Weber 38DGAS Power valve

[Tru-Craft]

My GT has been running great above 2,500rpm, pulls strong, plugs look great!
But below that, it runs rich, loads up, stumbles.............
If I turn the A/C on, it's worse!!!!

My GT:
1970 GT, 2.0, 265 Chevy pistons, 9.5:1, RallyBob designed ported big valve head, with a F306/290-8 cam from Cam Techniques, roller rockers, RallyBob intake, MSD/pertronic ignition, modified distributor, NGK plugs .040 gap, quick trip header, 2" exhaust, K&N air filter.

My 38DGAS:
Note: altitude is 20ft
Main-150
Air-180
Idle-50
Pump jet-55
Emulsion Tubes-F66
Power valve-6-1/2 pop-off

The problem is my idle vacuum is about 8-9 at 1,100rpm when warmed up.
When cold less than that.
When I accelerate the vacuum drops below 6.5 and the power valve opens, messing up the mixture.
When the A/C is on, the rpm drops slightly and the vacuum drops below 6.5 and the power valve again opens!
If I just open up the throttle plates (1,200-1,300) to compensate, then the transfer holes start feeding fuel and it runs rich.

Solution:
Eliminate the power valve!
I took an old one I had, cut the rod and removed the spring!

The car now idles smoother and partial throttle is better.
It still pulls very strong above 2,500 and the plugs look great!
Any thoughts?
Should I try cutting the power valve spring to get the pop-off vacuum to 4-5, or just leave the power valve off?
Lyle

[krewzer]

BUMP.......................
I need to hear about this! My setup is similar, and I'm experiencing nearly identical vacuum and throttle response problems. Have a new power valve to replace existing one, but would love to hear about options.

[jvandyke]

Me too. Struggling with a 38. I don't know if my power valve is involved. Still learning. In my case, I get stumbling and hesitation on acceration (under load), plugs look okay to me (ain't saying much).
Supposedly has a hot cam so I don't have much vaccum at speed either. I have to learn about Powervalves I guess.

[Gary]

There are a couple of good posts on this topic:

http://www.opelgt.com/forums/6245-post26.html

http://www.opelgt.com/forums/91336-post122.html

http://www.opelgt.com/forums/117050-post15.html

[Tru-Craft]

Gary, thanks for the posts.
I was going to cut the power valve spring, but, because of the low vacuum signal I have with the A/C on, the power valve would be set really low (about 4-5).
I thought this would make the power valve almost useless.
?
Lyle

[krewzer]

As I was scrounging thru a few carb bodies for another power valve, I found three power valve spacers, on a valve assembly, in an early Italian Weber 38. Do these act like cutting the spring down in order to modify the power valve?

[Tru-Craft]

As I was scrounging thru a few carb bodies for another power valve, I found three power valve spacers, on a valve assembly, in an early Italian Weber 38. Do these act like cutting the spring down in order to modify the power valve?

If the power valve spring was a low compression rate, then yes, if a spacer was added you could adjust the vacuum by adding or removing spacers.
Mine has no spacers, so cutting the spring is the only way to adjust it.

I still don't have my power valve hooked up, and its running good.

I am going to leave it off until I do a dyno day.
Then try putting it back on set to 4-5.
?
Lyle

[yellaopelgt]

I am wondering if the "stumble" with the weber 38 isnt just a coincidence. I have heard of a few opelers that have the same scenario. I was told the transition from idle jets to mains wasnt smooth and thats just they way it goes.It gets better with re-jetting but is still present no matter what you do. This seems to also happen with the 32/36 to a certain degree as well. So is there a possability that because the aftermarket weber carbs werent excactly set up for use in our opels? I could be WAAAY off here but, there are just too many people with the same issues.

just wondering............
Joe

[Gary]

I am wondering if the "stumble" with the weber 38 isnt just a coincodence. I have heard of a few opelers that have the same scenario. I was told the transition from idle jets to mains wasnt smooth and thats just they way it goes.It gets better with re-jetting but is still present no matter what you do.

F-66 emulsion tubes cured my "stumble".

[Tru-Craft]

Me too. Struggling with a 38. I don't know if my power valve is involved. Still learning. In my case, I get stumbling and hesitation on acceration (under load), plugs look okay to me (ain't saying much).
Supposedly has a hot cam so I don't have much vaccum at speed either. I have to learn about Powervalves I guess.

jvandyke, like RallyBob said in the other thread, your fuel pressure/volume is too low and can't keep your float bowl full.
Fix the pump problem first.
My fuel level in the bowl is about 1/2 way up the 2 larger holes above the main jets.

Your jets should be in the (155 mains/180 air) range after you fix your fuel delivery problems.
Lyle

[krewzer]

Lyle,
What ever happened with your disabled power valve on dyno-day? Curious if you kept it out of operation.
Wes

[tbags32]

Where is the power valve located and how do you cut the spring?????

