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View Full Version : engine backfiring at top end, please help?



DamDonzi
05-08-2000, 05:25 PM
I have a 1996 4.3 liter omc vortec v6. Between 4500 and 5000 rpm the engine feels like its not pulling right and it pops through the carb. It has a 4 barrel holly carb and I believe that the secondary butterflies are not opening. I ran it on the trailer with the hose attached and gave the engine full throttle and the secondaries never opened. I checked the diphram for the secondaries and there were no tears or rips in it. It could have too stiff of a spring in it, I was told. The boat starts to backfire about 5-10 minutes of running in the water WOT. The water pump sounds noisy, but the temp gauge never goes over 175 degrees.

johndmay
05-08-2000, 05:45 PM
Originally posted by DamDonzi:
I ran it on the trailer with the hose attached and gave the engine full throttle and the secondaries never opened.

Holy ****.

RickR,GroveCity
05-08-2000, 07:00 PM
If a boat motor backfires usually it is caused by water in Distributor Cap.
Use the boss's hair dryer to dry it out.
If there is any evidence of carbon tracking replace it.
If it is the original from 1996 replace it.
NEVER run an engine at wot without a load much less on a water hose. http://206.150.187.82/ubb/eek.gif
BTW you may have blown your power valve. In the future if you get a rich idle and she misses check the power valve


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RickR mailto:riggerb@aol.comriggerb@aol.com</A>

[This message has been edited by RickR,GroveCity (edited 05-08-2000).]

[This message has been edited by RickR,GroveCity (edited 05-08-2000).]

AVickers
05-08-2000, 07:01 PM
Secondaries won't open under a no-load situation -- as in running on the trailer.

Popping through the carb indicates a lean mixture (or arcing in the distributer or across spark plug wires, but if it's the distributer or wires it would occur at all RPM ranges and be especially bad at idle.)

As for leaning out, if, after 10 minutes of flat out running, can you idle down for a bit and then run flat out for another 10 minutes???

If so, I'd suspect the fuel system's ability to keep the float bowls full at sustained WOT. Volume/pressure from the fuel pump too low? Water separator plugged up? Tank vent blocked? Needle/seat too small (the marine version has a larger opening than the automotive unit...).

[This message has been edited by avickers (edited 05-08-2000).]

GeneD
05-09-2000, 07:33 AM
Wait, wait, wait...
I know the answer to this one!
I'll have Chevy motors for $1000 Alex.
I just went through this. Let's assume that the motor WAS running fine until one day at WOT the thing started backfiring through the carb...
Check your spark plugs and boots. Last month this happened to me and it was 3 cracked spark plugs and burned out boots.
Last weekend this happened to me and it was burned out boots, (3).
What gives? I don't know at this point, but I replaced the boots and now everything is fine.
Me? I'm thinking that the plugs I have are too hot. When I pull them, they look to be text book perfect. But I can't explain the burned out boots and the previous cracked plugs.
OR...
It could be the distributer cap, seen that too many times.
As for the secondaries opening up, or not opening up that is, they won't unless the motor is running right. If the motor can't take the fuel, the butterflies will just kinda flutter.

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GeneD
007
Melbourne, Florida

[This message has been edited by GeneD (edited 05-09-2000).]

Forrest
05-09-2000, 09:21 AM
. . . also the secondaries will not open up (at least not much) unless the engine is under load. Fix the ignition, power valve, trash or water in the float bowls, etc., then go out on the water and check the secondaries.

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Forrest

BillG
05-09-2000, 09:32 AM
First thing to check is to see if you have the sintered bronze filters at each of the fuel inlets on the Holley. I had this problem on mine a few years back. I removed these filters and the problem went away. The filters look like they are brand new but they restricted the fuel enough to lean the mixture after a few minutes at full throttle.

harbormaster
05-09-2000, 09:49 AM
Gene. I know why your plugs are cracking.
I think its all the jarring that they get from the way you drive that damn boat of yours!
repeat after me....
Its not an airplane. Its a boat.
http://206.150.187.82/ubb/tongue.gif

AVickers
05-09-2000, 12:15 PM
They do make a cellulous filter for the carb intake -- I forgot about that one... Seems it might "varnish up" if the engine sits for long periods -- like over the winter.

Still, being able to run at WOT for 5 minutes and then stuff falls apart... That doesn't sound like distributor or plugs -- unless maybe the coil is overheating and crapping out????

But why the backfiring through the carb? Maybe a crossfire -- arcing in the distributor cap or crossed wires or induced firing as one wire fires another plug. Generally, a bad coil/ignition system would cause a miss, dump raw fuel into the exhaust which (in a car) might cause backfire through the exhaust, but not via the carb. With a wet exhaust system -- I don't think so.

