'87 Merc. XR2

[Sparkieboat]

Lift pump? ?

Here's everything that has been done up to date, none of which has had any effect on this problem:
1. Changed all fuel lines.
2. Ran engine on remote fuel tank with separate fuel lines.
3. Checked compression, changed plugs, changed water separating fuel filter.
4. Checked condition of carbs (totally clean, floats at correct height. Though I will probably check this again if nothing else solves the problem).
5. Checked/corrected timing - 22 deg. BTDC in accordance with the shop manual that says this is the correct timing for this XR2 engine.

I run nothing but Mercury/Quicksilver Premium 2-stroke oil, and I run 89 octane gas usually from Union 76. Non-oxygenated gas is just not possible to get around here, so I run Star-Tron enzymatic cleaner in every fill up. This stuff is supposed to counter the effects of oxygenates in fuel.

I have NOT checked fuel pump pressure. One of the reasons I haven't checked this is that, in my thinking, if I did have a fuel pump problem it would run like crap at WOT, at least at times. It always runs balls-to-the-wall at WOT. It's that transition point between fast cruise and WOT where it is running inconsistently. It will cruise all day at slalom skiing speeds and below with no problems at all. It also never stumbles out of the hole. I can fire it up, let it sit at fast idle for a couple of minutes, idle away from the dock so that it gets warmed up a bit, and shove the throttle all the way to the stop and it storms out of the hole every time.

Anything else I can check?

Well here is my best thought on your difficult situation. IF the compression is good, IF the carbs are ALL good and float levels correct. Then it comes down to spark/electrical. I would get a DVA meter/adapter and a good multi meter and start trouble shooting EACH piece of electronics starting with the voltage regulator. You will want to test with the throttle in different positions, because your may have a short in a ground wire or timer base wires or something that could be moved in that 3/4 throttle range and cause a stumble. Good luck and keep us posted. You may want to test a bottle of synthetic oil and see if that helps, maybe as simple as a plug getting slightly fouled out. How do your plugs look by the way?

[croSSed]

Year 3, same thing... Last season (Summer, '18), it still had this hesitation issue. In fact, it was worse last summer than it ever was before. One time I had it out on the Columbia River upstream of Portland (near Troutdale if anyone is from there), and I was cruising up the river and it was surging SO BADLY that I was afraid it was going to die and not start. But - as before - if I opened it up all that went away. Just got it out of the shop AGAIN... the tech said he couldn't get the motor to replicate what I was talking about on his dyno. He checked everything else out and no problems. I am seriously wondering if it isn't those CCMS reeds... This problem didn't exist with the stock reeds. I'm almost tempted to put the stock reeds back in and see, if it wasn't such a PIA to do all that again. The CCMS reeds didn't help with making more power. The only difference I noticed is that it runs smoother at displacement speeds, and is noisier at those speeds because crankcase sounds get past those carbon fiber reeds better than the steel ones. Just thinking out loud here...

[Capt.Insane-o]

Put a clear hose in between the tank and motor.

[FORBESAUTO]

Reeds shouldn’t cause the problem, unless something didn’t go right or didn’t go back together right. If it still has air box in it, remove it and run it at idle and see if there is any spitting back on any of the carb barrels. You can also hold throttle wide open and shine a light in each carb throat and inspect for any obstructions of a gasket or obvious issue with reeds. Did bleed lines go back in right positions? May can try removing the backdraft jets on carbs and see if it changes it (if it has them, they are the ones between the carb Venturies on air box side) This will richen it up at part throttle and may help tune you in if it’s leaning out when you back out of it or going rich if gets worse.

[Merc 2.5]

What is the rpm change in the surge? U say almost die? 3500 to 1000? Or lower?

[JAFFA]

Im thinking you need carb baffles, you say at full rpm if you back off for a bit then hit it again it stumbles, just picture whats happening in the float bowl when you do this on off thing with the throttle, when circle racing we needed baffles for cornering

[FMP]

Fuel pump check valves going?

[Bruce2]

I would be looking at diaphragm fuel pump ...I rebuilt one and used the Merc mannual to build it and mannual was wrong and it stumbled bad coming out of hole..

[jeffg]

Look under the flywheel cover and check your trigger wires.Take throttle cable off and move throttle while watching to see if trigger is moving smothly.Trigger needs to rotate freely when you hit it.Platic wire cover gets stiff and or wire hold down clamp is crimped to tight.I dont know if it your problem but it would be my first look.god luck

[Glastron1987]

I vote for taking a real close look at the trigger, its connections, and its movement.