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Discussion Starter #1 (Edited)
Ok, long story short, the carb ran perfect for years.

Last season at random times it would stumble and die like it had water in the fuel.

This winter I drained the tanks, found no water. I replaced the fuel lines, the fuel filters, the tank selector switch and fuel pump. Leaving the carb as the issue.

This year it does the same thing. It is very hard to start hot and cold, when it does start there is lots of black smoke. It seems to be running very rich.

I took the float bowls and metering blocks off and cleaned them with spray carb cleaner. No change except now it surges at idle. To reduce, but not eliminate the surge I had to close the idle mixture screws all the way. At higher RPM's it runs great until the stumble occurs, always at part throttle. Full throttle is fine.

I want my reliable running boat back. Any ideas??

The carb is a Holley 750 VC on a Harman Marine 454.

Thanks
 

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I'm having similiar issues with my Holley except mine starts fine both hot and cold. Quite annoying actually. I won't take my wife and kid out until I get the issue resolved.
 

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i would replace the p/v :)...usually black smoke is the result of a blown p/v...sure cant hurt! :)...just my .02 cents
 

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X2 power valve. If you can (carefully) look down the carb with the motor at idle and the throttle plates are dry there are few other places that excess fuel can get into the motor.
 

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Ok, long story short, the carb ran perfect for years.

Last season at random times it would stumble and die like it had water in the fuel.

This winter I drained the tanks, found no water. I replaced the fuel lines, the fuel filters, the tank selector switch and fuel pump. Leaving the carb as the issue.

This year it does the same thing. It is very hard to start hot and cold, when it does start there is lots of black smoke. It seems to be running very rich.

I took the float bowls and metering blocks off and cleaned them with spray carb cleaner. No change except now it surges at idle. To reduce, but not eliminate the surge I had to close the idle mixture screws all the way. At higher RPM's it runs great until the stumble occurs, always at part throttle. Full throttle is fine.

I want my reliable running boat back. Any ideas??

The carb is a Holley 750 VC on a Harman Marine 454.

Thanks
How about buying a kit and rebuilding the carb, it's not that difficult, and it does sound like you have a problem if you have the idle air bleeds screwed all the way in. Did you check float level,, and make sure the needle and seats are working>
 

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Discussion Starter #6
I have checked the float levels and reset them to just a slight trickle out of the sight hole.

I removed the front needle and found the O ring disintegrating.

I guess a rebuild is in my future.

I checked the 'list" number which is 8572. Research indicates this is a 715 CFM carb, not a 750.

Thanks
 

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I just went through the same ordeal with the rear carb on my 565. Changed power valves, gaskets, checked float level, lowered my fuel press. etc., all to no avail. Finally I unbolted the carb, put a couple pieces of small wood between carb and manifold and watched a steady drip from where the PV dumps the fuel when open, called and spoke to a Holley tech rep, he suggested I check the main body surface where the metering body mounts, I put a straight edge diagonally from opposite corners and was surprised to find such an irregular surface, one pass with a med. file told me just how bad that surface was. I carefully made a few cuts with the file and brought the surface back to true, cleaned and blew everything with air, new gasket, reassembled, hooked up fuel lines with carb still sitting on my wood spacers, filled the bowl and watched for a very long time before I was convinced my problem was solved. Tech told me with a lot of assembly/disassembly those surfaces can become distorted and leak internally.
 

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Replace the power valve and clear the idle air bleeds in the main body to fix the idle surge,but
A "random " problem that causes it to die at part throttle sounds more like an ignition problem to me.
Or possibly a fuel delivery issue.
 

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Discussion Starter #9
I ordered a rebuild kit from summit. Should be here this weekend.

As for ignition, I checked for spark after one of the random episodes. It had spark at the number one plug.

Once the fuel issue is solved I was planning to replace the old mallory dual points system to a pertronix set up. I don't want to start chasing two problems at once.
 

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Discussion Starter #10
The rebuild kit came and I completed the process cleaning everything completely with carb cleaner and a toothbrush.

I used the carb cleaner staw to clean out all the ports in the metering bliock and the air bleeds. i dried everything with compressed air and made sure none of the passeges were blocked.

I reassembled carefully with all the new parts and gaskets, and reinstalled.

The problem is worse than ever. It takes forever to start, it runs very rich with lots if stumbling. It will not idle at all.

Just to verify the carb is the issue I put an old Holley 650 DP on it and it runs fine. Idles smooth and restarts instantly.

Any thoughts on this issue? The old Holley is useable but a little small for this motor and it is not a marine carb. I really need to get this figured out.
 

