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Holley Transfer Slot adjustment

57K views 19 replies 9 participants last post by  jbird 
#1 ·
I have my Holley Street Avenger carb off and am trying to adjust the transfer slots since I have a slight bog. From what I've read using the search, the slots are supposed to look like little squares and have around .02 to .03 of an inch showing. What I can't find is a simple explanation on how to make the transfer slots smaller. Is there an adjustment screw or something? I'm taking a look at it right now, but don't want to start messing with it until I know for sure.

Thanks for any help you could provide.
 
#2 ·
The transfer slots are supposed to be slots so that they are available to flow fuel over a slight range of throttle plate opening from the idle discharge openings upward. The top of the slot should be just visible above the throttle plate at curb idle.

They probably can be modified, but there would seem to be several easier ways to fix a bog. Working on the slots would be an extreme item for little gain.

I would start with accelerator pump adjustment, idle mixture adjustment using a vacuum gage, etc, and review where the timing was set.
 
#4 ·
The transfer slots are supposed to be slots so that they are available to flow fuel over a slight range of throttle plate opening from the idle discharge openings upward. The top of the slot should be just visible above the throttle plate at curb idle.

They probably can be modified, but there would seem to be several easier ways to fix a bog. Working on the slots would be an extreme item for little gain.

I would start with accelerator pump adjustment, idle mixture adjustment using a vacuum gage, etc, and review where the timing was set.
Thanks for the quick reply. Yeah, this was my second to last resort trying to fix the bog. I've adjusted the accelerator pump, set the idle mixture using a vacuum gauge and tried multiple timing settings...all to no avail. If this doesn't fix it, I figure I'll read up on accelerator pump cams and see if I should look into changing that. Before I did that though, I wanted to make sure that I've done all I could before I go and look into the accelerator pump cams.
 
#3 ·
ok, well I got impatient (I know I only posted a few minutes ago) and I figured out where the adjustment screw was. Problem now is that I backed it out as far as it can go and I don't have the perfect little square that I've seen in some of the searching I've done. It's really close, but not perfect. Am I ok? Its that all I can do to adjust it? It's a lot more "square" than what I had earlier.

Once the carb is back on, does anybody anticipate any problems I might have trying to fire the engine for the first time after this adjustment? Any tips/suggestions on how to get it close before I fire it up? I had the carb working in the car earlier today and won't get a chance to fire it up tonight (pesky neighbors).

One last thing. When I was tilting the carb around trying to locate the transfer slots and adjust them, a lot of the gas that was in the floats and other parts of the carb spilled out. Am I good just putting the carb back on, wiping it down and fire it up? Any issues from the fuel spilling out?
 
#6 ·
What secondary spring do you have in there?

Dave
The carb is a 770cfm vac secondary and I have the second stiffest spring in there (I think it has a brown tip). I have the spring kit and so far, that's what the car likes the best. I hope that is what you meant by spring...as you can tell, carbs are'nt "my thing"...but I'm trying!:thumbsup:

Does it matter what secondary spring I have in there for low rpm/throttle issues? I didn't think it did.??
 
#7 ·
Generally speaking, a low speed off idle bog or hesitation is caused by too small squirter, or too lean idle, or both. Make sure you don't have any vacuum leaks as well. Try adjusting the pump cam to give it a bigger/earlier squirt if possible. The next thing I would do other than that is go up a few sizes on the squirter. If it's a .028, try a .033 or .035. Good luck.
 
#9 ·
ok, so now I'm confused. I thought (after doing a lot of searching on the subject) that adjusting the transfer slots so they look like little squares is a good thing for overall carb performance and that a lot of other issues can arise from the transfer slots being too far open and allowing too much fuel to get through at idle while the carb is searching for enough air to keep the car from dying. The Holley website and an article in Chevy High Performance mag both state that proper transfer slot set up is important and to check that prior to getting parts for a problem you might not have..?

Thanks for the replies so far. I appreciate hearing from others and learning more.
 
#10 · (Edited)
I don't know about that type of Holley, but some Holley carbs ( like the 4500 model, and I believe the 4150 also) have an idle speed screw for the primaries, and one for the secondaries too. With the carb off of the intake manifold, you simply adjust the primary transfer slots with the primary idle speed screw. Then you install the carb again, and adjust the idle speed with the engine running, using the secondary idle screw only. That is how I was told to get the primary transfer slots "squared". ;)

BTW, I was also told NOT to go screwign around with squirter size changes, nor with pump shot adjustments until the primary transfer slots are squared first, and the secondary idle screw is then adjusted with the engine running to obtain the proper idle RPM.

Doing the above might solve your problems. If it doesn't, then, and ONLY then, should you go on to pump shot adjustments.
 
#11 ·
What BillyGman said, with the added part about timing first, carb second. Sounds like you've already got your timing set, so I'll say I hope it's in the 16-18° advanced range.

