performancems

Can anybody recommend a place I could send my holly 850's out to be rebuilt/tuned, or will I have to have them on the engines to get them running correctly. My carb experience is limited, and i've only really worked on edlebrock carbs or older smaller motorcycle carbs. Every time I mess with a holly I seem to make it worse. My issue is that the motors are running too rich anddumping too much fuel. I've tried adjusting the 4 corner mixture screws while idling but just didn't seem to do enough. they are putting out black soot/potential unburnt fuel through the exhaust.

OPIE272

What fuel you running?
Running too high of octane can do the same thing.
If motors are stock Merc, not the blue HP500 or bigger and you are running 91 you will black soot transom and smell like raw fuel when idling.
Im not sure if the HP500 even ran more than 87.

Rbesola

My stock 500s always ran better on 87. I ran 93 once in a while and in a few hours my transom would be grey every time

CDShack

Is this a new condition? if your motor backfires Holley's are susceptible to the power valve diaphragm rupturing and dumping fuel. If they were ok before, might bust them down, look on the power valve and get the number and stick new ones in and see what happens.
Also ck and make sure the floats are working properly. there is a flathead screwin "port" on the right side, put some rags or a catch-can down and open it. Normal level should be at bottom of the opening (dribble out) If fuel pours out, float is saturated or full (brass) or the needle and seat are shot. Holleys are dead simple carbs. Only a couple of thing generally can go wrong.

wingnit

All of the above is good info, but excess fuel flowing out the fuel bowl Float Level Port does not necessarily mean the float or the needle and seat are bad. On the top of the float bowls, there is an adjustment nut to move the floats up and down inside the bowl. If the float is set too high the fuel will pour out of that side port. If the float level is set too high the fuel bowls will overflow and dump the excess fuel into the venturies.

CDShack

True. I was thinking the rich condition is a new phenomenon, and that something went south. You are right, could just be a matter of wear on the needle and seat needs to be adjusted out. Whatever it is, I'd try some really simple things before I unbutted some major bucks on new hardware.

performancems

I got new needle and seat and re adjusted the float and that took care of part of it, but there's still some hesitation under load, and backfire when revving them up at times .The boat is drive able again but is lacking power. I don't know much about holleys, but i'll check the power valves.

as for fuel, I just assumed that I needed high octane. The only way to get non ethanol around here is to get 93.

CDShack

Backfires kill power valves. Still feel that's an issue. Change those and see what's happening.

Hesitation under load? Lean or fat? If lean: Does it have 50cc accelerator pumps? You can also ck the "lobe" that actives the accelerator pump. There is an adjustment there (so that it is pumping as the butterflies open) and there are different lobe colors you can install to change the profile of the fuel pump amount. Fat: power valves will probably help that part. May have to go in and reset the idles. All in, then back out 1-1/4 turn to get to Holley's "starting point". Keep up posted.

performancems

yes, there was some hesitation under load, as well as off idle, where revving in neutral would sometimes hesitate or cause a stutter with a backfire. after some more googling everything I see that indicates a bad power valve matches the symptoms I have. Gonna head out soon and pull the carbs and swap them out to see if it makes a difference.

MichiMike

Backfire is a lean condition.The accelerator pump circuit compensates until the discharge boosters flow by reading PV vacuum signal (low) and draw fuel from main jet well tube.Power valve maybe,upsize you pump discharge nozzle size if your PV checks out ok.I prefer the nozzles with tubes to aim the shot at booster to breakup the squirt as it hits it.
Also check for vacuum leaks that hide after the mixture.