Idle too fast, can't reduce

[Tinker]

Just got my first Datsun, a 1978 KC. It runs pretty well considering its age and being a farm truck for 40 years. 

Going through the tune up procedures, I've been unable to get the idle down to where it should be. Here are the symptoms: I have the throttle adjusting screw out far enough it's not engaging; when I adjust the idle mixture screw out I never get to a point where the idle slows down (indicating a mixture too rich). In fact, the RPM increases a lot in conjunction with backing out the mixture screw. The ONLY way I've been able to make the idle drop from ~1500rpm (initial) to ~900rpm (current) was to turn the mixture screw in until the idle speed went down. I'm at a loss for what else to try at this point. Here's what I've gone through:

- Confirmed the throttle screw is not touching - throttle valve is completely closed as far as I can see/feel. I can feel it close and hit the stop (either inside or the one on the outside)

- Disconnected the cable from the pedal to confirm that was not holding it open

- Disconnected the Air Pump belt (which eliminated the backfiring ? ). This was unrelated.

- Turned the BCDD adjuster out 2 full turns to confirm it was not stuck on at idle. No change. Returned to original position

- Checked AB valve, which does not appear to be working. I can't feel any vacuum from it when revving the engine and snapping the throttle shut.

- All vacuum hoses I can see appear to be in good shape and connected. 

 

It seems like there is air getting into the intake somewhere which is defeating the throttle being closed. I assume that's why I can't make the RPM reduce by turning the mixture screw out.

 

Any ideas?

 

[banzai510(hainz)]

what is the distributor timming set at?

I assume the choke is off and the choke plate is open.

distributor advance(air line) is going to the base of the carb and not the manifold?

make sure the 2nd barrel is closed at idle.

 

I think there is a fast idle kick down linkage also needs to be checked

 

maybe just time for a new carb.

[Tinker]

I measured the timing at 11-12° (at ~900rmp), so it's close.

Choke works - visibly full open after warm up

The 2nd barrel looks closed. I tried to gently press it with a screwdriver to see if it would open or close any more - and didn't see see it move.

I also confirmed the fast idle cam is not engaged - when warm it's rotated past the set screw as it should be.

I'll check the vacuum advance line.

 

One more thing: when playing with the carb yesterday without the engine running, I noticed that fuel comes out from the throttle rod when I move the throttle by hand. I don't think it happens when the motor is running because it gets sucked in. But, when I move the throttle not running it gets wet where the rod goes into the carb.

 

Is there another way air could be getting in to mix with the idle fuel? BCDD system? AB? Other spot?

No $$ currently for a new carb - are there any red flags to look for before I rebuild this one?

[datzenmike]

The BCDD needs to be turned counter clockwise to reduce the sensitivity.

 

Adding extra air will not rev an engine up. A vacuum leak will not rev an engine up. To rev up you need more gas and air.

 

Check the fuel level in the glass in the front of the carburetor. Should be at the line or the dot.

 

 

 

[Tinker]

Just to confirm - I don't see a BCDD solenoid on this engine. The Autozone repair guide indicated there is not one on the 78 year - is that correct? https://www.autozone.com/repairguides/Nissan-Pick-ups-and-Pathfinder-1970-1988/EMISSION-CONTROLS/Boost-Control-Deceleration-Device-BCDD/_/P-0900c1528004f465

I tried turning the BCDD screw 2 turns CCW - no change in idle

I then went back to original, and went 2 turns CW - The idle got very rough. So, I turned it back to the original position.

[datzenmike]

I had a '78.... no BCDD solenoid.

 

Turn counter clockwise and give several good revs. If the BCDD was too sensitive this will reset it. If it quits or tries to quit it's because you have the side speed and mixture turned way down.

[Tinker]

Great, I'll try that. Didn't rev it after turning the screw before. I'll check the fuel level as suggested too.

Thanks

[banzai510(hainz)]

I dont think it should leak when you cycle the linkage with the motor off is a sign already or a worn carb.

