Team Camaro Tech banner
Cancel
Create thread New Garage Forums

Adjustable VA - interesting findings

Jump to Latest
21 - 40 of 61 Posts
#21 ·
Mr. Fixit said:
I can easily adjust the one I have to be all in at idle vacuum. I currently have the static set at 16 (mechanical back to 20) and it is moderately happy there, starts easily, etc. I adjusted the can to be all in at idle as suggested and it is adding 8 more degrees of advance for a total at idle (static + VA) of 24. I have tried limiting the VA to 5 degrees and the results are the same.

With this setup, I am still dealing with a high idle which brings the mechanical in since the idle is over 1200 rpm. The carb blades are completely closed (including closing the secondaries). I could live with this setup, but due to the high idle, I get preignition again when I shut it off.

If I had a way of lowering the idle, it would probably be good like that. I suspected a vacuum leak but have tried all of the tests and don't find any.
Click to expand...
Have you tried changing one or both centrifugal advance springs to the next heavier spring, sounds to me like the mechanical advance is hanging on which is why your idle remains high, you need a spring combination that pulls the centrifugal advance out at or slightly above 1,200 rpm's?
 
#22 ·
If your throttle blades are completely closed your engine should stall. I've never had an engine that would run with no air.
 
Save
Reply Quote Like Like
Discussion starter · #23 ·
NH69Z28RS said:
Have you tried changing one or both centrifugal advance springs to the next heavier spring, sounds to me like the mechanical advance is hanging on which is why your idle remains high, you need a spring combination that pulls the centrifugal advance out at or slightly above 1,200 rpm's?
Click to expand...
That's what I am shooting for, but all of the springs in both kits that I have begin at 800 or 900.
 
Discussion starter · #24 ·
onovakind67 said:
If your throttle blades are completely closed your engine should stall. I've never had an engine that would run with no air.
Click to expand...
Actually, I have had other cars with built motors and have seen the same scenario since there is always some air around the blades. If I put my hand over even half of the throttle body it immediately stalls.
 
#25 ·
If there is air going around the blades, they are not closed. I've never had an engine that I couldn't throttle to submission with a properly adjusted carburetor.
 
Save
Reply Quote Like Like
#26 ·
IF the AFRs on the idle and the power and the cruise circuits of the the carb AND the secondary butterfly heights and accelerator pump are not adjusted correctly as per the manfactures instructions into ball park acceptable parameters....
Then it is a total waste of time messing with the idle/ power and cruise parameters of the dizzy .
yeah u can mess around wasting yours and everyone elses times with a hit in miss lottery method, and maybe eventually get something that kinda works.
Bottom line.. u dont put the paint on a car then do the panel work...
And the same goes for the engine dialling in
There IS a procedure
1/Get the carb circuit AFRS and basic out of the box adjustments ball park correct
2/Get the dizzy advance circuits ball park correct
3/ establish individually the tuning spec advance for the idle, total and cruise
4/modify those specs into the dizzy.
5/ALWAYS start over advanced and work back when dialling in tuning advance specs

And to do the above u NEED
1/A rivet bent at 90 degs and the end flattened in to a small blade screw driver (for a holly)
2/ AFR... preferably a data logger
3/ a in cab vacuum gauge
4/ A file
5/ A set of allen keys
6/ couple flat screw drivers
7 a pair of fine long nose pilers
8/the harmonic balancer timing markes extended out to 45 deg.. and if using a knock sensor 55 degs
9/ A std timing light.. many dial backs have issues with modern dizzy modules

Now IF one has not done these things or has not been using any of the above tools.
Go back to square one, forget everything u have done, and do it correctly

Its NOT rocket science..
I have posted many times why things are done, why done like that, talked other thru.. and just how senistive timing is....
What the hell .. there is A point (+/- 1 deg) between 12 deg and 18 deg ATDC (only a 6 deg ball park window) that a particular individual engine requires at a given rpm and load....to get the max cylinder pressure at the right point as the piston drops for a given dynamic compression ratio with a given fuel with a fixed formula and octane

like building an engine one is talking in a couple thu of an inch... well so is the timing ....
Its not rocket science, its like building an engine... Procedure and follow instructions and never short cut...correct sequence.
If that cant be done....take up needle point.

PS make use of the forum search function above.
 
