1985-86 brown band 75hp 4 cycl 2 stroke 2000 rev flat spot

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Aug 13, 2011
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Engine in gear runs on low revs, power on ok to approx 2000 revs then hits flat spot, some times it struggles through and then runs beautifully through mid through to full power. Powering down near 2500 - 2000 rev and she goes flat and often stalls/stops running.

New CDI unit, new rectifier, plugs, fuel lines, electrics, battery, lower carb...and still the problem with the flat spot...help
 

Coloradolakeboy

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Jun 21, 2010
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Re: 1985-86 brown band 75hp 4 cycl 2 stroke 2000 rev flat spot

Test for spark on all cylinders with an inductive timing light. This would be best done on the water in gear so you can test the problem RPM range with the light. You also could get a DVA and run some ignition tests on the muffs in your driveway.
 

Gomer50

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Re: 1985-86 brown band 75hp 4 cycl 2 stroke 2000 rev flat spot

Run some tests on your Stator, maybe the high side of your stator is bad.High side is for runing at higher Rpm.
 
Joined
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Re: 1985-86 brown band 75hp 4 cycl 2 stroke 2000 rev flat spot

Test for spark on all cylinders with an inductive timing light. This would be best done on the water in gear so you can test the problem RPM range with the light. You also could get a DVA and run some ignition tests on the muffs in your driveway.

Agree, my engineer to date hhas only conducted ignition tests on the driveway with muffs, although he has put o/b into gear using tank. Good spark recorded to date since new CDI and rectifier installed.

Need to get Engineer in boat on water...sound advice thank you
 
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Re: 1985-86 brown band 75hp 4 cycl 2 stroke 2000 rev flat spot

Run some tests on your Stator, maybe the high side of your stator is bad.High side is for runing at higher Rpm.

New unit applied, sound advice although the O/B runs very sweet when she gets out of low rev band; mid and high revs run beautifully. Dead spot is at or around the 2000 - 2750 revs. I believe the engineer has also installed a new stator unit.....big bucks spent over the past 5 weeks and still we have this low-mid revs dead spot on demand and when reducing power. Thanks for your ideas....I think that the engineer re-used the float and jets from my origional carb unit and installed them in a second hand carb body.......if the float was knackered the chances are he has reintroduced an over supply of fuel to air mix....which might suggest the problem in re-starting when she dies
... Thank you....every bit of sound advice get me nearer to questioning the engineer when he says he has fixed the problem.......which I've now heard 6 times!!!
 

jimmbo

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May 24, 2004
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Re: 1985-86 brown band 75hp 4 cycl 2 stroke 2000 rev flat spot

you say you put a used carb on it, what was the 'new' carb off of? And you say you changed the jets from the old carb to the "new" one. Were the jets on the 'new' carb different from the original carb?

FWIW.. Engineers are usually better designers than they are repairers. The ones I've dealt with were always crying "well the book says it should do this"
 
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Re: 1985-86 brown band 75hp 4 cycl 2 stroke 2000 rev flat spot

For a 1985 engine like this it's darn hard to get parts here in the UK. The Engineer replaced the bottom carb with a 2nd hand one that he had. I believe he did not use a carb kit and simply took parts of my carb and retrofiited them into his replacement. These parts may be the problem - question... if the float takes on fuel and his too heavy - would this swamp/over feed the combustion chambers No 2 & 4?

Would an over supply of petrol result in the flat spot? Would it not severely limit any increase in revs/power?

The Engineer booked to come out on the boat tomorrow afternoon...if he shows up!!
 
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jimmbo

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12,944
Re: 1985-86 brown band 75hp 4 cycl 2 stroke 2000 rev flat spot

The top carb feeds 1 and 2, the bottom carb feeds 3 and 4. If the carb is flooding it will cause issues/ What do the spark plugs look like?
As for the used carb. Unless the carb is an exact replacement, there could be issues regarding proper mixture. The main jets are only part of what affects the mixture, there are several air bleeds that might or might not be correct for that engine
 
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