No Measurable Vacuum at Idle???

Question

Here's something I've pondered for years... When I tune the Jeep, I have no measurable vacuum at idle. If I raise the RPM above idle around 1,000 and up (revving high), it shows plenty of vacuum; 16 inches or so I think. I have a '76 258 with the ole' Carter YF carb with minimal emissions. In fact, the only thing I have is the vacuum advance ported from the carb, PCV to manifold, canister to manifold and canister pulling vapor from around the bowl. The intake manifold ports were all plugged with bungs. I'm describing the plumbing from memory. So, don't quote me but I'm pretty sure that's it. The engine runs just fine besides being cold natured. but, it ain't a problem down here in Florida. All the hoses are in fine shape and I can't find any leaks anywhere. I've torqued the manifolds and they've never leaked either through various head and manifold swaps. The carb's tight too. So, what's the deal with an otherwise healthy engine that shows good compression. The distributor's new and I've ruled out a ruptured diaphragm. What could it be? Am I being stupid and there shouldn't be any measured vacuum at idle? o_o_o_o Best Regards, /| ,[_____], Jim, WP3JQ |¯¯¯L --O|||||||O- ()_)¯()_) ¯¯¯¯¯ )_) EM60qk 30.447439N 086.628959W

L.W.(ßill) · Accepted Answer

That's normal and is the venturi vacuum port, the stock and whatMike here likes to hook the distributor diaphragm to, that advances thetiming as the engine increases in revolutions per minute. You find fullmanifold vacuum down at the butterfly throttle plates about fifteeninches of mercury at idle. This port I favor for full advance at idle(makes for smooth idle) yet instantly retards for torque when you stickit, and gradually advances with the centrifugal weights to reach fulladvance together somewhere between two and three grand.         God Bless America, ßill O|||||||Omailto:

Drink · Answer

You lost me, I think, about where to connect vacuum advance.  From thepassenger side, there are two vacuum ports.  At the base, where I'mconnecting distributor vacuum, is all the way down on the right.  About 3/4inch higher, to the left is the other.  I checked vacuum at both and neitherpresents measurable vacuum but the lower (my connection) provides morevacuum at RPM.  So, where do you connect?  What benefit do you get by connecting your way?  It seems backwards untilmuch higher RPM since you have vacuum advance at idle and then no advanceupon acceleration.  At rev, it sounds like vacuum catches up and you're backto where you want to be.                               o_o_o_o  Best Regards,    /| ,[_____],    Jim, WP3JQ  |¯¯¯L --O|||||||O-                        ()_)¯()_) ¯¯¯¯¯ )_)EM60qk         30.447439N 086.628959W

L.W.(ßill) · Answer

I know I'll get an argument out of Mike Romain here, but manifoldvacuum will get you a better idle and fuel mileage. Try the port overthe number 59 in this view: You might go around ittighten

Paul Brogren · Answer

Hey Jim,I'm actually in the same situation of thoughts too at this time.  I have a'75 CJ5 258.  1 bbl Carter YF.  If you have the same, maybe we can worktogether on this.  I have my distributor vacuum line connected to thediagonal port on the carb which points toward cylinders 5 and 6.  This portonly provides suction once the throttle body plate opens.  I've consideredgoing direct to manifold vacuum, but don't know the pro's or con's.  If gotmy carb off right now for a needle/seat, rebuild and adjustment.  If youwant, when I re-install it, I'll switch the vacuum lines around to check fora difference.

Drink · Answer

Sounds good but I'm not sure we can collectively "fix" the YF:)  I pluggedinto the forward-most vacuum port because it showed more vacuum.  Are yougonna port straight into manifold?  I'm curious to hear if some morecomments pop up so I'll know a little more for performance/efficiencycomparison.                               o_o_o_o  Best Regards,    /| ,[_____],    Jim, WP3JQ  |¯¯¯L --O|||||||O-                        ()_)¯()_) ¯¯¯¯¯ )_)EM60qk         30.447439N 086.628959W

Drink · Answer

Hmm.  I might be stuck and this is the only configuration.  I've a YF,1Bbl carburetor.  If I really wanted to, I suppose there's more than one wayto get manifold vacuum; just to see what pros/cons it gives.  How much wouldyou offer for a Carter BBD, 2 Bbl carburetor (used)?                               o_o_o_o  Best Regards,    /| ,[_____],    Jim, WP3JQ  |¯¯¯L --O|||||||O-                        ()_)¯()_) ¯¯¯¯¯ )_)EM60qk         30.447439N 086.628959W

Mike Romain · Answer

You are tagging a ported vacuum source.This gives you timing advance with rpm and in my opinion give the enginethe best 'punch'.On the newer ones, they use a manifold vacuum to give the engine asmoother idle for the pollution sniffer and had a computer actuallyadjusting the timing advance.When I dumped the computer, I switched over to the ported advance.  Onmanifold vacuum timing my engine has a dead spot when you punch the gas,it bogs down and slowly recovers.With the timing ported and a punch on the gas pedal, it just perks upand lights up my 33" mud tires in both 1st and 2nd and takes off like ascalded cat.Mike86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '0088 Cherokee 235 BFG AT'sDr

Mike Romain · Answer

I just checked my book and it looks like you have 2 vacuum nipples.  Thebottom one is the 'spark' port and the upper one is the 'egr' port. That implies both are ported vacuum to me.MikeDr

L.W.(ßill) · Answer

You know the Exhaust Gas Recirculation valve may only workcorrectly using full manifold vacuum.         God Bless America, ßill O|||||||Omailto:      Roma

Mike Romain · Answer

The 258 one uses ported vacuum.Mike"L.W.(ßill) Hughes III" wrote:

Paul Brogren · Answer

Ok Guy's,This is what I've got.  I tried switching the vacuum ports.  Originally, Ihad the distributor line connected to the port on the Carter 1 bbl which issupposed to be connected to the (TCS) thing that works off engine temp.It's located mid to lower on the driver's side of the block.  Anyways, Imoved the vacuum line from this diagonal carb. (no vacuum at idle) port tothe "spark" port which supplies manifold vacuum.I must admit, Intown driving is lot nicer.  I've got good acceleration sofar (I think better).  Seems to not have upper RPM flatness anymore.  I'lltry it tomorrow on the highway on my way to work.  I'm going to post anotherquestion that relates to the timing in relation to the vacuum line move.Thanks all for your various suggestions.  By the way, It seems I got themetering rod adjusted correctly, but we'll see tomorrow on the highway.

No Measurable Vacuum at Idle???

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