Setting Timing on Thunderbolt IV, not V- Question

[Bry21317]

Engine is a 94 Mercruiser 305 Chevy with a Edelbrock 1409 Carb and Alpha 1 Gen 2 drive with 1.65 Gears.

I was having a slight bog after my 4bbl upgrade only on Take off, so I was told to check/advance the timing.

Well I finally got a chance to get in the boat last night on the trailer and check the timing. It seems that it was quite advanced. What I found is that it was set to about 14 degrees BTDC, I am guessing this as the marks only go up to 12 degrees. I turned it down to about 12 degrees and it seemed to sound a little more powerful on the trailer.

Would it being set this advanced have caused a slight bog on take off? It would only bog down for a second or split second when I nailed the throttle from sitting still. If I was idling around in Gear at a little over or at 1000 rpms it would not bog as bad, but would just go. I talked to Edelbrock and got some hints from them, but they told me to check the timing and advance it some, but its already way advanced.

Any ideas?

And this is not the version, from what the Merc Manual states to ground the wire out on, as stated in these excerpts from the Manual.

Thunderbolt IV Ignition
Timing

1. Connect timing light (91-99379 or similar) to No.
1 spark plug. Connect power supply leads on
light to 12 volt battery. Refer to ?Specifications?
for cylinder numbering and location.

2. Connect tachometer to engine.

3. Start engine and run at normal idle speed.

4. Aim timing light at timing tab, located on timing
gear cover and crankshaft torsional damper.
IMPORTANT: GM engine timing marks (on tab)
are in 2-degree increments. MCM and MIE LH engines
will have ?A? (Advance) mark to the left of
?0.? Timing must be set on the ?A? side of ?0?
(Top Dead Center).

5. Adjust timing by loosening distributor clamp and
rotating distributor body as required until timing
mark on damper or pulley lines up with the mark
on tab specified in ?Specifications.? Tighten
clamp and recheck location of timing mark.

6. Stop engine and remove timing light.

This is whats stated for the Thunderbolt V

Timing and Idle Adjustment Procedures For Thunderbolt V Ignition

Setting Base Ignition Timing

1. Connect timing light to number 1 spark plug.
Connect timing light power supply leads (if
applicable) to 12 volt source.

2. Connect a shop tachometer to engine.

3. Using a jumper wire, connect the ignition system
timing lead ?13? (PUR/WHT wire) to a good engine
ground (?). This locks the ignition module
into the ?Base Timing Mode?.

4. Start engine and run at normal idle speed. Allow
engine to reach normal operating temperature.

5. Aim timing light at timing tab, located on the timing
gear cover and crankshaft torsional damper.

6. Adjust timing using the conventional method.
IMPORTANT: Be sure to disconnect the jumper
wire from the ignition system test terminal before
attempting to resume normal operations. If the
jumper wire is left in place, the ignition module
will operate in the ?Base Timing Mode?. This
means that the additional timing advance features
would not function.

7. Make sure that the distributor has been tightened.
Remove the jumper wire from the timing
terminal.

8. Stop engine and remove timing light.

Adjusting Idle Mixture
The procedure for adjusting carburetor idle mixture
can be found in the appropriate engine service manual.

This procedure also requires that the ignition module
be locked in the ?Base Timing Mode?.

IMPORTANT: In order to properly set idle mixture,
the ignition module MUST BE locked in the ?Base
Timing Mode?. This is necessary because of the
?Idle Speed Control? feature that exists in the
ignition module. See information on the previous
pages about this feature.

1. Using a jumper wire, connect the ignition system
timing lead ?13? (PUR/WHT wire) to a good engine
ground (?). This locks the ignition module
into the ?Base Timing Mode?.

2. Adjust idle mixture following the procedure in the
appropriate engine service manual.

3. Remove the jumper wire from the timing terminal.

Adjusting Engine Idle Speed

This procedure should be done with boat in the water,
drive unit in neutral and engine at normal operating
temperature. Refer to the Operation and Maintenance
Manual for the correct idle speed.

1. Disconnect the throttle cable from carburetor.
IMPORTANT: In order to properly set idle speed,
the ignition module MUST BE locked in the ?Base
Timing Mode?. This is necessary because of the
?Idle Speed Control? feature that exists in the
ignition module. See information on the previous
pages about this feature.

2. Connect a shop tachometer to engine.

3. Using a jumper wire, connect the ignition system
timing lead ?13? (PUR/WHT wire) to a good engine
ground (?). This locks the ignition module
into the ?Base Timing Mode?.

4. Start engine and allow it to reach normal operating
temperature.

5. Adjust idle speed to recommended RPM.

6. Stop engine. Readjust cable barrel and reinstall
the throttle.

IMPORTANT: Be sure to disconnect the jumper
wire from the ignition system test terminal before
attempting to resume normal operations. If the
jumper wire is left in place, the ignition module
will operate in the ?Base Timing Mode?. This
means that the additional timing advance features
would not be functioning.

