5.7L Mercruiser Timing Advance Failure after 20 min of running

[snsrider717]

Thank you in advance for any help you can provide with this incredibly frustrating issue.

Boat: 1986 Sea Ray 21' Cuddy Cruiser
Engine: Original 5.7L 260 HP Mercruiser with Alpha 1 Gen 1

Problem: Engine runs perfectly for approximately 20 min at all RPM ranges, and then inexplicably starts to have timing issues. I should say I am 90% or more certain it is timing. After about 20 min the motor begins to struggle to maintain any RPM above 3000. It will however cruise steadily at about 2800.
After the motor completely cools off, say after a couple hours, it runs perfectly again.
I just replaced the Thunderbolt IV module thinking that it was going bad, but no change. It has new plugs from last year, and I replaced the sensor in the back of the distributor after I lost all spark about 4 months ago.

Other notes: Brand new Carb, new gas, new fuel filter, good compression and spark whenever I test it.

I'm desperate to get this resolved as I have a buyer who wants to take it for a test drive.

Thanks again for any advice you might have.

 

[Scott Danforth]

Welcome aboard.

a few questions

first, how do you know its timing? so far you have thrown a pile of parts at the motor, however what have you done for diagnosis?

I seriously doubt its the ignition system

what is your fuel pressure? it should be between 4 and 7 psi at the carb
if it starts to struggle, open the fuel cap. if you hear a hissing sound, you have a plugged fuel vent
have you checked the anti-siphon valve?

being a 34 year old boat, are all the electrical connections from the battery to the motor to the helm and back in tip-top shape. the ignition wire path thru the harness alone has 14-16 connections between the battery and coil.

 

[snsrider717]

Thanks for the quick response.

I have only replaced 1 part in response to this problem and that was the Thunderbolt IV.

I believe it is a timing problem because I as I understand it the thunderbolt IV digitally advances the timing as RPM increases maxing the advance around 3000 RPM. It is unlikely the RPM would be able to increase much past 2.5-3k without timing advance.

I put a timing light on and believe I saw unstable timing at one point but haven't been able to confirm that as of yet, and may try to do so tonight or tomorrow. The trouble is, I have to run the boat fastish (above 3k rpm) for more than 20 min to see this problem develop.

I have checked, cleaned, and re tightened a number of the electrical connections associated with the ignition system but haven't noticed any issues as of yet.

I will try opening the fuel cap if I can get the problem to manifest itself again and see if that makes a difference. I am not sure how to check the anti-siphon valve. Would problems with it show it self after 20 mins of running?

 

[Scott Danforth]

most boats are all-in timing wise at about 3000-3400 RPM range. however a healthy motor will still increase RPM. 99.9999% of the time its a fuel system issue improperly diagnosed as an ignition system issue.

you replaced the carb as well. I have never seen a carb I couldnt rebuild to date.

yes, if your fuel vent is plugged, after 20 minutes of running, the motor will fall flat on its face as the mechanical pump can not pull any more fuel.

no fuel, means no power.

out of curiosity, what are your compression test numbers? all cylinders should be between 135 and 150 psi. what is the fuel pressure?

 

[snsrider717]

I hope you're right and it's a misdiagnosis. I will try to put the boat back in the water this evening and replicate the problem, then troubleshoot the fuel. If it is just the tank vent, do you know of a good way to clear a block?

In the end, if it is a vent clog, it will actually save me money because my other thunderbolt module is fine and I can sell it.

Thanks again for the advice, I love learning about these motors.

BTW, carb was replaced because prior owner damaged the old one and a replacement was cheaper than a repair.

 

[snsrider717]

Oh, and I used to have a 1988 Cruiser Rogue also. Crazy coincidence. Loved that boat, wish I had never sold it.

 

[Bondo]

Ayuh,....... Welcome Aboard,..... I was gonna jump in here, 'n tell ya to forget about the timin';, 'n figure out why it's goin' Lean,......
But,....
Scott has ya pretty well covered,.... I agree with 'im, 100%,.....

The T-Bolt ignition is a Great system which always works perfectly, til it don't,......
Most deaths are owner inflicted, but it don't degrade over time, it works, or it don't,.....

Expensive to work on, so the Delco Voyager kit package is the "Go To" replacement,.....
Much cheaper, 'n nearly as good,.....

 

[peakay]

This sounds like a problem I'm chasing @snsrider717 . Can you let us knwo the eventual cure? Thank you.

 

[GA_Boater]

This sounds like a problem I'm chasing @snsrider717 . Can you let us knwo the eventual cure? Thank you.

Ain't been back in 2 years. You can keep waiting or start a thread.