50 horse Mercury jet only turning 4600 rpms

[moark]

Hello everyone! Been reading this board for quite some time and learned a ton but I now have an issue I hope some one can help me with.
I have 1990 Basstracker Pro 16 with 1996 50/35 horse 4 stroke Merc jet. I took it out it the other day and it ran 5200 rpms 26 mph for about 45 minutes then started to miss and lug down to 4000 rpms and would not get higher. I fixed several light gas leaks between tank and fuel pump and changed the fuel pump and now get 4600 rpms. The manual says the motor should run between 5500 and 6000rpms. The fuel tank is up front which is about a 12' run to engine. Are jets supposed to run rated rpms or lower? Any help would be appreciated and sorry for the long winded thread.

 

[lowe1648]

You should be turning rpm's in the recomended range. Is your boat oil injected? If it is not and you hae a fuel leak it can run lean enough to cause engine damage. I have a buddy that had the same thing happen two weeks ago and is now getting a new top end.

 

[moark]

No, it's a four stroke and I've checked all connections for leaks and found several that I fixed. It went from 4000 rpms to 4600 after fixing leaks but not close to 5500 and I can tell it's lugging down the engine. I bought the rig last fall/winter and just now started getting in to it because the rivers have been so high. I test drove it but for the life of me I can't remember the rpms when I bought it. I'd like to think it's because of old age but really it was just plain dumb. I was told to decarb the motor with Seafoam which I will do next. I will let everyone know the results when I'm done.
Thanks.

 

[Lil' Blue Rude]

I'd check the compresion. I don't think it's a problem with the jet, it's probably something with the motor and I'd guess it's fuel related. Might have something stopped up. Not sure if it carb or fuel injected but if it's carb I'd clean them. You might want to replace your fuel line. Alot of people have problems with the ethanol in fuel now a days. I wouldn't worry to bad about the recommended operating speed. Your only 300rpm short when it's runnning right.

 

[moark]

Yeah, I'm with you I think it's fuel related, I'm going in that direction with things I'm doing. Compression is good; 165,165,158, and 155. So you don't think I'll hurt the engine at 4600 rpms or should I back off wot a little when cruising.
Thanks.

 

[riverracer]

Have you checked ur impeller nut ? to see if it has loosened up and let ur impeller slide down into ur sleeve ? just a thought, I had a V-6 that did it and it wouldnt turn as many rpms. ur problem sounds more like a fuel problem, but it wont hurt to check !

 

[user 6672]

Since you have verified compression (which seems on the high end), your going to want to verify spark next. Check spark while cranking and make sure it can jump 7/16ths with a strong blue arc. If it seems to be dropping a cylinder take an inductive timing light out with you on the water and connect the timing light to each cylinder (one at a time) and hit WOT, this tends to be a 2 person job.. youll see the timing light go dark if you have an ignition miss at high speeds.

Let us know what you find out..

Sounds like your dropping a cylinder. When you did a comp test were all of your plugs clean?

 

[moark]

Checked the nut, it's tight.

I put new plugs in during winterization last fall.
When I did the compression check last month I checked each plug and #1 and #2 cylinder plugs were carboned up, #2 worse than #1. Cleaned plugs while out and put back in. I checked spark with a spark tester, all good, and pulled each plug and all had a good blue spark when grounded.

Went to the river today and decaboned the motor, smoked pretty bad so there was carbon. Took the plugs out afterwards and 3 and 4 were very good but 1 and 2 were carboned up again. I put another tank and primer bulb on and still missed.
I bought a dva and will check voltages next week but I think I need the carbs rebuilt, as Lil' Blue Rude suggested, four of them, ouch!!! When I pump the bulb it won't get hard until it starts leaking out the intake????, so I'm assuming a stuck open float.
I hope I explained myself so you can understand, not very good when it comes to keyboards.
I will update as I go.

Thanks.

 

[moark]

Almost forgot, the manual calls for 170-190 psi.
I will get a timing light and give your suggesttion a shot.
Thanks.

 

[Lil' Blue Rude]

I'd think your problem is something with the number 1 and number 2 cylinder. Carbon shouldn't be building up so quick especially since it's a 4 stroke. Which 2 cylinders had the lowest compression? I'd go through the 1,2 carb to just clean and see what that do's sometimes you can get away with just clean a carb and not rebuilding them.

 

[Ranchero50]

If the floats leaking by on a couple carbs it'll run rich and have troubles. Instant carbon is a pretty good sign.

I took mine apart on the Merc 9.9 I had, reused the gaskets by just putting a thin dab of grease on them before reassembly. Can or carb cleaner made them nice again. Should be a simple fix for you.