7. Remove the jumper wire from the timing terminal.

Thanks,
Bryan

 

[John_S]

Re: Setting Timing on Thunderbolt IV, not V- Question

I think you are still over advanced to spec. I think spec is 8 degrees BTDC. There isn't any advance, or curve adjustment with T4 modules. Whatever you set the idle advance to, the module will add advance based on the rpm curve of your module. Your total wot advance maybe too much for your engine, causing other problems. I would rebaseline with stock spec, and work from there. Assuming we are talking a 5.0LX the orginal carb would have been a Weber that is very close to the same as the 1409. I think it would be better to tune the carb to get rid of the bog vs adjusting the initial timing.

 

[Bondo]

Re: Setting Timing on Thunderbolt IV, not V- Question

Ayuh,....

You've got a T-Bolt VI,... Right,..??

So use the instructions for the T-Bolt VI,.... Not the T-Bolt V.......

And,....
I Agree with John,..... You're Way Over advanced,... Still.....
I'm thinking 8?,... Maybe even 6?.....

Check you Manual,+ Use what it says......

I also think that your Bog would be dealt with by tuning the Carb......

 

[180shabah]

Re: Setting Timing on Thunderbolt IV, not V- Question

Carb tuning time. Accelerator pump, idle mixture, and choke settings are the first place to start.

What is your idle speed set to? 650ish??? I your idle is set too high, the module may already be adding advance.

 

[JustJason]

Re: Setting Timing on Thunderbolt IV, not V- Question

You've got a T-Bolt VI,... Right,..??

holy crap they're on thumpervolt 6 now????? where have i been???

 

[nailem]

Re: Setting Timing on Thunderbolt IV, not V- Question

how do you know what version of the tunderbolt you have? i have the same bog that you describe but i have a q-jet 4bbl 1986 5.0l alpha 1.

 

[JustJason]

Re: Setting Timing on Thunderbolt IV, not V- Question

how many wires do you have coming out from the distributer cap?

 

[Bry21317]

Re: Setting Timing on Thunderbolt IV, not V- Question

I just know that mine had the motor cover that stated what it is. I also know that I do not have a Rev Limiter or any of the other advanced Items that the T5 have.

180 Shabah,

What is your idle speed set to? 650ish??? I your idle is set too high, the module may already be adding advance.

Thanks for that Information. My Digital Tach was showing the idle right at 710-720, while I was checking the timing. Maybe I am a touch high then also.

As to this

Carb tuning time. Accelerator pump, idle mixture, and choke settings are the first place to start.

You are correct, been doing this also. Got the choke adjusted perfect, per Edelbrock's spec, from the Tech. The tech is the one that told me to make sure the engine was advanced some and to check it, as he stated it would run much better there on the new gas. I found it was already way advanced.

He told me to move the Link on the Accelerator pump to the top hole, but this is where it already was set. So I talked to another tech and they said to move it to the bottom hole for the next test, if the timing did not help. Then they said I should start to tune the carb with Needles/Springs/Jets to get the most out of it.

 

[John_S]

Re: Setting Timing on Thunderbolt IV, not V- Question

Spec per Service Manual is 650-700 RPMs in neutral.

Timing is 8 degrees BTDC at idle.

What is the advance curve marked on your ICM? It will be something like V8-22A.

I'd be very leary of what the tech is telling you about the advance. I wouldn't try running at anything beyond 10 degrees BTDC. If you are going to run that high, at least be buying 89/91 octane.

You may want to check for vacuum leaks. Always a good idea after a carb swap, and especially if you also changed the intake. Vacuum leaks and idle mixture too lean, can cause the engine to seem to run better with more initial advance.

 

[Bry21317]

Re: Setting Timing on Thunderbolt IV, not V- Question

What is the advance curve marked on your ICM? It will be something like V8-22A.

Where would this be located?

Also I will turn down the Idle speed to within 650-700.

Also any easy way to check for leaks around the intake, carb? Take a piece of hose and put it to the seams and move it around while holding it to my ear, that should work and amplify the hissing if there is a leak, right?

Bryan

 

[John_S]

Re: Setting Timing on Thunderbolt IV, not V- Question

It will be located on the ICM. It may be on the part number or serial label, or have its own. Is your engine a L or LX?

The low-cost way to check for vacuum leaks is to spay carb cleaner around carb to intake gasket seam, and monitor engine rpms. A change in rpm when you spray a particular spot, would indicate a leak. Beware, it might remove some paint in the process.

The prefered method is attaching a vacuum gauge to an intake manifold port. I had to add a fitting to do this on my 350. It will provide allot more diagnostic capability than just gross vacuum leak test.

 

[Bry21317]

Re: Setting Timing on Thunderbolt IV, not V- Question

I had the L model originally. Just did the 4bbl and Intake upgrade.

