I have a question about setting the idle mixture screws on a 350 cu in engine. when hooking the vacuum gauge up, does the engine have to be in gear. i tried the holly procedure and the maximum vacuum obtained was with the screws turned all the way in, and i doubt that's the way they should be set. also, does changing the screws affect the air fuel mixture above at running speeds above 2000 rpms.
What holley carb? If its the spreadbore 650cfm, I believe the screws should be around 1/2 turn out. You should adjust to highest vacuum reading at idle.
That being said, the motor should definately stall with them turned all the way in. I think you have a internal fuel leak. You should check float height, possibly a leaking welsh plug etc. Hope this helps.
If you can't get the engine to stall turning the screws all the way in you aren't running on the idle circuit at idle. What you need to do is adjust the secondary throttle plates open a bit so you can close the primary throttle plates. Then you will be able to stall the engine using the idle mix screws. Thereafter start at 1 turn out on all screws. Turn all screw in in 1/4 turn increments until highest vacuum is attained or better yet an a/f reading for 14:1. You will most likely end close to the 1/2 turn out give or take an 1/8th turn. I agree 2.5 turns out would be way rich.
Good point. Before you get too carried away with setting the idle mix screws make sure there is no fuel dripping out the boosters at idle. If it is dripping it could be the float is set too high or the PV is blown if it has one. Either will negate idle mix tuning.
thanks for the great replies guys. I guess the question is why am I running so rich at 800 to 1000 rpms when i'm in no wake zones. can't believe i'm having these problems on motors that are only 3 years old
Remove the flame arrestor and look down the carb at idle. You should see no fuel dripping, none. If there is fuel dripping, loosen the float adjuster nut and turn it cw in 1/4 turn increments until it stops. Note that this must be done with the boat in the water. The attitude is different on the trailer.
If it is your power valves there could be one in the secondaries as well as the primaries depending on carb, and they have diffferent vac ratings. If you are taking the carb apart to change power valves take the base plate off the main body, and install back fire check balls if it dosen't have them. Holley # HLY-125-500. Just another thought,... hope you allready have it fixed.
I went down to the marina last night (had to work all weekend, I'm very lucky in this economy), It's a Holley model 80552 marine carbuerator. From what I can tell there are no float adjustment screws on those. I didn't run it as it was getting dark out to check for being able to see gas dripping at idle, i'll do that today. I did pull a couple of plugs, they are ac delco platinums (very expensive) and they are definitely black. The strangest thing (probably not in the world of boating lol) is that the boat has twins and both engines are running the exact same way. What started this whole problem is the first time out is that the chokes never opened up and I got fuel in the oil. I took care of that by changing the oil twice and I'm not getting any more fuel in the oil.
a little more history, these are new engines from pleasurecraft and they sat for two years
update. i went down to the boat and replaced the spark plugs in the port engine, they were platinum so kind of expensive. the ones in their were all carboned up but dry. after the change the motor ran fantastic. also, there is no external gloat adjustment on the carbs, but I saw no fuel leaking in the bores so I believe they are okay. i'll keep everyone posted
There may be many different opinions on this but I wouldn't run platinum plugs in a carbuerated application. They are more sensitive to fouling from excess gas and this occurs with carbuerators more so than Fuel Injection.