Carb loading

[krewzer]

I was in a time crunch, and that was the only one I could find quickly. I can't say that it worked, because it didn't solve my problem. It's time to replace this carb.

[BQS4]

Originally Posted by krewzer I just bought a $25 Spectre adjustable regulator that adjusts from 0-5 psi in 1/2 lb increments. It was all I could find in a short amount of time, locally. Autozone has them in their "performance" section.

I don't know if this piece of info will help, but, I too have a 38 weber, but, it will be quite a while before I get to use it. In the meantime, I have been reading as much as I can about the 38. I seem to recall RallyBob saying the 38 likes "volume" and not so much pressure. It might be that you need to increase the size of your fuel lines, more then you would need pressure.

[BMWonly]

Not sure if this will help, but here is my configuration:

New 38 Weber on a mildly modified stock engine: 1.9 with 60 over high compression pistons, 2.0 valves, some intake porting, combo cam from OGTS.
I am using the stock mechanical fuel pump, and it's running fine, although I had to increase idle speed a little. Otherwise, it will die every once in a while idling at a long traffic light. I am pretty sure I have adjusted the idle circuit correctly, and I use 135 main jets with the F66 emulsion tubes (changed from F50).

Dieter
71 GT

[RallyBob]

Originally Posted by krewzer I have good vacuum, around 12, and the accel. pump is in great condition.

Huh, and all along I thought this thread was about eating excess calories!

I'd look at the power valve. 12 inches of vacuum is about 2 inches below the point at which the stock power valve opens. If the power valve opens, you have unmetered fuel being dumped randomly into the carburetor in sync with the pulsing of the idle. Very common issue with Webers and big cams/low vacuum. You'd want to modify the power valve spring to open at about 10 inches....2 inches below your idle. This way the power valves stays closed at idle, and only opens when you step on the throttle (increased engine load=lower vacuum). With the power valve closed at idle, then only the idle circuit will be operating, and the carb shouldn't load up (I get it...carb loading!).

Bob

[krewzer]

Before there was even a power valve installed, I was having trouble with loading. For the past year or so, I have been using a block-off at the power valve mount. Truecraft and a few others have done this mod to the power valve, whereby the spring and shaft are removed, and the hole is blocked by JB weld. This has worked well for some time. I just reinstalled a new valve last Thursday at my mobile mechanic's suggestion. However, this power valve has three of the power valve adjustment spacers that go between the spring and the spring retainer at the end of the shaft. So it is an adjusted power valve. Unfortunately, I am unsure as to how to check that measurement. I have read the posts concerning the measurement, and have a Mighty-Vac, but don't really know where to hook the pump to check the measurement. Can you explain the hook up procedure for attaching the pump to the carb top?

[RallyBob]

Originally Posted by krewzer However, this power valve has three of the power valve adjustment spacers that go between the spring and the spring retainer at the end of the shaft. So it is an adjusted power valve.

Adjusted the wrong way unfortunately! You need to reduce the spring tension, not increase it with shims!

Unfortunately, I am unsure as to how to check that measurement. I have read the posts concerning the measurement, and have a Mighty-Vac, but don't really know where to hook the pump to check the measurement. Can you explain the hook up procedure for attaching the pump to the carb top?

There's no 'hook-up' to speak of. There is a passage in the lower carb body that reads vacuum from the intake plenum. This meets with a corresponding passage at the top cover plate of the carburetor...you can see where this is by following the cast-in channel from the power valve. I simply hold the end of the hose from the vacuum pump tightly against this passage (it's maybe .060" to .080" in diameter), and draw vacuum until the power valve*just* cracks open. Note the vacuum reading on the gauge...this is the opening value. If it's higher than 12" of vacuum, then the power valve is always open at idle and dumping fuel. In your case it should be about 10" before it opens, you would adjust this by clipping coils from the power valve spring. Try one half-coil at a time, rechecking after each cut. You don't want to go too far, as that's just as bad...it'll create a big bog.

HTH,
Bob

[bigjim5551212]

still at an idle and there is gas coming out of the auxilliary venturi, that tells me
1) fuel pressure is high
2) Float Level is too high
3) Needle valve worn
4) something keeping the needle valve open
5) sunk float

the power valve should not really have anything to do with how the idle is

[wrench459]

Well I've got a 38 in hand and looking.
Thanks for the loaner Mr. HC
What I'm seeing with this particular carb is one barrel throttle plate is right.
It's closed working on the idle circuit.
Now the other barrel is half way into the transfer slot.
Oh my its out of sync.
With a large cam the first mistake is to open up the throttle to get a half way decent idle. Now your out of the idle circuit.
Now add the addiction of extra fuel supplied from the power valve into the wells and there just might be a problem.

[krewzer]

So, what's the answer. Bigger cam with an idle requiring at least 1000 rpm or better for idle. You're on the fence between transitions. Is that all there is? It seems that the 38 is problematic with these engine combinations. Is drilling the throttle plates the solution to preventing the flooding at idle? I'm pulling this one off and trying a new 32/36 when it arrives. I have to relearn jetting for this one. I really like the 38, but am pulling what's left of my hair out, trying to keep her functional.

