Mr. Sinister
Contributor
Posts: 394
Age: 35
Loc: Fair Hill, MD
Reg: 05-18-09
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07-09-12 12:14 PM - Post#2246150
Hey guys, as some of you may know, I recently upgraded to a Holley 750 Street HP carb, and couldn't be happier with it.
I've got it set up pretty well, including jetting (had to go down 2 numbers on the primary, that's it). The last thing I want to do now is bump the in-gear idle speed up a bit. Currently, it will fluctuate between 600-700 (big cam), where with my old carb (Holley Street Avenger 670) it would idle in gear at 750, dead on, all the time, and never move more than 50rpm.
The car doesn't lurch with the lower idle like it did before getting the 670 set right, though. It idles smooth, even at 600. The problem is it doesn't generate enough vacuum at 600rpm, and my brakes get pretty stiff. And to a lesser degree of annoyance, it doesn't sound all that good at 600rpm. I'm not interested in installing any vacuum pumps or any of that stuff, because like I said, it would idle great in gear with the old carb.
I currently run 6psi of fuel pressure via electric pump and regulator, and fuel just barely trickles out of the sight hole in neutral at idle. It may be a hair low?
I set the curb idle with a vacuum gauge, it's set to where I get the most vacuum, but my understanding is this isn't ideal for all engines.
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Mr. Sinister
Contributor
Posts: 394
Age: 35
Loc: Fair Hill, MD
Reg: 05-18-09
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07-10-12 09:01 AM - Post#2246533
In response to Mr. Sinister
So any input on the correct way to increase this without screwing up the rest of my settings?
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models916
Frequent Contributor
Posts: 1879
Age: 62
Loc: Addison, IL
Reg: 05-28-10
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07-10-12 09:08 AM - Post#2246539
In response to Mr. Sinister
Turn the idle screw clockwise.
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 50hotrod
"2nd Year" Silver Supporting Member
Posts: 366
Age: 56
Loc: Wisconsin
Reg: 07-25-11
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07-10-12 09:20 AM - Post#2246540
In response to models916
Turn the idle screw clockwise.
About all you can do. The cam configuration dictates engine idle vacuum.
Well, you know what's wrong with the world today
People done gone and put their Bible's away
They're living by the law of the jungle not the law of the land
"Simple Man" By Charlie Daniels
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Mr. Sinister
Contributor
Posts: 394
Age: 35
Loc: Fair Hill, MD
Reg: 05-18-09
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07-10-12 10:54 AM - Post#2246589
In response to models916
Turn the idle screw clockwise.
That has no effect on in-gear idle, it just makes it drop further when I put it in gear. That and oepning the throttle blades more will expose too much of the transfer slot.
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Mr. Sinister
Contributor
Posts: 394
Age: 35
Loc: Fair Hill, MD
Reg: 05-18-09
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07-10-12 10:56 AM - Post#2246590
In response to 50hotrod
Turn the idle screw clockwise.
About all you can do. The cam configuration dictates engine idle vacuum.
Right, it's same cam I've always had, but with a new carb. If I had it working well with the old carb, I should be able to get it working just as good with the new carb that so far does everything else better but idle in gear.
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Mr. Sinister
Contributor
Posts: 394
Age: 35
Loc: Fair Hill, MD
Reg: 05-18-09
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07-10-12 11:10 AM - Post#2246595
In response to Mr. Sinister
The long and short of it is this: I'm not content to just drop a new carb on the car and call it "good enough", because what separates cars that perform "good enough" and the cars that perform exceptionally is taking the time to tune things the right way. Bolting on a carb and setting the curb speed is not good enough for me, and that's why this little engine performs the way is does. It's not a race car, but I do want it to perform on the street as well as it can within my knowledge and ability.
I tuned everything to work with the old carb as best I could, and now with a new, better performing carb on the engine, I have to start over. I asked questions, I listened to answers, and the difference from "good enough" to where it was is not an accident.
I'm asking what I'm asking because there is more than one single factor that controls the in-gear idle, and I'm looking for the best place to start, now that everything else is set where it needs to be set, including curb idle speed. I don't want to sound like a dick here, but if I didn't know the simple basics, I wouldn't be messing with it in the first place.
