If it basically has an off idle flat spot/bog, then you might need a bigger accelerator pump. Is the carb stock with the 30cc or the 50cc accelerator pump????

your way too far out on the idle mixture screws,,1/2 to 1 turn max..your stumble could be several reasons..first try to set the idle mixture correctly,start at 3/4 turn and turn the screws vary little at a time and let the engine calm bown..out is fat in is lein..if your idle screw is set to high then you will get a off idle stumble,set the idle with the secondary stop screw and leave the primary alone..

read this,

http://www.candsspecialties.com/tuning1.html

idle mixture screws,,1/2 to 1 turn max .i agree with mrfix.the jets set up sounds ok. let your motor idle take carb cleaner spray around the base of the carb to see if you have a leak and if the motor revs up the gasket is not seal . if thats ok i wounld check your timing put it at 34degrees at 3000rpm

I see your getting a lot of info here and don't want to add any more confusion to your problem however I would seriously pull it off and pull your bowl covers and metering gaskets. I have pulled down brand new carbs with similar problems and found the metering block gasket was way off from where it was suppose to be. In a nutshell it (they) were blocking holes in the carb base. It's just a manufacturing quality issue. It happens. Just pay close attention when you pull the bowls for insepection. The gaskets can just sometimes shift during assm. from Holley or ???

Carb thunderbolt V at 8 degrees???? I think my 1998 carb 500 hps had thunderbolt V ignitions. I think the ignitions had 20 degrees advance. At 34 degrees total advance, which is whate you need, you need to be at 14 not 8 at idle. Forget the purple/white wire deal. Put the boat in the water and run the rpms up to 3200 or so and set the timing at 34.

I believe all the timing was in at 3000 rpm for the thunderbolt V so you should be OK at 3200-3500.

Why can't you rap the throttle out of gear???

Also, setting the idle screws while idling in gear is the best way to set the idle adjustment. Just enough fuel to keep it from dying in gear.

You need to get down and start eliminating issues.

It can't be ..... should not be in your vocabulary right now.

I caught that also and forgot to mention it. His idle should clean up a great deal also. I'm just not certain what module he is using. As mentioned take the guess work out of it and time it at full advance. Personally I never check base timing.

you probably have done so however also be sure both primary and secondary squirters are working well.

thanks for explanation
--------------
Atlanta Press Release Service

Here is the TB V info as it different than the TB IV by alot. Its always changing advance so i dont think you can check the advance at at 3500 as depending on load it will chang itself. Thats why they want you to set base timing with wire grounded at 8 degrees....The knock sensor pulls and adds advance.

I have gotten alot of great ideas and info that i need to look into all of these ideas.

See they tell u in the manual to set idle and mixture screw adjustments with the ECM in base timing mode with the wire grounded, is this rally necessary? in gear what should rpms be.

General Description
The Thunderbolt V ignition system has several spark
control features that will be described following:
D
Idle Speed Spark Control
D
Acceleration Spark Advance
D
Mean-Best-Timing Spark Advance
D
Over-Speed Control (Rev-Limiter)
D
Knock Retard Spark Control
NOTE: The Ski models do not have the Mean Best
Timing feature.

Idle Speed Spark Control
The ignition module will control ignition timing to
maintain a calibrated idle speed. This is accom-
plished by making small spark advance adjustments.
This feature is only active within a certain RPM
range. This range may be slightly different from one
engine model to another. The approximate range is
400-700 RPM.

Acceleration Spark Advance
This feature is active during acceleration only. When
accelerating, the ignition module may add more
spark advance to the “Base Spark Timing Curve”.
The amount of spark advance added, is totally de-
pendant on how fast RPM increases (how fast the
throttle is moved). This feature is also active within a
certain RPM range. This range may be slightly differ-
ent from one engine model to another. The approxi-
mate RPM range for this feature is 1200-4000 RPM.
Within this range, the module can add approximately
10 degrees of spark advance to the base spark timing
curve.

Mean-Best-Timing (MBT)
Spark Advance
During light load cruising, the ignition module
searches for the optimal ignition timing. This is also
accomplished by small changes to the spark ad-
vance. At a given RPM, the module will try to add a
small amount of advance and wait to see if there is
an RPM change. If RPM increases, it will try to in-
crease timing more. The module will continue to ad-
vance timing until it no longer gets an increase in
RPM. Conversely, if it senses an RPM drop, it will
start to retard some of the spark timing. The approxi-
mate RPM range for this feature is 1200-4000 RPM.
Within this range, the ignition module can add
approximately 10-15 degrees of spark advance to
the base spark timing curve.

NOTE: The Audio Warning System is also connected
into the ignition module circuit. If the audio warning
system becomes activated by the closing of one of
the audio warning system switches, the MBT feature
is deactivated.

Over-Speed Control
The ignition module will prevent the engine speed
from exceeding a preset limit by stopping the spark.
This feature has an RPM range that varies from mod-
el to model. The over-speed limit for a particular en-
gine is set slightly higher than the top end of the RPM
range for that model. For example, if the recom-
mended range is 4600-5000 RPM, the over-speed
limit would be set at 5100 RPM. When RPM reaches
this limit, spark is turned-off until engine RPM drops
down to a “Reset RPM”, which would be approxi-
mately 4750 RPM for this example. At this point,
spark comes back on.

Knock Retard Spark Control
The knock control feature helps provide protection
from harmful detonation. Knock control is handled by
the Knock Control Module. This module receives a
signal from a sensor that is mounted on the engine
block. The knock control module works in conjunc-
tion with the ignition module to retard the timing if
spark knock is present.