Proper Transfer Slot Position

[jmanhoff]

Hi all,

I don’t mean to keep spamming new threads, but I also don’t want to merge a bunch of different topics into one thread.

For this thread, I wanted to go into the topic of t slot exposure. I’ve always heard square or about 20 thou. That’s what I was going for with my carb, but I have a ridiculously high idle when it’s set there.

For context, I have a 92 Dakota with a 5 speed, Magnum 5.9, mild Hughes cam (see attached card), edelbrock rpm air gap, shorty headers, 650 Holley DP. In the carb I have 65 PMJ, 71 SMJ, 28 MAB front and rear, 10.5PV with 0.045 restrictors, 0.070 t slot restrictors, 0.028 IFRs at the bottom of the well, and currently I have 0.063 IABs though I’ve been experimenting with different sizes to see how/if I can fix an issue of rolling into the throttle at low rpm without it going dead lean, it doesn’t buck it just shoots lean and I can feel it down on power. I have about 20 inches of vacuum at a 750-800r idle which is where I want it, but currently with the t slot square and the secondaries fully shut it’s 23”. 18 degrees initial timing, and vacuum advance is ported and is not coming in at idle. My mixture screws are about 3/4 out as well.

I would really like to idle down more into the 750-800 range mainly for volume, and long periods of idling I’d like it to be a lower rpm than 1000. I’ve checked for vacuum leaks and haven’t found any. Kind of out of ideas for what it could be. Any help would be appreciated

 

[Ant]

18 degrees initial timing

I can't help you with the carb, because I would go by feel and stuff I can't properly explain because I'm not a genius on that, but what I can tell you is that 18° of initial ignition timing will give you a higher idle.

 

[jmanhoff]

I can't help you with the carb, because I would go by feel and stuff I can't properly explain because I'm not a genius on that, but what I can tell you is that 18° of initial ignition timing will give you a higher idle.

It definitely will, but these engines seem to like a lot of timing at least from what I’ve been told by a lot of people including some on this forum

 

[Bewy]

Check the PCV. It should rattle if ok. I would also disconnect it & plug it to see the rpm change.
The sec t/blades may not be fully seating in the t/bores, which will increase idle speed. Hold the carb up to light & look for gaps around the blades. If all checks out, I would increase the IFRs to 033-4. Because you have hardly any T-slot exposed at idle, it means you have a lot of it exposed above the blades at idle.....& it acts like an air bleed leaning the mixture. As you roll into the throttle, you are getting too much air from the slots, leaning the mixture. So I would enlarge the IFRs to richen the mixture. The new idle mixture can be reset/adjusted with the mixture screws.

 

[Oldmanmopar]

Remove the air bleeds and thread the carburetor for removable air bleeds to play with air fuel. That is what I had to do to correct many. Also that 10.5 PV seems a bit much for that cam . Vacuum at idle divided by 2, For a factory cam the tallest I ever saw was a 6.5. I ran a 3.5 499 lift cam . big difference , I also put smaller low speed air bleeds in to richen the fuel of idle. Keep the transfer slot square and if you need more rpms drill holes in the venturi valves.

If you cannot idle down square with the secondaries closed. You are sucking air somewhere. Try another carb .

 

[jmanhoff]

I ended up with a 10.5PV as that’s where I’d notice with the stock jetting where the engine wanted a bit more fuel. My main body is setup to run adjustable bleeds now, and I use a bunch of brass set screws with a numbered drill bit set

I used to have a brawler 650 and I had a very similar problem with it. As a sanity check I’m going to spray some carb cleaner around all the vacuum hoses and connections just to verify again there’s no leaking anywhere. I might also go up a tad in IFR like mentioned to see if I can richen out the mixture which in theory would lower the rpm slightly

 

[JedIEG]

Check your brake booster. I chased an erratic idle/lean issue due to a leaky booster that never exhibited issues with the actual brake feeling.

 

[jmanhoff]

Check your brake booster. I chased an erratic idle/lean issue due to a leaky booster that never exhibited issues with the actual brake feeling.

That is actually a rabbit hole I’ve gone down before too lol. My pedal is firm but it sounds hissy, but it’s been like that since I’ve bought the truck and before the engine swap. I’m planning to do a swap to the newer generation brake booster because it’s smaller and stronger, just haven’t had the funds to do that quite yet.

Correct me if I’m wrong but they can leak vacuum internally and you’d basically never be able to tell right? Other than by just unplugging the hose and plugging it and seeing if it idles down

 

[JedIEG]

Yeah mine was leaking at the diaphragm, not the check valve like usual. The dead giveaway was I clamped off the hose to the booster and the RPM dropped.

