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Dying on deceleration
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PetulantCobra
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PostPosted: Sun Feb 12, 2012 5:42 am    Post subject: Dying on deceleration Reply with quote

This is a bit of a long one but I wish to provide all possible relevant details.

As long as I have had my bus it has exhibited the strange and mildly irritating behavior of dying when I take my foot off of the gas after running on the interstate for a while. My vw buddies and I just chocked this up to VW weirdness, the old girl wanting to take a break after some hard work.

I was content to leave it at that because I could just take my foot off the clutch and the engine would restart and run like a dream most of the time. Occasionally I would have to baby the engine speed down to idle from highway so it would not cut out but after it got to idle it would usually stay there until I asked it to do otherwise. This problem is usually worse when it is colder outside but seems to be directly related to how long it has been running at high speed. Longer=more likely to fail to idle.

Yesterday, oh yesterday. I drove all around town, all was well with the world. I even did some freeway driving and she decided to idle after coming off of the highway. I was home for a little less than an hour then I realized that I needed to get some groceries. I drove to the first grocery store, about 5 or so minutes away and all was still well. I was in the store for 20 minutes at most, probably less and all was still well. She started right up and drove to my next stop, the store directly across the street from the first. I was in there for maybe 10 minutes, probably less.

Here is where the fun started. I turned the key, the engine fired and then died. Ok I say, probably just need to give it a bit of gas. I did so and it ran but it did so a bit rougher than usual in that it sounded like it was missing a cylinder every few revolutions. Not once on my way home was I able to coax her into idling for any length of time. In fact combustion failure occurred immediately after taking my foot off of the gas. So much so that I had to powershift in order to prevent the engine from dying in between gears. I babied her back home using the handbrake where she died in my driveway. I got out and was greeted by a smell that I would only expect to encounter if I was actually inside of the gas tank. Keep in mind that the distance between where the engine was last running and where I encountered the smell was no less than 30 feet.

This morning I give it a go and she fires right up, less than 1 second of cranking and idles like a kitten with hardly any smell of gasoline at all. I let her run for a while. No problems, automatic choke is working just like it was yesterday. I did not drive it because I am parked in at the moment and it is way to cold for me outside right now, about 22 degrees F. It was about 55 degrees F yesterday when this problem happened.

Other details:
Idle cutoff solenoid has not failed
Spark plug wires are clean and firmly attached.
Electronic ignition
SVDA Dizzy
Fuel system not clogged
Recent oil change (not that it has anything to do with this)

Not yet fiddled with timing or valve gap as it is too ruddy cold. Maybe tomorrow if it gets significantly above freezing.
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gands
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PostPosted: Sun Feb 12, 2012 6:19 am    Post subject: Reply with quote

My 69 tin top displayed this rude behavior, I rebuilt the carb. Also had to flat file the base plate, very important, this was the whole problem. Spray carb cleaner around the carb and see if you have a leak. Carefully.
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Randy in Maine
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PostPosted: Sun Feb 12, 2012 9:02 am    Post subject: Reply with quote

At least adjust the valves and check the timing since that is free to do.

Are those heat risers getting all toasty warm?
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Wildthings
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PostPosted: Sun Feb 12, 2012 9:35 am    Post subject: Reply with quote

Sounds like it could be a failure of the float/needle valve causing flooding at idle. What carb(s) are you running and what is your fuel pressure?
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PostPosted: Sun Feb 12, 2012 1:18 pm    Post subject: Reply with quote

electronic ignition would be my guess as to yesterday. Your plugs were fouled proabably for lack of adequate voltage.

As for the dying off the freeway. If this is a single carb lean the idle mixture an 1/8 turn and check the dashpot action. Also check the throttle positioner as its job is to help stop that. If these are dual carbs, sync them and reset the idle mixture.
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grtfldan
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PostPosted: Sun Feb 12, 2012 3:10 pm    Post subject: Reply with quote

Could you go into more detail on "flat file the base plate"?
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PostPosted: Sun Feb 12, 2012 3:36 pm    Post subject: Reply with quote

it means pulling the carb off. Pull the studs if it has them. Lay a straight edge across the bottom and make sure it is flat. Side to side and corner to corner. Then you use a large file or piece of glass and wet and dry to make it flat again.

The reason an engine will sometimes die when you come off the throttle is that when the throttle closes it creates a rich running condition for a second. That mixture can be so rich that it quits burning and the engine dies. When you lean it a little so that it is just lean of perfect at idle the mixture may be lean enough to burn still. Too lean and it will spit and pop when you step on it, too rich and it will die after coming off the throttle.

I can remember my 1971 doing that from time to time with weather changes - summer to winter. VW and most car manufacturers added a dashpot and / or throttle positioner to slow the throttle close to prevent the engine from dying. By now most of those have been taken off or are worn out.

Image may have been reduced in size. Click image to view fullscreen.


Even though you may have a late bus with a progressive, the theory is the same. You'll see that engine go rich situation all the time at the track when the cars come off the throttle going into the corner. Many progressives had dashpots to stop the problem. Leaning the idle a tiny amount may be all it takes to stop the stalling.
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PetulantCobra
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PostPosted: Mon Feb 13, 2012 9:57 am    Post subject: Reply with quote

I am running a 34/3 and I have never seen a dashpot before in my life nor had I even heard the word. When I bought it there was a Brazilian carb, and while the Brazilians are a lovely people, their carbs are sub-par. I switched it out with a new German one at great expense and neither had this mystical dashpot.

I do like the idea of leaning it out a bit because it smells way too rich and I just now noticed that it blows a puff of white smoke out of the tailpipe every-so-often.

