What does the carb diaphragm do?

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Marc311

Marc311

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What does the carb diaphragm do? ISSUE SOLVED!!!

I posted the following in the 60 forum, but I figured there are probably several 2f/carb gurus in here that may be able to help me . . .Thanks

As the title states, What does the carb diaphragm do?:confused:

Here's the reason I ask. I have been having an afterfire (backfire out the tailpipe) problem on deceleration. After hours of testing, I am 99% certain it is the decel fuel cutoff system. Simple enough, but all the components of that system are working (vacuum switch, solenoid, wiring, computer). I have taken vacuum readings at the switch and it is getting plenty and it varies depending upon RPM. But the fuel cutoff solenoid doesn't seem to kick in properly. I can manually (applying and removing 12v) get it to function and it stops my afterfiring problem.

So my thought is that the vacuum signal going to the vacuum switch is funky. Since it gets the vacuum from the slow circuit in the carb, I am wondering if the problem may lie there. And since the diaphragm seems to be connected to the 2nd (slow) throttle valve, perhaps that is my culprit.

The strange thing is the car car runs fine - other than the afterfiring. Carb gurus, please help :frown:
 
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The cut off switch should get manifold vacuum because it needs to shut off the idle fuel when the manifold vacuum is high. I am not familiar with some of your terminology, but the only diaphragm on later 2F carbs is the one that operates the vacuum secondary throttle plate and this is unrelated to your problem.
 
I think we're talking about the same diaphragm Pin_Head. The vacuum for the vacuum switch comes from an orefice that appears to be controlled by that throttle plate (if I am reading the diagram right) as the throttle plate passes over the orefice.

The vacuum switch is supposed to turn on/off between 11-13". The vacuum at the hose is around 20 at idle, falls to zero during acceleration and then jumps back up. The change from 20" to zero and back happens pretty rapidly. So I was wondering if that diaphragm controls how quickly the throttle plate moves, therefore effecting the speed of the vacuum rising and falling. In other words, if the vacuum changes too quickly, it doesn't allow time for the solenoid to do anything.

It is just a theory on a subject I know very little about. So any enlightenment is appreciated.

The only other theory I can work off is that the Emissions computer (ECC) uses another input to determine when to activate the decel solenoid. And that other input is what is screwing up the engagement of the solenoid. I know engine RPM is a factor and I plan to test the wire from the coil to he ECC. But if that wire was shorted or open, it seems like it would cause the solenoid to be always on.
 
The vacuum diaphragm only opens the secondary at wide open throttle situations, which is the opposite of deceleration. It has no effect on the primary throttle plate.

Obviously, the idle fuel needs to be on at normal idle conditions where the intake manifold has 17-18 inches of vacuum. It needs to shut the fuel off when the intake manifold vacuum is higher than about 20 inches during deceleration or engine braking so that unburned fuel doesn't hit the exhaust manifold.

Why not measure the vacuum at the actual switch? Maybe you could post the diagram for your carb from the FSM. that would help.
 
Thanks PH. The measurements I have taken where at the switch. I will do it again and get specific numbers and post them. If the vacuum numbers look okay to you guys - then the problem isn't in that vacuum circuit.

If i am recalling correctly, my vacuum numbers were higher than what you posted (except when they fall to zero on accel). I will also snap a pic of the FSM pages.

Thanks for the help.
 
Marc311 said:
I have been having an afterfire (backfire out the tailpipe) problem on deceleration. :frown:
Click to expand...

Sound more like a faulty check valve on the air rail manifold to me...
 
The check valve to the air rail is new. However, the checkvalve to the exhaust pipe has not been replaced (I do have a replacement - it has just been a PITA trying to get the old one off). You bring up a good point, Grant, since a faulty air injection system can cause afterfire. And on decel, the air should should be diverting to the exhaust pipe (which it does). Perhaps a faulty check valve there is keeping the air from actually going into the pipe even the the AI system is sending it there.

I had considered this option, but disregarded it as I thought there is still unburnt fuel in the pipe, so why would adding more air solve that - but maybe it just causes the explosion sooner in the pipe - I don't know. I do know when I force the fuel cut solenoid to close on decel it solves the problem - but maybe that is because it is starving the system.

Here are my vacuum readings taken at the vacuum switch using a mightyvac as the guage:

idle=17"
If reved quickly vacuum goes to 0
If slowly reved, vacuum increases to about 20" just shy of 1000 rpm, then drops to about 15". As I go up to 1200rpm, vacum drops to 11", 1500 rpm = 5".

As I let off the gas, vacuum jumps up to 22-23" before going back to 17" at idle.

Do these readings seem right?

Also, I will get off my butt and replace that check valve since it needs to be replaced anyway . . .
 
If it is popping in the exhaust manifold, it is getting fuel and air from somewhere. Some air comes in from the intake. Are you sure that your idle speed/mix is properly adjusted so that no fuel is coming out of the main nozzles at idle or deceleration? How do you know the cut off valve isn't working? You could just put a test light on it and see if it shuts off at high manifold vacuum.

The vacuum readings are normal. it should get idle fuel at 17" and cut off at about 20".
 
