Down draught (draft?) Carburettor choices.

#1
Hi Guru's,
More fettling today. Since I've had the car on the road I've thought the idle was a bit high. I don't have a tacho and I know it's no big-block but it just 'felt' high. So I've been chasing vacuum leaks and found one today. Sealed it, can't find any more, engine still idling high-ish with the idle speed screw on the Holley backed all the way off. Any suggestions? Static timing is 10 deg adv, idle mixture screws are about 1 3/4 turns out vacuum is 17" in drive at idle, 20" in park.

Disregard I found my vacuum leak. Bloody throttle shafts on the carby. *sigh* So I guess now the question is, what is the best down draft carby to fit? Holley 390 Vs Holley 350 Vs Edelbrock 500 Vs Weber 34 ADM?

In addition, the manifold is a standard one that's been modified to accept a 4 barrel - problem being that at the bottom of the tower it's not much bigger than a 2 barrel anyway. I do have a nicely modified standard-to-two barrel manifold or should I just get an Edelbrock....?
 
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cobraboy

Well-Known Member
#3
Have you checked that the secondary throttle plates are not being held open on their screw and that they are not causing your high idle ?
I have run a Holley in the past with a lot of logger on the throttle shaft spindles and always managed to get a decent idle.
 
#4
I'll check that thanks Cobra. I did the old de-greaser trick and you could hear it sucking in and a drop in idle. I'll research that air screw though!
Edelbrock performer is a far superior manifold design. You should share some photos of the modified Rover one though
I'll take some pics of the 2 barrel - the 4 barrel has basically had the tower lopped off and a 1/2" alloy plate bolted on. The open area under the tunnel is about the same as a 2 barrel and even though they tried to taper the plate to flow it's....um....'agricultural'. In all fairness that's how the 2 barrel was done as well, but much neater. If I take it off I'll take a pic. :)
 
#6
All the maths I look at leans towards a 350.... (where my toddler brain goes "aww but I want a 4 barrel like the big boys" lol). Car is a cruiser, mild 3.5 with auto. 350 is also the cheapest option (was looking at Holley snipers for a bit) plus I have the 'correct' manifold so there's another saving....
Budget is an issue unfortunately, as always.

Apparently there is a super rare 4 barrel version of the 350 from 'back-in-the-day'......
 

ghce

Well-Known Member
#7
All the maths I look at leans towards a 350.... (where my toddler brain goes "aww but I want a 4 barrel like the big boys" lol). Car is a cruiser, mild 3.5 with auto. 350 is also the cheapest option (was looking at Holley snipers for a bit) plus I have the 'correct' manifold so there's another saving....
Budget is an issue unfortunately, as always.

Apparently there is a super rare 4 barrel version of the 350 from 'back-in-the-day'......

I imported my 500 and manifold direct from the States and the price was amazingly low, the local suppliers wanted 1200 for the carb and similar for the manifold, I got both for 400 from the USA direct (all NZ $)

But mind you that was in 2004 ish

Graeme
 
#8
I imported my 500 and manifold direct from the States and the price was amazingly low, the local suppliers wanted 1200 for the carb and similar for the manifold, I got both for 400 from the USA direct (all NZ $)
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Geeze that's cool. Who did you go though? It sounds like it's at least worth looking at. Saw one place here wanted over a thousand dollars for a new Holley 350 :LOL:
 

ghce

Well-Known Member
#13
Geeze that's cool. Who did you go though? It sounds like it's at least worth looking at. Saw one place here wanted over a thousand dollars for a new Holley 350 :LOL:
Hmmm prices have moved up in the intervening years here is what they are now from the same supplier I used https://sdparts.com/i-24117632-edelbrock-9904-reconditioned-carb-1404.html

And https://sdparts.com/i-24113155-edelbrock-2198-performer-rover-manifold.html

But I am sure there are other cheaper suppliers out there in the USA

Graeme
 
#14
Yeah Kangaroo tokens aren't worth many greenbacks these days either......



Been looking at heaps of carby calculators and converting cfm between 4 & 2 barrel carbs....the 350 still looks like more carb than I need....?
Currently I have a 450 Holley (used on tunnel rams so probably not the best carb anyway but I digress).

(215Ci X 5000rpm x .75VE)/3456= 233CFM. (311cfm@ 100%VE).

350 Holley measured at 3" = 350cfm. @ 1.5"= 248cfm
390 Holley measured at 3" = 551.46cfm. @1.5" = 390cfm
450 Holley measured at 3" = 634.5cfm. @1.5" = 450cfm

So for me if these calculations are correct then the 350 is the way to go. Even just the primaries of the 390 if measured as a 2 barrel carb flow 275.5cfm.

Does this sound right? I know other guys are running bigger carbs but my needs are cruising with the occasional spirited mountain jaunt, and my engine is fairly stock.
 
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#16
Hmm - looking at Holley 2 barrel base plates I think my 2 barrel manifold has been set up for a WW stromberg. Will have to take some measurements. It's never straight forwards is it.
Interestingly in Aus we have a very common engine (an inline 6) of 202ci (3300cc) Holden straight-six motor - Wikipedia . So I've been looking at recommendations for that as it's much more common than the Rover v8 here and of a similar age/capacity. 350's are quite common and recomended for engines that never see 4000rpm and 500 (2 barrels) are recommended as better for anything hotter or that see above 5000rpm.

So I'm thinking that with the extra 200cc (lol) in conjunction with the fact that my V8 has had some upgrades (but unknown cam, dammit) that the 500 is now looking good. Now, a 500cfm 2 barrel (so measured at 3"hg) would be a 350cfm if measured at 1.5"hg (4 barrel). So again the 390 4 barrel option rears it's head lol. Now, I believe a vacuum secondary 4 barrel is a much more sophisticated option and should therefore (once properly tuned) should give the best all round result........??

Best-case calculations (100%VE) still only warrant a carby of 311cfm, realistically 264cfm @85%VE. So why does everyone recommend something much larger??

Perhaps I should just get a reconditioned 390, put it on the current home-made manifold and save up for an Edelbrock (it feels so juvenile working to a tiny budget *sigh*).
My head is spinning. You guys know these engines better than me, what should I get?
 
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ghce

Well-Known Member
#17
There is much to recommend the 500 Weber in that it performs well with the smaller engine even though the 500 is a much larger on the surface flow rate. One of its main advantages is the gas speed is quite high on the smaller capacity engine belying the 500 rating.

The Holley has some disadvantages re-fuelling in that it potentially can flood the engine due to it physical layout.

Which reminds me of younger days and shit awfull 202 Holden engines and flooding on hills ( hill starts) with the stock carbies, some very hairy and scary moments with 500 plus meter drops on very ( extremely) narrow mountain roads ( fire breaks and barley roads).

The main reason there is no chance in hell of me ever buying a Holden!


Graeme
 
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