My C3 is quick now

[CaptainK]

Finally found the problem in my 1968 C3 - and its quite an obvious one when you think of it. For years now, ever since I've had my C3 to be honest, its never really been that quick, and never seemed to want to rev over 4k..... and getting there was a push as it was quick to 3.5k rpm, then slowly increase from there until it topped out around 4k.

When I first got the car, it had other driving issues, stalling, general not happy around town etc. Got a garage to swap out the carb (a fake chinesium Carter) and distributor for an Edelbrock 1406 and a HEI distributor for me. This solved all my problems except the revving above 4k issue.

Now, I'm not one for driving my C3 hard, I like to just cruise in it, and to be honest for a while I'd forget about its 4k issue. So I'd only really remember about it here and there over the years, and it wasn't a priority for me compared to other minor issues. Tried a few other things over the years - heat reduction, vacuum leaks, blah blah blah. Just resided myself to it not being that quick as it didn't bother me much and it'd be solved if and when I decided to do a 383 engine stroker like Andy's car as the engine builder would look at it etc.

Fast forward a bit more and I was in the engine bay last week checking levels and stuff and decided to take the air cleaner off to check inside that etc as not done that in years. It was then my brain had a lightbulb moment and thought something looked odd about the throttle cable. So I checked it, and it was all tense and seemed fine. But my brain wasn't happy, so I played with the throttle manually by hand at the carb, opening and closing it. All seemed fine. Then for some reason I decided to check it from the throttle pedal in the car, and noticed that the throttle was only opening around 1/2 - 2/3's from the pedal !!!

So in a nutshell, it turns out my throttle cable bracket on my carb is a bit odd and only held on with one bolt. It had slipped, and thus reduced the maximum throttle opening. I repositioned it, and tightened it up, but it slipped back immediately on first let off of throttle.

Anyway, fabricated a bracket extension to bolt to it that then forms an "L" shape around the base of the carb by the throttle bracket. This is tight and stops it slipping. I now have max throttle opening via the pedal. Took it for a test drive and fudge me when you put your foot right down it is a LOT quicker now and shoots up to 5k rpm with ease. I'm mechanically "huh?" with carbs, but I'm guessing before that with the limited throttle opening before that the secondaries weren't opening or whatever.

If anyone has a photo of what a proper SBC throttle bracket looks like on a C3, that'd be fab. I did Google and go on CK and other Corvette parts suppliers, but the brackets vary over the years and none of them look like what I have. I have a 1968 SBC too, so that probably doesn't help.

 

[Chuffer]

Finally found the problem in my 1968 C3 - and its quite an obvious one when you think of it. For years now, ever since I've had my C3 to be honest, its never really been that quick, and never seemed to want to rev over 4k..... and getting there was a push as it was quick to 3.5k rpm, then slowly increase from there until it topped out around 4k.

When I first got the car, it had other driving issues, stalling, general not happy around town etc. Got a garage to swap out the carb (a fake chinesium Carter) and distributor for an Edelbrock 1406 and a HEI distributor for me. This solved all my problems except the revving above 4k issue.

Now, I'm not one for driving my C3 hard, I like to just cruise in it, and to be honest for a while I'd forget about its 4k issue. So I'd only really remember about it here and there over the years, and it wasn't a priority for me compared to other minor issues. Tried a few other things over the years - heat reduction, vacuum leaks, blah blah blah. Just resided myself to it not being that quick as it didn't bother me much and it'd be solved if and when I decided to do a 383 engine stroker like Andy's car as the engine builder would look at it etc.

Fast forward a bit more and I was in the engine bay last week checking levels and stuff and decided to take the air cleaner off to check inside that etc as not done that in years. It was then my brain had a lightbulb moment and thought something looked odd about the throttle cable. So I checked it, and it was all tense and seemed fine. But my brain wasn't happy, so I played with the throttle manually by hand at the carb, opening and closing it. All seemed fine. Then for some reason I decided to check it from the throttle pedal in the car, and noticed that the throttle was only opening around 1/2 - 2/3's from the pedal !!!

So in a nutshell, it turns out my throttle cable bracket on my carb is a bit odd and only held on with one bolt. It had slipped, and thus reduced the maximum throttle opening. I repositioned it, and tightened it up, but it slipped back immediately on first let off of throttle.

Anyway, fabricated a bracket extension to bolt to it that then forms an "L" shape around the base of the carb by the throttle bracket. This is tight and stops it slipping. I now have max throttle opening via the pedal. Took it for a test drive and fudge me when you put your foot right down it is a LOT quicker now and shoots up to 5k rpm with ease. I'm mechanically "huh?" with carbs, but I'm guessing before that with the limited throttle opening before that the secondaries weren't opening or whatever.

