Guys,

I just replaced the Edelbrock 1406 on my 67 Chevelle with a new Holly 670 street avenger carb hoping to step up the performance. So far, the Edelbrock has the Holly beat! I'm sure I just need help with tuning to correct this.

Out of the box the holly had a terrible stumble (for like 3 seconds) when I got into it. I set the float levels (it was too high) and that did not produce much help. I also changed to a lighter vacume spring, no help. Tried a stiff vacume sping and that did help. But I still have a bit of a stumble on full throttle and it just does not seem to pull as hard as the Edebrock did. Jetting out of the box is 65 primary and 68 secondary.

Any suggestions to tune this up to a higher level of performance? Engine is a 355 vortec heads running about 325 horses at the flywheel and a 5 speed w/ 355 gears.

try a larger squirter, go up 3 sizes at a time.

i went from #30 to #37 on both sides of mine to really make a hard leaving launch. #35 cleared it up, and #37 worked better with cold water temps.

Whats your timing set at?

Remember to change one thing at a time! As stated, try larger squirters for your off-idle stumble. If it stumbles while cruising, you're likely running too lean and/or your secondaries are kicking in too soon.

Originally posted by mr 4 speed:
Whats your timing set at? Set at 16 initial and 34 total which the vortecs seem to like. I need to get a tuning kit with some jets, accel. pump cams, etc. The summit catalog shows the parts but the kits seem really high priced :eek: Guess I'll just go ahead and buy a kit. Better than paying someone else to tune it. I need to learn how to do this anyway.

Yes, the AP tuning kits are expensive. Try a squirter change first. You can get them individually Here is a graph (http://www.chevelles.com/cgi-bin/forum/ultimatebb.cgi/topic/4/21904.html?#000000) showing the various pump cams. Look at what you have and what you might need if the squirter change doesn't fix the problem. It is much less money to get the parts individually since you're not buying unneeded parts. Remember, you'll only be using one set of cams and one set of squirters. The rest will be pretty much useless to you.

Also, you can save about half using the standard squirter instead of the ones with the little tubes. I doubt you'd see any performance difference.

Yep, Pat's right, try to stay away from buy all the cams together. I ended up having to do that because the speed shop by my school was the only one that had the pump cams on hand when I needed them, but they wouldn't sell them individually. Now I have a bunch of cams I probably wont need until maybe years down the road when I have a set-up that requires a lot more fuel.

Anyway, I can't help with the off-idle stumble, as mine went away with ignition tuning. However, the full throttle stumble is kinda tricky, or was for me. At first it would stumble then really smoothly and slowly get on the ball again if I left my foot in it. When I opened it up, I saw that my carb came with the stiffest (black spring), I was reluctant to go lighter, thinking it would make it worse. I went all the way down to the plain spring, and that really helped out, no more stumble on the street with full throttle. Then I went even lighter, and it ran better on the track, but on the street it'll get a very quick bog when the secondaries open and quickly returns to normal.

So I think it is a very small bog when you go WOT, try a stiffer spring until it goes away. If is a long bog, try a lighter one until it goes away.

I think the avenger comes with the quick change cap on it? It's pretty much just trial and error with the secondary springs.

Good luck.

Since this is a vacume secondary carb, there is only one pump squirter nozzel, right? I saw a reference earlier to changing both sides?

Yeah, they only have one, at least mine only has one. Mine actually has a boss for the second one on the secondary side of the main body, but it is blind. I have a friend's older carb, same list number, that does not have the boss for the secondary squirter. I have no Idea if a secondary squirter could be added, I doubt it, just find it interesting that the main body has a boss for it.

That's one other thing that irritates me, I wanted to try a smaller squirter in my brother's 307, and nobody would sell me just one squirter, had to buy a pair. So I just didn't buy it.

Wow, if I'm not mistaken my 670 had 72 jets in it out of the box. Very interesting!

I have a 31 squirter in my 750 3310 (feels like it works good as is)..next time at the track I'll try a 35 and a 37 and see what happens...thanks.
Hopefully the conversion to a secondary metering block and #80 jets as a starting point will improve my 103.9 MPH

I have the same issue with the 770 on my big block. Bad stumble right off idle. I'm also dealing with a situation that acts like either a real high float level (but it's not, I've checked it - more than once) or a stuck choke (but can see the choke blade is open). Ironically though, I can get a good adjustment with the mixture screws at about 2 turns out, so I assume my primary jets are about right.

2 turns out? Isn't that a lot? Well, maybe it isn't for a big block? My small block (granted, really low compression, tiny cam, 355ci w/ a 600cfm carb) liked somwhere between 1 1/4 and 1 1/2 turns. After that the vacuum drops off, less than that, vacuum drops off abruptly.

That's just my experience though, I'm far from knowledged in carb technology.