ATTN: HOLLEY Experts...

I have some problems I'm trying to resolve with my new 7.4 michigan motorz (volvo) bogging out at above 2000, both under load and at the dock.

I had a fuel pump issue and along the way decided to get my carb rebuilt, just to eliminate some variables, since the carb was from my old engine. The reputable guy who rebuilt it said that my jets had been previously been installed backwards (the primaries were in the secondaries, etc.). So, the carb is totally redone and I'm expecting to see a boost in performance and fuel economy.

Problem is, now is when the bogging down issue started. It seems the only thing that was changed was the carb. Now, I also notice a vacuum leak. in the picture, you can see a nut with a hole in the center just to the left of the neg. term of the helix. It's a pretty strong vacuum and have no idea what's supposed to go here... or if it's supposed to be capped.

Now, if I cover the hole, I still get the bogging issue. I think I may still have a fuel pump issue but not sure... but I need to first solve this vacuum thing and then move on.

Thanks for any help.

Re: ATTN: HOLLEY Experts...

Well on a car that would go to the exhaust manifold "choke stove" (I think that's the name). Never seen one on a marine exhaust. Also haven't seen that feature on a marine carb either. If it's not connected to anything it should be capped, else you have a big vacuum leak. Don't know why it would be there with an electric choke either, usually it's one or the other.

Re: ATTN: HOLLEY Experts...

That's what I thought but thanks for confirming.

I'll plug it.

Re: ATTN: HOLLEY Experts...

Ever hear of the jets being in the wrong places?

Re: ATTN: HOLLEY Experts...

You say the carb was from your old engine. Was the old engine the same as the new engine?

Re: ATTN: HOLLEY Experts...

That's a metered vacuum and is supposed to be there. My Holley is the same way.

Re: ATTN: HOLLEY Experts...

jimmbo said:

You say the carb was from your old engine. Was the old engine the same as the new engine?

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yes, both 7.4 330hp

Re: ATTN: HOLLEY Experts...

skydiveD30571 said:

That's a metered vacuum and is supposed to be there. My Holley is the same way.

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cool... do you know the purpose of it?

Re: ATTN: HOLLEY Experts...

Holley calls it the heat tube inlet. The following quote is from a carb-related forum where someone asked the same question:

"That screened air inlet fitting allows air flow so the choke unloader piston can pull up in its cylinder to partially open the choke, immediately after starting, to keep the engine from flooding. It also serves as the connection for the heat tube if used with a system that heats the choke spring from a tube that connects to the exhaust manifold, in an automotive application that does not use an electric choke. The airflow through that bleed is (or should be) very small in comparison to what comes through the PCV circuit. The carb is calibrated to allow for that air flow."

Re: ATTN: HOLLEY Experts...

I see. Thanks for the info. I put tape over it and the cold idle is worse while the choke is opening... better without the tape.

Perfect.

Re: ATTN: HOLLEY Experts...

Dug up a pic of the Holley on our '88 Sun Runner, as it came from the factory, that same fitting is right there-- I never paid it no never-mind, interesting--



Now, as for your jets-- If your reputable mechanic just swapped the fronts and backs around, that isn't going to work if say, someone only changed out only one side or the other! But never fear, I remember exactly what jets were/are in mine (I still have the carb, put an Edlebrock in its place) It came with 76's in the rear and 72's in the front, and the boat was originally sold in Michigan, near sea-level. I remember this because at 6000' altitude where I did much of my boating, it was WAY too rich, and I put the 72's in the back and 68's in the front, and between the 6000' and 4400' altitudes I typically boat on, the thing ran great! (350 btw)... My carb is obviously older than the one in your pic, and my fuel inlets are on opposite sides, but I'm pretty sure they're the same otherwise. I'd be for pulling the bowls to check the jets, or at least asking your carb rebuilder what jets are actually in the thing...

