Author Topic: QJet tuning on SBC 350  (Read 7354 times)

Offline kfa670

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Re: QJet tuning on SBC 350
« Reply #15 on: April 14, 2013, 06:57:47 AM »
Thanks for the help so far guys.

I must now admit that I am an idiot...  My idle tubes, I didn't measure them correctly because... I didn't take them out!  They measured at .032-.033 or so.  I took them out very carefully and resized them to .036.  I also changed my accell pump hole like Omaha said to a lesser shot.

I still have the .071 jets and cliffs 44 rods in.  My idle is under control and is quite adjustable with the screws.  I'm a little snappier off the bottom with less of a pump shot but it is somewhat harder to start in the cold (just more pumping obviously)

My question is:

-You guys say to use a tapered rod.  I'm guessing that would be a "B" style?  It will work in the 75 and later carb?

I have my eyes on a .046B rod with some .074 Jets because that would give me about the same cruise/part throttle but get my top end fuel up.

-If i resized to smaller MABs, how exactly does this help?  How does it effect the tunability?

Thanks again fellas


Offline kfa670

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Re: QJet tuning on SBC 350
« Reply #16 on: April 14, 2013, 07:20:38 AM »
sorry not .046B I meant .048B

Offline omaha

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Re: QJet tuning on SBC 350
« Reply #17 on: April 14, 2013, 10:10:07 PM »
Sounds like you got the idle circuit set up good. The main airbleeds add air to the fuel when the main circuit is in use. So that means all cruise and heavy accel, WOT etc. The air from the air bleeds mix with the fuel and form an "emulsion" (just like mixing up some oil and vinegar for your salad) its kind of a frothy mix and this in turn mixes with the incoming air stream, the mix is pulled out if the  orifice in the middle of the center venturi booster (dual round horn shape things, one inside the other) now, in the late 70's and on, the emissions were getting tighter and tighter. GM decided to try large main airbleeds to form a more consistent emulsion, great but just one problem, they're not as "responsive" when it comes to performance. Note that a lot of chevy trucks came with these where outright performance was not as much of an issue. Also notice that larger jets were used with this and some appropriate rods to get the fuel curve in line. Much testing was done and on a completely stock truck etc., this worked ok. going to a smaller airbleed returns to the earlier setup where the rod and jet choice is not as "touchy" of a choice. So, in reference to your set-up, I think if you "shrunk down' the main airbleeds, it would be easier to tune the main circuit. I believe in recipe one would have a good starting point as far as size.
    Also, remember, that at your elevation, you can get away with more timing. Like if you are at 36 degrees total, you could probly bump that up to 38. You have less air up there and so the engine has less cylinder pressure. It like lowering the compression ratio at that altitude. IIRC, for every 2500 feet in elevation, you can go 1 more degree of timing, and that is every where on the timing, idle cruise etc. Might be something to experiment with.

Offline kfa670

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Re: QJet tuning on SBC 350
« Reply #18 on: April 18, 2013, 10:07:13 AM »
I've got the timing right on the edge of the engine hanging up a little when I try to start it, but I think the total advance is only about 34-35 all in about 2800 rpm.  I need to double check again, its been a while.

I've got a few parts from Cliff coming.  Let you guys know how everything is working once I get back from work here in a week or two!

Thanks