Two 620 carb issues. Related?

[ByStickel]

My '78 620 used to start just fine with the choke. After warming up, a tap of the throttle would set it to a nice, normal idle.

 

All of a sudden, it doesn't start easily with the choke on. Now, it requires 1/2-3/4 throttle to fire up. As soon as it fires, I can release the throttle and it'll idle quickly, as it should; it just won;t start without some throttle assistance. The choke does seem to be snapping shut when I press the throttle all the way, prior to cranking.

 

And, after warming up, the normal throttle tap DOES bring the idle down a little, but not all the way to the regular idle speed.

 

Since these both showed up at the same time, I'm wondering if anyone knows what could be causing this. I'd like to target the disassembly/cleaning, rather than dig into the whole carb, if possible.

 

Thanks.

[datzenmike]

The plastic fast idle cam may be stuck. Liberally spray the rear of the carb near the throttle cable with carb cleaner or WD-40. Hold the throttle part open and work the choke open and closed by hand. When the choke is closed you should see the fast idle cam drop into place and block the throttle return, holding it slightly open when the choke is opened you should notice the fast idle cam drops out of the way. I'm at work and can't post a picture of what I'm talking about.

[banzai510(hainz)]

I would just pump the gas to get mine to start by using the accell pump from carb to dunp gas in the manifold. But once started you have to keep it up till warm.

 

I went to a Manual choke carb after that

[ByStickel]

I finally started looking into this carb issue, but didn't have Mike's recommendations, available. I did notice some orphaned vacuum fittings, though. Can anybody tell me what needs to go where and what's important and what's not?

 

This seems to be from the idle compensator.

 

 

This is the bottom of the Temp sensor. The line still in place is connected to the vacuum motor on the same filter housing.

 

And here's the port on the manifold, capped:

 

 

The passenger side of the carb. In the foreground are the two vacuum hard lines, with the open one coming from the thermal valve on the thermostat housing and that tees into the line coming from the charcoal canister. The connected line goes to the distributor:

 

 

Not only that, there's this bit on the line heading over to the brake booster:

 

 

This, which I'm guessing went to the air injection pump that's long gone?

 

 

And this (for some anti-backfire valve?)

 

 

I just couldn't find a good diagram for '78.  Any help is always appreciated.

[datzenmike]

 

The small one is for the automatic .(if equipped)

 

 

 

Rear most line goes to the thermal switch on the thermostat housing and from it to the EGR . You can run with or without it.

 

The front port goes to the distributor vacuum advance AND to the purge port on the canister.

 

 

 

There should be a vacuum line on that spare opening to the thermal valve. When the air is warm enough the vacuum motor needs vacuum to pull the valve open inside the snorkel to mix cool and warm air. Right now you are sucking only hot air... not good for the summer.

[ByStickel]

Thanks, again, Mike.

 

So, I don't need to worry about the idle compensator?  What was it supposed to be connected to?

 

Another thought: if my brake booster diaphragm is bad, would that produce a vacuum leak?...I did have the leaky M/C right around the time the issues began...

[datzenmike]

I think the idle compensator opens when the under hood air temps rise above a certain point and leak air into that vacuum line. The vacuum line just connects to the intake and this leans out an over rich condition at idle caused by the hot air.

 

Leaky booster could affect the vacuum. Sometimes engine will stall at idle when you apply the brakes. Pinch the hose closed or remove and plug and see if this fixes the problem.. Have a care as braking is extremely compromised if you do.