#27 ·
Mr. Fixit said:
I can easily adjust the one I have to be all in at idle vacuum. I currently have the static set at 16 (mechanical back to 20) and it is moderately happy there, starts easily, etc. I adjusted the can to be all in at idle as suggested and it is adding 8 more degrees of advance for a total at idle (static + VA) of 24. I have tried limiting the VA to 5 degrees and the results are the same.

With this setup, I am still dealing with a high idle which brings the mechanical in since the idle is over 1200 rpm. The carb blades are completely closed (including closing the secondaries). I could live with this setup, but due to the high idle, I get preignition again when I shut it off.

If I had a way of lowering the idle, it would probably be good like that. I suspected a vacuum leak but have tried all of the tests and don't find any.
Click to expand...
For Holley users, the secondaries need to be open or you won't be able to lower the idle.

Back off adjustment screw until the throttle plates are fully closed (secondaries). Turn adjustment screw until it just touches the throttle lever and turn 1/2 turn more to position the valves.
Image
 
Discussion starter · #28 ·
COPO said:
For Holley users, the secondaries need to be open or you won't be able to lower the idle.

Back off adjustment screw until the throttle plates are fully closed (secondaries). Turn adjustment screw until it just touches the throttle lever and turn 1/2 turn more to position the valves.
Image
Click to expand...
Did that the other day. Thanks for the suggestion.

I am convinced that there is a vacuum leak somewhere. Just went for a spin with it totally warmed up. When I left, it was idling at 1200, when I returned the idle was at 1500 solid and would not go down.
 
Discussion starter · #29 ·
Steptoe said:
IF the AFRs on the idle and the power and the cruise circuits of the the carb AND the secondary butterfly heights and accelerator pump are not adjusted correctly as per the manfactures instructions into ball park acceptable parameters....
Then it is a total waste of time messing with the idle/ power and cruise parameters of the dizzy .
yeah u can mess around wasting yours and everyone elses times with a hit in miss lottery method, and maybe eventually get something that kinda works.
Bottom line.. u dont put the paint on a car then do the panel work...
And the same goes for the engine dialling in
There IS a procedure
1/Get the carb circuit AFRS and basic out of the box adjustments ball park correct
2/Get the dizzy advance circuits ball park correct
3/ establish individually the tuning spec advance for the idle, total and cruise
4/modify those specs into the dizzy.
5/ALWAYS start over advanced and work back when dialling in tuning advance specs

And to do the above u NEED
1/A rivet bent at 90 degs and the end flattened in to a small blade screw driver (for a holly)
2/ AFR... preferably a data logger
3/ a in cab vacuum gauge
4/ A file
5/ A set of allen keys
6/ couple flat screw drivers
7 a pair of fine long nose pilers
8/the harmonic balancer timing markes extended out to 45 deg.. and if using a knock sensor 55 degs
9/ A std timing light.. many dial backs have issues with modern dizzy modules

Now IF one has not done these things or has not been using any of the above tools.
Go back to square one, forget everything u have done, and do it correctly

Its NOT rocket science..
I have posted many times why things are done, why done like that, talked other thru.. and just how senistive timing is....
What the hell .. there is A point (+/- 1 deg) between 12 deg and 18 deg ATDC (only a 6 deg ball park window) that a particular individual engine requires at a given rpm and load....to get the max cylinder pressure at the right point as the piston drops for a given dynamic compression ratio with a given fuel with a fixed formula and octane

like building an engine one is talking in a couple thu of an inch... well so is the timing ....
Its not rocket science, its like building an engine... Procedure and follow instructions and never short cut...correct sequence.
If that cant be done....take up needle point.

PS make use of the forum search function above.
Click to expand...
Although the thought of needle point does attract me, I don't see a forum for that so I will continue to sort out the issue.

I am now on to something very substantial. The Merlin intake that came with the car has a provision for 16 bolts total. 8 on each side as is somewhat typical. The heads however, only have a provision for 12, (6 on each side). What I was never aware of is that the remaining 1/2" diameter holes connect to the intake runners. While trying WD-40 again to see if I could find a vacuum leak, I decided to squirt some into one of the holes and viola, big leak. :hurray: Not just one, but 4. Now to find the best way to plug them.
 
#30 ·
Mr. Fixit said:
Did that the other day. Thanks for the suggestion.

I am convinced that there is a vacuum leak somewhere. Just went for a spin with it totally warmed up. When I left, it was idling at 1200, when I returned the idle was at 1500 solid and would not go down.
Click to expand...
Is the choke fully open when warmed up?
 
