L20B and Weber dgav 32/36 will not idle when warm

[78_lilhustler]

Hello all,

 

I have a 1978 Datsun 620 with an L 20 B that is a factory AC and automatic truck I have a weber DGAV 32/36 carb installed that is only about two years old. This truck was running and idling fine previously with this setup but now it will start and idle fine for a few mins then just die when warmed up. I can get it to start right back up but it then dies shortly after as if the key was turned off. If I leave it sit to get cold it will fire right up idle til warm then die again. 
 

the Weber dgav 32/36 does not have a cut off solenoid or anti dieseling solenoid and I did not have any electric choke connected previously and it was fine. 
 

the only big change to anything was recently I had a battery catch fire which melted some wiring and few vacuum lines. I replaced all the batter wires, lugs, new ngk spark wires, replaced any melted or questionable wiring with proper gauge wires, all vacuum lines have been checked and replaced as necessary. 
 

I don’t think vacuum is it because there should be no advance at idle and that’s the only port to the carb. plus it starts fine and does not need to be throttles to maintain idle. 
 
Any one have any ideas or have this problem? Do you think it’s the carb? Possibly damaged circuitry?

 

[banzai510(hainz)]

DGAV is a electric shole carb

so the cholke flap should be closed when COLD.  when the truck warms up the choke should start to open after a little while. If not then wire fell off or no 12volt to the choke (coil) on the side of the carb.

 

Datzenmike would know more on these but one should time the truck in gear(automatic trans.

 

webers are really reliable. But never had a Automatic trans Datsun

[datzenmike]

Don't worry about the vacuum advance as it is not directly connected to the intake vacuum.

 

 

One thing I do know about Weber carburetor swaps is the adapter plate bolts come loose and leak and/or they get tightened too much and the adapter cracks making a vacuum leak. (this is the most likely problem) Check the carburetor bolts are snug too.

 

However check the vacuum line from the intake that used to go to the stock air filter to work the ATC (air temp control) servo. That fitting is on the side of the intake below the carburetor and must be plugged. It will run with a vacuum leak because the choke forces an over rich condition. Once the engine warms and the choke is off the vacuum leak takes effect.

 

Another possible leak is the power brake line to the brake booster.

 

A forth possibility is the very small hose coming off the fitting on the intake that the brake booster hose is on. This line goes to the automatic transmission's modulator valve to tell it the load on the engine. Check all the way to the side of the transmission, could be off or hose split. You might notice that the transmission has a delayed up shift because with no vacuum it thinks you have the throttle floored.

 

 

Is your EGR valve also connected to the vacuum advance??

 

 

[78_lilhustler]

2 minutes ago, banzai510(hainz) said:

DGAV is a electric shole carb

so the cholke flap should be closed when COLD.  when the truck warms up the choke should start to open after a little while. If not then wire fell off or no 12volt to the choke (coil) on the side of the carb.

 

Datzenmike would know more on these but one should time the truck in gear(automatic trans.

 

webers are really reliable. But never had a Automatic trans Datsun

Yes the flap is closed when cold. Definitely opens up when warm. 
 

i will time the engine w the auto trans in gear but have not been able to get it set since it cuts out as soon as it’s warm enough to go set timing. 
 

The auto trans and factory AC combo for this year definitely has many curveballs I hadn’t considered as compared to manual non ac vehicle etc. 
 

thanks for the advice!

[bilzbobaggins]

Why was the choke never hooked up?  Did you remove the actuation rod for it?

[datzenmike]

AC has nothing to do with this. 

 

Automatics have a vacuum line from the intake to the left side modulator valve. Look for vacuum leak.

[78_lilhustler]

1 hour ago, datzenmike said:

Don't worry about the vacuum advance as it is not directly connected to the intake vacuum.

 

 

One thing I do know about Weber carburetor swaps is the adapter plate bolts come loose and leak and/or they get tightened too much and the adapter cracks making a vacuum leak. (this is the most likely problem) Check the carburetor bolts are snug too.

 

However check the vacuum line from the intake that used to go to the stock air filter to work the ATC (air temp control) servo. That fitting is on the side of the intake below the carburetor and must be plugged. It will run with a vacuum leak because the choke forces an over rich condition. Once the engine warms and the choke is off the vacuum leak takes effect.

 

Another possible leak is the power brake line to the brake booster.

 

A forth possibility is the very small hose coming off the fitting on the intake that the brake booster hose is on. This line goes to the automatic transmission's modulator valve to tell it the load on the engine. Check all the way to the side of the transmission, could be off or hose split. You might notice that the transmission has a delayed up shift because with no vacuum it thinks you have the throttle floored.

 

 

Is your EGR valve also connected to the vacuum advance??

 

 

Thanks for the response datzenmike.

 

I will begin checking this list of of possible issues first thing tomorrow morning starting w the adapter plate. Good chance it is loose. 
 

I feel both the break line booster and or the line to the trans are hood possibility too have not really been looking in that direction yet. 
 

 

[78_lilhustler]

1 hour ago, datzenmike said:

AC has nothing to do with this. 

 

Automatics have a vacuum line from the intake to the left side modulator valve. Look for vacuum leak.

