Carb help please?

[Goatnut]

I am hoping to employ a club carb/tuning expert. I am on my 3rd carb in about 5-6 years. My car is not running correct again and my limited knowledge says...sell it! Well maybe later. But in the meantime, I was hoping to get together to discuss my symptoms, possibly purchase and install a new carb or whatever it needs. Carbs are not something that I'm comfortable with so, I am looking to pay for some help from a club member or maybe you could turn me onto a reliable mechanic.

[THOR]

Goatnut, I'm in the same boat, I need carb tuning smarts right away, my holley is idling too high and my air idle circuits need some tuning as well. I wish I could have gone to the CAR CLINIC over at Denny's place. I was wrenching on something else that day though.
Maybe we could do a carB CLINIC ?
Any members available to show us the light for a couple of hours?

[Goatnut]

I'm open to any suggestions. I have all of these "refreshments" just sitting around too!

[Lfowler]

Hi to all. I've been working on Holly carbs for awhile and tuned some successfully. There are a couple of common failure modes for this part which lead to an uncontrolled high idle. Perhaps one of them is what effects your car.

The first is when the float bowl screws get loose and allow the metering block gaskets to leak fuel across the idle transfer passeges from the power valve area. Then the engine runs a bit rough, slow and stumbles. Generally a fellow turns up the idle air screw (idle speed adjustment) and gets a temporary cure, but the engine still is a bit off and eventually the hot / cold warm up driveability issues lead an owner to compromise other adjustments until the car is so out of whack the carburetor gets rebuilt or more seriously worked over. New gaskets are the answer here.

The next phase of that failure is when someone notices the screws are loose and cranks down on them past the proper torque spcec. This might seal a deformed gasket for awhile but it bends the carburetor body and sometimes the metering plate. Then those parts won't seal correctly and get external air leaks and sometimes internal or external fuel leaks. A carb body can be surfaced in a mill to correct it. It needs to be flat to within .002 each side and the bottom. Squareness and angularity are not real important, just get it within a degree or two. The plates are harder to fix. They can be straightened in a press but one cannot mash the slightly raised gasket sealing ring ridge or the plate becomes junk.

The third major failure concerns power valves. A hard back fire or dieseling sneeze can rupture the valves diaphram. This causes a fuel leak into the main metering system which causes the engine to run rough and slow or lope leading one to open the idle air screw in an excessive compensation. When the engine picks up speed and clears out it's rich condition the idle is too fast so one turns the screw back down. As the fuel continues to leak it richens up the mixture again. This cycle repeats itself with a time constant of several seconds to several minutes. There are different quality levels of power valves, some with better protection of the diaphram than others. Lately the good valves seem to be scarce. They have a metal shield over the diaphram area partially covering it and are generally stamped with a vacum actuation calibration number.

There are several other tuning issues which can give a fast ldle or unstable idle but these are the things I look at first. Be careful working on carb's loaded with fuel on hot engines. Hope this helps.

[GPModJ]

Hey Thor:

How high is too high? Do you have the same symptoms I had? As you know from my post, my problem was a vacuum leak.

[GPModJ]

As an aside, now that my manifold gasket problem is solved, the car is running much better than it ever did before. I think I hosed the manifold gasket right at the start when I installed the engine and it got progressively worse until I had the engine backfire a couple of times, which is when the problem really showed itself.

Before fixing the intake gasket problem, I'd replaced the carb fuel bowl gaskets, the metering block gaskets, the bolt gaskets at the end of each fuel bowl, along with the power valves and accelerator pumps (though they didn't need to be replaced).

When I was done with the intake gasket repair, I reset my timing, used a vacuum guage to adjust the idle screws, set my fast idle screw, and now the car runs really well. It doesn't have quite as much lope as it did before. No stumbles.

I was also having a problem with the car dieing on me when hot and I stopped the car really quick. That doesn't happen anymore either. I think carefully setting the idle screws may have fixed that--along with the intake leak.

[THOR]

Hey Thor:

How high is too high? Do you have the same symptoms I had? As you know from my post, my problem was a vacuum leak.

Glad to hear that you've got your problem licked GP Mod, right on !

