Mariner 5 2 stroke won't idle

[ProDave]

As the title says.

Mariner 5 2 stroke. Engine stars and runs at high revs. As you reduce the throttle it slows down then reaches a point where it cuts out and stops abruptly

Done all the usual things. every jet and pasageway that I am aware of is clear.

Ideas please.

 

[Homer J]

Dodgy spark plug? Old fuel?

 

[ProDave]

Dodgy spark plug? Old fuel?

Don't think it is either.

I had this once before with a mariner 4 and it was the idle jet blocked. It's not that this time. bur not knowing all the drillings and how fuel passes through the carb, I am wondering if there is a blockage on the way TO the idle jet rather than the usual blockage in the jet itself?

 

[RichardS]

As it's a 2-stroke it's likely that the idle screw is an air bleed screw which will reduce the air and richen the mixture if you screw it in. What happens if you screw it gently right in and try a rich mixture?

Richard

 

[QBhoy]

Hi

Sounds like your not a novice, but I had a similar issue with a Yamaha Malta recently. Cleaned the carb and jets. Still the same. Took the carb off again and cleaned jets...along with a dose of sea foam and it’s better than ever now.
Failing that, only other thing I can think of is the fuel pump diaphragm having a pin hole or less likely the ignition coil intermittently failing. Often a sign of this is staining round the coil itself.
If it idles with the choke on or partially on, its usually a sign of carb and or jet. Worth taking off again I’d think. Even better sit it in brake cleaner or ultra sonic clean.

 

[ProDave]

The position of the idle jet makes absolutely no difference, screwed fully in, partly out, even completely removed, makes absolutely no difference. That's what makes me think no fuel can get TO the jet.

I have never understood the function of the square thing on the side through which the fuel enters. I have had it apart and there is no obvious tear or split in the diaphragm but never check in minute details for pin holes. Can someone better explain what this does?

It won't idle with or without choke.

 

[QBhoy]

It’s just a diaphragm fuel pump.
Just to rule out anything disastrous it might be worth doing a compression test.

 

[ProDave]

I haven't done a compression check, but it "feels right" when pulling the starter cord

 

[steve66]

Your slow running circuit on the carb is blocked either the ports or jet. Ultrasonic clean it by all means but I wouldn't recommend soaking it in brake cleaner. Any o rings, diaphragms or neoprene needle tips will be damaged

 

[ProDave]

Your slow running circuit on the carb is blocked either the ports or jet. Ultrasonic clean it by all means but I wouldn't recommend soaking it in brake cleaner. Any o rings, diaphragms or neoprene needle tips will be damaged

That's my point, I can't find where. I have blown / poked fine wire through every jet and opening I can see. Does anyone have an exploded view of this carb showing how the fuel flows so I can look for the bit I have not found yet?

 

[No Regrets]

Worth screwing the mixture screw in all the way (gently) and unscrewing it two turns as a base setting. Warm engine up and set idle, then turn mixture screw until engine runs fastest. Thats the best setting for many two strokes.

One is assuming:

1/ Clean fresh fuel
2/ Engine healthy (if it gives full power assume no hardware issues)
3/ Carb totally clean (Remove, strip completely and check every hole is clear. You mentioned idle screw does nothing which indicates a blocked jet or airway)
4/ I'm not going into plugs and ignition because it runs well otherwise...

 

[QBhoy]

Your slow running circuit on the carb is blocked either the ports or jet. Ultrasonic clean it by all means but I wouldn't recommend soaking it in brake cleaner. Any o rings, diaphragms or neoprene needle tips will be damaged

Fair point about the brake cleaner. I had perhaps too quickly assumed that the carb and it’s compements would be stripped and all perishable seals and gaskets removed. Worth you pointing this out, by all means. Thanks.