1972 Mercury inline 1350, hesitation at WOT

Just put the boat back on the water since being laid up over the winter.
Finnished the season with a few problems which I set about fixing over the
winter.

- Distributer bearings were shot - torn down and rebuilt...
- Top plug hole was threaded - new insert gone in...
- Bad missfire from splits in plug leads - replaced them all.

I also rebuilt the carbs with 0.0785 main jets as opposed to the 0.80 jets
that were fitted last season, floats checked and adjusted where necessary...

6 new plugs, dissy cap and bush fitted.

Rebuilt fuel pumps.

Now the problem, once on the loch I warmed her up the usual 10 mins whilst
parking the trailer etc.... Took the motor for a run and made the usual idle
needle adjustments till hole shot was spot on....

Once at WOT the engine leaps to 5400rpm and runs for 5 seconds fine then
what feels like a missfire brings the revs down to 4700??? then up to 5k
down to 4700 yet if I lay off the throttle and cruise at 4500 shes quite
happy....

I then do the same thing with my mate priming the bulb and the problem is
gone...like a fuel pump issue but it cant be that cause they have just been
rebuilt.....

I suspect that the 0.785 jets are just too lean and the 0.080's will sort it
but there surely isn't that much difference?

My tank is vented and clear, to be fair the male fuel fitting on my engine
is a little old and hasn't been checked for wear...

Could this be something else?

All help greatly appreciated....

Many Thanks


Stan

Re: 1972 Mercury inline 1350, hesitation at WOT

Ck your timing, but the main jets need to be change back to the original size

Re: 1972 Mercury inline 1350, hesitation at WOT

Timing spot on Laddies, thanks for the prompt reply, i was leaning off my mains as I suspected they may have been too rich, do you think this is my problem?

Maybe I should try some 0.082's in there?

Re: 1972 Mercury inline 1350, hesitation at WOT

If ya got 78's in there, and it does not bog/blat (in other words run crisp when ya nail it), thats not the prob. 82 might have ya flat on your face on a punch to WOT (what elevation ya at?). Far as I can see I would do a plug chop first...look see there will tell quite a story

I'd do new pump diaphrams first...(and make sure you have pulled any little tiny screens in the fuel intakes at the carbs..both side off the brass fits..service bulliten on it back in the 70's

And if ya got old gas hose, toss it...possible inside swelling..

Re: 1972 Mercury inline 1350, hesitation at WOT

I looked at all plugs and all seemed fine, a little oily, it was hard to rub off and the plugs are brand new which suggests to me the oil has been baked on due to overheating....lean mix...

As per my post, the pumps have just been rebuit but i didn't understand what you meant by pulling little tiny screens in the fuel intakes at the carbs..both side off the brass fits..service bulliten on it back in the 70's???

I'll change back to 0.080's first to see if that helps.... Are you sure this is deffo not the problem Scatty?

Re: 1972 Mercury inline 1350, hesitation at WOT

Anyone else got any ideas?

Re: 1972 Mercury inline 1350, hesitation at WOT

stant1man said:

I looked at all plugs and all seemed fine, a little oily, it was hard to rub off and the plugs are brand new which suggests to me the oil has been baked on due to overheating....lean mix...

As per my post, the pumps have just been rebuit but i didn't understand what you meant by pulling little tiny screens in the fuel intakes at the carbs..both side off the brass fits..service bulliten on it back in the 70's???

I'll change back to 0.080's first to see if that helps.... Are you sure this is deffo not the problem Scatty?

Click to expand...

Whoops, musta missed the pump rebuild part.
Middle 70's had small screens in some motors (found out REAL quick on my old XS150..ran up fine, then bogged out).
I can't remember now just where they were..either before the line went into the carb fitting, or after and just before the carb. They would plug up in time, and slow the fuel flow at WOT in short order. Yanked them, and problem solved.
Anyway, I would go with .080 jets. Just put together a 135, and (sea level) had a hard time deciding that or .078's. But the .080's work fine on a punch out. Still breaking it in, so I don't know what WOT will do, but I have tickeled it some, with no problem.
Sooo, I'd do the jet change...and as long as you are in there, look at one of the carb fittings to look for a screen.
Good luck...seems ya got the rest covered!