If anyone has a photo of what a proper SBC throttle bracket looks like on a C3, that'd be fab. I did Google and go on CK and other Corvette parts suppliers, but the brackets vary over the years and none of them look like what I have. I have a 1968 SBC too, so that probably doesn't help.

It`s often the simple things that get overlooked and make all the difference .

 

[antijam]

This is the bracket set up on my '71......



As you say, the bracket is secured by a single carb mounting nut but in my case it's stabilised by a strap (arrowed) welded to the metal vacuum take off line for the brake booster. Whether this is original I don't know; it looks a bit of a bodge and may have been added by some PO to cure exactly the problem you were having.

 

[CaptainK]

Thanks for that - it looks better than mine. Just below your bracket bolt to the carb, it looks like you have a rivet or something going through to the carb. I have two bolt holes in that area to attach to carb, but carb only has one hole. I also don't have a stabilising bracket to hold it together.

Side note: I really should have taken a picture of what I have. Doh!

 

[Roscobbc]

The throttle bracket that Antijam shows is 100% correct.......for a Rochester carb. It will open the carb 100% (with a Rochester). Use it with a Holly and useless you frig about with the pedal inside the cabin you'll struggle to get 3/4 opening. Ask me how and why I know this. It is also compounded by a design and metallurgy issue with '68 year Vettes. Brackets holding pedal assembly in place were dodgy and like to brake. Bending the throttle pedal actuating arm doesn't help to get additional movement as the temperature of the material used is too soft. There simply isn't enough pedal/linkage movement available to guarantee 100% carb opening on a Holley.....perhaps if lucky 90%.
I eventually bought an aftermarket billet carb throttle cable bracket assembly. This allowed for a far better cable alignment and full throttle opening. However it created other issues re. Air filter clearance.....
Another saga ensued.......
Am I still getting 100% throttle opening? - don't know, don't care......with the engines power output I doubt if even 3/4 opening would make any real difference to getting the power down.

 

[Roscobbc]

This is the bracket set up on my '71......

View attachment 27424

As you say, the bracket is secured by a single carb mounting nut but in my case it's stabilised by a strap (arrowed) welded to the metal vacuum take off line for the brake booster. Whether this is original I don't know; it looks a bit of a bodge and may have been added by some PO to cure exactly the problem you were having.

See the small cut-out just to the right of the carb nut (holding the bracket on) - if I recall correctly there is a small 'tang' on the underside that rests against the side of the carb base, effectively locking it in place and stopping it (to a degree) from rotating on the carb fixing stud.

 

[Oneball]

Thought I’d check mine, it might be quick now too!!

This was full throttle

 

[johng]

Is that important on a race car??

 

[Oneball]

Is that important on a race car??

Might be

 

[Roscobbc]

Thought I’d check mine, it might be quick now too!!

This was full throttle View attachment 27426

Hope you won't be offended if I comment on this...... especially with you being to all of us a well respected race driver.........as the usually verbose John Mackinroe says "You can't be serious" !

 

[Roscobbc]

Just remember fella's most people fit a carb that's often too big for the capacity and use of the car.......don't beat yourself up about it.......it won't make that much of a difference (secondaries being open just a third will.......Oneball!)

 

[Oneball]

Hope you won't be offended if I comment on this...... especially with you being to all of us a well respected race driver.........as the usually verbose John Mackinroe says "You can't be serious" !

I know! It is something I do check but I can’t remember the last time I did. Back at Santa Pod on Monday so we’ll see what difference it makes to 1/4 mile.

 

[antijam]

See the small cut-out just to the right of the carb nut (holding the bracket on) - if I recall correctly there is a small 'tang' on the underside that rests against the side of the carb base, effectively locking it in place and stopping it (to a degree) from rotating on the carb fixing stud.

Actually there is no tang, I think the cut-out is there to facilitate the swaging of the right of the bracket. Captain K is right though; there is a rivet/dowel next to the mounting hole that engages another hole in the Quadrajet flange and prevents rotation about the fixing. I guess that extra hole is not normally present on most after market carbs and if fitting one the rivet/dowel would be removed.
The amateurish looking strap welded to the vacuum line is apparently original though. I assumed it could be there to brace the bracket but actually I think it's the other way round; the attachment to the bracket is meant to stabilise the vacuum pipe since if no servo is fitted the pipe wouldn't be there.

 

[Roscobbc]

Actually there is no tang, I think the cut-out is there to facilitate the swaging of the right of the bracket. Captain K is right though; there is a rivet/dowel next to the mounting hole that engages another hole in the Quadrajet flange and prevents rotation about the fixing. I guess that extra hole is not normally present on most after market carbs and if fitting one the rivet/dowel would be removed.
The amateurish looking strap welded to the vacuum line is apparently original though. I assumed it could be there to brace the bracket but actually I think it's the other way round; the attachment to the bracket is meant to stabilise the vacuum pipe since if no servo is fitted the pipe wouldn't be there.

