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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I could try to put the venturis from the old carb on the new one. I wouldn’t be too surprised if they fit. I guess I’ll see
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Will do thank you
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Alright I’ll try to size up and size down jets and see what happens.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    It is very possible that my jet size is wrong, the carb came with extra jets of different sizes but I just went with the one already in the carb. Could incorrect jet size cause this?
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Ah I see
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I Will try that tomorrow, although I tried to smoke the system to check for leaks and did not find any
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    So the engine is audibly increasing and decreasing in rpms while sitting at idle, and the vacuum gauge is increasing and decreasing vacuum along side it. So I went with a timing light to check if timing was steady and it seems that timing is constantly advancing and retarding by itself...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Opened up the distributor again today and I noticed a bunch of rotational play of the whole spring/weight mechanism itself. (First video) I also showed that there is some distance the springs have to travel before the weights are contacted. (Second video) If any of this is abnormal please let...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I will try my 89 non-ethanol gas with octane booster… and then try to find 93 octane somewhere. Do you run non-ethanol?
  10. C

    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I haven’t unplugged the vacuum gauge for a while now. Once I get the dwell perfect, the needle still vibrates quickly and bounces between 13-14” of hg. Only way I have found to increase vacuum at idle is to increase idle speed. But then my idle would be too high and out of spec. It also might...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Yeah I wish I could get 92 lol. I’ve tried advancing and retarding timing within 10 degrees both ways it doesn’t seem to change anything unfortunately. Maybe I’ll throw some octane booster in my 87 gas and see what happens
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Re- did leakdown numbers #1 10% #2 16% #3 12% #4 10% I did this with engine cold, engine isn’t sitting-over-night cold but the coolant and engine felt cool to the touch.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    5 times was about what I did when waiting for reading to cap out. I will redo the leakdown test now. I will look into the dripping left Venturi on the carb. If anyone has any suggestions on what that dripping might be it would be much appreciated
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Okay so I just re-did the compression numbers and now, after the MMO soak, I’m getting these numbers. #1- 140 #2- 135 #3- 135 #4- 135 But I also noticed that if I crank it until the reading is steady and then wait for about 2 minutes it will slowly climb to 140 on cylinders 2,3,4 and 145 on...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    It states on the top of the engine that dwell needs to be 28-34. Mercruiser sent out a service bulletin for the MCM 470 that the timing needs to be decreased to 4 deg from 8 deg due to the different fuel used today. I thought the engine had to be idling in neutral to set the timing? (I could...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    After adjusting float it seemed that the spring loaded needle was set at 8/16 instead of 9/16 so I fixed that. And then the float drop was at 1 and 8/32 instead of 1 and 3/32 so I fixed that.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I have not watched timing while carb backfires with a timing light that’s a good idea I will try that next
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I took the top off the carb to check float level and it seems there is a spring at the bottom of the accelerator pump. That was not on my old carb but I suppose it could be normal I just thought I should point it out.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Okay so it seems that the left Venturi is dripping fuel. About one drop ever 1.5 seconds. The right Venturi is not dripping. After I adjust in gear idle to 650 rpm the left Venturi is dripping one drop every 4 seconds. Right Venturi is not dripping. It stops dripping after engine is shut off.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I’ll turn down my idle today and see what happens. My timing is 4 btdc as per the Mercruiser service bulletin. Not sure what you’re saying about the gasoline but I run non-ethanol 89 octane.
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