Carb rebuild?

[datzenmike]

Len... http://community.ratsun.net/topic/24412-my-76-710-goon/page-23 post 448 and on..

 

This thread got me motivated and I tore into my 710 carb. Remarkable results after taking it off and on 3 times. It's one of those non removable venturi types and someone had the thing apart and put a solid screw in where the hollow one was supposed to be. So no power valve. Didn't fix the problems but gave better acceleration. In the end probably some air bleeds or w/e were put in wrong and I swapped a '79 620 carb on. Top speed increased 25MPH!!!! so something was definitely wrong with the old one.

 

The 620 carb wouldn't idle but the solenoid clicked. Took the pin out and bang! idled. Was flooding but adjusted the float and it's a totally different car to drive now.

[FauxRoux]

So  im going over the hoses to rule out a vacuum leak.....ive encountered a few things..

 

I cant find the home for this... the hose that attaches directly below the hose im holding (not the large hose right above it...but the one closer to me) runs to a "T" and the open end off the "T" to the air cleaner.....

 

Im using the Chilton vacuum diagram and its...frustrating....My trucks carbon canister layout seems to match the diagram for a 75-77 by running over to the Dist VC and purge to the air cleaner....the 77-80 (cal except H.D. diagram) shows nothing going from carbon filter to dist VC...but shows the purge going to intake manifold and the dist vc line (the far right hose on the carbon filter) going to the air cleaner?.....am I using the wrong diagram in my naivete?

aaaand I seem to be missing my vacuum switch? no idea...

 

P.S. I want to take a second to thank both Mike and Len, you guys are remarkably helpful and patient and I truly appreciate any and all time you've taken with this.

[datzenmike]

 

This vacuum line goes to the air cleaner and is further divided up and provides vacuum to the air cleaner ATC (auto temp control) This is a valve that mixes exhaust heated air and the ambient air to provide a more constant temp range for more consistent running. In addition there is a temperature activated air bleed to lean the idle air mixture to compensate for an over rich condition at idle when it is extremely warm. California models also have an altitude compensator. All three need that vacuum line.

 

Carbon canister

The purge line should go to the vacuum advance.

The vent line comes from the gas tank.

Vacuum line simply goes to the intake.

 

Engine off.... Gas tank has non vented cap. Gas that evaporates escape the tank (and maybe the carb fuel bowl) and are stored in the carbon canister.

 

Engine on....

As soon as there is vacuum advance (signaling that the engine is running) it is used to open the purge valve and allow stored fumes to travel through the vacuum line connected to the intake to burn them.

[FauxRoux]

 

This vacuum line goes to the air cleaner and is further divided up and provides vacuum to the air cleaner ATC (auto temp control) This is a valve that mixes exhaust heated air and the ambient air to provide a more constant temp range for more consistent running. In addition there is a temperature activated air bleed to lean the idle air mixture to compensate for an over rich condition at idle when it is extremely warm. California models also have an altitude compensator. All three need that vacuum line.

 

Carbon canister

The purge line should go to the vacuum advance.

The vent line comes from the gas tank.

Vacuum line simply goes to the intake.

 

Engine off.... Gas tank has non vented cap. Gas that evaporates escape the tank (and maybe the carb fuel bowl) and are stored in the carbon canister.

 

Engine on....

As soon as there is vacuum advance (signaling that the engine is running) it is used to open the purge valve and allow stored fumes to travel through the vacuum line connected to the intake to burn them.

I ditched the Chilton and got a proper 78 service manual....things are making alot more sense now.

[FauxRoux]

Well the factory service manual certainly helped me clear up alot of confusion....but after 3 weekends in an F-ing row of fiddling I have come to the conclusion that there is something else wrong other then the carb as I cant even get the truck to idle no matter how I adjust the screws (ive come quite close but it just wont maintain).... I found the home for all the hoses... the last one was hidden on the bottom of the C.A.C. but now im hearing a loud, long hiss after it fails to idle...maybe the PO used the ATV sealer cause the phenolic spacers (there are 2 doubled up) are so scratched and chewed up their causing a vac leak? It sounds like its coming from the carb area...

 

Also I didn't do the valves or timing first (due to my inexperience) maybe that's it?

 

If its at all helpful info....prior to the issue leading to the carb rebuild, the truck wouldn't maintain idle when first turned on until it warmed up.... then with a loud pop on the freeway and loss of compression here we are......

 

Any suggestions on where to start looking would be appreciated as im getting quite frustrated.

[datzenmike]

I would definitely set the valve lash. A tight intake valve will leak intake vacuum. (you want as strong a vacuum as possible for good idle)

 

Good timing makes the engine more efficient. Very much needed at idle where all the gas and air is used solely for spinning the engine.

