1990 mercruiser 5.0 power loss above 3200 rpm

[ojohn]

This power train looks all stock, but I have only had the boat two summers and do not have much history before that. It is 21 foot American (Oregon made) glass cuddy hull. 5.0 Mercruiser Alpha. Has Quadrajet carb and Thunderbolt IV ignition. It starts fine, idles smoothly, runs up to about 3200 (on plane at about 2700 with 17 pitch prop) as long as throttle is advanced slowly. No trouble maintaining that rpm/speed. Any further advance of throttle and she starts to sputter badly and basicly just about quits running. I think it would die if throttle not backed off. It will do the same thing at low revs out of the hole if throttle pushed too fast. Now the oddest part. If I really "slam" it to WOT when running at 3200, it will burp a couple of times and then (most of the time) jump right on up to max rpm of about 4200, and stay there until I pull it down. If I try to "hammer" it before she is on plane, she won't go and just sputters and tries to die.

Chasing this problem I have had the carb rebuilt, new ignition amplifier and sensor, new coil, new plug wire set, and new plugs. New pick up tube in fuel tank with no antisyphon valve (valve wasn't there before) and new Racor water/fuel separator. Fuel filter at carb was cleaned or replaced with rebuild.

 

[Blk-n-Blu]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

Sounds like you may have to adjust your carb settings. There is an excellent article in the FAQ forum on just that.
8)

 

[ojohn]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

Sounds like you may have to adjust your carb settings. There is an excellent article in the FAQ forum on just that.
8)

 

[ojohn]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

Sounds like you may have to adjust your carb settings. There is an excellent article in the FAQ forum on just that.
8)

Thanks for pointing me to FAQ. After reading the post there I'm not sure that it applies to my I/O engine. The description of applications/carburetors sounds like he is talking about outboards. (the V4 talk and the description of setting the high speed needle valve) Is this really for Rochesters on V8 Mercruisers?

 

[Don S]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

No, it's not for Rochesters on V8 Mercruisers that post is for outboards. Rochesters only have idle mixture adjustments.
You probably just have some plugged passages, water in the carb or something of that nature.
Might want to do some compression tests and vacuum tests on the engine. Might reveal the problems.

 

[ojohn]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

No, it's not for Rochesters on V8 Mercruisers that post is for outboards. Rochesters only have idle mixture adjustments.
You probably just have some plugged passages, water in the carb or something of that nature.
Might want to do some compression tests and vacuum tests on the engine. Might reveal the problems.

Thanks for response. Are you thinking "plugged passages" in the carb? Where would you start with tests? Seems like we have just taken a shotgun approach so far. Is there a systematic order to test things? Right now I am feeling like I have to just get lucky with another mechanics best guess.

 

[Reel Poor]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

Have you changed the distributer cap and rotor ? Excessive gap between cap and rotor could cause thes symptoms

Have you checked fuel pressure to carb ? Low or intermitent fuel flow could cause these symptoms.

Have you checked ignition timming for proper base timming setting and operation of timming advance ?

Is there addiquate fresh air flow to the engine compartment ?

Is the fuel fresh ?

Have you done a compression test ?

Vacuum guage readings tell a great story.

All these things should be checked and repaired as/if needed before proceding.

To me, from your description, it sounds like you have some carburation problem. Although there is not yet addiquate information to jump to this conclusion.

 

[ojohn]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

Thanks Reel. Air supply to engine is good. Fuel is fresh. Timing was set with installation of new ignition amplifier and sensor, so am trusting that is ok (maybe a mistake right there). I have not checked fuel pressure (too much ignorance and not enough equipment) or vacuum, but will do it if I can figure out how. Or will have it done (gets expensive, of course).

And Don S. Thanks also for an old post of yours #5700 from 8/2004 which shows list of things to check, titiled "Engine wont reach operating RPM."

Compression test I understand and can do. Where would you check vacuum and how would I do it? Do you know of posts on "how-to" by any chance? I am still feeling my way around the forum search function. Only just figured out that the condition/problem I am dealing with seems to be called "bog" or "bogging." Any hints on "how-to" info or searches would be great. Thanks again.

 

[Reel Poor]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

Here is a great article on using and reading a vacuum guage. Every thing discussed will not be relevent to your marine engine, ie. pcv systems, emission control systems, transmissions ect. But the basics are still the basics.

How To Use And Read A Vacuum Guage

 

[ojohn]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

Thanks Reel; I will go take a look at what the auto parts stores have for guages. On my 5.0 Alpha LX is there any other place I would be checking besides intake manifold? Is there a port provided or do I need to take something loose? My Seloc manual doesn't have much to say about any of this. Is Mercruiser shop manual more helpful?

