1
News:
2000+ Members and Counting!
The best Quadrajet Carburetor Forum on the web.
Recent Posts
2
Quadrajet Carb Talk and Tips / Re: Base plate swap
« Last post by lightning boy on April 23, 2024, 01:36:50 PM »Robb MC makes a nice one made in USA.
3
Dialing in your rebuilt Quadrajet carburetor / Re: Mods on a 7041262 for a RAIV 455 engine. Are they OK?
« Last post by MattK2 on April 23, 2024, 11:18:29 AM »Coil gets proper voltage.
Jets went down in size from 77 to 73. Rods remained at 43.
APT at max rich.
The way I read your tip-in procedure, anything over 100 rev increase was out of range. You used 50-100 revs. I had nearly 260+ rev increase at tip in. Used the choke valve. Went from 2000 to 2260. That’s not “close” from what I read. Conclusion, it needs a little more fuel. Already increased the air by bumping the tubes from 36 to 38.
I’m willing to try other parts. I don’t have much to choose from so I’ll source what I think I need and buy it. Just looking for a suggestion on what a possible part change could be. Smaller rod? Perhaps a 75 jet. I know what you mean about the jets controlling higher rev and how the rods control lower revs when party raised.
Jets went down in size from 77 to 73. Rods remained at 43.
APT at max rich.
The way I read your tip-in procedure, anything over 100 rev increase was out of range. You used 50-100 revs. I had nearly 260+ rev increase at tip in. Used the choke valve. Went from 2000 to 2260. That’s not “close” from what I read. Conclusion, it needs a little more fuel. Already increased the air by bumping the tubes from 36 to 38.
I’m willing to try other parts. I don’t have much to choose from so I’ll source what I think I need and buy it. Just looking for a suggestion on what a possible part change could be. Smaller rod? Perhaps a 75 jet. I know what you mean about the jets controlling higher rev and how the rods control lower revs when party raised.
4
Quadrajet Carb Talk and Tips / Re: Little help here, please (part 2 of 2)
« Last post by King Scooter on April 23, 2024, 06:53:54 AM »Novadude, uncanny how similar our builds are. Makes me think there is more vacuum to be had. Thanks for the input.
Cliff, duly noted and I agree with everything you say. Hindsight being 20/20 I would do a few things different. Now I'm just working with what I have, to get what I can.
You sent me some good distributor/timing info and I will be doing some work on that once the car is back on the road. Maybe with some distributor and carb tuning, I can find some more vacuum.
I'll resize my lower IABs to .063-.067 and then follow recipe 2. I can always drill bigger, but it's hard to drill smaller!
Thanks
Cliff, duly noted and I agree with everything you say. Hindsight being 20/20 I would do a few things different. Now I'm just working with what I have, to get what I can.
You sent me some good distributor/timing info and I will be doing some work on that once the car is back on the road. Maybe with some distributor and carb tuning, I can find some more vacuum.
I'll resize my lower IABs to .063-.067 and then follow recipe 2. I can always drill bigger, but it's hard to drill smaller!
Thanks
5
Quadrajet Carb Talk and Tips / Re: Little help here, please (part 2 of 2)
« Last post by novadude on April 23, 2024, 05:56:48 AM »Cliff... My 9.6:1 355 with a 270/278, 217/225 @ 0.050", 108 LSA Hyd roller cam idles at 750 rpm with ~15.5" vacuum. I am running 12 deg initial with the vacuum advance on ported. Nice broad powerband with a ton of midrange torque (i shift it around 5500-5800 rpm). I can leave it in 4th gear (Muncie) going around corners in the neighborhood and can easily accelerate from ~1000 rpm without downshifting with no lugging, bucking, etc. I think the short duration is key.
I definitely agree with you on shrinking the lower idle air bleed on these late 70s Chevy carbs. Seems to make a nice improvement, and as you said, you can run much smaller idle tubes & DCRs.
I definitely agree with you on shrinking the lower idle air bleed on these late 70s Chevy carbs. Seems to make a nice improvement, and as you said, you can run much smaller idle tubes & DCRs.
6
Quadrajet Carb Talk and Tips / Re: Little help here, please (part one of 2)
« Last post by King Scooter on April 23, 2024, 05:04:25 AM »I can't say that Chris recommended these springs. They are the GM LS6 beehives that normally get installed on the GM HotCam combination. I had a brand new set in my inventory and both Chris and Pete (CPMotorworks) who did my heads, said they should be fine for what I was trying to accomplish.
