Vacuum vs Compression on 71 60 hp

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shirthero

Seaman Apprentice
Joined
May 6, 2008
Messages
34
I've been fighting with a 71 60 hp 3 cyl for some time, compressions 125 across, good spark on all 3. Not a very scientific test but I noticed today while idling it on the driveway, the vac at the mouth of the carbs was much stronger on 3 than 2 and especially #1. I can completely choke of #1 and it makes no difference, #2 causes some stumbling, # 3 kills it right out. The lack of vac on 1 confuses me...how can compression looks good but yet vac seems so poor?
 
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F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
Re: Vacuum vs Compression on 71 60 hp

Because "that's the way it is". Seriously, compression is only from the time the ports are closed till the time the piston reaches TDC. Crankcase pressure/vacuum is dependent on the piston & rings, but also on crankcase sealing and reeds. You could take the reeds clear out and still have good cylinder compression, but obviously zero crankcase pressure.
 
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shirthero

Seaman Apprentice
Joined
May 6, 2008
Messages
34
Re: Vacuum vs Compression on 71 60 hp

But is this a problem? I'm brgining to think it just isnt drawing fuel into #1, if so what is my next step?
 
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F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
Re: Vacuum vs Compression on 71 60 hp

I don't know. The closed throttle plates block off most of the airflow, so there isn't much vacuum there to "feel" at the open end. The actual vacuum is on the engine side of the throttle plates. But there obviously is a problem with the upper two cylinders, especially #1. That carb may be plugged up. Other things can cause it to not run on one or two cylinders, including water getting in them. Or fouled plugs. Or carbs not sync'd right, or....
 
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