1987 OMC Cobra 4.3 hesitating/missing on acceleration

Knewfie

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Aug 24, 2016
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Hi There!
We have been having a hard time trying to figure out what is causing a hesitation or misfiring on our boat. When you first get it going and warm it up it is initially fine. After 10 min or so the problem arises. If you try to push it past 3000 or so it starts to hesitate, miss, bog. What I've done so far:

New distributor cap, rotor button, points, condenser.
Had distributor springs, weights, etc serviced
New wires and plugs
Checked anti-siphon valve
Changed out fuel
Changed filter/water separator
Checked timing
Checked dwell angle at various rpms
Cleaned carb and during this seems like the bowl is nice and full of fuel
I haven't checked the vacuum but the place where I had it serviced did, they say it is fine. Although it is definitely sucking air through a hole next to the steel line for the vacuum for the choke. Is this supposed to be open? If I plug it the boat runs really rich.
Ordered a new carb from I5. Worked OK when installed but ran rich and I couldn't get it to consistently idle correctly. Sometimes OK, sometimes it would just stall. When I got it up to speed it still kinda seemed like the hesitation/misfire was there. Maybe not as bad..not totally sure. So, reseated mixture screws and turned out 2.5 turns. Was careful not to seat too tight. After this I couldn't get it to work very well at all. Would eventually get it going, but very rich idling. Black smoke, etc. Ended up putting the old carb back on and of course the same issue.

Wondering if anybody can offer any suggestions at all?
Thanks in advance for any help you might give!!
 

Knewfie

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Aug 24, 2016
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Oops couple more things I've done:
Cleaned flame arrestor
ESA module changed 2 years ago
 

alldodge

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Bogging, hesitation and running rich are all from the carb not supplying the correct amount of fuel when needed. Things like this can also be caused by a worn out motor with low compression, and a vacuum leak. Suggest doing a compression test to see how its doing.

If compression is good. Rich running at idle is the idle circuit and from what I see how it reacts to adjustment, things are not right. Hesitation can be the accelerator pump and/or power valve (if it has one). Black smoke is to much fuel.

Although it is definitely sucking air through a hole next to the steel line for the vacuum for the choke. Is this supposed to be open?

No, it should not be open. It should have a tube which goes to the intake hole. It gets heat from there to work the choke.
 

Knewfie

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Aug 24, 2016
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Thanks for the response! Actually we did a compression test and numbers are all good. Sorry I forgot to put that in the list before. Have a look at the attached image. This the hole that is sucking air. Is this the one you are talking about? In any event if I block it up it starts running really rich. With the old carb it runs beautifully at idle. It starts to bog or hesitate only when I'm trying to get it up on plane, and generally it's fine when we first take it out. The issue seems to develop after running it for a bit.
 

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alldodge

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That is something like the one I was thinking about but not quite in the same order. In any case there should be no air being sucked in thru the hole. Where does the tube run to? going past the choke.

fetch


With the old carb it runs beautifully at idle. It starts to bog or hesitate only when I'm trying to get it up on plane, and generally it's fine when we first take it out. The issue seems to develop after running it for a bit.

This is telling me the old carb is setup better then the new one, but needs some cleaning to get the main jets opened up. The new one is flooding and needs adjustment, might be the float setting.
 

Knewfie

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Aug 24, 2016
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The other tube wraps back around and connects to the choke pull. Or the black circular thing in the right of the image. Another rubber tube comes from the base of the carb to the gold coloured piece in the top left of the image also connected to the butterfly on the top of the carb. I've had the jets out of the old carb and cleaned and blew them out with compressed air. Also did the same with the whole venturi assembly.
 
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alldodge

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Pump might be part of the issue, I haven't said much about it because of the issues with just idling and running rich. Shouldn't be running rich with a weak pump, that is unless its dumping fuel in thru the bypass hose
 

Knewfie

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Hi again,

It's definitely not running rich with the old carb, the running rich was happening with the new carb I put on. With the old carb it idles perfectly and there is no sign of it ever running rich. Ran it again (old carb installed) last night and there are no issues at lower RPMs at all, just as we get up near 3K or so. Sometimes it kicks in for a fraction of a second and runs fine....feels a little fuel starvation but who knows. When I had the new carb on I set the mixture screws at 2.5 turns but what I didn't check is whether the choke is opening all the way...maybe that needs adjustment. Thanks again for your responses by the way.
 

alldodge

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Idles fine and has issues at higher rpms, then that would tell me its in the main jet, venturi and emulsion tubes. The think the still bother me with plugging the hole and it running rich
 

HT32BSX115

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Although it is definitely sucking air through a hole next to the steel line for the vacuum for the choke. Is this supposed to be open? If I plug it the boat runs really rich.
Howdy,

Welcome aboard!

Yes, there should be a slight vacuum and air flow at the little hole next to the choke "tube" That tube is actually a "loop" of tubing that goes into the "exhaust" area of the intake manifold. Then comes out to the choke bi-metallic spring actuator. The vacuum is actually from the choke actuator. They port a small amount of vacuum there to suck air from outside through the "loop to the choke. Otherwise there's be no way to get that heat into the choke "spring" area.

#27 in the picture below is the choke tube that is the little "heat exchanger" in question. The only way you could eliminate it is to switch to an electric choke (or NO choke) but the carburetor would have to probably be rejetted and/or adjusted for eliminating that intentional vacuum "leak"
intake%20manifold_zps1sbmkypl.jpg



Since the engine runs ok at lower RPM, your hesitation above 3000 RPM sounds more like a fuel starvation issue, maybe due to a fuel pump, anti-siphon valve (restriction), (partially) clogged fuel pickup, fuel filter etc.....or even fuel tank vent......


Regards, ,


Rick
 

Lou C

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You said it initially runs fine and starts bogging after a while. I wonder if the ignition coil is starting to break down with heat.its about the only thing you have not changed....
 

HT32BSX115

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even fuel tank vent......
Here I go quoting myself again!!!:eek::eek:

Before you do anything with other stuff, you might try running it on an external fuel tank to eliminate the fuel vent, pickup, anti-siphon from the picture.......

Just saying.....
 
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