kooky idle

[GRpufnstuf]

1973 620, stock engine setup

L16

Hitachi 2bbl

4spd manual

single points diz

 

my truck has had a fast idle I can't seem to beat. Sometimes 1500-2000 rpm when cold or hot, sometimes perfectly mellow at 800rpm. No rhyme or reason that I can find.

 

I've checked the vacuum lines for leaks, tightened any nut or bolt I could find and rechecked for vacuum leaks.

 

The carb was rebuilt by a shade tree guy b4 I bought it in July. Then tuned by a shady Datsun mechanic and it ran pretty well for a while.

 

 

NOW

 

I have fast Idle and the car occasionally 'almost' dies then restarts and idles fast again.

 

usually when I'm driving stop and go. It has also stopped running a couple of times this week while I was hitting bumpy road type stuff, then restarted and I kept rolling on.

 

 

 

SO

 

I seem to have 3 issues,

 

1. Fuel delivery issues, but the filter is clean and full and the bowl is at a good level whenever I look.

2. Ignition issues, I have new points, condenser, cap and plugs. The connections at the coil +/- were loose so I crimped the connectors down and they fit firmly now.

3. Vacuum leaks, or stuck choke or...? The choke seems inoperational and is stuck 1/2 closed but there are hoiles drilled in the plate so there should be plenty of air running thru, all the vacuum lines are new. I'm going to hose clamp them all in a moment or 2.

 

any ideas/procedures are greatly appreciated!

 

thanks

[GRpufnstuf]

i should also mention that I have a matchbox diz to put on, I just need cap, rotor, and coil.

But I'd like to figure this out before I throw that into the mix

[ggzilla]

If the fuel bowl is always at the right level, then you don't have a fuel delivery issue.

[ggzilla]

Matchbox uses same cap & rotor as a 1974. You can use the 1973 coil & resistor with it.

[ggzilla]

High idle is not usually caused by vacuum leak, but by an air leak. Make sure the jet covers are finger-tight as a loose one acts just as you stated.

[datzenmike]

Check the throttle cable is free to move and non binding. Also make sure it is not set too tight and not allowing the carb to come back fully to idle position.

 

Check that the throttle shaft is not worn or has worn an oval hole in the carb casing. A worn shaft or case will let the throttle plate rub the inside of the throat and gang up.

 

Next time it idles fast shut off and look down the secondary barrel. It that throttle plate closed fully?

 

The BCDD is set too sensitive to the vacuum signal. The BCDD is that roundish thing on the drivers side of the carb...

This thing...

 

Under the center rubber cap is an adjusting screw. Counter clockwise sets the vacuum signal higher or less sensitive.

.

[GRpufnstuf]

ok,

 

High idle is not usually caused by vacuum leak, but by an air leak. Make sure the jet covers are finger-tight as a loose one acts just as you stated.

 

 jet covers are more than finger tight.  is that too tight?

 

Matchbox uses same cap & rotor as a 1974. You can use the 1973 coil & resistor with it.

 

Is the '74 cap and rotor the same as '73? or is that the dual point setup, or?  I'll research

 

Check the throttle cable is free to move and non binding. Also make sure it is not set too tight and not allowing the carb to come back fully to idle position.

 

Check that the throttle shaft is not worn or has worn an oval hole in the carb casing. A worn shaft or case will let the throttle plate rub the inside of the throat and gang up.

 

Next time it idles fast shut off and look down the secondary barrel. It that throttle plate closed fully?

 

The BCDD is set too sensitive to the vacuum signal. The BCDD is that roundish thing on the drivers side of the carb...

This thing...

 

Under the center rubber cap is an adjusting screw. Counter clockwise sets the vacuum signal higher or less sensitive.

.

 

Throttle cable is loose and free.  It actually has slack before it engages the pulley.

 

Throttle shaft has side to side but no up down play.  I wasn't in a good position to pull the carb tonight.  I'll get it out in the AM.

