72 620 runs rough when warm?

[72patbo620]

Alright guys I am sure you guys get this stuff all the time so i'll try and be as direct and thorough as possible.  I bought a 1972 620 few months ago and have been tinkering with it but am really stumped.  The truck will start right up, idle high, drop off high idle and will run like a champ for like 8 minutes then it all goes to shit.  The idle is all over the place, backfires under load, bogs and stumbles but has slight sweet spots in the throttle to limp me home.  Where I am getting confused is why it starts and warms up like a champ and then just starts running like crap.  Pardon my inexperience with carbs and distributors I'm working on obtaining a proper 620 manual but most of them go from 73-78... but mine is a 72 so i'm kind of at a stand still.

 

Truck:

L16 motor, 4 speed (bone stock)

DAF328 Carb, Electric choke (Choke seems to work fine)

Vacuum advance distributor with dual points (Distributor diaphragm holds vacuum) 

 

 

What I have done so far:

• Changed Fuel pump- No change
• Changed Fuel Filter- No change
• Blown out the lines back to the tank- No change
• Changed the carb base gaskets because it did have a leak when I first got it
• Sprayed the hell out of the carb with carb cleaner when it was off
• Points (Dual) gap was re-set to .019 (It will backfire a bit more in 3rd gear and i've read that the second set of points changes the dwell for something to do with 3rd and 4th gear?)-  Started a bit stronger once adjusted
• Cleaned the cap and rotor that appear to be in good shape
• Plugs wires look fine 

 

What I plan on checking tonight:

Plug gaps- .023-.035in

Re-Time- 7Deg BTDC

(From my limited experience with older cars it sounds like it could be too far advanced causing this issue?)

Do you time with the vacuum advance disconnected from the carb/distributor?

Compression Test- Just for good measure

Valve Lash- Just for good measure

 

Is there anything else that I might be overlooking?  I keep reading everywhere to exhaust all testing options before thinking its a carb... Pic for shits and giggles.

 

 

[datzenmike]

Check the valve clearances for sure. Possibly an intake is tight when warmed up.

 

 

 

I'm thinking that when the choke is on the over rich mixture is masking what's really wrong. When warmed up and the choke and richer mixture is removed the problem presents itself. If I'm right it probably idles not bad....

 

The primary jet is partly plugged and not letting enough gas through so the mixture is lean. Drive it around when warmed up for a while and then pull the plugs out. If running severely lean, the porcelain tip on the plugs will be bright white and clean.

[72patbo620]

Alright... After a few hours of checking and adjusting... still noting.  Although I definitely am narrowing down a list of things that it is not? 

 

Plugs were all gapped at .037, so I brought them down to .035

All plugs are still a nice tan/mocha color so its burning clean

Compression was as follows:

(1) 135psi

(2) 135psi

(3) 140psi

(4) 135psi

 

Timing was a bit advanced, but I backed it down to 7° BTDC at idle with the vac advance off and plugged

I checked the pcv valve and it was clear, although the line was a little kinked it still flowed air. (I took it off, kinked it by hand more than it was in the truck and blew through it with no real hesitation)

Lastly I lashed the valves to the proper .008 and .010.  some valves were a little tight and some were a little loose, but never more than +/- .004 on the intake and +/- .005 on exhaust.

The pads were all there and in place with no issue and sealed it up. (edit: I set the lash when it was cold, I just didn't type this in order)

 

Started it, realized my throttle cable was a bit too tight because my idle screw wouldn't really do anything, loosened cable so throttle could return to the original position.  

Checked throttle shaft for end play and it was pretty minimal if any.  

Started it, high idled, choke slowly opened vertical 

Timed it to 7°

Warmed it up, adjusted idle and mixture to where it was running smooth

Did a quick base gasket leak test again with carb cleaner and it passed, same with the intake.

 

Started driving it, ran great and after about 5 minutes of around town driving then it happens.

It starts to load up/ chug and buck and sputter and occasionally backfire.

The only way to get it smooth is to have your foot to the floor.

It comes to a stop without dying and idles fine. 

 

That's where i'm at.  

Its a 72' so its got no emissions or anything crazy, so whats left to check?

 

Any thoughts or input are appreciated!!

 

 

 

[datsunaholic]

Sounds like the primary jet is partially plugged.  Pull the 2 jet cover plugs (under the fuel bowl) and blast them out with compressed air, but it's easier if you take the carb off first.  Fuel will run out when you pull the jet cover plugs because they're at the bottom of the bowl.  I've cleaned them out on the side of the road, but it's not fun.  If they're really bad the jets themselves might have to come out to clean them, but usually it won't run at all if they're totally plugged with varnish.

[banzai510(hainz)]

Try Mexican tune up first.