Tom

[Tru-Craft]

Lyle,
What ever happened with your disabled power valve on dyno-day? Curious if you kept it out of operation.
Wes

Wes, still driving my Opel without the Power valve.
It's running good.
I hope to do a Dyno test day at the end of the month.
Following the other dyno day thread.
Lyle

[Tru-Craft]

Where is the power valve located and how do you cut the spring?????
Tom

Tom, the power valve is located in the top half of the carb.
Between the float.
3 screws hold it in.
Cut spring with a small diagonal cutting pliers or a dremel with a cutoff wheel.
Lyle

[bigjim5551212]

With the power valve there are several different ones. but what you are doing is nothing new.

on the DGV webers for Formula Ford Race cars, shops like Ivey Engines take out the power valve diaphram and use 2 brass tubes that pick up gas from the bottom of the float bowl and then they go thru the carb lid and then they exit just above the main venturis. This to get more gas at full throttle.

The 40 DFI and the 40 DFO do not have power valves. You do have to go a lot larger on main jets to make them run good.

[bigjim5551212]

My GT has been running great above 2,500rpm, pulls strong, plugs look great!
But below that, it runs rich, loads up, stumbles.............
If I turn the A/C on, it's worse!!!!

My GT:
1970 GT, 2.0, 265 Chevy pistons, 9.5:1, RallyBob designed ported big valve head, with a F306/290-8 cam from Cam Techniques, roller rockers, RallyBob intake, MSD/pertronic ignition, modified distributor, NGK plugs .040 gap, quick trip header, 2" exhaust, K&N air filter.

My 38DGAS:
Note: altitude is 20ft
Main-150
Air-180
Idle-50
Pump jet-55
Emulsion Tubes-F66
Power valve-6-1/2 pop-off

The problem is my idle vacuum is about 8-9 at 1,100rpm when warmed up.
When cold less than that.
When I accelerate the vacuum drops below 6.5 and the power valve opens, messing up the mixture.
When the A/C is on, the rpm drops slightly and the vacuum drops below 6.5 and the power valve again opens!
If I just open up the throttle plates (1,200-1,300) to compensate, then the transfer holes start feeding fuel and it runs rich.

Solution:
Eliminate the power valve!
I took an old one I had, cut the rod and removed the spring!

The car now idles smoother and partial throttle is better.
It still pulls very strong above 2,500 and the plugs look great!
Any thoughts?
Should I try cutting the power valve spring to get the pop-off vacuum to 4-5, or just leave the power valve off?
Lyle

I have not checked vacuum on cars for a long time. But this sounds really low. Like if you had a camshaft in it. I had a Datsun with a vacuum guage on it and it was about 15. When I took a tune up course once, it was said to tune your car to the highest steady vacuum. You might have other issues like ignition timing and such, but thats my 2 bits worth.

[Tru-Craft]

Yep, the cam is the low vacuum "problem".
Can't have an agressive cam profile and idle good.
Still don't like the way my GT runs at idle/partial throttle.
But, it runs so good above 2,500rpm's!
Waiting for RallyBob to come to Florida next month!
Need some tuning help.

Lyle

[Tru-Craft]

I ordered some jets to try and get my bog below 2,500 a little less.

My 38DGAS before:
Note: altitude is 20ft
Main-150
Air-180
Idle-50
Pump jet-55
Emulsion Tubes-F66
Power valve-blocked off

My 38DGAS now:
Note: altitude is 20ft
Main-152
Air-185
Idle-55
Pump jet-70
Emulsion Tubes-F66
Power valve-blocked off.

Talked to a Weber Tech guy (Joe) and he said Formula Fords (low vacuum engines) block the power valves, like bigjim5551212 said in a previous post. He never tried it on Opel's but thought it should work.

Joe suggested:
*Richen up the pump jet.

*Go richer on the idles and leaner on the mains.

He also said the idle jets control the fuel flow up to about 2,500rpm's and they still are dumping fuel in at higher RPM's. That's why he suggested richer idle's and leaner mains.

The bog is less, but still present.
Seems to be running better at higher rpm's, and pulls very strong above 3,500 to 7,500.

Oh, I put the power valve back on, and the bog was worse. Removed it.

Will try 057 Idle jets next.

Getting close.....amazing what a couple thousands of an inch larger hole can do!
Lyle

[jmbinjax]

Please keep us informed. I'm in the process of building an identical engine except for the roller rockers and the header, I'm using a sprint exhaust manifold.

[Tru-Craft]

If your motor is close to mine this would be a good starting spot.

My 38DGAS now:
Note: altitude is 20ft
Main-152
Air-185
Idle-57
Pump jet-70
Emulsion Tubes-F66
Power valve-blocked off.

The low end response is pretty good below 2,500 now.
Lets remember what we have here.
Only 2,000cc, 2 valve motor, not a cross flow head, aggressive cam, carb, older ignition system.........
Upper end power is very good!

We are so spoiled with modern motors, 40 years of technology is hard to beat! Fuel injected, 4 valve heads, variable port runners, electronic everything!
But, expensive and most of us can't work on them.

Let's not forget the advantages of our Opel's. I can rebuild my carb & ignition system and be running in 2 hours! For less than a $100 bucks!
I will take my GT.
Lyle