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 gotta56forme
"8th Year"Silver Supporting Member
Posts: 3282
Age: 50
Loc: West Seattle
Reg: 09-19-03
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07-10-12 11:51 AM - Post#2246606
In response to Mr. Sinister
The more learned here will correct me if I'm wrong, but when you went from a 670 to a 750, I'm wondering if the primary bores are a smidge bigger which has caused your idle vac signal to not pull as swiftly as the smaller bore carb. In other words, the smaller bore of the 670 may have had higher flow which sucked in more gas that made for a crisper idle.
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Mr. Sinister
Contributor
Posts: 394
Age: 35
Loc: Fair Hill, MD
Reg: 05-18-09
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07-10-12 12:22 PM - Post#2246618
In response to gotta56forme
The more learned here will correct me if I'm wrong, but when you went from a 670 to a 750, I'm wondering if the primary bores are a smidge bigger which has caused your idle vac signal to not pull as swiftly as the smaller bore carb. In other words, the smaller bore of the 670 may have had higher flow which sucked in more gas that made for a crisper idle.
I was thinking along those lines as well. The 670 had the choke removed and one of those stub stacks which are supposed to increase flow velocity, so not only were the bores smaller, but the velocity though them was probably higher as well. The 750 is an HP model, with no choke horn and the smoothed bores, as well.
It's still odd to me why it idles smooth even at 600rpm on the 750, when 600rpm on the 670 would make the car lurch and shake like crazy. It may just be a case of "it is what it is". I can accept that, but I want to know why anyway. I may bring the float level up a tad and maybe try adjusting the mixture screws with it in gear. It's a street car, so idle manners are important, particularly in gear.
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gotta56forme
"8th Year"Silver Supporting Member
Posts: 3282
Age: 50
Loc: West Seattle
Reg: 09-19-03
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07-10-12 12:32 PM - Post#2246621
In response to Mr. Sinister
Sorry, Mr Sinister, I guess I had it backwards, I thought the car was idling poorer on the 750 as compared to the 670. Sounds like it's idling better, yes? Maybe it's the difference betweeen a new, untampered-with carb vs. a carb that may have seen a variety of handiwork.
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Mr. Sinister
Contributor
Posts: 394
Age: 35
Loc: Fair Hill, MD
Reg: 05-18-09
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07-10-12 12:40 PM - Post#2246625
In response to gotta56forme
Sorry, Mr Sinister, I guess I had it backwards, I thought the car was idling poorer on the 750 as compared to the 670. Sounds like it's idling better, yes? Maybe it's the difference betweeen a new, untampered-with carb vs. a carb that may have seen a variety of handiwork.
Well, depends on what's better. The idle speed was better on the 670. It would idle at 750 in gear and not budge. I can't seem to get the 750 idle speed the same in gear as the 670, but for idling lower, it idles smoother than the 670 did at the same rpm. Where the 670 would lurch and shake at 600-700 rpm, the 750 is pretty smooth. The only issue this causes is a lower vacuum reading in gear, which makes my brakes stiff.
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gotta56forme
"8th Year"Silver Supporting Member
Posts: 3282
Age: 50
Loc: West Seattle
Reg: 09-19-03
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07-10-12 12:44 PM - Post#2246629
In response to Mr. Sinister
If it's been asked already, I apologize. Do you know what your idle vac was on the 670 vs. your new 750, both in & out of gear? If there was a measurable drop, maybe you have a vac leak?
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Mr. Sinister
Contributor
Posts: 394
Age: 35
Loc: Fair Hill, MD
Reg: 05-18-09
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07-10-12 12:50 PM - Post#2246638
In response to gotta56forme
If it's been asked already, I apologize. Do you know what your idle vac was on the 670 vs. your new 750, both in & out of gear? If there was a measurable drop, maybe you have a vac leak?
Actually, I have 13" of vacuum at 950rpm on the 750, where I could only get 11" with the 670.
I've never checked it in gear. I don't suspect a vacuum leak, since it runs like a champ otherwise.
Edited by Mr. Sinister on 07-10-12 12:51 PM. Reason for edit: No reason given.
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50hotrod
"2nd Year" Silver Supporting Member
Posts: 366
Age: 56
Loc: Wisconsin
Reg: 07-25-11
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07-10-12 01:02 PM - Post#2246642
In response to Mr. Sinister
Do you have room to advance the timing? That could correct some of the problem.
As far as air screw adjustments, put the car in drive, set the idle speed at 750, now adjust the air screws. If the RPM increases re-adjust the idle speed screw. Sounds like you could use a little more fuel on the idle circuit.
