Ok, so I've got a 400SBC with an aftermarket intake, headers, ported large valve Iron Heads, and a pretty radical cam (for a stock TC that is) The cam specs are 224 234 Duration@.050, .465 .488 Lift and 107 117 Lobe Seperation. I have the stock torque converter on a 700R4. I put a Holley 750CFM Vac-secondary carb on it.

My problem, is that I CANNOT get the mixture right! I've purchased a air/fuel mixture gauge that works with the stock O2 sensor (I replaced it with a new one to make sure it was accurate), and first of all, it says it runs to rich while cruising, like WAY to rich (the spark plugs were also rather black). So I took it down 4 jet sizes, and it wouldn't even run, It would idle (because the jets don't matter there) but when I applied throttle it just leaned out and didn't do anything. SO I took it back up to only 2 sizes smaller than stock, and it seemed to run well. Although the mixture gauge still says it's running a little rich at cruising.

Here's the kicker though, it leans out HORRIBLY at any throttle between 5-10% throttle, and 80% I mean it just doesn't hardly do anything it leans out so bad! and I've put a vacuem gauge on it, and installed one size larger powervalve than stock, but I only have about 13Hg of vacuem at idle in gear, so I can't really go any bigger than 85PV can I? The thing is, it does it even if I right out the gear and am not excellerating any more, as long as I'm at "medium" throttle, it's WAY lean!

I havn't gone through the steps in the "holley tuning guide" yet, I"m going to though, but I don't see anything there that will help me? am I wrong? Thanks so much guys.

Stuart

 

Hmmm...did you set the floats correctly? Was the carb in good running condition when you put it on? How is your timing set? The stock O2 sensor probably wont be worth much until your AFR is really close to ideal. Also, with a 107LSA, the O2 sensor will be completely worthless at idle.

 

Yeah, I don't take what it says seriously at an idle, I figured the cam sould screw with it to much at that rpm. I think the floats are correct, and the timing was set by the racing shop down the road, He said he set it fairly conservative for the engine (didn't want to run it on the edge, it's a daily driver). I bought the carb from someone on craigslist, and replaced all the gaskets with the standard holley rebuild kit.

 

I'd say go through the Holley tuning sticky steps first and ignore the O2 for now.

As stated, narrow band doesn't give you a very good picture. On the other hand, wide band can make you pull your hair out trying to get it to behave under all conditions, even though the car seems to be running just fine.

 

ok I'll do that. I was actually going by the sound and feel of the engine, the gauge was just for confirmation of what the plugs and my own feelings were telling me. I should have said this before, it doesn't put out any more power above 20% throttle until you get to 80%, than it has like, 3In-HG, and pulls HARD. I would just like it to be smooth, not three settings, none, a little, and OMG.
Thanks again!

 

If I remember to I'll try to check my Holley Tuning book if I get home this weekend.

 

thanks man!

 

Ok, so I've been doing some research, and for street applications, a 750 might be a little to big a carb for easy tuning. I was thinking about going to a 650CFM Holley, would a double pumper be a good idea? or just a regular vac-secondary 650?

Also, does the age of the carb matter? I mean, I would think that they would still be making improvements, and that a 2 year old carb would be all around better than a 30 year old carb. but is that the case? Thanks.

Also, anyone have any experience with the 670 Street avenger? it's one I'm looking at. Thanks Again.

 

A 750 on a 400 shouldn't be too big. A 650 might have slightly better street manners, though.

A 30-year old Holley would be lacking power valve protection, but if otherwise "healthy", shouldn't be an issue (the PV check valve can be added).

I wouldn't recommend a VS carb. All negatives, no positives.

 

I was just saying "30 Years should make it better right? I mean, newer implies better desing etc..." Anywho, Why not a vacuem secondary? I figured that was best for streetability? I would LOVE more info on this.

I think I found a good carb though, it's a 670 Street Avenger.

 

That is the one that I bought but have yet to put on the motor. I've been too busy with work and just trying to get the motor running again otherwise it would already be on the motor.

Sorry I forgot about looking in the Holley tuning book, once again because of trying to get my motor running again. Let me know if you still want me to look in it.

 

I have a Holley Street Avenger 770 vacuum secondary on my 350 while I build my 406. Right out of the box it runs great except for a stumble when you put your foot down too far. I'm not going to chase the problem until I get the 406 in though.

 

The bigger the cam, the less it will like a factory/stoich A/F ratio, which is all your narrow-band can tell you.

I know your gague is reading "rich" but your symptoms are "lean". The fact it got worse when you went 4 sizes leaner really speaks to that.

In the few cases I've had access to a wideband on a cammed/modified motor, it always ran best with the A/F ratio in about the 13.5 range at part throttle. By the time it was up in the mid 14s it always ran pretty lousy as far as throttle response. And 13.5 is going to read as "pig rich" on a narrow band.

 

Oh, I know it's lean at accel, that's my problem, at mid-throttle, it leans WAY out, and doesn't really move any faster than at 10% throttle.

 

Ok so my 670 seems to work fine, although the accel-pump doesn't really work AT ALL. the wierd thing is, I used the bladder, pump cover, and cam from my old carb! and yet, it just dribbles into the carb. The thing is, I don't know which passageways etc to check in the carb. And where is the check valve for the nozzles anyways? I'm sooooo confused...

 

it comes out of the discharge nozzles on the top under the butterfly opening. here is my thread i posted for info on it. mid throttle problem can be transition slots or the power valve. remember your jets dont come into play until off idle.
https://www.thirdgen.org/forums/carb...e-nozzles.html

 

Hopefully I won't need to adjust this once I get my car started this weekend with the new Holley on. I have a 350 with the 670 Avenger so I'm hoping that I won't have that flat spot that you refer to.

 

ok, today, I'm going to try and take the nozzle from my old carb and put it on this one. If that doesn't do anything, I'm going to have to take the whole front appart, and test where the restriction in flow is coming from. Argh...

 

lol, wow, I feel stupid. I COMPLETELY missunderstood how the pump itself works, and put it together wrong. it was just pumping gas INTO THE BOWL lol... No wonder it sounded wierd... Anyways, the car runs GREAT now. I'm going to go through the "holley tuning guide" steps and everything, but I've got a great baseline I think.

I have one question though, the tuning guide talks about a "secondary gap" or something like that, and I have NO IDEA what that's referring to. Thanks.

 

There is an idle slot in each venturi, including the secondaries.

 

I'm glad you got it running good!

 

never mind, I take it back, it doesn't work at all... I just realized why I had to jet it up 5 notches at first, there is either something stuck in the carb, or it's badly coroded, because it can't get any fuel hardly at all. It has to be pulling HARD, and it has to be jetted WAY to rich, in order to get enough to barely run. I'm going to tear it apart and try and find the blockage. Any advice on where to look for a blockage inbetween the venturi's and the bowl? I mean, which passageways go to the venturi's? thanks again guys.