OMC 460 backfire under load

[Trevthefox]

Back again with another 460 KC question. As always I appreciate all your help.

I have a '89 460 OMC King Cobra. It's equipped with a Holly 4160 650 CFM carb (newly rebuilt) and a comp cams xtreme marine camshaft. Stock camshaft was replaced. Points distributor that has been rebuilt with a fresh set of new points and condensor about a year ago.

Problem:
Engine will backfire once I reach about 2800 RPM under load. It will not backfire at 2800 rpm in neutral.

I've been searching online and I've read that it could be timing or compression problem. Initial timing has been set at 10º BTDC at idle. Dwell has also be properly checked at adjust with a dwell meter. I've been debating whether I should change the points out to an electronic ignition. I haven't checked the mechanical advance yet, nor did I change any springs when I rebuilt the distributor, but just assumed that it was functioning properly since initial is dead on and it revs up to higher RPMs with no problem in neutral.

Fuel lines have been checked and there is no blockage. Carb is being fed by an electric fuel pump and a holley fuel pressure regulator set at 6 PSI.

Should I be checking timing advance that is shown in the OMC handbook? or does that change now since I have an aftermarket camshaft?

Looking for some tips before I start throwing money at new spark plug wires, EFI ignition, new coil, etc.

 

[Scott Danforth]

You are experiencing a lean sneeze. Go back to the carb.

 

[stresspoint]

buy some new ignition parts , points, cap , rotor , coil and condenser ( i know you posted these have been replaced but start again anyway).
if you have a ballast resistor replace that to.

I'm guessing the points are cooked "but " a condenser or coil will give similar symptoms.

 

[Bondo]

You are experiencing a lean sneeze. Go back to the carb.

Ayuh,..... Agreed,.....

 

[kenny nunez]

Quite often when a Holley suffers a backfire it blows out the power valve in the metering body. When the carburetor was rebuilt did you use the correct kit for the marine carburetor? What is the vacuum at idle? How many turns is the idle stop screw in and how many turns out from seat is the idle mixture screws?
When a Holley is right the idle stop is just over 1 turn and the mixture screws are 1.5 out. If you cannot get close to these settings then the cam overlap is affecting carburetor settings. You may have to start drilling a small hole in the forward area of the primary butterfly plates to bring the stop screw back to around 1 turn in.
There is a tutorial on the Holley web site on the procedure or get the Holley rebuild book from Barnes & Noble.

 

[Trevthefox]

You are experiencing a lean sneeze. Go back to the carb.

I feel like I rebuilt that carb perfectly and checked every setting that Holley had to offer. Jets and power valve are correct according to the book. The only thing I can think of is that maybe the surface of the metering block and carb are a little warped causing a vacuum leak.

 

[Trevthefox]

buy some new ignition parts , points, cap , rotor , coil and condenser ( i know you posted these have been replaced but start again anyway).
if you have a ballast resistor replace that to.

I'm guessing the points are cooked "but " a condenser or coil will give similar symptom

I thought about replacing the coil. I might give that a try. I don't have a ballast resistor but the engine does have those OG resistor wires that only give the coil 9 volts while running. I did check a while back to make sure those resistor wires were still functioning correctly. I can check that again. I do have a jumper wire from the slave solenoid to the coil so that the coil gets a full 12 volts when cranking. This is according to the manual.

 

[Scott Danforth]

I feel like I rebuilt that carb perfectly and checked every setting that Holley had to offer. Jets and power valve are correct according to the book. The only thing I can think of is that maybe the surface of the metering block and carb are a little warped causing a vacuum leak.

if you have enough warpage to cause a vacuum leak, you would be pouring fuel out of the bowls and metering block into your bilge

99% of the time its dirt in the down leg boosters

did you chase every passage with a piece of fishing string?

are the emulsion tubes clear?

 

[Scott Danforth]

I thought about replacing the coil.

its never the coil

 

[Trevthefox]

Quite often when a Holley suffers a backfire it blows out the power valve in the metering body. When the carburetor was rebuilt did you use the correct kit for the marine carburetor? What is the vacuum at idle? How many turns is the idle stop screw in and how many turns out from seat is the idle mixture screws?
When a Holley is right the idle stop is just over 1 turn and the mixture screws are 1.5 out. If you cannot get close to these settings then the cam overlap is affecting carburetor settings. You may have to start drilling a small hole in the forward area of the primary butterfly plates to bring the stop screw back to around 1 turn in.
There is a tutorial on the Holley web site on the procedure or get the Holley rebuild book from Barnes & Noble.

Hey Kenny,
Yes the correct marine kit from Holley was ordered according to the list number on the carb. I verified that the correct power valve and jet numbers were used as well.

