84' 3.0 Block & 91' HO Head Tuning?

[Coastiebrewer]

Good evening all,

Recently swapped heads on the old 1984 MCM 140 3.0 in the Chris Craft. Couldn't find the old style large port head, so had to go with the 1991+ high output head. Still using old manifold and carb, despite non perfect port alignment.

Engine starts and runs well, but backfires out the carb and dies if I jump on the throttle too much while in gear, but not in neutral. If I take it slow and let the engine catch up, I can reach full rpm.

I would guess that the accelerator pump is pushing lots of fuel for the large ports that are no longer there, which is causing problems.

Have any of y'all had any luck tuning for the newer style head? Do I need to buy the new manifold and carb for proper response? Thanks in advance for your time and advice!!

 

[Scott Danforth]

you realize that the 91 head is for 1991 and newer motors, your 1984 head requires parts from 1990 and older motors.

the whole motor went thru a re-design in 1989-1990 for the 1991 model year.

 

[Coastiebrewer]

Good morning Sir,

Yes, I am aware that the parts are different. Found a hole in the old head and needed to replace. After talking with 3 different shops/vendors determined that the old big port head is not made anymore, and could not find one. So time to adapt and overcome!

Will likely replace the rest of the top end. Rest of the engine is happy.

 

[Scott Danforth]

how did you get the water-ports to align?

 

[Coastiebrewer]

The head bolted up to the old block without any issue at all Sir! Direct fit!

 

[Coastiebrewer]

Update! Found Solution!

After two hurricanes, a road trip, and a bunch of time in the office, I got back to the boat. Noticed it wasn't getting gasoline to the carb.

Replaced the 34 year old fuel pump, and cleaned the carb all the way through, the engine ran like a top today.

3 hours on the new 1991 style head on the 1984 block; no issues noted. Will continue to update as I accumulate engine hours.

Hope everyone is well!