#32 ·
Mr. Fixit said:
Did that the other day. Thanks for the suggestion.

I am convinced that there is a vacuum leak somewhere. Just went for a spin with it totally warmed up. When I left, it was idling at 1200, when I returned the idle was at 1500 solid and would not go down.
Click to expand...
Check intake gasket. I was after this same exact symptom for a month. Turns out , my intake gasket was shot (800 miles on new motor). After I replaced the gasket, my idle is steady again. I also heard an ever so slight miss in the exhaust. You would really have to listen to hear it. Replacing the gasket solved all of my problems.
 
#33 ·
If u have a vacuum leak.. u cant set mixtures or set secondary butterflies... and the secondaries if adjusted a smidgen too far open... that is a vaccuum leak...
To set secondaries .. use the rivet described in my previous post in list of tools
establish what the idle advance is going to be... say 18 for a mild cam.. make sure the PVC valve is working correctly.
fix the idle advance... turn the primary engine speed to be 1 1/2 turns out +/- 1/2 turn.
now adjust the secodaries and mixture screw as iff the secondaries are the idle speed screw... BUT DO NOT have the secandairies closed.. Period as mentioned by other above.
If u do then u are liable to bend the shafts, misa ign the butterflies , damage the bores because they will jamb and cause damage
IF u cant get the secondaries adjusted with the idle adjustement screw 1 1/2 turns out +/- 1/2 turn then
1/ u do have a vaccuum leak
2/ the butterflies primary and/ or secondaries are misaligned
3/ worn butterfly shafts
4/ have the wrong bottom gaskets...cab to base and/ or base to manifold
5/Wrong or dirty PVC valve
6/ hole in the VA diapham

note all of the above are vaccuum leaks.

To find leaks... use a small LPG flame torch/ soldering iron.. turned off, no flame but flowing LPG... when u find a leak it sucks in and the rpm,s increase

OH and make sure all the spark plugs HT leads etc are in excellent working order
Drop a spark plug on a hard floor... just throw it in the bin.. no questions...
 
Discussion starter · #34 ·
Thanks all..

Last night I machined 4 button plugs slightly shorter than the depth of the manifold flange and .010 smaller in diameter. I applied RTV sealant to them and rotated them as I pushed them into each hole. The RTV formed a small ring of sealant under the heads of the buttons. Since the static pressure is so low on max vacuum, I am pretty sure that this will resolve the huge Swiss cheese leaks that I was never aware connected to the intake runners. If not, I will remove the manifold, bore and tap the holes for pipe plugs.

_________________
Thanks,

Mr. Fixit
 

Attachments

Discussion starter · #35 ·
Just a quick update. The vacuum leak issues resolved. I did the propane test and all good.

I started over with the tuning process and found that the engine is very happy now with 10-12 static. I tried the adjustable VA can on the lowest vacuum setting and it drifts up and down. I just ordered a vc1862 (AR31) can which works from 2-4 hg and applies 8 degrees of advance at 6-8 hg. I am hopeful that I can make it work..

Thank you all for the insight.
 
#36 ·
As suggested before .. on std and adjustable cans one can make stops and file out the slots to further adjust the vaccuum range they work in.. with limits.
ball park.. u will need a unit that starts about 6 to 9" of vacuum and all in around 10 to 13" of vaccuum... which is basically the range most adjustables work in anyway.
starts low as 2/4 inches... well I described at what vaccuums u need in previous post.. ie engine loads 60 mph
think about it WoT is about 1 to 1.5 inches...
So for to get full advantage at cruise u will need to be cruising well over 100 mph for bit to work correctly
As I said before... you NEED and in cabin vaccum gauge to KNOW the engine vaccuum charactoritics in the real work car moving when carb and dizzy ball park specs are set
 
Discussion starter · #37 ·
[QUOTE=Steptoe
ball park.. u will need a unit that starts about 6 to 9" of vacuum and all in around 10 to 13" of vaccuum... which is basically the range most adjustables work in anyway.
starts low as 2/4 inches...

I am seeing a contradiction or I am confused, as you and others have posted that the "VA needs to be all in at idle". My idle vacuum with the initial timing set at 10 BTDC is at about 5hg. As previously stated, the adjustables only go down to about 5hg at a minimum. With this setting, the VA comes in and out and will not work.