 

 

 

Vacuum leaks.  (a vacuum gauge is handy because you can tell at a glance if high or low and any changes are readily apparent).

 

Make sure the gasket/spacers between carb and intake are sealing. Buy them or cut your own using the spacer as a template. A good gasket does not need RTV.

 

Make sure the hose to the brake booster is not leaking and there are no other hoses to the intake that are open, cracked or leaking. Pinch the rubber hoses closed and try to idle it.

 

 

Fuel

 

As Len said. Remove the idle cut solenoid and take the little pin out of it and put the solenoid back on. The solenoid allows gas into the idle circuit. If not working you will never get the idle below 1,200 RPM.

 

While the idle cut solenoid is out, unscrew the idle mixture screw and remove it. Count the turns so it can go back in the same amount. Get a can of carb spray that comes with a small red straw. Direct the carb cleaner spray into the idle cut solenoid hole... shove the straw right in there. Now stick it into the idle mixture screw hole and do the same thing. Hopefully carb spray will come out of these two holes showing that there is nothing blocking them.

 

The idle mixture screw has been previously adjusted without result so it's very possible that it's way way too rich or lean for idle running. You may want to try several different settings once you get it running on a fast idle. Slowly lower the idle while turning the mixture in or out.

[distributorguy]

" A good gasket does not need RTV."

But an imperfect manifold, spacer, and carb surface appreciates a little Permatex #2.  

[FauxRoux]

" A good gasket does not need RTV."

But an imperfect manifold, spacer, and carb surface appreciates a little Permatex #2.  

So lo and behold.....I figured I would give the PO enough credit to know what he was doing (after all HE got this thing running) and add some permatex to the gaskets and spacers like I found it since the spacers are chewed to shit...and the truck now at least idles :rofl: .... I had the needle steady at 15 for a minute then a slow slide to 1 then died... now she idles with the needle jumping a bit from about 10-12 on the vac gauge. So maybe theirs a slight leak still and it looks like those valves are in need of a good lashing. probably timing too....I just wanted her to idle first.

 

Pressing down on the accelerator causes her to bog and die.

 

aaaand now that she idles im hearing something rattling under the valve cover.....hopefully just everything thats not tuned right..

 

But on the plus I no longer wish to end my shame by bludgeoning myself with a rock...despite the embarrassment of how long its taking me to get her back on the road :blush:

 

P.S. thanks again for all the help and suggestions guys. You all rock. :thumbup:

[datzenmike]

Tight valve lash is quiet but valve doesn't always close tightly and may leak vacuum or cylinder pressure

Loose lash will make tapping noise.

Just right fixes both of the above.

[distributorguy]

Glad to hear you got it started,  but...

If you didn't wait overnight for the gasket sealant to set up, you likely sucked a passageway (leak) through it.  But I'd definitely lash valves first.  

Specific order for a tune-up:

lash valves

repair and adjust ignition timing

repair and adjust fuel pressure and metering.  

Any other order will have you chasing your tail and trying to fix fuel forever, but fuel needs are reactive to other "floating" problems like a sticky advance mechanism in a distributor or a valve that isn't opening and closing consistently.  

[FauxRoux]

weeeell.....valves lashed...timing check.....neither were really in need of adjusting.....compression starts at 20 and is solid upon cold startup (idles high) then while I attempt to adjust idle and the engine gradually warms up the compression slowly drops until it dies......It doesn't seem like exhaust blockage as there is exhaust present (granted it was white, but that disappears as I lower the fuel mixture)...ideas?

 

Could it be that I am simply this inept at tuning the carb?

 

 

Also I have a spare non-cal Hitachi I could swap out....worth a shot?

[datzenmike]

....compression starts at 20 and is solid upon cold startup (idles high) then while I attempt to adjust idle and the engine gradually warms up the compression slowly drops until it dies......

 

wtf does this mean?

 

 

You can't adjust idle while engine is warming up. The choke and fast idle are on (or should be) and this forces a rich mixture. Idle speed and mixture are set only when the engine is warm and the choke is OFF and not accecting it..

[banzai510(hainz)]

WTF is right!!!!!!!!!!!

 

this motor is pretty much intact so should be ez to figure out.

 

 

you list this as a 78 620 so is this a electric ignition or a point distributor as I see a ballast resisitor but cant see the wire coeming off the back side(hose in the way). I think I see something missing. Like its hooked up wrong. Usually on the + side coil is 2 wires. One for the Hot start then the ON wire which come from the ballast via the blk white wire.

[datzenmike]

I don't see the ballast resistor. There shouldn't be one as this should have a remote igniter EI distributor in it. That is not the stock coil so it replaced with a points coil it would need a ballast.

[banzai510(hainz)]

MIke

look close there is a ballast resisitor riding on top of the coil with a wire to it. but cant see one comming off of it.