 

[Inertia]

Have you used the secondary throttle stop screw, accessed from under carb.
Try closing it a very small amount

 

[crackedback]

Plug everything off and isolate the carb. The slot setting is a starting point.. not a strict must have it here setting. As suggested take an 1/8 turn out of both sides and see where your idle speed sets up.

The 1/2 idle vacuum for PV is holley covering their behind to get folks with no clue into a "safe" PV range. Works OK for a drag car, idle or WFO applications. Not best selection method for a streeter.

That cam should idle like a stock engine, nothing crazy. 700-800 with minimal drop when in gear. Could pull a couple degrees of timing out of it. Make sure your aren't getting mechanical advance bleeding in with the high idle.

 

[jmanhoff]

Have you used the secondary throttle stop screw, accessed from under carb.
Try closing it a very small amount

Yeah, I do need to slightly open it actually as I think it’s closed a tad too far out because right around half throttle when the secondary linkage starts to move there’s a slight bind in the cable, but I have verified they’re fully shut by pushing on the linkage itself.

Plug everything off and isolate the carb. The slot setting is a starting point.. not a strict must have it here setting. As suggested take an 1/8 turn out of both sides and see where your idle speed sets up.

The 1/2 idle vacuum for PV is holley covering their behind to get folks with no clue into a "safe" PV range. Works OK for a drag car, idle or WFO applications. Not best selection method for a streeter.

That cam should idle like a stock engine, nothing crazy. 700-800 with minimal drop when in gear. Could pull a couple degrees of timing out of it. Make sure your aren't getting mechanical advance bleeding in with the high idle.

Next time I have a chance to poke at it I’m going to pull all the hoses off and just let the carb sit with nothing hooked to it and see how it runs. I did set the initial timing with it idling at about 800rpm and the timing is the same when it’s sitting at 1000, I believe my mechanical advance comes in around 1200-1400.

The way I got my PV measurement was with a combination of vacuum gauge, afr gauge, and seat of the pants. Basically just loaded the engine up until I noticed it felt weak (obviously it went lean), and then noticed at what vacuum it was at, which happened to be around 10 inches so I went with a 10.5. Given the engine is basically stock, in daily driving if I’m driving light footed I rarely dip into the 10s or lower

 

[jmanhoff]

So, interestingly it seems like I have a vacuum leak at my PCV valve. It’s the gasket though it seems like because I can seal the valve with my finger and it will idle down, same goes for if I pinch the hose, but with it in the valve cover it idles up.

The PCV is new, and I’m running LA covers on a magnum, but the covers themselves are new as is the gasket, and I’m running the LA PCV although I’m not sure if they’re very different from the magnum.

Do PCVs normally cause the idle to go up a bit or is mine actually leaking at the gasket

 

[Bewy]

The crankcase [ & valve covers ] are 'pressurised' by blowby passing the rings. With the PCV in the v/cover, some extra air is being forced past the PCV, which is causing the rpm increase. I think your PCV is ok.

 

[jmanhoff]

The crankcase [ & valve covers ] are 'pressurised' by blowby passing the rings. With the PCV in the v/cover, some extra air is being forced past the PCV, which is causing the rpm increase. I think your PCV is ok.

I think just for sanity I’m going to get the “magnum” style grommet and put it in there, the hole is strangely smaller than the one for an LA, and I noticed that the magnum valves seal along the smaller part of the valve, but on this valve cover the wide part of the pcv is what sits inside of the grommet if that makes any sense. I wonder if this whole time that’s not been sealed properly, I noticed it when I first installed it and just figured that’s how it was

 

[jmanhoff]

could I potentially have the wrong PCV valve for my setup? Currently I have a Carquest PCA1005 which is a stock replacement for a 360 LA or magnum motor from advance auto, and given my mild cam and high vacuum I figure I can get away with a stock pcv, but just wanted to double check

 

[Hysteric]

Holley 650 list number?

 

[jmanhoff]

Holley 650 list number?

0-4777SAE

 

[Hysteric]

0-4777SAE

Is there no - X number after the 4777? Is the base plate a standard thickness base plate?

 

[jmanhoff]

Is there no - X number after the 4777? Is the base plate a standard thickness base plate?

