Stalling out when decelerating. HELP!

[Birdie]

You raise a very interesting point. I thought it was quite odd (maybe too minimalist) that he has no fuel return valve / fuel return line. I actually think that the brake booster and fuel pressure may be related. Remember he drove it happily for a year. Then he got the MC replaced. Now it stalls. I wonder if there is a correlation between a much stronger brake vacuum (not exactly sure how) and now too much fuel on deceleration. I'll bet he needs a fuel valve and return line with his new brake setup.

 

Unfortunately, I don't have a fuel pressure gauge handy but just so I understand the test, if I see the steady fuel pressure at high rpm and then see it spike very high at deceleration, that would be an indicator that the full pressure is jumping at deceleration and may be flooding the cylinders? A fuel return valve and line would likely address this.

[Son of Marty]

I would make sure the distributor advance is free swinging and not getting hung up at full advance, add a few 3 or so drops of light machine oil under the felt pad that's covered by the rotor. Plenty of   cars run that weber and never had the fuel return ever fitted, it would take a real big shot of brake fluid to stall the engine.

Edited May 20 by Son of Marty

[Birdie]

That sounds very promising. Can you please give me a very quick instruction on accessing the felt pad: what is removed etc.?

 

 

[Son of Marty]

Remove the rotor and look into the center of shaft and you'll see the pad, hook it with a bit of wire or tweezers and pull it out. Not a great picture but the brown in the picture is the pad, the dist. pictured is disassembled way more then you'll need.

Edited May 20 by Son of Marty

[John76]

3 hours ago, Birdie said:

an indicator that the full pressure is jumping at deceleration and may be flooding the cylinders?

Jeremy,

Increased fuel pressure plus increased vacuum on deceleration could cause a "flood stall" if the float level is too high.

Yes, the fuel return valve with the manifold vacuum port will decrease the fuel pressure when vacuum is high (by routing more fuel back to the tank}.  Fitting a dashpot will reduce the extra high vacuum on deceleration by cracking open the throttle butterfly.  This really helps with the "smoke screen" on deceleration if the rings and/or valve guides are worn.

Also try reducing the float level to see if it makes a difference.

John

[Birdie]

Thank you for all of the input. I have just returned from the patient '02. I didn't expect to really cure the issue but to simply gather more information. Well fixing it would have been nice.

 

I should mention that all of my prior assumptions went out the window.

 

1. let the car warm up. it doesn't really seem to stall consistently until warm. make sure it is still happening. high rpm -> deceleration -> stall

2. added oil to the distributor advance.  high rpm -> deceleration -> stall

3. crimped the booster hose with vice grips to avoid all vacuum relation to booster. high rpm -> deceleration -> stall

 

I should mention that I can avoid the dead stall by hitting the accelerator linkage again when it is at low rpm... and when I do I hear the carb hesitate and gasp for a split second. Then it switches to what I would assume is the proper jet and finds its idle again.

 

Therefore, before I read @John76 latest "adjust the float" post it started to feel like the carb.

 

His inline fuel filter was new but had some tiny debris in it - like the filter material had deteriorated and it had barely enough fuel to reach the paper filter. We replaced the filter and it immediately filled up to a normal level. Considering the debris on the unfiltered side - it is possible that debris made it into the filtered side and into the carb.

 

I then noticed that the passenger side barrel of his weber was quite fouled compared to the driver side. And this a "newly" rebuilt carb. I hadn't planned on spending too much time on this today but.. off the carb cover came just to do a quick visual. The floats were not filled (leaking). Unprepared to clean the primary jets and adjust the float level, we put it all back together and confirmed that: high rpm -> deceleration -> stall

 

When I returned home I saw john76's suggestion that we adjust the floats and I think it is a great idea. Can you explain to me a bit more about how the high float level or a severely clogged jet could cause this stalling behavior?

 

thanks,

 

[John76]

4 hours ago, Birdie said:

Can you explain to me a bit more about how the high float level or a severely clogged jet could cause this stalling behavior?

Sure... the high float level is subject to the extremely high vacuum in the intake manifold when deceleration from a high rpm.

This sucks fuel from the float bowl into the intake resulting in a very rich mixture that cannot be ignited in the cylinders. Result is a stall. 

A clogged idle jet will not allow gas and air to the engine when the throttle is closed ... resulting in a stall.

