Carb idle circuit issues

[sublimemike]

That was the plan. The car has the typical L88 fan set up...no shroud so the vicous fan just blows around doing nothing. The previous guy fitted a decent 20 amp electric one onto the Harrison rad and that works well though I'm not sure I'd be 100% confident In 30degree traffic jam

 

[sublimemike]

Idles ok without getting too hot . Deiseling sometimes an issue, hence I put an idle stop solenoid on the car. 12.5 comp...too much advance...bang?

 

[teamzr1]

I mentioned in another post on this that it is important to know what the CAM grind the engine has and esp
what the LSA is as that highly affects how the engine functions from startup to about mid RPMs
It of course effects how the car idles, cold, hot, A/C on or off and what type of tranny used

You can calculate the LSA using the centerlines of the intake and exhaust lobes
The formula for LSA is:

Intake Centerline + Exhaust Centerline ÷ 2​

For example, if the intake is 107° and the exhaust is 117°:

107° + 117° = 224°
224° ÷ 2 = 112° (LSA)​

How does it affect performance?

Combined with Duration, the LSA determines Valve Overlap. This overlap has a significant impact on Engine Vacuum and idle quality.

• Camshafts with a "tight" LSA (106° - 109°) will usually have more overlap.
• Camshafts with a "wide" LSA (110° - 118°) will have less overlap.

Vacuum is the difference in air pressure between the inside of the intake manifold and the outside atmosphere.
The pressure difference creates suction and helps to draw air into the engine.

Engine vacuum is a required condition for a gasoline engine to run.
It helps control the engine rpm. Without proper vacuum, the engine won’t start. A vacuum leak can also cause high idle and run-away rpm.

How is it measured?
Vacuum is measured in inches of mercury (in. Hg).
An air pressure difference of 1 psi will move a column of mercury 2.036254 inches.
If your Vacuum Gauge reads 20 in. Hg, the manifold pressure, is about 10 psi less than the atmospheric pressure.

What creates vacuum?
The engine creates vacuum under the following conditions:

1. The piston is traveling down on the Intake Stroke.
2. The intake valve is open.
3. The exhaust valve is closed.
4. The throttle blades are closed (or slightly open).

How does it affect performance?

Vacuum is actually a measurement of the engine’s inefficiency. Higher vacuum indicates that the engine is working harder to breathe.
This limits performance. However, it is necessary. Restricting the airflow and creating vacuum helps slow the engine down and limit rpm.

The vacuum created by the piston pulls air from outside, through the intake, into the cylinder. The throttle blades create restriction. This reduces the amount of air/fuel that goes into the cylinder. With less air/fuel, the engine cannot turn as fast.

As the throttle blades open, restriction and vacuum are reduced. This allows more air/fuel into the cylinder and rpm increases.
With the throttle wide open, airflow is maximized and vacuum is at its least.

When you let off the throttle and the blades snap shut, the RPM is still high. This creates a spike in vacuum.
The restriction starves the cylinder of air/fuel. The result is falling rpm and vehicle deceleration.

The table below lists vacuum ranges for most engines:

​

Engine Function​	Vacuum Range​
Cranking​	3-5 in. Hg​
Idle​	14-20 in. Hg​
Cruising​	10-15 in. Hg​
Deceleration​	21-25 in. Hg​

Other vehicle systems also use engine vacuum. This can include:

• Brake Boosters
• Distributor Advance
• PCV Systems
• Etc.

In short, the more narrow the LSA is, the more loss of vacuum there will be, and you need to forget what stock GM settings are and make adjustments as to what the total makeup is to allow correct startup, and idle with all the conditions I mentioned above

If not knowing the CAM grind then at least measure to see what vacuum is at idle when engine is cold and hot

Example what I had to do to make up the loss of vacuum where stock LSA was 118 and with custom CAM where LSA was reduced to LSA
and had to change stock timing on left to changes I made on right to have proper startup and idle

 

[sublimemike]

Thanks for the detailed reply. Looking at your posts you know your onions and have far superior knowledge to myself. I think the cam could be a ZL1 grinder profile but without heading out to michigan where it came from I'm guessing. When I brought the car over and rebuilt the carb I had the initial knocked back a bit so I could use our 99 octane and some booster. It never liked the booster so I switched to avgas and it liked it much better. My suspicious are what Ross's advice is will cure it. I don't have a vac gauge but if I still have issues after this then I will be looking for one..
Car is a 4 speed no A/C. Thanks for the help

 

[teamzr1]

Just doing some number crunching and not knowing your engine specs other than a big block and ZL1 Comp CAM grind
That show an LSA of 110, so it would have lower vacuum from startup, idle, to maybe 2,500 RPMs
So would require more timing and higher RPMs at idle to be stable in cold and hot temps

Good torque but not so good on the HP side, see results of dyno at bottom left

 

[Roscobbc]

Interesting stuff Jon - it would be all too easy looking the the ZL1 cammed chart above to presume (looking at the .050" lift duration) and perhaps the difference between that and seat-to-seat duration figures that the 'old school' cam is a relatively mild grind.........I'm sure you will tell us otherwise?

 

[sublimemike]

Usefully stuff team zr. Thanks. Hopefully push the advance forward tomorrow.