 

[Reel Poor]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

There may be a 1/4 inch pipe plug either in the base plate of the carb or in the intake manifold just inches behind the carb. You could remove this plug and install a temporary fitting to do the testing.

The Mercruiser repair manual is far better than the others avaliable.

 

[Mahoney]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

have you had someone(carefully) watch the carb opperation while you are on the water? With the flame arrestor off and face not directly over the carb, are the plates and butterflys all working correctly? can you see that fuel is being delivered but not soaking the carb with gas?

sounds like it could possibly be something as easy as your secondary butterflys not openning when needed, and when you hammer it, the intakes can develope enough vacuum to firce them open. this would explain why gunning it before you are on plane wont do it, but after you got over that initial hump of rpms, that gunning it will get em open.

Thats for hoping. there is a screw on the choke side of the carb that also needs a very small allen wrench to adjust the spring tention of the secondary air butterflys.

 

[ojohn]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

Hope is good. Thanks Mahoney.
After a careful re-read of the Seloc book I have in hand, I think I get what you mean. Yes, we did run it with the spark arrestor off (face out of the way) but I was not the one looking at the carb. I was just helper driving the boat, and I think the guy checking the carb was most concerned with accelerator pump, which he declared to be working. My book says the "secondary throttle valves" are opened by linkage with primary throttle valves. (are the throttle valves what you are calling "plates"?) And "butterflies" are the air valves? And air valves are opened by a combination of atmospheric pressure on top and vacuum from below. I will go home and take the lid off the carb and see if this still makes sense. (Sooo much ignorance to remedy.) Thanks much.

 

[Mahoney]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

I think you got it, right, the throttle linkage starts to engage the secondary throttle plates when the primaries are at about 30 degrees, and reach wide open at the same time the primaries do at Full throttle.

The butterflys are not linked per se to the throttle linkage and are "sprung" open when air is needed from the engine through vacuum like you said. Your secondary needles are attached to the butterfly as well, so if those butterflys aren't opening, the needles arent coming out of the jets, and no fuel.

 

[ojohn]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

Well, I got a vacuum/pressure guage at Napa. I found the plug in manifold to hook up to, and have looked over the carb parts Mahoney pointed me to. I did not find the allen head adjustting screw on the carb, but I did just now find it (I think) on the blowup in manual. It is on the side of air horn body, kind of hiding under the lever which carries the rod from vacuum unit, right? I think I will not mess with this until I can confirm vacuum levels under load, and whether the secondary side looks to be opening correctly or not. Now all I need is time to go putt around a while. Any other suggestions for what I might check on the next run are welcome. Thanks guys.

 

[Mahoney]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

that is correct on the allen screw. You use a screwdriver to hold the position of the butterfly tension screwon the side as you loosen the allen screw underneith. Be VERY careful to keep your screwdriver on the small screw, as it is now spring loaded and will want to self unscrew itself when you loosen the allen screw. Not a fun trick when you have to pull the carb to put that little assembly back together.

I am only suggesting that might be the issue, or one of the issues. As with all carb adjustments, do yourself a favor regardless of how tempting it is, and make ONLY ONE adjustment at a time. This is more time consuming, but far more effective in narrowing down the issue and adressing it.

Good luck and keep us posted.

 

[ojohn]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

Well, it has taken me much longer than I thought to get someone to go for a ride with me so I could check this carb out. Finally did it. I found the choke linkage seemed like it was hanging up, not letting the air valve open on secondary side. I took it loose and the valve itself was moving very smoothly. Adjusted the linkage rod (slight bend with a pair of pliers) and put it back. We changed something. Engine seemed to be running right again. Will be taking it to the lake for the Labor Day weekend. Hoping the fix was really that simple. Never did mess with adjusting the spring tension on the valve, but thanks Mahoney for putting my nose right there to find it.

 

[Mahoney]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

Well at least I was pointing you to the right area! I have actually had that issue before too, you can actually put that choke linkage on backwards or turned around or whatever that will actually prevent the secondaries from opening.

Let me know if you are still having issues with how that linkage is binding, and I can take a pic of my carb for you and see if its what you have going on.

Glad to here you are back in business!

 

[ojohn]

Re: 1990 mercruiser 5.0 power loss above 3200 rpm

Spent three days on the lake and not a bog or burp in sight. Seems like the linkage was the issue, maybe. Thanks again to all, and special tip of the cap to you, Mahoney.

Now on to find out if the grumble/thump in the stern is my old man ears, a problem brewing, or just the way this one sounds. I'll be back for help if it gets interesting.