Misfiring when hot

[pekkak]

I have Mercruiser 7.4 from 1988 with Thunderbolt IV Ignition. It starts fine but after abt 15-20 min when reaching the normal running temp starts misfiring. All other exept the distributor body is new, new ign cables and plugs, new coil. Carburettor was overhauled last winter. Engine has been running totally 1660 hours. Compressions when cool are fine. Starting to run out of ideas...

 

[Scott06]

Did you change the sensor in the distributor ? Google thunderbolt ignition troubleshooting you'll find a trouble shooting flow chart, would suggest trying this when it acts up, should be able to determine if it is the sensor or ignition module.

Coupke other thing sis to disconnect the tach from neg side of coil, and verify that the fuel tank doesn't gave a venting restriction .

 

[pekkak]

Thanks Scott,
Both sensor and ignition module are new. I am pretty sure that tank venting is working, since it is misfiring if the tank cap is removed or in situ. The tach is unreliable. Stays mostly on when engine stopped, but can be zeroed when switching the ignition on shortly.
Pekka

 

[harleyman1975]

Try disconnecting the gray wire from coil (tach lead) perhaps this is the trouble.

 

[Scott06]

Definitely if the tach is squirrely disconnect the grey wire on negative side of coil. Tachs have a habit of grounding out the ignition.

 

[achris]

Did you change the sensor in the distributor ? Google thunderbolt ignition troubleshooting you'll find a trouble shooting flow chart, would suggest trying this when it acts up, should be able to determine if it is the sensor or ignition module.....

No need to google, you can read all you need to know about the Thunderbolt systems HERE (including the troubleshooting flowchart).

.. The tach is unreliable. Stays mostly on when engine stopped, but can be zeroed when switching the ignition on shortly.

That's eactly how the Merc tachos work. They are 'unsprung', which means the needle will float until power is applied.

You need to determine what is causing the misfire, ignition or fuel. You could use a timing light to see if you are missing sparks, or cup your hands over the carb to richen the mixture..

Chris.........

 

[pekkak]

Tach wire disconnected, still misfires when hot. Hand cup richens the mixture ok. No spark in number 4 plug measured from ign wire. Some other plugs weaker. Plugs, wires, distrubutor cap and sensor are new. Ign mdule is new. Coil is also new, but both primary and secondary resistance abt 8 ohms. Wires disconnected. Strange since when cold start engine runs perfectly abt 15 minutes. When reaching normal running temp (in the dashboard gauge) starts misfiring.

 

[achris]

Cold engines run rich. Weak sparks fire a rich mixture easier than a 'normal' mixture. That's why it seems good while cold. How are you measuring the resistances?

Chris..........

 

[pekkak]

Thanks Chris,
I studied how coils work and noticed that I have measured wrong. Now it looks like ok and resistancies are according to specs. I haven't found mixture adjustment screws, yet.. Enriching mixture by pushing the flap with fingers carefully doesn't help.
Waters are freezing soon here.
Pekka

 

[pekkak]

My plugs are Champion RV91MC, all new. They probably are quite hot plugs. Am I correct if I say, that's what inboard Mercruises need. The gap is 0.8. How if I reduce them to 0.6?

 

[Bt Doctur]

measurements are well and good COLD, but it seems to be breaking down only when hot. Suspect a coil going bad internally or feel the coil to see if it is hot,the ` manual 23 lists MR43LTS or RV12YC .045 gap Cooler plugs can`t hurt

 

[pekkak]

Reducing plug gaps to 0.6 doesn't help. Tach jumps occasionally up and down about 600 rpm. Should I get a new tach?

 

[Bondo]

Tach jumps occasionally up and down about 600 rpm. Should I get a new tach?

Ayuh,.... Just disconnect it from the coil, to test yer theory,...

 

[biggjimm]

You said you had no spark on #4 & week spark on others? Was this when the engine was warm or cold? Right there is your starting point. Measure resistance on plug wires to make sure they're good.
Remember, just because the parts are new does not mean they work correctly. What brand of parts. Oem or aftermarket?
I'd get those weak & dead cylinders firing & see how she does when firing on all 8.
If you have to you can swap wires to different cylinders to see if the condition follows the wire to the new cylinder. Just make sure to switch both ends of the wire. Lol.
Jim.