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Old 30 July 2005, 17:52   #11
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Quote:
Originally Posted by Turbodiesel
Is it an EFI model or a Carb?
Carb one.

Fuel starvation...Wouldn't that get worse the wider the throttle and bigger the demand for the fuel?

The only other things I can think of that hasn't been mentioned is a vacum device on the throttle linkage and a timing problem Adance/retard
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Old 31 July 2005, 14:26   #12
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Andy,

If its a carb engine. Then shouldn't the carbs have two sets of jets. Idle and fast run. Maybe the idle side is playing up. That might explain why you can rev it hard and its okay but on tickover it plays up.

Best of luck.
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Old 01 August 2005, 03:39   #13
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If the problem goes away when you squeeze the primer bulb its almost definatly going to be the fuel pump diaphragms. When you squeeze the primer all your doing is doing the job the fuel pump is doing. Usually you can hear the pump is noisy, it make a ticking noise if it has no fuel in it. If you think you have air in the system try putting a piece of clear hose in the fuel line so you can see what is going through your fuel hose. You should have no air in the fuel line.
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Old 01 August 2005, 03:47   #14
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Turbo D
So you are saying I have a problem with my fuel pump as well?
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Old 01 August 2005, 03:52   #15
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Possibly, really you need to have the fuel pressure checked. But as i was saying usually if the problem goes away when you squeeze the primer its probably going to be a fuel pump problem. Have a listen to the pump though, the usually noisy if there is not alot of fuel in it.
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Old 01 August 2005, 05:32   #16
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very occasionally we have a similar problem on our merc 4 stroke carb 90. engine will lose some power at full throttle but pick up as soon as the bulb is pumped. Fuel pump diaphragms are ok and quiet. Might be an idea to check that your tank is venting ok? Ours is so very infrequent as to not be a problem to us, it has been going on for a couple of years but by the time it goes for a service I have forgotten to mention it to our mechanic.
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Old 01 August 2005, 05:39   #17
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Quote:
Originally Posted by wavelength
very occasionally we have a similar problem on our merc 4 stroke carb 90. engine will lose some power at full throttle but pick up as soon as the bulb is pumped. Fuel pump diaphragms are ok and quiet. Might be an idea to check that your tank is venting ok? Ours is so very infrequent as to not be a problem to us, it has been going on for a couple of years but by the time it goes for a service I have forgotten to mention it to our mechanic.
The tank vents no problem, too well, so that when filling if you are not careful it will pour out of the vent B4 it clicks off the pump
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Old 01 August 2005, 07:50   #18
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See if this forum can help (always assuming I can post the link OK)
http://www.iboats.com/cgi-bin/ubb/ul...ubb=forum;f=29
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Old 01 August 2005, 08:27   #19
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Am I right in saying there is only one accelerator pump/damper on the Merc 90 and that it seems to work on the throttle linkage and not direct to the carbs?

How much would one of these cost?

PS: I can't see that it'll be a fuel starvation/air inline prob, surely this would make the problem worse the faster the revs!
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Old 01 August 2005, 08:38   #20
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Have you checked the engine fuel filter for water? Had a Mariner 75hp fourstroke with the same symptoms. The idle jet had become blocked so would run rough below 2000rpm, then the main jet kicked in and the engine would run fine above 2000rpm.
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