When you adjusted the slot, you've lowered or raised your idle rpm. You will likely have some issues when you first start it if you lowered it, might need a second person to keep it running while you adjust the secondary screw. (Typically a screw or allen key accessible from under the carb plate on a VS model - not the easiest to get to)

What color accelerator pump cam do you have? Different pump cams shoot different amounts either faster or slower than other colors. See chart for an example:


Heavier cars with automatics need more "shot" and sooner than a manual car and/or lighter car. I have a bunch of different color cams, if you can post what color you have now and want to try one that shoots more gas earlier (like the blue cam) let me know, I could stick one in the mail. Given your location you'd likely have it in 1 day.

After this, time to look at the size nozzle in the shooter, as already mentioned. Jump 3 sizes at a time. I also have extras of those that I don't need anymore, I might have a size that would fit your need.
 
#13 ·
What BillyGman said, with the added part about timing first, carb second. Sounds like you've already got your timing set, so I'll say I hope it's in the 16-18° advanced range.

When you adjusted the slot, you've lowered or raised your idle rpm. You will likely have some issues when you first start it if you lowered it, might need a second person to keep it running while you adjust the secondary screw. (Typically a screw or allen key accessible from under the carb plate on a VS model - not the easiest to get to)

What color accelerator pump cam do you have? Different pump cams shoot different amounts either faster or slower than other colors. See chart for an example:


Heavier cars with automatics need more "shot" and sooner than a manual car and/or lighter car. I have a bunch of different color cams, if you can post what color you have now and want to try one that shoots more gas earlier (like the blue cam) let me know, I could stick one in the mail. Given your location you'd likely have it in 1 day.

After this, time to look at the size nozzle in the shooter, as already mentioned. Jump 3 sizes at a time. I also have extras of those that I don't need anymore, I might have a size that would fit your need.
Mark- thanks for the reply. I have the initial timing set at 18*, have tried 16-18 and it seems to like 18* the best. The total timing only gets to 32*, which will soon be solved (hopefully) after I get a DUI/performance distributors unit, which I plan on ordering this week...but that's another story.

As far as the idle adjustment screw, I first backed that all the way out and I still didn't get the "square" I was looking for on the transfer slots. The only way I got it (which I now think was incorrect) was to adjust a bolt that was attached to the primary throttle shaft, on the opposite side from the trottle lever, which I now think might have been the choke cam (don't know I'll take and post a pic once I get the chance today). Is the secondary srew you are talking about #185 in this diagram?

http://www.holley.com/data/TechService/Technical/4150-4160 Exploded View.pdf

Regarding the accelerator pump cam...I have what came with the carb, which is the black one. The car is a 68 with a 396 big block and TH400 transmission. You think a different one might help? It would be great if you would send me one that you think might work:beers:. I'll pm my address to you, just let me know what you think.
 
#12 ·
There's no specific adjstment to "adjust the transfer slot".

What changes it, is the idle speed adjustment screw... the one that the throtle linkage comes to rest against at idle.

The goal is to back the idle speed adjustment screw off until the blades are in the correct relationship to the transfer slot, by opening the secondary blades, using their screw, to let in enough air.

Easiest way to do it, is to adjust the idle speed correctly on the car, the normal way; take the carb off and turn it upside dow; observe the transfer slot, and how far the primary blades are away from the desired relationship; and adjust the idle speed screw while watching for the correct relationship. Then adjust the secondary adjustment to change them an equal and opposite amount to what you changed the primary ones. For example, if you closed the primary throttles .050" to get the slot right, open the sec blades .050". Put the carb back on and see where you are. Repeat until perfect. On some carbs, you can take the sec screw out and turn it upside down, so that its head is accessible from above. Not sure about the Street Adventure carbs.

LEAVE THE SQUIRTERS ALONE!!!!! They are the VERY LAST thing to adjust anything about, until every other detail about the carb has been dialed in. From the point of view of carb design, they are merely a Band-aid for the carb's inability to deliver fuel as quickly in a throttle transition as it delivers air. The better you get the carb tuned, the less they are needed. They are almost always TOO BIG, and feed TOO MUCH GAS, and are one of the reasons Holleys have a bad reputation for poor gas mileage. People use them too much.

The correct way to tune a Holley for the street is to first get the ignition timing right. Usually this means limiting the mech advance in the dist so that when the "total" (static plus mech, no vac) advance is at the 36° or so that it needs to be for best power and efficiency without detonation, the static can be at around 16-18°. Meaning, NO MORE THAN 20° of mech advance. Most stock distributors give 26-28°, which results in the static (low-speed) timing being WAY too retarded. Inadequate advance at low speeds will make the motor generally run lazy, no "snap" to the throttle.

Once you get that set right, move to the carb. Get the floats right (pri with the level just barely below the sight plug, sec the fuel level should be about .050" below the sight plug), and do the transfer slot setting as described above.

Next thing to do to the carb is to set the main jets right. Holley main jets are usually a couple of sizes too large. Get yourself a few sets of jets, maybe 2 sizes larger than the stock ones, and 2, 4, and 6 sizes smaller. (a change of 1 size is imperceptible; 2 sizes is a fine tweek; 3 sizes is a standard step; 4 sizes is a large step; 5 sizes is HUGE) Decrease the jet size and drive the car on a normal road in a normal way. Keep dropping the size until it just begins to surge going up a slight hill inthe higest gear at the lowest speed you would customarily drive at; then go back up 2 sizes from there. Ignore all other behavior, good or bad, while making this test.

