7.4 carb with Thunderbolt V - Cant get into base timing

[snowbrd84]

I had wondered why it seemed my timing was bouncy when setting base timing. I have been.grounding the purple/white lead. Was reading merc manual and saw the adaptive advance feature which I now understand why it moves around. Problem is that is supposed to disable with the purple white grounded.

With purple white grounded, reving advances the timing all the way through 32 deg at 3500 rpm. Atleast I know the advance is working, but this is obviously a problem.

Potentially connected issue, I have low voltage on the Ignition circuit.
When running:
-at the alternator: 14v (v drop test to battery = 0.20v)
-at each battery: 13.5v
-at coil: 11.8v
-at purple white timing lead/knock signal: 6v (should be 8-10)

Checked continuity of purple white a d it carries all the way through the Ignition Module connecter to receiving pin on module.

Cleaned all alternator, battery and starter connectors with rotary wire brush. No change.

Pulled alternator and Ignition Module. Will take alternator to have tested today. Not sure if anything to bench test on ignition module.

Thoughts?

 

[Grub54891]

Did you clean the grounds at the rear of the motor? Most often overlooked area. Sometimes hard to get to so they get missed.

 

[Scott06]

Are you starting the engine with the timing lead grounded or grounding it after starting? Procedure says to ground it then start engine.

If your total advance is in the ballpark I would use that to back into a base timing setting.

If you are loosing voltage could be in the engine harness, cannon plug, ignition switch . Grounds are equally important as positive side. Might be worth running a jumper direct to positive terminal on coil and see if it changes the voltages or how it reacts.

Not sure if that voltage will make it go into retard mode due to knock. May be able to tap the side of the block by sensor with a C wrench to see if it retards the timing

 

[snowbrd84]

Yeah I can set the timing based on total I suppose, but I can't set my idle speed correctly without getting into base timing.

Either way, need to figure out whether there is an issue with ignition module or if it's related to the low voltage issue. Will take a look at main engine ground tomorrow.

Good idea on the jumper to coil positive. And yes I am starting engine with timing lead already grounded.

 

[Scott06]

Yeah I can set the timing based on total I suppose, but I can't set my idle speed correctly without getting into base timing.

Either way, need to figure out whether there is an issue with ignition module or if it's related to the low voltage issue. Will take a look at main engine ground tomorrow.

Good idea on the jumper to coil positive. And yes I am starting engine with timing lead already grounded.

You can set your idle speed to 650 or so on the water and in gear, close enough.

There is no test per see for the module. TBV is a great system until you need a module as they are NLA or $$$$. Most take it out and go back in with a Delco EST if they need a module.

Since you have voltage drop - maybe ground the base mode jumper back to the battery negative see if it kicks into base mode.

 

[snowbrd84]

Did another test today, and with the knock module completely unplugged from its harness, there is still 8.5v on the purple/white wire. The merc TB V diagram shows that purple white has 3 ends, the knock module where 8-10 v outputs, the Ignition Module where that signal is received, and the timing lead where that signal can be grounded to enter base timing mode.
It would seem the Ignition Module is supplying 8.5v on this wire but that just doesn't seem right.

 

[snowbrd84]

Voltage issue solved. New ignition switch and immediately everything at 14v immediately after start with no fluctuation. I had cleaned contacts on outside but must have been corroded or burned out inside.

Still no base timing mode. I'm assuming at this point the Ignition Module is damaged. It does still advance all the way to 32 total in neutral. Don't know about under load.

 

[Scott06]

Voltage issue solved. New ignition switch and immediately everything at 14v immediately after start with no fluctuation. I had cleaned contacts on outside but must have been corroded or burned out inside.

Still no base timing mode. I'm assuming at this point the Ignition Module is damaged. It does still advance all the way to 32 total in neutral. Don't know about under load.

I wouldn't worry about the base timing if it works 32 is a reasonable total advance for your engine. In the old days when they would put the advance degrees on the module I cant recall if a 7.4 was 32 or 36. I suspect now that you aren't loosing voltage it probably runs fine. If it does leave it alone as unless you can find a used module your only option is to convert to delco est.

 

[snowbrd84]

I wouldn't worry about the base timing if it works 32 is a reasonable total advance for your engine. In the old days when they would put the advance degrees on the module I cant recall if a 7.4 was 32 or 36. I suspect now that you aren't loosing voltage it probably runs fine. If it does leave it alone as unless you can find a used module your only option is to convert to delco est.

The issue is more setting the idle speed and idle mixture on the carb. The Ignition module has to be in Base Timing Mode to disable the Idle Speed Control when doing so otherwise it constantly changes the timing to try and smooth out idle. So you can't go through the standard setup procedure.

 

[Scott06]

The issue is more setting the idle speed and idle mixture on the carb. The Ignition module has to be in Base Timing Mode to disable the Idle Speed Control when doing so otherwise it constantly changes the timing to try and smooth out idle. So you can't go through the standard setup procedure.

It really is not an issue that matters. You don't use the boat in base mode. Set the mix and speed on the water in gear. I usually do it in my hoist or tied to the dock.

Or replace the module ...your call

 

[snowbrd84]

Went back out today to try and set idle. If I get the idle in hear to 650, then neutral revs up to 1200 is.

Gave it gas with rmrge engine cover off and this certainly doesn't sound right. See video. Came back and puller port side exhaust elbow to find this, which looks pretty beat.

 

[Grub54891]

Yeah that flapper looks pretty bad. Replace it. But that isn’t the issue you have with the idle part. Wish I had an answer right now but it’s late and I’m tired.

 

[snowbrd84]

Yeah that flapper looks pretty bad. Replace it. But that isn’t the issue you have with the idle part. Wish I had an answer right now but it’s late and I’m

Discovered throttle return spring wasn't connected (rebuilt the carb 2 days ago - hence the reason for adjustments - and this was first time out). Had also replaced sloppy throttle cable which I think needs a little more preload to push lever back in neutral.

 

[Grub54891]

Lets get a pic of your throttle spring. It really shouldnt have one unless there is something I'm not aware of on your setup.

 

[Scott06]

Discovered throttle return spring wasn't connected (rebuilt the carb 2 days ago - hence the reason for adjustments - and this was first time out). Had also replaced sloppy throttle cable which I think needs a little more preload to push lever back in neutral.

That's what I was gonna say something is wrong the throttle cable should push it back against the idle speed screw. the few mercs that use a throttle return spring (357 Alpha does) it is there as a back up - really the throttle cable should be adjusted such that it pulls it back against the stop. Could be your cable is hung up

 

[snowbrd84]

I'll take a pic next time I am out. Need to run through the throttle cable set up adjustments, I just adjusted new one to match old one, but old one had a lot of stretch in it.

 

[Scott06]

I'll take a pic next time I am out. Need to run through the throttle cable set up adjustments, I just adjusted new one to match old one, but old one had a lot of stretch in it.

Only two things to check on throttle cable adjustment does it positively close the throttle against the speed screw, and does the throttle open wide …