I adjusted per the FSM lean drop method - although I can't say it was right as the difference in rpms was barely noticeable as I changed the idle mixture. However, thinking a rich mix could be the culprit, I also randomly toyed with the mixture screw and it did not cure the problem. It did seem to vary the severity of the problem, but the afterfire was always there.

What I can't tell you is whether there is a problem in the carb that effects the mixture, i.e. bad jets, a bad needle, etc. . . Based upon the appearance of the plugs, it doesn't seem to be running rich.

Another piece to the puzzle - this afterfire problem first appeared when I replaced the OEM bead-type cat with an aftermarket honeycomb-type cat. The cat was just advertized as a replacement, and not specifically a free flowing cat, but perhaps the honeycomb-type is inherently more free flowing. I don't know if replacing the cat caused a slight leak which allowed air in to make the afterfire (of course, there still is unburned fuel). Perhaps the afterfire was always there, but I couldn't hear it because the previous cat muffled it better.

I don't know, but the problem became evident after the new cat.
 
It is the idle speed and float level that determine whether fuel is pulled out of the main nozzles and not the idle mix screw. You can see the fuel coming out of the nozzle just by peeking in the carb at the center of the venturi nozzles.
The new cat may just provide a hotter ignition source for the unburned air/fuel mix. Toyota put in the fuel cut off on later carbs to prevent this problem. If you disconnect power to the fuel cut off solenoid, will it still idle? If so, it is getting fuel when it shouldn't.
 
The car won't idle with the fuel cutoff disconnected. I can keep it running by reving the engine, but clearly the cutoff is working to stop the fuel flow at idle. Also interesting is that when I do this I don't get the afterfiring problem, which I guess is obvious.

When I adjusted the mixture (sprawled over the top of the carb) I didn't notice any fuel flowing in the carb - but I wasn't specifically looking.

I was thinking/hoping along those same lines about the new cat. Although I have heard stories about people changing to a free flowing exhaust and the lack of backpressure caused afterfiring.
 
I'm just not buying the carb thing or change in exhaust as the culprit.. seems to me unwanted air is entering where it is not needed.. have you checked the inverter valve for correct operation? And when was the last time the valves were adjusted ? A valve train with too close clearances and not sealing off properly can cause problems also..
 
If the idle fuel cut off works when you disconnect the power and this stops the backfiring problem, then it sounds to me that the electrical side of the vacuum switch is stuck on not working to interrupt the circuit even though it sees the appropriate vacuum signal. It takes both fuel and air to make it pop and Toyota chose to regulate the fuel because it is easier and more reliable that shutting off all the air. On later cruisers, the idle fuel solenoid is switched by completing the ground, so put a test light or meter on the ground side of the solenoid before the switch (yellow wire with green stripe) and look for power at high vacuum. If there is no power, the solenoid is still grounded and fuel is flowing. When it shuts the fuel off, there should be 12V on the solenoid side of the ground wire.
 
I did buy an extra ABV (diverter) from cruiserparts. Mine appears to be working fine, but I have been testing and retesting for so long maybe I need to doublecheck.

Let me ask this . . .If the valve is NOT directing air into the exhaust pipe (just before the cat) would that cause this? I mean, it is supposed to blow air there to burn off extra HC (fuel), but wouldn't burning off HC just add to the afterfire problem, or does the burn occur previous to the cat so it does not result in the popping?

Thanks again for helping me hash this out.
 
Marc311 said:
Let me ask this . . .If the valve is NOT directing air into the exhaust pipe (just before the cat) would that cause this? I mean, it is supposed to blow air there to burn off extra HC (fuel), but wouldn't burning off HC just add to the afterfire problem, or does the burn occur previous to the cat so it does not result in the popping?
Click to expand...

No.

It needs both fuel and air to pop. The cat is not supposed to deal with this much unburned fuel. If you shut off the fuel solenoid, the popping goes away, so this is likely the source of the fuel. Chances are the vacuum switch is not working.
 
Well, my afterfire issues have essentially been eliminated. And the winner is . . . .Grant. At least his answer was closest.

The main culprit indeed seems to be the faulty check valve. I had already replaced the check valve to the air rail, but had not been able to get the check valve to the exhaust taken off. It was worthless and you could feel the exhaust coming back to the diverter valve.

It took quite a bit of propane, but I finally got the sucker off and the new one on. Started it up and no more afterfire/backfire.

There is still just a faint hint of a pop so I do think I am getting some extra fuel through the exhaust. I guess the extra air into the exhaust (through a check valve that works) was enough to burn off the fuel before it caused a problem. Well, that's my theory.

Thanks all for the help. Hopefully I will get it to the emissions station this weekend and get more happ news.

EDIT: Just an FYI should anyone have the same issue . . The check valve alone did not cure my issue. In fact the backfire returned (although not as bad) after further testing (i.e. I let it warm up more). The second part of the issue appears to be the idle mix (which I had adjusted several times). So, theory two, the rich condition took both a new check valve and some further leaning out to burn the HC. I am also suspecting my idle circuit may be a bit gummed up as the mixture adjustment seems flaky. But nonetheless - it's better.
 
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