Not looking to set the world on fire, just wanted a good running, efficient but aggressive cruiser. Max RPM under 6000 rpm. BUT...the engine does begin to cut btwn 5700 & 5800.
Could be any of these things that I haven't begun to chase down yet...1. Soft springs, as you mention. 2. Pertronix not happy above 5800. 3. The rev limiter dial on my Pertronix, set at 6000, isn't really 6000. 4. OEM tach isn't accurate.
Once I get the car back on the road, I'll be looking into this.
Not looking to set the world on fire, just wanted a good running, efficient but aggressive cruiser. Max RPM under 6000 rpm. BUT...the engine does begin to cut btwn 5700 & 5800.
Could be any of these things that I haven't begun to chase down yet...1. Soft springs, as you mention. 2. Pertronix not happy above 5800. 3. The rev limiter dial on my Pertronix, set at 6000, isn't really 6000. 4. OEM tach isn't accurate.
Once I get the car back on the road, I'll be looking into this.
7
Quadrajet Carb Talk and Tips / Re: Base plate swap
« Last post by Cliff Ruggles on April 22, 2024, 08:28:02 PM »If you only need a different linkage arrangement install the primary shaft from the donor baseplate only and keep the original baseplate with the carburetor.
I'm not sure what's out there for regulators these days. So much of this stuff comes from off-shore difficult to say what's going to make the grade and what isn't. For decades I ran a Holley regulator in my fuel system and it was flawless. At one point I bought a really expensive Mallory regulator and it was a piece of bovine excrement right out of the box and so was the one they replaced it with under warranty.
The swap was done to provide a bypass back to the tank. I ended up going back to the Holley regulator and making my own bypass......
I'm not sure what's out there for regulators these days. So much of this stuff comes from off-shore difficult to say what's going to make the grade and what isn't. For decades I ran a Holley regulator in my fuel system and it was flawless. At one point I bought a really expensive Mallory regulator and it was a piece of bovine excrement right out of the box and so was the one they replaced it with under warranty.
The swap was done to provide a bypass back to the tank. I ended up going back to the Holley regulator and making my own bypass......
8
Quadrajet Carb Talk and Tips / Re: Little help here, please (part 2 of 2)
« Last post by Cliff Ruggles on April 22, 2024, 08:22:52 PM »"Timing: 17d initial at 800 rpm, 34d total at 2850. Vacuum advance limited to 10d all in at idle. Cliff feels I may have too much final so I will work with this
Vacuum: 14.5” at 850 rpm idle, 12” at 700 rpm curb."
If it takes 27 degrees initial timing to get 12" vacuum at 700rpm's the engine is telling you that it could have used another full point of compression and/or the cam moved out to a wider LSA.
For some reason that I'll never understand is running mid-9's for compression then installing a cam on a really tight LSA. Tight LSA narrows the power curve, pulls power down in the RPM range, peak VE occurs earlier, and spikes cylinder pressure pretty high lower in the RPM range at the same time.
A good rule of thumb for compression vs cam events is that your engine build should produce good vacuum at idle without a lot of timing in it. I like to see around 13-14" vacuum at 750rpm's with about 10-12 degrees initial timing. This tells me the engine is pretty happy with the cam choice and I always use higher compression and longer seat timing so the same engine has strong upper mid-range and top end power but still idles well and efficient in the normal driving range.
Anyhow, I'm not surprised in the least that a 350 engine build with 9.6 compression and a decent size cam on a 108 or 109LSA is going to like, want, need and respond well to a LOT of timing at idle plus require good idle fuel delivery to the mixture screws at relatively "low" vacuum.
That's just the way it is with these things and why I provide "recipes" to help get carburetors up to par for what they are being used on.
If you really want to help out a 17057204 or any other Chevy Q-jet from the mid to late 1970's reduce the size of the .078" lower IAB's to .063-.067" before you do anything else. That is the first vent point above the idle mixtures screws and once you size them down EVERYTHING gets better and you'll find you don't need large IFR's, or DCR's either.......
Vacuum: 14.5” at 850 rpm idle, 12” at 700 rpm curb."
If it takes 27 degrees initial timing to get 12" vacuum at 700rpm's the engine is telling you that it could have used another full point of compression and/or the cam moved out to a wider LSA.
