Merc 18xd will only run on choke!

[Tha Big Chief]

Gday all...I am new to the forum so please bear with me...lol I have a 1985 18hp merc XD which I am using for the second season and has always ran great! Had the kids out for a fish the other night and as we approached the launch afterwards I closed the throttle and the idle got rough then stalled about 3 seconds later!!! Now it will only run with the choke fully on, at idle or high speed. As soon as I turn the off the choke, the engine stalls as if it is starving for fuel. I figured i picked something up in the main jet...pulled carb/bowl...everything is a clean as a whistle...compression is good...new plugs now and still the same result! had the same fuel in the tank for the last few trips so I am thinkin its fine. Obviously not safe to run with full choke(without burnin it up right shortly) I am guessing next step is to check for a vacuum leak somewhere but not too sure how to go about it! all help is greatly appreciated...dont want to waste the long weekend not being able to go bassin!!! thanks in advance

 

[CharlieB]

Re: Merc 18xd will only run on choke!

The IDLE jet is quite a bit smaller then the main jet, a small bit of crud will restrict the fuel flow enough the motor will not run.

Pull the carb apart again, use a solvent based carb cleaner, not the soapy water based kind, spray thru EVERY passage BOTH directions, blow dry with compressed air is ideal but not always available. Wal- Mart Super Tech works great and is one of the cheapest available, about 1.40 can.

Remove the idle jet and thread a bread bag wire thru it, roll the jet a few times so the wire will scrape any mineral accumulation off. Spray the jet again before reassembly.

Set initial idle mixture adjustment at 1 1/2 turns out from lightly seated. Make final adjustment on the water in forward gear to best idle speed PLUS about 1/8 turn out to provide enough enrichment for acceleration. Test by rapidly cracking the throttle to WOT. ANY lean cough, miss, bog or hesitation, then open idle screw an addition screwdriver blade slot width and repeat acceleration test. Repeat until perfected.

 

[madanorak]

Re: Merc 18xd will only run on choke!

CharlieB is right, but it is worth taking off the small plate, held down by 2 scews, on top af the carb, where you will find the end of the idle jet, and 2 small holes into the venturi. Ensure these are clear and there is no crud in the little chamber. I use one of these engines on a Yam 330s inflatable, almost impossible to keep at WOT without being thrown out of boat, great fun!! It's worth fitting an emergency kill switch as these engines didn't have one as standard, just in case you fall overboard.

Kev