Fevre

Does adjusting it out have any effect on the fuel flow rate of the transfer slots? From the diagrams I have it looks like if you open it up(adj the screw out) there would be less signal to the transfer slot and fuel flow would be reduce.

norvalwilhelm

I thought they were 2 completley seperate systems. Throttle position affects transfer slot signal. I actually drilled and taped my holley carbs for screw in transfer slot jets.
You want your throttle plate as closed as possible so the transfer slot doesn't get too much signal. That is why the throttle plates are often drilled. To admit air without opening the throttle plates and keep the transfer slots covered.

CGGorman

Norval's partially correct.

The idle mixture screws do NOT affect the transfer slots, but the Idle Feed Retrictors DO.

Fevre

From my Holley Carb Handbook:

It appears to me that if you increase (adj the screw out) the fuel to the idle circut it will decrease the avail fuel to the transfer slot so if you have the idle circut too rich you will have less fuel for the transfer slot to provide when you apply some throttle.

I have a nasty off idle stumble that I can't seem to get rid of and was trying to figure out the effects (if any) of the idle screw on the transfers slot.

stingr69

The idle speed adjustment screw setting will affect the flow through the transfer slot but you might not want to use that as a "tuning aid". The transfer slot needs to be only partialy exposed at idle. When you crack the throttle to take off, the rest of the transfer slot will get exposed. This will pull in extra fuel so it will act like an auxilliary accellerator pump untill the main accellerator pump shot can do its job. If you have too much exposed at idle, you do not get the benefit of the additional fuel enrichment as the carb transitions from idle to accelleration. It can run rich if it pulls too much fuel because too much slot is exposed all the time. Lars has written a few papers that describe how to check your transfer slot position setting. If you are still rich with the adjustment in the ballpark, you might want to look into further restricting the idle feed restrictions.

Hope this helps.

-Mark.

Fevre

I have read Lars' paper and using some of his tips I still cannot get rid of the slight hesitation off idle. I have tried the main thottle blades in many diff position starting from being fully closed (all air coming in from the sec's) and up from there. It may be that the transfer slot circut it plugged.

Solid LT1

You failed to give us the model number of Holley you have. There are Holleys out there with "reverse idle circuits" where turning the mixture screws outward will LEAN the carb. You also have to make sure the transfer slot is not too far exposed due to the throttle plate being excessivley oped for good response to mixture screw adjustments. What is the "List #" on the choke housing on the drivers side (4 digits on most Holleys.)

Fevre

It is a ProForm 750 main body and base with 750 double pumper metering blocks and bowls. I have tuned it before with a a/f meter and know that it leans out when the screws are turned in. Will have to dig out the holley main body to get the numbers.

big632

Have you messed with pump cams or set it to #2?

Fevre

No, I seem to be getting a good shot (31 shooter) when you move throttle. I am getting some lean cut out when I keep it at a constant speed with the throttle just off idle. Main jets are 78's, it cut out real bad with 76's, might try 80's but that seems kinds high for 9/1 comp 350 with smallish (216/226 .470/.490) cam, plugs do look a little on the sooty side but would need fresh/clean set to get good reading. Seems like once the PV kick in I am fine but it still should not run lean with the throttle just cracked.

MotorHead

I am suprised it runs OK with 78 main jets, I had the same setup as you on my 406 and it ran like crap ( loss of power ) with 78 jets and ran fine with 76. My 825 Race Demon comes wth 76/84 and it runs great like that.

Here's what I would do, go back to square one. Make sure there is no vacuum leaks. Plug everything except the power brakes, you should be able to inspect that short piece for any leaks.

Next take the carb off and expose the transfer slots on secondary and primary the same amount, try and make them look square against the throttle blades. Put the carb back on, a new base gasket is always nice to prevent air leaks.

Now just using the idle mixture screws try and get the best idle, if it idles too high you can go back and close the secondary plates to close the secondary slot a little more and try again.

THe best setup is too have the idle transfer slots square and the mixture set perfect and it idles at around 800RPM. THis might not be possible because other things come into play like the timing. You might have it set for 34 deg at 3000RPM but at idle it is too high, this is where the MSD and other after market distributors are nice to have because you can extend or shorten the amount of mechanical advance.