 

 

must not be inportant since he hasnt wrote back yet

[FauxRoux]

sorry...out of town for a death in the family .....I was talking about the readings on a vacuum gauge (was a tad tipsy...sorry for the incoherent ramblings)....when the engine is cold and choke fully closed I would first fire her up and get a reading of 20 (on vacuum gauge) with the needle nice and steady and as the car warmed up and the choke opened it would slowly drop to 0 and die...leaving me no time to even try to adjust the idle or mixture screws....So I was thinking I must still have a vac leak (or blockage maybe?) but upon trying to pinpoint where that leak might be with carb cleaner I found nothing.

 

I have the the remote igniter type (with the correct matching distributor)...the unit is mounted under the passenger side dash...If you guys think I am missing something due to a swap out of the coil just tell me where to point the camera and ill take all the pics you want...hell, ill throw a bikini on it and sexy that bitch up if that helps :D....

 

 

 

 

[distributorguy]

Loss of vacuum is a result of the car dying, not the cause.  Is your carb running empty?  Can you squirt the accelerator pump after it stalls?  Is it stalling because the choke isn't working?  Do you have a breather line hooked to a vacuum port?  Disconnect and plug all non-essentials to get it tuned.  

[banzai510(hainz)]

well your getting spark. to me soembody put a older older point coil on there and it needs a Ballast. a true 78 datsun with ignitior type distributor needs only a 78/79 coil as its made to run on the stariaght 12-14 volts.. a point coil will burn up thats why it needs a ballast resisitor

 

I think you need to adjust the mixture screw.

I ran into a sitting 521 . Found out the carb was loose. then would run if I pumped the gas real fast. but still die. then I found out the mixure screw needed adustment.

 

photo of the distributor and tha carb would be nice

 

 

then your read olddatsuns.com the datsun tech section.

[distributorguy]

Looks like a Bosch red coil, which is 1.5 Ohms with a 1.7 Ohm Chrysler ballast resistor.  Good stuff, although your main coil lead looks like its not fully pressed into the coil.  .  

[FauxRoux]

Loss of vacuum is a result of the car dying, not the cause.  Is your carb running empty?  Can you squirt the accelerator pump after it stalls?  Is it stalling because the choke isn't working?  Do you have a breather line hooked to a vacuum port?  Disconnect and plug all non-essentials to get it tuned.

 

If I hit the accelerator she dies instantly....although the mixture screw is only about a turn and a half from being bottomed out

 

 Good stuff, although your main coil lead looks like its not fully pressed into the coil.

 

The coil lead IS super slutty and won't press in better...falls off with minimal pull. I was thinking of replacing it.

 

I will grab a shot of the carb and distributor shortly.

[distributorguy]

The coil wire has 2 "tabs" on the clip that hold it in place.  Just bend them out so you get more pressure in the hole.  Like plug wire Viagra.  

 

Make sure your accelerator pump is squirting.  Sounds like maybe its not.  If you were running points I'd cry bad condenser.  Could still be a bad engine ground.  

[banzai510(hainz)]

what Distributor Guy said

 

on carbs you have a accell pump when you cycle the linkage there will be gas squirting in themain barrel to makes you for the extra air that is needed immediactly. so it enriches the circut.

 

Like before for you start your car you pump the gas this is like pouring gas down the carb then it shoudd start also.

[FauxRoux]

The coil wire has 2 "tabs" on the clip that hold it in place.  Just bend them out so you get more pressure in the hole.  Like plug wire Viagra.  

 

Make sure your accelerator pump is squirting.  Sounds like maybe its not.  If you were running points I'd cry bad condenser.  Could still be a bad engine ground.  

Pump is squirting...

 

 

photo of the distributor and tha carb would be nice

 

 

 

 

[FauxRoux]

I got the lead to the coil on snug (although the boot still sits further back then it used to) When opening up the mixture screw a full turn of so she fought dying a tad longer and im seeing gas fly through the fuel filter, but still dies....when I pump the accelerator she still immediately falls on her face and dies...

 

Am I being too dainty with opening the mixture screw? How many full turns from dead bottom should I reasonably expect to go? (generally speaking)

 

After she dies I can pump the pedal and she starts back up.....I think you guys are on the right track with it being a lack of gas issue at this point.

[datzenmike]

Back the mixture adjustment screw out 4-5 turns. Better too rich than no gas at all.

 

Turn the idle speed up a bit also.

 

Check the idle cut solenoid is working by turning ignition on off on off while listening near the carb. You should hear a soft clicking sound. Must have a 12 volt supply that turns on and off with the ignition.

 

Get it to run, even if too fast and/or to rich or lean and you can begin 'dialing it in'.