91 40hp 4cy 2-stroke Power loss

[dmorulez]

Got a 91 40hp 4cyl 2-stroke that starts, idles, and cruises fine at low rpm. Get on the throttle a bit and I get what I'm assuming is an intermittent misfire. I was thinking coils, but now thinking it may be one of the carbs. Plugs and gas are new. In the link to the youtube video you can hear the motor lose power for a second and fire again while in WOT.

 

[Faztbullet]

Check each cylinder with a timing light..

 

[chris.olson]

tGot a 91 40hp 4cyl 2-stroke that starts, idles, and cruises fine at low rpm. Get on the throttle a bit and I get what I'm assuming is an intermittent misfire. I was thinking coils, but now thinking it may be one of the carbs. Plugs and gas are new. In the link to the youtube video you can hear the motor lose power for a second and fire again while in WOT.

That thing is not even running on all four cylinders. What you think is a misfire is actually one pair of cylinders firing now and then. Do the old hand over the carb test with it running and see which one has no vacuum. That will be the pair of cylinders that has the broken reed block.

 

[jimmbo]

Could be a Fuel issue, but It could Ignition. What rpm was it running in the video, and you say it was WOT? You replaced the Plugs? Being that is in the same sentence as replaced the gas, did you disassemble the Carbs, cleaned all the passages and replaced the Welsh Plugs? Or was that referring to the Spark Plugs?
You say it had fresh Gas, fresh gas mixed with some old stuff remaining in the Tank(s), or emptied and cleaned Tank(s)? If there was old gas in the Fuel System, how old? Is the Fuel Filter on the Engine Clean, as well as the Pickup Filter/Strainer in the Tank?

While Reeds can fail, it is very rare that it happens, Very Rare

 

[Faztbullet]

Reed damage will not cause the symptoms shown.....

 

[chris.olson]

Not the reeds themselves, the reed block. I've replaced so many of 'em in those engines I've lost count over the years. The first thing you do is put your hand (or three fingers) over the carbs when it's running. If one will suck your fingers in, but the other won't, you got a busted reed block. The reed blocks are also the #2 and #4 main bearings and they break in the thin area of the spider where the two halves of the reed block bolt together. If you got good vacuum on both carbs, then look elsewhere. But when you got a busted reed block you can rebuild carbs and throw ignition parts on it till you spend more on it than the engine is worth, and it's not gonna fix it. So when you got a Merc inline-four or tower of power six that loses a pair of cylinders, the first thing you do is the vacuum check. Run it too long with a busted reed block while you're throwing other parts on it and you'll ruin the crank.

 

[chris.olson]

Oh
To

 

[chris.olson]

Oh, to diagnose your engine problem; first of all it's only running on two cylinders in that video. It's not even close to reaching full power. It's pretty easy to verify you have ignition on all four cylinders. If you have ignition, you have a fuel problem.

So start it and just let it idle. Reach in there and put some fingers over the carbs. You should get a definite suction when you cover the carb and it should immediately flood those two cylinders on that carb. If you got one where it doesn't seem to have much suction and doesn't flood when you cover the carb, don't go any further with bolting parts on. Pull the powerhead and split the crankcase to see what's causing the no-vacuum condition. 9 times out of 10 it's a broken reed block. Mercury Marine still had those in the warehouse in Fon Du Lac a month ago - I just put a pair of them in a '92. Mercury calls it a main bearing assy, part #534-3267T25.

If both have good vacuum, then look at why the carb is not letting fuel get to it. The older inlines had a fuel pump on each carb. The '91 has a common fuel pump because they have oil injection. The main jet circuit in the carbs is a pretty big hole and it's highly unlikely it can plug it unless the carb is full of water, and you're well off the idle circuit at full throttle. BUT - if the fuel pump output is low because of a bad diaphram or check valve it will starve the top carb on fuel.

The main thing is, if you find it has poor vacuum on one carb DO NOT RUN IT THAT WAY. That is the very first thing you verify on an inline Merc two-stroke. Those reed blocks have a bunch of grooves in them that fill with oil and that's what seals between the two cylinders in that pair. When they break, half the bearing part of it separates from the body and comes in metal to metal contact with the crank. It will ruin that crank journal if you run it that way for an extended period at full throttle running on only two cylinders.

 

[Faztbullet]

The reeds or broke /cracked reed block will not cause symptoms shown in video.. it is a ignition problem.