I found that the timing is spot on and that the idle cutoff solenoid had wiggled itself loose as did the carburetor nuts. I changed the gasket and tightened everything down. That fixed the idling problem. Now it wants to sputter out when accelerating from a stop. I remedied this problem somewhat by reattaching the vacuum hose from the dizzy to the carb (whoops Embarassed).

I like the fouled plugs idea and in fact I had the same idea myself and when I went to check them I found that I had loaned my ratchet extension to a friend and had forgotten to ask for it back. More on that later.

As for my fuel pressure I am certain that if it is in any way off of spec it will be a bit lower on account of the enormity of my fuel filter which is a WIX 5 micron fuel filter/water separator and has a capacity of roughly 3 cups of fuel.
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PostPosted: Mon Feb 13, 2012 10:56 am    Post subject: Reply with quote

The size of the filter does not affect the fuel pressure.

And are the heat risers working well?
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PostPosted: Mon Feb 13, 2012 12:08 pm    Post subject: Reply with quote

those loose items will certainly cause issues. The problem with the rich mixture as I mentioned is that due to carb design they go lean when acclerated and rich when decelerated. Happiness is in a mixture neither too rich nor too lean. I always knew when I got the 1971 too rich because it would die on decel especially at the end of the freeway ramp. If you lean it do it only like 1/16 to 1/18 turn leaner at a time. See how it does. If it is worse go back.
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PetulantCobra
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PostPosted: Mon Feb 13, 2012 4:28 pm    Post subject: Reply with quote

I did not think that the size of the filter would reduce pressure. The heat risers seem to be working fine. they are getting warm after all.
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PostPosted: Tue Feb 14, 2012 10:03 am    Post subject: Reply with quote

SGKent wrote:
those loose items will certainly cause issues. The problem with the rich mixture as I mentioned is that due to carb design they go lean when acclerated and rich when decelerated. Happiness is in a mixture neither too rich nor too lean. I always knew when I got the 1971 too rich because it would die on decel especially at the end of the freeway ramp. If you lean it do it only like 1/16 to 1/18 turn leaner at a time. See how it does. If it is worse go back.


I haven't heard anyone mention the fact that the '71 used an active idle retard distributor so that the 34 Pict3 idle circuit *could be opened up to proper airflow*.

I have seen sooooo many stalling issues come back to a failed or removed retard diaphragm where the idle speed is now too high without that retard, and people adjust the idle speed down at the big brass screw to the point where the engine cannot hold its idle after deceleration.
Colin
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PetulantCobra
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PostPosted: Sat Feb 18, 2012 8:18 pm    Post subject: Reply with quote

Well, it has been a few days and I took a 40 or so mile highway trip and I encountered no problems. No bucking, stuttering or dying on deceleration which is odd as I have not fiddled with anything since the last time it did that.

I really think it has something to do with the ambient air temperature because that seems to be the only constant factor. Next week is supposed to be rather warm so I will take a trip some place and see if anything happens
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PostPosted: Sun Feb 19, 2012 11:45 am    Post subject: Reply with quote

I, too have noticed it is dependent upon ambient temps and weather(pressure).
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PostPosted: Sun Feb 19, 2012 1:35 pm    Post subject: Reply with quote

Huh. Good thread as I was having this exact problem with my bus! Drive down the highway perfectly, and occasionally when I'd put in the clutch and the engine would die. Generally I could avoid the problem by waiting until the bus was at near idle speeds before using the clutch. Now I know why.

My problem resolved itself when I switched to my programmable ignition setup. I had assumed it was a timing issue with my DVDA/petronix setup. But I'm guessing it had more to do with my modern coil producing a stronger spark that could ignite the richer mixture. And possible the fact that mine would add advance at engine speeds below the desired idle speed.
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PostPosted: Sun Feb 19, 2012 4:54 pm    Post subject: Reply with quote

Amskeptic wrote:
SGKent wrote:
those loose items will certainly cause issues. The problem with the rich mixture as I mentioned is that due to carb design they go lean when acclerated and rich when decelerated. Happiness is in a mixture neither too rich nor too lean. I always knew when I got the 1971 too rich because it would die on decel especially at the end of the freeway ramp. If you lean it do it only like 1/16 to 1/18 turn leaner at a time. See how it does. If it is worse go back.


I haven't heard anyone mention the fact that the '71 used an active idle retard distributor so that the 34 Pict3 idle circuit *could be opened up to proper airflow*.

I have seen sooooo many stalling issues come back to a failed or removed retard diaphragm where the idle speed is now too high without that retard, and people adjust the idle speed down at the big brass screw to the point where the engine cannot hold its idle after deceleration.
Colin


Colin that is a good point.
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PostPosted: Tue Feb 21, 2012 5:57 pm    Post subject: Reply with quote

Might want to check to see if the float is sticking in the up or closed position.
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PostPosted: Sat Feb 25, 2012 2:29 pm    Post subject: Reply with quote

Float seems to be fine. I think I figured it out. It did that same not starting after being shut off after running fine for a while. The anti-dieseling solenoid had rattled itself loose again. Is there anything I can do to stop this from happening that does not epoxy?
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PostPosted: Sat Feb 25, 2012 3:34 pm    Post subject: Reply with quote

try a little locite, permatex #2 or perhaps some teflon thread tape to make it tighter?
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PostPosted: Sat Feb 25, 2012 4:37 pm    Post subject: Reply with quote

This thread inspired me to add a dashpot back to my 34pict-3 carb:

Image may have been reduced in size. Click image to view fullscreen.


Hopefully this will help improve things when the engine is returning to idle.

I'm also going to go back and add some retard to my timing at idle and see if that also helps smooth things out.

This is all starting to make sense as I had a hard time getting it to idle at speeds below about 1100 RPM's, and ended up having to keep the idle higher than I had wanted.
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