BAD-HABIT

I have had a problem with my Holley 750 Vac Sec. ever since I got the damn thing.....When I set the idle and idle air screws I set it to get the highest vaccum. So fine I get that done and set the idle to 700 in gear. Which puts me at about 950 out of gear right.....WRONG it goes up to around 1200 and after about 45seconds or so will creeeeep down to about 950. I have tried setting the air/fuel ratio in gear and out of gear. At 700rpms and at 1000rpms.....My fuel pressure has been double checked and is at 4.75lbs. Floats have been checked and re-checked and are set with boat in water and have been set with boat out of water. Each time with fuel just below the hole. Also tried with it a little high and a little low. When the idle hits the high spot I have checked and no fuel is dripping and the secondaries are closed. Choke has been adjusted to specs and re-adjusted. Finally I have just unhooked the damn thing and set it all the way open. Any ideas? I am running about 13lbs of vaccum at idle out of gear. Sometimes the thing will idle great. But most times especially after a run I have to re-adjust the idle speed. I really hate doing this...ANY IDEAS???? Thanks for listening to my rant. Good news is I did get the boat out today though!

BH

Reed Jensen

What ignition system are you running?... does it have a mechanical advance?... check your timing settings both when it is idling correctly and when idling too high.. make sure that the timing is returning to the original settings.

Madmax

The secondaries are staying cracked open a little bit, sometimes returning to seated, and other times not, hence the radomness. Next time it idles fast, grab the secondary shaft and close it. Bet the idle will come back to normal.

If this is the problem (and i'm betting it is), adjust, lube, and yer good to go.....

Reed Jensen

It's been a long time since I've worked on a vacuum opening secondary Holley... what color spring do you have under the diaphram?.. I don't remember if there is a "dump circuit" for the vacuum when the throttle is closed...

racinbird

Ive had the holley 750 (3310) carb on my built SBC and mild BBC, both times I have some idle issues like you are describing. One thing you may want to try is remove the carb, turn upside down, check and make sure the primary throtle plate is in the middle of the transition slot, if it isnt and the whole transition slot is shown, then you need to open the secondary plates slightly to allow some air flow, this then will allow you to adjust primary idle circuit with idle mixtures. I had to do this on both of mine to get it right. Also you can drill small holes, (forget the size) in the primary plate to allow more air flow.

BAD-HABIT

I am running a D.U.I. (custom built H.E.I.) ignition. I will check the timing thats one thing I haven't looked at yet....
BH

formula31

Yup, very familiar with these carbs and they sometimes do this. Another thing to check is slop in the shafts where they go through the throttle body. This will also cause idle variances you described.

formula31

This is also good informaton and especially applies if your idle adjustment screws have little effect on idle quality or rpm. I have hole drilled in my primary plates and needed to to get my idle and start quality where I wanted it.

These carbs are great when they are set up right but that takes some time.

BAD-HABIT

What effect does drilling a hole in the primary plates do and what is the downside or danger in doing this? Is it something I can do and not hurt anything or damage the carb?

BH

racinbird

drilling holes does nothing but allow more air in, which is what opening the secondaries also does. This is only benefial if your plates are already adjusted higher than the transition slots. Doing this means more air in so you can turn back down the primary plates where they are supposed to be in relation to the transition slots to get the correct idle settings.

Down side, too much air if not needed and be careful drilling with carb on engine so no metal shavings in intake

You need to have the plates adjusted properly at the slots to be able to get the idle adjusted properly