On mine with a stock distributor and 15 deg mechanical advance I would idle around 21 deg. and that would screw up the carb tuning, but with the MSD distributor I can change the mechanical advance and I set it for 20 deg. so now I only have 16 deg at idle and it makes it easier to tune the carb

Oh yeah and jet down a little 72-74 primary and try the 78-80 in the secondary

Brettmc

From Super Tuning and Modifying Holley Carburetors by Dave Emanuel, pg. 31:

"A traditional idle circuit contains a curb idle discharge port and a transfer slot. Fuel discharged through the slot is derived from the idle system but is not affected by the idle mixture screw."

Are you running a spreadbore/square flange style intake?

Brett

Fevre

Square bore manifold.

Brettmc

I’ve noticed that sometimes the proform throttle plate will not seal exactly right on the passenger side towards the front. I’ve only noticed this on the spreadbore/square flange combination intakes. Just for grins, pull the carb and look at the gasket on the carb side and make sure it’s sealing.

The off-idle stumble you’re having a problem with will take some trial-and-error tuning. I put a 750 on my BB to run while my 850 is down. It had a terrible stumble right off idle. I solved this using the following method: Start by backing out the primary idle almost all the way. You will have to crank up the idle on the secondary side to compensate. Gradually put more idle in with the primary until you find the sweet spot where you get no stumbling when baaaaarely opening the throttle. After you find that spot, fine-tune the idle using the secondary idle screw only. BTW, this should be done after you have the idle mixture screws set. Have fun!

Brett

Fevre

Thanks for the help Brett, I am going to pull the carb and give it a good going over.

I have adj the primary throttle from being 100% closed and then adj both from there but still get the stumble, it is as if the transfer slots are not feeding any fuel on the primary side, this would happen if I had the primary adj too far open but it happens when I have it 100% closed and pretty much any where. I will check the relationship off the transfer slots to the thottle blades when I pull it. Think I'll also soak the blocks in some carb cleaner to see if they are somehow gunked up.

Brettmc

Things to do when you pull apart the carb: buy some carb cleaner or brake cleaner and put the red tube on it. Squirt it into the holes in the main body and make sure it comes out in all the right places. Especially watch the idle circuit. Make sure the carb base to body gasket does not cover up any holes in either the body or the baseplate.

CGGorman

Good adivce here. Compressed air is your friend, too.

Fevre

Great advise, all things i have used in the past but the base plate to main body gasket I have not thought to check.

SpyderD

Sounds like you are covering everything but I want to add a few more ideas:

You have a 31 pump which really is not that large, I think that's stock on the double pumpers. I know you say you are getting enough pump shot but a 33 or 35 might be enough to cover the hesitation. Also it sounds like you are running out of pump shot, play with the pump cam to give you a longer shot.

Also you can always either drill open the idle feed restrictions a bit (we are talking about micro measurements, you need a wire drill or something) to richen it up more at idle and transfer (then you can lean out the idle with the idle mix screws) OR you can buy one of those proform metering blocks which I understand have removable idle feed restrictions.

The symptoms you are giving is with an engine with a large carb/huge cam and cruising at low RPM. You are getting to the end of the transfer slot and the carb can't read the signal to start flowing the main jets yet. I see you don't have a big cam but the proform body I heard actually flows in the 800 range. It's adjustable but not without making idle or the main slightly richer - richen the idle to get more gas in the transfer circuit, richen or extend the pump shot, or richen the jets to get the main flow earlier.

Don't the proform bodies have adjustable idle air bleeds?

Edit - also I wonder if you are lugging it at low RPM with a standard or maybe in overdrive while cruising. Cruising at higher RPM will avoid this problem since you will be working off the main circuits.

Fevre

Yes the Proform does have replaceable idle air bleeds but if I increase their size would that not also lean out the transfers lot? The hesitation is just off idle at any speed or rpm. but only if I cruise or hold the throttle for a little bit at that position.

I tore down the carb tonight and noticed a couple things, 2 of the throttle body to main body screws where a little loose and there are no power valve channel restriction openings in my sec meter block. Odd that there is a threaded opening there but no way for the fuel to get to main well. I am assuming that even though I had a pv in there it was really doing nothing since there was no where for the fuel to go. Also pulled a couple plugs and they where real sooty.