So I was timing my 8ba today. And when I was finished. I revved the motor up. No more than 3500 Rpm and when jt was de accelerating it started knocking. I shut if off instantly. So I tried retarding the timing and it still knocked. It was running rich so that made me think it couldn’t be pre ignition. I then pulled one plug wire at a time and the knock continued. So I pulled the pan and all of the rods are tight and only have play side to side like they should. I’m totally stumped. I’ve heard piston slap but I’m hoping it’s not that. Wanted to get some other ideas that it could be from you guys. Sent from my iPhone using H.A.M.B.
Need more info. Brand new rebuild/first time start up? Running without the knock before you worked on the timing?
So it’s a merc motor. High compression heads,bored .040 over, cam, adjustables, it ran fine before. I even drove it up the block. It had 60psi of oil pressure idling which leads me to believe that it’s not a rod bearing. And the rods have no play on them which means it’s not a wrist pin. So the only other thing I can think is pre ifniton. It’s regular fuel with a tiny bit of lead additive. Sent from my iPhone using H.A.M.B.
It is freshly rebuilt. No more than 10 miles on it. I was also thinking something on top of piston. But I don’t want to pull a head if I don’t have to Sent from my iPhone using H.A.M.B.
In my experience, knocks that only occur on deceleration have been rod bearings. The only real way to check is to remove the rod caps. However, it could be something like a loose flywheel or pulley. It would have to be something that is free to move when unloaded.
I checked on a few and I have .060 on them. And I’ve run the motor for a long amount of time and gotten it hot. So why would it happen when it’s not fully warmed up? Sent from my iPhone using H.A.M.B.
It wouldn't unless something changed to let the piston rise higher,like excess rod clearance. So...maybe pull the plugs and turn the engine over by hand feeling for resistance on any cylinder in case there's something stuck in a piston top. If you find a suspicious spot where resistance is higher,see which pair of cyls is at tdc and try to inspect the pistons...but the heads may need to come off if a scope isn't available. I'd try to do anything I can to troubleshoot this before taking it apart. Like turning the crank back and forth every few degrees of rotation to see if you hear anything that sounds/feels like excess clearance. It's a knock just on one cyl, correct? Terry
Sounds like you checked piston to head clearance. Did you check valve to head clearance ? Don't recall what cam you are using, but, a 400jr will contact most heads if not massaged for clearance.
It turns out it was pre detonation. I went to retard timing and actually advanced it. So I finally got the timing right and it ran good. It’s very retarted. And it somewhat diesels when I turn the key off. So I bet if I put some hightest in it, it will run better and will let me advance the timing more. It also has 170psi max on a few cylinder Sent from my iPhone using H.A.M.B.
Are both your distributor and carburetor compatible? That is, if you have a Ford Loadamatic distributor, do you have the correct carburetor to allow it to work properly? It seems like these things usually work better if converted to a later model mechanical advance distributor and an upgraded carb of some sort. Search this site for "Loadamatic" and you'll likely find more information than you'd hoped for.
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