Again the question: Can the boat run WOT until it starts acting up, idle down for a while, and then WOT for another run?

GeneD
05-09-2000, 03:14 PM
Well, maybe I should drive a bit more sane. I drive way too fast, take too many chances, scare too many fish, and I'm hated by most sailboaters.
Yes, I think a re-evaluation is in order...
NOT!!!



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GeneD
007
Melbourne, Florida

GeneD
05-09-2000, 03:17 PM
Oh, and one more thing.
If all your observations on the ignition don't pan out, take a compression test. It could be a bad head gasket?
It's a sickness, and it's going around.

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GeneD
007
Melbourne, Florida

[This message has been edited by GeneD (edited 05-10-2000).]

major
05-13-2000, 04:08 AM
Originally posted by DamDonzi:
I have a 1996 4.3 liter omc vortec v6. Between 4500 and 5000 rpm the engine feels like its not pulling right and it pops through the carb. It has a 4 barrel holly carb and I believe that the secondary butterflies are not opening. I ran it on the trailer with the hose attached and gave the engine full throttle and the secondaries never opened. I checked the diphram for the secondaries and there were no tears or rips in it. It could have too stiff of a spring in it, I was told. The boat starts to backfire about 5-10 minutes of running in the water WOT. The water pump sounds noisy, but the temp gauge never goes over 175 degrees.
The secondaries will open load or no load.one healthy backfire through a holley carb will blow the power valve resulting in a rich mixture, poor idle, and a slight decrease in intake manifold vacuum [ the force nescessary to open the secondaries ] after eliminating the obvious in the ignition system and given the fact the secondaries don't open and your remark " the engine feels like its not pulling right"the most likely cause of popping through the carb is a bad exhaust lobe on the camshaft or weak valve springs pull the rocker covers, crank the motor and check the exhaust rockers for lift, if you find one that barely opens the valve or if you can open any valve with hand pressure ,you will have found your problem. good luck

Barry Phillips
05-15-2000, 08:35 AM
Don't get nuts check the simple things first.
Run the boat in the water not on the trailer,
load it at WOT. Check timeing, make shure
light is grounded, I have seen a dealer
mechanic get this one wrong. Check your cap
for condensation, check plug wires, check if
choke is sticking, check linkage to carb,
check float level, can the carb be dirty
from winter lay up? Did you use fuel
stabilizer, did you top of your fuel tank
before layup, if not thier can water in the
fuel from codensation. If the motor ran
good last season than it is logical that
something happen over the winter. When did
you last change the plugs, fluel filter
cap etc? Do a complete service. If this fails
than it is time check the exspenive stuff.
Good Luck!

Jamesbon
05-16-2000, 12:37 AM
Try sticking another ignition coil in there, then test in the water. Long ago my boat popped for years, I changed everything, then one day I threw a spare coil in there, and it never ran better. My old one corroded, leaked out the fluid and caused my motor to run like s**t. (It's a cheap test anyway.)
Nate
jamesbon@ij.net

harbormaster
05-16-2000, 09:27 AM
Since your power plant is so new, perhaps you could take it somewhere and hook it up to a diagnostic computer. It would cost you money, but if would have to be easier and cheaper than the trial and error method of replacing things.

http://www.donzi.net/sig.gif

GeneD
05-22-2000, 10:10 AM
You know something?
Now I'm gettting popping through the carb at WOT too! AGAIN!
I'm going to take a look today, but I guarantee it's the spark plug boots again.
Maybe I'm running too hot a plug?

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GeneD
007
Melbourne, Florida

Forrest
05-22-2000, 01:36 PM
What plugs are you running GeneD? Try a set of NGK UR5. DAP sells them.

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Forrest

DamDonzi
05-22-2000, 04:27 PM
I fixed the problem i think, there was a vacume leak on the left side of the carb near the electric choke, and secoundiarie pump. It looks kinda like a compression fitting orafice. It was open and sucking air, i capped it off. Where does that go on this carb, what does it do, please tell me if i did the right thing, the backfireing went away. Thanks guys bill

BillG
05-23-2000, 08:15 AM
It is an air bleed to the choke assembly and should not cause the problem.

GeneD
05-23-2000, 09:56 AM
I'm willing to try anything!
NGK's huh?
So that is the right plug for a 350?

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GeneD
007
Melbourne, Florida

Forrest
05-23-2000, 11:27 AM
The NGK UR5 has the same heat range as AC R42TS to R43TS and seems to do well in small block, big blocks, and even F-word marine engines that use 14mm taper-seat plugs.

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Forrest

[This message has been edited by Forrest (edited 05-23-2000).]