Stuck Throttle

[ZINGER-D95]

On my 1978 Datsun 620, I have been having this repeating problem where after I rev up, the rpm's do not return down to 950. but if I reach behind the carb and push the throttle rocker forward it drops back down.... there is a vacuum powered valve that has a little arm that pushes the throttle rocker. could this be getting stuck? and I also do not know the official name for it.

[]2eDeYe]

Can you take a picture of your linkage?

 

 

[datzenmike]

4 hours ago, ZINGER-D95 said:

On my 1978 Datsun 620, I have been having this repeating problem where after I rev up, the rpm's do not return down to 950. but if I reach behind the carb and push the throttle rocker forward it drops back down.... there is a vacuum powered valve that has a little arm that pushes the throttle rocker. could this be getting stuck? and I also do not know the official name for it.

 

That's the vacuum secondary. 

 

 

If you haven't messed with the carb lately, spray down the back with lots of WD-40 and work the linkages. Probably just dirty.     

 

                                     

[ZINGER-D95]

i will snap a picture as soon as i get home. On that vacuum secondary, how do i adjust it? There's a rubber cap that covers a small screw. If i turn it clock wise the idle increases slightly.

[paradime]

2 hours ago, ZINGER-D95 said:

i will snap a picture as soon as i get home. On that vacuum secondary, how do i adjust it? There's a rubber cap that covers a small screw. If i turn it clock wise the idle increases slightly.

 

What are you trying to accomplish by messing with the secondary? I would start by detaching the linkage from the carb and the peddle then isolate what's sticking by moving it by hand. Instead of using WD40, I would recommend lithium grease in a can. Although WD is a lubricant, it's also a solvent that penetrates and eventually will gum shit up again.

[G-Duax]

Yup, WD40 might be a quick fix, and will help to dislodge road grime, to flush it away, but long term it isn't very good.

When I had my gun shop 3-4 decades ago, there was always a customer or two that had lubed up their rifle with 'problem in a spray can' the season before, and when it came around to deer season again, they couldn't get the bolt to open!

I would have to disassemble it the best I could, and soak it in acetone to take it all the way down, to get all the gum & varnish out.

 

I second the lithium spray grease idea  :-)

[datzenmike]

5 hours ago, ZINGER-D95 said:

i will snap a picture as soon as i get home. On that vacuum secondary, how do i adjust it? There's a rubber cap that covers a small screw. If i turn it clock wise the idle increases slightly.

 

The screw that's covered with a rubber cap is the BCDD. Boost controlled deceleration device. It turns on with a vacuum signal from the intake when decelerating and allows more gas and air in to help burn off emissions. If the screw is not adjusted properly it won't shut off and a fast idle can result. Turn the screw counter clockwise to lessen the sensitivity so it turns off sooner and allows the engine to idle down properly.

 

Turn one turn left then rev the engine and hold for 5 seconds then let the throttle close quickly. It should rev down and then suddenly lower to idle. If not, try another turn and keep trying.

[ZINGER-D95]

2 hours ago, datzenmike said:

 

The screw that's covered with a rubber cap is the BCDD. Boost controlled deceleration device. It turns on with a vacuum signal from the intake when decelerating and allows more gas and air in to help burn off emissions. If the screw is not adjusted properly it won't shut off and a fast idle can result. Turn the screw counter clockwise to lessen the sensitivity so it turns off sooner and allows the engine to idle down properly.

 

Turn one turn left then rev the engine and hold for 5 seconds then let the throttle close quickly. It should rev down and then suddenly lower to idle. If not, try another turn and keep trying.

 I loosened the little screw and throttled a few times. Huge difference! The linkage was also part of the problem, I sprayed down the back with some crc brakleen, then with a light lubricant. Now its slick as snot!! I would supply pictures of the BCDD but I cant upload from my phone. I'll post them off my desktop. 

[datzenmike]

I've seen one before.  ?

 

I also had a throttle hang up, actually a misnomer... it didn't catch on anything, it was getting extra air and fuel from the BCDD that was not shutting off properly. High vacuum sets it on, but as the engine lowers in RPMs the vacuum drops and it should sense this and shut off. Turning it counter clockwise reduces the sensitivity. The point of the BCDD is to reduce a severe lean condition on sudden deceleration which causes a badly burning mixture producing un-burnt hydrocarbons in the exhaust.

[ZINGER-D95]

8 hours ago, datzenmike said:

I've seen one before.  ?

 

I also had a throttle hang up, actually a misnomer... it didn't catch on anything, it was getting extra air and fuel from the BCDD that was not shutting off properly. High vacuum sets it on, but as the engine lowers in RPMs the vacuum drops and it should sense this and shut off. Turning it counter clockwise reduces the sensitivity. The point of the BCDD is to reduce a severe lean condition on sudden deceleration which causes a badly burning mixture producing un-burnt hydrocarbons in the exhaust.

Well luckily, my idle is staying around 950. And as i throttle it, it returns back down and doesn't hang up. As i was tinkering along with it, i came across a handful of small vacuum leaks... one of which was the manifold vacuum that goes to my brake booster. 

[datzenmike]

Correct idle is about 200 RPM lower.