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More on Ye stalling on decelleration and nose down

(71 bus, stock 1600DP engine, standard carb/air cleaner)

Thanks to everyone who has provided suggestions to this pesky problem(engine conks when foot goes off gas. Sometimes. Almost always after engine has been revving for a minute or so, like hitting offramp, or cruising street at 3,000 rpm. But not always. When it does, it conks quickly, no hesitation. When it doesn't, tach shows that it slows to 950 rpm. Braking not required for conk. Engine can be hot or cold. It's an either-or situation: either it conks or it does not. No in-between low-idle situation where it it trying to make up its mind.).

So far I've been able to eliminate:

  1. fuel pressure or delivery to bowl (engine runs fine at WOT for as long as I like),

  1. ignition (timing light mounted in rear window shows I have spark, even as engine is stalling out)

  2. brake boost vacuum leak (no problem when braking, I was wrong earlier)

as possible causes for this odd behavior

This suggests that it isn't mixture (no black smoke, no smell of unburned fuel, and, as I mentioned, it's either-or), nor a vacuum leak (either-or, engine hot or cold).

Prevention/Recovery Technique.

I have evolved this method of dealing with the problem: While driving I can stop it from happening by keeping an eye on the tach when foot goes off gas. If I am not in a panic stop situation, I have enough time to see if the needle is dropping slowly or more quickly. If it is going down to a stall condition, pumping the rpm's up a few times fixes it. Pump, watch tach, pump, watch tach. The first couple pumps usually don't do the job. The third or fourth does. Can tell right away by how quickly the tach is falling whether the engine is in a nosedive or not.

If it does stall, the engine starts up right away with application of starter and a touch on the accelerator.

The problem occurs more frequently and is harder to recover from when the nose of the bus is downhill.

Unless my "Prevention/Recovery Technique" suggests otherwise, I'm thinking I'm dealing with an idle circuit problem.

Jan proposed that when it conks I take this newish aircooled.net 34 PICT-3 Pierburg carb apart and inspect the bowl to see how much fuel is in it. See if there is some idle circuit problem, or a fuel delivery problem. It's the really right idea, but I don't see myself pulling over to the side of the road and waiting for the engine to cool enough to pull, disassemble, inspect, re-assemble and re-install the carb while I am going about my daily stupid errands and stuff. Heck, I am not 100% certain that I can get the carb out without pulling the engine.

I bet there are plenty of Real Men here on RAMVA that would undertake such a task without hesitation. I am, however, a mere squirrel. Timid.

I might just get another carb and install it to see if the problem goes away. Direct, and simple. Can sell the unused one on eBay. I would take a loss, yeah, but I would narrow down the problem. Other than the fact that no Real Man would use this method, is there anything I should consider or try before going this route?

Reply to
Mike Rocket J. Squirrel Elliot
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No no no,

just disconnect air cleaner from the carb, disconnect carb lever return spring remove the 5 top screws, and carefully lift the carb top off. That's all there is to it. One fairly short flat head screwdriver needed. Make sure power is off when you are doing it and watch those exposed alternator terminals. Don't want any metal shorting on them. One os hot at all times.

Once you have the carb top off, you can inspect the fuel level in the bowl with a small mirror if you can't peek in there. I know it's hard to get a good view in there in a bus engine bay.

Don't throw parts at it at random. The new carb might work for a while, but soon enoough you might end up with, for example, ANOTHER rust particle in the idle passage that got there from your rusty gas tank. Plan on buying a new carb again? Better get a full crate of them then.

Hang in there. People replace parts that have nothing wrong with them because they don't bother, or understand how to properly maintain them or repair them. Half of the time they aren't even broken.

Jan

Reply to
Jan Andersson

Squirrel,

Have you done as suggested but multiple posters, i.e. remove the idle jet and clean with comprtessed air...? Also, again a repeat suggestion, simply take the top cover off the carb and look (carefully) for small chunks of dirt, if so, clean. I have had your symptons (though in a bug), same carb same motor, a couple of times; the cleaning of the idle jet fixed the situation twice and once I actually noticed a small piece of crud when I took the top of the carb off. You do not need to do this on the side of the road...

Some have already suggested the above but Jan had the best write-up.... and, in amongst all that fine mechanic talk, he mentioned doing the above....just clean the damn idle jet (one screwdriver) and see what happens....

