secondary throttle butterfly

Jts · Post by **Jts** » Mon Nov 27, 2006 5:39 pm

Can someone tell me if the secondary throttle butterfly ( the one closest to the drivers side) is supposed to open while the gas pedal is fully open? I've been trying to pinpoint a mid throttle stumble, and while it wont stumble at full throttle, it is definetly low on power. I was looking at the carb to see if there were any vacuum leaks and noticed the secondary throttle butterfly does not ever open via the throttle cable. This is wrong, isnt it? It should fully open at full throttle. Or is it entirely vacuum operated. Can someone take a look at their carb or tell me if they are certain that it is supposed to open with the throttle? I am guessing that the linkage is broken or worn out and will check for both.

keith · Post by **keith** » Mon Nov 27, 2006 6:34 pm

Its vacuum operated, but the primary releases it. That is, it won't open when the primary is closed, but when the primary is opened about 3/4 or more, then the vacuum can start opening the secondary as it builds up.

Mac · Post by **Mac** » Mon Nov 27, 2006 6:43 pm

mine is spring operated

if you open the throttle fully by hand and the engine is off, it should open all the way, or you should at least see it move slightly.

but when it does open the car will slow slightly as the secondary throttle opens abruptly and suddenly introduces a bunch of air into the intake (no accelerator jet to compensate for the sudden addition of air before the venturi starts atomizing fuel)

Jts · Post by **Jts** » Mon Nov 27, 2006 7:50 pm

Well, i feel a bit foolish asking if its vac operated and then reading the manual and seeing it clearly is. Is the vac system able to open itself while reving it in neutral, or does it only open up at high load? Thanks for the replies so far.

takza · Post by **takza** » Mon Nov 27, 2006 8:11 pm

I haven't seen the secondary open with the car sitting still.

You can test this by having someone drive the car...while you sit on the radiator with the air cleaner off and watch. Wear a helmet?

Typrus · Post by **Typrus** » Mon Nov 27, 2006 8:25 pm

It'll overrev in neutral long before you reach the secondary. Perhaps if you had someone braking and clutch-slipping while you rev it? Or something along those lines. I've actually never even seen the thing open.

You can get carbs with linkage secondaries, linkage/vacuum, or pure vacuum.

Jts · Post by **Jts** » Mon Nov 27, 2006 9:32 pm

haha thanks guys. I had a feeling that might be the case. So i'll continue trying to track down my stumble.

coltarms · Post by **coltarms** » Tue Nov 28, 2006 10:14 am

I think I have the same stumble. I have replaced all the vacuum line I can get to, all the check valves, rebuilt the carb, replaced the PCV valve, replaced the intake/exhaust manifold gasket, replaced the headpipe gasket, replaced the entire IIA unit along with a new cap and rotor and Belden wires and 2 different types of NGK plugs, new fuel filter and pump, adjusted the valves, and most recently replaced the entire carb with a Holley rebuilt (I didn't trust my rebuild anymore.) I still get the stumble now and then.

Next on my list are the PCV and HIC hoses along with the breather filter. Then I'm going to be replacing valve seals and while I'm in there I'll be triple checking the manifold bolts, pulling and cleaning the EGR, and RTV'ing my carb and plates and shields etc back down to the intake manifold (to ensure no vacuum leaks there.)

Fun, eh?

Mac · Post by **Mac** » Tue Nov 28, 2006 5:40 pm

i wen't to the J-yard and took all those check valves and vacuum reldated gizmo's off a newer 4A-C equipped corolla, that helped out alot. I also replaced all 8 feet of vacuum hose (thats right, EIGHT feet) i have no stumbles at all anymore.

I think the tercel slowing down momentarily when the secondary throttle opens is normal.

