Hot Rod Forum banner

1 - 20 of 21 Posts

·
Registered
Joined
·
157 Posts
Discussion Starter · #1 ·
I am in the middle of changing camshafts in my 383 from a radical hyd roller to a more mild hyd roller (230/236 112lsa).
My previous cam produced no vacuum at idle, had a lot of overlap and the duration was too high. It was on a 108lsa. Couldn’t tune the carb for nothing. I’d say there was a flurry of reasons as to why. I drilled the throttle plates. Tried just about everything I knew how. During disassembly I noticed the accelerator pump arm was just rubbing the intake. So I put a 1/2” spacer in it.
my main concern is trying to get the carb back as close to factory settings as possible. I turned the idle mix screws out 1 and 1/2 turns. Fast idle doesn’t engage the cam.
my question is does anyone know the factory setting of the transfer slot in the base? I’ve always heard to open the up until enough untilabout a square in of the transfer slot is exposed. Not on issue on the primary side. The secondary side is my question. Ill post pictures for reference but I backed the secondary adjust screw all the way back and I get just more than a square exposed.
I also replaced the throttle plates to ones that aren’t drilled.
Thanks!
616815
616816
 

·
Registered
Joined
·
423 Posts
Your primary looks ok, but secondary I find is best with transition slot just covered with none open. But bigger cam often like more. I’ve found some stumble with secondary transition showing. In fact it was suggested here to close it up.
 

·
Registered
Joined
·
157 Posts
Discussion Starter · #4 ·
[
What he said^^^
I’ve backed the secondary adjustment screw all the way off the diaphragm linkage. The hole will not cover all the way unless I push on the blade. When I let it go it reverts back to being slightly open. maybe the throttle blades are catching. I cleaned the bores good with carb cleaner.
 

·
More for Less Racer
Joined
·
20,110 Posts
[


I’ve backed the secondary adjustment screw all the way off the diaphragm linkage. The hole will not cover all the way unless I push on the blade. When I let it go it reverts back to being slightly open. maybe the throttle blades are catching. I cleaned the bores good with carb cleaner.
With the stop backed well off, loosen the throttle plate screws and re-center the blades, this will often allow the shaft to fully close, then retighten the plate screws.

Same thing should be done on the primary shaft too....actually, any throttle shaft after the blades have been removed and re-jnstalled or replaced.

Once that is done, then check the secondary closing link....this recent thread is all about it.
(21) Vacuum secondary link setting | Hot Rod Forum (hotrodders.com)

You are in effect, "blueprinting" the carb.
 

·
Registered
Joined
·
157 Posts
Discussion Starter · #6 ·
With the stop backed well off, loosen the throttle plate screws and re-center the blades, this will often allow the shaft to fully close, then retighten the plate screws.

Same thing should be done on the primary shaft too....actually, any throttle shaft after the blades have been removed and re-jnstalled or replaced.

Once that is done, then check the secondary closing link....this recent thread is all about it.
(21) Vacuum secondary link setting | Hot Rod Forum (hotrodders.com)

You are in effect, "blueprinting" the carb.
before reading this I wen into the secondaries and took the throttle shaft out.Cleaned it real good and put it back in. Then I took my new throttle plates and just lightly hit the edges of them with 1500 grit sandpaper to knock the rough burs of he edges. Then I put them in loosely and opened he secondaries up and dropped it to center the plates. Now the secondary transfer slot is covered. If I even turn the secondary stop screw a 16th of a turn it starts to crack open.

this is a pic of my current secondary transfer slot.

please advise
616817
 

·
Registered
Joined
·
4,172 Posts
What exact Quick Fuel carb do you have? I need to know and I can help you with what the factory settings were. Is this a vacuum secondary or a double pumper and is it a two corner idle or a four corner idle? I read your post real fast so I might have missed it but I did not see it from re reading it. If you say your accelerator pump arm hit your intake then that sounds like you have a double pumper as on some Edelbrock intakes the rear of the double pumper accelerator arm always will 99 percent of the time make contact with the pipe plug if your using it on the intake manifold or if you try to use a connector piece for either your pcv valve or your brake booster then it will most likely not clear and that is even with the low profile vacuum fitting you can get that has a lower head on it.

The Quick Fuel and other carbs you must be careful trying to equal the transfer slot with the front and back even with having a big cam because the rear transfer slot sits up about .030 higher on the Quick fuel brand carbs on either the billet or cast base plates and compared to a Holley cast base plate regardless of it being a double pumper or vacuum secondary, the quick fuel base plates will have a slightly longer and wider transfer slot on the front and rear vs a Holley brand. Also on a Holley brand carburetor the rear transfer slot will sit up even as much as .040 higher and trying to equal them out will end up with the secondary side open up to much and your idle will be erratic and not positioned enough for the throttle blades to be set right.

