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Lacking Mid Range Power (454)

8

805HD

Regular Contributor
'87 Cobalt 454 w/ alpha1


Boat is lacking mid range power almost like a "dead zone" between 2000-3500 RPM. Like a misfire but way more subtle, more of a slight stutter or hesitation. When I push through it I can get passed said "dead zone" and she runs great in the top end.

No backfires, carb farts, and temps are perfect.

Shes hard to start on first fire of the day, but once there's some heat on the engine she starts right up split second.

I'm thinking a timing issue maybe? Perhaps its a bad ICM? Plugs and wires, dist, coil all new. Could be the carb, (Q-jet) it was rebuilt about 2 years ago.

Just wondering where you guys would start..

Thanks in advance.
 
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I'd check the timing advance (ICM) to make sure it is still in spec...on the carb, you can check the power piston raising and lowering the primary rods...and old plastic straw works as a gauge in the forward vent hole...depending on the spring used, the rods will start to lift once the vacuum has dropped low enough...my guess is the piston could be stuck in the lean (down) position...
 
makomark said:
I'd check the timing advance (ICM) to make sure it is still in spec...on the carb, you can check the power piston raising and lowering the primary rods...and old plastic straw works as a gauge in the forward vent hole...depending on the spring used, the rods will start to lift once the vacuum has dropped low enough...my guess is the piston could be stuck in the lean (down) position...
Click to expand...


yea I had a old timer set the timing by ear. But that was on the trailer on the hose. The backseat sits so damn close to the crank pulley it’s impossible to get a light in there. Will be removing the back seat this weekend to have that checked out with a light.

he thinks it’s carb related.

I rebuilt the carb and had zero issues with it on my last engine. Just curious where you guys would start. I appreciate the advice.
 
Ayuh,.... I agree that the timin' needs to be set with a light, 'n it sure sounds like a carb problem with the lack of power,.....
 
Hey jack!

Perhaps, same carb I was running on the last 454. Never had a issue with it. It’s got to be timing and or carb related. Going to tackle timing next weekend and see what I get.

boat would be 100% if I could just sort this issue out!
 
If timing is close to ~ 8*BTDC and when you rev to 3000 and you see timing advance by at least 20*
Then you timing module is good.

I find it hard to belive you could have high speed with a bad timing module.....typically you would not be able to get past that 2500-3000 rpms......you would be flodding out i would think.

Take off flame arrestor and visually confirm secondaries are opening (under load in water).

You actually should be able to hear it if engine cover is open

Post back what u find.
 
makomark said:
If its 'running good' over 3500 RPM, the secondaries are likely to be opening fine...
Click to expand...

Without actually being there to fully understand his description I am not sure that we can say that the secondaries are functioning correctly.


""When I push through it I can get passed said "dead zone" and she runs great in the top end.""

The fact that he says he "pushes" through it says that the primaries are allowing this to occur. I guess I am saying that it is running on the primaries and not the secondaries.

If the secondaries are not functioning properly, then the sluggish behavior at 2500-3500 may be a result of lack of fuel when secondaries open. Like the large needles are not lifting out of the secondary jets and the primary supply eventually "catches up" allowing higher rpm. Similar to a two barrel function.

Again, Tough to say without being there to hear and see what is and what is not happening.

I am also not convinced the timing module is at fault. That is typically a ON/OFF type device. it works or it does not. You get full advance or you do not.

I have never seen the timing module start to work then not work and then start to work again at different rpm's...........................

I have seen the Distributor cap reek havoc at various RPM due to carbon streaks or cracks. Basically creating a miss fire....

also in some cases the pickup in distributor has caused some weird issues but still basically works.....
 
by “push through it” I mean continue to in crease throttle till it gets passed the dead zone and runs “normal”.

On a Quadra jet, aren’t the secondaries initially controlled by vacuum then taken up the rest of the way mechanically? If so, could t there be a vacuum leak?
 
the secondaries (throttle plates) are mechanical and are driven by the control cable. The air valves are lockout with high intake manifold vacuum and once it gets low enough, the dashpot will allow them to open....with the opening rate controlled by the spring on the shaft....the secondaries fuel flow is driven by the air valves angle which control the metering rods.
 
Basically what Mark just said is,

The throttle blades are mechanical and open based on linkage.
No real fuel will occur at secondaries if large butterfly doesnt open which then supplies air and raises the secondary metering rods.
The rate of the butterfly is controlled byvthe spring he refers to.

So what you need to confirm is,
Under load in the water, when throttle is applied to get boat on plain, the secondary large butterfly is opening like it should.

If it is not, based on my opinion, that lag or hesitation that you " push" your way through is the primary side " catching up" so to speak.

If you look at the secondary butterfly plate in the center there is a cam and a hanger (stamped with letters/numbers) and two metering rods that when the butterfly plate opens, the cam raises the hanger which in turn raises the metering rods up and out of secondary jets allowing fuel to flow.

So all of that is dependant on the secondary butterfly platevopenening.
No opening no fuel.
No fuel then a bog/lag/hesitation until primary fuel supply "catches" up.

Thats my theory for now....
 
So timing and carb both checked out. I am beginning to suspect my ICM and here's why.

Initially the engine that I got the boat with had a really bad over heating problem. That exhaust elbow mounted ICM had to be getting cooked! Finally threw in the towel and replaced the engine. Second engine had a similar issue to the one I'm having now but much much worse. It would miss like crazy trying to get up on plane, if it would even get on plane. I eventually noticed the problem would become much more "manageable" if the engine hatch was left open. Okay so its heat related, must be electronic right? That second engine was short lived and ended up detonating its self to the moon.

