454 Stumbling

[trace elements]

All,

I am having an issue with one of my 454s stumbling when getting on plane. As I accelerate past 2200 RPMs it falls on its face and then recovers. The problem is consistently happening around 2400 RPMS. After the stumble, it will turn all the way up to WOT. Any ideas? I replace the racor, the carb fuel filter, and plugs. I am going to replace the fuel pump today and check the timing advance, but I don't think it would turn all the way up if there was a total advance issue.

https://youtu.be/dMIoYLq9Cyg

Secondly, what's everyone's recommendations for an ignition system upgrade? Thinking something along these lines to replace the current Prestolite system.

https://bpi.ebasicpower.com/shop/all-ap ... -edm550002

[John F.]

I had carb'd 454s in my old B31 the Anna E. I replaced both of the old and problematic (mine were) Mercruiser Thunderbolt Ignitions with electronic ignition systems from EBasic Power They were a lot cheaper then, and used a basic coil, Except for one coil going bad after a few years--motor wouldn't run right and the oil was leaking out of the coil = easy diagnosis--the systems were fine. The replacement coil I had to buy could be had at any auto parts store for cheap.

I replaced the Hollys on my 454s with Edelbrock 600 cfm marine carbs from Summit. The Hollys were bad, leaking, and automotive carbs and were basically a really bad kaboom-type accident waiting to happen. I never had a problem with the Edelbrocks.

To help with diagnosis of fuel supply issues, I had a section of fuel hose with a glass inline automotive filter in it that I could put on whatever motor I was having problems with. I could easily tell if I had a fuel starvation problem or not. Just an idea.

[scenarioL113]

Excellent video. I always say a picture is worth a thousand words...and a video is worth a thousand more.

Could be a few things. My first impression is electrical but could def be fuel issue.
Sorry I know that does not help.

You did not mention the distributor cap, did you replace that as well? (I think if it was dist cap issue or plugs it would not recover the way yours does)

Is there any condensation in the cap or rotor? Or corrosion?
Is the grounding good?

I am just mentioning "basics"

What distributors are you currently running? Points?

Is this issue constantly repeating itself?

[trace elements]

I am running original Prestolite distributors, I originally thought it may be an issue with the advance weights "sticking" when first accelerating. However, yesterday the boat got on plane fine, then a minute or two later, it started bogging like shown above, and then there was a single backfire and I backed off the throttles.

Based on how the engine "recovers" leads me to believe its some sort of fuel delivery issue. All the filters are new, which leads me to believe maybe there is an air leak in the fuel supply line, or maybe the aftermaket electric fuel pump is acting up.

[scenarioL113]

A fuel pressure gauge would be helpful, even just a temporary setup. These are the type of probs that can be very frustrating. Just keep working the problem and prove to yourself that each component in each system is performing properly.

Plenty of old school guys on this forum that have played around with big blocks whether automotive or marine.

It will be figured out, just might pull some hair out BUT you will figure it out.

Having 2 engines is helpful sometimes if you can swap some things from one to the other to eliminate probs

[Tony Meola]

When we had the 454's we had the same issue. For us it was when the engines were warm and we would be drifting along the beach fishing. Pick up to run and the Starboard engine would always start to come up to speed then stumble and then recover. For us it was the automatic choke, so we wired it open.

[trace elements]

When we had the 454's we had the same issue. For us it was when the engines were warm and we would be drifting along the beach fishing. Pick up to run and the Starboard engine would always start to come up to speed then stumble and then recover. For us it was the automatic choke, so we wired it open.

Tony, was your choke somehow closing even when the engines were warm? I'll have to take a look at that tomorrow.

[scenarioL113]

If the choke is closing (even if only partially) it will give this type of symptom

[trace elements]

I just reconnected the chokes on both motors to fix a hard starting problem. They are the old mechanical coil spring-type chokes. I don't know how they could close after the engine warms up, but I am going to wire them open and run the boat today nonetheless.

[Rawleigh]

If that doesn't show the problem, swap carbs side to side and see if it follows the carb. Then do the same thing with the coils. On second thought maybe do the coils first since they are the easiest swap.

[pschauss]

Be careful about swapping the coils. I did that on mine and set off a whole chain of problems. I found out that my port and starboard engines had different ignition systems. One needed a ballasted coil and the other did not.

[pschauss]

Does your Prestolite distributor have the original electronic module? I had a similar problem with mine where the engine would cut out completely at 3000 rpm. I could restart it immediately but it would do the same thing a few minutes later. Replacing the module with the equivalent Sierra part solved the problem.

[ccroke13]

If I could add my two cents from my own experience. Had the same issue on my old 28. Accelerating around 2400 rpms the starboard would would do the same as yours. Went through all the fuel and ignition systems. Came to find out that a wire on the back of my start/ignition switch was broken. Fixed the wire and replaced the switch cause I was in there anyway and issue was gone. Only discovered it because a friend with the same boat had it happen to him.

