Merc 150 XR4 Idle problem.. Need Help :(

[btwitche]

I have a 1988 Mercury 150 XR4. The Engine has aproximately 150 hours on it. For quite some time now the motor has had idling/low speed problems.<br />Just recently I completely overhauled the carburators. I installed new float needle valves and seats and cleaned all jets and orfices(also setting the float levels to the mecury manual specs). I rebuilt the fuel pump and replaced every piece of hose associated with the operation of the the feul system and oil injection system. Both fuel filters (inline and water separator) have been replaced.I also threw in a set of new plugs to boot. <br /><br />I just went of on a fishing trip and dropped the boat into the water. I took it for a test spin and it ran beutifull; however, when I went to crank the motor the next day to go fishing, it continually stalled and idled rough, even when warmed up. It sneezed through the exhaust repeatedly(unburned fuel I thought) and konked out several times. I was able to get it out and on a plane, at which time it ran great at every rpm all the way to 5600 rpm. I opened it up (running at 68 mph, which is what it always ran topped out) and held it at there for 4 minutes with no surging. After getting the boat off a plane it idled for about 2 minutes before repeating the same low speed problems. What the Heck?? I cant think of what to do next. Is it ignition? Plug fouling? Im using the new synthetic oil from quicksilver. Is that ok? Im lost with this thing. Ps: The boat was having this same problem before I worked on the carbs/fuel system.

 

[btwitche]

Re: Merc 150 XR4 Idle problem.. Need Help

Can someone give me a hint? Im almost to the point of scrapping the motor even though its like new. Ive had this prob for 2 years now with no resolve.. Please help!!<br /><br />I was thinking there may be a problem with the feedback lines from the ports(or bleed lines). Could this be? What about weak spark?? Could the above water exaust relief be restricted?

 

[corm]

Re: Merc 150 XR4 Idle problem.. Need Help

Hi,<br />It sounds to me like it is running out of gas. You could be getting an air leak from the gas connector or your anti-siphon valve in your gas tank could be sticking closed at low pump vacume<br />while idling. The anti-siphon valve is in the tank<br />pick-up tube. It is not legal to remove the valve,<br /> but their are many tanks out there with out one.

 

[petryshyn]

Re: Merc 150 XR4 Idle problem.. Need Help

I have never worked on that motor, but the first thing one must do is determine whether it's a fuel, ignition or mechanical problem.<br /><br />>get a pump spray bottle and spray a little pre-mix in the carb when idling poorly. If it improves, its fuel starvation.<br />.<br />.<br />.<br />>to test ignition, run the engine with this

installed on each cylinder, one at a time. Compare the spark quality at idle.<br />>check compression<br /><br />One of these tests will show up something....don't give up! Get back to us with the results, and we'll steer you right!!

 

[12Footer]

Re: Merc 150 XR4 Idle problem.. Need Help

As Schematic, I have never operated on one of those.<br />But the plugs will tell all..Well,allmost all.<br />Definitely check compression.<br />If the compression is within spec,start the engine, run until warm,then at a fast idle,shut the key off. imediately pull the plugs,and examine thier condition..Yes, they will be a tad hot to handle,but will give a good indication of what gives AND WHERE.

 

[btwitche]

Re: Merc 150 XR4 Idle problem.. Need Help

Thank you for the help.. Ill give it a try <br /><br />One question tho. Does the backfiring through the exhaust indicate unburnt fuel in the exhaust, possibly pointing to ignition or compression? This was my wacky idea, does it make sense?

 

[petryshyn]

Re: Merc 150 XR4 Idle problem.. Need Help

its certainly possible

 

[ODDD1]

Re: Merc 150 XR4 Idle problem.. Need Help

Bassin, I can think of two things that could cause your problem, assuming the mechanical health and spark on all 6.........#1 your bleed system check valves have failed, causing individual cylinders to run rich or lean...this will only affect the motor up to about 1600 rpm.....#2 your ignition idle timing is retarded to far...idle timing spec is something like 2 to 9 degrees after TDC....some engines will continually fire out the exhaust[backfire] with the timing near the 9 degree end of the spec,a rich fuel mix at idle compounds the problem...

 

[btwitche]

Re: Merc 150 XR4 Idle problem.. Need Help

Thanks fellas. Now I have a list of things to check, and will know what tools to bring(It will be one month before I can go back to Okeechobee to work on the boat). Althogh I tested the compression within the past year, it's something I cant overlook. The spray bottle, bleed check valves, and bleed hoses are also on my list. Going to check the spark jump distance too.<br /><br />Excellent ideas everyone.<br />This forum is great. Ill try and give some back too.