| Author |
Message |
Steve (Steve)
Member
Username: Steve
Post Number: 317
Registered: 2-2001
| | Posted on Sunday, March 02, 2003 - 3:09 pm: | |
Bill , thanks I think I got it.I guess I still don't know how the brake servo both gets full vacuum and how does it store it except for the check valve in the line. Also as I do the timing belts I'm going to recheck the valves. I may have a few to tight and will see after I open it up. I guess this could also cause low vacuum. I still didn't here from others on what their readings are , I'd just like to get a baseline here. |
Bill Sebestyen (Bill308)
Member
Username: Bill308
Post Number: 414
Registered: 5-2001
| | Posted on Sunday, March 02, 2003 - 11:12 am: | |
Steve,
Weber performance carbs are best at non steady flows. This is a consequence of having an individual carb barrel per cylinder. In the case of SU's , Strombergs , Carters, and Holleys, these carbs are always feeding multiple cylinders. Typically a plenum is feed from which the individual cylinders draw their air/fuel mixture.
An interresting example is the case of the Lotus Twin cam engine. Prior to emissions, these engines were fitted with Weber 40DCOE or Dellorto 40DHLA carbs, one barrel per cylinder. Later, to meet US emissions requirements, the carbs were changed to Stromberg's, which don't like the pulsed flow that Webers love. To accomodate the Stromberg's, the intake manifold was changed to tie the barrels together with a log. This smoothed out the flow making it compatible with the Stromberg's. The same thing is done for SU's where a single carb normally feeds two or more cylinders. Holly's and Carters are similar. A typical 4-barrel carb will feed a plenum chamber from which the individual cylinders draw their air/fuel mixture.
When a single carb barrel feeds a single cylinder, the flow is pulsed or unsteady. When a carb feeds multiple cylinders, the flow becomes more steady as the multiple pulses blend together. If you were to put an air flow restriction upstream of the air cleaner assemblly on a 308 and then measure the resulting vacuum at the air cleaner, you would find a near steady flow, assuming no air leaks. In the case of the 308, the vapor recover manifolds tie several cylinders together but the flow through these pipes is very small even at idle. This means that measurements of the vacuum in the inlet ports will be unsteady, especially at idle. If rpm is increased, the flow becomes more steady because the vacuum pulses occur so rapidly that the vacuum gauge cannot respond fast enough to distinguish them individually. |
Steve (Steve)
Member
Username: Steve
Post Number: 316
Registered: 2-2001
| | Posted on Sunday, March 02, 2003 - 10:27 am: | |
Bill I like the idea of single taps in each carb/cyl. base. Would be a good gage to help set up engine. I believe that the tube are press fitted into the carb base so all you would need to do is pull them out and tap them. What's got me a little nuts here is in all of my years with SU's , Strombergs , Carters and Holleys I usually get a steady reading at idle somewhere around 17"-20". I did recall the more radical the cam the less vacuum you would get at idle due to the duration. I still don't know if anyone of the 308 carb. cars are getting a steady 17" at idle. Also on my car the vacuum lines are hooked together on cyls. 5,6,7,8 and then a rubber line connects 3 and 4 . The Weber for cyl. 1&2 is hooked together but seperate from all of the others. Also the 1,2 carb has the large vacuum fitting in the base for brake booster. |
Bill Sebestyen (Bill308)
Member
Username: Bill308
Post Number: 413
Registered: 5-2001
| | Posted on Sunday, March 02, 2003 - 10:04 am: | |
Steve(s),
I don't think there is anything abnormal here. If you put a vacuum gauge on the 3-4 vacuum tap, you are in effect monitoring the sum of 2-vacuum pulses for cylinders 3 and 4. If the vacuum signals for cylinders 1-2 and 5-8 are plumbed together, you are measuring the sum of 6-vacuum pulses, this reading is likely much more stable.
The best way to perform this measurement would be to isolate each cyclinder's pulse and then do the comparison. To do this, one would have to replace the metal tubing manifolds with individual pressure taps. In time I plan to do this as individual vacuum taps in the intake manifold are an excellent indication of flow. If one were to connect the individual vacuum ports to a gange of manometers, you would have a powerful instrument for balancing the cylinder-to-cylinder flow, as is done on multicylinder motorcycles using a CarbTune I mercury synchronizer or a CarbTune II non mercury manometer. Alternately, one could also use Merriam manometer fluid (specific gravity~3)in 8-transparent fexible tubes.
Does anyone know how the vaport recovery manifilds are attached to the intake manifolds? Is this a press fit? I'd like to be able to remove them without damage and replace them with individual barbs. When not used for carb balancing, they could then be plumbed with flexible vacuum tubing to fulfill their normal vapor recover function. |
Steve (Steve)
Member
Username: Steve
Post Number: 315
Registered: 2-2001
| | Posted on Sunday, March 02, 2003 - 9:49 am: | |
Steve the brake booster hose is OEM and is not collapsing it is pulsing. I am taking the vacuum from the rear bottom tubes on the front (rear of car) carb. which has the brake booster line coming off of it. I will take the booster hose off an block the port and see if it improves. Also the cyl. down on a little comp. is #6 assuming you read the cyls. 1-4 front (rear of car) and 5-8 rear (cockpit side) and left to right stand at the back of the car. I also get the same readings with the front carbs which are hooked to gether with 1 little tube. I kinda of thinking that the throttle plates on the carbs are set to high and also maybe the timing is a little off but I would suspect if I had bad valves I wouldn't get the comp. readings I'm getting??? I'm just reading my Weber manual and although I haven't fooled with them since I got the car 2 years ago maybe I reset the adjustments. |
Steve Magnusson (91tr)
Intermediate Member
Username: 91tr
Post Number: 1464
Registered: 1-2001
| | Posted on Sunday, March 02, 2003 - 9:02 am: | |
Steve -- How are you hooking up the vacuum gauge? (to the shared little tubes on the intake manifolds?). Are you saying that vacuum-wise you've confirmed that the other 7 cylinders are OK and the only strange vacuum behavior is at the cylinder #4? One thing you can try is removing the brake booster hose and block off the fitting to the int. manifold (to simulate a properly sealed brake vacuum system) and see if that affects the behavior.
On a side note -- your comment that you can readily see the the brake booster line collapse/expand under changing vacuum conditions makes me wonder if it's been replaced with the wrong style hose (i.e., not super-thick-walled vacuum hose). |
Steve (Steve)
Member
Username: Steve
Post Number: 314
Registered: 2-2001
| | Posted on Sunday, March 02, 2003 - 7:29 am: | |
I just did a vacuum check yesterday on my 77 GTB and am confused with the results. My vapor recovery system was gone when I got the car and the vacuum lines were connected from the front (cockpit side) to the rear right carb. When I took a reading on the rear left carb. I get a rapid fluxuating needle whit a 5"-15" reading. Engine idles rock solid at 1000rpm and hot comp. is around 180psi in each cyl. except 1 . Also the main vacuum line for the brake servo is pulsing and low with same type of reading. Engine seems to have good power all the way up to 7500 rpm. I would normally think this would be a valve problem , weak springs or carbon valves but I seem to have good comp. Is this timing or carb. setting? Any suggestions would be appreciated. Thanks |
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