1987 OMC 7.5 popping and not reaching higher rpm range.

[JerryRigged]

Hey guys!Need a little help here. I have a 1987 liberator with a OMC 7.5 ford 460 stroked to 514.
Been having trouble this summer with the ignition system.
I have replaced the spark plugs, that didn't fix the problem. They were wet fouled when removed and replaced.
I replaced the cap and rotor, that didn't fix the problem.
I changed the fuel water separator, that didn't fix the problem either.
We took apart the Holley carb and found the power valve was stuck and rebuilt that portion and cleaned it.
It then wouldn't run.

A ford mechanic looked at it and found the points were fused.

I changed them. And now it runs. I had it running on the muffs through the operating temp in the driveway and all went good. However, when I got it out on the water and put it to the metal, the engine pops and sputters intermittently and doesn't reach top RPM or speed.

I'm not sure where i should have the timing set. the engine has been worked. 460 stroked to a 514 with a 4 bolt main and aluminum forged pistons.
currently running aluminum heads and manifold with a holley 750 carb
MSD ajustable ignition with coil
over 93 octane
competition cam
prestolite distributor
platnium plugs

Any ideas??

 

[bruceb58]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

What is the ignition advance doing over RPM...within spec?

Checked your dwell with a dwell meter?

Checked compression?

Have you double and triple checked your firing order?

 

[JerryRigged]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

Haven't checked the dwell. Not sure how to do that to be honest. I set the points gap to .017

We changed out the fouled plugs 3 times while trying to nail down the problem. I'm pretty sure the firing order is correct.

The wires weren't removed from the bracket attachments.

No one around here wants to work on the boat since it is over 10 years old and has been modified from stock.

The boat runs and makes it to around 40 to 50 MPH. then starts to pop. In the 4000 plus rpm range

God help me if I have a compression problem! This is a 12000 engine build with aluminum heads!!!

 

[Silvertip]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

You set dwell with a dwell meter. You then set tming to the 460 spec as a starting point. This engine should not have a vacuum advance unit on the distributor. Verify mechanical advance operation. The engine builder should have provided thetune up specs.

 

[Bondo]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

Haven't checked the dwell. Not sure how to do that to be honest. I set the points gap to .017

We changed out the fouled plugs 3 times while trying to nail down the problem. I'm pretty sure the firing order is correct.

The wires weren't removed from the bracket attachments.

No one around here wants to work on the boat since it is over 10 years old and has been modified from stock.

The boat runs and makes it to around 40 to 50 MPH. then starts to pop. In the 4000 plus rpm range

God help me if I have a compression problem! This is a 12000 engine build with aluminum heads!!!

Ayuh,.... Sounds like it's goin' Lean at high fuel usage,....

 

[Silvertip]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

Fuel fouled plugs would say "rich" rather than "lean". Sounds like you are guessing at some things. Timing? Dwell? Advance Curve, Firing Order? Carburetion? Timing and dwell are "builder questions". If he/she can't answer that, I wouldn't term them a builder! Firing order doesn't change with stroke change. A rebuild on an engine without an adjustable valve train requires special push rods and/or valve stem length tweaking. Was that done and by whom? If an adjustable valve train was installed, who adjusted the lifters and was it done correctly.

 

[HT32BSX115]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

I changed the fuel water separator, that didn't fix the problem either.
We took apart the Holley carb and found the power valve was stuck and rebuilt that portion and cleaned it.
It then wouldn't run.

Oh Gawd, not another Lib!!!!


Howdy!


You have a 26 year old boat! (just like mine!)

It REALLY sounds like a fuel related problem.

Pull the back seat and remove the cover over the fuel tank gage-sender.

remove the tank sender, fuel pick-up and anti-siphon check valve.

You're looking for either debris in the tank (plugging the pick-up screen, or anti-siphon valve)

Then work your way to the carburetor. I had a similar problem being unable to reach max RPM and found everything to be ok from the tank to the carb.

I even rebuilt the carb only to find the only thing wrong with my Holley 4160 was the destroyed gaskets from taking it apart.

(Turned out my camshaft was flat)

Also, check your ign wires........ It'll run pretty good with a couple of wires switched!! ..........and ensure that you're timed correctly. ( If memory serves me, A stock 460 should be timed at 10 degrees 91oct and 8 for 87AKI fuel. Your machine shop should have told you where they wanted the timing and that you are probably going to have to use premium.)

If you're using the stock dist, you might have the wrong (total) advance and /or advance curve for your cam and comp ratio etc......... You may have to pull the dist and have an auto electric place run it and change teh advance springs etc........ or replace it with a newer MSD marine dist programmed for your engine. (the machine shop that built the engine should be able to tell you what you need)

I don't think you want to be running a King Kobra over about 5500 RPM......

Now having said all that, if you have fouled the plugs 3 times You either have FAR too rich jets or the choke is stuck/staying partially or fully closed!!

