1979 mercury 90..Will not run past 1500 rpm

Overpoweredpony · Aug 28, 2010

Hey all, i would first like to start off with how big of a help this community is.

I have a 79 merc 90hp switchbox motor. I been having some issues with it for sometime now but just learned to deal with it for the season but now it is much worse.. The problem just seemed to gradually get worse and worse.

It starts and idles perfect! but under any load at all, it bogs and just hits a flat spot from 1/4 throttle all the way to WoT. It will run up to about 1500rpm. Compression tested ok, all cylinders are at 140ish except number 6 which was at 125, 15 psi isn't gonna cause this big of a problem, at least i think. One thing i did notice though is that at an idle the bottom carb spits gas out and none of the others do. Possibly reeds? Does anyone know of a way to test them? I took it to the local marine and they pretty much told me they didn't know what was wrong with it. They tested they ignition and said everything was ok with it, and they also said the carbs were fine. But for some reason never said anything about the bottom carb spitting gas out. So i decided to take a piece of cardboard in front of every carb and the top two i had no gas, the bottom one though after bout 30 seconds was pretty wet. So i am guessing im gonna start there.. Anyone have any ideas? Anything would be muchly appreciated!

ray914 · Aug 28, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

Is each cylinder getting fire? I just had a similiar issue and it was the switch box. It idle fine but would not get any rpm's under a load. I check the fire on each plug and the lower two were dead.

79Merc80 · Aug 28, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

I have an 84 90 Mariner that did the same thing about not going to WOT. Found out that at about 1800 #3 spark plug would stop fireing and #1 was intermitin. Turned out to be the stator. It was sending irratic voltages to the switch boxes. changed it and it runs like a scarred rabbit.

Hope this helps.

Craig

Overpoweredpony · Aug 28, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

ray914 said:
Is each cylinder getting fire? I just had a similiar issue and it was the switch box. It idle fine but would not get any rpm's under a load. I check the fire on each plug and the lower two were dead.

Yes each cyl is getting fire. Every one has a nice blue spark.. When under a load im not sure.. My local boat shop checked the ignition system and said everything was ok..Atleast they said lol

For some reason the bottom carb spitting the fuel out is bothering me, like im missing something

Overpoweredpony · Aug 28, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

Here is a pic of the plugs in it.. 4 of them look pretty good( First pic), but the bottom two 5-6 look like water is getting to them. Especially 6.. I know it isn't good but does anyone think that this could cause the running issue?

Any help is greatly appreciated!

Chris1956 · Aug 29, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

Sure, Water will put out the fire in the bottom 2 cylinders. Sounds lie you need to pull the powerhead and replace the crankshaft seals.

starcraft1982 · Aug 29, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

I would first suggest,before pulling the powerhead,that you pull that lower carb and clean thoroughly/rebuild.While i have no personal experience w/that motor,i would assume those two cylinders share a carb.and while youre at it,throw a fuel pump kit in it.less tthan $100 and couple hrs work.Just saying i would verify cylinders are getting gas/oil since ingition is "verified".couldnt hurt your engine in the long run,even if it turns out to not be the problem.Now,im not saying your mechanic didnt thoroughly check the carbs,but if he didnt pull/clean them at least,it could very well be the issue.Especially w/gas coming out of it!Do this and report back,then it can be discussed about pulling the powerhead.

Chris1956 · Aug 29, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

Starcraft's advice is fine, however, if you have water on the plugs, it will have no effect.

starcraft1982 · Aug 29, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

So,do your 5 and 6 plugs look cleaner than the others or do they look "steam cleaned"or spotless?i do agree that if its water intrusion to the cylinders,carbs will do nothing.Hpwever,my suggestion was baseed on the carb spitting gas.Also running poorly and also lower compression down there.you just cant run cylinders lean.and if gas is coming out carb then its not going to the cylindeTHATS BAD!.and lastly you could very well have two(or more)seperate issues.good luck.

