1999 Mercury 90 HP 3 CYL no power & rough idle

[vrundquist]

Hello all!
I have run into a very hard to diagnose problem. I have a 1999 90 HP 3 cyl marketed under tracker. At the end of last season the motor would run for about 5 minutes with reduced power and then nothing. It really was not maintained well so I decided to look at the usual suspects. I have done the following to the motor:

R & R carbs with rebuild kits
New fuel filter and hoses
Fuel pump diaphram kit
New timer base (old one had a crack in the epoxy)
New stator and regulator/rectifier (old one had wires melted together and was not charging)
New coils (thougt it to be a good idea since other ignition parts were being replaced)
New reed valves on #1 cylinder

Compression 115,115,120
Removed bleed system tubing and check valves and tested with vacuum gauge.

When the motor is running it will sputter every once in a while and RPMS will increase then drop back down. Almost like 1 cylinder is going in and out of operation. When under load (pushing boat against dock) I can put my hand over the #1 carb intake and the motor will not bogg down. Placing hand over #2 and the motor bogs down but still runs. Placing hand over #3 and the motor dies instantly.

Yesterday I removed the covers on the cylinder head and the exhaust housing. Found what looked like a small water leak around the gasket in the area where the exhaust port is for #1. On the cylinder head side, I found some sand wedged between the gasket and the cover between #1 and #2.

Any ideas on what else to check. This problem is very frustrating. I have done a spark test and all 3 fire fine at idle and while under load (after replacing ignition system). The only item I have not replaced is the flywheel and I found no signs of damage to it.

thanks in advance!

 

[jeffersondodson561]

Re: 1999 Mercury 90 HP 3 CYL no power & rough idle

Check the head and block for distortion with a straight edge and feeler gauges. #1 is obviously not hitting. If when you pulled the carbs off did you inspect the reed valves for damage?

 

[vrundquist]

Re: 1999 Mercury 90 HP 3 CYL no power & rough idle

I repolaced the reeds on #1 because I thought I saw what looked like discoloration on one petal. And that petal did not have the spring that the others did. i will check for warpage and report back...

 

[CharlieB]

Re: 1999 Mercury 90 HP 3 CYL no power & rough idle

Have you found the service manual available thru the links in the first postings in this forum, if not, do it.

Careful attention to the carb link and synch proceedure can make a substantial differnce in how a motor runs.

Next, using either an inductive mini-tach or timing light, ensure you still have spark on each cyl when the problems are occuring.

Testing is THE most cost effective method of determining a problem, as you have already learned, throwing parts on often just costs you a LOT of money and gets very frustrating when the problem still occurs.

 

[Maxz695]

Re: 1999 Mercury 90 HP 3 CYL no power & rough idle

Check for dry rotted or cracked coil wires they will work fine sometimes and miss at others. Just a suggestion

 

[vrundquist]

Re: 1999 Mercury 90 HP 3 CYL no power & rough idle

I do have the online service manual from Seloc and the mercury manual @ manual1.com. this motor had lots of real issues. When I started on it, it would not run at all.

thanks for the advice, I will re-check the carb sync.

 

[vrundquist]

Re: 1999 Mercury 90 HP 3 CYL no power & rough idle

when you speak of the coil wires, I assume you mean the high voltage ones going to the plugs? When I replaced the coils, they came with new wires.

 

[vrundquist]

Re: 1999 Mercury 90 HP 3 CYL no power & rough idle

Checked all spark plug wires, and no cracking or dry rot. Checked the mating surfaces of the exhaust cover end head cover and found no evidence of warpage. Used mirror thru the #1 exhaust port to inspect cylinder and piston. Nothing to note except the top of the piston is brown, where the other two are slightly brown but not as dark as #1.

Maybe the problem is in the bleed system... As stated before, I checked the bleed hoses and check valves. I did nothing to the fittings on the block. If one of the fittings was plugged, would this cause the symptoms described above?

Also, what about the carb idle adjustment. I get conflicting instructions from Seloc and the merc manual. Seloc suggests all idle screws backed out 1-1/2 turns for initial start, and the Merc manual says 1-1/2 on #2 & #3 and 1 turn on #1. I would tend to listen to the Merc manual but the Seloc manual is mostly written from experience. What do you guys think?

 

[CharlieB]

Re: 1999 Mercury 90 HP 3 CYL no power & rough idle

Brown pistons are too lean. Brown to tan is ideal coloration for racing motors but is so lean that the slightest fuel problem will scorch a piston, like simply running out of fuel.

Check the main jet sizes compared to the parts manual, it is possible that you have a high altitude jetting.

Initial idle adjustment is only to enable starting the motor, final adjustment must be made running, IN gear, while IN the water, to best idle speed PLUS almost another 1/8 turn out to provide the additional fuel for acceleration.

Float height can be critical, make SURE that they are set correctly, then synch the carb linkages.

Ensure the throttle cam and timing are exactly as spelled out in the factory service manual.

Check fuel hose connections to ensure no air is sucking into the system anywhere before the fuel pump by substituting a temporary clear hose between the fuel pump and carbs. Observe for any 'bubbles' passing, indicating air. Under loads the pump may not be filling the upper carb(s) causing a lean surge and/or browning the pistons.

I do not recommend continuing to operate this motor except for short testing as continued operation lean will sonn damage the motor.

 

[vrundquist]

Re: 1999 Mercury 90 HP 3 CYL no power & rough idle

I did find that the #1 float height to be 1/8" too high. I suspect that might have been flooding #1. I went ahead and repleced it with a new one and adjusted it to 9/16".

What do you all think of the upper seal being the culprit to the symptoms? My local mechanic suggested I look at it. He said a leaking upper seal could affect the vacuum created in the #1 cyl crank case. I found some oil under the timer base and on the seal area. I also used a feeler gauge around the seal and one side was not as 'tight' as the other. Got the old seal out no problem, then SEVERE THUNDERSTORM WARNING. I am in for the rest of the evening. Will have to continue in the morning.

I will report tomorrow as to the status. Thanks for all the advice.

 

[vrundquist]

Re: 1999 Mercury 90 HP 3 CYL no power & rough idle

Replaced the upper seal. Now the engine will respond to me placing my hand over the #1 cyl intake. I did this while connected to a hose and running at 2300 rpm. Interestingly enough, when the engine came up to full temperature again it started to stumble at idle.

I have not run a compression check when the engine is warm. Should this be the same as a cold compression check?

thanks,

Victor