83 Chris Craft 210SL - Mercruiser 5.7 running badly after warm up

[chevyace68]

I recently purchased a 1983 Chris Craft 210SL - Mercruiser 5.7, and first time on the water, it ran great for 3-4 hours. After stopping for one of our swim breaks it started running badly. Off idle it just wouldn't accelerate over 600-700 rpms. Just chug chug chug back to marina. I was figuring it needed tune-up so I started checking things. Ended up replacing plugs, cap, rotor, wires, ignition coil, distributor pick up, fuel filter/water separator, and inline carb filter. (distributor shaft felt fine. no shaft slack) When I take it back out, twice now same result, it starts great cold, idles great, and runs smooth. WOT it is at appx 3500 rpms and around 33 mph if speedo is accurate. Both times it ran great until I stopped and let it idle. Never shut it off just let it idle for a minute. Then it will not accelerate above 600-700 rpms or so. Not sure if there is fuel pump issue, carb problem, something in fuel lines from tank to engine, or what. I am an auto technician by trade, but not necessarily familiar with carburated boats. Kind of hard to test in driveway with no load, and harder to do on the water LOL. Any suggestions? Thanks in advance

 

[harleyman1975]

well for starts it should be turning between 4600-5000 WOT so something is wrong. I suggest to start with compression test. if that is good then re-check the firing order. if just 2 wires are reversed it can still run O.K. but not make full power. last thing to look at is fuel system but check other things first.

 

[Scrub868]

Two suggestions. With boat running well remove spark arrest or (air cleaner) shine a light down into the carb to see what ur good fuel delivery looks like. Then observe fuel delivery when u are having problems.

If you can't see any difference with throttle full forward--while the boat is not running good for a little bit and turn the key off. Immediately pull a plug and inspect to see if you are running lean or rich-- would suspect lean.

Good luck

 

[chevyace68]

Yea, tried that sort of Scrub. Only thing is you can't really go full throttle when it's bad. I did try though, of course secondaries aren't opening when its barely running, and when i tried to open and "peek" it backfired and blew flames in my face. My fiance thought it was funny. she almost had to swim back in LOL

 

[Pete104]

Does it still have the TB IV ignition? If so, you can check the timing advance easy enough in the drive way.

FYI, that engines WOT range is 4200-4600.

 

[Pete104]

Does it still have the TB IV ignition? If so, you can check the timing advance easy enough in the drive way.

FYI, that engines WOT range is 4200-4600.

 

[Bondo]

Ayuh,.... I'd start by lookin' at what's in the fuel filter,.....

 

[blamtro]

I had similar issues when getting to know my boat with same engine of similar vintage. Its either one of the two: get in the habit of checking for condensation under the distributor cap (you have a non vented cap) after a run and it sits for a bit or a fuel/water filter separator dump.

 

[chevyace68]

Distributor cap is new and it is vented. Fuel/water separator is new also. It does still have the TB IV ignition amplifier. Maybe check timing without jumping connector? Is that what you mean? Watch to see if it advances when running well vs when it doesn't?

 

[Pete104]

I mean, you can see if the timing is advancing. Timing set at 8 degrees take it up to 2500 rpm or so & you should see just over 30 degrees TOTAL advance. You could go with timing tape & use a plain old timing light.

Thunder IV, there is no base timing. No jumper connected to anything, timing light & wrench that's it!

 

[NHGuy]

What pitch is your propeller? Should appear as part of an identifying number either on the barrel of the prop or in the rear center.

 

[chevyace68]

Update...got it acting up in the driveway with muffs on it. Basic same scenario, after running 1500-2000 rpms for a few minutes...slowed down to idle. Actually shut it down for a couple of minutes. Started it back up, ran bad...can't get to 1000 rpms. Shut it down again, pulled a spark plug...its soaked. Went ahead and checked compression...150 psi.

 

[NHGuy]

Soaked plugs, stuck or sunk float over fueling things. The choke opens ok, right?

 

[chevyace68]

Yes, choke opens fine

 

[nola mike]

Soaked plugs, stuck or sunk float over fueling things. The choke opens ok, right?

Soaked plug or soaked plugs?

 

[chevyace68]

I only pulled one...#6. Just because I could get to it easy with hot exhaust. I had the carb where i could see in it after choke opened and it seemed to be dumping a lot of fuel.

 

[NHGuy]

Spark plugs might get fouled by the extra fuel. So if you do have the stuck float it could need spark plugs too.

 

[NHGuy]

Also, if there is fuel in the yellow translucent argon tube from the fuel pump to the carburetor it will over richen the mixture. So if it's that change the fuel pump and try it.
The tube is an indicator when the main fuel pump diaphragm fails. It lets you get home but adds fuel in a way that makes things obvious to someone who know what to look for.

 

[chevyace68]

Replaced the carburetor. Ran great in driveway with muffs on for 20-30 minutes. Back on lake for test drive today...no good. Same thing as before.

 

[chevyace68]

Ended up replacing the Thunderbolt IV ignition amplifier. Runs great now.