help with tuneup

I am trying to adjust my carb. When I close the smaller screw and then turn it back 2.5 times, I then set the big (volume) screw to idle at 900rpms. No problem so far but when I go back to the smaller screw and turn it slowly the rpms don't drop nor if I turn it back clockwise no change at all. So its idling at 900, smells rich (fumes), sometimes pops from the tailpipe. I did check the fast idle screw turned it all the way out and then back to the point it touched and then another 1/4 turn. Sometimes the idle will accelerate then back down.

1970 Beetle
new carb
new points (checked out a 44-50%)
timing is dead on
new vacuum tubes, plugs, plug wires
new fuel pump
new distributor cap
Distributor was replace with one from another car
(this is something that was done when the vacuum tubes were put in by a volkswagen mechanic)
New fuel line and tank

I keep thinking something is wrong because it tends to hesitate a bit in 4th gear.

Any help would be appreciated.



Kara

Assuming that the carburetor isn't in need of cleaning, and that the idle mixture screw seat isn't damaged by someone turning it too tightly closed, the idle mixture screw shouldn't be any further out than 2 full turns from completely closed. Normally the idle mixture should be about 1 3/4 turns out from the closed position. If the idle mixture screw in turned closed, the engine should die at idle. If it doesn't, then the carburetor is probably in need of disassembly and cleaning by soaking in cleaning solution for a few hours.

If I remember correctly, the idle RPM should be somewhere around 700, but it's been so many years that the exact figure is long forgotten.

That "popping sound:" Does it sound like a muffler noise or does it only occur during hard acceleration? There was a problem with head studs pulling out of the case with the years 1968 through 1972 because the crankcase metal was made too soft. The only cure for that was to completely disassemble the engine, remove the head studs, drill out the threads in the case to a larger size, and install what Volkswagen called "Case Inserts." They looked like a larger bolt drilled and threaded out in the middle to accept the stock head stud, in effect giving the head studs a bigger grip on the crankcase. That was and is the only cure for the loose head stud problem. Other non-factory "cures" didn't work. One was a self-tapping larger head stud that was forced into the case. One would think THAT would have worked, but it didn't because the metal in the case was already being pulled by the original head stud. Trying to anchor the self-tapping larger head stud into metal that was already pulling loose from the crankcase was a fruitless effort but it sounded like a good idea. Some people used "Loctite," but that was totally worthless. The people who did that had no idea that the crankcase threads being pulled loose was the problem. They thought that the studs were unscrewing themselves. However, at this date in time I'd be surprised if the loose heads problem is with your engine because by now, years and years later, most of those engines have been modified by having the case inserts installed.

I guess that there is a remote possibility that your engine may be the exception. With the loose head problem, during hard acceleration the engine would sound like plopiety-pop sound....not quite like a leaking muffler noise. In the initial stages the noise would only occur during full throttle acceleration. As the problem worsened, the noise would become more constant. With a leaking muffler, the noise would be constant but vary in intensity depending on the amount of throttle. The easiest thing to do is stop by a Volkswagen repair shop and have the mechanic listen to your engine as you accelerate away from him/her. Once you've heard the loose head problem you can immediately identify that same problem with any other VW.

If the popping noise is directly from out of the tail pipe, then the loose head studs isn't your problem. It could be anything? Too rich an idle mixture setting with the timing way, way off. You are setting the timing with the vacuum line off, aren't you? Otherwise the timing will be too retarded. Hesitation is usually caused by timing that is too retarded. One cure for hesitation is to advance the timing a few degrees. Sometimes that has to be done to compensate for a difference in gasoline. Your carburetor could be jetted wrong too, especially if it isn't a stock VW carburetor. Interestingly, the air-cooled VW's are supposed to run on 91 octane, minimum...what used to be regular gasoline.

Another possibility could be defective spark plug wires, wire connectors at the spark plugs, or a defective distributor rotor. Easy to check the wires: Wait until night time. In total darkness, start the engine. If the wires are leaking energy they'll have a glow to them in the darkness. See...you don't even have to buy expensive electronics to check for that! Such a problem would also show up on the oscilloscope ...if you had one. The engine was designed to have metal plug wire cores with resistor (1000 ohms) plug connectors and a resistor (5000 ohms) distributor rotor. Especially with the Square Back that has a computer control, if you use anything other than stock type wires, spark plug connectors, and distributor rotor, the engine often won't run correctly. The computer misinterprets the engine function and will cause problems with the way the engine runs.

How is your valve adjustment? .006 cold, intake and exhaust. Unless your valves are way, way out of adjustment they probably wouldn't be the cause of your problem, but anything is possible.

Other than what I've mention above, unless I was there to look and listen to your engine a good diagnosis would be pretty hard for me to do. The engine is very simple to maintain. The problems occur when people start getting away from the stock engine. Too often after-market conversions have their own set of problems that can baffle a mechanic.