[Tony Meola]

If the choke is closing (even if only partially) it will give this type of symptom

Yes it was,.

[trace elements]

Wired the chokes open, no avail. Boat stumbles getting on plane once or twice and then does it intermittently after that. Once the boat recovers and climbs on plane, it will run great and pull to WOT.

I switched the carbs but didn't get to run the boat because the fuel inlet fitting was leaking. Need to figure that out. I have had a lot of trouble with these if anybody has any tips.

Also, not sure if I have mentioned this before, but the troublesome motor has a seachoice electric fuel pump on it, model 20341. This model provides 60" lift but only 3.0-4.5 psi of pressure. The crusader service manual I have calls for 4.5-7.0 psi at 1800 rpms, so I ordered a new electric pump with these specs. I don't believe this to be the issue though as the engine ran fine last year with this pump.

I hate to start throwing parts at this thing, but I am going to try the following:

Run with switched carbs
Switch Fuel feeds on tank.
Change ignition coil
Bypass GFS-10 fuel flow sender
Check Wire Harness on engine
check ignition wiring behind dash

If none of these work, I am going to try a remote tank. I just don't like having remote tanks around inboards.

I really need to figure out whether it is a fuel or ignition issue.

[scenarioL113]

1st run with carbs switched. Is 2400rpm right around the time the secondaries kick in?

Maybe the secondaries are kicking in early and over fueling??? I dont know if that would cause a problem tho

[Rocky]

I'll give you my scenario,
Before I pulled my 454s I had my port engine stumble when WARMED up only, after 2500RPMs as long as you stayed in throttles. If backing off it would get happy again. Tried fuel and ignition every possible switch over nothing followed.
When I sold them for converting to 6BTA, my friend who drag races and bought my 454s told me there was a bunch of "goop" stuck in the exhaust valves. I now know it was ethanal eating my OEM tank. If it popped through carb I'd know intake valves. But didn't. It was exhaust valves.
A leakdown check and boroscope would have found the "goop". So, in my experience don't forget to look outside of fuel/ignition and perform a engine health check with leakdown.
It's only time and no money spent to make sure not mechanical fault. (If you perform it yourself).

[Midastouch73]

I had a similar issue. Come to find out there was a lose connection on the electric fuel pump. The electric fuel pump fed the manual fuel pump. When it became disconnected it would stumble then stall when trying to get up on plane. Once I reconnected the pump it ran like a champ. It took months to figure it out and no mechanic was of any help with this simple issue....

[CamB25]

My 2 cents only. Looking at the video, the engines are in sync until 2400 rpm and then the STBD engine drops immediately to exactly 1/2 rpm - 1200rpm and then recovers immediately to 2400 rpm to match the Port engine. Not really a stubble, more like falling on your face on a trampoline. That's a huge clue that I think points to an electrical issue with the advance/distributor system. Full advance should be in by around 2500 rpm I would guess.

Good luck with it!

[trace elements]

So I switched the carbs, and added some fuel. Was not able to replicate this issue. Below find a video of the engine accelerating with a timing light to verify the ignition. 10 degrees at idle 34 total advance. The acceleration is only to about 3000 RPMS, (I wasn't at helm) but i believe the secondaries should start to open shortly thereafter.

https://youtu.be/xMOZT7g7yss

[scenarioL113]

Does that mean the problem is gone (for now)???

[trace elements]

Makes me wonder if the mechanical fuel pump was having issues overcoming the head from the low fuel level in the tank. I would really like to convert this motor back to a mechanical fuel pump. I'll add that to the project list....

[scenarioL113]

I am not a fan of electric fuel pumps. I am sure they are great and necessary for EFI etc...

I use a mechanical fuel pump on my 454 (automotive application). I just think on something that is not an everyday driver that having a mechanical fuel pump will give less headache. Things dont like to sit and not used everyday, so if you can simplify your systems they will serve you more reliably.

I am not condemning your fuel pump, I am just generally speaking. I do hope that it is the gremlin in all of this

[trace elements]

I am not a fan of electric fuel pumps. I am sure they are great and necessary for EFI etc...

I use a mechanical fuel pump on my 454 (automotive application). I just think on something that is not an everyday driver that having a mechanical fuel pump will give less headache. Things dont like to sit and not used everyday, so if you can simplify your systems they will serve you more reliably.

I am not condemning your fuel pump, I am just generally speaking. I do hope that it is the gremlin in all of this

Yeah, when I bought the boat it had the electric fuel pump installed. I have no idea why.

I've read that the pump rod in the motor can become seized, I need to investigate if a mechanical fuel pump can be installed or if this rod is frozen or non-existent.