TB IV ignition
 

last summer I got my twin 502's rebuild project back on the water, everything feels 80-100hp better but cant make any more rpm (4700) than previously, thought I would be shopping for bigger props. looking at posts on tbIV read where the module must be grounded besides the ground wire coming from plug- I had drilled my module off plate and didnt have it mounted just ty-wrapped in wire bundle could this possibly cause me to have no ignition advance hence my expected rpm increase is non-existant Thanks Randy

My Tbolt module was bolted to the distributor. Some are bolted to to the exhaust manifold. I believe that "bolting" the module bracket is necessary for grounding, but can not swear to that.

Also, are you sure it's a Tbolt IV and not a V? The Tbolt V has a built in rev limiter set at around 4700 RPMs. The IV has no rev limiter.

I'm sure someone with more know how will chime in on this.

Yes its a IV 1991 502 mag carb no rev limiter was bolted to manifold before I changed exhaust and re-located it

Did you have the engines dyno'd or are you giving an educated guess as to the increase in hp after the rebuild? Dynoing an engine before dropping it in the boat can be a real plus as you get to semi tune it before hand.

I don't know how far you've gone as far as chasing the gremlin, but the issue may or may not be Tbolt and or ignition related. Could be fuel related (not enough pressure/volume) or mechanical, ie parts used in the rebuild. I'm no guru, and just shooting from the hip. If you post more info regarding the build I'm sure one of the more learned ones here will be able to help out.

Tbolt IV has no limiter.

Put a timing light on it and see if the timing is advancing.

I have mine bolted to the back of the cylinder head to get it out of sight.

mine is mounted to the distributer and i have a 1993 502 mag dont know if it is V or IV but id dosent have a limit. i have had the prop cavitate and get to 5400 or so.

thanks for the input guys I will be checking in about 2 months or so when winter lets go, just doing the winter thinking and wanted to see if anyone had had this issue or knew for sure if that grounding would cause my issue. will be going thru the steps in the spring if thats not it to find my estimated hp increase with no top end gains Randy

I can tell you that when my case/bracket wasn't grounded that I didn't get enough spark to even fire the engine. I don't know if a weak ground would give enough spark, but not allow the module to advance.

Like was said by Jim, put a timing light on it and set to 34* total at 3700rpm. That should be about 10* base timing.

What was all done to the engine to add hp????

Its a TB IV.

It ran fine I set up idle, timing 8 initilal and did plug checks at 3600 for a cruise all looked good got some time on it felt much stronger than before but when I went for wot couple tries wouldnt break 4700. I totally went thru both eng heads bowl blended valves new ss and inconel spriings etc for comp xm284hr, roller rockers, orig bores stock pistons blocks decked aligned etc,stayed with original intake with holley 800's and did 496 take-off exhaust manifolds. was just glad to be back in the water so ran what summer had left now doing the winter thinking. I do know my props have too much stern lift- with the added weight removed with alum exhaust may be big culprit also. I cant raise bow, but I thought I added enough hp to see a difference and will have to get it sorted out this spring thanks Randy

I'll guess not enough fuel.

You need to check timing above 3700rpms. Set it to 34* or at least bump base timing to 10*.

Your modifications should be good for about 50-60hp increase. You should have milled the heads as well. I would also add an open carb spacer.

If you can't raise the bow of a 311 with twin BBC's, you definately need different props.

Did you do the heads or have them done somewhere else ? I have seen guy's loss HP grinding on a set of heads. And with out running the on a dyno it's all just a guess . Commonly known as auto trade HP. :eek:

thanks for the input guys, I put it toghther with 2' 4 hole carb spacer have rounded up 1' open for the spring . my head clean up and bowl blending was done by me with guidance from my eng shop, will start back with my original props for a baseline and go from there in the spring, wish it would come sooner thanks again Randy

I thought you were still using the same props????

You should see a 300-400rpm gain with your changes and the same props.

yes Griff I had same props on no change in rpm I have set from 4yrs ago back to 3 bladed see what that does ( they do lift bow with old power)

With stock inlet manifolds and upgraded exhaust + a bigger and I'm assuming better carb, the spacer will help but my thoughts are you intake manifold probably sucks too!
It'd be hard to get a gauge on what's wrong where with combo that has a choke point like the stock intake. If your ignition is stock, grounding issues would cause a missfire or dead engine. Lead (trigger) wire flaws may cause limit to RPM on an opticaly triggered ignition or something like yours, but my suggestion would be to test your engine under load and then off load (rev it in Neut... does it go past 4700rpm that way?) Then you'll know if it's the ign, or the hp. If it's the power (or lack of) look to your fuel supply. Did you upgrade your fuel pump? Do you still have that little widget thingy in the fuel line from the tank? (The one USCG mandates that the boat be built with I think). Apparently that's a big inhibitor. You've done some big flow increases and are putting a large increase in demand on your fuel system, I'd look there.
Hope that helps... Roll on Spring
(or move to Australia, it's always boating season here!!!)

The stock fuel pump is plenty for what he doing.

You should have picked up some rpm. My guess is more timing and carb tuning.