Doug

The only thing I saw wrong with the carb proceedure was the fast idle screw should not touch the choke cam, there should be a small gap. Make sure the accelerator cable isn't too tight & turn the coke cam all the way off to set the gap. Hopefully between the cable & screw you can get the idle below 900 with the lagre air volume screw. Then if the smaller fuel screw still makes no difference & can be turned in all the way & still runs rich you may have a carb problem (even the new carbs are problem plagued). The little backfires at the tailpipe are a symptom of running too rich. Were the plugs gapped to .035"? Have you checked the condition of the ignition points & gapped to .016" & where is the timing set? Also what distributor are you using (vacuum, centrifugal)

As usual, boschmann is giving correct information again. However, we both forgot about another thing: What do your spark plugs look like? Are they all black with soot? That would indicate excessively rich running. Eventually the spark would short-out via the carbon in the soot and your engine would develop a miss.

Tonight I checked the timing, with the timing light and a dwell meter, both are still on time. It seems the more I've tried to adjust the carb the more it seems to backfire. Running worse than ever. Guess I need to pull the plugs and check them out. It seems as I adjust the volume screw and the small mixture screw...the rpm never changes except when I close one of them. When I had a volkswagen mechanic do the tubes and replace all of the above he showed my dad a vacuum hose that he said wasn't needed. I wasn't there so I'm not sure what hes talking about. I know the there are like three holes that are plugged in the front of the carb and one on the side. Also when I was trying to adjust the carb I connected a hose from the bigger of the three hole to the open hole that on the side of the engine (would be your left side if you were looking at it.) Both had a sucking and it did seem to stablize the rpms. Might have fouled the plugs from so much adjustment? I had it running pretty darn good, wish I had never messed with it. Does that seem strange that I adjust the mixture screw and get no change in the rpms? And I have to really turn the voume screw in order to move it from say 800 rpms to maybe 1200. I really appreciate any advice you can give me.

Don (Kara's project I've seen to inherited)

If you can't get the idle slowed down then maybe the timing is too far advanced. Try checking the point opening with a test light rather than a strobe light. You'll have to use a socket on the generator/alternator pully nut to rock the engine timing mark left and right to see where it averages out. Turn on the ignition, pop the distributor cap off, pull the rotor out of the way and connect the test light to the points and touch the tip of the test light anywhere on the crankcase to complete the circuit as you rock the timing mark at the crankcase split. I suspect that you have a centrifugal advance and not the stock vacuum one. That being the case, are all the vacuum leaks plugged? Also, I'd reset the mixture to 1 3/4 turns rich from full-in, and leave it there until you figure out any other problems.

Did you reset the fast idle screw? I still would need to know which type of distributor & to which mark are you setting the timing. Also, does the carb have an electric solenoid on it?

First things first.

What are you timing it to? Original specs, or to the distributor?

Many of these cars have had the distributors changed, usually to the dreaded .009 centrifugal advance only distributor.

Some have been changed to a different model SVDA or DVDA dizzy.

Make sure you're timing the engine to the particular distributor, and ignore original engine year specs.

When i set the points, I try to set them at .016 on the lobe of the distributor. After I do this I set the crank pulley and set my test light to set the distributor in the right position. Light never comes on. It will only come on when I set the points to the flat spot of the distributor. Should I be setting them at the lobe or the "flat" spot?

Don

I appreciate everyones help. I've done as all have suggested and nothing works for me. I GIVE UP!! I'll stick to replace automotive glass, that after 20 yrs I know I can do. If anyone has any question concerning glass I can help.

Thanks Again

Don

Hey Don,
Don't give up. This whole problem can be something as simple as a vacuum leak. Do you know anyone who still has one of those old type exhaust gas analyzers? Maybe that's the situation with the fast idle speed and mixture adustment screw required to be set so rich. Eventually you'll discover what the problem is/was. We're all curious to know what the fix was.

Doug,


The other day I replaced the plugs, distributor cap and rotor. I set the points. Used the test light to set the distributor with the crank shaft at 7.5. Then started the car and made sure the timing was still right. Even checked it with a dwell and it was 45-50%. Shut the car off and set the idle screw where it just touched the fast idle and then turned the screw another 1/4 turn. Close the mixture screw and opened it back up as you suggested at 1 3/4 turns. Had the volume screw set so that it was 800-900 rpms. Pushed the accelerator and sounded good. Took it for a test drive and I didn't think I would ever get back home. It would lunge and I drove. Got it back to the barn and made more so many adjustments now all it wants to do is back fire. Thats the things Doug, I've done everything I think is right and it still doesn't run right. I won't give up but you probably now how frustrating it can be.

Thanks for your interest

Don

Sounds to me like the next logical thing to do is take your car to a shop that can check your ignition on the scope. If nothing else, a scope check can either confirm or rule-out problems with your ignition. That's what I'd do next if it were my car.