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Hesitation during acceleration

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AlfaDon View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AlfaDon Quote  Post ReplyReply Direct Link To This Post Topic: Hesitation during acceleration
    Posted: December-02-2018 at 4:27pm
Hi,
I continue trying to solve a problem that's been happening for a couple of months. Some of you know the history, but Ill explain it again.

I had the boat running last summer and despite it running rich on an 750 CFM Quick Fuel carb, it ran great, without any stumbling. At that point I was using a common Airtex electric fuel pump. I was concerned that the Airtex wasn't delivering enough fuel and it was also only putting out 4psi at idle. I tried a couple of different Holley electric pumps and they weren't designed to pull fuel so I went with a mechanical pump. With the Holley pumps, I was also getting a big drop in power under acceleration. The boat would come up on a plane and then lose power. Repeatedly. I would just start to accelerate and it would lose power.

I'm running rubber hose from the regulator that's mounted on the manifold and found that the line to the secondary inlet was kinked. So I re-plumbed it with some 1/4" brass fittings to eliminate the kink on the secondary side. Primary side still has the rubber hose going from the regulator to the primary inlet. So there are some differences in inlet tubing I.D.'s from the regulator. At this same time I exchanged the 750 cfm for the same carb in a 650 cfm which is waht I have now.

I've got a 5* manifold wedge adapter so the carb is tilted forward at idle but then levels out underway. I haven 't changed the float level from the factory settings.

Could the floats be shutting off fuel delivery when it comes up on a plane, and fuel sloshes around in the bowls? Are the differences in fuel feed lines not allowing air in the system to purge out?

On another side note of concern, I've got an LC-1 O2 controller mounted on one of the manifolds. I've got the idle mixture screws adjusted to where I've got max idle vacuum at about 16"hg , and is about 14:1 air to fuel mixture. When I was able to get underway one time, the engine rev'd quickly to almost 6k rpm and by the time I backed off and looked at the LC-1 it was at 17:1. Way too lean. I'm sure that's why it rev'd so quick. Plus it's fairly cold outside at about 50*. From the factory it came with a 6.5 power valve, 71 primary and 78 secondary jets. I thought I'd follow the standard rules of by increasing the PV to a 8.0 which is half of the vacuum reading at idle. Shall I try going up 2 jet sizes on the Primary and secondary?

On another side note, I ended up buying 20' of Tigon 3/16" clear fuel hose for the pump vent to the carb. If anybody wants a piece let me know and I can send it to you.

Don
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SNobsessed View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SNobsessed Quote  Post ReplyReply Direct Link To This Post Posted: December-02-2018 at 6:28pm
Don - What displacement engine do you have? 850 is quite big, maybe it can't react to sudden changes in throttle.
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Jonny Quest View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jonny Quest Quote  Post ReplyReply Direct Link To This Post Posted: December-02-2018 at 8:51pm
How are you running an O2 sensor (LC-1) with your wet exhaust?
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AlfaDon View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AlfaDon Quote  Post ReplyReply Direct Link To This Post Posted: December-02-2018 at 9:35pm
Hi
Sorry about the error in carb size. I was on hold with Comcast and not thinking clearly. I've since corrected the initial post.

The original carb was a 750 cfm, and it's now been replaced with a 650 cfm. The engine is a modified 351W in my 96-176.

JQ- The LC-1 wide band controller is mounted in an adapter like this https://howellefi.com/product/adaptor-o2-sensor-for-marine-use/
I've also got an oil fouling extension to keep it away from the steam. Having the water enter in lower at the exit of the elbow helps too. So far I haven't had any problems.
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KENO View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: December-03-2018 at 7:31am
I guess I'm confused about why you have a fuel pressure regulator with your mechanical fuel pump and why you would want the carburetor tilted forward at idle.

Looking at your previous posts it seems like you were getting a Carter M6389 marine pump that doesn't need a regulator

When did the stumbling start, after the carburetor swap or after the fuel pump swap{s}?

Maybe you got a "not so good" mechanical pump.

Why not try the original Airtex setup again and see how things are running with the 650 cfm carburetor? At least that'll tell you if you have fuel pump or carburetor issues
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GottaSki Quote  Post ReplyReply Direct Link To This Post Posted: December-03-2018 at 9:43am
Seems more than this going on but consider that setting max vacuum at idle-neutral is not best, Results too lean when you apply a load.
Needs a load by setting idle mix leaving it in gear, your will net a stronger off-idle transition (no dull lean stumble) and less rpm delta in/out of gear
Mimics auto-trans idle mix setting procedure of years past.

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AlfaDon View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AlfaDon Quote  Post ReplyReply Direct Link To This Post Posted: December-03-2018 at 2:20pm
The problem started when I went from the Airtex to the Holley electric pump. It has gotten a little better since I went to the mechanical pump. One thing I thought of to try was watching the pressure gauge to see if the pressure is dropping when it hesitates or cuts out.
I guess I left the regulator in place because it’s part of the dual feed assembly.

The beveled carb adapter was an effort to keep both bowls level when underway.

When I was reading about idle mixture adjustment it mentioned doing it with the car’s automatic trans in drive but also said that it could be done in neutral with a manual trans. I can see how if the idle mixture was set under load it would be a smoother transition from the idle circuit.
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