Author Topic: Changing Motors - What do I need to change on Cliff's carb?  (Read 92 times)

Offline wheelz

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I have stepped up my engine from a low compression 400 to a 462 with E-Heads, 11:1 compression, headers, 60919 cam.

My Cliff's carb (17081276 2890 DPA) with #72 Jets and .042 Primary Rods setup OK, but runs lean in the mid-range on the 462. (pinging 2200-2600 rpm/white plugs). WOT runs hard up 5K, then tach flutters so I have not pushed it much further. Could be distributor or fuel.

I tried 1 counter-clockwise turn of the APT...with no noticeable affect.

 I do not know the main air bleed size or which recipe was used by Cliff..probably #1

I had a .77 jet and 50P rods available to try. It cured the pinging, but it now falls flat on it's face when flooring it, a second or 2 later and it takes off ..on the secondaries I assume.

Any suggestions would be appreciated.


Offline wheelz

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Re: Changing Motors - What do I need to change on Cliff's carb?
« Reply #1 on: June 14, 2017, 01:48:13 PM »
I did find that if you do not jam up the secondary putting the air horn on...it runs much better.  :/
No more bog.

Offline wheelz

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Re: Changing Motors - What do I need to change on Cliff's carb?
« Reply #2 on: June 14, 2017, 08:23:34 PM »
After a run tonight I find I still have pinging at partial throttle and white plugs. And it is sometimes backfiring when opening secondaries.

I cannot figure out why it is so lean at partial throttle.


Offline Cliff Ruggles

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Re: Changing Motors - What do I need to change on Cliff's carb?
« Reply #3 on: June 16, 2017, 03:03:50 AM »
77 main jets are HUGE in that carb and a big jump from 72's.  I would go back to 73 or 74 main jets and work with the APT to fine tune the primary side for the new engine.

I'd look at the timing to cure the light throttle ping.  How much timing is being added by the vacuum advance?.......Cliff

Offline wheelz

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Re: Changing Motors - What do I need to change on Cliff's carb?
« Reply #4 on: June 16, 2017, 07:58:23 AM »
I was going to try 74's with 44 rods later today...or would sticking with the 42's be better?

The vacuum advance is about 18-20, and higher than I want. I cannot seem to find an adjustable vacuum advance that will fit on a Pontiac HEI, without reclocking the HEI drastically.

How many turns of the APT (counter-clockwise, I assume) might I be looking at here?
I tried one turn with the 72's in and no noticeable affect.

Concerning the APT, is a 1/2 turn considered a lot, is 5 turns?

Got out for a late evening test last night, and it ran well with no bog or lag.
But it is still pinging at part throttle and plugs are very white.

Offline 77cruiser

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Re: Changing Motors - What do I need to change on Cliff's carb?
« Reply #5 on: June 16, 2017, 08:30:00 AM »
Pretty easy to limit the vac. advance. Found a pic on the interweb.

http://67-72chevytrucks.com/vboard/attachment.php?attachmentid=795080&stc=1&d=1314984995
Jim