Timing thunderbolt V ignition

atrick

Active Member
Mar 4, 2017
546
Penna, N Carolina
Boat Info
1998 400 Searay Sedan Bridge
1995 Searay 290 Sundancer
1990 21 maxum
2001 24' FlotBot Pontoon Boat
Engines
Twin 3116 Cats
Twin 4.3 mercrusers
305 chevy with a alpha one drive
Yamaha 90 HP Outboard
Have 1995 4.3 v-six engine with 4 barrel carb and had thunderbolt V ignition. On the fuel injected engine you have to ground the purple/white wire to set base timing but was told on carbed engine you don't have to,so is that correct or do you still have to set base timing. the carbed engines don't have the computer.
 
I am pretty sure you have to ground the purple/white wire on a carb'd engine....I did on a 1998 boat with the TBV ignition and it was carb'd but that boat may have had a more advanced control system than your 1994 boat....

cliff
 
Read the timing procedure here: http://www.boatfix.com/merc/servmanl/16/16b4r2.pdf.

The need to put it in base timing mode is determined by the ignition system regardless of whether it is carbureted or not. Thunderbolt V should have a purple/white wire that needs to be grounded to put it in base timing mode. The Thunderbolt IV does not have a base timing mode. One way to tell is just put a timing light on it, if it is a Thunderbolt V and not in base timing mode, the timing will be jumping all over the place - this is the advance and knock sensor at work. Ground the purple/white wire and the timing will steady out so you can set it properly.
 
checked with two mechanics locally and both verified with carbed engines no need to ground to set timing only with fuel injection as then the computer controls comes into play
 
Ok - I just know on my 1999 carbed 4.3 you have to put it in base timing mode - following the TBV timing process in the document I attached to set the timing correctly. Are you sure you have TBV and not TBIV? Because on a TBIV you are right, no base timing mode, just point the light and set it. If you put a timing light on it, if you have TBV you will see the timing jumping around, but if you ground the purple/white wire it will steady out and you can set the timing. The Mercruiser does not say anything about carbureted vs fuel injected.

Now, on newer MPI engines there is no setting the timing, they have a crankshaft position sensor and the ECM handles the timing. Any checking or adjustments you need the Mercruiser scan tool.
 
Its written on the flame arrester cover V and put the timing light on both motors and no jumping around. So one mechanic only works on merc motors and been doing it since the 70's so I'm sure he's forgotten more than I know about these motors.
 
Ok - Mine just says Thunderbolt Ignition on the flame arrester cover, also says to set timing at 10deg BTDC and Special Timing Procedure Required. Yours must be different, because on mine it is very obvious if you try and time it with out setting base timing mode. I guess on yours just point and shoot.
 
mines got everything on the cover 4.3LSX with thunderbolt V ignition has a separate tag with all the serial #'s for drive and outdrive , list timing 8 degrees and all the numbers match all componets so they must be original
 

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