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Starts great, but stumbles and dies
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BruceJ
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PostPosted: Mon Jan 25, 2016 4:34 am    Post subject: Starts great, but stumbles and dies Reply with quote

To summarize, my 40hp "1385" Beetle was running great, but it was time for a regular tune-up. Less than 2000 on the engine since rebuild, most of which trouble-free.

So, I set the valve gap (cold, of course). A couple were a little tight but nothing alarming. Checked/set the point gap, set the static timing (seems to be happy around 8 BTDC with the .009 it's been running). Fired up the engine to check the dwell, and to double-check the total advance with my strobe. Shut it down and changed the oil. Also, double-checked and tweaked the electric choke action with the colder weather arriving. Took the car for a 20-minute spin.

For the first 15 minutes it ran beautifully, like it has been. But on the way back in I noticed it was starting to stumble and miss. This was initially present cruising along in forth gear, but started to spread to all gears and more often, to the point where I barely made it back home.

Naturally I figured I must have done something wrong during the tune-up, so I double-checked everything. I also checked for good spark, and even took the top off the carb (bowl was full, float/valve okay), and accelerator pump spray is good.

When I go to start it up now, it immediately cranks right up, like it always has, but as soon as the initial start-up rev is done, it slowly starts to die on me, within about ten seconds or less. I give it some gas and that seems to only make it worse, and the engine quits. For the few seconds it's running normally, there are no clanking "expensive" sounds, nothing alarming, it just starts to cough and sputter and that's it. There is no backfiring. It sounds like a fuel issue, to my admittedly inexperienced ear, but like I said, carb is getting gas, as until now the rebuilt Pierburg pump has not given any trouble.

So, I've spend a month of Sundays pulling my hair out over this one. I have done the following, often more than once:

-Double-checked valves.
-Swapped to spare points, condenser, rotor, cap, condenser, ignition wires, coil.
-Reset point gap and timing (static -- can't check with engine running now, obviously)
-Pulled plugs, which were a bit black from failed attempts to keep it running for more than a few seconds. Cleaned them, checked gap, reinstalled. They were gassy too, so I know fuel was making it to the cylinders.
-Checked to make sure the new leads went where they are really supposed to.

I even swapped out the entire carburetor. But nothing has made a whit of difference. Starts right up, runs nicely just long enough to offer the brief hope that I actually fixed it, sputters, then dies. Again, carb is wet with gas so I'm assuming the fuel pump's okay.

The fact that giving it throttle makes it worse made me think maybe a vacuum leak, but I'm not sure what would cause that to happen so suddenly after so many trouble-free miles. I double-checked either end of the manifold anyway, but obviously can't keep it running long enough to further investigate for vacuum leaks.

I'm still thinking it must be something I did initially, during the tune-up, but I guess there's also the small chance that something was destined to act up that day anyhow, but I'm at a loss.

Any ideas?
Thanks!
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paf
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PostPosted: Mon Jan 25, 2016 4:52 am    Post subject: Re: Starts great, but stumbles and dies Reply with quote

Hello,

I faced such a problem once, and it was the choke that was not disabling by itself. Actually it was not an automatic choke, and it was staying on overfeeding the engine with gas. After a while the engine has exactly the same trouble. As you act on the choke, check if this is not the cause.
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KTPhil Premium Member
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PostPosted: Mon Jan 25, 2016 10:25 am    Post subject: Re: Starts great, but stumbles and dies Reply with quote

Does your oil smell like gas?

With cold weather a fuel pump diaphragm that was limping along may have ruptured, which will leave the carb starved for fuel and then dilute your oil with gas.
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boomtown
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PostPosted: Mon Jan 25, 2016 12:01 pm    Post subject: Re: Starts great, but stumbles and dies Reply with quote

BruceJ, having exactly the same problem. Ive changed things three times and rechecked too. my thought was not enough fuel but the carb settings are as per the book. My 1600dp has an automatic choke, I wonder how to check the choke? does it heat up and expand or contract the spring. Ill be checking back for other suggestions and will report back if my trials are successful.

I do have a second running 1600 dp that I am tempted to remove the carb and try it in the engine that does not run for longer than five minutes.

