• If you log in, the ads disappear in the forum and gallery. If you need help logging in or getting registered, send request to: webmaster@southernairboat.com

454 missing under load

H

huntingjman73

Well-known member
1998 model 454 runs sluggish under throttle, misses and backfires occasionally. Got really noticeable today, rough as heck waves, and both muame off at collectors. You could really hear the misfire etc. I have ahad a little troucle with it starting, seems rich at times. It has an Edelbrock carb, running too rich??? too lean?? How hard are jet changes, I am used to Holleys not Edelbrocks.

thanks guys,
 
se if one of your plugs porcelain is craked.i did an engine swap on a jeep one time put new plugs in it and it was a claimed rebuilt iron horse motor,it had a weber 2bbl on it and it ran like crap.racked my brain could not figuer it out asked my dumba$$ brotherinlaw to give me a fresh perspective.he seen one of the plugs cracked and $1.28 later it was fixed.but hey i never said i was a genius.
 
Well before I get off into a novel of ideas......My first suspicion is that the timing is too retarded. Maybe your base timing is OK, but the centrifugal isn't functioning properly. I'd check the timing curve with a timing light before I started chasin a bunch of other stuff.

Felber
 
You can quite easily test and tune on the trailer.

After she reaches operating temp and RPM where she misses, run her a bit hard watching water temp and oil pressure#. Run her at 70ish percent load for a couple minutes then reach over and shut the key off. After the exhaust has cooled enough to not burn you, and after you dis-connect the battery cable from the the starter by whatever means it takes to dis-able the starter, then you can pull all your plugs to see what your engine is telling you?

The ground strap wraps around the electrode. This is the gappable part of your plugs.
What do the ground straps look like? This bit of evidence is always there to examine.


Read the plugs one at a time. After reading the plug, apply just a small amount of anti-seize to the plug threads and re-install if the burn meets your liking.
Spark plugs could show you which cylinder(s) is the culprit.

Have you changed the spark plugs and then ran her to operating temp and RPM to see if this cures your problem?

What are the condition of your spark plugs? Have you checked your rotor, cap, and the condition of your spark plug wires? Have you checked your hot wire for ignition voltage source to see if the voltage is proper? This voltage check preformed at the distributor.

Metering rods?? If you cannot read a plug, you cannot properly re-callibrate your circuitry. I bet you can and have. Careful though. You get that big block running proper, she may get away from you.

If you have a reduction unit, I truly hope Santa brings you a 750ish holley for that BBC. :wink:
 
huntingjman73":1emlj7b8 said:
1998 model 454 runs sluggish under throttle, misses and backfires occasionally. Got really noticeable today, rough as heck waves, and both muame off at collectors. You could really hear the misfire etc. I have ahad a little troucle with it starting, seems rich at times. It has an Edelbrock carb, running too rich??? too lean?? How hard are jet changes, I am used to Holleys not Edelbrocks.

thanks guys,
Click to expand...
:D o540!
 
i just put new plugs in it prior to this trip. I have only been out on it less than 8 trips. I bought this used, but through a very reputable dealer on here. They had justinstalled a new distrubitor, plugs, wires, intake and carb. Not real sure if it got tuned properly under use. When I changed the plugs they all looked normal, and most important they all looked the same.
 
it has a 2:1 reduction, holley 750 sound right? Vacuum secondaries or DP?

sorry, used to be a hotrodder, but that was 20 yrs ago.
 
have ya checked for moisture under the dist cap, hate to sound simple but sometimes its the simple stuff that eludes me
 
I am not saying anything about the carb causing your problem. Good possibility the carb will not fix your problem.

If you just gotta have a Holley, I strongly suggest you buy one from Thunder or Felber. A carb that is setup for airboat use. I would venture to guess a vacuum secondary is out of the question.

Idle circuitry, and squirter size for sure out of the box are not right. IMO-
A simple jet change still does not make a new holley an airboat carb!!!!!

I just popped off about the holley. Sorry! I got off the subject.
On a used engine there is an extremely long list of what might be causing your miss under a load. Just an engine you are dealing with and you are smarter.

I am an optimist. I still lean towards ignition. After exhausting all ignition possibilities, only then would I search elsewhere.
 
I think everyone here has given good advice. Honestly when a problem is described over the internet it opens up a lot of posibilities, since it can be so hard to describe the problem. I know if I heard the engine, I could probably diagnose it in 5 minutes. But going by what you describe, I can come up with a hundred different things from ignition to valvetrain issues. That's why my idea of where to start was the timing. Honestly it's my gut feeling based on what you typed. But I guess I should have asked if the backfire is through the carb or out the pipes? That can narrow it down quite a bit.

Felber
 
same thing i was thinkin felber theres a whole bunch of stuff it does sound like timing even something as simple as two plug wire mixed up , i like to carry a plastic straw with me if ifeel something wrong i just touch the straw to the headers to see if it melts consistantly if not ive narrowed it down somewhat an go from there, old dragracin trick between rounds when time was at a premiun
 
plumcrazy":1fjc1m8p said:
same thing i was thinkin felber theres a whole bunch of stuff it does sound like timing even something as simple as two plug wire mixed up , i like to carry a plastic straw with me if ifeel something wrong i just touch the straw to the headers to see if it melts consistantly if not ive narrowed it down somewhat an go from there, old dragracin trick between rounds when time was at a premiun
Click to expand...

Another new trick I learned....When not readin EGTs or armed with my temp gun, I fling spit off my finger at em! Maybe now I won't gross people out as much or stain the nice shiny headers anymore! LOL

Felber
 
backfire was out the pipes. I'll check the cap and rotor and go from there. Boat sits at a friends house, as i live in town and going through a dvorce, so it may be a while before I can verify. THanks for all the ideas.
 
You must log in or register to reply here.
Back
Top