Why does my 4 barrel bog down?

[Jeaux]

IT was working fine then one time flooring it the secondaires kicked down then bogged down chocking the car and slowing it down.
I cleaned everything I can think of with no luck.
This 1975 280s only has one inline fuel filter at the carb.
Suggestions?
Thanks

[oscar]

Hi Jeaux

I had a similar poblem.  It sounds to me as that a plastic cam on the secondary air valve shaft is broken.  It controls the flow of fuel coming through the secondary main fuel jets. 

Is this what's happening?

http://www.youtube.com/v/Sjrgrvb5tjU&hl=en&fs=1


Have a look at the next link.  I just glued the plastic cam together and it worked.  There was a few other issues too but that broken cam is what causes fuel starvation when you try to accelerate.
http://forum.w116.org/test-drive/got-a-new-project-pictorial/msg56492/#msg56492

[Jeaux]

Thanks Oscar for the reply.   Your video is exactly what my carbarator is doing.  I see the picture of the broken cam however i do not see the cam location in the first picture. I will however take the top half of the carb off right?  I will see then?  May a breakdown picture will show me.  I do have the secondaries sticking.
I did remove it and cleaned and adjusted as good as I could but not as freely moving as you say it should be.  This cam sounds like my problem.  Thanks so much.

[wolf_walker]

With the breather off, press down with your finger on the top secondary air flapper, you should see the little arm going to the two metering rods raise up higher the further the air flaps move, this is what allows increased fueling as the secondary's open further.
If they do not move, or very slightly, your cam is broke, if they do, your cam is fine.  They do break but not real often.
The 4A1 is fairly well known for a good secondary opening bog, later versions seem less prone to it.

If your secondary air flapper is sticker either open or closed, do try cleaning and lubing, which is hard to do well on the car, but it seems to usually be physical interference with the top body of the carb.  Look real close at the rear of the carb top where the two long screws attach the carb top to the main body, right around there they seem to bind up pretty often.  I just file or sandpaper them till they clear.
A warped top plate or carb can do this too.  Take care not to get crud in the motor if you file on it.  The best way is with the carb top off and holding it up to a light so you can see where light does not pass, and there is interference.  But I hesitate to pull a top plate unless I have to, many a decent running 4A1 never worked right again after having the pressure relased and then being re-torqued(warping).
And always watch close for leaks afterwards if you do pull it off, and tightening up more is not often the answer.
Good luck.

[oscar]

Wolf is spot on and a better way to go before a big overhaul is done like I was thinking.  It's such a simple test to see if the cam is good and should be done before digging any further.  I've taken a pic with labels in case there's any doubt but in particular I've tried  to show where the cam is.  It's hidden within the lever numbered (3) and has the shaft going through it.  If you were to pull the shaft out, the cam  would stay there and slide of the shaft but stay within that lever.  From memory I think I took the top half of the carb off then found the cam broken when I took apart the air flaps and shaft.

Wolf also mentions the air flaps sticking which can also cause the bogging down or add to poor stage II  performance.  The couple of times I put the flaps on the shaft, I had to reposition them a few of times due to their leading edges binding against walls inside the carb.  When reassembling them, the four screws that hold the two flaps on should be done up finger tight at first then push the flaps down and release a number of times to check for clearance problems and readjust their position on the shaft before tightening the screws with a screwdriver.  Only small adjustments are possible but will make a difference.  If there's contact no matter what you try then filing or sanding the edges of the flaps is the way to go.  Wolf also mentions an important point regarding over tightening screws.  The third photo shows how overtightening can cause deformation of the barrel walls.  The space between flaps and barrel wall should be parrallel.

Spring tension on the shaft and a faulty vacuum pot also affect the secondary air flaps and stage II performance but wait and see how you go investigating the cam and freeing up the air flaps.  Setting the spring is guesswork without the proper tool and I'd leave that spring alone until you find out about the cam.

About the fuel filter.  That's normal but what some people do is buy a cheap generic plastic inline filter from an auto shop and place it before the fuel pump.

[wolf_walker]

Oscar makes me look lazy with all his fancy photos. 


While on the subject sir, do you know what the function exactly of, if your 4A1 has it, the spring loaded needles on the primary side.
They look to be held closed by vacuum and spring up/open under load allowing more air in the two larger upper air bleeds.
My 73 4A1 did not have them, this 85 I recently installed does.  Other than a little lean on cruise it seems to work well, and the secondarys come on quite smoothly for a change.