 

Both throttle plates are fully closed after a warm up and 2000rpm fast idle shut off.

 

The BCDD is disconnected, I mean no vacuum line runs to it.  The guys at  Bill's Datsun Shoppe did this.  should I reconnect?

[datzenmike]

I may be wrong but the vacuum is sensed inside the carb by the BCDD. The hose may go to a solenoid on the intake that opens to bleed away the vacuum to shut it off when it's not needed. Hose off should bleed the vacuum but won't prevent it from being turned on in the first place. Maybe it's sticking? Maybe turn the screw a couple of turns anti clockwise just in case.

[GRpufnstuf]

I'll try that out first thing tomorrow.

 

I'm more and more tempted to order up a Weber and start 'new' with the carb and matchbox and go from there.

But when this one is running right it is very nice so a little more tinkering won't hurt.

 

I'm going to re-read the carb tuning guide just to get my head straight

[GRpufnstuf]

Hey DatzenMike,

Here is the BCDD on my car and the vacuum inlet I was talking about. I don't see it on your pic

 

Throttle shaft doesn't move like I described last night ( i was getting itred I think) and it looks good.  I can't find any stickiness in it's movement.

 

BCDD turned anti/counter clock and no change that I could tell.  turned back to original start position now.

[GRpufnstuf]

I found a 'diagram' of the vacuum lines in my shop manual. Not great but it will get me back to square 1 for that anyway.

 

 

It's really just evap/emissions that were disconnected but it feels better to get looking like the book

[GRpufnstuf]

Spinning wheels and confused.

 

This choke position gets me 2000rpm on a warm engine:

 

 

And this gets me down to 1200rpm

 

 

...

 

Just went back out to start and check something else and it's idling well at 1200 rpm.

The choke seems to have opened some:

 

 

I'm going to button it up and run a couple errands to really put some heat into it. See if that gets more change or random fluxuations going.

 

Idle adjust screw doesn't seem to be having an effect on the current idle. I think that means I'm idling off the main circuit so that's not great.

 

I'm now concerned that if I can't beat the factory carb with a datsun shop manual, Haynes book, and Ratsun then I'll be crap at tuning a weber onto it.

 

If stabilizes as it is I'm going to run it as is until the end of next week. I want to go to Peterson Museum meet and Eagle Rock meet this weekend and I work tomorrow thru next Friday otherwise. The wife is starting to tap her foot at me spending so much time revving the truck in the driveway.

[ggzilla]

All pics show the choke partly on. Hold the choke all the way open and see if that makes a difference.

[GRpufnstuf]

choke held full open gets me the 1200 rpm idle.  I initially thought the choke wasn't working but it opens some when the heat really soaks in.

[ggzilla]

Unless you need to pass smog test, you can leave it at 1200 rpm.

[GRpufnstuf]

I know it's usable and if it were reliably idling at 1000 or so I'd be happy.

 

But as it sits it isn't right. The idle behaved the rest of today and I've got it running pretty nice though I didn't do much other than tighten screws and re-route the emissions lines.

 

Still though, the idle doesn't adjust at the idle screw as it should and there is still a mystery stall and restart very occasionally.

 

I'm going to drive for now and hope it doesn't permanently stall.

 

Also, I suspect my battery is bad. It cranks ok but dies fast if I have the key on for testing and such.

 

Waiting for a paycheck and that will get replaced.

 

I'll get my meter on some wires next chance I get and look for a broken wire in the ignition system. And start getting prepped to install the matchbox diz.

[GRpufnstuf]

Well my super fast idle seems completely cured after a day and a half driving it around. It's staying around 1200-1500 now.

 

I think my throttle cable was so loose on the throttle pulley that it was occasionally jumping the track and holding the throttle partially open.

 

At least that's the only thing I significantly changed (tightened the cable to remove slack). The cable looks a little kinked in the right areas for this to have been happening for a bit.

