I am thinking of putting a Holley 670 Street Avenger on my recently rebuilt SBO350. I've had the q-jet adjusted by a very knowledgeable q-jet/olds afficienado but it still idles poorly -


my question is my intake is spread bore and I will need to use an adapter (e-brock manifold) - are they problematic? Also, how much trouble getting the throttle linkage modified for the Holley?

 

If your going to change carbs change the manifold too. In all seriousness the Quad is the way to go for your mild build. I'd see if you can get it squared away. There should be no major mods to your linkage by changing the carb. You may have to adapt the way it connects to the carb and make sure it goes to WOT.

 

does anyone else make a spreadbore carb? My manifold is new and was pricey

 

What manifold is on there? Holley has a few spreadbore replacements, they are 650 and 800 CFM.

 

2711

 

Either a properly rebuilt Qjet, Street Demon or the Holley 650 spreadbore. All should work with out an adapter.

 

If you are going holly spend extra go quick fuel.JMO

 

The 2711 manifold is compatible with a spread or squarebore carb.

 

What are the specs on your 350? You should be able to run the Street Demon 625 or 750, which is a 3 barrel based off the Thermoquad that fits square and spreadbore flanges. The Quick fuel carbs are more refined versions of the classic Holley carb. The Holley spreadbore has a base that looks like it came off a Qjet, so it will bolt right on. Anything other than Qjet will require fuel line mods.

 

I run demon carbs and not a fan of the 625. The 750 is a different story.I know of three other guys using demons with no problems besides myself. I have two demons.One is a 750 street demon the other a 850 mighty demon.My next carb will be a quick fuel.

 

The confusion comes because Demon makes mostly Holley style carbs, but the Street Demon is totally different. It is basically a Thermoquad with a bunch of improvements and new 3 barrel castings. I was very close to buying the 750 but figured after buying the carb and geometry corrector for my 2004R, a custom tuned Qjet was actually cheaper since I had a good 78 800 cfm core.

 

It idled fine from the factory, so maybe you should figure out what's wrong before swapping carbs. You may very well have a problem not related to the carb and end up spending money on a new carb that doesn't fix the problem. Unless you have too much money, try free troubleshooting FIRST.

 

My 750 street demon looks nothing like a thermoquad. I did see a street demon 750 on summit site that is a basic thermoquad go figure holly changed the demon. Joe you maybe right about the op qjet but i hate them and chuck them in the trash.My nick name for them is quadrajunk.LOL

 

What does "idle poorly" mean. What kind of vacuum is it getting, what idle rpm does it like the best? Is it getting hot? Smoking?

-I had a quadrajet on my 307 for 10 years that quit when the sh*t gas ate away the needle/seat- great carb.
-When it died I panicked and let somebody talk me into an edlebrock 1406 - Worst carb I've ever had. Bog, erratic fast idle..just don't get an edlebrock.
-I'm still in the process of tuning my quick fuel but it's a real dream to work with- float adjustments, air bleed access/ single screw secondary adjustments- readily available parts...find me a 750 cfm carb with an electric choke near $300.00 that can match it.

 

it is a rough idle, thready - the 350 has a 262 cam in it and while I don't have the numbers, my olds buddy in Plano TX has torn this down and replaced parts, made sure all was working properly and the vacuum was fine - he's a long time q-jet expert who is a member here. my el camino, with a cam, also did the same thing with 2 different e-brock carbs and a q-jet..... switched to Holley and all issues went away 2 years ago and not adjusted since

 

Is it thready because it is starving for gas or because the seat in the q-jet has been eaten away and it's choking to death on fuel?

 

The idle circuits will need enlarged and probably also needs more bypass air. Unless the internal circuits are modified, new parts won't do squat. What is the duration on that cam?

 

To tell the truth on the 12th of never would i buy that 750 demon {new street demon 750} carb. The carb i have which is a 750 street demon looks just like a 750 mighty demon.Looks like holly made a switch a roo to to get more cash out of their crabs they are selling. Not all street demons are equal in looks or price.LOL To the op which ever carb you end up with i hope it works good for you.

 

Why not? The reviews for the new Street Demon have been great. They fixed a lot of the Thermoquads short comings, mickey mouse choke, leaking O rings that caused flooding were eliminated, Aerospace grade polymer vs Bakelite, optional aluminum body and spread or square bore 3 barrel configuration. The Thermoquad and Qjet both got a bad rap running ultra lean calibrations on terribly anemic engines. Holley's just couldn't pass emissions and were only aftermarket in the ugly 70's. They were also tempermental junk for street use. It sounds like refined carbs like Quickfuel's version are very daily driveable.

 

UMM i will tell you why not.Because it is fugly looking.The street demon i have that is now called a mighty demon is much better looking in style and never had a problem i use mine on the street and at the track.You should be a salesman for Holly.In my case knowing the difference in street demons it would be a no sale.

 

As was mentioned the idle circuit needs modifications. Nothing wrong with a Q-jet, it's a very versatile carb.