Well, you know what's wrong with the world today
People done gone and put their Bible's away
They're living by the law of the jungle not the law of the land
"Simple Man" By Charlie Daniels
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Mr. Sinister
Contributor
Posts: 394
Age: 35
Loc: Fair Hill, MD
Reg: 05-18-09
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07-10-12 01:18 PM - Post#2246649
In response to 50hotrod
Do you have room to advance the timing? That could correct some of the problem.
As far as air screw adjustments, put the car in drive, set the idle speed at 750, now adjust the air screws. If the RPM increases re-adjust the idle speed screw. Sounds like you could use a little more fuel on the idle circuit.
Eh, not much if any. I'm at 15* now (vacuum disconnected). It's just on the cusp of making the starter labor. I went with a low vacuum advance can to get my vacuum advance working properly last year. I did have to fab a plate to control vacuum advance at part throttle, and it needed adjusting when I put the new carb on. Probably due to the added vacuum? Either way, not really much room to go up. I did bump it up 1* when I put this carb on to get the in gear idle where it is now. I could try 1* more and see how it goes....
I suspected a lean idle as well. I had the air/fuel screws turned out 3.25 turns on the 670 to get the highest vacuum reading, where it's 2.5 on the 750. So I'm using less fuel at idle with a bigger carb. Kinda counter-intuitive in my mind. Part of why I mentioned peak vacuum reading at idle not always being ideal for all engines in my original post. This HP is more sensitive to changes than the 670 was, this is why I'm being so anal about it.
Edited by Mr. Sinister on 07-10-12 01:19 PM. Reason for edit: No reason given.
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 Rick_L
Honored Member
Posts: 24741
Loc: Katy, Tx, USA
Reg: 07-06-00
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07-10-12 01:20 PM - Post#2246652
In response to 50hotrod
You are making this too hard. Idle fueling is just a controlled leak. Throttle bore size has nothing to do with the way it idles. Besides there simply isn't that much difference between a 670 and 750.
Likewise on the transfer slot. It's only going to take a tiny part of a turn to increase the in gear idle speed if you really want to increase it.
Just do it, if there's a problem after doing it, then post back.
And once you do it, you may or may not like it.
Usually the problem to solve is the opposite one - idles too fast.
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Mr. Sinister
Contributor
Posts: 394
Age: 35
Loc: Fair Hill, MD
Reg: 05-18-09
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07-10-12 01:29 PM - Post#2246657
In response to Rick_L
You are making this too hard. Idle fueling is just a controlled leak. Throttle bore size has nothing to do with the way it idles. Besides there simply isn't that much difference between a 670 and 750.
Likewise on the transfer slot. It's only going to take a tiny part of a turn to increase the in gear idle speed if you really want to increase it.
Just do it, if there's a problem after doing it, then post back.
And once you do it, you may or may not like it.
Usually the problem to solve is the opposite one - idles too fast.
I have a tendency to do that, lol.
I seem to remember trying that to no real avail (the curb speed screw), but maybe not....
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Mr. Sinister
Contributor
Posts: 394
Age: 35
Loc: Fair Hill, MD
Reg: 05-18-09
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07-12-12 05:14 PM - Post#2247550
In response to Mr. Sinister
Finally got a chance to mess with it earlier. Tried the curb speed screw and it would increase the idle in park, but not really anything in gear. I would put it back in neutral and it would go back to idling at 950 where I had it set unless I gave it a good rev. Figured something wasn't right, so I reset the float level so get a little more fuel trickling out, and reset the curb screw. That seems to have done the trick. Idling great in and out of gear now.
This carb is nothing short of amazing with the overall difference it's made. More power, better manners, and better fuel mileage due to less pedal effort at low speed. Throttle response is razor sharp now with the idle sorted, and it just takes a little blip of the throttle to spin the tires. People kept telling me a 750 was too big for this engine (11:1 350, Ported Performer RPM heads, big Comp roller cam). Shows what they know.
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50hotrod
"2nd Year" Silver Supporting Member
Posts: 366
Age: 56
Loc: Wisconsin
Reg: 07-25-11
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07-13-12 10:10 AM - Post#2247789
In response to Mr. Sinister
Great Job Sinister! Never settle for good enough.
People told me not to run a Tri-power set up because they never run right. I guess that's why I got it dirt cheap from the guy. He is still shaking his head in wonderment every time I see him. Go figure.
By the way, you have a beautiful car there. I listened to your engine on the youtube clip. Did you build that engine? Sounds perfect to me.