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I just went through the same ordeal with the rear carb on my 565. Changed power valves, gaskets, checked float level, lowered my fuel press. etc., all to no avail. Finally I unbolted the carb, put a couple pieces of small wood between carb and manifold and watched a steady drip from where the PV dumps the fuel when open, called and spoke to a Holley tech rep, he suggested I check the main body surface where the metering body mounts, I put a straight edge diagonally from opposite corners and was surprised to find such an irregular surface, one pass with a med. file told me just how bad that surface was. I carefully made a few cuts with the file and brought the surface back to true, cleaned and blew everything with air, new gasket, reassembled, hooked up fuel lines with carb still sitting on my wood spacers, filled the bowl and watched for a very long time before I was convinced my problem was solved. Tech told me with a lot of assembly/disassembly those surfaces can become distorted and leak internally.
First, I qouted this because it worth reposting.
Second, if you bought a universal kit that had multiple gaskets that you did not end up using, you need to DOUBLE, then TRIPLE check the base gasket you used between the carb main body and the base throttle plate. The 850 throttle plate gasket is different than all the others. The idle circuit does not line up with any other carb's main body. Tear the throttle plate off the carb and check the gasket you use to see that the idle ports in the throttle plate line up with the holes in the gasket.
Its a very very common mistake.



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Discussion Starter #12
OK

I took it apart to verify the gasket matches correctly with the idle ports. All appear to match correctly.

Reinstalled. Still runs like crap, very rich and won't idle below 2500 RPM.
 

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I'm going off of memory here, so give me a break if I'm not spot on (or even close).

It sounds like your primary butterflies may be open too much, which will bypass the idle circuit altogether. I seem to remember having to measure the transfer slot, or the distance from the butterfly to the base plate (or something like that) in order to make sure the "idle" is actually running on the idle circuit. One of the fixes (again, from memory) if it won't allow enough air to idle and remain on the idle circuit was to drill a small hole in the down side of the primary butterflies. Not sure what size hole to drill... Double check everything and do a little research about this. It does sound like you might be idling on the "cruise" cicruit.

Does adjusting your idle mixture screws change anything?

Some of the others around here can confirm or debunk this, but pretty sure it's an accurate and common problem/fix.

Good luck!
-Seth-
 

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A dead giveaway that your idle circuit isn't working is nothing chances when you turn the idle mixture screw in and out.

Its almost always what Budweiser posted. The throttle blades are open to far and the idle circuit no longer functions. The easiest way to know if you are where you need to be with the throttle blades is that the exposed portion of the transfer slot appears square. That means you have about as much transfer slot exposed as the slot is wide. It it appears to have length, you have the throttle blades open too far.
Too much open blade opening will also lead to stumbles off idle on top of a shitting pig fat idle.



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Discussion Starter #15 (Edited)
I checked the butterflys as suggested.

It does seem they are too far open. I backed them down so the slot appears square.

It still won't idle and it runs like crap until it gets up to 3000-3500 RPM, then it seems to smooth out.

just grasping at straws I swapped base plates with an old 3310 I had laying around just to see if that helped, it didn't.

Do metering blocks ever "go bad"?

Just out of curosity and frustration I swapped the metering blocks into the 650 DP I have and it ran like crap with them. Switched the blocks for the 650 DP back in and it runs good.

At this point I think I'll use the 650 DP for a while even though its a bit small until I can purchase a new Marine 750 VS. Its just a cruiser and ski boat so it should be ok for a while.


Thanks for all the suggestions.
 

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I checked the butterflys as suggested.

It does seem they are too far open. I backed them down so the slot appears square.

It still won't idle and it runs like crap until it gets up to 3000-3500 RPM, then it seems to smooth out.

just grasping at straws I swapped base plates with an old 3310 I had laying around just to see if that helped, it didn't.

Do metering blocks ever "go bad"?

Just out of curosity and frustration I swapped the metering blocks into the 650 DP I have and it ran like crap with them. Switched the blocks for the 650 DP back in and it runs good.

At this point I think I'll use the 650 DP for a while even though its a bit small until I can purchase a new Marine 750 VS. Its just a cruiser and ski boat so it should be ok for a while.


Thanks for all the suggestions.
Yes. If water gets into the fuel, and even water in the E10 can cause white rust corrosion in the metering blocks. I can happen inside the fuel channels and you'll never see it, and its all but impossible to get out.
Its also possible to have the warp and not seal. Go back and reread Bubbletops409's post. Sometimes its the main body, and sometimes the metering block that causes the problem, but the symptoms are the same regardless.

If you think it is corrosion in the blocks, you can try soaking them in about a 50/50 solution of CLR and water for a few hours, rinse and repeat. Sometimes it helps, sometimes it doesn't. White rust(zink corrosion) can be the death of a metering block.