Disconnect that choke and adjust or wire it permanently OPEN to ensure that it is not your problem......

Boatinfo - OMC 1986-1998 Service Manual

There's a procedure in the above manual link to for setting the ignition points, timing and a description on DWELL and /or setting points gap.

(I "fixed" my 460 king Kobra a different way)


Let me also add that I hope you're either NOT operating in salt water OR you have closed cooling with those aluminum heads.......OR it won't end well......


Regards,


Rick

 

[JerryRigged]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

Oh Gawd, not another Lib!!!!


Howdy!


You have a 26 year old boat! (just like mine!)

It REALLY sounds like a fuel related problem.

Pull the back seat and remove the cover over the fuel tank gage-sender.

remove the tank sender, fuel pick-up and anti-siphon check valve.

You're looking for either debris in the tank (plugging the pick-up screen, or anti-siphon valve)

Then work your way to the carburetor. I had a similar problem being unable to reach max RPM and found everything to be ok from the tank to the carb.

I even rebuilt the carb only to find the only thing wrong with my Holley 4160 was the destroyed gaskets from taking it apart.

(Turned out my camshaft was flat)

Also, check your ign wires........ It'll run pretty good with a couple of wires switched!! ..........and ensure that you're timed correctly. ( If memory serves me, A stock 460 should be timed at 10 degrees 91oct and 8 for 87AKI fuel. Your machine shop should have told you where they wanted the timing and that you are probably going to have to use premium.)

If you're using the stock dist, you might have the wrong (total) advance and /or advance curve for your cam and comp ratio etc......... You may have to pull the dist and have an auto electric place run it and change teh advance springs etc........ or replace it with a newer MSD marine dist programmed for your engine. (the machine shop that built the engine should be able to tell you what you need)

I don't think you want to be running a King Kobra over about 5500 RPM......

Now having said all that, if you have fouled the plugs 3 times You either have FAR too rich jets or the choke is stuck/staying partially or fully closed!!

Disconnect that choke and adjust or wire it permanently OPEN to ensure that it is not your problem......

Boatinfo - OMC 1986-1998 Service Manual

There's a procedure in the above manual link to for setting the ignition points, timing and a description on DWELL and /or setting points gap.

(I "fixed" my 460 king Kobra a different way)


Let me also add that I hope you're either NOT operating in salt water OR you have closed cooling with those aluminum heads.......OR it won't end well......


Regards,


Rick

Air-Check

Fuel-Did the tank check. Pumped out 12 gallons and found it clear and clear. I initially thought it was water in the tank.
this is my 3rd liberator and had to do that before. Carb had debris in the power valve keeping it open which we th
ought was probably causing the plugs to foul. Rebuilt that. I may need to do some adjustments to the flow though because it does seem like it is running lean when under demand.

Spark-A mechanic friend of mine pointed out that the points were fused. I replaced those. Along with the cap, rotor and condensor. My confusion is whether the MSD adjustable ingition could be the culprit or a advance wire that could be becoming aged. The timing was set at 16 degrees and the wires aren't crossed.

The engine has a competition cam and it has run well since the rebuild around 200 hours ago. No signs of significant engine component wear are being seen. It is a freshwater only boat and a trailer queen to boot.

 

[HT32BSX115]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

Air-Check

Fuel-Did the tank check. Pumped out 12 gallons and found it clear and clear. I initially thought it was water in the tank.
this is my 3rd liberator and had to do that before. Carb had debris in the power valve keeping it open which we th
ought was probably causing the plugs to foul. Rebuilt that. I may need to do some adjustments to the flow though because it does seem like it is running lean when under demand.

Spark-A mechanic friend of mine pointed out that the points were fused. I replaced those. Along with the cap, rotor and condensor. My confusion is whether the MSD adjustable ingition could be the culprit or a advance wire that could be becoming aged. The timing was set at 16 degrees and the wires aren't crossed.

The engine has a competition cam and it has run well since the rebuild around 200 hours ago. No signs of significant engine component wear are being seen. It is a freshwater only boat and a trailer queen to boot.

VERY COOL!

I have had my 211 Lib since 2005. My brother bought it new (With the OMG 460 King Kobra!)

In 2006, I replaced the entire mess with a complete (stock) 1997 Mercruiser 454/Bravo III. Top speed didn't change much but the hole-shot is unbelievable!


I cannot help you with the MSD other than your problem (if verified NOT fuel related) sounds a lot like timing.

 

[bigdirty]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

A few thoughts theorys.. plugs possibly wrong heat range? Is it 'pinging' or detonating at high rpm or just doesn't have any more go? Is there a possibility the valves are floating? (valve springs not heavy enough in the heads) Something else I've seen is the anti siphon valve in the tank gets plugged up, decreasing flow.. u ever look into a 4 barrel carb at 4K+ rpm? lol wow, they need LOTS of fuel!

