Blvd_cruiser Posted September 16, 2015 Report Share Posted September 16, 2015 Well I am sick...my 46 p15 special deluxe seems to dropped #3 and #4 cylinders...don't understand it was running fine..parked it went to start it and it was missing and backfiring...did a tune up thought i'd fouled some plugs or wires broke down...replaced everything ...ran better but still popping...pulled wires off one at a time those two cylinders made no change in the way it ran.....wth...don't know a lot about flatmotors..lol... Tony Quote Link to comment Share on other sites More sharing options...
Don Coatney Posted September 16, 2015 Report Share Posted September 16, 2015 Did you do a compression check? Quote Link to comment Share on other sites More sharing options...
greg g Posted September 16, 2015 Report Share Posted September 16, 2015 It is common for the head gasket to erode between the pairs of cylinders, most prevalent is between 5/6 but 3/4 not unheard of. What happens when the gasket goes is that the cylinders will swap compression, leading to the poor running and back firing. Since there is no coolant in the area it doesn't act like a typical head gasket failure which produces coolant loss and white smoke. Not a difficult fix, while the head is off it is a good idea to check the surfaces for true flat. And having the head milled a bit will give you somemore responsiveness, and allow you to crank in a bit more initial ignition advance. The head bolts need to be reinstalled and retorqued in a certain sequence but over all the job can be done in the garage or driveway. The compression test will verify the problem but symptoms indicate a head gasket problem. Quote Link to comment Share on other sites More sharing options...
Blvd_cruiser Posted September 16, 2015 Author Report Share Posted September 16, 2015 Yep ..found head gasket between cyl's blown...ran a straight edge over head appears to be flat... Tony Quote Link to comment Share on other sites More sharing options...
greg g Posted September 16, 2015 Report Share Posted September 16, 2015 For 50 to 60 bucks you can take the head to a machine shop and have 30 to 50 thousanths milled off. Really wakes these old engines up. Most bang for the buck of any upgrades. You might want to check into getting new head bolts. Ask at the Machine shop when I did mine he supplied some pioneer brand for about 90 cents a piece. Good idea to chase the threads in the block while you have it apart. You can use an old head bolt, make a couple grooves vertically along the threads with a cut off wheel or dremel disk. Then run the bolt in a out a couple of times with some solvent or penetrating oil then wire brush them out when finished. new head gasket goes in dry with may a bit of copper gasket spray or a light coating of AP grease on the block side. 1 Quote Link to comment Share on other sites More sharing options...
Blvd_cruiser Posted September 17, 2015 Author Report Share Posted September 17, 2015 Thank you for the quick replies...engine back together...running great Tony Quote Link to comment Share on other sites More sharing options...
TodFitch Posted September 17, 2015 Report Share Posted September 17, 2015 Thank you for the quick replies...engine back together...running great Tony Glad it worked out and thank you for the update. So often there is never feedback about if the problem got fixed and if so how. 2 Quote Link to comment Share on other sites More sharing options...
Blvd_cruiser Posted September 17, 2015 Author Report Share Posted September 17, 2015 I didn't mill the head... was afraid to boost combustion...I know on older small block chevy's if you redid the head 9 times out of ten the raise in combustion would strain the lower end and it start knocking..lol...just my experience from many years of running chevy's...this is my first old plymouth..but not my first mopar Tony Quote Link to comment Share on other sites More sharing options...
Ulu Posted September 18, 2015 Report Share Posted September 18, 2015 You can raise the compression easily and not blow the bottom end, but it does put more load on the wrist pins, which is what will usually go first. Also it puts more load on the starter. Mine was sometimes slow to crank when hot, because of the high compression. Quote Link to comment Share on other sites More sharing options...
Plymouthy Adams Posted September 18, 2015 Report Share Posted September 18, 2015 how can it affect the wrist pins but yet at the same time not affect the big end bearing....your speaking as if there is cushioning built into the rod assembly Quote Link to comment Share on other sites More sharing options...
Ulu Posted September 18, 2015 Report Share Posted September 18, 2015 You are sorta right. The total max physical pressure is the same, but the bearing pressure per sq inch is MUCH higher on the wrist pin because it's much smaller than the crank journals. But the wrist pins are actually fairly generous & the extra wrist pin wear is really due to weak oiling IMO. More heat too. The wrist pins stay hotter than the crank because they're so close to the combustion. 1 Quote Link to comment Share on other sites More sharing options...
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