Setup Questions

[Chief_Wiggum]

I have a 17059212 that I'm rebuilding with Cliff's book and rebuild kit.  It's going into a full size truck ~6000lbs with a new 350 L31 Vortec truck engine 9.5:1 CR.  No engine mods except 1.6 ratio rockers.  Trans is a 700R4, 37" tires, 4.56 gears.

I'm looking at using Cliff's "First Recipe" as a starting point.  Tore the carb apart, cleaned it and started measuring.

Idle channel: .055"
Upper idle air: .070"
Lower idle air: .067"
Bypass air: .110"

That's as far as I got since the idle channel seems big, and the bypass air seems huge.  Do I need to try and resize these?  If not, what are the implications?  Thanks for the help.

Mike

[Shark Racer]

With the idle channel that large I'm sure you'll find that the idle tubes have been enlarged as well. I'd probably go down a bit on them.

Is this the stock truck cam? If so, it's likely very large on bypass air and idle circuit.

[Chief_Wiggum]

I already replaced the idle tubes and enlarged them to 0.036" as per the suggestion in the book.  Should I try and resize the idle channels and the bypass air?  What will be the result if I do not?  Thank you for the help.

Mike

[Shark Racer]

The idle DCRs will add some fueling to idle. With the idle tubes at 036 it will minimize the effect somewhat. They work as a unison. Not sure how big the effect will be.

Bypass air will make the engine idle higher. You can test it and see if it's a low enough idle. For that motor you could probably idle comfortably between 550-650 rpm.

[Chief_Wiggum]

Finally got the carb on and the engine running.  I do have a wide band O2 sensor and gauge.  Unsurprisingly it's running super rich at 10.2:1 at idle and low speed cruise.  I can only get the idle down to ~800 rpm.  The throttle blades are closed all the way. 

Also, the mixture screws are fully seated.  I took the springs off them to make sure.  Where is the carb getting its idle fuel if the mixture screws are closed all the way, the transfer slots?

So it looks like I'm going to need to deal with the down channel restrictions.  Can I buy replacements or am I going to need to make something?  Do they come out just like the idle tubes? 

Is it possible to size the bypass air smaller while I'm in there?  I'm not sure how I would approach that.  Thanks for the info.

[omaha]

   Ok, as far as the DCR's go, yes they can be removed and replaced. Little heat and tiny screw yada yada.  Bypass can be resized at  it's exit point by using a steel ball and peening it into the hole in baseplate, makesure to use a ball large enough so it don't get stuck in the hole, the idea is to peen the hole smaller, not jam a steel ball into it. so if bypass at .110 use a .150 ball at least maybe .175 even better. Alternative method would be to drill tap plug redrill etc.
   The thing im lookin at here is where is that fuel coming from?? If the primary blades are closed AND the needles are seated the only ting left is the transition slot and it should only be showing about ~.020-.030 and with no extra fuel from the needles (which is normally there) the only thing I can think of is that the primary/idle well plugs are leaking OR the needles are the wrong type (don't know if that's possible, jus grabbing at ideas at this point).
   Are you sure your secondaries are closing all the way? Kinda hard to blow a 10.2:1 on just the transition slot and bypass air. Got me stumped for sure!!

[Chief_Wiggum]

Thanks, omaha.  That's what has me confused as well.  My carb number is 17059212 and the idle screws look like these:

http://cliffshighperformance.dreamhosters.com/metric-idle-screw_popup.html

but the ones that came out of it look like this:

http://cliffshighperformance.dreamhosters.com/early-idle-screw_popup.html

Is there a way to check to see if they're actually seating all the way?  Since the ones Cliff sent screw right in, I would guess they're the correct metric idle screws.  But at this point it sure seems like that must be the problem since there shouldn't be a way for it to get idle fuel except by the idle screws correct?  Even with the idle down channel restrictions at that size I should be able to kill the engine with the idle screws seated correct?

I double checked the fast idle cam as well, and it's definitely off the fast idle cam when the choke is open.

When I rebuilt the carb I did double check that the primary and secondary throttle blades closed all the way as that is one problem Cliff mentions is common in the book.  I also checked the idle well plugs and sealed them with epoxy.  They were staked and weren't leaking. 

[omaha]

Yes I think you are onto something there. The needles should physically fill the holes if you look at the exit of the idle port (into the venturi area, from the bottom). you should actually see the needle's sticking out quite a ways when the needles are bottomed. I guess you could blow some air through from the top of the idle passage while spraying some soapy water at the exit point to see if they are actually closing the port. Of course, air will blow through the transition port too but try to concentrate on the idle ports exit. maybe block the transition port with some thing. You wont need much air, should have very little to no bubbles with the needles seated. makes me wonder about the base plate if it is the original one. different years had different needles and styles of needles. Good luck!

[429bbf]

i think omahas on the right track . id install the old idle needles and screw them all the way in . ether someone tapped the holes for the old style needles or you got an old base plate . the metric needles will screw right in an old baseplate and act like there right . but they wont slow down the fuel.

[Chief_Wiggum]

Ahhhh..

I'm hoping to work on it some tomorrow.  I'll post back on results.  Also, I called Cliff and he set me up with replacement down channel restrictions so I'll install those as well.  He also indicated I can completely plug the bypass air as he doesn't think I'll need it with this engine at all.  So, some work to be done yet. 

[Chief_Wiggum]

Finally had some time to work on this yesterday.  Tore the carb apart and replaced the DCRs with .043"s.  Also checked on the mixture screws.  The replacements that Cliff sent me do not seat all the way.  The threads bottom out before the needle will close the passage.  I put the stock ones back in and those seem a lot better.  You can see in the picture how far they screw in.  From what I can figure out, the carb must be getting all of it's idle fuel from the transfer slot?  Is that even possible?

Now showing AFRs of 13.0 - 14.0 at idle.  However, I have the mixture screws in as far as they will go and I cannot lean it out any more.  Idle speed with the .110" bypass air ports is ~1000rpm.  The primary butterflies are all the way closed.

Also, another oddity.  This carb has no timed vacuum ports.  All the vacuum ports are manifold vacuum.  When I had it off I verified that the port usually used to connect the distributor (driver's side front of carb) comes out below the butterflies.  Is this normal?

I think my next steps will be to block off the bypass air and see if I can lower the idle.  Will that result in a leaner idle mixture?  Any tips on what I should do with the vacuum port?

[v737d]

I can't help you with the idle or needle seats, but I can tell you your distributor vacuum source should be above the butterflies=no vacuum at idle.

Cheers

Chris