1974 Mercruiser 888 - Symptoms of Exahuast manifole leak???????????

[pdr1966]

background: Last Summer intermittent running on 7 cylinders after heat soak, then clears up and runs fine after 1min running.
This spring: constant 7 cylinder running - alos very 'laboured' and can't get down to idle, backfires and dies.
In my experience JUST running on 7, should still be able to idel fine, but rough.
Smoke both blue and black/grey when trying to keep running.

Have been all over the ignition, new plugs, wires, coil, dist cap, rotor, (points are gapped and need new, but ok for now).
Compression test is good on all cylinders 145-155 psi verified twice (plugs out, throttle open cranking).

Latest observation is that Plug #8 is much more carboned than atleast two others (all of which are fresh during this troubleshooting.)

My next tact is to pull the rocker cover and determine if the valves are operating fully.
Other idea at this point - and looking for suggestions on the likely hood - is if the exhaust manifold is either blocked with carbon, or water is going into #8 causing the no fire, AND a loaded thump which causes it not to be able to continue running at lower throttle.

Conversation / Suggestions?
Pete

 

[Don S]

Re: 1974 Mercruiser 888 - Symptoms of Exahuast manifole leak???????????

Could be water getting in that cylinder from a manifold to elbow gasket leak. Next time you run it, after it sets for a bit, pull #8 plug and crank the engine over and see if you get any water out of it.
Might even be gas if the carb is leaking and raw gas is flooding into that cylinder.

 

[pdr1966]

Re: 1974 Mercruiser 888 - Symptoms of Exahuast manifole leak???????????

Well #8 is dry, had running with finger over the hole.
Took the rocker cover off and all valves on that side are operating normally.
You mention carburetor leak, is this something likely?
Tomorrow going back to the basics, verify my base timing and point gap.
One other idea is if for some unkown reason a cylinder or more I carboned up and basically plugged - I think, unlikely, but I am running out of ideas?
Pete

 

[pdr1966]

Re: 1974 Mercruiser 888 - Symptoms of Exahuast manifole leak???????????

FUEL too much of a good thing.
Following your thought and another similar post which had good feedback.
This morning took the flame arrester off, and the coil +12v, cranked .....
Once fuel pumped up it continuously was running down the venturi walls.
Pulled the carb off will take to the city and repair.

Pete