Holley carb tuning?

[biggsie]

Last weekend was the 1st time out, boat ran fine but I noticed dark smoke from exhaust at slow speeds/idle and the back of the boat was all dirty from the unburned fuel. I discovered that there was no thermostat installed, ordered one and got it in. Set the mixture screws to 1-1/2 turns out and took my vacuum gauge with me on a test drive.
New t-stat worked fine, about 150-160 and holds steady. Less soot now but I think it's still a little rich. Turned in the screws all the way, no noticeable difference? What rpm to use while adjusting for max vacuum? When I adjust screws the idle changes?

Can someone explain the idle circuit of these carbs to me? I'm a motorcycle mechanic and these big holleys are foreign to me. I want to put a smaller pilot jet in but I'm pretty sure they don't have one lol.

Boat seems to run great otherwise, maybe I should just leave it alone!


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[chris young]

Yours is the 454 IIRC so the jets etc won't be the same as mine, but if you get the list number off the horn of the carb Holley will tell you which power valve and jets the carb was shipped with. Sounds like someone may have tried to re jet the carb for more power. Assuming it's the 4160 2 1/2 turns should be the initial setting on the idle screws. If you can close them all the way, and the throttle plate is closed, and it doesn't stall, you probably have either a blown power valve or the primary boosters are dripping fuel. There are no replaceable jets on the idle circuit on that carb nor switchable air bleeds. From all the reading I've done on various forums, and my own experience, it seems that these carbs are really hard to get to idle properly until you get either a wedge spacer to level the carb or adjust the front floats low enough not to flood the primary boosters. I chased that problem around all last summer and after rebuilding the carb over the winter, it actually got worse this spring. Rather than lowering the front float any further, I decided to put in a wedge spacer and everything just worked right away. (so far, I haven't had a chance to run the boat properly to get it properly tuned but it idles really well and runs up to ski speed smooth and strong)

If I had to guess, I'd say your float level is too high but that's easy to see as the boosters will be dripping fuel. It's crazy how far away from factory settings I had to adjust the floats to get that to stop on my carb, an as I said is started up again after the overhaul. The right fix is to level that carb. Check the last or second last page of the "weird heat soak" thread, I posted the Holley P/N for the wedge kit (it's actually meant for marine applications) The problem is that with any wedges I could find, none of them had a vacuum port and it would have been crazy long studs to keep the original spacer, so right now I can't tune with a vacuum gauge and I don't have any PCV. It's good for now, but I will have to drill the spacer and put in a port at some point.

BTW if you go to both the Summit pages and the Holley pages there is tonnes of good info on how to tune these carbs.

Hope this helps.

[biggsie]

Mine is a 351.

So if the floats are set too high I will see a drip (overflow) from the tubes?

Also the manifold is an Edelbrock if that makes a difference.

I had to remove a plug on the intake and install a nipple to get full vacuum for my gauge. I had no ports at all anywhere.

My valve covers are leaking and now I'm wondering about the PCV. I have a valve on the cover with a hose to the air cleaner but no vacuum to the crankcase.


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[cjtpilot]

A bad PCV will cause idle issues and keep you chasing your tail if you never think to check it. I just rebuilt my carb and ended up having my idle needles turned out 2 turns. yes adjusting your idle mixture screws will effect idle RPM. Set your screws with the idle adjustment all the way out, then after you set the mixture screws with a vacuum you can adjust the idle speed. Typically around 650-750 RPM. Which trans do you have 1:1 or 1.23:1? if you have the 1:1 then you have to adjust float level to make up for the carb sitting on an angle. Otherwise the engine and carb should set level, at least it does on a Comp.

[biggsie]

1:1
So when I adjust the screws will the idle be really low? I haven't tried to see if the engine will idle with the idle screw all the way out.


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[cjtpilot]

1:1
So when I adjust the screws will the idle be really low? I haven't tried to see if the engine will idle with the idle screw all the way out.


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Yes set the idle mixture screws out 1.5 turns to start, back out idle speed screw all the way out. It should still run even if it is a very low idle. As you are adjusting the idle mixture screw to greatest vacuum the rpm will go up or down. Then you can set the idle speed screw. I actually still have my idle speed screw backed out and am @ 750RPM in neutral and 650-700 RPM in Fwd idle.

[chris young]

Interesting you have no vacuum port on the carb, I would think that motor should have PCV but maybe it's an oddball? My mistake, 1 1/2 turns is the correct starting point (I could have sworn it was 2 1/2) but I just checked the Holley video and 1 1/2 is what they say. http://holleytv.com/?p=426

Yes, if your floats are set too high you will see fuel dripping into the primaries through the booster nozzles. You'll know for sure if it continues to drip after the motor is shut down. In my case it was only one nozzle so I must have been just on the edge of having the float in the right place. BTW I just did a bunch of reading on PCV in order to determine if it was ok to run the motor for a while without it so I don't have to deal with installing a vac port right away, and it is a bad idea to use a manifold port for PCV (in case you were thinking of doing that) as the cylinders closest to the the port will run leaner than the rest.

[biggsie]

I'll have to look into the PCV thing. I have valve covers from a mercruser that say 188hp on them :/

Right side has one port with a PCV valve going to air cleaner and left side has a vented filler cap. No place to run a vacuum line to.


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[cjtpilot]

Here is what is on my 351, rh with filling port goes to flame arrester LH side goes from PCV to manifold. That's where I get vacuum from.


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[cjtpilot]

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