Well, you know what's wrong with the world today
People done gone and put their Bible's away
They're living by the law of the jungle not the law of the land
"Simple Man" By Charlie Daniels
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Mr. Sinister
Contributor
Posts: 394
Age: 35
Loc: Fair Hill, MD
Reg: 05-18-09
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07-14-12 07:49 AM - Post#2248089
In response to 50hotrod
Great Job Sinister! Never settle for good enough.
People told me not to run a Tri-power set up because they never run right. I guess that's why I got it dirt cheap from the guy. He is still shaking his head in wonderment every time I see him. Go figure.
By the way, you have a beautiful car there. I listened to your engine on the youtube clip. Did you build that engine? Sounds perfect to me.
Thanks, I did build it myself. This was my first full Chevy build, I did a few ford engines back in my mustang days. Used a 10k mile vortec 350 out of a Chevy HD pickup (so it's a 4 bolt main block). Didn't have to touch the bores, they looked brand new. Reused the entire bottom end, but reassembled it with arp fasteners, and put a set of Keith Black 10.8:1 slugs in it. With the heads, it equals out to 11:1. Never did the math on the DCR, but it's also no race motor, so I'm not terribly concerned with it, since it runs pretty strong. Cam is a Comp XR294HR piece, comp solid pushrods, scorpion 1.5 rockers, and a performer rpm air gap intake. There's a few pics under the hood in my photobucket link. The car runs pretty well for what it is, but some shorter gears and more stall would make it an animal. I do need a new rear though.
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MikeB
Senior Member
Posts: 9388
Loc: Plano, TX
Reg: 08-28-03
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07-14-12 02:39 PM - Post#2248189
In response to Mr. Sinister
I recently fixed a problem where idle speed would drop from 900 RPM to 500 RPM when I put the trans in Drive. After playing with the idle mixture screws for a long while, I ended up increasing manifold vacuum by 1-2 inches, and idle speed now dropped only 200 RPM in Drive.
You have probably already done that, but I thought I had, too. But if I did, it was 7-8 years ago!
The 350 engine has a fairly mild cam (that now makes 19" at idle), and the Hughes GM11TOW torque convertor is 100-200 RPM looser than stock.
Ah, the beauty of a stick shift in a car with a big cam. 
Real Hot Rods have a Clutch!
1955 210 2dr: 327, Brodix IK180 heads, Jones cam, Muncie M20, Wilwood front brakes
1969 C-10 pickup: 350, TH350
My car pictures
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Edited by MikeB on 07-14-12 02:40 PM. Reason for edit: No reason given.
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fuzzyff
Forum Newbie
Posts: 45
Loc: CA
Reg: 06-13-12
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07-17-12 12:12 PM - Post#2249128
In response to Mr. Sinister
I really think that the use of Rhoads lifters and/or a vacuum reserve storage tank would totally eliminate your problem, and would be an overall benefit with your setup.
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Mr. Sinister
Contributor
Posts: 394
Age: 35
Loc: Fair Hill, MD
Reg: 05-18-09
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07-18-12 05:19 AM - Post#2249362
In response to MikeB
Ah, the beauty of a stick shift in a car with a big cam.
One of these days, I will put a manual in it.
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MikeB
Senior Member
Posts: 9388
Loc: Plano, TX
Reg: 08-28-03
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07-19-12 05:48 PM - Post#2250042
In response to Mr. Sinister
Didn't read all the posts above, but what happens to the timing when you go from Park to Drive? May be a slim chance, but it could be the mechanical advance has just started to kick in at 950 RPM, then totally goes away when you put the trans in drive. So you'd have both the torque converter and spark advance working to drop the idle.
Real Hot Rods have a Clutch!
1955 210 2dr: 327, Brodix IK180 heads, Jones cam, Muncie M20, Wilwood front brakes
1969 C-10 pickup: 350, TH350
My car pictures
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Mr. Sinister
Contributor
Posts: 394
Age: 35
Loc: Fair Hill, MD
Reg: 05-18-09
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07-19-12 09:17 PM - Post#2250124
In response to MikeB
Didn't read all the posts above, but what happens to the timing when you go from Park to Drive? May be a slim chance, but it could be the mechanical advance has just started to kick in at 950 RPM, then totally goes away when you put the trans in drive. So you'd have both the torque converter and spark advance working to drop the idle.
Never checked the timing in gear, but I seem to have the problem solved. It was a combination of curb speed and float level.
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