I'll need to recheck the vacuum at idle. But when I did the initial test (on muffs) the vacuum was fine. Idle screws were about 1.5 turns out. I did not check how many turns the idle stop screw is, so I couldn't tell you that. I just remember playing with it to achieve correct idle rpm after the engine warmed up.

 

[Trevthefox]

if you have enough warpage to cause a vacuum leak, you would be pouring fuel out of the bowls and metering block into your bilge

that makes sense

99% of the time its dirt in the down leg boosters

did you chase every passage with a piece of fishing string?

Yes and blasted carb clear to make sure it came out on both ends

are the emulsion tubes clear?

Yes those are clear too.

 

[kenny nunez]

If the idle stop screw is turned too much to compensate cam overlap it causes the off idle circuit to start bleeding fuel.
Hopefully that is not happening. Try to get a vacuum gauge connected to the manifold. Readings will be lower when the boat is in the water verses when on a flushout which is normal.

 

[Trevthefox]

If the idle stop screw is turned too much to compensate cam overlap it causes the off idle circuit to start bleeding fuel.
Hopefully that is not happening. Try to get a vacuum gauge connected to the manifold. Readings will be lower when the boat is in the water verses when on a flushout which is normal.

When you say the idle stop screw needs to be just over 1 turn. 1 turn from what?

 

[Trevthefox]

I have an update.
I was thinking about buying a rebuilt carburetor. The 650 cfm that is on my engine currently is supposedly undersized. The original carb was a 750 cfm. Before making the purchase I decided to look over the problems one more time since the engine did fine last year with the 650 cfm.

First, I checked the condensor in the distributor. Seemed to check out with a multimeter. I noticed the rebuild kit i purchased a year back came with a condensor meant for the 3.0 OMC engine of the same year (even though this kit was meant for the 460's according to mult. web sites). Cap and rotor looked fine. All spark plug wires were in good shape and firing order was correct. Since the condensor passed the multimeter check, i kept it in the distributor.

changed out spark plugs. 6 Old plugs showed evidence of lean condition. (plugs from cylinder 2 and 5 looked like possible carbon fouling)

Last time I rebuilt the carb I purposely set the float levels so that the fuel in the bowl was slightly lower than the recommendation. I did this because I was having problems with engine flooding at idle after coming down from higher rpms. Anyways, I set the levels parallel to the bottom of the fuel bowl according to manual and holley specs. This time on the way out of the marina I was able to get passed 2800 rpm and plane with no back fire. I was happy that I solved the problem. However coming back into the marina at the end of the day I got multiple backfires at about 3100 rpm. This time we were going against current and wind.

I'm guessing she's still starving for fuel under heavier loads. My only guess is adjust float levels again so fuel level in bowls are higher than what's recommended in the manual.

 

[Trevthefox]

forgot to mention. I had a spare coil and swapped that out before trying any of the above mentioned changes. I wanted to make sure it wasn't my ig coil. Both coils would backfire at 2800 rpm. So the coil was not at fault for the backfire. Also found out I was getting about 10 volts to the coil through the resistor wire. I know it should be about 8-9 volts. Hopefully the 10v doesn't burn out my coil.

 

[Scott06]

forgot to mention. I had a spare coil and swapped that out before trying any of the above mentioned changes. I wanted to make sure it wasn't my ig coil. Both coils would backfire at 2800 rpm. So the coil was not at fault for the backfire. Also found out I was getting about 10 volts to the coil through the resistor wire. I know it should be about 8-9 volts. Hopefully the 10v doesn't burn out my coil.

10 v is fine voltage drop is for the points life not the coil.if coil isnt getting mad hot don’t worry about it

lowering float level can only lead to leaner fuel mxitures.

 

[Trevthefox]

10 v is fine voltage drop is for the points life not the coil.if coil isnt getting mad hot don’t worry about it

lowering float level can only lead to leaner fuel mxitures.

Makes sense. I'm going to raise the levels slightly above the recommendation and see if that fixes the issue.

 

[Trevthefox]

Adjusted the float levels yesterday. Fuel now sits above recommended levels to allow for more fuel. Did not fix the problem. Still backfires. If anything it backfires around 2900 rpm, so the problem got worse. But we also have 2 extra bodies in the boat this time. At this point I'm ready to just buy a new carb. Can't figure out the issue. But then I wonder if the carb is even at fault.

 

[Scott Danforth]

Go back to the carb and make sure every passage is clear. If you can't pass a piece of fishing line thru them, they are not clear

 

[dubs283]

Sounds to me like the jets are too small. 650 could be fine for a 460 with the right jets. Fords are thirsty motors, maybe better off with a 750 in the end

I would do a plug burn to determine proper jet size