[opelbits]

The 38 shouldn't be a problem. Even on my relatively stock 2.0e it can idle really nice. Right now I am producing about 15-16 in. of vacuum (which seems low to me).
It seems everytime that I find the fuel dribbling into the venturi, it's because the idle speed screw is too far in (throttle plates opened and high idel speed) and due to a vacuum leak.
Disconnect and plug all the vacuum ports at the carb/intake and see what you get. The check the carb base.

Don't give up on the 38 just yet. The 32/36 will be very disappointing compared to it.

A little addition here:

If all else seems okay (fuel pressure, no crud clogging jets, floats, etc.), there are only two things I can think of to cause fuel to dribble out at idle.

1-Too lean: vacuum leak requiring the throttle to be opened more to so that it masks the problem, or your idle jets are too small, also requiring the throttle plates to be opened to get an idle. If you find no vacuum leaks, try larger idle jets and see if it changes things. I don't know how many steps higher to go, but I'd try to get some that you know are too big. Theoretically, if the problem swings back the other way (idle mixture/speed screws too far in), then you are on to something...maybe.
2- Too rich: Power valve. If it is bad, it will allow fuel into the venturi nozzles. I have found several bad powervalves lately. I do not know if a backfire thru the carb will damage the Weber PV like it will a Holley PV, if it can, then all it takes is one pop.
Or it is as RallyBob says and the vacuum produced is incorrect for the power valve.

[bigjim5551212]

Originally Posted by opelbits ...........: Power valve. If it is bad, it will allow fuel into the venturi nozzles. I have found several bad powervalves lately. I do not know if a backfire thru the carb will damage the Weber PV like it will a Holley PV, if it can, then all it takes is one pop. Or it is as RallyBob says and the vacuum produced is incorrect for the power valve.

The power valve goes bad in one way.

I have seen the rubber go bad under 2 conditions, 1, removal of the power valve tears the rubber, 2 there is a burr that tears the rubber.

IMHO, a power valve should always be replaced when taking apart the carburetor. A careful inspection should be made of the surface where it sits as well. The rubber will usually be hard and brittle on anything that is older than a year.

To be honest, i have never seen a burned one. I think the passages are too convoluted for fire to get there.

[hrcollinsjr]

Originally Posted by bigjim5551212 The power valve ... To be honest, i have never seen a burned one. I think the passages are too convoluted for fire to get there.

When an engine backfires with a Holley carb in place it blows the power valve with pressure not actual fire. It is designed for vacuum not pressure. IIRC there are aftermarket kits to put a type of check valve in to prevent power valve damage.

Harold

[jmbinjax]

Krewzer:

Have you corrected your problem?

[krewzer]

I did. I was able to fully rebuild my earlier Italian made 38. I also installed the throttle shaft bearing/seal kit from bigjim5551212. The car idles and runs well. Plug color is still a little lean, but not far off. I believe that there is better performance to be had, like from recurving the distributor, and tweaking the jetting for a slightly richer mix. Idle is as smooth as possible with this cam, and acceleration is fierce from just under 3000 rpm. I am searching for a little more torque in gears 1 and 2, but with the rebuild, and the smooth pedal feel from the Weber bearing/seal kit (cured a possible minor vacuum leak), I am more than pleased with the outcome. I tried, or checked a number of the suggestions from all responses that this thread received. I really appreciate all of the invaluable information, that is shared with everyone. I am learning as I go.
Wes

[wrench459]

Way to go Wes
I like to get the car to idle first and respond in the lower end.
Retard the timing a few degrees..4,6 depending on the motor.
Starting as low as possible in the rpm range try and get the A/F ratio where you want it.
""With a N/A motor it's somewhere around 13.5-1 to 14.0-1 for max power but I would not run those mixtures for very long.""

After getting back into a safe A/F ratio range then start increasing the spark timing in small increments until the torque starts to drop. Back the timing 2 degrees from the peak and your done. Until parts are changed.

P.S. You'll need at a min. 10x glasses to read the sparkplugs. Unleaded fuel is a pita.

[jmbinjax]

FYI:

I've had a similar problem to the one described in this thread and wanted to pass on my info. I have the Bob's recipe 2.0. I realized (with Bob's help) that my base timing was off. I adjusted it to 18-22 and was able to close the throttle plates enough to stop the loading. I then had a stumble off idle. I had already modified the power valve as described in other threads. It pulled real good after about 2,500 rpm. I then lowered the pump jet from the standard 70 to 60 and virtually eliminated the stumble. It idles real nice at about 1,100 rpm and accelerates smooth. I'm going to try a 55 pump jet and see if that's too much but will probably keep it at 60.

[tcav]

Wes have you been able to incorporate your compufire back in yet?
tk

[krewzer]

Nope, I have a wiring problem. Me

[bigjim5551212]

the other reason for fuel to be coming out of the auxiliary venturi is that the throttle plate is so far open, the idle system no longer works.

Is the idle speed screw cranked open "a lot", like 3 or 4 turns past where the screw contacts the stop?

Only vacuum or fuel pressure can pull fuel from the auxiliary venturi.

Do your idle mixture screws work? does adjusting them make any difference in how the car runs?