[oscar]

Oscar makes me look lazy with all his fancy photos. 

LOl, no not all.  I just wish there was a solex tuning guide like what holley has.  Search youtube for holley tuning and there's a user called boxwrench or something similar.  If there was a parts illustrator and tuning guide with commentary like that for the 4A1 we'd be ecstatic and I could put the camera away.

About those spring loaded needles, I've read it before but wouldn't be able to repeat it now and make sense.  I'll have to hit the books later on to refresh the memory. 

[wolf_walker]

I agree about the tuning information, calling it spotty isn't even a start.
What puzzles me about the primary metering rod dealys is they seem to permit more air in high load situations, and were not present at all on older carbs.  I suppose it's a more complicated going on that that.  I have what I'm assuming are the primary jets off my original carb, the large ones that sit down in the carb and unscrew from the bottom of the top plate sorta, but I'm weary of putting them in this carb not knowing what these air metering jets are exactly.

[Jeaux]

Thanks Wolf and Oscar.
You are both right.  I took the upper half of the carb off last night and fould that the cam was not broke but moved to the right of that area. All I did was take a flat screw driver and move it back to the center however i feel that this was installed wrong form the person before me who said he rebuilt the carb or it moved on it's own.  I feel the need to put small spacers  on the back pin.  This will help keep the lever from moving left and right.  Unless the cam moves?
You also said this will rase my RPM?  I hope so because I have not been able to get my RPM down to 900?  I will put the carb back together maybe tonight.
By the way the pictures are good.

[oscar]

What puzzles me about the primary metering rod dealys is they seem to permit more air in high load situations, and were not present at all on older carbs.  I suppose it's a more complicated going on that that.  I have what I'm assuming are the primary jets off my original carb, the large ones that sit down in the carb and unscrew from the bottom of the top plate sorta, but I'm weary of putting them in this carb not knowing what these air metering jets are exactly.

From what I can tell, the spring loaded things are for Air Correction Stage I.  This air correction is still there on carbs without the  spring valves as well but exist just as an air jet without a valve.   The manual says something like, at increasing speeds as the fuel level in the mixing tubes drops it passes up to three air correction bores so that the mixture becomes an emulsion which prevents excessive enrichment.   Sounds a bit counter productive.  But my take on it is that they're not necessarily talking about high load situations like hard acceleration but rather high fuel usage conditions like coasting on a freeway.   Once at freeway speed the demand for fuel is less than it was getting to that speed.  The enrichment to maintain the speed is less and it does this leaning out to maintain a bit of economy.  As to why there are versions with valves and some without and when they come on, I haven't found anything at all and I'll have to think about it some before I can even start guessing.  I can't understand what it is that overcomes the spring pressure unless it's vacuum.  There's nothing mechanical linking them to the accelerator butterflies I don't think.

I feel the need to put small spacers  on the back pin.  This will help keep the lever from moving left and right.  Unless the cam moves?

I'm not sure I understand sorry Jeaux.  The cam shouldn't have any free play.  It's wedged in that slot and surrounded by the lever, then the shaft goes through it.  You should not be able to move it.  The only time it can move is when the shaft turns.  In other words, the air flaps turn the shaft, the shaft turns the cam, the cam lifts the lever. 

[wolf_walker]

Oscar, that is a sensible explination, I figured it had to be something for leaning out cruise somehow.  The piston is drawn down by vacuum, much the same way the primary metering rod piston is on a Quadrajet if you have ever had them apart.
As an aside, I put my lower primary jets from the original carb in this one and it seems happy, the slight lean surge is gone.
It did not like my original secondary rods though, it was quite rich when the secondary's opened at highway speed.  I put the ones from the 250 back in and it's happy.  I suspect this is because the secondary's are smaller, at least the bottom plate opening is.
I actually can tell it has less punch at wide open throttle than with the original carb, wide open being the only thing it ever did correctly.
I'm glad I did not spend the money for the Weber conversion, this motor clearly needs more flow than it can provide at full tilt.
Still waiting on a fuel mileage readout, first tank was up a mpg or two, but there was a lot of idling and hard acceleration and testing in there, so who knows.  I'm not expecting a miracle.