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Mercruiser 1983 305 has to warm up forever

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briang0

Regular Contributor
My 1983 Mercruiser 305 with Thunderbolt Ignition has to warm up for about 20 minute before it runs good. I have replaced plugs, wires, cap, and rotor. Cleaned the carburetor and made sure that the choke was working and not sticking. When I start it when it's cold it fires right up and goes up to high idle and will run at high idle for a while and then will drop down and runs nice and smooth. If I try to rev the engine it will bog down sometimes back fire through the carburetor. If I try to leave the dock I can't go much higher than 2,000 RPM or the engine will bog down. After about about 20 minutes or so I can then run the RPMs up and get good performance out of the engine. The plugs I put in were AC MR43 T and I gaped them at .35 I got this information off the tag on the engine. I checked to see if there any wires arcing but I couldn't see any. I would think if it were wire related it would not get better. All the parts I purchased from this site except the plugs. My last thing to check is timing which I will do tomorrow. What did I miss?
 
Did you run a compression test on the cylinders?
If I try to rev the engine it will bog down sometimes back fire through the carburetor.
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Lean condition can cause it. So can a burnt exhaust valve...run the comp. test.
 
I have not done a compression test yet. When I pulled the plugs they looked great nothing to suggest any cylinder was not doing it's job. Is there a map for the vacuum on this engine like in a car or is it all in the intake manifold? This issues seems to be getting worse because when I first put it in the water she ran pretty well. When I took the boat out of the water on Friday it was at it's worst.
 
OK I did some investigation. My engine is an 898 with a 2 barrel Merc carburetor on it. There is no vacuum lines running from the intake to the carb. or heat tubes. There is one clear line which comes from the fuel pump. The choke actuator is electrical. I made sure that all the screws and nuts were tight and double checked the operation of the choke plate and linkage. Then I checked the fuel filter on the carb. What I found was that it was half full of what appeared to be a fine powder similar to saw dust. I cleaned it out real good and re-installed it and ran the engine on the flushing ears. It was hard to tell how much of a difference it made because the engine had been sitting in the sun most of the afternoon so it had already heated up and as I watched the choke it only was closed for a short period of time. It did open and the engine did not run at a higher idle. I did try manually closing it and when I did I stalled the engine out it did not pick up speed.

If I want to check for vacuum I would have to remove one of the intake manifold plugs because I cannot find any hoses with vacuum on them. The distributor is electronic and with no vacuum advance.

I'm heading to the store to pick up a new filter/water separator since it must not be in to go a shape if the one in the carb was the way it was. This is a new to me boat and the previous owner said it sat all last year. He was a mechanic so I "thought" he took good care of it but I guess I was wrong.
 
Then I checked the fuel filter on the carb. What I found was that it was half full of what appeared to be a fine powder similar to saw dust.
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The powder was brown...dried fuel varnish. The same powder will be inside the carb bowl and inside the fuel pump dome filter...change it too.
If I want to check for vacuum I would have to remove one of the intake manifold plugs
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There may be a plug near the top of the manifold behind the carb to use for a vacuum nipple. Check the carb base nuts for tightness. You can check it for a vacuum leak when the engine is idling by spraying carb cleaner around the carb's base gasket.
 
I replaced the fuel filter/water separator and cleaned the filter up at the carburetor started the engine and let it run for about 5 minutes. Tried to bring the RPMs up past 1500 and it started to break down and would not go above 1200 - 1500 and back fired through the carb. I shut down the engine and checked each wire on the cap one at a time looking at each to make sure that none were discolored and looking for any sign of arching. Guyjg where is fuel pump dome filter? I'm not familiar with that. I restarted the engine and it ran fine and I was able to run the RPMs up past 3000 with out any problem. Now I'm thinking it might be something in the fuel line or floating around in the fuel tank. I had a similar problem a few years ago on a different boat but that would kill the engine altogether. With the tank under the floor and only fuel sender hole to look into it is not going to be easy to figure out what is in there if anything. Last time I connected a vacuum gauge to the fuel line and watched it change as the engine started to stumble.
 
i had the same issues with my boat. i ended up putting a carb kit in, filter/ water separator and screen and it ran great for a week. then the same issues arose. I changed my fuel pump and the boat runs great. I am not a fan of simply cleaning the filters, they are cheap replace them. I was told to change the fuel filter/water separator every year. On my engine there is a glass bowl attached to the fuel pump if that has fuel in it the pump is finished. I am not a mechanic by any means but i would suggest pulling your fuel line off of the pump remove your distributer wire turn the key and check to see how much flow you have. Its a shot anyway good luck
 
i would suggest pulling your fuel line off of the pump remove your distributer wire turn the key and check to see how much flow you have.
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No...don't do that. Install a "T" in-line w/the output and attach a pressure gauge to it. Pump should put out 4-6 lbs. pressure.
 
guyjg my pump does not look like the one in the url. At this point in time I'm not sure what I'm going to do next. I want to put the boat back in the water. I think adding the fuel pressure gauge off the line to the carb is probably what I'm going to do so I can watch it when it acts up again. The fuel pump is pretty easy to get at so it won't be to hard to replace if it comes to that.
 
Does the engine temp. gauge show about 140 F.? If not, as crankbait stated, thermostat probably needs replaced.
 
The engine runs solid at 140. I put a new thermostat in May when I did the water pump and housing. This is strange I will recap the symptoms

Cold start after about 5 minutes choke opens...
try to accelerate past 1500 rpms and engine will bog down sometimes backfire through the carb
Shut of the engine
Restart after maybe 5 minutes and engine runs fine.

This has happened on several occasions. Also it cleared up once without having to shut the engine off I just backed off the throttle for a minute and then applied more throttle and it was fine. (I thought it just took forever to warm up)

So far new plugs cap rotor wires filter in carb and water separator double checked wires for connection. A gear head buddy thought it sounded like ignition possibly electronic ignition module.

Does the advance of the timing all happen in the thunderbolt ignition module????
 
The advance is done electronically inside of the T-Bolt IV module...if you had the dust like crud inside the fuel system, it is highly likely it is inside the float bowl of the carb. You need to thoroughly disassemble to carb to get the junk out of the passages...not just throw a kit in it...verifying the fuel pump's operation is a prudent step now.
 
I think I have figured out the issue. I pulled the distributor cap to check for moisture and what I found was that the center post was all pitted. The cap was only about 2 months old I looked at the rotor which was the same age and it showed signs of arcing. I read in the manual (after the fact) that you have to make sure that the height of the metal contact piece of the rotor is suppose to measure 1/4" above the plastic. The other issue which I had was it was twisted slightly. Not only was it low but it was also twisted. When I looked inside the cap several of the brass contact pieces were also a little corroded. I replaced the rotor made sure it measured the right height and was flat and put a new cap on. I started the engine and increased the throttle up past 1500 rpms with no problem. Right now the boat is out of the water because I was working on a trim hose issues but I'm pretty confident it all set.
 
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