Mercruiser IO, Loss of power

[skyking_22]

Can anyone help me out? I have a 2000 mercury I/O Alpha I. Has a chevy 305 engine with Thunderbolt V ignition. This is a carburated model. My problem is I have lost power with the engine. Originaly the boat ran at 4800-5000 rpm and a speed of 46kts. Now I can only get 3800 rpm and 15kts. I have verified the prop pitch and hub are fine. Changed the plugs, wires, cap & roter. Verified timing was correct. But it only seems to be getting worse. I have a closed cooling system so I don't think it could be a rusted valve, or could it. I am at my wits end. Any suggestions would be great. I see all kinds of posts here and there seems to be a lot of knowledgable people here. Thanks in advance.<br /><br />Bob.

 

[Trent]

Re: Mercruiser IO, Loss of power

Water temp go up?<br /><br />Engine running smooth? All ranges?<br /><br />Carb butterfly opening all the way?<br /><br />Need just a little more hints???<br /><br />Compression ok?

 

[skyking_22]

Re: Mercruiser IO, Loss of power

Water temp remains constant at 170 deg. Engine runs smooth except for occasional backfire/pop from carb at wide open, (have replaced flame arestor just in case) with no difference. I can work the butterfly and when the throtle is full it is all the way open. Have not done a compression test, don't have a gauge yet. When I changed the plugs all were evenly worn and no deposits on them. They looked like they were 2 only years old. I have also checked fuel pressure from tanks to carb and all is fine here. I even changed the in-line filter at the carb.

 

[f_inscreenname]

Re: Mercruiser IO, Loss of power

Just a guess. Water in gas. Sticking valve maby. My first guess though (Backfire/pop from carb) sounds like a timing thing.

 

[daveswaves]

Re: Mercruiser IO, Loss of power

Bob, there are only 2 things that can cause a backfire in the carb, running way too lean or really bad timing. Its unlikely the timing just "changed" unless your dist hold down bolt was or is loose. You mentioned that you changed the cap, wires etc. Is there a possibility that you mixed up two wires, firing a cyl on the intake stroke will cause a backfire, I have done a lot of chevy V8's and helped pals get them going again after they "fixed" them. Swapped plug wires are responsible for a lot of trouble. If there was a wire swapped you would only be running on 6 cyl which could explain your rpm/speed. Check the firing order on your block and trace the wires from the dist to the cyls. the firing order should be 1-8-4-3-6-5-7-2<br />Remember, the chevy is 1,3,5,7 on the port side, counting back from the bow to the stern. 2,4,6,8 is on the sbd side, distributor rotates clockwise. Your No 1 cyl should be marked on the dist cap.

 

[skyking_22]

Re: Mercruiser IO, Loss of power

Thanks Dave for the input. I verified the wired were correct, I will have to double check the timing. The mark I have is not like your standard mark on a car. It only has 1 indent in it (like a giant V) and 6 hash marks on the crank shaft. The book said it should be @ 8 TDC. So I put it on the 4th hash mark thinking each one was 2 deg.

 

[Down South]

Re: Mercruiser IO, Loss of power

Hi Bob,<br />A bad or sticking intake valve can cause the very problem you described. Get a compression guage and check the compression before you start changing a lot of things.<br /><br />Good luck

 

[DJ]

Re: Mercruiser IO, Loss of power

Bob,<br /><br />I believe it is timing also. Did you notice anything strange happen, all of a sudden, when this began-large backfire, hard start, etc?<br /><br />I jumped tooth in a timing chain will act like that also.<br /><br />One of the best tools for diagnosis is a vacuum guage. Vacuum should read about 18 in. hg. and steady at idle.

 

[skyking_22]

Re: Mercruiser IO, Loss of power

I am getting a vacuum and compression gage from my neighbor tonight. I will check my readings and let you know. Thanks again for the help I am pretty savy with cars and I am new to boats. I dread paying the dealer a small fortune for things that I can do myself. Any thoughts on it possibly being the coil? To answer the other question, nothing unusual happened, it just began to slowly loose power and then all at once down to 3600 rpm.

