idle help

[77cruiser]

73SS have you checked your damper for TDC.
Nice carb old cars.

[Cliff Ruggles]

"Next I converted it to DMAB."

Old Cars, could you describe this modification?

Pics?

[73ss]

73SS have you checked your damper for TDC.
Nice carb old cars.

Yes, during the build with a dial indicator, spot on. I would like to think that nothing  slipped on the damper, but anything is possible. The entire rotating assy came as a kit.

The car is starting to come around as I get some miles on it. Was able to close the throttle a tad and get it off the transfer slots some more. I'm going to leave the lower bleeds at .070 and try some .039 tubes this weekend. If it doesn't respond well to that I'll put the .038's back in.

[77cruiser]

"Next I converted it to DMAB."

Old Cars, could you describe this modification?

Pics?

Don't know if he did his the same, but mine I blocked original bleed & added 2 about where they would be on an older DMAB carb. I also reduced the LIAB to .063 & moved the upper to the lid.

[Cliff Ruggles]

It's all in the details.

Blurbs about making modifications without specifics don't provide the readers with anything they can work with.

So basically not helping the situation any if at all.

Generic answers don't help either.  Like optimal advance at idle being 32 degrees.  Where does that come from?  Why is 32 degrees the magic number for these engines?  Is that proven fact and well documented or did someone just happened to find an engine once that like that much timing at idle and ran poorly if you tried anything else? 

I've tuned engines that HATED much more than 8-10 degrees at idle, and others that had to have 35 degrees to even have a snowballs chance in hell of idling under 1000rpms.

Once again, it's all in the details and generic or blanket statements don't really help much when it comes to these things.......FWIW.....

[old cars]

Was simply referring to the setup I used. "77 cruiser " understood.
No need to revert to "bashing"

[Kenth]

Was simply referring to the setup I used. "77 cruiser " understood.
No need to revert to "bashing"

Are you saying you plugged the Truck style single main air bleeds (sizes?) and drilled holes in airhorn and float bowl to copy Passenger car style main air bleeds (sizes?)
Pictures?

[Cliff Ruggles]

"Was simply referring to the setup I used. "77 cruiser " understood.
No need to revert to "bashing"

If you consider asking for "details" bashing then you are once again mistaken.

You came on a thread were someone is asking for help and made a few blurbs but not enough detail for them to have something to work with.

I know full well how a SMAB carb could be converted to the earlier design but it would NOT be a good course of action.  The SMAB design is much more efficient and why the factory experimented with it in 1977-78 on the divorced choke truck units, then went to that set-up across the board in the early 1980's. 

It simply works better and doesn't require nearly the metering area and metering area change from the upper section of the rods to the power tips. 

Even so it really has NOTHING to do with the issues that are going on here.  The problem is idle and for some reason this engine build isn't making nearly the vacuum that it should.  I've built smaller engines with less compression and cam and they made more vacuum at idle with a LOT less initial timing that the big 489 is making here. 

I'm suspecting a more fundamental issue like late intake closing, but that's just a guess on the information that I've seen to date......Cliff

[old cars]

Are you saying you plugged the Truck style single main air bleeds (sizes?) and drilled holes in airhorn and float bowl to copy Passenger car style main air bleeds (sizes?)

Yes

[old cars]

picture

[Cliff Ruggles]

Thanks for the pics, as with anything else a picture is worth a 1000 words.

For the readers, using a tiny direct main bleed/blocking the indirect bleed and relocated the upper IAB's to the airhorn would pull a LOT of fuel from the idle and main system.  Haven't tried that and woln't be in one of those but I be assured it will fatten it up everyplace and no doubt it would be fine with .031 idle tubes and .046" DCR's at 14" vacuum at idle speed, especially if you didn't open up the LIAB in the truck big block carb. 

Those carbs were pretty "fat" without any modifications and why Edelbrock patterned their "850" cfm model 1910's after them.  In stock form with a .038" idle tube and .055" DCR's the 1910's have enough idle fuel for a Pro-Stock dragster engine!.........Probably why a lot of customers call up here bitching about them being a little "fat" at idle on their "mild" street engine build......FWIW......Cliff

[77cruiser]

If that thumb wasn't in the pic I'd swear that was my carb.
If you have time & an extra core try one.

[Cliff Ruggles]

PLENTY of spare cores, no time and no need to go that direction.  My personal carb is custom tuned using one of the "recipes" in my book.  Nothing will outrun it, and believe me the dyno shop I use has tried, and I've ran every size and type of carb out there against it at the track and it's always posted quickest ET and highest MPH.

For certain applications I will relocate the upper idle bleed to the airhorn and use a small size and block the ones in the main casting above the DCR's.

The factory actually did this on a few of the "high performance" units.  Coincidentally was working on one of them today from a 1970 Ford 429 Cobra Jet engine.  Talk about a "special" carb. It had tiny MAB's and about the smallest upper IAB's you will find on any Q-jet in the airhorn.  They worked well for what they were designed for as the 429 CJ engines were heavy breathers and poor signal to the carb at idle, off idle and low RPM's..........

[73ss]

other than getting away from large idle tubes and dcr's, what are the benefits of this modification?

[Cliff Ruggles]

Basically blocking the indirect upper IAB in the main casting and moving it to the airhorn requires a much smaller size to be used.

Keep in mind that an upper bleed in one location or the other is required or the fuel bowl will syphon down after the engine is shut down. 

When the throttle plates are closed and engine is idling there is high vacuum or strong "pull" at the mixture screw holes and any exposed transfer slot.

For most stock engines the signal is very strong and fuel pulled easily into the engine.  The factory used a lower idle airbleed just above the throttle plates as a vent and to reduce the signal some, and other upper airbleed to reduce it even more and provide a vent to prevent syphoning.

The sizes of these holes vary considerably depending on carburetor part number and application.   It is part of the total calibration since it not only provides idle fuel it also provides fuel thru transition to the main system as more transfer slot is exposed.   

Once in a while I'll venture in that direction with a carb I'm building here.  When I do it's typically when the engine "builder" has went with a pretty "hefty" can on a tight LSA and the compression ratio isn't going to be high enough to provide good signal at idle speed.  Experience has taught me when to do this and we almost always increase base timing at the same time, so I may have the owner send the distributor here as well so I can shorted up the curve some.......Cliff