 

[sublimemike]

Gave the timing a nudge on the advance - revs raced up, definitely wants to go...I've got nothing left to wind in on throttle screw to bring it back..?
Idle screws are 3 turns out both sides - revs about 750rpm

 

[sublimemike]

I should add that was 750rpm AFTER I had put the dizzy back to its original setting

 

[Forrest Gump]

Depending on what distributor you have you may be able to adjust the amount of mechanical advance which will allow you to increase the initial timing without altering the all-in timing. If there's no means of adjusting then a mod will be required (I had to mod my HEI type dizzy to limit the bob-weights movement) I've got 25 degrees of initial timing and all-in timing of 35 degrees - so just 10 degrees of mechanical advance. Modified engines do like a lot of initial timing.

Three turns on the idle screws sounds like too much, but I would get the timing right and then tinker with that. When I increased my initial timing I was able to back-off the carb idle settings.

 

[sublimemike]

Of course...the bob weights. It's a delco remy. It's another route I can take. It must have ran for the last owner so will scratch around a bit more. I was thinking to squeeze up the transfer slots a bit more and nudge the advance or am I going down the wrong track? Tim mentioned the vacuum cannister on the side of the dizzy. How do I test that?

 

[Roscobbc]

Mike - as Andy says if the idel improves when you add more timing you'll definately need to restrict the total advance..........but for now and bearing in mind we're only discussing idle speeds and a little above.......if you are having issues 'holding back' the rpm at idle (with more timing) why not experiment and close-off the throttle plate adustment screws (from where you were at when previously 'tweaked') and see if now idle screws will pull the revs back. Normal adjustment of idle screws is between 1 1/4 and 1 3/4 turns out.......

 

[sublimemike]

Yes Ross can give that a go with the hex key and reduce the transfer slots. My carb has only one hex screw that you can get to (with difficulty) from underneath. I suppose I could wind the throttle adjustement screw up to 850 -900 to help vacuum for the cam and utilising the idle stop solenoid set that up to help shut her down minimising chances of any run on? Then again closing up the transfer slots should also aid this anyway? I'll reset the idles to 1 and 3/4, everywhere says 1 and 1/2 turns as starting point and have a play. Weather is pissy tomorrow so won't be then. Be good to get this done before winter and while its still mild.

 

[Roscobbc]

Mine idles happiliy at 750 or so rpm........tbh I used to run it at about 550 to 600 rpm (much like stock) as I loved the choppy idle sound at low rpm - and it was fine.........it was a bit of an issue in winter though as without a choke it would take about 5 minutes or so from cold to maintain the lower idle speed without the engine cutting........so I just leave it a the higher idle rpm and during summer months it will hold a steady idle after about 30 seconds running................the lower idle speed also meant that the plugs would foul-up, eventually going open-circuit after a year of so of use (being as the idle mixture needs to be fairly rich to maintain the idle)........the primary accelerator pump orifices are the largest size that Holley produce and are needed to get over any potential hesitation when cranking the throttle open.................

 

[Oneball]

Of course...the bob weights. It's a delco remy. It's another route I can take. It must have ran for the last owner so will scratch around a bit more. I was thinking to squeeze up the transfer slots a bit more and nudge the advance or am I going down the wrong track? Tim mentioned the vacuum cannister on the side of the dizzy. How do I test that?

Take the cap of the dizzy, disconnect the vac tube and give it a suck. The vac advance mech should move. When you suck it put your tongue over the tube to trap a vac and make sure the mech stays where it moved to. This method will probably give you cancer or something so other methods are available!

The idle mixture screws shouldn’t be used to reduce idle speed.
If your idle is now too fast close the secondaries a bit.

if you moved the dissy and have now moved it back and it’s made a change there’s definitely something going on with it. It should do that. I’d also check that the mech advance is working you might have broken a spring. It’s pointing to timing to me.

 

[sublimemike]

I've closed the transfer slots on the primaries as advised by Ross...not run it yet. Can tell by the feel of the thread they hadn't been there before. I advanced the dizzy and the motor raced up , had to move it back to where it was. I set the idle at 750 on the throttle screw, it would have taken 550 as has been at that idle before. You think close the secondaries?

 

[Roscobbc]

I've closed the transfer slots on the primaries as advised by Ross...not run it yet. Can tell by the feel of the thread they hadn't been there before. I advanced the dizzy and the motor raced up , had to move it back to where it was. I set the idle at 750 on the throttle screw, it would have taken 550 as has been at that idle before. You think close the secondaries?

I would guess that the secondaries are letting too much air in at idle - and the reason why it wants to race-up. It's a shame we don't know the cam spec's - it would give us an indication of carb needs.......

 

[sublimemike]

As I've said I've done the primaries slots but not run it. The secondaries slots are much deeper , How do they adjust as there is no allen screw on that side? In the mean time I will check for broken spring.

 

[Oneball]

As I've said I've done the primaries slots but not run it. The secondaries slots are much deeper , How do they adjust as there is no allen screw on that side? In the mean time I will check for broken spring.

I thought you’d opened the secondaries with the Allen headed screw that’s the secondary throttle stop?

The idle speed screw is the primary throttle stop

 

[sublimemike]

Sorry Tim, you are correct . Must concentrate more. Like I say I'm not great with carbs, pump diaphagms, setting fuel floats ok but this is a new journey. All logged now. So having done the secondaries try and push the timing forward again?