Carb tune ...

[Panoz Paul]

Greetings ... I'm in to my second year w/ a carb ... I've installed a air / fuel meter to monitor & have obsearved to lean spot in the mid-range ... It leans out at mid 4000 rpm's w/ a light load ; meter will jump all the way up to the 17's ... I've learned to step through it & get firmly in to the secondaries ... Is this a jetting or power valve tweek ... Running a Mighty D. 650 on a 302 w/ 368 hp & 355 t. Thanks Guys / pmw

[NZGTRA17]

Originally Posted by Panoz Paul

Greetings ... I'm in to my second year w/ a carb ... I've installed a air / fuel meter to monitor & have obsearved to lean spot in the mid-range ... It leans out at mid 4000 rpm's w/ a light load ; meter will jump all the way up to the 17's ... I've learned to step through it & get firmly in to the secondaries ... Is this a jetting or power valve tweek ... Running a Mighty D. 650 on a 302 w/ 368 hp & 355 t. Thanks Guys / pmw

Interesting problem. We run the same carb but have not picked this up as we do not tune for part load conditions (not practical on an inertia dyno).
only full throttle.

I wonder how much of an issue this is as given the engine is only lightly loaded this will not occur very often or for long in race conditions. It sounds as though it is a "transitory condition" so may be caused by the overlap area between the systems (idle / main / power valve) and is just a fact of life with a carb.

What primary and secondary jet sizes do you have in the carb at present? Have you got power valves fitted in both the primarys and secondarys?

Kel.

[Panoz Paul]

Good Morning Kel ... I was hoping to hear from you ; Thanks !!! I have 70's pri. / 74 sec. / 6.5 pv's both ... The contition is when i'm in traffic & holding for a pass ... I was thinking a easer flow pv would help ; i think there is less vacuum in that condition ??? Plus go to 72's in the pri. Thanks Again / Paul W.

[NZGTRA17]

Originally Posted by Panoz Paul

Good Morning Kel ... I was hoping to hear from you ; Thanks !!! I have 70's pri. / 74 sec. / 6.5 pv's both ... The contition is when i'm in traffic & holding for a pass ... I was thinking a easer flow pv would help ; i think there is less vacuum in that condition ??? Plus go to 72's in the pri. Thanks Again / Paul W.

Paul, I would make a change to one system at a time. If the rest of the rpm curve is OK fuel ratio wise I would try the Primary circuit PV first. If that doesnt work I would increase the primary jet size by say 2 as you have suggested. If that works, I would swap out the primary PV and check again.

If none of the changes work I would mash the throttle and turn the music up............!! Let us know how you get on. May have to get out a carb book and do some study to relearn stuff long forgotten.

Kel.

[NZGTRA17]

Originally Posted by Panoz Paul

Good Morning Kel ... I was hoping to hear from you ; Thanks !!! I have 70's pri. / 74 sec. / 6.5 pv's both ... The contition is when i'm in traffic & holding for a pass ... I was thinking a easer flow pv would help ; i think there is less vacuum in that condition ??? Plus go to 72's in the pri. Thanks Again / Paul W.

Paul, re your question on power valves and vacuum. Power valves are held closed by high vacuum and open when the vacuum drops below the rating of the power valve (to prevent a rich condition close to idle).

In your case if you are feathering the throttle when the lean condition occurs, then manifold vacuum will have increased not decreased so the power valves will most likely have closed taking fuel out of the overall mixture.

A good thing to do before you start making changes would be to hook up a vacuum gauge to the manifold and note what the actual vacuum is when this condition occurs. That will assist you with appropriate PV selection.

Kel.

[Panoz Paul]

FYI ... Talked to an old hot rodder ... He stated to set the idel w/ the secondaries & richen their two idle corners by a 1/8 to 1/4 turn ...
The theory is to keep the secondaries a little " wet" in the closed / low transition & slightly advance the trans into the seconds ... It did reduce the lean spot & also elimiminated a small stumble at transition ... Still need to step through it ; but a lot smoother ...
Thanks / pmw

[NZGTRA17]

Originally Posted by Panoz Paul

FYI ... Talked to an old hot rodder ... He stated to set the idel w/ the secondaries & richen their two idle corners by a 1/8 to 1/4 turn ...
The theory is to keep the secondaries a little " wet" in the closed / low transition & slightly advance the trans into the seconds ... It did reduce the lean spot & also elimiminated a small stumble at transition ... Still need to step through it ; but a lot smoother ...
Thanks / pmw

Paul, great to hear you found a simple solution.