Right didn’t really think the AC had any connection to this issue was just noting that stock ac and auto has proven to have specific parts and specific issues I hadn’t previously considered as a whole. 
 

thanks again 

[78_lilhustler]

1 hour ago, bilzbobaggins said:

Why was the choke never hooked up?  Did you remove the actuation rod for it?

No it had been installed by someone else and since it ran fine and due to my previous inexperience with Weber carbs I did not notice. The flaps still open when warm so not sure how thralls works?

 

I did notice however as I started going through all the electrical in the engine bay and finding both that lead for what I assume is the choke and the lead for the old anti dieseling solenoid taped up and tucked away. 

[banzai510(hainz)]

this is a long shot here.

since carb seems to work correctly

maybe the idle jet is plugged. since when choke is ON the gas is being bypassed and when warms up should go thru the idle circut.

but usually you can run the motor by pushing on the pedal using the carbs accell pump to pump gas in to keep running but when you stop pumping the pedal the car will run out of gas when in the idle circut if plugged.

 

 

A loose carb late will also not idle if everything else is OK.

 

a 78 has alot more emmisssions so Im not familiar with there later gizmos

[78_lilhustler]

3 hours ago, banzai510(hainz) said:

this is a long shot here.

since carb seems to work correctly

maybe the idle jet is plugged. since when choke is ON the gas is being bypassed and when warms up should go thru the idle circut.

but usually you can run the motor by pushing on the pedal using the carbs accell pump to pump gas in to keep running but when you stop pumping the pedal the car will run out of gas when in the idle circut if plugged.

 

 

A loose carb late will also not idle if everything else is OK.

 

a 78 has alot more emmisssions so Im not familiar with there later gizmos

It is definitely worth looking in to. I did pull the idle set screw out to check it to see if it was gunked up but it was clean. The way it behaves it does seem like the idle circuit is not working properly 

[Crashtd420]

I have another possibility ....

When was the last time you adjusted your valves?

 

[banzai510(hainz)]

11 hours ago, 78_lilhustler said:

idle set screw out

Why you do that.?

Now you wont know if that is set right as that can compound the proplem.. As since it dont idle putlling it back in you mikght be off unless you bottomed it out then counted the turns where it was

idle jet just unscrew it from the side of carb by the accell pump diaprham and pull jet ouy and see if clean then reinstall

[Crashtd420]

14 minutes ago, banzai510(hainz) said:

Why you do that.?

Now you wont know if that is set right as that can compound the proplem.. As since it dont idle putlling it back in you mikght be off unless you bottomed it out then counted the turns where it was

idle jet just unscrew it from the side of carb by the accell pump diaprham and pull jet ouy and see if clean then reinstall

I'm thinking he ment the jet 

[78_lilhustler]

1 hour ago, Crashtd420 said:

I have another possibility ....

When was the last time you adjusted your valves?

 

This should have taken place just over a year ago after I had the truck for about 6-8 months a cylinder lost compression and I took it to a very professional engine shop, one that rebuilds any thing from small boat engines to full on detroits. Owner very knowledgeable about old Datsun/Nissan  engines. Couldn’t believe I had this truck in such good condition. Anyway he checked over everything replaced the timing chain and I would think went through the valves then. 

[78_lilhustler]

20 minutes ago, banzai510(hainz) said:

Why you do that.?

Now you wont know if that is set right as that can compound the proplem.. As since it dont idle putlling it back in you mikght be off unless you bottomed it out then counted the turns where it was

idle jet just unscrew it from the side of carb by the accell pump diaprham and pull jet ouy and see if clean then reinstall

The idle adjust has always been set all the way in since I got it. Opening it up just ramps the idle too high. 

[78_lilhustler]

7 minutes ago, Crashtd420 said:

I'm thinking he ment the jet 

Oh yeah jet I’m sorry. 

[datzenmike]

If adding more idle fuel causes the engine idle to go up, tells me it was set too lean.

[78_lilhustler]

Sorry for late updates on the progress here all, trying to work on this in between my 10 and 12 hr shifts as a sous chef in Philadelphia. Needless to say I don’t get but a few hours after and or before work to get at this. 
 

so this morning did a lot of  looking around, adapter plates and studs in the intake for mounting the carb were definitely loose. So as I’m sure you all are aware, it’s not easy getting a wrench or socket down between the carb and everything else to tighten the nuts. I decided if I’m going in might as well remove the carb get proper tightening on the studs and clean the carb. 
 

i also investigated the vacuum brake booster lines and the line leading to the auto trans. Brake booster lines are solid and still the stock ones. The vacuum lines leading to the trans that datzenmike mentioned are steel but had a fitting from the manifold to the steel line to the trans cover in a rubber sheath. The rubber looked ok prob original and hard to move up but it exposed the connection which is a flanged steel tube leading to another steel tube to the trans. Looked as hard as I could for any damage and could not find any there. 
 

this is about as far as I could get this morning before work. I have some Puc for reference but need time to upload and post. 
 

so in summary all other vacuum line look good, definitely needed to get carb set tighter on intake, also going to clean carb while off for good measure. 
 

when I get done with my shift here I will start on the carb clean. I’ll keep you updated. 

[datzenmike]

Use lock-tite on the manifold threads.

 

For vacuum lines try pinching them closed. If there is an improvement then there is a leak somewhere that's not visible.