Need help with a promax 300

[Lake rat Skater]

Ok guys I bought a 250 xb powerhead last year to replace 300 promax that put a hole in the block. I swapped the heads and ecus over to the xb to essentially make it the same motor as the promax 300.

Well under a load it had a bad bog and just didnt seem right. This is on a twin engine cat so the other motor helped pull it along but it was no where near as responsive as it should be. I went through the fuel system, had injectors cleaned, changed coils, installed new champion cc plugs, swapped out the stator, inspected the reeds and nothing changed. Compression was good and the motor runs fine in neutral but way down on power in gear. I ran it a little more than normal and pulled the plugs to have a look. The left bank of cylinders all looked like normal brownish 2 stroke plugs put thr two upper cylinders on the right bank were super clean as the one pictured below.

I pulled the head and as you can see below the two cylinders matched the cleanness of the plugs. I thought the two o rings looked small and did some research and learned mercury changed part numbers and went to a smaller diameter around the time the motors were built. I thought maybe water was getting sucked in to the cylinder on the intake stroke and causing this. I got the old part number for the larger diameter o rings and installed three new ones, headed to the ramp and no improvement.

Tell me what I am missing or what to do next, kinda at a loss. Thank You

[Lake rat Skater]

I should of asked if by chance I dont have spark could it be fuel washing down those two cylinders?

[Chaz]

Lake ,

Just throwin some things out there ..

When I opened the picture of the plug , it looks like the center electrode is hot and maybe boiled the end of the porcelain a bit.

Do you have a manual to see which color wires go to which injector. Could have a couple of em crossed. I gotz a manual in the shop .. I will look if need be.

What do the piston tops look like on that bank ..???

When the other motor broke .. was it on one of the two that are acting up now ?
Same ECU's , harnesses , CDM's , etc.

Did you try parts from the known good running motor .. ??

Here's the electric parts ohm/volt spec's ..

[Lake rat Skater]

Chaz, thank you so much for the response and I think you might be on to something with the injectors crossed. I do have a manual and will try to take a look later today.

When I bought the powerhead the dealership said they just cleaned the injectors. I bought a different set that were serviced by bracuto and just swapped them out individually, never even considering the plugs might have been crossed by the dealer previously.

I will attached a picture of the pistons. They have some previous minor carbon build up but then the bare aluminum parts are shiny in the two problem cylinders just like the heads. The motor that blew was on the other bank and the cylinder looked nothing like these. Just looked like the rod broke. I switched to premix.

The only parts I have not swapped between motors is the coils and the plugs are sparking so I just assumed they were good.

Thanks again and I'll report back

[Lake rat Skater]

[Chaz]

All show good fuel wash , so the injectors are firing. Just as said , maybe not at the right time.

If you can find an old aluminum 17, 19 , 21" 3 blade prop , you can cut the blades back to make a test wheel. I mark the flywheel for all six holes , put the wheel(s) on and hit the ramp. Best time is at night , you can find bad plug boots , arcing plug bodies to the cyl head .. etc. best thing is you can warm it up , turn it wide open and walk back to the motor and look around , check out the spark pattern .
I needed a left hand wheel as well , Todd found a 4 blade clearance sale , dropped shipped them to me and I clipped them and send them to there rightful owners.

Here's the Merc bulletin from back in the 3.4 days .. LOL still applicable ..



Here's mine .. they get some funny looks at the gas station ...

[Lake rat Skater]

All show good fuel wash , so the injectors are firing. Just as said , maybe not at the right time.

Chaz - Bumping up an old thread as I don't get much time for the boat. First off thank you! Injector wires were crossed and I got them corrected and the motor idled and ran perfect until........ I went to get the boat up on plane. Still a dead spot or lack of engine response. With the boat as a twin the other motor will get the boat on plane and then the bad motor seems to come a little more alive with more speed perhaps when the exhaust is less restricted? If you hold them both wide open the bad motor will slowly come around and my tach will show both motors get up to a little over 5000 rpms. This is still low so I really think the bad motor is holding the boat back. 25 motion with 15x32 three blades, will run a little over 90 on GPS currently

Cruising with both motors showing 3000 rpm if I only give the good motor more throttle it is very responsive, if I only give the problem motor throttle is just bogs and barley gains any RPM. Here is what I have done and checked out to date with no solution

1) Had spare stator so I swapped them out
2) Installed Injectors that were serviced and checked by Brucato
3) Checked the TPS and voltage is within Spec
4) Swapped out both fuel and ignition ECUs with known good ones
4) Swapped out intake air temp sensor
5) Replaced Vac lines and made sure correct line went to the Map sensor
6) Swapped Map sensors back and forth
7) Swapped coils back around
8) I had a spare complete VST that I switched to
9) I took a look at the reeds and did not see anything wrong with them visually

At this point the items below are on my mind, let me know if you think any of these would make sense or if I am missing something. Again the motor idles great and runs perfect in neutral, also shows great compression but under a load is when the problem shows up.

1) Check and clean out the tuner - Is there a chance major carbon build up is doing this?
2) I have pretty much messed with all fuel parts but I do need to check fuel pressure
3) Maybe a new Pulse Pump?
4) Might Replace the head temp sensor
5) Might need to look closer at plug wires and coil / test the stator that I assumed was good

Thank You!

[nephew22]

You wouldn't know where I could find 300 pm heads would you?

[Chaz]

Needle in a haystack issues ... I thought I was the only one those landed on ..

Did you ohm test the stator, coil low side, map, head + IAT sensor and crank trigger eye.
Set crank trigger air gap.
Coil wires can corrode where they meet the coil.
Same for harness plugs where they connect to the pancakes.

Throw away the screen under the fuel pressure regulator.

The reason I like the test wheels is that you can have someone sweep the throttle / hold it wide open, while your standing next to the motor. You can check / watch pulse pump pressure / high pressure / run a timing light on all cylinders, see if any are erratic or firing at the wrong time.

Do you have a repo man switch that you can plug in at the motor, to eliminate the boat side from being an issue.
(cannon plug .. couple feet of harness and an old switch)

Let us know .. please.

[Lake rat Skater]

Chaz - got rid of the screen under the fuel pressure regulator and cleaned the tuner - running great now. Thank You!

[Chaz]

Haw-Haa ! ... great news ...

[rgsauger]

That’s crazy that sometimes the simplest little crap can be the problem. Chaz, what does the screen do under the regulator? I mean, besides mess things up

[Chaz]

That’s crazy that sometimes the simplest little crap can be the problem. Chaz, what does the screen do under the regulator? I mean, besides mess things up

A lot of guys have an oil filter size (Mercruiser type) filter between the tank and pulse pump, then another large filter at the VST . There is a screen at the bottom of the high pressure pump. A small basket in each injector (I throw them away too) and then the tiny azz screen below the regulator that dumps back into the VST tank.
How many filters does one man need ... LOL

The little injector baskets clog up over time and make the motor go lean ...

The little screen under the regulator clogs up over time and makes the motor go pig rich. Better than going lean I rekkon. But as Lake described ... no holeshot and down on power thru-out the RPM range ..

[Dave S]

Pig rich......Yaaaa.... forgot that one.......on the regggg....Ya fixxin more than ya Eatiiinnn.....''''happy druneeee......

[Chaz]

Ya fixxin more than ya Eatiiinnn.....

Brother Dave ... You know I never miss a meal ..

Yellow tail ... Tuna steaks .. caught fresh this mornin. Black beans ~n~ rice and a YETI full of Ice Tea ... <---{sprinkle o lemon juice (=-