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

    Okay so I am trying to set dwell with a meter but my dwell is bouncing sporadically. It hits 27 degrees and 52 degrees at the extremes as well as everything in between. I’m not really able to accurately set it as it’s bouncing so much. I replaced the condenser and points just last year.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I seemed to have seen your suggestion to drain coolant before leak down test too late. I will test it when I do coolant change.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Okay so leakdown test finished - finding TDC on all cylinders is a B***H btw- and it seems that the results are good, but I’m not sure I left in compression numbers for reference #1 126psi, 9% leak (most of the air out of oil fill cap but a little out of intake) #2 103psi, 10% leak (all air...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Ah yes now I understand that’s a great visual, yes I would agree that would be extremely unlikely for it to only blow in that tiny little landing, thank you. I’m about to perform a leak down test so hopefully that will shed some light on this
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Oh, no I didn’t have the cooling water drained so I assume those results are invalid. But I did get a combustion leak tester, used it on my cooling system while running and no combustion gases were detected in the cooling system. When you say it’s unlikely for combustion to transfer between...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    All right sounds good thanks, I will rule out dwell tomorrow and I will do leak down test today to check for worn rings or valves etc. Is it possible that the head gasket blew between cylinder 1 and 2? Causing high compression in cylinder 1 (126 psi) and low compression in 2 (103psi) (Based...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    No no sorry the cooling system itself holds pressure just fine it’s my coolant cap that wasn’t holding pressure it’s just old and needs to be replaced. Yeah I just picked up a dwell meter and cylinder leak down tester and I’m going to do tests in the next day or two. I’ll post results when done.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    So I used an ever tough combustion leak tester and I had zero combustion gasses in the coolant, which tells me my gasket isn’t blown on a cylinder-to-coolant portion. I’m not sure if that’s helpful I just thought it might be.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    All right I’ll have to pick up a dwell meter. I set the points with a feeler gauge and I have made ignition timing spot on with a timing light numerous times. I read online that you only needed to set points with a feeler gauge but I guess that’s not good enough.
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

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

    So back to the carb backfiring problem and hesitation on acceleration. Is it possible that the head gasket is leaking between cylinder 1 & 2 which is causing low compression in one and high compression in the other. And would this leak cause the carb to backfire only under heavy acceleration...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    All right sounds good thank you
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Oh trust me I am well educated in the shortcomings at this point 😂. Do you believe that the head gasket is the only problem?Would you or anyone else have any tips for removing and replacing head gasket?
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Just finished compression test and numbers are as follows Cyl #1 - 126 Cyl #2 - 103 Cyl #3 - 114 Cyl #4 - 114 Engine was brought to operating temp before removing spark plugs to begin test. Ignition coil was grounded, all plugs were out, throttle neutral button pressed and then opened to WOT...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Okay so I forgot to take the spark plugs out but I grounded the coil and this is the video I got of the valves, I can’t really see a problem but if I’m being honest I definitely wouldn’t have the eye for one. I tested the pressure on the coolant cap and it is in fact leaking pressure… it takes...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I’m not sure what this metal tab is directly above the oil hole but it’s slightly off center compared to the other ones. (Left) Reference (Right) A couple others arent 100% straight either, almost like they are bent a tiny bit?
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I have taken the valve cover off and I discovered that about 7 out of 8 screws on the cover were barely tightened down and a couple I could quite literally unscrew with my fingers. But here are the pictures I have of the rods and rockers, no blatantly obvious problems except some weird rust like...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    Well I hope not but we’ll see. Posting compression results later today, yeah hopefully I have all my cam lobes lol
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I will make sure to do that and I’m sure a broken intake valve spring will be easy to see but how might I see a worn exhaust cam lobe? Do I have to open it up further? Also it’s funny you mention the cap because I recently replaced rotor, points, condenser, springs and cap on the distributor (I...
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    1986 Mercruiser 470/170hp/3.7L Carb backfire, low vacuum, other weird issues

    I’ll make sure to pressure test the cooling system today and post results thank you
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