1985 merc 150 cutting out at high rpm

**Keaten** · 10-07-2015, 09:23 PM

Hello I've got a 1985 mercury 150 on my bass boat I've had a on going problem with my engine cutting out at high rpm.. I've changed a lot of parts on the engine trying to fix the problem I'm thinking it's starving for fuel some times at 4800rpm or higher..I can run full throttle with lil bit of trim ok but it's only Turing about 4900rpm..but when I add more trim and get rpms higher around 5000 to 5600 it cuts out for half sec them pulls hard again.. when it cuts out its very fast and random... ruff water usally makes it cut out a lil more often... I'm think that the carb bowls are not staying full and the ruff water slamming the transom up and down is affecting it some how..the motor will run perfect all day below 3500rpm with no problems.. starts perfect never dies... just cuts out at random times in 1,2 foot waves.. some times with glass slick water It will run good wot at 5700rpm perfect until I find waves.... any help would be very helpful....

Parts I have replaced are.....stator,trigger,all coils, switch boxes, votage reg., rebuilt carbs twice,rebuilt fuel pump twice, new plugs, plug wires, wires going to coils, engine harness, removed both black boxes, fuel hoses on engine and hoses from tank to engine ran on different tank with same problems...

**Go Time** · 10-08-2015, 06:09 AM

I'll bet its not fuel but ignition. you may have a chafed wire that finds ground when the motor is shaking. Check your harness and all grounds...sounds like you done everything else. Gas rarely goes out and come back rapidly. Just a guess based on your description....good luck

**Keaten** · 10-08-2015, 11:56 AM

Thank you very much, for your help ill do every thing you said... Its driving me crazy...

**David Borg** · 10-08-2015, 04:13 PM

I agree with Go Time , sounds like its cutting spark . I would check / eliminate the tilt limit switch , could be playing around.

Good luck

**j_martin** · 10-08-2015, 05:50 PM

My XR4 did that when I got it. I put a fuel pressure gauge on it and sure enough it was starving. The culprit turned out to be an AtWood fuel bayonet. Couldn't flow enough fuel.

Pull the 1/8" plug out of the bottom carb fuel rail, and fit a hose to it for a gauge in yer lap and take it for a ride.

**Keaten** · 10-08-2015, 08:00 PM

Thanks guys for your help...today I cleaned every single ground and pos..conection even on trim stuff on air box bracket. Took for river ride after work at low speeds in medium ruff water.. problem still there cut out twice for half a sec or so just enough to scare me in to turning around and heading back to trailer.. I'm thinking it's not gas related now bc of it cutting out at low rpm today...like 2200 at 25mph turning in big circles.. I'll try by passing that trim limit switch tomorow...and try it out sat on the water thanks again..

**Go Time** · 10-09-2015, 01:16 PM

Look for a chafed wire and make sure that all black/yellow wires are connected to the switch boxes or the tilt limit switch are properly connected and don't have any loose ends or cuts in the insulation.

**Wayne Cammidge** · 10-10-2015, 06:33 AM

Why not remove all the Black/yellow wires from the switch boxes and try it like that. you will not be able to switch it off but then at least you know that you do not have any cut out circuits that are causing the issues.
Do you have any of the ignition control boxes connected? Try with them removed.

**Keaten** · 10-10-2015, 07:59 PM

Well I will say this...I've found some trim wires around the transom area inside the boat were someone spliced in a trim switch bc the motor doesn't have one on it bc of its age.. but the switch was cut off of it and the wires weren't taped up so maybe.... they may have been hitting each other one red and one black wire when waves were present..I've repaired that area with heat shrink and wire loom.. do you guys think that would affect the ignition..I really hope that was my problem.... I'll try it tomorow at lake... I want to try one thing at a time so I will know wat fixed it... next will be all the other stuff you guys said...with black,yellow wires and trim limiter

**j_martin** · 10-10-2015, 09:12 PM

If that haywire was on your boat, perhaps there's something haywire with the multiple kill circuits on the boat. Like Wayne said, if you disconnect the black/yellow (kill) wires from the switch boxes, you eliminate that possibility. You'll either have to rig a test kill switch or starve it ti kill it.

**Wayne Cammidge** · 10-10-2015, 11:45 PM

Or Flood it on the choke to kill it. Easier and quicker sometimes.

**capnzee** · 10-13-2015, 12:00 AM

Would your engine happen to have a mercury tilt/kill switch. Some of the older engines had a mercury (fluid) switch that grounded out the magneto when the engine was tilted up. This same switch would often activate the magneto ground circuit in rough water. I agree with the guys who have suggested removing all of the yellow/black grd wires. Could you have a hi-speed stator problem? Some times the wires coming from the stator can rub on the bottom of the fly wheel as they pass underneath it going to the power packs.