Carb tuning.....
 

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

Larger cammed motors need a lot of intitial advance. If they don't get enough advance, they will also exhibit the symptoms your having.

#1 rule when tuning carburetors is:
Make sure your ignition is correct
Make sure you don't have any vacuum leaks
Make sure your valves are adjusted correctly
Make sure you have correct compression in all cylinders
Make sure you have proper fuel pressure
Make sure your flame arrostor top is not touching/ or almost touching your choke tower. 3" or more is good.

Then, go tune your carb if this still seems like the culprit.

Talk about information overload........

The problem it seems is that there are now too many cooks in the kitchen............many of which don't understand the workings of a Holley. But hey, It's your money they are spending. I am now done posting in this thread. Good luck.

BTW...the info posted by SB above my post (post #11) is good info.

Anytime someone posts a question on an open forum ,there will be replies from people with varying knowledge of (whatever) .I don't call this too many cooks ...I call it input .You've hopefully got enough sense to sort out truth from fiction .Not many "cooks" yet on this short thread.....Certainly you don't think your point is the only one that matters. .I'll go make another drink .............

If all else fails buy a new carb...been there done that.But bad performance can be so many things.

I certainly don't think my point is the only one that matters. I am certainly always willing to learn new things. I am certainly very open minded. I certainly know when to back away from the keyboard. I most certainly have enough sense to walk away from this thread.

http://www.nickersonperformance.com/ Usally tune my own carbs but set ups like my dual quad engines send my carbs here to make them identical.

Nickerson is Great, but I don't think he's there yet.
SB, all great info as usual! I was leaning towards this being a new condition, kinda of everything was fine then "suddenly went fat" and leading him to the simplest/fastest fix with the facts he gave. All the procedures you sited should be followed up on in this situation, even IF the carb PVs are the problem, and especially if they are NOT the problem.
Performance, keep us posted!

It's a new boat to me, and this is the first time I have had it in the water. It's complicated to be a mechanic over the internet to somebody that's not quite sure what they're doing. I appreciate all feedback and suggestions. I've never worked with a holley before, and the cheap rebuild kit I got from advance didn't have all the parts match what I took out of mine, so I reused a few things, including the power valves. Just found a guy down the street that had the right size ones in his spare parts drawer, and they feel much better than mine....one of them actually felt stuck. Timing is the next thing I'm doing, but being a gen VI motor there aren't timing marks like i'm used to, so my old timing light does me no good. I'm taking it to my buddy this afternoon or tomorrow to have him do it for me. From what I've gathered from google, I should be at 8-10 at idle and advance to 30? does that sound right?

8-10 idle is good, makes them easy to start. My experience with naturally aspirated BBC is they like around 34, give or take. More timing isn't necessarily more power. Last BBC carbed motors we dynoed maxed at 36 degrees and started losing HP any higher.

You shold get all your parts from Quick Fuel. If its an older carb it might not have back fire power valve protection. QFs stuff is much stronger and they have retro fit anti back fire kits. Just installed them in a 830 I have for sale complete rebuild using QF parts.

so after replacing the power valves both motors seemed to run a tiny little better, then moved onto timing and I think i'm finally good to go. One motor was at only 26 degrees while other was at 30. Both are now at 33 and throttle response is perfect, they run better than they ever have since i've owned it. Sea trial tomorrow, should hopefully be good to go. Before with the hesitation from both and crappy idle out of one i was only able to see 4500 and 4600 rpm and 65mph, and there were still some notches left in the throttle sticks. I'm really hoping I can get into the 70's. Everything I've seen says max rpm should be around 5200-5500 rpm, correct?

When setting total timing with Mercruiser modules, absolutely make sure that you find out where the module ends advancing timing. A lot of people assume they'll stop at some rpm but don't check.

I ran into one (525SC) that advanced up to like 4200rpm.

Previous mechanic (marine mechanic) mind you didn't. so, guess what, yup, about 10 degrees too much.

You do this by reving motor until you see it stop advance, and then rev it higher to make sure.

So yeh, I didn't like it, but I went to 4500rpm on those 525SC's.

You may want to pull your distributors and inspect the gears. Worn gears lead to retarded timing. Eventually when you run out of gear the rotor will not turn at that point it's time to get towed in.

5200-5500 correct