After a THIRD engine (current) and some reading, I decided to relocate the ICM to another location as far away from the exhaust as I could get it. And relocated the ground to the block. Misfire became even less and I no longer had to leave my hatch open. Considering all that and what the ICM does, that HAS to be it. I know Jack, I know, they almost never go bad! But after being cooked for who knows how many years I would say its definitely possible. And considering the problems I'm having I don't think anyone could argue that it COULDN'T be related to s faulty ICM.

Questions, would there be any benefit to changing out to a second generation thunderbolt IV? Is that even possible? Or should I just stick with the first gen like I have (if i can even find one)?

THANKS
 
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805HD said:
So timing and carb both checked out. I am beginning to suspect my ICM and here's why.

Initially the engine that I got the boat with had a really bad over heating problem. That exhaust elbow mounted ICM had to be getting cooked! Finally threw in the towel and replaced the engine. Second engine had a similar issue to the one I'm having now but much much worse. It would miss like crazy trying to get up on plane, if it would even get on plane. I eventually noticed the problem would become much more "manageable" if the engine hatch was left open. Okay so its heat related, must be electronic right? That second engine was short lived and ended up detonating its self to the moon.

After a THIRD engine (current) and some reading, I decided to relocate the ICM to another location as far away from the exhaust as I could get it. And relocated the ground to the block. Misfire became even less and I no longer had to leave my hatch open. Considering all that and what the ICM does, that HAS to be it. I know Jack, I know, they almost never go bad! But after being cooked for who knows how many years I would say its definitely possible. And considering the problems I'm having I don't think anyone could argue that it COULDN'T be related to s faulty ICM.

Questions, would there be any benefit to changing out to a second generation thunderbolt IV? Is that even possible? Or should I just stick with the first gen like I have (if i can even find one)?

THANKS
Click to expand...
So you have a ealry Tbolt 4 ignition? Did you try changing out the sensor in the distributor ? These can also cause similar faults easpically the older ones with the bare posts for wire connections.

Would think based on what you are seeing it is eother the sensor or the ICM you cnan generally find used ICMs off ebay. I got my Tbolt 5 for $200 when I repowered. You also can step up to a newer Tbolt 5 with a conversion harnes .
 
I replaced the sensor in the dist to the newer one from the threaded posts to the wires.

so they do make a conversion harness to utilize the newer ICM?! Would I have to change the distributor to do that? Aside from splicing in the new harness is it just plug and play?
 
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Dieter said:
So you have a ealry Tbolt 4 ignition? Did you try changing out the sensor in the distributor ? These can also cause similar faults easpically the older ones with the bare posts for wire connections.

Would think based on what you are seeing it is eother the sensor or the ICM you cnan generally find used ICMs off ebay. I got my Tbolt 5 for $200 when I repowered. You also can step up to a newer Tbolt 5 with a conversion harnes .
Click to expand...

I replaced the sensor in the dist to the newer one from the threaded posts to the wires.

so they do make a conversion harness to utilize the newer ICM?! Would I have to change the distributor to do that? Aside from splicing in the new harness is it just plug and play?
 
kghost said:
Yes,
Bullet connectors to distributor connections the rest would be the same.
Click to expand...

well hell, no point in sticking with the old one. Newer style is much more available. So it would just be plug and play? Simply just splice in the new harnes, mount the module and go?

in theory..
 
The one I linked to includes the plug and wires. There is only three wires you need to redo.


The white with red stripe from the timing module will need the correct bullet connector and the White with green stripe from Timing module and (the White with green stripe going to Shift interrupter switch if an Alpha drive will need to be combined into ONE bullet connector).

I forget if you have a Bravo or alpha drive.

The other two wires are to coil, Purple to Coil + and Grey to Coil - Both look to be ring terminals in image.

Its very important to get the connector with the wires otherwise you cannot use timing module alone!!
 
kghost said:
The one I linked to includes the plug and wires. There is only three wires you need to redo.


The white with red stripe from the timing module will need the correct bullet connector and the White with green stripe from Timing module and (the White with green stripe going to Shift interrupter switch if an Alpha drive will need to be combined into ONE bullet connector).

I forget if you have a Bravo or alpha drive.

The other two wires are to coil, Purple to Coil + and Grey to Coil - Both look to be ring terminals in image.

Its very important to get the connector with the wires otherwise you cannot use timing module alone!!
Click to expand...

Its a Alpha. and yes that's how i have it now after updating the pick up in the dist from the old to the new style. cant remeber the colors but two of the wires were combined and ran into one wire.

last question, what do you mean by "correct bullet connector" I'm I just reading that wrong, or is there some specific connector I am going to need to get?!
 
UPDATE! So as usual, I’m a idiot. Lol fired up the boat and stuck my head down in the engine compartment to really listen to it running. Noticed a ticking sound. Almost that of a electric stove or BBQ igniter. Very faint, would have never heard it without getting down in there.

#2 spark plug wire was sandwiched between the block and the manifold and was arching off to the manifold. I had previously pulled that manifold (at night) to fix a oil leak coming from the valve cover. It was late, I was tired and a bit frustrated as those big exhaust manifolds are not very fun. Anyways new plug wire and we are up and running like a top! Problem solved!
 
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