Carb size?

[richpet]

Howdy all! I'm sure this has been addressed, but...

Building a 347 (details in Race/Dyno section) and going back to a carb from a TBI system for simplicity and less wires.

Was thinking of either a 670 Holley Street Avenger, the Demon street 625, QickFuel Brawler 650, or the Edelbrock AVS series 650.

I used a 570 Holley on a previous build and liked it. Thoughts?

[85MUSTANGTGT]

I'd think any of those carbs would be fine. Ran a 670 on mine for quite awhile, ran well. Currently running a 750, personally think it runs better than the 670.

[richpet]

I would have thought the 750 range too big, though it can be jetted down easily enough. You still get real good response at/off idle and such?

[85MUSTANGTGT]

I didn't notice much, if any difference. I did have to make accel pump adjustments to both carbs though. Oddly enough, the 670 liked a larger cam and nozzle than the 750 does.

[Broncojunkie]

I have a 750 ProForm on my 331 and have also ran a 650 Brawler on the same car/engine. Driveability is fine with both. I really didn't have to mess with jetting much at all. Well, I did but ended up going basically back to the stock jetting. I did change idle air bleeds on the 650, which seemed to cure a slight stumble at part throttle. I believe my next carb will be a Summit. I've heard good things about them.
https://www.summitracing.com/parts/sum-m08750vs

[richpet]

I have actually heard good things about them too. Maybe I will just go with the 750 or so cfm, maybe even the Summit brand.

[Scott Peters]

Richpet,
Holley’s site as a Carb CFM chart on their site to help you pick what Carb size you will need.
I am using a 570CFM street Avenger and it’s plenty of carb for my 302, which is basically stock except for 86-93 factory headers and cat back.
There “might” be a difference in carb jets and power valves between a manual choke and Electric choke.
I didn’t realize this until years later.
You might want to check the specs to see if there is a difference between them on the 670.
The street Avenger worked for me right out of the box. I run the manual choke model.
Scott

[richpet]

I used their 'chart' and it came in at the 650 range. I also used the 570 SA carb on my Mustang in my signature and liked it. I may go with the 670. Debating as the Street Demon 625 looks nice, etc. Sigh.

[gr79]

A carb with annular boosters is a plus.
Helps a large carb's drive ability in the low rpm band.
Had a 585 Holley center pivot bowl carb on my 67's stock 289 2v manifold.
Used a 2v adapter and the stock chrome lidded air cleaner. Drove fine looked awesome.
A 780 would be interesting. 347 is almost a 351.

[richpet]

I am getting a 625 street Demon today to use on the current motor. May use it to start up the 347 when that time comes, though by then I may have a bigger carb on the shelf.

[richpet]

Got the Demon on. Everything I read said they were rich out of the box and those were folks with larger motors. So I dropped one size from factory both primary and secondary, and I also put the next step larger (leaner) metering rods in.

Idles great and feels more responsive and faster responding to throttle inputs - but I didn't do a lot of tuning/adjusting of FiTech parameters.

Test drove today. Think I am lean as it sounds sputtery at very light throttle openings, but good with more throttle input. Gonna read the spark plugs when it cools down later today.

So I called Holley tech.

Going to reinstall the stock/smaller metering rod and re-test drive. Apparently a little is always going by the metering rods so going to start with that. Might even jump two sizes. The tech thinks the metering rod alone should do the trick. If not, I will bump the primary jets back to factory. Hoping the rods do it as the jets are on the bottom of the carb.

[richpet]

Update:

Bumped the metering rods two sizes so they are now one size smaller (richer on both steps) than factory and left the one size under factory jets in. Who wants to pull a carb full of fuel off if it isn't necessary, right?

Test drove.

Seemed to run real good! I got it warm and then hit some level ground at a constant speed and I did not notice any missing or sputtering at all.

Tomorrow we are taking the car to a picnic up at Odell lake, so I will go from 800' elevation to close to 6000' feet over the Willamette pass, the lake is north of 4000' feet I believe on the other/East side. If anything the altitude should richen the mixture a bit.

As I knew I was lean earlier today I did not pull any plugs. I think we will take this drive tomorrow, I will spend some time driving it 'normally', and then take a look at the plugs next week or so. As long as I do not notice/hear any sputtering/preignition/etc. in the meantime.

[richpet]

Yet another update (not sure if anyone is interested, but meh. Lol.)

Once it fully warmed up there was a little sputter/miss, though it was a lot less pronounced, but went home and switched cars anyway just to be safe. So today, off came the carb, stock/factory main jets put back in, dropped the metering rod back one to stock size. Left secondaries one step leaned out. My thoughts are it SHOULD be good to go now. The motor will probably never need the fuel provided by the stock/factory secondaries so I can fine tune with the metering rods (I have the tuning kit which provides something like 3 steps richer than the stock, give or take). Worst case is I will pull the carb again and swap the secondaries. Took me an hour or so to do all the work, so not bad.

I also noticed the guy I bought it from had the choke rod hooked up incorrectly. I was looking at the angle in which it enters the choke housing and it looked wrong. Found a pic in the manual and sure enough, it appears it should be on the bottom, which makes it line up with the choke housing opening much more linearly. I test started the car and it was fine. This might help explain why yesterday it seemed fine if the choke was not operating fully as it should.

The real test will be a level ground test drive at full operating temp. Tomorrow?

[85MUSTANGTGT]

I'm still following along. Glad to hear you're having some success.

[richpet]

Glad to hear it!
But, ...
Test drive was better, still just a hint of the sputter (it sounds like a gentle 'whoop' or 'pop' from the exhaust, like a bunch of bubbles in the sink and one softly pops once ina while?) and I could smell gas, especially when I got on it.
I have yet to pull any plugs (shoulda sooner) so that IS next. If I am rich I am wondering if the either 1- the floats are set a bit too high or 2- the needle isn't fully seating. Having the carb essentially back to stock (minus the secondaries, and it smelled rich when those engaged) makes it hard to believe I am overall lean on a motor this size.
So, plugs get checked next. Period. Any sign of a rich burn means the carb comes off again and gets a total go through. I suppose one hazard of buying a used part, even one slightly used, is that the previous owner MAY not be 100% honest about what they did or how.
Pretty confident I will get it figured though.