 

[daveswaves]

Re: Mercruiser IO, Loss of power

Hi Bob, still sounds like timing, but it sounds like your timing at idle may be OK but perhaps you are not getting advance at speed. Check to see that the dist advance plate is free to move????

 

[chiefmike58]

Re: Mercruiser IO, Loss of power

Bob G, if that timing proves to be ok, I think the next obvious place to look would be the carb. How old is it, and has it had a recent rebuild? Also, though a long shot, but I've seen it happen more than once, is the space between prop and lower unit housing clear of all fishing line, etc., because that will show similar symtons as you are describing?

 

[SS MAYFLOAT]

Re: Mercruiser IO, Loss of power

Hey! Sounds like a fuel filter ristricting your gas flow to me. This would cause lean condition causing back fire. Been there, Done that! It's alot cheaper than all the diagnostic work.

 

[Walter]

Re: Mercruiser IO, Loss of power

You have a water/fuel separator fuel filter? When was the last time you changed it? You mentioned you changed the filter screen at the carb, but I don't recall reading you changed the spin on filter. Go there...do that (if you haven't already)...before you do anything else.<br /><br />Walt

 

[skyking_22]

Re: Mercruiser IO, Loss of power

Well, I have good compression in all cyl's. Vacuum is a little strange. With the timing set for best performance it is around 15 in, when I set the timing at 18 in vacuum, I have no power. I have not checked the dist advance, but I will. Prop area is clear of line. As for the fuel/H2O seperator, I changed it last year, maybe time to change it again. I really appreciate everyone's help.

 

[flashback]

Re: Mercruiser IO, Loss of power

You H a v e a vaccum leak.

 

[daveswaves]

Re: Mercruiser IO, Loss of power

So.....Bob, did you get it fixed????

 

[n-4ih]

Re: Mercruiser IO, Loss of power

Bob G, I'm curious about the outcome of your problem. I have a 1998 2452 with a Mercruiser 5.0 (220HP, carbureted). Just recently I developed a problem with it suddenly losing power and running rough. I had been running at 3600 RPM for about 30-45 minutes with a short spurt to WOT. Five minutes after I dropped back to 3600, the RPMs started dropping to the point where I could not stay on plane. The motor began running a bit rough. Operating temp did not vary from my normal 175. I brought her down to about 1200 RPM for a few minutes while I looked for obvious problems in the engine compartment (none found). I was able to get her back up to 3600 RPM but she continued to run rough. My mechanic's first look determined that the carb appears to be alright but that I've lost compression and need a valve job. Three plugs looked burned. I've got about 420 hours on the motor (in six seasons). She is raw water cooled and operated in the brackish waters of Chesapeake Bay. I flush the motor/outdrive with fresh water at the end of each weekend's run. Any suggestions that you might pass on because of your experiences would be greatly appeciated.

 

[skyking_22]

Re: Mercruiser IO, Loss of power

N-4ih<br /><br />I think it must be the Chesapeake Bay, no really. I am still struggling with the same problem. I have spoken to Mercruiser and they suggest I have my Gimble bearings checked. They think it might be a bad bearing causing the loss of power. I have had the engine checked 7 different times for the same problem and no one can seem to solve it. They tell me when I pick it up that it was a bad plug wire. I must have gone through 4 complete wire sets in 6 boat trips. Sounds crazy to me. What I need to do is find a mechanic that will run the boat and not just diagnose it on the DYNO machine only. I had to drop to a 13 pitch prop just to get 4000 rpm. Hope you have better luck than me.

 

[bert6315]

Re: Mercruiser IO, Loss of power

I had a similar problem. I changed the plugs , but found that one of the wires on the distributor cap was not fully seated. The boot had slipped down but now the wire was not securely seated in the cap. It must click to be seated.<br /><br />Bert

 

[skyking_22]

Re: Mercruiser IO, Loss of power

Strangely enouth I also have a 2452. Maybe it is a systemic problem with that model boat/engine?