David
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BruceJ
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PostPosted: Mon Jan 25, 2016 12:30 pm    Post subject: Re: Starts great, but stumbles and dies Reply with quote

KTPhil wrote:
Does your oil smell like gas?

With cold weather a fuel pump diaphragm that was limping along may have ruptured, which will leave the carb starved for fuel and then dilute your oil with gas.


You know, I'm away from the car for a few days, probably a good thing, so I can think. And I too was thinking fuel issue, specifically, the pump. Practically the only thing I haven't touched. When I did the tuneup, I made a point of having a big ole whiff of the oil and it smelled like, well, oil. But like I said, I didn't have the problems until after the tuneup. On the other hand, the engine dies but the carb throat is wet with gas, and bowl is full.

Still, is it possible that there is residual fuel siphoning through after the engine quits, even though the engine quit, and that by the time I get the carb top off it has filled the bowl again? Or that the pump is working just some, but not enough to keep the engine lit, because obviously fuel is making its way to the carb?

paf wrote:
I faced such a problem once, and it was the choke that was not disabling by itself.


Will double-check the choke, or maybe just disable it completely for diagnostic purposes. I'm pretty sure it's working properly -- I sat and watched it open with the ignition ON -- and has happened with two different carbs and chokes. Still, wouldn't hurt to triple-check it.
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PostPosted: Mon Jan 25, 2016 12:38 pm    Post subject: Re: Starts great, but stumbles and dies Reply with quote

Another common problem is a faulty inlet valve to the float bowl. Carb gasket kits you buy these days have notoriously crappy valves, and a cleaning of your old one is usually a better idea. If the replacement carb was on the shelf for long its gaskets/valves may have dried out, giving you similar symptoms.
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67rustavenger
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PostPosted: Mon Jan 25, 2016 12:41 pm    Post subject: Re: Starts great, but stumbles and dies Reply with quote

Rotate your choke back to the position where is was running without dying. If you disable it, The Bi-metal coil will not heat up, causing the choke to stay closed. The electric choke needs 12v to heat up the B-metal coil and allow the choke to open completely.
Good Luck.
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BruceJ
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PostPosted: Mon Jan 25, 2016 12:41 pm    Post subject: Re: Starts great, but stumbles and dies Reply with quote

boomtown wrote:
BruceJ, having exactly the same problem. Ive changed things three times and rechecked too. my thought was not enough fuel but the carb settings are as per the book. My 1600dp has an automatic choke, I wonder how to check the choke? does it heat up and expand or contract the spring. Ill be checking back for other suggestions and will report back if my trials are successful.

I do have a second running 1600 dp that I am tempted to remove the carb and try it in the engine that does not run for longer than five minutes.

David


I checked the choke a couple of different ways. For starters, I pulled the air cleaner off, pulled the arm off the stepped cam, and made sure the choke sprang to the closed position (cold engine, unheated garage). Then I turned the ignition on and watched the valve slowly open. I think I've read it takes a minute or so, but mine takes more like three to reach the completely open position. Cam moves along with it until it's at the lowest, warm idle position, and the choke valve is completely vertical.

To double-check, once it was cool again I undid the three screws completely and pulled the choke off the carb. Leaving it plugged in to the -1 terminal of the coil, and lying on top of the generator for grounding, I turned the key ON and sat there watching the thing unwind. Also, I made marks on the choke housing corresponding where the full open and full closed positions of the thermostat hook, for reference.

Can't remember if it winds or unwinds as it heats, but the main thing to note, in my way of thinking, is the position of the hook that contacts the valve pin.
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BruceJ
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PostPosted: Mon Jan 25, 2016 12:45 pm    Post subject: Re: Starts great, but stumbles and dies Reply with quote

67rustavenger wrote:
Rotate your choke back to the position where is was running without dying. If you disable it, The Bi-metal coil will not heat up, causing the choke to stay closed. The electric choke needs 12v to heat up the B-metal coil and allow the choke to open completely.
Good Luck.


