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JWray707

High Idle speed?

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So, have been making multiple trips to the smog station with my 1978 620 and have been so close to passing every time. Been doing small fixes as they come up but the consistent issue has been the idle speed won't drop. The technician found a leak on the middle gasket of the carb. Luckily I my warranty was still active so I traded the carb out and got a new one...much cleaner...no missing parts (at least visually). It's a Hitachi.

 

The difference with the new carb is the BCDD valve has a vacuum port and I'm not sure where its supposed to be plugged into.

 

Anyways, the idle is super high when the BCDD is plugged into manifold and won't drop. When I put the truck into reverse or drive the engine dies.

When BCDD is not plugged into manifold the engine just dies when the choke opens fully.

It seems to surge either way.

Checked vacuum pressure and it seems fine. Timing is good. I haven't messed with either the mixture or idle speed screw because I think it was set from the factory. Adjusted the BCDD pressure and it helped to drop RPMS a little bit, but not much.

 

Wondering if anyone has suggestions or information that could help. Long post...I know, just trying to give some background and context.

 

Thanks all!

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I take that back, I did turn the mixture and idle speed screw but it didn't do anything significant so I just put it back to where it was set when I got it.

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Is the throttle cable too tight?  Or you still have a vacuum leak even with the new carb?

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th_DCH340-1dx.jpg

 

The BCDD is like a mini carb within the carb. It adds air and gas on deceleration to reduce emissions, but it should shut off. The BCDD may be set to too low a threshold and it won't shut off.  If you have this style pull the rubber plug out of the center and turn the screw counter clockwise to reduce it.

 

 

As to disconnecting it and it quits when warmed up that may be fixable by setting the idle mix screw and the idle speed screw. Remember the choke forces a rich idle condition which will hide a bad idle adjustment. Take the choke away and.... it refuses to idle.

 

Also make sure the idle cut solenoid is working. Key on/off/on/off yopu should hear a soft clicking sound at the rear of the carb. If not check the electrical connections. You should have 12 v on the red wire to the idle cut.

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Is the throttle cable too tight? Or you still have a vacuum leak even with the new carb?

Checked throttle cable earlier and it's not tight. Don't think there's a vacuum leak...reading 20hg...the gauge is shaking a bit, but I think that's from the idle speed not being steady. Recently replaced all of the vac lines so no chance of cracks or worn hoses.

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th_DCH340-1dx.jpg

 

The BCDD is like a mini carb within the carb. It adds air and gas on deceleration to reduce emissions, but it should shut off. The BCDD may be set to too low a threshold and it won't shut off. If you have this style pull the rubber plug out of the center and turn the screw counter clockwise to reduce it.

 

 

As to disconnecting it and it quits when warmed up that may be fixable by setting the idle mix screw and the idle speed screw. Remember the choke forces a rich idle condition which will hide a bad idle adjustment. Take the choke away and.... it refuses to idle.

 

Also make sure the idle cut solenoid is working. Key on/off/on/off yopu should hear a soft clicking sound at the rear of the carb. If not check the electrical connections. You should have 12 v on the red wire to the idle cut.

I will decrease the pressure tomorrow (maybe if the rain lets up!) Earlier, like I said I did turn the screw counter clockwise and it made the engine idle a bit steadier but I will turn it some more and see if that helps.

Is there a point where I should stop turning or just go for it?

I will also check the idle cut solenoid by ear and make sure there's 12v coming from the wire connection.

 

I need the BCDD connected because I'm trying to pass smog and they don't like a open vacuum port!

Do you happen to know where the vacuum port on the BCDD is supposed to connect to? Manifold?

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I would start by leaving it disconnected and get it idling properly then maybe ad it in.

 

You said when disconnected it only quit when the choke opened so I assume it idles lower, or the same?

 

 

Fast idle...

 

Fast idle cam is engaged, stuck? badly adjusted.

BCDD is badly adjusted.

Throttle cable too tight

Idle speed screw turned in too far

Secondary not closed all the way..... had this once causing a fast idle that made me pull my thinning hair out. Poked the secondary throttle plate with a screwdriver, it dropped into place and it was fixed. Just stuck in place cracked open.

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I would start by leaving it disconnected and get it idling properly then maybe ad it in.

 

You said when disconnected it only quit when the choke opened so I assume it idles lower, or the same?

 

 

Fast idle...

 

Fast idle cam is engaged, stuck? badly adjusted.

BCDD is badly adjusted.

Throttle cable too tight

Idle speed screw turned in too far

Secondary not closed all the way..... had this once causing a fast idle that made me pull my thinning hair out. Poked the secondary throttle plate with a screwdriver, it dropped into place and it was fixed. Just stuck in place cracked open.

Yeah, when the BCDD is disconnected it idles lower rpms but it stumbles quite a bit until it eventually dies.

 

The fast idle cam screw looks like it was bent a little and the screw was not sitting on any of the steps. I managed to bend it back so it will catch on the lowest step so I will check that again and make it so it will catch on all three.

 

I will certainly check the secondary throttle plate as well! Have a feeling the problem is something small and elusive like that! Thanks!

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I am hearing a rattling in the carb aswell...Maybe something like a air jet or something loose?

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Might be the idle cut solenoid. It's an electrical valve that allows fuel into the idle circuit. When the ignition it turned off the solenoid closes off the fuel to help stop the engine without run on or dieseling. A loose wire being shaken by the engine would do this. With the engine off and the key ON wiggle the red wire to the solenoid and see if it begins clicking. Yes? you have the problem.

 

Hold the throttle part open and close the choke. You should see a linkage move down allowing the fast idle cam to lift up and hold the idle speed screw open. Open the choke and the linkage releases the fast idle cam and it drops down out of the way allowing the throttle to close to it's proper idle position.