Wingedwheel · Aug 29, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

Try this. Start the motor and at an idle cup your hand over the carbs one at a time. Don't use the flat of your hand as you want some air to get in. See how much of a difference there is between the upper two and lower carb as far as a vacuum. I once had a 950 on a '62 SeaRay that was doing the same thing. It was the reeds in one of the lower cylinders. Unfortunately it was on a family boat and one in-law kept running it like that untill the reed completely broke and was sucked into the cylinder. I'm sure you know what happened next.

Overpoweredpony · Aug 29, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

I have all the carbs off and went through them and they were all spotless.

I tried that with my hand in front of the carb idea, and you could definetly tell a difference between the top two and the bottom carb. the bottom carb you could almost feel it. The top and middle carb not so much.

by the look of the #6 plug water is definitley getting in there. So today i'm gonna pull the powerhead and have a look at the crank seals and reeds.

Since the #6 cylinder compression was a little off, should i go ahead and put new rings in?

Faztbullet · Aug 29, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

I would check the reed block on 5 & 6 as I bet it has a chipped/broken reed and if cylinder is down 15 psi doubt rings will help as cylinder is likey scored....

asm_ · Aug 29, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

Before you start taking things apart to check the reed. I would check condition of your stator/switch box. A simple DVA adapter and CDI's instruction can tell you a lot about the ignition system.

I'm also working on a 82 that would bog down at 1800RPM. A quick check with DVA adapter shows stator is producing voltage that are just a bit too low to one of the switch box. Haven't get the chance to water test the new repair, but the motor run much smoother even at idle with ear muff.

B

Overpoweredpony · Aug 29, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

Update!

I finally got the powerhead off! The crank seal is definitley leaking. After getting the seal out i can kinda see inside the #6 cyl and can see some surface rust.
It seems someone has already been in here before too as there is some Orange rtv sealer around the seal, you can see it in the pic

The seal also had a lot of water in it. Do you think think this thing is even worth fixing? Any opinions? I will be tearing it the rest of the way down to check the Reeds on #5 and #6.

Also should water even be making it to the spot where the lower unit shaft is?

Overpoweredpony · Aug 29, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

Update!

Now were on to something! i don't have the crank out of it yet, need to get some tools from work to go any further, however i did find this! Number six reed petal broken.

My next question is what kind of reeds to get? and should i replace them all?

Hopefully i can put a new crank seal and reeds in it and be done.

Any suggestions would be greatly appreciated!

Faztbullet · Aug 30, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

I bet it has a chipped/broken reed

Guess I won that one!

I would check with a dealer/shop to see what they would charge to set crank after you pull it. Inspect # 5 & 6 real good for scored cylinder /piston due to water in it..

Overpoweredpony · Aug 30, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

Any thoughts on reeds? it seems all i can find are boyesen reeds... Any thoughts?

Chris1956 · Aug 30, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

Stock reeds are fine. Boylesens are supposed to be a bit better, but pricey.

Overpoweredpony · Aug 30, 2010

Re: 1979 mercury 90..Will not run past 1500 rpm

Well, i got the crank out tonight and the cylinders don't look bad at all! Kinda made my day!

I took the reed block off of the #5-6 cylinder and the labrynith seal kinda looks tapered some. Kinda odd though the motor didn't have any idling or starting issues.. What do you guys think, Replace it? From what i read they only cause issues with starting and idling, is this correct? I guess if i find one cheap enough i'd go ahead and replace it as i'm kinda on a budget here, (trying to pay for a wedding in a month!) But anywho if anyone would wanna part with one let me know! Again, thanks a lot for all the help everyone!

1979 mercury 90..Will not run past 1500 rpm

Cadet

Seaman

Senior Chief Petty Officer

Cadet

Cadet

Attachments

Supreme Mariner

Petty Officer 1st Class

Supreme Mariner

Petty Officer 1st Class

Lieutenant Junior Grade

Cadet

Supreme Mariner

Petty Officer 1st Class

Cadet

Attachments

Cadet

Attachments

Supreme Mariner

Cadet

Supreme Mariner

Cadet

Attachments