Carb issue. Fuel bypassing or leaking

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1978 fj40!stock carb

It seems like while decelerating and in gear. I get some torque to the wheels as if the intake is still getting gas the car stutters and feels like tourque moving the fj40 forward

And

I can start from second gear and not give it gas…,


So I’m thinking carb issue but gas in sight glass is good.. runs awesome… no dieseling… no backfire…no gas leaks outside of carb…

Might need to tear it apart but any ideas as to what to look for. What is stuck open or cracked???

Thanks
Eaj71
 
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It just seems off…. And makes the cruiser shake or stutter when it should be just more “smoothly” decelerating.

Didn’t do it until a few weeks ago

Doesn’t do it in neutral only in gear
 

flx

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Throttle Positionner that stays engaged?

Check that just after you start the engine the diaphragm on the back side of the carb pulls the 2 metal rods connected to it.
 

flx

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1655167683824.png
 
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I didn’t mention it is desmogged and that diaphragm gets no vacuum …

Is effectively I don’t thi it does too much. But I will try to move both all the way in and all the way out….. and see what happens.
 

flx

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This diaphragm itself is not related to smog. A valve inline of the hose between the base of the carb and the diaphragm is for emissions but you can still connect the vacuum hose directly bypassing the valve.
This diaphragm also control the Choke Breaker that reopen slightly your choke after start.
If you have no vacuum at all the TP stays active all the time, so it's the opposite of what you want to do removing emission stuff.

A quick fix to check if it's your issue is to unscrew all the way the screw under my arrow in the previous post. It should not touch the little plate.
But I would really reconnect this diaphragm to the base of the carb, at least for the CB. TP will do nothing without a valve controlling the vacuum.
 
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This diaphragm itself is not related to smog. A valve inline of the hose between the base of the carb and the diaphragm is for emissions but you can still connect the vacuum hose directly bypassing the valve.
This diaphragm also control the Choke Breaker that reopen slightly your choke after start.
If you have no vacuum at all the TP stays active all the time, so it's the opposite of what you want to do removing emission stuff.

A quick fix to check if it's your issue is to unscrew all the way the screw under my arrow in the previous post. It should not touch the little plate.
But I would really reconnect this diaphragm to the base of the carb, at least for the CB. TP will do nothing without a valve controlling the vacuum.
Groovy. I will give it a whirl
 
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Sadly it did not do the trick. I am thinking I have vacuum leak near the base and somehow gas is getting into the intake. May just need a rebuild but I was hoping not…

Thanks again for the data!!!
 

flx

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If no high-iso is engaged, what is happening if you try to turn the idle speed screw? Can you lower your idle speed like that? Does it stall or run rough when lowering it?

If your carb was only dumping fuel in the intake with no extra air I think it would flood the engine.
While desmogging did you cap all vacuum outlets from the carb?
 
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The idle screw does function fine

Yes all capped ports except to dissy

I took it apart last night. It looks like maybe some schmutz kept the secondary cracked open a bit. More than what it should be but on a little. This may do it? But will have to se
 
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Hmmm. Still an issue post carb rebuild. We won’t get into that damn whistling

But

The throttle positioner is now active. What exactly does this do …. Before I start to tinker

Does anyone know ?

Thanks
 

flx

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Throttle Positioner = TP
Choke Breaker = CB

Both the TP and CB are connected to the same diaphragm. In countries with only the CB this diaphragm is directly connected to the base of the carb. In countries with the TP you have a valve on the vacuum line that the emission computer can control.

Not sure if you reconnected the diaphragm with the valve or without.

The TP functions:
- Actived it imposes a high idle that you can set with the screw indicated before.
- Deactivated the screw is pulled to not face the little plate so don't impose a higher idle than normal.

The basic function without the valve (or inactive which should be the case at start):
- When you start the engine you get vacuum from the base of the carb that will pull the diaphragm.
- The diaphragm pulls the CB that will slightly reopen the choke if it was fully pulled (to avoid to the engine to choke to a stop.
- The diaphragm also pulls the TP which deactivate it.

If you have emissions valve on the vacuum line:
- In some driving conditions the emission computer will close de the valve which will activate the TP.
- It may change a little depending of the years but the common in case of TP activation is when you suddenly release the accelerator at speed. Keeping a little acceleration for a few seconds instead of going back to idle immediately avoids to have unburned gas in the exhaust.
 

flx

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Now if your valve is bypassed and the TP is pulled not in front of the little plate. Or you have unscrewed the TP screw so much that it can't touch the plate it should not do anything to your idle speed.


If I understood correctly your issue you have a high idle all the time so th engine is accelerating.
You also said that the idle speed screw was working correctly.
Here I'm lost, if the idle speed screw is working it means you can lower the speed of the engine so you should be able to lower it to a regular idle speed or encounter an other issue?
 
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Yes. It seems off.
Update

1. So I discover a small nick which grew big very quick on my clutch line before the slave.
I was hoping that was a problem maybe the couch was keeping engaged or something. Nope. Not it after replacement and shower of DOT 3.

2. The throttle positioner is fully functioning but haven’t tinkered with it too much I have it turned about 1/2 in

3. The idle speed after a lean drop is set at about 700rpm and timed well…

4. I’m still thinking im
Missing something obvious. But maybe it is just the way it is…. I will recheck that idle screw again and return spring.

5. Again on decelerating and I hear it seems to still be delivering torque to engine

6. Maybe a clutch issue?

Thanks
 

ToyotaMatt

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The throttle positioner positions the throttle ...........
 
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Hahah.
 

flx

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I'm not sure to understand the issue?

So with the idle speed screw you were able to set the idle at 700rpm and the engine runes fine at this speed?
But after driving when you are decelerating you don't feel engine braking and to the opposite you feel pulled by the engine while you try to brake?
At a stop after decelerating the idle speed of your engine is noticeably superior to the 700rpm it should be?
Does the engine goes back to 700rpm at some point?
 
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I'm not sure to understand the issue?

So with the idle speed screw you were able to set the idle at 700rpm and the engine runes fine at this speed?
But after driving when you are decelerating you don't feel engine braking and to the opposite you feel pulled by the engine while you try to brake?
At a stop after decelerating the idle speed of your engine is noticeably superior to the 700rpm it should be?
Does the engine goes back to 700rpm at some point?
At a stop it returns to regular idle
 
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I will check my advance one more time as well… it seems to like it maybe a just a bit past 7…about 9-10 btdc. It has been that way for a while. But my issue is new
 

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