500HP Carb issues
 

Need some help here. 1997 hp500 with carb. All stock. Nothing has ever been touched on it judging by the paint showing no wrench marks. Have fuel pouring out of secondaries when trying to start. Pulled plugs so didnt hydro lock it. Pulled rear bowl off and put in new needle and seat and adjusted tab on float up. With float screw turned all the way down (clockwise) it still overflows thru sight screw and into carb. Checked fuel psi at mechanical pump and although I didnt have correct fitting I did have a rubber cone style to shove into port and it read 8.5psi. Question is A, will a fuel filter cause higher psi as I have not changed this year, and B, will a bad pump cause higher psi? Its been perfect all year until today. If I crank long enuff it will come out front holes as well. The spigot looking spouts at the very top of the venturies will spill fuel too. Not much of a carb guy so hoping to get some advice. No kinks anywhere as all hard lines except from tank to pump.

Thanks and Happy Labor Day!

You said “Pulled rear bowl off and put in new needle and seat and adjusted tab on float up”

what do you mean adjust tab on float up ?

adjustment on float/fuel is how far the needle and seat is adjusted up/down.
Proper fuel level is when fuel just dribble out of side-window plug. A little morevto this procedure but that’s most of it.

Yea I understand that all of that. The tab is where the needle sets on the float. Its a procedure to lower float level if need be. Thats 100% not the problem as with the needle all the way closed off almost its over filling the bowl. Thinking more of a fuel psi issue at this point. Just not familiar with these mechanical pumps.

Only doing it to secondary bowl, not the primary?

Knowing your feed pressure before tearing everything apart would go a long way to eliminating potential culprits.

8 1/2 lbs is probably pushing it on a Holley carburetor. That being said if nothing has been done to the carb since 1997 it is well overdue for at least a re-gasket job. Especially if you are running ethanol fuel. I just did one for a car that was about that old and all of the gaskets were starting to fall apart. The gaskets available in the Holley kits now days are made to withstand the ethanol fuels.

Also I am not sure if your carb has a power valve in the secondaries but they will deteriorate too and start to dump fuel.

Every time one of my Holleys do that, it's one of the power valves. I agree, 8.5psi is pushing it. I've been running 7.25 psi via a press regulator (15 psi mechanical fuel pumps) for 10+ years with no issues.

You want to set/check float levels with the boat in the water.

Wouldnt power valve only be in primary metering block? If so my issue is with the secondaries.

Possible. What part number is the carburetor ? Its on the front of the choke housing. I dont know what carb was on those engines.

Were the needle and seats that you purchased genuine Holley parts ? You really should never have to bend the float tang to get the correct adjustment on a Holley with the adjustment screws.

yes holley oem. I bent the Tang just to make sure I could get the float to zero to make sure that was not the culprit.

Depends on the carb.

Double Pumper carbs typically have a secondary power valve. It's a large factor in tuning the top end.

Otherwise you secondary jets need to be massive, and you lose fidelity of the fuel control.

The carb is a Holley 800cfm dp like the 9022.

There is a primary powervalve, no secondary one.
Here is mercruisers parts list on it;
https://www.mercruiserparts.com/bam/...13459/28183947

Is there a good alternative to the stock mechanical fuel pump or is 861677T the best out there?

My bad, wasn't aware Holley made DP's without secondary power valves.

Airtex 60932
Carter M60932
Sierra 18-8860

An oso member or two stated their merc pumps where airtex. I can’t vouch for it, but a search will find those threads. Lots of threads on these tears ago.

I just checked the Quickfuel 850 DP I got used. It doesn't have a PV in the Sec metering block either.

Good to know. I was planning on upgrading to Quickfuel carbs someday.

With no PV, the needle/seat assy is abt all I can think of that would cause secondaries to flood. (Assuming the fuel pressure hasn't increased somehow).

I've seen shards of Teflon tape get tangled with fuel pump check valves preventing them from sealing. If any fuel line fittings got taped recently, shards could prevent a needle/seat assy from sealing.

New fuel pump will be here today so will let yall know. I talked to a few who have said they have had pumps go bad by increasing psi. Only thing I can think of at this point. We shall see.

Have you put a new o-ring on the needle body? In my experience that is the #1 cause of flooding like your experiencing. Also you MUST lubricate that o-ring when screwing the body into the fuel bowl. If you don't, odds are very high that you will tear it and be right back where you started.
When rebuilding the Merc 500 carbs remember that they have a funky metering jet layout on the. Put the jets in the wrong corners and your going to have lean/rich issues.
As for power valves on the secondary side of a 4150/4500, they are primarily used in carbs intended to be used on street cars. True high performance carbs rarely have them on the secondary side. Some race only carbs won't even have them on the primary side.

Update:

Im embarrassed to even update this but maybe it will help someone in the future. While at Jegs I was talking to my salesman and had the float in my hand. I looked down at it for whatever reason and saw a super tiny air bubble move around. Needless to say it was so full of fuel I couldnt even tell without seeing that bubble. I guess if someone was familiar with carbs they could tell by the weight of the float but honestly it wasnt much difference. Either way a new nitrile float and back together and its good to go. Well other than a new outdrive issue but thats another topic for another day....lol. Thanks again for all help! And yes I know it was mentioned on here about the float. I just couldnt see it until I saw that air bubble.

Thanks!

Chris

Thanks for the update. Nice to know the solution for future reference.