For some reason that I'll never understand is running mid-9's for compression then installing a cam on a really tight LSA. Tight LSA narrows the power curve, pulls power down in the RPM range, peak VE occurs earlier, and spikes cylinder pressure pretty high lower in the RPM range at the same time.
A good rule of thumb for compression vs cam events is that your engine build should produce good vacuum at idle without a lot of timing in it. I like to see around 13-14" vacuum at 750rpm's with about 10-12 degrees initial timing. This tells me the engine is pretty happy with the cam choice and I always use higher compression and longer seat timing so the same engine has strong upper mid-range and top end power but still idles well and efficient in the normal driving range.
Anyhow, I'm not surprised in the least that a 350 engine build with 9.6 compression and a decent size cam on a 108 or 109LSA is going to like, want, need and respond well to a LOT of timing at idle plus require good idle fuel delivery to the mixture screws at relatively "low" vacuum.
That's just the way it is with these things and why I provide "recipes" to help get carburetors up to par for what they are being used on.
If you really want to help out a 17057204 or any other Chevy Q-jet from the mid to late 1970's reduce the size of the .078" lower IAB's to .063-.067" before you do anything else. That is the first vent point above the idle mixtures screws and once you size them down EVERYTHING gets better and you'll find you don't need large IFR's, or DCR's either.......
9
Dialing in your rebuilt Quadrajet carburetor / Re: Mods on a 7041262 for a RAIV 455 engine. Are they OK?
« Last post by Cliff Ruggles on April 22, 2024, 07:54:49 PM »Just a note on tuning with jets. Changing jet size tunes both heavy and light part throttle.
Tuning with primary metering rods only effects light part throttle or the "normal" driving range.
Any heavy or full throttle rods are up and you are on the jets plus the .026" end of the rods.
It is best to tune for jet size first by doing some heavy part throttle driving without the secondaries. Once you have found the ideal jet size then tune light load/light throttle driving with the metering rods. Idle system mods have an impact there as well since some of the fuel comes from the transfer slots.
The "tip in" test is used to tell you when you are close but final testing is always done by looking for the best results. Timing is a player for part throttle tuning as well as the vacuum advance is applied when engine load is light and engine vacuum is high.
It's really not all that complicated but for tuning in the normal driving range you need to do both timing and fuel to get the best results. What makes it not so complicated is that all N/A engines run pretty much in the same range for A/F. I also know what needs to be done for any particular engine combo having spent nearly half a Century figuring all that out.
What I've seen so many times over the years is that folks want to yank the top off these carbs and start throwing bigger jets in them, when more times than not the jet size was adequate and they just needed to change to slightly smaller primary metering rods.......
Tuning with primary metering rods only effects light part throttle or the "normal" driving range.
Any heavy or full throttle rods are up and you are on the jets plus the .026" end of the rods.
It is best to tune for jet size first by doing some heavy part throttle driving without the secondaries. Once you have found the ideal jet size then tune light load/light throttle driving with the metering rods. Idle system mods have an impact there as well since some of the fuel comes from the transfer slots.
The "tip in" test is used to tell you when you are close but final testing is always done by looking for the best results. Timing is a player for part throttle tuning as well as the vacuum advance is applied when engine load is light and engine vacuum is high.
It's really not all that complicated but for tuning in the normal driving range you need to do both timing and fuel to get the best results. What makes it not so complicated is that all N/A engines run pretty much in the same range for A/F. I also know what needs to be done for any particular engine combo having spent nearly half a Century figuring all that out.
What I've seen so many times over the years is that folks want to yank the top off these carbs and start throwing bigger jets in them, when more times than not the jet size was adequate and they just needed to change to slightly smaller primary metering rods.......
10
Quadrajet Carb Talk and Tips / Base plate swap
« Last post by hugginjugs on April 22, 2024, 07:36:51 PM »I have a 1978 Transam with carb number 17058276. It hasnt been used in a few years as I decided to run a Holley Sniper. Part of the reason I chose to go Sniper was because it has a throttle blade that will work with a 200 4R TV cable. The Sniper has let me down for the last time, and I'm switching back. I picked up a parts carb, number 17083253. I has the proper throttle blade for use with a TV cable. It is going to need new bushings, but I would like to verify that this base will work with my original carb before I invest in a bushing kit.
Thank you.
PS I have decided to keep my electric fuel pump and use a regulator. Whats my best choice for a regulator?
Thank you.
PS I have decided to keep my electric fuel pump and use a regulator. Whats my best choice for a regulator?