Merc 850 idles good but stalls when throttled

Ronguor

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Jul 9, 2012
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I Have a 1976 Mercury 850 thunderbolt. It starts great and idles great. And shifts in and out of gears. It will run good at anything at or below 1/3 throttle, but as soon as I start to throttle past that it will stall. Now this only happens with the intake cover on. If I take that off, it will run fine through all throttle positions. but as soon as I put it back on it won't. I have checked for exhaust leaks and can't find any. Besides it idles too good and runs too good at lower ranges for me to think it might be that. It seams to me that it isn't getting enough air. there is nothing blocking the air ways though. I have tried adjusting the fuel mixture each and every way. It actually does just a little better running rich, unless the cover is off. Any clues would be awesome.

Just a note, this is all done without the cowl and the wrap around on, these covers seam to make no differance, only the intake covere does.
 

Chris1956

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Mar 25, 2004
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28,798
Re: Merc 850 idles good but stalls when throttled

What are you calling the "Intake cover"? There will be plastic gratings over the carb throats, and a cowling support.
 

Ronguor

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Jul 9, 2012
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Re: Merc 850 idles good but stalls when throttled

What are you calling the "Intake cover"? There will be plastic gratings over the carb throats, and a cowling support.

cover.jpg
That is a parts picture of it. YOu remove this cover to adjust the carbs, you are able to see the butterfly valves and ports as well with this off.
 

Chris1956

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Mar 25, 2004
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28,798
Re: Merc 850 idles good but stalls when throttled

Very interesting. Neither my '73 Merc 850 or my '77 Merc 1500 had anything like that. Since I have never seen one, I cannot comment on it. sorry
 
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