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Discussion Starter #17
I don't see any corrosion, but i have had water in the fuel. I will soak them in cleaner and see if that helps.

I did check the blocks and the carb body for fit. Filed down a few high spots but nothing that seemed so bad it would cause the gaskets to not seal.
 

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You can soak them for a year in carb cleaner and never move one ounce of corrosion from the blocks. Not one bit!

When you get it back together, see if there is any change if you block the idle air bleeds, (furthest from the accel squirter) with your fingers. It there isn't a significant change, the blocks are corroded up pretty bad or the bleeds are still blocked. Blocked bleeds or corroded emmulsion tubes will give the symptoms you're seeing.



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I'm going off of memory here, so give me a break if I'm not spot on (or even close).

It sounds like your primary butterflies may be open too much, which will bypass the idle circuit altogether. I seem to remember having to measure the transfer slot, or the distance from the butterfly to the base plate (or something like that) in order to make sure the "idle" is actually running on the idle circuit. One of the fixes (again, from memory) if it won't allow enough air to idle and remain on the idle circuit was to drill a small hole in the down side of the primary butterflies. Not sure what size hole to drill... Double check everything and do a little research about this. It does sound like you might be idling on the "cruise" cicruit.

Does adjusting your idle mixture screws change anything?

Some of the others around here can confirm or debunk this, but pretty sure it's an accurate and common problem/fix.

Good luck!
-Seth-
not positive about holleys. on demons, you must set the primary and secondary blade stops with 0.020, or less, of the transition slot showing under the blades. carb has to be off to do that. then set idle with the idle mixture screws. if the engine can't get enough air with the blades closed that far, 2 5/64 holes in each primary blade can work. on newer demons, there's an idle air adjustment screw under the center post, so you don't have to drill holes. with demons, you never change blade position in an effort to adjust idle.

but with all the symptoms described, hard to believe this is only a tuning issue...
 

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A dead giveaway that your idle circuit isn't working is nothing chances when you turn the idle mixture screw in and out.

Its almost always what Budweiser posted. The throttle blades are open to far and the idle circuit no longer functions. The easiest way to know if you are where you need to be with the throttle blades is that the exposed portion of the transfer slot appears square. That means you have about as much transfer slot exposed as the slot is wide. It it appears to have length, you have the throttle blades open too far.
Too much open blade opening will also lead to stumbles off idle on top of a shitting pig fat idle.
That was most of the fix on my 660's X2 deal...2 .080 holes in each primary throttle plate got the idle speed screw down from almost 3 turns off the bottom to about 1 1/2 and the transfer slot is almost square but, the OP said
"Ok, long story short, the carb ran perfect for years." so something has changed.

I checked the butterflys as suggested.

It does seem they are too far open. I backed them down so the slot appears square.

It still won't idle and it runs like crap until it gets up to 3000-3500 RPM, then it seems to smooth out.

just grasping at straws I swapped base plates with an old 3310 I had laying around just to see if that helped, it didn't.

Do metering blocks ever "go bad"?

Just out of curosity and frustration I swapped the metering blocks into the 650 DP I have and it ran like crap with them. Switched the blocks for the 650 DP back in and it runs good.

At this point I think I'll use the 650 DP for a while even though its a bit small until I can purchase a new Marine 750 VS. Its just a cruiser and ski boat so it should be ok for a while.


Thanks for all the suggestions.
Well alrighty then, problem narrowed down to metering blocks. And you have carefully taken a look down the carb with the motor as close to idle as you can and it's not dripping, right? (Float level/needle seat)

You have good carb cleaner? i.e. gumout...Might try sticking that straw in each hole in the block and then go to the matching hole on the opposite side of the block and make sure the flow and where it comes out and matches from side to side...when in doubt, cross check it against the block that worked ok.

The metering block gasket has a nice even print?
Blow it out with compressed air with decent pressure.

And there is an old school trick you can try..Hard to explain but i will give it a shot. Get the motor running and make like you are going to shoot a rev....just as you begin to open the throttle and it revs maybe 3 k stick your other hand over the choke horn and open the throttle more....the motor will try to load up and die/suck your hand down the carb..... remove your hand from the choke horn, try to avoid letting it die, it will be a hard start pig....just hold the throttle open far enough to keep it running while it cleans itself out....avoid shooting revs to clean it out...the acc pump just adds excess fuel that you already have to much of. it might sound odd explaining it...but see it done and it will make sense.

The theory is..the massive amount of vacuum generated (hand suck) might pull anything loose through the carb...it could run better...good deal, the same..oh well ....or worse...something still stuck in the carb.
 
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