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1991 OMC 5.7 Cobra - WOT top end surge/bog

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Brian O.

New member
Greetings all,

First time poster here looking for some direction on my recently acquired 23’ Chris craft cruiser. It has a 5.7 OMC I/O setup and runs very well for its age overall. Very happy with it.
My one gripe is that at full throttle, on plane and cruising in calm lake water I notice a surge that changes my cruising speed by a good 5mph - about a 400 rpm difference in engine speed. It’s inconsistent and usually happens more when the engine has come up to full operating temp. Some items I’ve addressed in my pursuit on rectifying this issue:
-Full carb rebuild (Holley 4bl 80238 750 CFM marine)
-Fuel system purge (new lines/fuel wrx)
-new f/w separator
-new plugs/wires
-added adjustable vacuum secondary on carb - thought my secondary’s were not fully opening.

I’ve not checked timing as I was told that was set recently. There are marks on the distributor cap where the previous owner/mechanic was likely adjusting that in. That’s what I’m going to check next.

Beginning to think it’s timing advance related but I’m not sure. I’m pretty good mechanically - I own my own CNC based side business and manage a fabrication plant full time so I am definitely not afraid to take things apart but I know where my limitations exist. Hoping those in the know would be able to point me in the right direction focus my attention to.

Just a reminder this isn’t a mission critical issue. Boat runs great and serves my family well…I just wanna go fast!…and avoid a larger issue down the road.

I appreciate any feedback you’re willing to provide!

Kindly,
Brian
 
Most likely a fuel supply issue, being inconsistent

I would connect a fuel pressure gauge between carb and pump.
Things to consider, sticking antisiphon valve, fuel tank pickup tube, power valve, or loose connection going to fuel pump
 
AllDodge,

Thank you for the input.

Your response triggered some thoughts based on work I'd done that I didn't mention:

1. I checked the pickup tube going into the tank. It was clear. That was one of the first things I looked at.

2. I did pull the anti-siphon valve off. You could readily see the exterior had been corroded especially on the barbed portion where the fuel hose connects via a hose clamp. I cleaned off the corrosion and put it back in the top of the tank. I'll order a new one - they didn't seem terribly expensive.

3. As far as the power valve is concerned, it is interesting you mentioned that. During the rebuild, I replaced it with a 4 port vs. the 2 port it originally had. Naturally, the motor ran like crap with the 4 port (completely wrong kit) as it was drowning in fuel, so I cleaned up the original 2 port and put it back in. It ran fine after that - lesson learned. I ordered the correct marine rebuild kit for it which had the proper 2 port power valve (6.5), but I have yet to install it as I didn't give it a second thought. I may go ahead and just do that for posterity.

Much appreciated,
Brian
 
Last edited:
Greetings,

I wanted to update those who might have been following this as to the latest on my quest to solve this issue. Since originally posting I've:

1. Replaced the anti-siphon valve.
2. Cleared out the tank vent (it was partially clogged but not completely)

Based on some observations I noted from when we were out on the water this past weekend, the issue more closely seemed to mimic a rich bogging scenario. I decided to pull the carb off when we got back and re-jet the primary just out of posterity. Ended up finding that the electric choke had previously been disabled by the previous owner and the choke flap was loose. I'm guessing that under high vacuum it was being pulled partially closed and richening out the air/fuel mixture (bogging) until it rattled back into the fully open position. I solved that by resetting the choke and validating its operation using a 12V power supply. All works fine now. I stepped down on the jets one size (I know Holley recommends moving up or down two sizes at a time but I didn't want to risk leaning out too much at full load). I also replaced the power valve with a brand new one of the same size that came out. I took measures to also correct another issue where my accelerator pump was out of adjustment. It's now where it should be

The wife and I are planning another cruise this Sunday. Fingers crossed that I might have finally found the issue.

Take care,
Brian
 
How about the opening of your secondaries? Vacuum operated I'd assume. If they open too soon, you could have that same effect.
Also is this carb original to the engine? 750 CFM seems big for a 5.7, has it always had this problem?
I'd think an engine that size would come with a 600 CFM.
 
a surge and a bog are two different beasts....that said, I've seen a couple 'loose choke plates' but there impact was much more significant...either way, making sure the choke opens and then stays open with the engine warmed up is a good step in the right direction...could be the culprit...
 
Louc/Mark,

Thank for your input. Here are my answers:

"How about the opening of your secondaries? Vacuum operated I'd assume. If they open too soon, you could have that same effect.
Also is this carb original to the engine? 750 CFM seems big for a 5.7, has it always had this problem?
I'd think an engine that size would come with a 600 CFM."

-I've got an adjustable vacuum secondary on this carb. I'm still dialing that aspect in. I can feel them open now at about 3/4 of the way through the RPM band. I would provide a specific RPM but I think my tachometer is not reading correctly. If I go by that gauge, they open at about 5k RPM. Not sure if that's too soon. When the boat is cruising at full throttle, I'm running about 6100 rpm. This boat is a new-to-me purchase and this was the carb that came with it. I can say for sure it was not the original. I think it came with a Rochester vs a Holley.

"a surge and a bog are two different beasts....that said, I've seen a couple 'loose choke plates' but there impact was much more significant...either way, making sure the choke opens and then stays open with the engine warmed up is a good step in the right direction...could be the culprit..."

-Sorry, my description was vague. The more I used the boat the more I'm leaning towards a bog. Very similar to when on a 2-stroke the mixture is too rich and you lose RPM. I can tell you for sure the boat starts much better now that the choke is actually functional. I had it out last Sunday and for the first 10-15 minutes I thought I had the issue licked. Shortly after, it started up once again but it was much better. What I mean by that is that the bog previously would last like 3-4 seconds then clear up and the cycle repeated (fluctuation of about 200rpm). Now, it only lasts a half second or so between bogging and clearing up. I also put number 61 main jets in vs. the 62 that were in it. I am thinking about going down to a 60 size main jet as that seems to have helped considerably.

"I checked in some of my OMC literature and the 5.7 was supposed to come with a 600 CFM carb....."

-Given that we're confident the carb is a little too big for that 5.7, is there any further modification that I can do to better suit it to the application short of replacing it completely? Would stepping down on the jets further be a good idea to continue with?

Much appreciated again!
Brian
 
I'd also suggest getting the tach checked/corrected...6100 RPM at WOT is way too much and the engine won't last long if it is really running at that speed. I use an 'optical tachometer' as truth...

Getting a vacuum gauge hooked up and noting its reading at varying points in the operating band will also help you understand what is happening and to optimize your tuning components.
 
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