Can't go WOT

I'm working on a 83 Searay 21' Mercruiser 350 w/4 barrel that starts right up and runs fine as long as I don't push past 3400 rpm's or accelerate too quickly. If i do it coughs and spits and sounds terrible. Do you all have some pointers on what to try first? Does it sound like a carb' problem or something else like timing. Since it starts up so well i kind of discounted the timing but plan to check it soon anyway. Thanks for your help, you guys are a real wealth of info and I have learned a ton from reading your responses over the years.

what have you done to it so far? tune up,filters, etc?

I really haven't done much except add a new temp sending unit+gauge and a new thermostat and installed checkballs that were missing since I bought it. I did a sea trial after I bought it since i got it so cheap and really only tested it farther after eliminating an overheat issue I was told it had. Because it sounded so sweet and started immediately on the muffs I didn't tune it as the PO said he had recently had it done along with a new outdrive pump and risers. He also change the distributor and coil so it is not using the original thunderbolt setup. I don't know the age of the fuel water separator or any other filters it may have. It now runs at about 200 degrees on the gauge and hold there steady. Using a laser gauge it shows cooler temps than that everywhere I pointed it. No more than 180 something.

If PO changed the distributor and coil to a "non thunderbolt"...
a) Is it at least a marine distributor?
b) Is the advance curve of this "non thunderbolt" correct for the engine?

Re: 200 degrees and 180 w/laser
a) laser reads surface temps, internal WATER temp is usually a bit higher
b) Is this a RAW water cooled engine or does it have a "fresh" water conversion, i.e. a heat exchanger (FWC)?
If Raw water cooled, engine WAAAY too hot. If FWC still a bit warm. Raw water cooled engines (if run in salt water) have a Tstat about 130 degrees. If FWC, a 165 degree T'stat.

Check the compression on the engine...

yes it is marine distributor and it is raw water cooled and i'm not sure about the advance curve.

The question is not is the distributor a marine, Is it a point ignition or electronic?

If it is points a complete tune up is in order, points, rotor and condenser. then make sure the mechanical timing counter weights are moving properly. Then set base timing to ~ 6*-8* btdc

If electronic let us know and we can discuss options...

it really sounds like carb rebuild time at this point.......

Also your temp is tooooo high!!

that motor should be running a 143* thermostat. Running temps should not get above 170+/-.......ever......

Well the distributor is electronic and I just now 30 mins ago, I changed the thermostat to a 140(as close as i could get) and last Friday i replaced the water pump in the outdrive for good measure although it was in good shape.When I tested it on the muffs it still went to about 180 and held. I bought the sending unit from NAPA and the gauge from West Marine so I'm still not sure they match or are telling me the real temp. I also cleaned the breather (flame restricter carb cover), whatever the right name is, which was quite dirty. I am also hearing what I think is a faulty flapper in the exhaust on the starboard side. When running I was hearing a louder than expected engine noise but figured it was because the covering of all the transom area was re- invented by me because it was non existent.(I made it out of a half inch starboard type material.) Anyway it's too noisy and I believe I have now isolated it to that exhaust. It's not so noisy and idle or lower speed. I don't know is it is related to the original problem or not. I checked the timing and it's right at 8 degrees BTDC. So what do you think now?

My suspision is you have an exhaust manifold/elbow rerstriction.

Are the exhaust manifolds rectangle in shape? If they are you have a older outdated style, Not very efficient. That bieng said, there are basically three issues regarding high water temps. Incomming water supply, good impeller and no restrictions or air bubbles. Thermostat and housing in good shape (not rotted away). Water exit, if the manifolds or elbows are clogged or restricted they will not allow the hot water to exit the motor fast enough.

As far as replacing the water pump impeller in the outdrive, Did you put the large 1/4 inch square oring on top of the water pump housing? This oring keeps exhaust gasses from entering the incomming water stream.

DId you look up where the copper tube connects? The tube inserts into a rubber "boot" into a plastic diverter in the upper gear housing. Sometimes and I have seen this myself many times, the rubber boot gets pinched by the copper tube and partialy clogs the water stream......May be worth looking at.

Note: the exhaust flappers are there to keep water coming back wards into the exhaust manifolds while in the water. Running on a hose they have NO affect on anything.

markbabb said:

I'm working on a 83 Searay 21' Mercruiser 350 w/4 barrel that starts right up and runs fine as long as I don't push past 3400 rpm's or accelerate too quickly. If i do it coughs and spits and sounds terrible. Do you all have some pointers on what to try first? Does it sound like a carb' problem or something else like timing. Since it starts up so well i kind of discounted the timing but plan to check it soon anyway. Thanks for your help, you guys are a real wealth of info and I have learned a ton from reading your responses over the years.

Click to expand...

Ayuh,.... I donno's it's the Carb, but it sure sounds like a Fuel Delivery issue,....

Thanks for all the help. I'm unsure of which way to start first. I am most worried about the loud noise at above 2500 rpms when running. I don't know if the knocking is coming from a rod or if something related to the exhaust could be making this sound that you can't hear at lower rpm's but is very loud and constant. The temp issue I will have to take the suggestions and try to sort out by process of elimination but having it drop by replacing the 160 degree thermostat to a 140 is encouraging and to tell you the truth I can place my hand anywhere on the engine and not be uncomfortable like it would be if it was overheated, even before the thermostat switch.