Next Step Carb

[66Sprinter]

there is only the small pot attached to the DUI like the original..its probably just a bellows operated lever direct to the advance mech...

I dont know about a motor or gearing within this....! Is it something new or that I should have gotten?- :

 

[66 Fastback]

The small pot is what I am talking about. Inside there is a rubber diaphram and a spring that governs the amount of tension that the diaphram has to pull against. Some of these pots can be adjusted with a hex key stuck inside the nipple to adjust the spring range. Many of the vacuum cannisters are not adjustable. I do not know if the DUI is adjustable.
Doug

 

[66Sprinter]

Let me see if DUI has any info on this... the guru's there says it a standard setup with no adjustment.

It goes 15 deg total when engaged.

Does this alter your opinion.

 

[66 Fastback]

The 15 deg that DUI mentions would be about right for the amount of vacuum advance you should get from the vacuum advance. But it does not tell you at what vacuum level it starts and when it finishes or is fully advanced. I assume that with the bigger cam that you have, you may not be producing as much engine vacuum and the stock vacuum can may not be advancing the timing enough at idle or at cruise conditions.

Basically, I think the timing needs to be confirmed with a timing light and the timing mark on the balancer should be verified too. I don't necessarily think that the vacuum advance is causing your problem, but it is all part of finding out what your timing curve looks like and if it is appropriate for the engine modifications that you have done. If the idle vacuum advance is not correct, then the tendency is to open the throttle plates more in order to get a stable idle. This can lead to some stumbles and hesitation.

The 15 degrees of vac advance coupled with say about 10 deg of initial timing would give you about 25 deg of advance at idle which is in the ballpark of where you want it. At higher rpm's (3000 rpm or more) and with the vac line disconnected and plugged you should have about 36 degrees of total advance from the 10 deg initial and 26 deg of centrifugal.

Rather than jump on buying a carb first, I would confirm the timing is correct and also investigate any potential flooding issues you may have with the current carb.
Doug

 

[66Sprinter]

thanks Doug,

I have relayed the plan to my son and hopefully tomorrow he will get the necessary equipment and setup the test for this weekend. My friend will also help if he gets off work early.

I will know for sure tomorrow if this will work out. I picked up the other carb for a fix...I want my car home and not at the shop where it is now!

Even if it sits for a few months, I want it in my garage and not somewhere else. I havent found a place for storage yet in Dallas. When I get back stateside, I will look for a better place then just storage.

Jim

 

[66Sprinter]

OK...heres the recent update.

I added a Holley fuel regulator after the pump and dialed in at 2.5psi.

The car started and settled into idle at 800 rpms with no flooding. Even after reving. Ran smooth and pure.

Vacuum, at idle, is ported but only measured 2". So if I am correct, that means I have full advance on the dizzy? (Works opposite right)

I will try to switch from ported vac to manifold vac..otherwise I will need a can for this!

Any other thoughts?

 

[66 Fastback]

2" of vacuum on the line to the distributor at idle would likely mean that it is a ported vacuum source at the carb. At that vacuum level, the vacuum advance should not be contributing. Connecting it to a manifold vacuum source should yield full vacuum advance.

Do you know how much manifold vacuum is being generated at that idle of 800 rpm (and the distributor connected to ported vac)?

A fuel pressure regulator in fron of my Weber helped solve some of my problems too and increased the fuel economy. If the higher pressure was causing problems with the float and dumping too much fuel to the engine during idle, that may explain the drivability problem you have off of idle.
Have you test driven it now with the fuel pressure regulator and no change to the timing?

It would probably be best to change one thing at a time and test drive. Lord knows I have made the mistake of "adjusting" too many things at once only to find that I had to go back to square one to get the thing running decently.
Doug

 

[66Sprinter]

I have a 5200 carb and an adaptor...but am hesitant to put it on.

It has two ports for vacuum on it. A ported and manifold. I dont want my dizzy advancing at idle...

So I will try to get it driven...(but not by me...I need my hands and feet where I'm at!)...and see.

My thinking is the same as yours...i would like to see it the vacuum pulls the dizzy in during acceleration....when the throttle opens up!

Wouldn't I see that by bumping the throttle and not in gear?

 

[gtm1086]

One of the best Mods I did was to install the Holley 2300. I'm running a larger cam and Mallory ignition. I started out with the little stock carb. Jumped up to the tripple carb set up and ended with the Holley 2300. 350 cfm. It works

 

[66Sprinter]

Can you tell me what the numbers are on your cam? Also with the little carb, did you ever have fuel flooding or geysering?

