1985 90hp firing problem

[captain catfish]

i was really hopeful i had fixed this thing, i have fixed the charging and the tach problem. i took the boat out this morning and it ran great (1st time since the reg/rect change) . i only ran it about 15 min because of the weather. we took it out again tonight and ran it for a good 30 mins, and everything was perfect, best its ever run. until we stopped in a slew for about 30 min, when we went to leave it was obvious that something wasnt right, we could only turn 3000 rpms vs the 4200 we were turning prior to stopping.

i didnt have any test equip on the water with me, but when we got home, i check for spark (while running) some little hand held test that lights up, i have spark at each cyl. but when i pull the plug wire from 3 cylinders i get a reaction, but on the 4th nothing, i hooked up a spark test tool, and have a good spark, i didnt set it to 7/16s probably a 1/4 judging by eye, but it does have spark, i swapped leads from the power pack with cylinder above (the lower right is the problem) and plug wires, but the problem didnt follow the "coil" so i did a compression test and both cylinders were 116 and 120.

pulling the plug wire off the bottom right cylinder does nothing to motor, each of the others will have a noticeable impact on the idle, but not this 1 cylinder? someone please help me.

thanks
mark

 

[mfkadz]

If it has spark and does not move to the other cylinder it should be fuel. If you spray carb cleaner or ether in that one cylinder and things pick up then a carb cleaning would be in order.

mike

 

[emdsapmgr]

The bottom right cyl is not contributing to the engine, if nothing happens when you pull the plugwire off he plug. We know your test as the "cyl drop test." Either it has no spark, or it's not getting any fuel on that cyl. Either will prevent combustion. I'll bet it's not firing. Takes more juice to fire when it's under compression, so check for spark when it's running-with an inductive timing light. Hopefully, you'll find it's not firing. If it is, then the worse outcome is that cyl's not getting fuel. That's a real problem, as that means that cyl is running dry-a possible fatal problem. The coils are generally very reliable, certainly the most reliable of all the ignition components-considered on a long-term basis.

 

[captain catfish]

i will test it with a timing light tomorrow, if its a fuel issue, is each cylinder fed fuel directly? what im asking is, every car i have worked on has a "manifold", this engine has 2 carbs supplying 4 cylinders, i have been through these a few times, they have always been clean, i dont understand how a cylinder on the same bank could be getting fuel, but another 1 might not be.

thanks for the responses,
i will check spark with a timing light tomorrow.

 

[emdsapmgr]

Your engine has two two barrel carbs. Each barrel feeds one cyl. If the main jet on one side of one carb plugs up, you get no fuel/oil to that cyl.

 

[captain catfish]

good to know, i sure hope its not caused damage if thats the case.
thanks, i will look at both of those tomorrow after work. i will say this the plug was wet when i removed it.

 

[emdsapmgr]

Good thing to check on your part! A wet plug indicates that it's getting fuel (and lube) That's good. Another simple test you can do: That engine has dual power packs. Each pack control spark to one head. You can swap them for testing purposes. If the non-firing problem moves to the other head after the power pack swap, you've got a bad pack.

 

[Bosunsmate]

good to know, i sure hope its not caused damage if thats the case.
thanks, i will look at both of those tomorrow after work. i will say this the plug was wet when i removed it.

Spark, yet wet plug.... a wet plug shouldnt be wet if sparks good so it might be poor spark, unless you a getting water in there.
Take a lighter with you and hold it to the wet plug, it should go poooof straightaway. If it evaporates off then its got water in it.
Ive had that problem and many other posters have on here too.
If thats the case you then need to order the parts quick as you need to run the motor to get all the water out before it corrodes the internal surfaces of bearings etc.
Or drop the lower unit and run it for a while with no water to get the engine lubbed up properly again

 

[captain catfish]

thanks for the tips on the water in the cylinder. i pulled the head last night and took a look, i took a lot of pics, and will make a new post, if any of you are inclined to see the pics please look at the new post.