Lars, my 355 sbc produces about 11"hg at 650ish idle rpm. It has the VC1810 (B28) vac can. I've read your articles many times over BTW. If I hook up to full manifold vacuum the engine runs/accels much better but it simply will not idle down below 1000+ rpm without a blip of the throttle every time. No such issue with it hooked up to ported. Per your article it seems to say that I will have these issues with this can. Is the VC680 (B1) the correct can and will is solve this erratic idle issue for me? Thanks bro... See sig for engine specs

 

I am NOT Lars, but I when I am working on my car, I wish I was! The vacuum advance unit that you have installed, B28 (VC1810), starts advancing at 3-5 'MG and is fully advanced at 5.75-8 'MG. How much vacuum does your car produce at idle with the vacuum advance not hooked up (with the vacuum port plugged) or hooked up to port vacuum (the reading should be the same)? If it is higher than 8 'MG, then it is most likely not the vacuum advance unit.

What do you mean by blip the throttle? Do you mean that it idles at 650 RPM, then you accelerate and when it comes back to idle, it is at 1000 RPM. Then you have to quickly tap the throttle to get it to come down to 650 again??

If so, it sound like sticky carb linkage or maybe the throttle plates are binding a little leaving the transfer slot overexposed.

If your vacuum with the vacuum advance unplugged is OVER 8 "MG, I think your vacuum advance unit is okay, but it is weird that is only happens when the advance unit is hooked up to ported vacuum.

Just something to think about while you wait for Lars to answer…..
Thanks,
Joe

 

Primary throttle shaft is binding. I used to have an old 600 CFM holley years ago that did the same thing. On mine it was due to worn shaft bushings. You can try closing the secondaries some which would let you open the primaries more and avoid this problem.

11" of vacuum sounds like a pretty decent cam and I'm wondering why you are idling it that low? 800 rpm might be a better idle speed.

 

, but the wierd thing is that he said it only happens when he has it hooked up to manifold vacuum. He said when it is hooked up to ported, it does not do it......
Thanks,
Joe

 

Joe, vac line off and plugged I get 11" at idle. Carb has only a couple summers on it. I do not believe it is a linkage issue, even with the linkage unhooked and revved it does the same thing.

 

No, not weird. Mine was the same way. With ported vacuum the engine is less efficient at idle so the idle speed has to be turned up (throttle plates opened) to get the same idle speed. Manifold vacuum with no other changes increases idle speed so the throttle has to be closed. If it is sticking the last little bit you get this problem.

 

Try this: With manifold vacuum get it to where it is idling high and then push directly on the throttle lever on the carb. I bet you will see it move slightly and idle speed drops to normal.

 

Hey Dave,
Long time no see! Maybe when you get this problem corrected it'll be time for a road trip down to Palm Springs?
See ya buddy,
Greg

 

My 73 L82 auto idles at 1000 rpm in park and 700 rpm in drive. When Lars checked it he had me put a B28 vacuum advance motor on it, I was running a B20. He also change the centrifuge advance springs to a lighter setup. The combination of vacuum advance and centrifuge advance was kicking in around 800 rpm and jumped up to 1000 rpm so the idle speed in the range of 800 to 1000 would be very hard to get. I believe the initial advance is set to 8 BTDC.
Roger

 

That is plenty of vacuum for that unit. I would have to agree, it sounds like the throttle blades are staing open.
Thanks,
Joe

 

 

Dave -
If you can "tap" the throttle and have the rpm drop, you have a sticky throttle blade issue - not a vacuum advance control unit issue.

Often what happens is that with vacuum advance applied to direct manifold vacuum, very little throttle blade opening is required. Any misalignment of the blades in the bores can cause the throttles to get a little sticky, especially when there is vacuum under the blades. When you use ported vacuum, you have to set the idle speed up just a tad, so the blades get out of the "sticky zone". It has nothing to do with the vacuum advance itself - it's a function of the blades getting sticky in the bores at a certain blade angle with the manifold vacuum pulling on them from the underside.

To correct this, you can back the idle speed screw all the way out so that the blades are closed in the bores (with the engine off, of course...). Then, take a long screwdriver and insert it down the venturi. Loosen the throttle blade screws each about 1/4 turn. If you go any more than this, you will snap off the staked screws. Once loosened 1/4 turn, take the screwdriver and "tap" on the blades to make them center themselves in the throttle bores (you can wiggle the throttle arm a little while doing this to make the blades center themselves up with the closed throttle position). Then snug the screws back up. Now, re-establish your idle speed and see if the stickiness is gone.

 

1. Add an extra throttle return spring
2. Check the choke to see if that is what is hanging the throttle open
3. Check the primary and secondary throttle stops to see if they are set right.
4. All else fails try reducing the initial advance. This will alllow more throttle opening for the same idle speed. Might crutch the problem.

-Mark.

 

Thanks for the input fellas. I did install another return spring a while back and that fixed the problem, but I darn near got shin splints holding the throttle at freeway speeds. Methinks I need to center the throttle plates as Lars and others have suggested.

 

Dave -
I have TedH's carb here right now for setup and test. He was having the same problem with his carb, and it turned out to be badly worn shaft bores in the throttle plate. I had to put a pair of throttle shaft bushings in his carb, and it fixed the problem with the carb not returning to the base idle setting. It's not often that I see the bores worn so bad that bushings are really needed, but it does happen. If you find that your throttle shaft is really loose you can send me the carb or the baseplate for a bushing fix if you'd like.
Lars

 

I cannot emphasize enough to agree what Lars wrote.
Correctly adjusting the blades/butterflys is the way to do it permanently...one time...finished product.


Adding springs unnecessarily is the leading contributor to premature throttle plate blade-shaft bore, and blade-shaft wear...end results will be:
vacuum leak
fuel leak
stiff accelerator...
all the above.
...then; the only repairs are to correct those *extra* problems are: bushings, blade shaft, or new(er) throttle plate...all of them.
Not to mention adding a spring is not correcting the problem...it’s just a temporary solution.
If you think you are scratching your head now; add too much spring and see what happpens...can you say "grey hair"?...
Heat and tension (and the fact the shaft is not solid, but partially slotted) can even bend the throttle shaft...then more grey hairs come.

[TIP#1] Take Carb off.
...if not, pay close attention to what Lars wrote about how far to back out the tiny screws...they are etched/grooved on purpose to not back out all the way. (this prevents the screws from falling into the intake if they accidentally vibrate loose)
[TIP#2] have a NEW screwdriver that properly fits the tiny screws...buy a new one, or make sure the screwdriver blade tip is in very good condition, and correctly fits the slot of the screw head....you'll be glad you did...they don’t always loosen that well.
[TIP#3-kinda important] if possible...take the carb off to do the work.
Reason:
If a small piece of screwdriver chrome comes off the screwdriver blade, it isnt in the intake if carb is off.
If a piece of screwdriver blade breaks, it isnt in the intake if carb is off. If the tiny screw gets mangled and a piece comes off, it isnt in the intake if carb is off, or even shears by accident; it is not in the intake if carb is off...you get the idea.

If the carb is off, and you are comfortable with removing the throttle plate; it makes the repair easier by accessibility, and allows the use of a tiny amount of lock-tite on the backed out threads when re-tightening... use small amounts, and left-over lock-tite is cleared off of blades/butterflys and shafts.
The throttle plate removed also allows the use of a small file or *lightly* using a Dremel to smooth the screw heads if they become boogered up upon the loosening procedure, and allows clean-up of any debris before start up.

I know I wasnt asked...coffee made me chime in and add to what was already well written.



Good luck,
Jim