In terms of the thickness I’m not sure to be honest, but this is the listing on holleys site if it helps

https://www.holley.com/products/fue...1gIXTUvcBGt6h4HpGlOdsnmvvgzppEAoaAp6lEALw_wcB

 

[Hysteric]

In terms of the thickness I’m not sure to be honest, but this is the listing on holleys site if it helps

https://www.holley.com/products/fue...1gIXTUvcBGt6h4HpGlOdsnmvvgzppEAoaAp6lEALw_wcB

If its a dead stock holley with a standard base plate why would it have a T slot restrictor?

 

[jmanhoff]

If its a dead stock holley with a standard base plate why would it have a T slot restrictor?

I drilled and tapped the holes in the main body to allow me to tune the transition circuit as I’m trying to set it up for lean cruise, it was a suggestion on a different thread of mine awhile back

 

[Hysteric]

I drilled and tapped the holes in the main body to allow me to tune the transition circuit as I’m trying to set it up for lean cruise, it was a suggestion on a different thread of mine awhile back

The reason for the Tslot restrictor is the slot is too long and the restrictor improves signal. Its a band aid for bad design.

If you are generating enough vacuum (20hg is plenty) then you can lean it out via the the IFR/IAB relationship. Also when you have a lot of vacuum you can close up the Blade/ Tslot as the engine will still pull air and fuel from the slot.


If you are chasing efficiency then try a Thermoquad or Rochester as those designs are inherently better.

 

[Mattax]

If its a dead stock holley with a standard base plate why would it have a T slot restrictor?

You'll have to backread. LOL!
Somewhere he went from a Browler to a 4777 but it seems the timing is still at the bottom of the list of things to measure and tune.

Post in thread 'Holley 650DP Sticky Throttle'

Mar 27, 2026

If that's the case then your timing should be the same at 1000 rpm as it is at idle. Next time it is stuck at 1000 put a light on it and verify that is the case. Good luck!

^^This^^

The carb will respond to the conditions and the timing is a big part of that.
Heat soaked is the condition to set the tune for.

• Mattax

Search for your own posts back here

Post in thread 'Fine Tuning a 360 Magnum with a Brawler "Double Pumper"'

Feb 8, 2025

Lol I’m also just a really stubborn traditionalist because my dad grew up with mopars and holleys so I’m kind of just following the same road.

To be frank you're throwing darts blindly. Pin the tail on the donkey without seeing the donkey.
The first thing you need to do to seperate wheat from chaff is know the difference.

Go read the booklets from Chrysler on carburetor fundementals.
Its free at available at your electronic fingertips. Urich and Fisher will have to be found or ordered from ebay or 'zon so you won't get it for a week. Its not that its a great book, but the...

• Mattax

Seems like you have as much ignition timing tuning to do as you’ve had carb tuning.

 

[jmanhoff]

You'll have to backread. LOL!
Somewhere he went from a Browler to a 4777 but it seems the timing is still at the bottom of the list of things to measure and tune.

Post in thread 'Holley 650DP Sticky Throttle'

Mar 27, 2026

If that's the case then your timing should be the same at 1000 rpm as it is at idle. Next time it is stuck at 1000 put a light on it and verify that is the case. Good luck!

^^This^^

The carb will respond to the conditions and the timing is a big part of that.
Heat soaked is the condition to set the tune for.

• Mattax

Search for your own posts back here

Post in thread 'Fine Tuning a 360 Magnum with a Brawler "Double Pumper"'

Feb 8, 2025

Lol I’m also just a really stubborn traditionalist because my dad grew up with mopars and holleys so I’m kind of just following the same road.

To be frank you're throwing darts blindly. Pin the tail on the donkey without seeing the donkey.
The first thing you need to do to seperate wheat from chaff is know the difference.

Go read the booklets from Chrysler on carburetor fundementals.
Its free at available at your electronic fingertips. Urich and Fisher will have to be found or ordered from ebay or 'zon so you won't get it for a week. Its not that its a great book, but the...

• Mattax

He does have me there lol. I’m currently in the search for a different style distributor because the HEI on a magnum in a Dakota is practically touching the firewall so I want something with a smaller footprint, but I was actually planning on remapping my timing curve again and then messing with the springs. If I recall correctly Mattax, given that I have a heavier truck, I don’t want the timing to come in right away, but I don’t want it to be lazy either. From what I can tell, my timing right now is all in at about 3000 with 20 degrees, and it starts to come in around 1300 rpm or so, about 12 degrees at 1500, 16 at 2000. And my initial currently is 18.

18 degrees at 800rpm
30 degrees at 1500rpm
34 degrees at 2000rpm
38 degrees at 3000rpm

Looking at that mapped out though it looks a little, off, so I’m definitely going to recheck that and post an updated one. This is not mapped with the vac advance as well