Tried that, didn't work. What I meant by "disabling" the choke was to leave it plugged in, just rotate it far enough so that, wound or unwound, the valve is always open.
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BruceJ
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PostPosted: Mon Jan 25, 2016 12:49 pm    Post subject: Re: Starts great, but stumbles and dies Reply with quote

KTPhil wrote:
Another common problem is a faulty inlet valve to the float bowl. Carb gasket kits you buy these days have notoriously crappy valves, and a cleaning of your old one is usually a better idea. If the replacement carb was on the shelf for long its gaskets/valves may have dried out, giving you similar symptoms.


Yes, I had heard about this. I didn't open up the new carb (and it is new) to check, but I have a bunch of spare carb parts and have swapped out the valves on the existing carb.

Still focusing on the pump, though, i.e., if it's possible, in this case, to have a bad pump even though the carb appears wet and the bowl is full?
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PostPosted: Mon Jan 25, 2016 3:06 pm    Post subject: Re: Starts great, but stumbles and dies Reply with quote

Perhaps you should give yourself some piece of mind and check your pump pressure-a gauge isn't that expensive (I think I paid $16)-pressure should be about 3psi.
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PostPosted: Tue Jan 26, 2016 3:11 am    Post subject: Re: Starts great, but stumbles and dies Reply with quote

Gary Haberman wrote:
Perhaps you should give yourself some piece of mind and check your pump pressure-a gauge isn't that expensive (I think I paid $16)-pressure should be about 3psi.


Good idea Gary, I'll do that. Not sure I can keep the engine running long enough for a meter to give me meaningful info but it might be worth a shot, thanks.
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PostPosted: Mon Feb 01, 2016 11:24 am    Post subject: Re: Starts great, but stumbles and dies Reply with quote

Ok, had some time (to waste, as it turns out) today.

Dipstick did not smell like fuel, but as ideas were very limited I pulled the Pierburg pump off and rebuilt it (again) with parts I had on hand from WW. (Yes, I preset the diaphragm with a jig I made). Reinstalled pump, cranked it up, and it did the exact same thing . . . ran wonderfully just long enough to make me think I fixed it (10 seconds, maybe), but slowly dies, with no throttle response. Just when I thought I might try to get a video with my smartphone, the engine quit.

I double-checked all the fuel lines. Tank was resealed within recent memory, and when I opened the line at the filter I installed (underneath, near the clutch lever) plenty of fuel came out in a healthy stream.

I changed out the spark plugs. I don't know why. Next I'm going to check the tire pressure, and make sure the distributor is compatible with the upholstery.

Also, I don't smoke, but I'm thinking I might start. What's a good brand?
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PostPosted: Mon Feb 01, 2016 2:57 pm    Post subject: Re: Starts great, but stumbles and dies Reply with quote

Try running it with the fuel filter off completely just using a vacuum connector splicing the line for test purposes.
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PostPosted: Mon Feb 01, 2016 3:32 pm    Post subject: Re: Starts great, but stumbles and dies Reply with quote

Is the vacuum port on the carby plugged ?
Have u tried driving it again since?
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PostPosted: Mon Feb 08, 2016 3:31 am    Post subject: Re: Starts great, but stumbles and dies Reply with quote

I had a similar problem and found that the auto electro cutoff valve was bad, that's the little cylinder coming off the right side of the carb wired to the coil along with the choke heating element. I replaced it with a jet valve from an extra carb I have. Runs great again without run on which was surprising.

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PostPosted: Wed Mar 02, 2016 6:15 am    Post subject: Re: Starts great, but stumbles and dies Reply with quote

Update: I had a highly experienced friend look at it, and it turns out that it was the condenser after all. Which is frustrating, because the old one checked good (even though I swapped it for a new one anyway). It took him a while to pinpoint; eventually he swapped out the whole dizzy for a known good one (the one thing I had not tried) and the car ran great. From there he tracked it down to the condenser.

So somehow, the condenser was allowing the car to start and run briefly, but not stay running or accelerate.

boomtown wrote:
BruceJ, having exactly the same problem. Ive changed things three times and rechecked too. my thought was not enough fuel but the carb settings are as per the book. My 1600dp has an automatic choke, I wonder how to check the choke? does it heat up and expand or contract the spring. Ill be checking back for other suggestions and will report back if my trials are successful.


^^^ Hey David, did you figure out what your issue was?
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