I do wonder where is the fuel pump on that engine? Possibly it's running lean on the bottom and rich on the top because it can't feed enough fuel for the cylinders away from the pump? I guess you could install an inline pressure guage and check it out or even running a short hose and tank by the engine. If it's lugging under a load they should all be dark I would think... Just thinking out loud to help isolate the problem.

Jamie

 

[moark]

New water pump 2 weeks ago, not a rebuild, that's another story in it's self but I won't bore you with the details. It is a mess getting to the carbs but I'm going to give it a try but I'll have to take in to synchronize the carbs, need special equipment for that.
This motor has been nothing but trouble but I think the guy before me didn't do much to care for it and actually did some crazy things to keep it running to sell it. I'll get her going and give some tlc and see how she goes from there.
If I remember correctly the top 2 cylinders were the low ones but are well within the the 10% limit of each other.
I guess I'm in for a carb cleaning (4), for sure the #1 and 2. I was hoping the intense Seafom cleaning would help but no such luck. When I get her going I'll let you guys know the fix.
I'm learning, thanks again for all the replies and help, much appreciated.

 

[user 6672]

You get this figured out?

 

[moark]

Not yet. Went to sharpen the impeller and liner was grooved so bad it's a wonder it even planed. Ordered a new one and going to do the spark test at wot when it gets in as you suggested. I checked all the idle voltages with the dva and all within spec so the wot spark test is about it electrically.
With wet plugs it's either that, float stuck open, or the carbs could be out of synchronization. If it's not electrical I'll tear into the carbs and then take it in to synch the carbs. Have to have special equipment for this......... should have bought a two stroke.
I will let you know the out come, thanks.


Gotta get this thing going for fall fishing.

 

[moark]

Well, I'm stumped.
Did leak-down test, all cylinders good, around 7-8% air loss.
Compression tests good.
Took all four carbs off checked and cleaned. They were in very good condition.
All dva voltages good.
Resistance checks good.
All wires good, jumping 7/16 gap.
Also took the boat to the river and ran wot with timing light and the coils were firing.
#1 still not firing. #1 plug shooting nice blue arc when grounded and when I put #3 plug in #1 cylinder it still does not fire.
I'm going to get all new plugs and start from scratch. If I EVER figure this out I'll let you guys know. I'll probably end up taking it to a certified mechanic, even if my pride takes a hit. On second thought, never was much of a mechanic, just a wanna be.

 

[user 6672]

You do a high speed shutdown and check the plugs for water?

Everything might be testing out but if you have a bad inner exhause gasket or form of hairline crack it may let a little water in at WOT..

just a thought..

 

[moark]

Never did that one but it's worth a try. When I pull the plug it's wet with gas. Could that happen at idle, it misses all the time?
I keep hearing the stator on that particular engine is a big problem. Listed at $520 (at Mercury website) I was hesitant to buy but I found a new one, by luck, for $350. I broke down and bought it. I home rigged a flywheel puller but it bent before the wheel budged at all. No one rents these things and my dealer wanted $100 just to pull it so I broke down again and bought one, ouch again. Waiting on both to arrive.
When I get this thing going I'm just going to look at it.................I'll be afraid to drive it with the luck I've had with it.
Thanks for the tip.

 

[moark]

I installed a new fuel pump and the readings are on the low side. The vacuum is about 8" of mercury, spec is ~10, and the output pressure is around 2 psi with spec being 3-6. Also the manual for the gauge says psi should hold for a couple of minutes after shutting motor down but mine goes down pretty quickly. i have checked for leaks and cannot find any. I'm getting gas to the top carb because I took the line off and it shot out pretty good.
Is the output pressure (psi) adequate even though the manual says no? I know sometimes these manuals are a little off so experience at times is more reliable.

 

[moark]

Finally got her going, bad stator.
Spoke with guys at OPC Marine and was told you cannot always count on all cylinders firing at idle with my set up, 1 ignition coil firing two cylinders, because one side of the coil sometimes goes to ground instead of firing plug, crazy but he was right. I was basing most of my tests at idle and short bursts at 2000 rpms using the flushing port, by the way it did not hurt my water pump.
And although I checked the the spark at wot with a timing light apparently the spark was not strong enough to fire the plug at that speed.
Took it out today and it ran great.
Caught two drum (on small jig and pig), several Kentuckys, and a few small smallies, man those drum fight on light tackle.
I want to take everyone who took the time to help me out, I appreciate it.

 

[user 6672]

Way to get it chased down!!

What RPM's are you pullin now at WOT?