1968 Mercury 1250ss still runs ROUGH after new top end!

TomSawyer

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Joined
Jun 26, 2011
Messages
21
I bought this engine and it never ran right. I tore it down and found that the lower main bearing and seal went out and leaned out #5 and #6. I replaced all pistons and rings honed and new upper and lower main bearings and seals. Its got good even compression on all cylinders. It still misses very bad. The idle fluctuates up and down but not quite like a vacume leak. It will not rev or get on plane. The plugs are wet and carboned. I jet it for 8500' (.074). Its got good strong spark on all cylinders. While I was in there I noticed the reed valve blocks were loose on the crank. The reed petals were within spec but there was no tolerance for the block on the crank in my Seloc manual so i left them alone. I could rock them in and out (like you would a rod bearing) and there was a little play. Now thinking back I can see how there would be blow by between the cylinders. Could this be the problem?
 

Gomer50

Chief Petty Officer
Joined
Sep 5, 2009
Messages
507
Re: 1968 Mercury 1250ss still runs ROUGH after new top end!

I dont think it would be the reeds,after bolting the block and the intake cover back together the reeds are tight there not going anywhere.With the pins/dowls in place they cant move.I would shoot more towards the timing.You could try a leak down test.When I put my inline 6 together at first she kinda sputtered and had a slight miss. I discovered that one of my fuel port covers had a slight leak so I sealed that back up and the miss went away all good.

crank.JPG
 

TomSawyer

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Joined
Jun 26, 2011
Messages
21
Re: 1968 Mercury 1250ss still runs ROUGH after new top end!

I dont think it would be the reeds,after bolting the block and the intake cover back together the reeds are tight there not going anywhere.With the pins/dowls in place they cant move.I would shoot more towards the timing.You could try a leak down test.When I put my inline 6 together at first she kinda sputtered and had a slight miss. I discovered that one of my fuel port covers had a slight leak so I sealed that back up and the miss went away all good.

View attachment 109240

Thanks for the reply. Im talking between the reed block and the crank. The bushing surface that it runs on. There is play in there. I will lync and sync again.
 

TomSawyer

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Joined
Jun 26, 2011
Messages
21
Re: 1968 Mercury 1250ss still runs ROUGH after new top end!

What type of operating symptoms would you expect from having these labrynth seals worn out?
 

TomSawyer

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Joined
Jun 26, 2011
Messages
21
Re: 1968 Mercury 1250ss still runs ROUGH after new top end!

The #!$?#@ plug wires were crossed!!! Whoever replaced the wires last had #2 and #5 switched at the distributor. Whew! she runs good now! Thanks for everyones help!!
 
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