Vmax

Dragged theboat home today and changed fuel water separator. Lots of gunk and water trapped in it.

Looked around for ignition module. Module is a Thunderbolt IV P/N 8k14A V8-24. I'm assuming this is the module with 24 degrees advance. Started looking at the HP 500 Carb noticed the two rear idle mixture screw were both removed so it is sucking air threw the threaded screw ports. Carb was installed by friend at work who builds NA marine engines as business on side. Carb was a left over from a engine build where owner went to a dominator carb. I figured 400 bucks for a HP 500 carb with 6 hours on it how could I go wrong since that was an installed price and dial in for proper jetting. I'm thinking maybe he and his partner removed the idle mixture screws to secondaries to get enough air coming in. Have not talked to him yet to confirm. Can anyone speak on this practice? Thinking this might be reason boat stalls out with idle set at 750 RPM and placing in gear when boat in the water. Still have the old carb thinking of installing idle mixture screws that are in the secondaries in new HP 500 carb and drilling primary throttle plates with 1/16" holes like it had. The old carb would idle somewhat decent occasionally.

Also dug my old timing light out of attic. Looks like it is the type that will not let me measure timing at full advance. Will tinker some more at the lake tomorrow and find out idle timing and whether fuel pressure issue is fixed.

mcollinstn

Dude,

You need to slap your mechanic friend in the side of the head.

That is by no means a "normal practice".
Might as well drill a hole in the intake manifold to get "more air".

You also need to get a refund and spend your carb money on one that has been setup for blower use, not a stock HP500 carb with the idle screws left out.

HiPerf2000

iTrader: (2)

it really sounds like you got a vacuum leak somewhere.....

but i would send the carb to dean and tell him exactly what your motor specs are.....and he will do you right.

ohhh yea....make sure your disrtibuter is good....that can be way out of wack if it is bad.

also the v-6 module will help

Griff

It does sound like you have a vacuum leak from all the symptoms you have listed. If I remeber correctly though, my HP500 carb I had did not have a 4 corner idle adjustment. The rear metering block just had spots on the outside where the screws would go. I'm not 100% positive on this though.

You NEED a different ignition module. At a bare minimum, you need an HP500 module with 20*advance. A V-6 module would probably be even better. You want total timing at no more than about 32*, so withyour current stock 454 mag module, you would have to set initial at 8*. That cam will not idle well unless it has at least 11-12*.

I would not drill holes in the plates till you have to and have done everything else. Set the intial timing up to 12* or more and watertest it just at idle speeds. Don't run it at more than idle speed or you will get detonation. This way you can find out how much advance you need at idle and then determine what module to get. If it idles fine with 12* then get a 20* module. If it needs more, then you will have to get a V6 module.

BTW, 400 bucks is no deal on the carb. I got mine for $200 and have seen many in the $250 range. Once you get it close, I would still send the carb to get reworked and boost referenced. You could even send your 850 to get done instead.

Gary Anderson

Are you sure the idle screw holes in the seconday block are sucking air? They are usually blocked internally and no screw is used. Dont wory about it if they're not leaking air.
Yes, that's a 24 degree module. Buy a variable advance timing light. You'll need one with the new V6 module you buy. Install it and set total advance at about 28 degrees at 3200 rpm. See my earlier post.
Gary

RLW

I agree with Mcollinstn, get the refund on that carb and then slap him in the head. Forget dealing with friends that build marine engines on a part time basis.
The answer to all your problems is with Nickerson Performance. I know that I do promote Nickerson but having been through everything you are dealing with now. The outcome was nothing more than fantastic. After Dean and/or Brad get you squared away, you can deal with pitot tubes and over trim situations.
Like I said before, Leave it to the experts.
http://www.nickersonperformance.com/marine/marine.html
215-781-1370

Vmax

Initial timing was set at 6 deg. Timing does advance with RPM. Could not determine how much with out a variable advance timing light. I like Griffs idea of advancing timing at idle and seeing how the boat goes in and out of gear. That will help me diagnose how much of the problem is a timing issue. The rear mixture screws were blocked off there was no vacuum covering the holes with my fingers. So this must be the standard HP500 set up. As far as carb prices a new marine carb from Holley would have set me back $550 and they no longer build the supercharger models. The 850 that the boat had was a supercharger model for auto and idle had to be high with it. Talking to a Holley representative he led me to believe I could use the equivelent to the HP500 model for my set up and being boost referenced was not imperative.

Fuel pressure was up at first at idle at 5 PSI after filter change but dropped considerably after running boat conservatively under 2600 RPM. So I'm thinking fuel pump is heading South. I am considering one of the Holley Marine mechanical fuel pumps roughly 70 dollars for the 120 gph model.

I have read some other posts regarding similar idle issues and they range from a bad coil to contamination getting into the idle circuits.

How much does Nickerson charge usually charge ballpark cost to boost reference a carb and how long does it take and are there any more performance gains.

Gary Anderson

Change your fuel water separators, change the fuel pump. That almost certainly will solve that problem.
Try the carb w/o boost referencing. I know, it's better to have it referenced, but try it without it. It will probably work fine. See archives for effect of boost referencing.
Get the V6 module (from 91 4.3L, PN 15247A1). Cost is about $275. Buy a variable advance light ($70). Set your timing at 28 total at 3200n rpm. Turn your idle down to 800 rpm. That will probably do it.
Check your plugs after a few short WOT runs and see how they look, you may be able to run a little more advance.
Didn't I post this earlier?
Gary

HiPerf2000

iTrader: (2)

Yea...what he said

Just tell Dean exacly what you have and he will decide on boost referencing or not.

And if you want...i'll sell you one of the V6 modules for half...let me know if you want it.

John B

I hope this HP 500 carb was jetted for the supercharged application. If not you may want to do a compression and or leakdown test. Could be the motor is going south from other problems. You don't want to hear this but you might be chasing the wrong problem. Supercharging will require up jetting 4 to 8 numbers, per the book.

Just trying to be helpful, John B