After thinking about my situation, I realized that I sort of forgot that
I made a change to the way I’m tapping manifold vacuum and wonder if that’s part of the problem.
I need to have a vacuum source for my 4 wheel power disc brakes. The vacuum source used to be a tapped bung on one of the intake runners and not the best source. Now I have tapped manifold vacuum via a hold drilled into my carb spacer. Ever since this was done, I’ve had the higher idle. Can’t seem to get it below about 1050 rpm, so this checking the new power brake vacuum system connections will be among the things I need to do

[Lfowler]

Hi. I've had several plates deform when tapped creating gasket or crack induced leaks. Propane is a quick way to test them. No flame, just put some cardboard around the air horn a foot or so tall with duct tape to isolate air coming through the carb vs air from other sources (leaks) then put a small dia hose on your torch a few feet long to make a probe. It is safer than carb spray and doesn't risk messing up your engine detailing.

Sometimes gaskets don't fit the plates too well. Air can draw up the adaptor studs if your using a Q-jet to square bore plate.

Boosters are not fussy about vacuum sources. The volumn of air per brake cycle is very low compaired to air flow through the engine. Many boosters have about 200 to 300 cc's of space in them vs 6500 to 7000 cc's of engine displacement. Divide by eight to get, more or less, 800 to 875 cc's per cylinder and figure 50% effective volumetric efficency per intake stroke still leaves plenty of suck using a single cylinder. Not all vacuum is lost in every brake application either. That said, boosters are fussy about the size of the supply hose and fittings. Never undersize a booster hose or use a restrictive fitting. The recovery time for the booster can become unacceptable if engine vacuum is delayed while sucking through a small hose.

Power brake systems can be leak checked with propane the same as manifolds or any other air ducting. If your running a tunnel ram or a lot of spacers so the booster is lower than the carburetion you might want to install a vapor seperator and flame arrestor in the booster hose from a Corvette. Its a real suprize when you blow the booster up with a back fire because vapors accumulated in the low spot. Ignition inside a booster can be from as goofy a thing as an extra long plug wire drapped over the diaphram housing creating a static charge between the case and rubber.

Some of the disc brake conversions get the pedel linkage a bit too long on the actuation side of the booster and make it draw all the time. Then they make the push rod to the master cylinder a bit shorter than it could be to compensate giving a low sorta soft feeling pedel. Listen for a hiss real soft under the dash at the linkage. That's worth only 50 RPM change at most though so odds are your troubles are in other areas. Good luck.

[GPModJ]

Lfowler:

Wow! Some good info there in your previous post and the one above. I visited your site and it communicates a "down to earth, practical experience" type of operation (that's a compliment :P ).

Thor:

Your symptom sounds very similar to mine. Lfowler's suggestions are great and seem to come from years of experience (something I don't have) but here's another one anyway.

Jim Hand, via the PY board, suggested using mechanic's stethoscope to find the vacuum leak. First remove the end of the stethoscope and add a length of hose to the end so it will reach without having to stick your head next to the engine. He said to "calibrate" it first by listening to the sound of the carb venturi. Then run the end of the hose everywhere there might be a leak. It worked great for me. I found my leak in about five minutes.

I'm told if you don't have a stethoscope, you can also simply use a length of hose. Just hold it close to your ear and do the same thing.

[THOR]

Thanks for this info guys, very helpful
I'm Trying to get this all together before the OldTyme Drags and Car show
along with the scattershield.

[Lfowler]

Thanks for the complements. I'll have to update my site to reflect some of my more recient jobs / projects. The stuff shown is 5 - 10 years old and there is nothing from the machine shop.

On the hose vs. stethoscope deal a hose works fine but CAUTION is needed. If you have one end stuck in your ear and the other end finds the leak your seeking the engine will try to inhale your brains via your ear drum. And both your hands are tied up holding the hose ends. Better to put the hose in your mouth, hold it with your teeth, and use your tongue as a sensor leaving one hand free.
Either way has advantages as your ear is more sensitive to small leaks but ones tongue is more durable.

Question? I've got a console mounted vacuum gauge for my GTO that a friend gave me years ago. It has the right part numbers and all that. It seems there is no provision for illumination that I can see on my gauge (a wire or bulb socket hanging out) but I remember watching a gauge work in another friends folk's new (back then) '66 Grand Prix when we were in High School that was lit. Are there different gauges that look alike or am I missing a part of my gauge? or perhaps just remembering wrong. And what fitting was used in the engine compartment to draw off? Thanks.

[GPModJ]

Thor:

He's right. As I mentioned in my other post, I almost had my eardrums sucked outside my head. My advice would be to hold the hose end a bit away from the potential source of leaks.

[THOR]

I didn't realize that manifold vacuum was so danged strong !

[GPModJ]

Thor, have you been able to track down the source of your idling problem?

[THOR]

Thor, have you been able to track down the source of your idling problem?
Not yet GP Mod, I hope to have time on Friday to have a whack at it !