Why won't my 4hp mariner OB rev high

Engage

Engage

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28 Oct 2001
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Preston, Lancashire, UK
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Iv'e got a real head scratcher and I don't know the best way to go about diagnosing the problem. I have a Mariner 4hp 2-stroke (6EO) and last season on one occation it was hard to start and run until I took the cowel off but even then it would not rev very high and would labour as if the spark timing was out. I cleaned out the carb which made a little difference but what I did notice was that if I restrict the fuel from the jet by placing a flat screwdriver on top the revs would increase. All of this was done in my garage so not under load. There was no apparent exhaust leakage into the cowel. I don't know if this OB has points nor how to test spark timing / strength. The non-return valve after the carb seems fine. She will start after a few pulls from cold but just can't get it to rev up like it used to. When running it does not splutter or cut out. Can someone suggest some tests I could carry out that might point the way?
Cheers.
William.
 
I've got the same engine, and the carb does foul up easily - I think I've finally got into the habit of running it dry before stopping (ie turning off the fuel tap & letting it run out).

No idea what your screwdriver trick implies, but if I may ignore this...

1 - maybe you didn't clean the carb as well as you thought - especially the needle
2 - did you take all the slack out of the throttle cable when re-assembling?
3 - have you tried giving it a little choke as you try to rev? Mine responded to this on 1 occasion, and eventually revved OK, at which point I shut the choke.
 
Max, thank you for your reply.
The screwdriver trick was simply to cover the top of the main jet within the carb body to restrict fuel.
1. You might be right but I am unable to detect any further contamination visually.
2. I tested by turning the butterfly valve directly so any slack is irrelevant here.
3. Choke worsenes the problem.
 
Its CDI ignition

Check that you have a good strong blue spark.. if it starts easily then most likely spark is good.

Tried a new plug.

If plug and spark good then turn attention back to carb. Clean again
(Dont know what you are doing with the screwdriver)
Make sure throttle is opening fully

Using an integral tank or remote tank? check fuel flow to carb and filter in integral tank outlet
 
If the choke makes it worse you are maybe running too rich, is the valve in the float chamber passing and flooding the engine?
 
gordmac said:
If the choke makes it worse you are maybe running too rich, is the valve in the float chamber passing and flooding the engine?
Click to expand...

Check for this by turning off the fuel.

As the fuel in the carb bowl is used up you should get a short period during which it runs normally before speeding up a little and then stopping
 
I only had a mariner 2hp but if yours is built the same way (likley) gthen the problem will be the water pump at the bootm of the leg. The impellor runs in a stainless steel cup which is a push fit into the ally casting of the leg. As corrosion builds up between ally and stainless the drag on the pump gets higher and is shown in difficulty of starting and low max revs. Cure is simple - take the leg to bits, clean out the corrosion and replacde th distroted cup
 
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