Yet another '68 C3 introduction component that was soon updated?

 

[Emc]

This is the bracket set up on my '71......

View attachment 27424

As you say, the bracket is secured by a single carb mounting nut but in my case it's stabilised by a strap (arrowed) welded to the metal vacuum take off line for the brake booster. Whether this is original I don't know; it looks a bit of a bodge and may have been added by some PO to cure exactly the problem you were having.

The bracket is held in place with a carb bolt and a stud going into the carb base plate

 

[Grahamred70]

When I changed from a 600 cfm Holley to a Brawler 670 cfm on my 1970 C3 the throttle linkage worked fine and gave full opening with the throttle pedal all the way down but when I changed from an Edelbrock to a Holley on my Chevy C10 truck I had to make up a throttle linkage to increase the output to the carb to get full throttle, like a multiplier if you like, works OK if a little heavy on initial opening off idle.

 

[Mad4slalom]

Finally found the problem in my 1968 C3 - and its quite an obvious one when you think of it. For years now, ever since I've had my C3 to be honest, its never really been that quick, and never seemed to want to rev over 4k..... and getting there was a push as it was quick to 3.5k rpm, then slowly increase from there until it topped out around 4k.

When I first got the car, it had other driving issues, stalling, general not happy around town etc. Got a garage to swap out the carb (a fake chinesium Carter) and distributor for an Edelbrock 1406 and a HEI distributor for me. This solved all my problems except the revving above 4k issue.

Now, I'm not one for driving my C3 hard, I like to just cruise in it, and to be honest for a while I'd forget about its 4k issue. So I'd only really remember about it here and there over the years, and it wasn't a priority for me compared to other minor issues. Tried a few other things over the years - heat reduction, vacuum leaks, blah blah blah. Just resided myself to it not being that quick as it didn't bother me much and it'd be solved if and when I decided to do a 383 engine stroker like Andy's car as the engine builder would look at it etc.

Fast forward a bit more and I was in the engine bay last week checking levels and stuff and decided to take the air cleaner off to check inside that etc as not done that in years. It was then my brain had a lightbulb moment and thought something looked odd about the throttle cable. So I checked it, and it was all tense and seemed fine. But my brain wasn't happy, so I played with the throttle manually by hand at the carb, opening and closing it. All seemed fine. Then for some reason I decided to check it from the throttle pedal in the car, and noticed that the throttle was only opening around 1/2 - 2/3's from the pedal !!!

So in a nutshell, it turns out my throttle cable bracket on my carb is a bit odd and only held on with one bolt. It had slipped, and thus reduced the maximum throttle opening. I repositioned it, and tightened it up, but it slipped back immediately on first let off of throttle.

Anyway, fabricated a bracket extension to bolt to it that then forms an "L" shape around the base of the carb by the throttle bracket. This is tight and stops it slipping. I now have max throttle opening via the pedal. Took it for a test drive and fudge me when you put your foot right down it is a LOT quicker now and shoots up to 5k rpm with ease. I'm mechanically "huh?" with carbs, but I'm guessing before that with the limited throttle opening before that the secondaries weren't opening or whatever.

If anyone has a photo of what a proper SBC throttle bracket looks like on a C3, that'd be fab. I did Google and go on CK and other Corvette parts suppliers, but the brackets vary over the years and none of them look like what I have. I have a 1968 SBC too, so that probably doesn't help.

I found that the new stock throttle cable was too long , I had 5cm taken out and works perfectly with the holley carb bracket

 

[Roscobbc]

OK fella's please see below images of the Quadrajet 'specific' throttle cable bracket as OEM fitted to my April '68 L36. As you'll see it does have a small 'tang' that rests against the side of the carb base to prevent it rotating in use.
The aftermarket billet throttle cable bracket fits to driver side carb fixings. This does 'present' the cable in a far better position than stock, with great adjustability and better alignment.
The stock throttle cable bracket is shown next to carb stud where it would normally be fixed. The 'tang' on the underside. Presuming that later cars perhaps have a improved bracket?
Other images show the air fllter base almost 'sitting' on the bracket. GM's stock air filter base is the lowest filter base available but 'sat' on the bracket neccesitating replacement with a 'higher' aftermarket filter base and reduced thickness 2" air filter element (replacing stock 3" element).
You will notice the K & N 'Airstream' filter 'top'. This more than makes-up for the reduced thickness air filter element. Independant test have shown the top itself flowing far better than the stock style air filter.

 

[CaptainK]

OK fella's please see below images of the Quadrajet 'specific' throttle cable bracket as OEM fitted to my April '68 L36.

From memory, my 68 is built a month or two after yours. My current throttle bracket is different to yours. And yes, I keep forgetting to go out to the garage and take a photo.