If you can't get the idle circuit rich enough with the right idle air bleed combo and transfer slot position without having to open up your throttle so much that you expose the transfer slot on the primary side beyond .040 then that is the time you have to start drilling holes in the butterflies to let enough air through to keep the right transfer slot position in relation to what idle speed you need. I have had cams making very low vacuum and with the correct idle calibration on my carb I did not have to drill holes and also the first important thing is you have to have enough initial timing as well for your camshaft.

I once had a comp cams big mother thumper hydraulic roller cam with a whopping 243/[email protected] on a 107 lsa and I had only 6 inches of vacuum in park. It was a beast to tune my 650 double pumper I had on the engine but with time and patience and changing the idle circuit a little at a time I finally got the sweet spot and I did not have to have my secondary side open up so much to show any transfer slot and my primary transfer slot was between the .020 and .040 max and I did not have to drill any holes in the plates and I kept my plugs clean and never fouled any of them out and the idle was just enough to be right for the cam and all things work well.

I just had it for a few months give or take one month or so and I had enough of it and all that massive amount of overlap and low vacuum and my truck did not like cruising around with it as its just to big. I listened to comps cams marketing junk about those cams and thought it would be fine well I learned the hard way.

Anyways post what exact carb you have and if you have changed out your idle feed restrictors and stuff I can help you out. Also you don't want your secondary side completely closed as you need them open up some in order to keep the throttle plates from sticking. You at least at a minimum want a 1/4 of a turn open. The final amount will depend on all things the position of your primary side and the correct idle speed you need and also keep your primary transfer slot opening in the range of about .20 to no more then .040 range. You want it to be as square as possible on the primary side but you don't want to have it completely closed off either.
 

·
Registered
Joined
·
4,172 Posts
Also on the throttle plate screws on all the base plates that use the thin throttle shafts they don't stake the screws because they don't protrude like the regular ones they use on there basic carbs and they use the short screws so they don't stick out to allow more air flow and they normally use red loctite and many times it can be kind of hard getting them out without stripping them or worse breaking off the heads in the shafts. I have been lucky to 99 percent of the time getting the out without them breaking or stripping but have had a few here and there and was lucky I got them out by drilling the brass styles ones out but the stainless ones I never had any of those break but did have a few that stripped.

I just took a tiny cut off wheel and cut a notch in it and used a straight blade screw driver and got it out that way. I always just use blue loctite on them if putting good screws back in but if ever I rebuild a base plate or put screws back in, I don't care if they stick out and don't worry about a few cfm and I use longer screws and stake them along with using blue loctite on top of them.
 

·
Registered
Joined
·
4,172 Posts
I looked from your previous posts and your using a 750 Quick fuel vacuum secondary slayer carb. I think I remember from some of your other posts you got a secondary metering block and you have it set up as a four corner idle and you put in .070 idle air bleeds front and back and you have been changing out the idle feed restrictors on your metering blocks on the front and back if I am correct trying to tune it with your bigger cam that your changing out to a more milder one. I know you had another Quick fuel carb as well but I can't remember what size or one if your the same person I talked to about it. Been many posts and been a while so if I am wrong my apologies.
 

·
More for Less Racer
Joined
·
20,110 Posts
What's the consensus on throttle plate screws , ? It doesn't appear the OP staked his ??
I don't stake on more serious performance carb builds, I Loctite since i use button head allen screws. I also trim screws flush to shaft.

Daily driver or mild build may get stock screws put back in and staked
 

·
Registered
Joined
·
423 Posts
LOCTITE is fine I just think the manufacturer didn’t want anybody messing with the plate alignment so they crimped. Could be less people are willing to go thru the process of filing down the crimps and source new screws......
 

·
Registered
Joined
·
4,172 Posts
I myself prefer to loctite but with blue and then also stake my screws. To me a few cfm lost is nothing compared to a 5 to 6 thousand dollar engine. Grinding the screws down is not that difficult for me. In all out racing I can see you want every square inch you can get but mine never sees any track time and my truck still hauls it when it need it to regardless of a few screw ends sticking out in the bores of the main body. Everybody has there way of how they like to do things. Nothing wrong either way.

My engine is not all out race and I also don't go for the most power and every ounce I can get and go conservative on things for my builds.
 

·
Race it, Don't rice it!
Joined
·
8,065 Posts
Lock tight or super glue works great and a pen torch releases them easily.

100 percent on the staked thing. That sorta effort of the detail doesn't net anything to the average guy at home other than kill some time.
 