Jon

64 urban baja
Reply to
joneischeid

Jeepers -- that's a heck of a lot easier than removing the whole carb!

Inspection mirror, check,. Small flat-blade screwdriver, check.

(Called a "minus" screwdriver in Japan. The Philips they call a "plus" screwdriver. Makes sense to me).

Fuel filter element is clean, so I don't think I have a gigantic rust problem. But yeah, your point is taken.

Crate of carbs, check. Hey -- wait a minute!

Thanks for the ass-kicking. I will continue to work at this thing.

Reply to
Mike Rocket J. Squirrel Elliot

But wait . . . before I take the top off the carb while at the curb in order to inspect the inside of the bowl to see if it has fuel in it . . . couldn't we expect the bowl to have fuel in it if the car has no trouble accelerating and cruising at road and freeway speeds? Doesn't all the fuel that goes down the throat come from the bowl?

I guess I mean: why bother with the roadside inspection procedure? Just open the thing and clean out the idle as you and others have said?

Stupid questions # 156, 157, 158, and 159 in the series. Collect them all.

-- mrjs

Reply to
Mike Rocket J. Squirrel Elliot

Carburetors are kind of a mystery to me so . . . to answer your question, I've not done that yet.

What stumps me about this, is something like this (which I read at

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): "The carburetor idle circuit controls how much fuel you have at idle. Where many people run into problems is that they don't understand that the idle jet is the circuit that they do most of their driving on (2500-3000 RPMs). This jet is critical to engine flexibility and economy."

I am having zero -- 0 -- trouble driving. The engine's behavior has not changed even a little bit when driving.

Yeah I didn't understand that. I am not the brightest star in the sky, never claimed to be.

Now that I know that the jet can be removed easily, I will do so. Still . . . shouldn't a fouled idle jet or idle circuit affect driving performance . . . ?

Reply to
Mike Rocket J. Squirrel Elliot

Keep in mind I swore off carbs years ago... Each circuit has overlap into the next. The idle circuit is solo at idle unless the enrichening circuit is active (even so, if the enrichening circuit is merely a choke, it just restricts airflow enough to cause the fuel level to rise enough to richen the mixture, still using exclusively the idle circuit) In carbs with an intermediate circuit, it comes in as the engine rpm rises and more demand for fuel exists. The idle circuit is still working. The main jet circuit is generally considered exclusive at full throttle, but not so. It accounts for the largest percentage of gasoline flow but the idle circuit is still working.

All that said (and it is in a very general way correct - kinda generally!) when your up to speed, you're mostly on the main jet. A plugged or partially plugged idle jet will be hardly noticed then. Consider also when the accelerator pump is activated, a massive amount of gasoline is squirted directly into the throat...

SO - a flake of something could be blocking the idle jet in some attitudes but not in others and may flop over the opening sometimes and not other times; may fully block at times and partially block at times.

Fuel Injection is SOOOO much simpler!

I actually think it's an intermittent blockage in your exhaust system, myself! Try removing the exhaust system completely from the engine and drive it like that until... ;o)

-BaH

Reply to
Busahaulic

Mike,

Check your setting of the "Fast Idle Adjuster screw" You could be blocking off the transfer hole with the butterfly.

Setting help here

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Grahame from Australia

Reply to
Grahame Rumballe

Hope you weren't offended. Somehow I don't think you are the kind of person who would be. :)

jan

Reply to
Jan

There may be enough fuel in the bowl at all times for driving around, but if the fuel level is LOWER than normal, there may not be enough of it for the idle circuit to work properly. Low level = lean mixture and high level = rich mixture.

Fuel in the bowl is not a Yes /No situation, but how MUCH is in there is also critical.

Then, if your fuel level is just a little too low, and you have ok Idle when you are standing still... the stalling you experience when coming to a relatively sudden stop, might be due to fuel sloshing about in teh bowl.. levels are all over the place. The idle circuit may drain completely or go VERY lean for a second, and that's all it takes.

Jan

Reply to
Jan

Mike,

if you want to better understand the workings of a carburator, I can go through it with you in private. I know you like them private shows anyway.

It would require drawings and pictures, so this forum doesn't really work for that purpose.

Jan

Reply to
Jan

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