 

[JerryRigged]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

A few thoughts theorys.. plugs possibly wrong heat range? Is it 'pinging' or detonating at high rpm or just doesn't have any more go? Is there a possibility the valves are floating? (valve springs not heavy enough in the heads) Something else I've seen is the anti siphon valve in the tank gets plugged up, decreasing flow.. u ever look into a 4 barrel carb at 4K+ rpm? lol wow, they need LOTS of fuel!

thanks, but it's not pinging,it's miss firing.

it's getting gas. the plugs were fouled. cheecked heat range. standard plugs for a 460 are recommended. mine are platnium and the MSD can be adjusted for hotter spark.

It's not getting into the range it used to. 5500 to 6000 max.

After 4500 rpm it starts to miss fire.

 

[bruceb58]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

It's not getting into the range it used to. 5500 to 6000 max.

No way you should ever run the engine at that RPM. You need to check your tach.

 

[HT32BSX115]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

No way you should ever run the engine at that RPM. You need to check your tach.

I think this is balanced/blueprinted/stroked 460 "cammed" to run that fast..... You would not want to run a stock 460 that fast though. My brother routinely ran mine mine at 5000 RPM before I got it and it was stock. (and it flattened the cam!!:facepalm

 

[JerryRigged]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

I think this is balanced/blueprinted/stroked 460 "cammed" to run that fast..... You would not want to run a stock 460 that fast though. My brother routinely ran mine mine at 5000 RPM before I got it and it was stock. (and it flattened the cam!!:facepalm

I'm starting to think my carb is the problem.

I think it's a holley 750.
The engine is blueprinted with forged aluminum piston, trick flow aluminum intake, aluminum heads, 4 bolt main, stroked to 514 and designed to handle up to 6000 rpm.

Everyone has been saying it's either fuel or running lean.

I made some adjustments after going over all the electronics today.

Got it out on the water and got up to 4900 rpm. it flattened out there instead of continuing upwards. While this was going on, the valve cover breather started puking oil!!!

When the smoke cleared, I checked the pcv and it was ok. (Rattled) This leads me to believe there isn't any vacuum causing the crankcase gases to back up. I checked the vacuum lines and they are clear. This leads me to believe the carb isn't creating vacuum at this point. (most likely clogged internally) which also could be cutting off my fuel demand too.

What do you guys think? Does it sound like the carb is either needing a full rebuild or could it be too smalll for this set up?

 

[HT32BSX115]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

Mine was a stock 4160. It produced a LOT of crankcase vacuum.

I would go though the carb again.

 

[JerryRigged]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

You converted your liberator to mercruiser? Do tell!!

How difficult was it?

Plan B for me down the road! The boat is is great shape and worthy of a transplant and updating.

It would be great to go with an MPI

 

[HT32BSX115]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

How difficult was it?

It's pretty easy.

1. Remove OMC

2. Install Mercruiser.

You do have to relocate/rebuild the (side) engine mounts, but it's just about as easy as above. The transom cutout is the "same". You must drill two additional holes. (the Bravo transom has 8 through-bolts, OMC has 6 but they are in the "same place")

 

[JerryRigged]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

It's pretty easy.

1. Remove OMC

2. Install Mercruiser.

You do have to relocate/rebuild the (side) engine mounts, but it's just about as easy as above. The transom cutout is the "same". You must drill two additional holes. (the Bravo transom has 8 through-bolts, OMC has 6 but they are in the "same place")

I might be going this route.

My local marine guy, another one who refuses to work on this boat because it's a highly modified OMC said he'd bet money that I shaved one of the pistons because of a timing issue which is causing the blow by.

I was never able to track down the timing issue. Matters worse-the guy who built this boat (the second owner out of 4 last being myself) doesn't want to return my calls regarding questions as to where exactly the timing should be set.

The power loss, combined with the blow by is makin' me queasy thinking at least the rings are bad in one or more of the cylinders. Could also be that piston or even valve seats. UGH!

I am going to try a compression test and see what that indicates.

 

[Maclin]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

What type of fouling did the sparkplugs show? Wet fouling? (spark problem) Sludgy deposits clogging the gap? (extreme oil fouling exaceratbing potential spark problem)

Got any pics of the sparkplugs?

 

[HT32BSX115]

Re: 1987 OMC 7.5 popping and not reaching higher rpm range.

The power loss, combined with the blow by is makin' me queasy thinking at least the rings are bad in one or more of the cylinders. Could also be that piston or even valve seats. UGH!

.

Yeah. if you have a LOT of blow-by, and poor compression, you're probably looking at a complete rebuild. It might not be worth going much farther. If your drive is in good shape, it's still worth quite a bit. (I only got $150 for the engine!) I sold drive + trim pump/trim cyls for $1450 about 5 years ago. (Over a 3 year period, I sold all the King Kobra "stuff" for approx $3500) The "big" Cobra drives are getting harder to find, not easier.


And used Mercruiser Bravos, (I, II, III) are also easier to find and more plentiful than Volvo's.......