Anything that can dilute the fuel/air mixture (such as brake fluid from the master/booster) will also result in a stall.

Sounds like you have eliminated the brake fluid contamination, so I would concentrate on the carb.

Also check that the choke is functioning properly.

Hope this helps,

John

[Son of Marty]

When you twist the rotor and let is go does it snap back to the idle position or does it get hung up.

[Chris_B]

I believe he said he has a 123 dizzy.

 

 

[Birdie]

10 hours ago, John76 said:

Also check that the choke is functioning properly.

Could you explain to me how the choke would come into play when the car is fully warmed up and running at 4k+ rpm then decelerating? I was under the impression that once warm the choke linkage and butterflies have been removed from the equation.

thanks in advance.

['76mintgrün'02]

Fuel pressure/vacuum gauges are not expensive and both functions are very useful.  It's easy to =T= one into the fuel line.  

 

I'd also check the carb's baseline settings.  Sometimes the idle speed screw gets turned too far in (past 1-1/2 to 2 turns in) and it draws fuel through the secondary side.  When that's the case, the idle mixture screw has little to no effect.  

 

You could back the idle-speed screw out and count the number of turns until it comes off of the lever and turn the idle-mixture screw in until it seats and while counting those turns.

 

You might as well write down the jet and air corrector sizes while you are in there.

 

The power valve comes into play based on vacuum.  I played around 'testing' a couple of mine for the fun of it.  

 

 

There's also a little spring loaded valve at the bottom of the float bowl that it pushes on.

 

I made cardboard shims when setting my floats.  Benchtop paper cutters can be very precise.  I'm not sure how I came up with these numbers though, since 1.535 is only 39 mm and the recommended setting is around 40-41.  I remember changing it, but couldn't phind those photos.

 

 

Is the rubber tip on the float valve still nice and soft?

 

Tom

[John76]

2 hours ago, Birdie said:

Could you explain to me how the choke would come into play when the car is fully warmed up and running at 4k+ rpm then decelerating?

With the car running at 4k+ rpm, the inrush of air into the carb will keep the choke plate open.

When the throttle is snapped shut, the air flow stops. If the choke linkage is loose or the small clip to the choke flap is missing, then the engine could stall because both the throttle butterfly and the choke plate are closed. You have extra high intake vacuum sucking gas into engine...but no air.

Note: Revving and snapping the throttle closed under no load (car in neutral) may not stall the engine as would happen when decelerating on the street.

Tom @'76mintgrün'02 is on to something (see above). Any debris in the idle circuit will stall the car when fuel is no longer supplied by the transition ports and main jet(s). A worn float needle valve will increase the float level and cause "overflow" flooding issues, as will a heavy plastic float. A brass float is not subject to the degradation of the plastic floats by the alcohol (ethanol) in our fuel.

I think you mentioned this stalling problem started rather suddenly, so I would double-check the choke linkage and thoroughly clean the idle jets. All the other probable causes occur gradually.

[Birdie]

14 hours ago, Son of Marty said:

When you twist the rotor and let is go does it snap back to the idle position or does it get hung up.

yes. the rotor snaps back into place quickly.

[Dudeland]

How are the motor mounts?  if the engine is loose, it can move enough to lift the dizzy cap and cause issues. 

 

You can get a good idea if the car is flooding on deceleration by not restarting it when it stalls, taking off the air filter and looking down the carb's throat.  If it is flooding, you should see the fuel dripping out and a pool at the bottom of the intake if you open up the butterflies.  

[Birdie]

While I did not make it back to my friends house for another round of troubleshooting, a seasoned mechanic did get involved and the verdict is in. I hate to say "everyone gets a trophy" but... honestly this car had just about every issue that was mentioned on this thread:

- @'76mintgrün'02 mechanical fuel pump pushing ~8psi (and with no dashpot or return...)

- @'76mintgrün'02 while the rotor passed the 'snap-back' test, it turns out that this dizzy had the wrong cap. Therefore, it would only pass this test with the cap off. When the cap was on the rotor was being restricted. As a matter of fact, the cap was not only keeping the rotor from going back into place after acceleration, it had worn out the contact plate.

- @John76 the floats were too high and had to be set tot he proper height

- and several other less critical issues that were addressed.

 

It was a very interesting issue(s) to troubleshoot. Thanks for all the input everyone.