Also, did you use a stock F-pump. I would appreciate your input...if your mods are similar to mine.

Did you mount the carb with an adaptor?

 

[66 Fastback]

...i would like to see it the vacuum pulls the dizzy in during acceleration....when the throttle opens up!

Wouldn't I see that by bumping the throttle and not in gear?

When you step on the gas, the engine vacuum will decrease and the associated vacuum advance will decrease. Blipping the throttle when parked in the driveway is not the same as openning the throttle when the car is under load. In the drive way in idle, there is no load on the engine to bog it down and prevent it from revving freely with very little throttle movement, so the engine vacuum will increase. However, when driving down the road, openning the throttle will cause the engine vacuume to drop. The engine cannot rev freely, because it is attempting to move the load faster.

I saw your question in another post about a timed or ported vacuum source. Think of the throttle plate as a choke or restriction. You have atmospheric pressure above the Carburetor and something less than atmospheric pressure below the throttle. At idle the throttle is choking air flow increasing the difference between the pressure above and below the throttle plates. A Manifold vacuum source has a port below the throttle plate. A ported vacuum plate is located above the throttle plate but very near the throttle blade. At idle, the ported source is open to the air above the throttle plate and therefore sees no pressure difference. It reads atmospheric pressure or zero vacuum. As the throttle opens slightly, air running through the venturi and around the edge of the throttle plate increases in velocity and decreases in pressure, so the ported vacuum plate starts seeing a decrease from atmospheric pressure. As the throttle blade moves above the ported vacuum port, both the manifole and ported vacuum ports are now located below the throttle plate and act similarly with similar vacuum readings. At wide open throttle, they both will have low vacuum readings because the "Choke" or the throttle plate is open and there is little difference between the pressure above and below the plates.

Anyway, some of your posts keep mentioning fuel stains, flooding etc. The flooding may be causing your bog. But it should be fixable without resorting to another carb, especially since you have so much money tied up in a Pony Carb.
Doug

 

[66Sprinter]

I really appreciate the lengthy explanation Doug. I have the fuel regulator on now and when it stops raining..will test drive it!

From there I want to see what the difference is between timed and manifold vacuum for starting or performance. I am wondering if I need a vac can to help with idle conditions.

 

[66Sprinter]

I am waiting for a text msg on how the car drives in the current setup. I need to just see if I can get the car out of the shop and back in my garage for when I get home at the end of the month.

However, is it true that vacuum advance will not start until about 4", so at idle, (2") I am still at full advance.

My question concerns the driveability issues with ported vs manifold. Was ported vac supposed to be the main vac sourcce for dizzies? Why would one choose manifold over that...or because of the cam...I have no other choice.

I am also wondering about timing. I don't have a back dial timing light, but still feel the setting of timing by ear is masking another problem here. I havent touched the timing (as set by Gateway on the DUI) since the start.

Since everyting is so new and different I really dont have a good feel of where to start and in what order. Doug gave me the best setup to work at, so I am ordering a back dial light from Summit so it will be there when I get back stateside.

At least with the fuel regulator, I dont have anymore flooding issues. Still pissed I ruined my paint job on the engine. (I used ceramic heat paint)

 

[Harte3]

"However, is it true that vacuum advance will not start until about 4", so at idle, (2") I am still at full advance."

Someone correct me if I am wrong but that sounds backwards to me. I think that on a timed port there is no appreciable vacuum vacuum to advance the timing until the throttle is opened as per 66 Fastback's explanation. BTW, what is your full vacuum reading at idle?

Although the application is different, the principles are basically the same. When setting up the first carb I used on my engine the differences between ports were minimal so I used the full vac and everything worked. When I recently changed carbs my engine would not idle well on the timed port unless I opened the throttle blades some via the idle speed screw. So, I went to a full vac source and turned the idle speed screw back down and got a good idle...which means when one gets to the point you are with your engine there is going to have to be some trial and experimentation to find the settings your engine wants to run at. With these modified engines the stock rules and specifications are pretty much history and timing, carburetion, etc have to be re-engineered just as the rest of the engine was re-engineered.

I'm sure you will feel better when you get back to the world and can get your own hands on it and feet in it .

