briana

Recently, I performed an arizona speed upgrade on my 1998 502 efi motors, new dart heads, cam, rockers, msd ignition computer remapping,cmi exhaust etc. in theory they should be running close to 500hp at the prop.


here is the problem that I need help with:

my port motor backfires at idle but the timing is set perfect and all clearly obvious reasons for the backfiring have been checked. the motor runs 400 rpm less that starboard at wot and the performance of the boat is 4 mph less than stock before i performed the upgrade.

the boat is proped to run 5200 rpm wot .

oh by the way the boat is a 1998 35 fountain

any advise at this point would be great.

liquid asset

Im dealing with a similar prob, my fuel pressure regulator was coroded and was showing 90 psi fuel pressure, after repair back to 41,

mr mercruiser

maybe you have a bad injector-did you do a cylinder drop test?-

briana

wouldnt the bad injector come up as a code on the ddt

stevesxm

no... as long as the pulse is present, there will be no code. you might want to look at something REALLY simple like the firing order... it is really easy to get the wires mixed up on the cap and and will give you that idle backfire yet SEEM to run sort of ok...

did these things run on the dyno when complete ?

Griff

Have you rechecked the valve lash?????

I would do a leak down to rule out a valve that is sticking open.

briana

There was no dyno time. I will perform the leak down test and see about a valve. as far as spark plug wires go they are all in the right spot, double and triple checked.

one other idea is, az speed did the computers, could one of the computers not have been flashed correctly?

Vinny P

Try to swap ecu's between your 2 engines. If the problem follows the ecu, you got the problem found. If not, get into checking the motor.

briana

the reason why i post this question is this:

prior to the motor upgrade the motors were stock 502 mpi, the boat ran 72mph at wot with bad compression ot of the port motor spinning 26 hydromotives non labbed @5100 rpm. obviously we all look for more speed so an upgrade was performed instead of a valve job.

here is where my performance numbers are at now after the 20k upgrade

5000 rpm starboard
4600 rpm post this is the backfiring motor

68mph wot spinning the same props before the upgrade

Is the port motor causing me to lose this much speed, i was thinking i was going to gain 10 mph with the upgrade not lose 4.

az speed told me the motors should now be producing 520 at the crank and about 500 at the prop. comparing this to 35 lightnings model year 96-99 running 500 hp they perform 80 to 82mph

some thing is wrong here?

Payton

Sounds about how mine ran with a rounded cam lobe.