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

    Oh I just realized that it might be important to state that if I floor it at idle and it backfires through the carb I have to pull back on the throttle or the engine will sputter for 2 seconds and die. But if it’s at like 1000 ish rpm’s when I do it, it will hesitate and then accelerate normally.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

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

    All right I will turn the idle rpm down to 650 in gear in the water tommorow and report back. It’s possible that the engine will stall if I completely lean out the fuel mixture at that rpm so I will post results. I’m not sure if I already said it but the carb will only backfire under heavy...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I spent about two weeks last year thouroughly cleaning the old carb and I put a rebuild kit on it and it didn’t seem to get much better at all. It would take 2.5 seconds of cranking to start the engine no matter if it was cold or hot or whatever and it had the same backfiring problem. I threw on...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    When you say “650-700 rpm’s idle engine” are you saying idle in gear or in neutral. Also if I turn down the idle like 100-150 rpm’s the vacuum will drop like 1.5 to 2” Hg. I haven’t personally adjusted the float as it is a new carb but I can do that tomorrow.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    In neutral it’s about 850 rpm, but in gear it’s at 750 rpm which is what I’ve read the in gear specs should be. I have only one idle mixture screw. I don’t recall seeing any gas coming out of the bores during idle. But I can give it a closer look tomorrow.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Compression numbers were #1 126psi #2 103psi #3 114psi #4 114psi Yes, vacuum is low, fluctuating and about a 1 to 2” hg rise and drop. I mean I’m not sure how the leakdown numbers would be wrong I put the cylinders at TDC, plugged it up to shop air at 100 psi and read the leakdown %.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Here is the link to the new carb. https://www.amazon.com/gp/aw/d/B0B4N3T92P?psc=1&ref=ppx_pop_mob_b_asin_title
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Yeah I can hear start stumble at about 3/4 turn but I’m able to turn it all the way in without it stalling, although it will stumble hard. It’s possible my idle rpm is too high, which is in turn creating enough vacuum to continue to suck fuel through the Venturi’s. Although I’ve adjusted it to...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I already rebuilt my old carb and that did not my fix the problem so then yes I went with the Amazon special. Yes it most definitely runs rich when I turn it 2.5+ turns and I can smell it but I’m not able to make it stall by increasing fuel mixture.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    When I turn the idle mixture screw all the way in there is definitely a stumble but it will not stall. It doesn’t seem to stumble nearly as hard when increasing fuel mixture but there’s a slight rpm decrease and stumble when I get up to 2.5 to 3 ish turns out. It’s a brand new carb so I can’t...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    It’s a brand new carb so I don’t think my idle circuits plugged and I was having this problem before I replaced it too. It is possible that the new carburetor is jetted down however, although I’m not sure if that would these problems.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I’ve tried to adjust the mixture extremely rich (like 4-5 turns) and that seems to help slightly but it doesn’t completely solve the problem. When you say adjust the choke do you mean adjust it so where it isn’t fully opened when warm? I don’t floor it when cold unless the electric choke plate...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    After adjusting dwell it did seem to steady my vacuum but it is a consistent bounce between 13-14” of HG at idle with a rapidly vibrating needle as well. The needle will steady completely if I increase throttle. I am able to get up to 20” of HG at idle if I increase idle rpm to about 1150 which...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I put my old points and condenser and no changed but instead of just cranking engine I actually started it up and checked swell at idle. That gave me a much more stable reading and I was able to adjust it to about 30.5 degrees of dwell.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Yeah 0 side to size play. Since the oil in cylinders helped with compression I put Marvel mystery oil in cylinders through spark plug hole to hopefully free stuck piston rings if I have them. Letting it soak for 48 hours. I also put some in fuel tank and engine oil. I plan on doing an oil change...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Rotational play yes, I will look into distributor bushings. I added a spoonful of oil to each cylinder and 2,3,4 all came up to 125 psi while 1 came up to 136 psi, so bad piston rings maybe? I am trying to find where the bushing would be on my distributor on the diagram but I can’t seem to find...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I also noticed that I could rotate the shaft by hand about 1/16-1/8 inch before the advance springs kicked in making it harder to turn, and the rotor arm probably had 1/8 inch play in it.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

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

    I had the spark plugs out but I only pulled the ignition coil wire out of the coil, I didn’t ground it, do you think that could be the issue? Also when i tried to set the dwell, whenever I cranked the engine sometimes the spark would jump from the points and I could see it hit engine parts like...
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