 

I'm going to try the idle adjust screw again in morning. Hoping to get the idle down to about 1000rpm before I head out for the. Peterson show. I'll have a look at the new plugs I put in in case I messed up mixture along the way.

[ggzilla]

Smog test requires idle under 1000. Datsun spec is usually around 750 rpm.

[GRpufnstuf]

No smog test for 1973 here. I'm in the clear

[datzenmike]

Well my super fast idle seems completely cured after a day and a half driving it around. It's staying around 1200-1500 now.

 

I think my throttle cable was so loose on the throttle pulley that it was occasionally jumping the track and holding the throttle partially open.

 

At least that's the only thing I significantly changed (tightened the cable to remove slack). The cable looks a little kinked in the right areas for this to have been happening for a bit.

 

I'm going to try the idle adjust screw again in morning. Hoping to get the idle down to about 1000rpm before I head out for the. Peterson show. I'll have a look at the new plugs I put in in case I messed up mixture along the way.

.

 

It's better but not nearly cured. You should be able to turn it down until it stalls. 

 

 

Something is letting gas and air it for the idle to be that high.

 

Look at the primary where the throttle cable joins it. Check the choke levers, maybe one is bent and not letting the throttle return fully.

 

Look at the accelerator pump lever. It's the arm that has the large spring on it.

 

 

 

Check the secondary is closed. Push down on it with a pencil. Try pulling down on the vacuum diaphragm lever. See where the thumb is? See that rod sticking out the bottom.

 

At high speeds vacuum pulls that rod upward and this linkage pulls the secondary open. It may not have closed all the way.

 

 

 

.

[GRpufnstuf]

Victory!

 

 

I checked all DatMike's suggestions and found everything normal and fine.

 

Then I looked really hard at the primary with the carb in my hand. It was partially open, but only just!

 

 

I missed it the first time I pulled the carb and it wasn't visible from the top when the carb is mounted.

 

I adjusted the stop screw for the throttle pulley (it was the only thing holing it open)

 

 

Mounted everything up with the idle screw cranked all the way down and it wouldn't start. Great!

 

I backed the idle screw out 3 turns and it starts but stalls. Backed out 6 and it's close etc...

 

Now I'm idling on the idle circuit and it's backfiring thru the carb when I try to come off idle and transition to the main circuit.. Damn.

 

I do a little digging online and in my books. I check the plugs after some running... Lean.

 

I adjust the main 1.5 turns richer and the pop from the carb goes away but I'm still a little lean according to the plugs so now I'm 2 turns from my original start position and it's running nice. I'll check the mixture again tomorrow.

 

So now I'm letting it cool down so I can check cold start and choke adjustment. It was turned all the way rich so I zeroed it.

 

So for now... thanks so much for the help! It calms me to be able to vent on here and get opinions about what to check next. I'm good at the big strokes, the little ones drive me batty.

 

Until next time... Probably soon ;)

[datzenmike]

That set screw is for the fast idle cam so I don't see how that would change unless the cast idle cam was stuck in the on position.

 

 

Here is the plastic fast idle cam. Both pictures show it in some form of the ON position. When the motor is warmed up and the choke OFF this cam should be free to drop down out of the way of the set screw allowing the throttle to close fully against the idle speed adjusting screw.

 

[GRpufnstuf]

We'll shoot. I'll get back into it today and see if the fast idle is free to move

 

This seems to have solved my issues so far.the only other changes were the choke cover moving from rich to the middle "zero" mark and just general snugging of all the screws holding things together. None were loose but a few took a 1/8th turn

 

If I'm working with the cold carb on a bench would the fast idle cam want to be holding the primary open just a hair?

[GRpufnstuf]

Well you're totally right about that set screw. But it was holding my throttle open and causing the fast idle.

My choke seems only partially operable and wasn't letting it kick off as far as I can tell.

As of now it starts well but doesn't have a fast idle. This suits me fine, I can let it warm some while I fast idle it myself. It doesn't get too cold here anyhow.

When I move back north I'll get more worried about it