 

Rev the shit out of it then pput a rag over the carb and maybe the clog will get sucked thru. Use a clean rag cause you dont want to suck the dirt out of the rag.

 

I never have gapped pluggs in my life most are already gapped pretty close. Matter of fact I never buy plugs i pull them out of used heads.

 

I would ck the dist for sideplay wear and make sure you have a stock coil and the ballast resisitor. otherwise points gets hot and arch out.

 

If datsunaholic is right pull the choke wire so the choke stays CLOSED this will enrich the circut and you should be able to drive around a little better. what this does is make the second barrel suck the gas . if better then the main barrel is likely plugged. I had a small pepple in my weber DGV and I could drive home with the choke pulled.   A stock vacuum carb might be harder

[72patbo620]

So just to confirm, Mexican Tune-up, and if that does not work, clean out the jet pointed in red? 

 

 

[datzenmike]

No.... #66

 

 

Won't hurt to clean #69 too. I had a dead mosquito in one once and another time a flake of casting. Hit a large bump and it would jam in the jet. Stop engine and count to ten and it would settle down, start, and it would run just fine.

[datsunaholic]

Yeah, #66 and 69.  Don't have to take the carb apart to get to those, they're behind hex plugs under the fuel bowl.

[bilzbobaggins]

Probably possum fur in them jets. Lol

[72patbo620]

HOLY CRAP ITS FIXED!!!! I can finally dive it!

Its still not 100% but it sure it a hell of a lot better than it was before.  

 

Mexi-tune was no bueno, so I pulled the jets, and although I did not see any debris, I sprayed like a half a can of carb cleaner on everything.  Sure enough started up, started to stumble when driving and after a few full throttle punches it cleared up.  Thank you guys very much for your input, if we ever get a chance to meet in person, i'll gladly buy you gents a beer. 

Cheers! 

[BluEvo210]

I think I need to try this next.

 

Your Datsun ran rough when it warmed up?

Mine won't run at all when warmed up.  Either I nurse the gas pedal (hold it at 2000rpm or so, if I'm in the driveway), or it sputters out.

 

I bet I just have a worse version of the problem.

[72patbo620]

I think I need to try this next.

 

Your Datsun ran rough when it warmed up?

Mine won't run at all when warmed up.  Either I nurse the gas pedal (hold it at 2000rpm or so, if I'm in the driveway), or it sputters out.

 

I bet I just have a worse version of the problem.

Yeah, just pull off the carb, remove the 2 12mm bolts, flat head screw driver the jets out and spray the hell out of them as well as inside the carb body.  It made all the difference in mine. 

[datzenmike]

I think I need to try this next.

 

Your Datsun ran rough when it warmed up?

Mine won't run at all when warmed up.  Either I nurse the gas pedal (hold it at 2000rpm or so, if I'm in the driveway), or it sputters out.

 

I bet I just have a worse version of the problem.

 

If no idle it could be your idle cut solenoid is unplugged. The idle cut solenoid allows fuel into the idle circuit. Shuts it off when the ignition is turned off to help stall the engine and prevent 'run on'. Without starting it, have someone turn the ignition on off on off on off while you listen near the carb. You should hear a soft clicking from the carb.

 

Possibly the idle circuit is dirty or blocked. Unscrew the idle mixture screw but count the turns so it can be put back exactly. Remove also the idle cut solenoid... generally they are hexagon shaped and have a red wire going to them. Have a care as there is a couple of loose parts and a spring inside that will jump out and get lost. Now get a can of carb spray that comes with a small red plastic straw and spray liberally into both holes. Don't get that shit in your eyes, it stings. The carb spray will dissolve or force out any obstruction. Put back together and see if that helps.

[BluEvo210]

If no idle it could be your idle cut solenoid is unplugged...

 

The wires are completely broken.  When I touched them, they felt brittle.

 

I ordered a new solenoid, thinking it would be nice to fix. I had no idea it could be causing the whole problem.  :hmm:

 

...And RockAuto sent me the wrong part. I filled out their online return form yesterday.

Are they pretty good about returns?

[datzenmike]

What you can do in-between, is remove the spring and that pointy thing inside, put the solenoid back on for now. This will get you your gas down to the idle circuit. Then you can probably turn your idle down and set your idle mixture. The new one won't affect your new settings.

[BluEvo210]

What you can do in-between, is remove the spring and that pointy thing inside, put the solenoid back on for now.

 

Yep, that worked!  I had a triumphant drive around town this afternoon.   :thumbup:  

 

It still idles quite rough, as if it's not using all the cylinders.  I may yet want to look for jets and clean them out.

 

But, should I also take the spark plugs out and see if they all look the same?