Then get yourself some power valves. Holley almost always puts too low of a number in their street carbs, meaning, no power enrichment occurs until you have given the car ALOT of throttle, which is why they make the jets too big. Raise the PV in steps until the bog goes away.

THen last, tune the secondaries. Find the loosest spring that prevents a bog when flooring it from a stop, and then out the next stiffer one in. You'll probably end up with the brown one; almost cetainly not the yellow one. Then set the sec jets for max mph in the ¼.

If you tune up a Holley by this method, you'll be able to get better driveability and gas mileage than you ever thought possible. As well as spark plug life.
 
#15 ·




Here are two pictures I took of the bottom of the carb and the only other screw that I could find that offered any adjustment to the primary throttle blades, other than the curb idle speed screw (which I have all the way backed off and alone, didn't get me the "square" transfer slots). Does anybody know what this screw is(my finger is pointing to it in the first pic)? After looking the diagram, it appears to be the choke cam adjustment screw, but I've never heard of anybody adjusting that (other than me:( right now) to get the transfer slot problem solved. Do you think this will work? You think the choke will be all jacked up now? I'm guessing yes, but that's secondary to getting the carb to run correctly.

As far as adjusting the secondaries...my carb doesn't have the #185 screw in the secondary blade, as it appears on the diagram. However, I did find another screw that offers adjustment on the secondary throttle blades (seen on the first pic, below my finger on the small tab on the baseplate - flat-head screw), but I thought that you were not supposed to have any of the secondary throttle slots showing. If I open them up any more than they are now, I'll expose the secondary transfer slots. Any thoughts?
 
#16 · (Edited)
The way that it was explained to me, you only need to be concerned with squaring the primary side slots, and then the carb goes back onto the engine, and you merely adjust the secondary side to get the idle RPM correct, without worring about the secondary slots. as far as your question about the choke screw, I don't really know about that, because I'm used to double pumper carbs without chokes. Sorry.
 
#17 ·
Well, you've got the lowest cam in there now.... I'll send you an orange and a blue cam to try out.

Really, all the advice you've got here is good advice - it's up to you to decipher it and figure out what works for your combo. Just try to change only one thing at a time, and just to confuse things - you can hide the real problem by changing something that appears to fix your problem. Get everything as close to what you feel is perfect, then work on the one thing that fixes your remaining problem IMO.
 
#19 ·
in the first pic the screw you're pointing at is the fast idle adjusting screw. Unless somebody has really boogered the carb it should only enter into things when the choke is on. The choke is not on, is it?

in that same pic look straight below the fast idle screw to see a threaded hole in the baseplate, it has like a set screw in it. it's a straight slotted screw that down in the hole. it's what adjusts the idle or closed position of the secondary throttle. from the side you can see the tab on the of the secondary shaft resting on it. You'll notice it's a little hard to adjust with the carb on the car. It's that way because it should never need adjusting.

I recommend you leave it alone for the time being.

try moving the pump cam to the other hole or a different cam that's more aggressive in the early part of the travel.

And don't worry too much about the shape of the transfer slot below the throttle. There's many thousands of cars out there running just fine that don't have little squares there. It's not magic or anything. The problem is when the throttle is way too far open at idle and all the idle fuel is getting sucked out the slot. Then there's none left over to go past the idle mixture screw needles so the idle mix adjustment has no effect. This usually happens when there's a big cam and not enough ignition timing.

I recommend you try a different pump cam, make sure the pump linkage is tight at idle and try a bigger squirter. Holley has traditionally been very conservative on the squirter size. If it's a 28 try a 31. if it's a 31 try a 33.

remember, one thing at a time. if you make a change and it doesn't help put it back the way it was before moving on to the next thing.
 
#20 ·
in the first pic the screw you're pointing at is the fast idle adjusting screw. Unless somebody has really boogered the carb it should only enter into things when the choke is on.

in that same pic look straight below the fast idle screw to see a threaded hole in the baseplate, it has like a set screw in it. it's a straight slotted screw that down in the hole. it's what adjusts the idle or closed position of the secondary throttle. from the side you can see the tab on the of the secondary shaft resting on it. You'll notice it's a little hard to adjust with the carb on the car. It's that way because it should never need adjusting.

I recommend you leave it alone for the time being.

try moving the pump cam to the other hole or a different cam that's more aggressive in the early part of the travel.

And don't worry too much about the shape of the transfer slot below the throttle. There's many thousands of cars out there running just fine that don't have little squares there. It's not magic or anything. The problem is when the throttle is way too far open at idle and all the idle fuel is getting sucked out the slot. Then there's none left over to go past the idle mixture screw needles so the idle mix adjustment has no effect. This usually happens when there's a big cam and not enough ignition timing.

I recommend you try a different pump cam, make sure the pump linkage is tight at idle and try a bigger squirter. Holley has traditionally been very conservative on the squirter size. If it's a 28 try a 31. if it's a 31 try a 33.

remember, one thing at a time. if you make a change and it doesn't help put it back the way it was before moving on to the next thing.
:D There you go.:yes:
 
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