But MIke is more familiar with the BCDD stuff than I am. as mike said More air usually give a gives a MORE NO IDLE situation. Like a cracked carb adapter . soemhow thru vacuum is its sucking gas thru the carb more than it needs thus giving a higher idle . This is all arm chair guessing on my part.

 

a vaccum leak before the carb isnt going to be the proplem

a vaccum leak after the carb will usually be a low idle or none at all(one compensates by giving it more gas) ex... speed screw turned in.

 

1500 rpm your using the main fuel jet

Edited December 21, 2018 by banzai510(hainz)

[datzenmike]

I had a secondary stick part open. I shoved up on the diaphragm lever and it came loose and closed properly. Bit of rust? debris? who knows but engine stalled and had to turn the idle speed back up and re adjust the mixture. Worked perfectly after. 

 

 

If you have a small vacuum leak the idle mix could/would be adjusted richer to compensate for the added air but of course the idle speed would also go up. This may be where you are now.

 

I would check the carburetor mounting gasket and nuts are snug. Maybe remove the carburetor for a really good look and also check visually the secondary is fully closed. Check ALL hoses connected to the intake for cracks or looseness, (like anti backfire valve, brake booster, hose to automatic modulator valve if so equipped) maybe pinch them closed and see if you can turn the idle mixture down and reduce the idle speed to normal.    

[DanielC]

This sounds very much like a vacuum leak.  You probably know that.  A good look can find bad vacuum hoses, but something the the vacuum hose goes to may also be leaking air.  It could be the brake booster, for example, and not the hose going to the brake booster.  It could be the end of the hose has hardened, and does not seal on the fitting.

Check the manifold bolts where it bolts to the head.  Check the bolts holding the carb to the manifold.  Is the PCV valve being held open by some crud in it?  Is part of the EGR system leaking air into the intake manifold?  Make sure the secondary is closed completely.

It might be the throttle shaft where it enters the carburetor.   Squirt carb cleaner where the throttle shaft goes into the carb, both sides.

 

Check mechanical things.  Has a gasket slipped into the manifold, leaving an opening for air to leak through?  this can happen at the carb base, and at the cylinder head.  Squirting carb cleaner at mating surfaces can help find an air leak.   But try checking bolts first. 

You may need to get some plastic caps to plug all the places where a vacuum line attaches to the manifold.

 

It might just be one larger leak you have not found yet, or it could be a lot of little leaks.

Edited December 21, 2018 by DanielC

[Tinker]

Thanks for all the help, and sorry for the delay in updating.

I rebuilt the carb last weekend. Lots of crud on the bottom of the secondary valve. I also went through and capped all the lines that weren't used anymore (anti-backfire, thermal switches on air cleaner, etc.). I had already removed the air pump because it wasn't working, so all the air injection stuff is now a paperweight.

Result:

The truck runs better and seems to have a little more oomph to it. It does idle well - but I still can't do the "typical" idle adjustment of turning the mixture screw in/out until it has the highest revs. With the throttle screw backed out completely (throttle closed), the idle speed simply keeps increasing as I turn the mixture screw out. That said, it does idle well, I just have an interesting time setting the idle. I can either have the throttle screw completely closed and use the mixture screw to attain ~800rpm, OR I can open the throttle a little, then lean the mixture to attain ~800rpm. Both cases seem to idle the same. Maybe it's time to just drive it...

Here are things I noticed during the rebuild:

- The choke pull-off was leaking vacuum and not actuating. Ordered a new one and will install this weekend. Covering the vacuum leak with a finger didn't change the idle.

- The throttle shaft is very loose where it enters the carb. I can wiggle it quite a bit. As I understand, there is nothing to do about this except save for a new carb. This one has 40years and 250,000 miles on it.

- I'd like to replace the secondary diaphragm, but can't find one. Anyone know where to get a secondary diaphragm?

- I set the fast idle per the carb kit instructions, and I think it is much too high now. What is the fast idle speed supposed to be?

 

Again, thanks for all the help!

 

 

[datzenmike]

Fast idle is 1,800-2,200. Not that critical.