Vacuum Leaks

[Matthew E. Herd]

As I've said ten million times (ok, maybe not that many) I have twin sidedraft webers and problems (what a surprise). As I suspect that there are vacuum leaks between carb and manifold, what sort of symptoms would I see in such a case?

As an aside, when I turn the mixture adjustment screws, nothing really seems to change, at least not significantly. The idle speed barely, if at all, picks up. However, the stumble, which was something of a nuisance, has been removed by richening the carbs a bit. The troubling fact is that though this problem is cured, the idle speed does not change measurably. Should this be the case, or should the idle change noticeably with only small turns of the screws?

 

[Dave Russell]

[ QUOTE ]

As an aside, when I turn the mixture adjustment screws, nothing really seems to change, at least not significantly. The idle speed barely, if at all, picks up. However, the stumble, which was something of a nuisance, has been removed by richening the carbs a bit. The troubling fact is that though this problem is cured, the idle speed does not change measurably. Should this be the case, or should the idle change noticeably with only small turns of the screws?

[/ QUOTE ]
I think it means that the idle jets themselves are about the correct size & all is well. The mixture screws should normally be 1 to 1.5 turns out. If you wish to change the idle speed, turn the idle stop screws (hex head) to set the butterflys.
D

 

[BACLVBob]

Until you acertain if you have an air leak at the manifold, all adjustments to the carbs are worthless. To check you can spray water or if you are rich, carb cleaner along the manifold joints and listen for a change in the engine note with the engine at idle. Air by-pass screws etc. will be ineffective as air, like electricity, will always take the easiest route!

 

[Dave Russell]

Bob,
From what I gathered, a slight richening of the idle mixture screws fixed the problem. Given the normal limited idle mixture adjustment range of the Weber carbs, mixture is mostly controlled by the fixed idle jets, it's pretty unlikely that there is a vacuum leak? Or did I miss something?
D

 

[Matthew E. Herd]

That's the thing though, Dave. I probably wasn't clear about how much I turned the mixture screws out. I was playing w/ WD40, but I could not detect leaks (I was planning on trying vaseline next). However, the idle mixture screw settings are 3.5 turns out -- way too much! It would seem that, if they exist, the vacuum leaks are causing the mixture to be too lean as a result of the leaking air. I plan on doing more exploration, but your suggestions seem to be along the same track as I was thinking ... basically, it runs good up top, but not so well at idle (speed is adjustable indeed, but I question the mixture quality).

I suspect that this may be the cause of my mysteriously 'missing' cylinders. When I pull the plug wires on 1 and 3, nothing really changes. But when I added some choke and tried it again, there seems to be some change in engine speed when the plug wires are pulled.

If my reasoning is correct, richening the mix by adding choke seems to make the cylinders contribute, so perhaps the mixture was simply too lean ... It could also be that there is a blockage which causes this difficulty.

 

[spritenut]

Check for dirt or something in the idle jet.
I just had this problem, no idle but great at speed.
Turned out to be a bit of dirt stuck in the idle jet.
I found this out ny swapping the idle jets from front to back and my idle miss moved from 1&2 to 3&4 so a little compressed air and all was fine again.

 

[BACLVBob]

[ QUOTE ]
Bob,
From what I gathered, a slight richening of the idle mixture screws fixed the problem. Given the normal limited idle mixture adjustment range of the Weber carbs, mixture is mostly controlled by the fixed idle jets, it's pretty unlikely that there is a vacuum leak? Or did I miss something?
D

[/ QUOTE ]
The origonal comment was that an vacuum leak was suspected. My comment was that if that was indeed the case then it needed to be rectified 1st before spending time and money on setting up the carbs.

 

[Dave Russell]

[ QUOTE ]
That's the thing though, Dave. I probably wasn't clear about how much I turned the mixture screws out. I was playing w/ WD40, but I could not detect leaks (I was planning on trying vaseline next). However, the idle mixture screw settings are 3.5 turns out -- way too much! It would seem that, if they exist, the vacuum leaks are causing the mixture to be too lean as a result of the leaking air.

[/ QUOTE ]
Matt,
Vacuum leaks could be a problem, but you seem to have eliminated the possibility.

Sorry, I wasn't paying closer attention earlier. A few questions:

Are we talking about your 76 Midget with two Weber 40 DCOE's.

If so, what size main chokes (venturis) do you have. You can probably look down the throat with a flashlight & see a number such as 32 on the edge. If you need to pull one to look, there are two set screws on the bottom, secured with lock tabs. One for the Aux. venturi, & one for the main venturi. You probably already know this.

Did the engine ever run really well & idle well or has this always been a problem.
If it is a recent problem, plugged idle jet passages may be the problem, as suggested. Either in the jet or the passages. If you don't have compressed air, a bicycle pump will work.

If it never did idle & run well unless the mixture screws are 3.5 turns out, & the jets not plugged, you may have the wrong size idle jets installed. Something like a 40/F9 or 45/F9 should be in the correct range. While you're cleaning the idle jets note the size marked on them.

A ballpark jetting setup would be:
Main venturis = 30 to 32
Auxilliary venturis = 3.5
Main jets = 130
Air correctors = 150
Emulsion tubes = F11
Idle jets = 45/F9
pump jets = 35
Aux. vents = 4.5

Forgive me if you already know this info, I just am not clear on exactly where you are on this project.
D

 

[Matthew E. Herd]

From memory, those jets seem to be about what I had installed. I recall that (this was a year ago) I was unable to order the appropriate sized aux. venturis, but everything else was available. I will do some looking, and determine exactly what the jets/chokes are. I had determined some baseline sizes for all the parts via the internet and have been using them since, with the exception of the aux. venturi. It runs fairly well, except for a rough idle and a miss. As spritenut has suggested, I will examine the idle jets for dirt. The carbs were used 40mm DCOM's and were not perfectly clean (I had problems with the accelerator jets, so the car would not start initially). I also plan on getting some more of the 'softmounts' but have been very unhappy with their quality (the possible cause of the leaks). The carbs were installed, then removed a day later to clean them. When attempting to reinstall, the rubber O-rings had swelled and would no longer fit properly. This is why I suspect leaks. Also, I have what looks to be a vacuum port on the front of each carb (with respect to the car's direction of travel) which is parallel to the ground and angles diagonally forward about 45 degrees. It doesn't seem to actually be a vacuum port as when plugged, nothing changes. It was proposed that it was a float bowl vent? I am unsure about what this is.

Thanks again,

 

[Matthew E. Herd]

Thanks for the help, esp. to spritenut. I blew out the idle jet passages today and magically, the idle picked up 1000 rpm. Just gotta readjust the carbs, but the car is quite happy now. No evidence of stumble, even when leaned back a bit more, and no smoking whatsoever (a bit on acceleration was detected before). Looks like my problem is solved! Love the easy solutions.