Sad motor - what to check next?

**gmacrae** · 03-27-2018, 10:02 PM

EDIT: Motor seems sorted - will update this post with details in the next day or so...

Hi guys, wondering if you might be able to help me diagnose this old girl, motor is a Merc Laser 220

I was coming home across the lake the other day, started perfect, came out of slow zone, put the foot down, got on plane as per usual, then after about a minute cruising at about 4000rpm the revs dropped about 1000rpm, continued running though, then as i backed right off it started kinda bucking (would be the best way to describe it)... It taking it real slow to the nearest beach @ 1000rpm it would continue to buck, with the occasional sneeze. The bucks/misses would often cause it to stall outright. Start back up straight away and carry on at 1000rpm, bucking, sneezing, stalling. Got towed the rest of the way back.

The bucks would send thru the gearbox and initially made me think the gearbox had crapped itself. Have since drained the gearbox and there were no metallic bits in there, oil looked fine.

Have also tested compressions thinking maybe a ring picked up, but all are 120-125 after 2 seconds cranking (all plugs out) and come up on the gauge quickly.

New spark plugs installed (they were quite old) and its still not happy. Will run/idle ok but gave a solid backfire once (exhaust) and is running pretty rough/weak

Timing light shows all cylinders are firing.

Plugs show all are getting fuel/wet.

Gave it new coils/leads/switchboxes/stator last year, triggers about 7 years ago.

Reeds maybe the culprit?

Where would you guys say to look next? It is an old motor and i've had a pretty good run, but hopeful she's not dead yet, after seeing the compression numbers...

Thanks for looking

Gene

**Capt.Insane-o** · 03-27-2018, 10:14 PM

Index the flywheel for each cylinder and check spark again,then check fuel pressure then start shopping for an ecu

**gmacrae** · 03-27-2018, 10:26 PM

Originally Posted by Capt.Insane-o

Index the flywheel for each cylinder and check spark again,then check fuel pressure then start shopping for an ecu

Ahh right, mark tdc for each cyl then check each is firing *when* it should? Can do! Fuel pressure, can do! ECU... dammit haha, lets hope you're wrong there - cheers Cap'n!

**Capt.Insane-o** · 03-27-2018, 11:18 PM

If that's the original ecu and ran this long it deserves to be bronzed and hung up as a monument.

**gmacrae** · 03-27-2018, 11:25 PM

haha fair enough, I'll send you an invite to the unveiling

**gmacrae** · 03-28-2018, 04:31 AM

marked all 6 tdc's on the flywheel and each pot is firing in time, I have a gauge coming to check the fuel pressure (and an aftermarket adjustable FPR to replace the stocker anyway)...there is a lot of fuel pumping thru this thing

**Motv18** · 03-28-2018, 11:02 AM

Would start with fuel supply myself. Running fine then flat means a clog or broken in my experience. Easy restart leads me more to a clog.

I am 90% wrong most of the time and 80% on the right track for the wrong reason.

**billybob** · 03-29-2018, 07:46 AM

I would check your fuel line first. If it,s old gray fuel line it,s probably coming apart on the inside. after you remove it cut it lengthwise & inspect it. Sometimes when these motors do that, it could be both switchboxes need replaced, because of bias circuit, in switch boxes. or stator dropping out. switch boxes tell injectors when to fire so if you have ign. problem you may also have injector problem.

**outasite** · 03-29-2018, 10:25 AM

Take a look at the reeds as well.

**gfinch** · 03-29-2018, 01:10 PM

Fuel quality and supply.

**Capt.Insane-o** · 03-29-2018, 03:27 PM

Check for debris under the VST needle too, but that will show up on the fuel pressure test.

**gmacrae** · 04-02-2018, 10:41 PM

Thanks very much guys. Finally back home after the long weekend to start looking at this thing.

I replaced the main fuel line and prime bulb about 6-7 years ago, so don't expect that to be an issue. Might replace it again in the next few days anyway, probably with EFI hose.

Have installed the little fuel pressure gauge and pressure sits at 38psi after letting the EFI pump prime (without starting engine). After about 10 minutes sitting it's dropped back to about 30psi which seems fair enough to me.

The little mesh filter on the VST was basically clean

Prime bulb pumps up fine and stays hard.

Assume the solid fuel pressure suggests supply to the VST is fine, but I'll pop the top off the VST anyway to check that needle now.

Next is to remove the intake and have a look at those reeds...

**gmacrae** · 04-03-2018, 12:47 AM

What do these reeds look like to you guys? None have any significant gaps (my manual says <0.51mm and no gaps are anywhere near that). But there are a few small chips and cracks here and there. Worth replacing them anyway I guess...