FWIW, about the Edlebrock I put in its place:



--- I changed it hoping for maybe a little faster on plane time, maybe some better mileage, yada yada-- and as much as I prefer Edlebrock/Carter carbs to Holley's, I gained absolutely zilch with the Edlebrock in every possible category! Absolutely nothing changed, except the ease with which I could tune it if I needed. And that's why I still HAVE the Holley!

Re: ATTN: HOLLEY Experts...

Thanks, that's a big help. Your carb is a little different but not much. I have single fuel inlet and I also see you have the external adjustments at the bowls - which I don't. But, your jetting info is very handy to know and gives me some real numbers to work with.

Thanks.

Re: ATTN: HOLLEY Experts...

Are you sure there is a secondary metering block on your single fuel inlet Holley? I have not seen every Holley carb made, but usually carbs with secondary metering blocks (into which the changeable jets are installed) have dual inlet. A single inlet has a balance tube going from the front bowl to the back bowl. I have never seen one that has a balance tube going into a secondary block first, the tube in the ones I have worked with go from bowl to bowl and the secondary metering is a small plate that mounts to the carb body on the inside and has no adjustable jetting. The pic you show, is that a "stock" photo, or of your exact carb?

Re: ATTN: HOLLEY Experts...

That's a stock photo that shows the brass fitting I was asking about.

Mine has the balancing tube and single fuel inlet.

Re: ATTN: HOLLEY Experts...

I would like to see a pic of your carb from the side opposite the choke, always want to know more

Re: ATTN: HOLLEY Experts...

Good point on the bowl setup with the single fuel inlet- I've only seen the metering plate setup on single inlets also? I too would love to see a pic--

Speaking of single inlets, not sure about the old days, but current Holley's only have single inlets on the 650 cfm spreadbore or 600 cfm or less 'typical' carbs. My carb is the 750, so I'm assuming the jets will be somewhat smaller in a 600.

If you by any chance find out you have the non-adjustable metering PLATE in the secondary side and not the metering BLOCK with removable jets, then I'm not sure what to tell you about who told you the jets were installed backwards!

Re: ATTN: HOLLEY Experts...

Here are some pics from all sides. I've been scouring the internet with my carb numbers and have found similar posts over the last 10 years - but no one ever posts a solution.

The numbers on my carb listed top to bottom are:

3854057
80378-1
2225

according to Holley, my jets should be Prim 56 Sec 73, but it runs lean at the 2000-2800 rpm range and then runs normal when the secondaries open, I guess.

I have put 72's in the front and that solves the lean condition but now runs rich at idle (strong smell and I can see fumes at exhaust). It could be running rich at all speeds but it's hard to see/smell.

Re: ATTN: HOLLEY Experts...

Here is my 2 cents.
I would think that 68 in the primaries and 72 in the secondaries would be just about right. 56 like Holley says is way to small.
Jets have nothing to do with the idle mix. That is controlled by the idle mix screws on the sides of the metering plate. The only way they can affect idle is if the throttle plates are open at an idle. If this is the case you have another problem. Also, I would plug off the "vacuum leak" even if it is metered as has been stated, its still not helping in my book. Also those have not been used since the 70's. I can't understand why it would be there.
If you are smelling unburned fuel at an idle, it may have to do with the ignition system more than the carb. Low spark with also cause all the problems you are having.
If you know everything else is good, start with the mixture screws about 2 turns out from lightly seated.
If you have one, hook a vacuum meter to the engine and adjust the screws to achive max vasuum at an idle.
Running the engine under a load, slam it to WOT from an idle. If it stumbles then you need to adjust the screw out a bit more. About 1/4 turn at a time. Turn each side the same amount until you find a sweet spot.

Re: ATTN: HOLLEY Experts...

GilligansIsland said:

It could be running rich at all speeds but it's hard to see/smell.

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This is where you need to learn to read spark plugs.

Re: ATTN: HOLLEY Experts...

Thanks. My plan now is to try different jetting and checking the plugs for results.

Holley's stamped code calls for 56 in the front and 73 in the back... what do you mean, "Like Holley says, is way too small"?