 

I have NEVER seen a vacuum leak cause a faster idle, ever. A vacuum leak allows more ai,r in leaning out the mixture and destabilizing the idle quality. NOW... if you were to be running excessively rich to begin with, then the added air might raise the idle speed BUT generally it everything is correct the idle would drop.

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So just to be sure...the fast idle cam looks something like this...

 

MSUlQB.jpeg

 

Should it be on the 4th or 1st step when starting cold?

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It's depends on the choke.  When the choke is full on, it's on the 1st step but if warm out it will quickly drop down as soon as you step on the gas and drive. When very cold the choke will stay on more firmly and take much longer to warm up and release the fast idle cam. Then it will be much more noticeable as it goes down each step and the idle reduces gradually. 

 

The fast idle speed is adjustable but it is calibrated by the maker and the cam steps designed for your engine and pretty much never needs nor should be adjusted. Usually the linkage from the choke is bent out of the adjustment or it gets dirty and sticks on fast idle. Or the choke set to rich or lean. 

 

WD-40  or carb spray will rinse off and lube the linkages usually. Hole the throttle part open and open and close the choke plate to free up the linkages. The fast idle cam should flop freely around.

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I've been spraying all movable parts with carb cleaner aswell as using it to check for leaks.

The cam linkage is bent so I'll bend it back so it's straight and see if that helps too.

Thanks again for the advice...I'll try to check all of that tomorrow and update.

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Soo...

Went out there and reset (bent back) the fast idle cam screw so that it catches all of the steps.

 

Adjusted the BCDD pressure and it dropped rpms a little bit but not a significant difference. Now the adjustment screw seems to have lost resistance when I try tightening it again.

 

The throttle cable is not tight (I can loosen it all the way and the RPMS are still high)

 

Adjusted the idle speed and mixture screws and didn't seem to have any effect on the speed.

 

Also, I just tightened the idle cut solenoid with my hand and it seemed to get rid of that rattling. Thanks for that tip Mike!

 

Can't figure out why the RPMS are so high! The engine continues to die when I put it in reverse or drive.

I've been doing all of this with the BCDD vacuum port plugged into the manifold because (still) the engine will not even start when the choke is open with the BCDD vacuum unplugged.

It's running really choppy and seems to be getting boosts of high rpms and then will drop...I assume from the BCDD.

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I'll be working on it today trying to trouble shoot the problem. Any suggestions are welcomed!

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Leave the BCDD unplugged or disconnected for now.

 

Did you check the secondary is closing tight?

 

Check the primary barrel is closing tight. If too high, the idle adjustments really won't work because fuel and air are getting in through there.

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Just pulled the BCDD off to clean and inspect it and I found a problem, maybe THE problem. A washer which controls the opening and closing of the valve is stripped and is not catching on the screw that holds it in place. Hence the lack of resistance when trying to adjust the screw. So, the BCDD is not working the way it's supposed to.

May have to swap the carb out once again for a new one...cause I don't see any way of fixing it and I know that part is hard to come by.

It'd be cool if I could just close it off...but I don't know if that would fly at the smog place. Maybe some way to leave it on and make it look like it's actually doing something when it's really not doing anything?

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The BCDD comes into play when at highway speeds and suddenly decelerating. The throttle shuts off the air and the mixture (idle mixture) goes lean and doesn't burn properly and the intake vacuum spikes. The high vacuum signals the BCDD to open a small 'carb within a carb' passage supplying gas and air to promote better burning. When the vacuum level decreases to normal as the speed lowers, the BCDD resets to off.

 

I doubt they would check this as you would need a dynamometer. If you disable the BCDD internally it would still have to pass a visual and 'look' like it worked.

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Yeah, I'm probably just going to exchange the carb due to the faulty BCDD valve. I need everything to be working for smog.

See if the new carb helps with the idle speed and I'll be sure and inspect it in a little more detail next time, before I put it on.

Thanks for your help and advice Mike.

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These carbs are rebuilt on the cheap, been through 4-5 of them till I found one that works good, not just for Datsuns but others too.

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I would rebuild the original before I would pay for a piece of crap. I trust myself.

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Still waiting on the carb to come in. They could only get ahold of a Federal emissions carb (not entirely sure the difference from the Cali?) Figure I have the warranty, so it doesn't cost me any extra to exchange it for a possible better rebuilt one. Seems like it's been very hit and miss when it comes to all the parts being there and working. May rebuild it in the future...but after it passes smog will probably just convert it over to a different carburetor.

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California emissions generally have smaller jets and run leaner. The choke may be riveted on and not adjustable by the owner. The idle mixture screw may have a limiter cap on it to prevent the owner messing with it too much. I believe the two venturies are staked in and also not removable where federal ones have set screws holding them on.

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Not sure if this should be in this thread, let me know if not.

 

Got the new carb and there were a few little things that needed fixing after inspecting it...but I got it on there.

So now, the engine won't run or idle.

It WILL turn over and start with a push of the gas and then I have to continue to have my foot on the gas or it will die, slowly. It's really chugging when it does run and lots of shaking going on.

One thing is that the choke housing is cracked away in a few spots. Would that affect anything?

The fast idle cam wasnt set the right way so it could catch on the steps and that's one of the things I fixed. Not seeing how the cam is supposed to get back to the first step when choke closes again. If that makes sense? There's no tension on the cam to bring it back to its starting position.

Anyways...just wondering if someone has ideas of why its not running?

Fuel/Air? Haven't touched the adjustment screws yet.

Timing? Haven't checked with a light yet.

Pretty sure theres no vacuum leaks, but will check better tomorrow.

Float is perfect level.

Suggestions?

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