Logo
FULL SITE MENU

Thunderbolt IV timing issue

W

willytv

New member
Hi,
Long-time reader, first-time poster so bear with me, please. I have an '83 Searay that my wife and I restored some time ago. Original engine was the Merc 228 (GM 305) and it has always run great -until this June when it laid down on me out on the water. With the help of some buddies, I got it out and it had dropped a valve on #3 cylinder. I decided to go back with a crate 350 marine engine (long block). I used my old intake, carb, exhausts and ignition components. I have it in now and it's doing great so far. Only thing is, I have an issue that carried over from the old engine to the new one. I think it may be coming from the Thunderbolt ignition box. On the old engine and the new, when I start up at idle, it sometimes seems to be running too slowly and sort of rough. But after a minute or so the RPM's will jump up instantaneously to about normal and the engine will smooth out. I caught it doing this one day while I had a timing light in hand. While it was running rough, timing was showing zero degrees, however, when it jumped up as described earlier, timing went to the correct value of 8 degrees BTDC. So what am I looking at here? If it is the ignition box then I will get a new one. But I'd like to be sure because those things are like $500. Any help and/or advice would be greatly appreciated...
 
Thanks for the reply - it does have the shift-kill switch. Plus my shift cables and linkages all look good. Also, it doesn't do it while shifting. It does it in neutral or while in gear - either FWD or REV. I've never heard of the shift assist thing. Shift-kill basically kills ignition momentarily as you shift from FWD to REV and vice versa. Engine dies for a split second to minimize gear damage. When shift is complete, kill-switch releases which restores ignition. At least that's how I think it works.
 
I'd suggest that you rid yourself of that Merc TB system.
If it played any role in the demise of the previous engine, you certainly don't want a repeat!

.
 
I don't think it had anything to do with the old engine going down. I mean, it was 30 years old. An exhaust valve came apart which took it down. In reference to the Thunderbolt ignition - what would I replace it with? I've only been looking at getting a new TB IV box. I didn't think there was an alternative.
 
Merc's TB ignition system and ignition module are proprietary components. These are also EST.
If you are comfortable with that, by all means repair what you now have.

Alternatives would be the Delco Voyager (EST), Mallory's YLM (VR w/ mechanical advance), or an array of others.
IMO, I'd steer clear of the Hall Effect units.


.
 
willytv said:
Hey Rick, pardon my ignorance here - but what does EST mean?
Click to expand...

EST represents Electronic Spark Timing.
With EST, all but Base Advance is accomplished via electronic circuitry.
The system sees full advance, but then delays the advance as per engine demands.... I.E., engine rpm, and with the more sophiticated systems, other engine data.


Other than where the housing is positioned for BASE or Initial advance, the distributor itself offers no means of advancing the spark any further.

When they work, they can work fairly well. But when they quit working, you're basically done.


.
 
Last edited:
My bad (duh), yeah I know what you're talking about. Yes, all the advance other than baseline setting is done via the TB IV box. In reference to your last sentence, that is what has me stumped. I figured that it would work or not work, i.e. once it craps out it is ready for the trash. Mine has an intermittent issue so I don't know if it is the box or not. Can you think of anything fuel-related that could be causing this? I've never had the carb rebuilt, because it seems to work fine. Fuel pump isn't that old and all filters are in good shape. That's why I'm leaning toward an ignition issue.
 
Disconnect the grey wire on the coil.

retest, if you have a bad tach it can cause issue with ignition.

Also when you say the timing went to zero was the rpms also very low and was it running on all eight cylinders?

Difficult to analyze all the info as some details are missing.

If the timing module goes bad it goes bad.......typically but not set in stone.

base timing of 8 degree before top dead center is done normally by hooking up timing light and turning distributor.

TB V has a different timing module and some have a knock sensors therefor a wire must be shorted to set a base of 0 degrees.

TB IV does not have this. if it did you would have a knock sensor near the starter where the block drain plug is located.

It could be fuel related but it is not the carb. the carb even though it is from a smaller engine all it would do is lean out at high end due to jetting/hangers (calibration) not to say that a carb issue could not be the culprit........

Is it possible you have a vacuum leak?

possible a bad valve adjustment??? maybe a bad lifter???

I will say after rereading your original post, It sure sounds like a miss adjusted choke issue.

Any carbed motor when first started will run a bit rough until the choke butterfly opens. if the choke is not opening correctly maybe that is your issue.

How clean is the flame arrestor? it must be cleaned or it will act like a choke is closed.........
 
RE kghost - hey thanks for the info. Yes, when I checked timing it was at operating temp, RPM's were low and it was firing on all 8 cylinders. You have me wondering about the tach though. I have replaced it but it's been a while - maybe 15 years ago. I did notice it act flaky one time last summer but I haven't seen it malfunction since and I try to watch it really closely. FYI - I'm running without a choke. I put two of the choke coils on there and both of them kept the engine choked for almost 5 minutes after engine was up to operating temp. I got tired of it blacking up the back of the boat and struggling to get out of the marina no-wake zone so I removed the choke coil and wired the butterfly open a while back. It starts easy enough now with no choke and doesn't load up on fuel unnecessarily. I'm pretty sure I don't have a vacuum leak. The engine is a fresh rebuild and I took meticulous care while installing intake manifold and other components. BTW - new engine has no noise and runs smoothly most often with the exception of this one issue so I don't think it's any lifters or stuff like that. Also, and this is the big thing = the old engine did this very same thing so that's why I'm thinking it might be ignition related since I reused those parts. You've raised some really good points. If you have any other ideas, please post them. Thanks man...
 
Update Got in the boat last night and fired it up with timing light in hand. It was running a little rough, idle was too slow and timing was at zero degrees per the light. I tapped on the TB IV box with a wrench and idle jumped to normal RPM, engine smoothed out and light showed timing at normal value of 8 degrees BTDC. So as best I can tell my problem is in the box. I found a used ignition box on ebay for $150 that I'm going to try out. Hopefully it will get me through the end of this boating season then I'll buy a new one if need be. Thanks to all you guys for your input and suggestions.
 
Hey thanks! The Thunderbolt box I ordered should be in today. I'll put it on and let you know how it goes. I'm pretty sure it's bad.
 
Hi everyone! I wonder if there is any software with device adapter to test mercruiser thunderbolt IV ignition modules. I had conversation with Mercury but seems that they are way behind with all that)
 
No
TBIV is older, replaced by TB-V and now all are EFI so no plug in diagnostics for old simple electronic ignitions.
TB-IV and V are a basic electronic timing module/ignition.

Set base timing- Module then controls timing based on pre canned tables within "software" in module.

What is it you are trying to do?
 
If you have a running engine the only way to tell is to hook them up one at a time and with a timing light, rev engine to 2500-3000 rpm.
If timing mark advances from Base (~ 8* btdc) to ~ 25-30* btdc then the unit is good. If no advance pass base timing or engine wont run right then unit is bad.
 
Back
Top