I will check to see if there are any leaks again on the Intake.

Bryan

 

[MikDee]

Re: Setting Timing on Thunderbolt IV, not V- Question

Bryan, Just a thought here, if it's a new carb, and you've gone through the fuel system, & filters, ahead of this, then it seems you've done all the right things, an engine will bog if the timings retarded some,,, Just maybe, the weight, & mark, on your harmonic balancer has slipped some, it's easy enough to check by taking out #1 spark plug, and verifying TDC by alternately moving the crank by hand, clockwise, & counterclockwise, and finding the midpoint between them (true TDC).

 

[John_S]

Re: Setting Timing on Thunderbolt IV, not V- Question

It is probably a V8-22

Identification Mark: V8-22
Module Advance: 22 Degrees
Initial Timing: 8 Degrees BTDC
Total Advance: 30 Degrees

Advance starts at 900 RPMs
@ 2400 RPMs should add 17 degrees advance
@ 3400 RPMs all 22 degrees are in

 

[Stinky_1]

Re: Setting Timing on Thunderbolt IV, not V- Question

Just thought I would mention that I think this thread helped me solve my problem on my cousins boat. It seemed to be flooding out at anything over half throttle. The timing was part of the problem, but also a VERY bad connection on the ignition coil. Someone some time had popped it off, then put itback on wrong, it folded the tab over so it did not stick on the post like it should. So the spark would have been considerably weaker than it should have been.

For some reason the guy before that owned it had WAY too much oil in the boat too. I drained about 3L of oil out to get it back to where it should be.

All in all, setting the timing back to where it should be, draining the extra oil, and fixing the coil wire seems to have fixed it. We test it tomorrow so I am hoping its good news.

Thanks to the guys on Iboats.

 

[wca_tim]

Re: Setting Timing on Thunderbolt IV, not V- Question

I'll bet you a half a tank of gas that the following will cure the problem:

1. set the timing right according to factory specs (isn't it sposed to be more like 8 btdc? with ca. 20ish a few degrees of advance curve from the module for a total advance of 30+ degrees of total timing on that engine?)
2. try one metering rod richer (unless already a little on the rich side)
3. if the above helps and doesn't take care of it, open the idle mixture screws up a turn or two
4. try change the step-up springs (controls when carb is in cruise vs power mode)

not an expert, but have had recent experience with similar issue...

after reading this, I thought I might chime in on the very clear off idle lean spot on the edebrock carbs. The problem as I understand it is that the idle cirtuit has a lot to do with the fuel mixture even when the throttle is open a little and can't suply enough fuel in many cases. I had a 1409 on my 4.3, and have tuned a couple for friends. I also have a 1410 on my 383. In every single case, there was an off idle lean condition that made the engine hesitate, etc... and took some playing with to get rid of. never once have the edelbrock techs been right on the money, they do a better job than most (comp cams techs recommended a set of roller rockers that clearly wouldn't work with my heads... we had to send them back...).

What jets and metering rods are you running? Typically if you move one metering rod richer, it will tkae care of it. On my 383 I was about to throw the carb away I was so frustrated with the hole right off idle - everything else was great, plugs looked perfect, etc... It finally dawned on me that if the problem was right off idle and the idle circuit controlled the fuel mixture to some degree.... I turned the idle screws out a bit and the dead spot went away... You about have to use a tach and watch carefully to set the idle according to the way that edelbrock instructs you to but if you go a little richer (say 1 turn out on each idle screw) it will help as well. On that engine I had to change the step-up springs as well, but I don't think you should have to unless yuour running a cam with a significantly higher duration / overlap than stock.

 

[MikDee]

Re: Setting Timing on Thunderbolt IV, not V- Question

Good Luck! This just reminded me of an incident that happened to me about 12yrs ago. I was about to buy my 79' SeaRay 24' Cuddy with a 350/260hp Mercruiser. It was a steal at $4k, all the hatch covers were totally shot (disintegrated), but the hull, & engine were hardly used, just faded. The marina I bought it from was good as gold, they put in a new steering cable, and actually got the boat shipshape for a sea test. My girlfriend, & I, went for a sea test, and I watched the gauges like a hawk, all seemed well, and I loved the way the boat drove, & handled, but it only did 35mph , no real problems. When I went to drive it home to my marina, about 8 miles by water, it stalled a few times, but restarted, finally at the only part of the Great South Bay that had rocks at the shore, it stalled, and wouldn't start, in no time with the incoming waves I was almost on the rocks, and couldn't find a boat to flag down for a tow I jumped into the shallow water below my waist, and tried to fend it off the bottom, & rocks, from the swimdeck, when a jetski came by, luckily he gave me a tow a little ways to a large cove, where I had time to check out the engine. Sure enough, it didn't take long to find the 2 small coil terminal nuts, & both wires, very loose! I tightened them up, and never had another problem with that boat for years.