·
Registered
Joined
·
157 Posts
Discussion Starter · #18 · (Edited)
I looked from your previous posts and your using a 750 Quick fuel vacuum secondary slayer carb. I think I remember from some of your other posts you got a secondary metering block and you have it set up as a four corner idle and you put in .070 idle air bleeds front and back and you have been changing out the idle feed restrictors on your metering blocks on the front and back if I am correct trying to tune it with your bigger cam that your changing out to a more milder one. I know you had another Quick fuel carb as well but I can't remember what size or one if your the same person I talked to about it. Been many posts and been a while so if I am wrong my apologies.
I hadn’t done that yet. I parked it for a c10 LS project and just sidelined it for a while. Now I’m back and just decided to go to a more streetable camshaft so thank god I didn’t do all that yet. The only thing drastic I did was drill the plates. I’ve already put them back to factory though. But you are right. It’s a 750 slayer carb. And I have put blue loctite on the plate screws. Didn’t stake them as they weren’t from the beginning. I also crudely measured the transfer slot on the primary and secondary and one of them was a bit longer. Cant say now which one. But I just covered the secondary with a square showing on the primary side. Idle screws are set to factory and choke is setup factory. Idle speed screw doesn’t engage yet. Jets are the same, metering blocks are the same.
 

·
Registered
Joined
·
157 Posts
Discussion Starter · #19 ·
What exact Quick Fuel carb do you have? I need to know and I can help you with what the factory settings were. Is this a vacuum secondary or a double pumper and is it a two corner idle or a four corner idle? I read your post real fast so I might have missed it but I did not see it from re reading it. If you say your accelerator pump arm hit your intake then that sounds like you have a double pumper as on some Edelbrock intakes the rear of the double pumper accelerator arm always will 99 percent of the time make contact with the pipe plug if your using it on the intake manifold or if you try to use a connector piece for either your pcv valve or your brake booster then it will most likely not clear and that is even with the low profile vacuum fitting you can get that has a lower head on it.

The Quick Fuel and other carbs you must be careful trying to equal the transfer slot with the front and back even with having a big cam because the rear transfer slot sits up about .030 higher on the Quick fuel brand carbs on either the billet or cast base plates and compared to a Holley cast base plate regardless of it being a double pumper or vacuum secondary, the quick fuel base plates will have a slightly longer and wider transfer slot on the front and rear vs a Holley brand. Also on a Holley brand carburetor the rear transfer slot will sit up even as much as .040 higher and trying to equal them out will end up with the secondary side open up to much and your idle will be erratic and not positioned enough for the throttle blades to be set right.

If you can't get the idle circuit rich enough with the right idle air bleed combo and transfer slot position without having to open up your throttle so much that you expose the transfer slot on the primary side beyond .040 then that is the time you have to start drilling holes in the butterflies to let enough air through to keep the right transfer slot position in relation to what idle speed you need. I have had cams making very low vacuum and with the correct idle calibration on my carb I did not have to drill holes and also the first important thing is you have to have enough initial timing as well for your camshaft.

I once had a comp cams big mother thumper hydraulic roller cam with a whopping 243/[email protected] on a 107 lsa and I had only 6 inches of vacuum in park. It was a beast to tune my 650 double pumper I had on the engine but with time and patience and changing the idle circuit a little at a time I finally got the sweet spot and I did not have to have my secondary side open up so much to show any transfer slot and my primary transfer slot was between the .020 and .040 max and I did not have to drill any holes in the plates and I kept my plugs clean and never fouled any of them out and the idle was just enough to be right for the cam and all things work well.

I just had it for a few months give or take one month or so and I had enough of it and all that massive amount of overlap and low vacuum and my truck did not like cruising around with it as its just to big. I listened to comps cams marketing junk about those cams and thought it would be fine well I learned the hard way.

Anyways post what exact carb you have and if you have changed out your idle feed restrictors and stuff I can help you out. Also you don't want your secondary side completely closed as you need them open up some in order to keep the throttle plates from sticking. You at least at a minimum want a 1/4 of a turn open. The final amount will depend on all things the position of your primary side and the correct idle speed you need and also keep your primary transfer slot opening in the range of about .20 to no more then .040 range. You want it to be as square as possible on the primary side but you don't want to have it completely closed off either.
quickfuel slayer 750cfm vacuum secondaries. Right now I’ve got the secondary covered (they open and shut seemlessly right now but I can crack the set screw open a hair to make sure they don’t catch) and the primary open to about a square. Carb isn’t on the motor yet. I’m hoping to idle this thing around 800 rpms with the 230/236 112LSA cam I’m going to.
 

·
Registered
Joined
·
4,172 Posts
Good luck with your next cam install and with the 750 slayer you should be able to tune it pretty easy with a little bit of patience as if I can tune a carb with a Comp Cam big mother thumper cam with only a two corner idle Holley double pumper and a previous Proform 650 vacuum secondary 4160 carb, and not drilling the base plate, I am sure you will be successful with this cam. I had a lunate voodoo hydraulic roller with very similar specs at .050 mark with a 110 lsa and I had only 12.5 inches of vacuum at idle and it took me all but an hour and I had the idle setup perfect with the came with all the proper adjustments and the idle not being pig rich but just enough to what the build needed.
 
1 - 20 of 21 Posts
Top