Jts · Post by **Jts** » Tue Nov 28, 2006 6:01 pm

what I'm really trying to fix is a mid throttle stumble. I can drive along and when i'm about 3/8 to 5/8 throttle, the engine will just die. It usually happens about 2500-3000 rpm. If i just floor it around, it wont happen. I've read the forums and alot of people have had this problem it seems. I now have working vacuum advance, which helped the problem alot, and making the stumble less sensitive, but it is still there. I've blocked off the egr to see if that was the case (hard block off, with a gasket on the intake mani. This didnt do anything, which was expected as my egr valve was totally blocked off. I will fix that soon. Does anyone think that a non functioning egr valve could cause the stumble? Perhaps the carb is tuned to account for it at mid range throttle and rpms? My next step is to go to a jy and snag all the vacuum parts i can, and replace all the vac hoses. Ive already fixed one major leak in the vac system. I'm a bit stumped that this problem is so hard to find. The car is driveable to me, but its definetly not something i could hand off to someone else. I would like to get it to that point. Any suggestions on what to try next?

Post by **ARCHINSTL** » Wed Nov 29, 2006 12:07 am

Check this out, vis-a-vis cleaning the EGR valve:
viewtopic.php?t=1496&highlight=egr
takza also has the first treatise on the cleaning of it. It is a real PITA, though (the cleaning, not his treatise!).

Once I renewed all of the vac tubing, cleaned the EGR valve, and got a new OE EGR valve modulator ($63), I had no more stumble/hesitation - and Goldie also passed the emission test (by this time I had already installed a remanufactured carb and OE distributor vac advance). The modulator was a key, though - maybe you can snag one from the junqueyard - 4AC maybe? I tried to fix mine, but the diaphragm i/s the plastic case leaked.

I did/still have an RPM cycling/hunting problem from 1900 to 2300 and back, but sorta resolved that by blocking the "A" switch - do a Search for that thread.
Tom M.

Jts · Post by **Jts** » Wed Nov 29, 2006 12:26 am

Thanks. I will go to the JY soon and get a modulator and possibly an EGR assembly. I took mine off to clean it and broke the head off the screw holding the vacuum line that goes from egr to modulator. It was also completely clogged. Hopefully i will find one that is clean to begin with.

Arch, can you tell me what part make the biggest difference? Or did you do it all at once and not able to pinpoint it. I'm thinking i still have a vacuum leak that is affecting my idle (rough) but that something else, like the egr, is causing my complete loss of power at mid throttle.

takza · Post by **takza** » Wed Nov 29, 2006 5:12 am

Have you checked the larger tubing surrounding the charcoal canister? It's also possible that the vac modulator itself can be leaking...even if the EGR is plugged. Could be one of the vac diaphrams around the carb?

Post by **ARCHINSTL** » Wed Nov 29, 2006 11:15 am

I cleaned the EGR valve, checked the modulator/renewed its filter and it seemed to work - then it flunked the emissions test and still stumbled.
I cleaned the EGR the second time using oven cleaner (as detailed), and rechecked the modulator and discovered that it leaked, so bought and installed the new one - and it passed and stopped stumbling. I think the modulator was the key to making the EGR work, rather than the oven cleaner flush, but I would still recommend the flush.

The little screw you mentioned - see the following link - and then visit a bicycle shop or look at an old clunker for a screw.
viewtopic.php?t=906&highlight=egr

There are so many places that can leak vacuum - check the torque on the manifold bolts - renew the vapor canister tubes (mine looked "cooked"). Again, long before I did the EGR thing, I renewed all of the tubing, including the fuel lines at the back and in the engine compartment. I am convinced the EGR cleaning and new modulator did it for me.
Tom M.

Jts · Post by **Jts** » Wed Nov 29, 2006 9:18 pm

i went to a jy today, and got a clean unclogged egr valve. Also get another modulator. I installed both today and theres no change in the stumbling. Even when blocked or unblocked the egr didnt help. Its nice to have a functioning system and all, but i guess it wasnt the problem. The more i play with it, the more i think its a fuel system issue. When it cuts out, it sounds like the fuel is totally cut. When it catches, it sputters for a few seconds before it picks up and takes off. My new theory is that the fuel cant keep up with the carb, causing it to go lean and fire sporatically. This doesnt explain why it would run when its floored all the way to redline however. I'm going to take a look at the fuel filter and see if its clogged.

On a side note, anyone need a vacuum advance? I grabbed 2 working ones at the jy today, from 3a's and 4a's. All the ones i got work suprisingly. That makes 4 that ive found in the jy. Roughly half of all the ones i checked were bad. Let me know if you need one.