 

[james singleton]

Harte3 or anyone else, I am curious, are you saying that you can hook up a low-vac carb (or ported vac carb) to manifold vac by simply adjusting the idle mixture screw??? You mention that your idle got better; but what about on acceleration, how does it run. I am curious since I have a low-vac/ported carb with a Load-A-Matic dizzy and am wondering if I could switch to manifold vacuum since I am not sure my SCV is working correctly. I did replace the vacuum bisquet on my Load-A-Matic dizzy so the slight hesitation I am getting on acceleration seems to be from a lack of vacuum at the carburetor. Thanks, Jim

 

[Harte3]

What I am saying is that I tried running the dizzy vacuum both ways...timed and full vacuum to see which would run better on my specific application. What I am suggesting is to try it both ways to see which works best for a specific application.

Also, depending on the differences in carbs and applications, there may be more then one timed port on a carb...one for EGR and one for Dizzy with different activation points depending on the position of the throttle plate. (per 66 Fastback's explanation) At least that is the case with some 4v carbs which is what I tinker with on my application.

On my present carb there is one timed port and it worked I think because I opened the throttle plate enough with the idle speed screw that it uncovered the vacuum port in the throttle bore putting some advance to the dizzy at idle and getting a smooth 650 rpm idle. On full vacuum I achieved the same result...a smooth 650 rpm idle with the throttle plate opened less. There was no discernible difference while actually driving.

Your hesitation might be from the accelerator pump not moving immediately at throttle tip in and you are getting a momentary lean condition. With the engine off look down the carb and move the throttle open...you should get fuel immediately from the accelerator pump squirter with the slightest movement of the throttle...at least that is the proceedure for checking Holley 4v's and there is an adjustment for that. On your application it probably is different. At any rate, try the different vacuum source first...if it works, great. If not, put it back to the original source and then check the carb.

Trial and experimentation required. Your results may vary or differ.

 

[66 Fastback]

Was ported vac supposed to be the main vac sourcce for dizzies? why would one choose manifold over that...

A manifold vacuum source as opposed to a ported vacuum source will yield more advanced timing, smoother idle and lower combustion temperatures. At idle, the fuel charge is more diluted with exhaust gases and needs more advance due to the slower flame propagation under these conditions. For maximum snap in performance you want to use all of the advance that you can short of causing detonation. Ported vacuum was not developed for performance enhancement, but rather as an emission strategy to reduce the advance at idle and increase combustion temperatures. Ported sources provide less vacuum and less advance at idle. At idle, this retarded the timing (when compared to a manifold vac source) promoted higher combustion temps and less hydrocarbon emissions. But to get it to idle better with reduced ignition advance, they typically had to open the throttle plates a bit further to smooth out the idle and raise the idle speed. That is what Harte3 encountered on his vehicle.

and am wondering if I could switch to manifold vacuum since I am not sure my SCV is working correctly

James, using manifold vacuum will not solve your problem with the Load-a-matic. Review the sticky at the top this section to see how the SCV & Load-a-matic works. The SCV provides both manifold and venturi vac to the distributor. If you stomp on the gas to accelerate, the manifold vac drops to zero. So if this was the sole source of vacuum to the Load-a-matic, the distributor would not advance the timing and in fact it would retard the timing. Timing would stay retarded until the load on the engine was lessened and the throttle plates were more closed. Acceleration would be doggy. A conventional distributor has both vacuum advance and centrifugal advance. It uses the centrifugal action of some weights to move the distributor plate as engine rpm's rise. The Load-matic does not have any centrifugal advance and relies on Venturi vac to advance the timing when the throttle pedal is stomped on. Unlike manifold vacuum, venturi vacuum increases at wide open throttle because of the vacuum created by the increase in speed of the air flowing through the carb. So the SCV switches from manifold vac to venturi vac when the level of manifold vac falls. As rpm's increase and air flow increases, venturi vac increases and the Load-a-matic advances the timing.
Venturi vac is not to be confused with ported vac.
Sorry for the length, but I hope that helps clarify things.
Doug

 

[gtm1086]

Jim.

I started out with a pretty much stock motor and kept having trouble with the 1100 always flooding or hot soaking. Went to the trip set up using one barrel Holley’s. Worked pretty well. Lots of tinkering never could quite get the flat spot out. Next built a new motor every thing balanced, block decked head milled cam from Mike. one of his dual pattern 272/268 I think. Dual roller timing chain, harmonic balancer, Mallory centricifical advance, Mallory High fire and a Holley/Webber 5200. It was ok but just didn't feel right. Broke down and bought the Holley 2300 350 cfm. Great set up. Turn the key and go.

Gary

 

[james singleton]

Harte3, Doug, & Gary, thanks for taking the time and giving me this info on the carbs & dizzies. I have been reading different posts, and the "stickys" on a lot of the questions that I asked, but just wanted to make sure that I was not missing something! Thanks, Jim