 

I don't get your post. Post up a picture of what carb was "formerly known as street demon" but now is "know as a mighty demon."
Here's a street demon, you can get the basic 625 on sale at advance auto web site for $270 shipped.

 

Better yet go and type in mighty demon 750 and you will see what my carb looks like.It looks similar like my other demon carb and it is a 850 mighty demon.A few years ago before holly bought out Barry Grant there was no street demon that looked like a thermoquad.

 

This is what I don't get.

 

I know what you talking about.Google 750 might demon then 750 street demon.The new street demon is plain ugly compared to what i have which is what holly now call a 750 mighty demon. When i bought mine new a few years back the 750 street demon looks just like the new 750 mighty demon. Neither carb looks like that hunk of junk that Holly is saying is a street demon 750. Just bear this in mind i bought my carb when Barry Grant owned his business.Now Holly owns the brand a put their spin on things.

 

They didn't have the street demon when Grant was running the show. I think your thinking of the speed demon. Street demon has only been out since 2012-13. And the 750 came later.

 

I agree with the name but that was the street/race carb not that junk Holly is selling.So i see why you say you didn't understand. The style is much better for the speed demon verses a street demon! Hows that is that better.I think you knew what i was referring to all along.LOL

 

You obviously hate anything non Holley. Function counts as much as style. I think the all black version looks real good.

 

Who are you talking to? If you are talking to me i hate holly's.I also don't care for quadrajets. I like my demons just not that street demon that looks like crap. You must hate what Barry Grant made!

 

Problem is Barry Grant was big time hit and miss, one would work great, the next was full of filings. The Demon line has great features and maybe now has the consistency it needs. Most go to Quickfuel for Holley style carbs, tough to beat them dollar for dollar. The Qjet is great, IF you get an properly built one. The Edelbrock line of AFB and AVS have given people a lot of issues. The Street Demon, I have seen one, yes one, negative review.

 

Baaa haaa coming from a know it all like you that is special. I have owned and drove more qjet car than you will ever have. I know of five good working Barry Grant carbs you know of what you read some where. Of these five all were in race cars no issues. Oh i have use carter carbs too AFB's when i was younger and ebrock competition carbs which are junk. So feel free to tell me any and all your info. You know who i am and you are stiring the pot.

 

Whatever, I have drove many Qjet cars and my family owned them too, much better carbs for the street than Ford or Dodge ever had. Th Street Demon fixes many of the Thermoquad's issues. Have you ever used a Thermoquad? I have. Many, many problems with Demons on quality control, gee why did Barry go bankrupt? Done here, you are a strange guy.

 

I know you were in diapers with a snotty nose when i was driving. Of course i have used a thermoquad and i didn't find them that great. Yes i have used Rochester also. I lost count of cars when i got past three hundred owned and sold. Why did Barry go bankrupt because he was a bad businessman.When a couple of these so called guys were bitching on ROP about there junk Barry Grant carbs i offered to buy all of there junk carbs at junk price guess what none stepped up and sold them you know why? Because they were not as bad as thought and some of them didn't even know how to tune one. Which is why Barry was getting a bad rap.

 

I'm the lucky guy that worked on the carb. John, sorry it didn't settle down nicely for you. In my experience idle is hit or miss - some carbs it'll work out great, others just seem to be cursed.

For the group, this is a shop bought reman, I forget the number offhand but it's a 7042XXX for big block manual transmission (7042953???). It was actually in really good shape without the normal rebuilder shortcuts (hot idle air was there, piston stem wasn't cut, etc). The idle bypass air was blocked off - I opened to 0.070" and there are no holes in the primary blades. Opened idle tubes to 0.037".
Ignition is an HEI with base at 15 degrees. I switched vacuum to manifold instead of ported which seemed to help quite a bit.

With all that I was able to get the blades nearly fully closed. IIRC, vacuum was low - 12" or so - with a rough idle at 700 in gear. I didn't replace the power piston spring and didn't check to make sure the piston was seated. It wasn't leaking fuel out the venturis. It responded well to being slightly choked, which tells me it wants more fuel. Idle screw holes were enlarged a bit, screws were out 4 turns or so.
The idle tubes that were in there were 0.042"!

I'm no expert, but I try to figure these things out. John, just let me know when you want to swing around again.

 

Hey John, thanks for sharing. Sending you a pm.

 

I'm Jason. JPM is John.

 

Sorry... Why don't we switch to a known good carb, put the timing at 18, and plug the vacuum advance for now. Set idle RPM at 750.

 

hi Jason, yes it is better but only when warmed up now... it stalls when cold and now stumbles a little more off the line. I thought about maybe trying the other q-jet that I have - who knows? Maybe it's just cursed

 

I've got a 670 AV if interested. 200 bucks plus shipping. Worked fine on my mild 455 build. I have bigger carb on it now so don't need the 670.

 

A 670 is to small of a carb period for a 455!For a stock 350 or very mild might be just right.