Quote:=Steptoe

Are u runing EGR? if not ideally u should have manifold vaccuum and the idle made up of intial + VA
So if your ideal (best stable idle at lowest prefered rpms and mixture slightly on lean side) idle is 18 .. And your best performance/ power is 36 deg with a rich mixture....it is un wise to have more than 40/ 42 deg initial+cent+ VA without dialling in with a knock sensor
so 41 deg less 36 gives a VA max of 5 deg
And if ideal idle is 18 18 minus 5 gives an intial of 13 degs
The VA needs to be all in at idle vacuum.. or no more than 1/2 to 1" above.. MAX
It also need to be all in at approx 60 mph flat road and '1/4 acceration... and start to come in when hit a gentle slope

Of not thu.. with modern pump fuels, cam designed stc I doubt very much that 36 deg is your best total (intial+cent) advance... ball park would more likely be around 32 @ around 3000/ 3200 rpms
This would then give 41-32= 9/10 degs in the VA
idle still 18 intial 18-9= 9 deg and
cent 32-9= 21 deg .

Now a couple things to watch
I mentioned above where a counterweight can fall off the sliders and over the edge above
If a VA is not all in at idle (non egr on manifold vaccuum) the u are most likey get a idle that speeds up and slows down... as the VA pulls advance increases ,that increases the rpms.. then sice u are running on the fixe metering carb idle circuits.. the engine runs out of fuel to maintin those rpms slows..the Va start pulling again....repeating

So with that said, I selected a VA to try that begins to pull between 2-4. Otherwise, no VA
 
#38 ·
I am seeing a contradiction or I am confused, as you and others have posted that the "VA needs to be all in at idle". My idle vacuum with the initial timing set at 10 BTDC is at about 5hg. As previously stated, the adjustables only go down to about 5hg at a minimum. With this setting, the VA comes in and out and will not work.
Click to expand...
As I have said before.. u MUST establish what idle advance is needed, What Power (total advance is needed and IF not dialling in with a knock sensor do not go over 40/ 42 degs intial+cent +VA
Have u established these parameters individually.. locking the dizzy, also I have suggested using the search forum search function? how tos...

Now IF u have been messing around and established these parameters then u will have the intial around 8 to 12 degs... and IF a mild cam, AND secondaries adjusted correctly u will have a a low idle vaccuum at this intial advance... no manifold VA around 7 to 9"
Then when connect the VA, it will start to advance the engine... the engine will increase in rpms, the vacuum wiil increase, the advance will increase more and the VA will be all in at you ESTABLISHED best idle advance that is made up of initial +what is in the VA
IF this is not happening then u have not followed procedure.. and something is wrong...ie secondary , or also running on venturi mixtures... or u have a wild track cam and engine way over carbed suite for track locked or magneto timing .
 
Discussion starter · #39 ·
Here is the latest. I believe that I am making great progress. I found the PCV to be faulty and have tried several new ones with varying results. This did help to lower the idle a bit.

I have the static timing set at 10 and the VA limited to 8 for a total 18. I understand that the this scenario gives me a total mechanical of 31 and I may want to change the static to 12 and reduce the VA limit to 6 in order to get a total static and centrifugal of 33.

When I adjusted the secondary blades, I didn't have the linkage between the primary and secondary connected. What I didn't realize when I selected the particular progressive linkage that the carb came with, the bend in it was such that it didn't let the secondaries close as much as I wanted and also pulled slightly on the primary. I corrected this and then connected my data logger AFR and set the idle to 13.5. This gave me the ability to keep the idle below the 900 mark eliminating the centrifugal from joining the party.

Although the jury is still out, it seems after a cruise of about 5 miles, the temp is a bit lower and the preignition is at bay. It is screaming fast now more than ever. At low speed cruise, the AFR is at 12.4 which I think is a bit rich, correct? Air bleeds? Any other suggestions are always welcome.
 
21 - 40 of 61 Posts
About this Discussion
60 Replies
11 Participants
Last post:
cv2065
Team Camaro Tech
posts
2.4M
members
90K
Since
1998
A forum community dedicated to 1st generation Chevy Camaros owners and enthusiasts. Come join the discussion about performance, builds, restoration, modifications, classifieds, troubleshooting, reviews, and more!
Full Forum Listing
Explore Our Forums
Bench Racing Engine Body Shop Performance Brakes, Suspension & Steering