes and need to look at the manual they work in reverse of every other carb with respect to rich and lean calcs.
 
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Thanks for the input Dave, the motor actually has some pop and starts the accelleration portion of the curve but then falls on its face. The plugs all look pretty good, dry, not burnt, and these carbs are all hard jet set, no real adjustments.

If I was going to try playing around with the jets, which jet should I focus my energy on first. I was thinking of reducing the idle bleeder down a little smaller to .05, am I picking the right jet to focus the changes on otherwise I have like way too many options to test.

Thanks again,
Steve
 
Steve,

I think focusing on the idle air is the right choice, I forgot too look in the manual a pop on accel or even at idle is usually a lean condition. But the confusing thing is you say it does not drop pressure which mean it has emptying one of the bowls OR loading a cylinder via the bleed lines, which is another possiblity but you say the plugs are clean.

You did setup the primary pick-up right I think I read that earlier in the thead. I will look at my master chart for the carbs in the am and post back what I find.
 
The carbs all look pretty good, a little bit of fuel/oil residue when I take the front off. This weekend I was thinking about first off taking the vent jet out, somone on another site recommended it, then go back with the .076 and then to the .074. If that doesn't do anything, I was thinking I'd keep focusing on the idle bleeder adjustment and maybe try reducing it again to .050 or even try .048. Lastly, if none of that works, I'd go ahead and invest another $240 in carb kits and go through them in great detail, making sure to check the jet holders and floats. Sound like a plan? At least it's somewhere to go from here. Thanks to everyone for the suggestions so far, I'll keep you all posted on the progress.

Steve
 
Steve,

Actually I was talking to a chum the other day and he thinks the motor is rich. Try upping the idle air not lowering them this should lean the mix not richen it.

Sorry for the delay, I had a crankout rebuild on a 250XS opti to complete.
 
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