Carburetor Idle Issues

So I got to cleaning my engine a while back and ended up tearing out a bunch of parts. I cleaned, fixed and replaced most everything that I could get my hands on or wrench off. After I got everything back together, I can't seem to get my new carb to idle correctly.

The engine block number suggests that it is a 1968-70 Pontiac Firebird engine. I recently bought a Rochester Monojet #7040014 to replace my old rotten Rochester B carb. On starting up, the engine will rev all the way up and hold at close to full throttle. Nothing I can do about it, short of turning it off. After a couple starts that go straight to full throttle, it will start to calm down and idle correct for a second and then want to die. I have tried messing with the fuel mixture screw on the bottom and the bolt on the throttle linkage, but nothing really stops it from dying completely or revving up like it's possessed.

Now I'm not sure if this is the wrong carb for my engine, I just went off some basic descriptions when I bought it. There does seem to be a few extra ports on the side that faces the valve cover. Not sure if those were supposed to go somewhere else on a firebird engine that the van just doesn't need.

I have included a bunch of pictures. Some of just the carb itself and others of the carb installed. Any help would be appreciated.

I am not 100% sure but I think that the bore on the B carburetor is different. Make sure that you have all of your vacuum hoses hooked up and all are in good condition. If it sucks just a little air it wil not idle. I just finally fixed a similar problem on my 69 Chevy van.

did you plug the unused vacume ports?

You need to measure the bore on the bottom of the carb & make sure it's the same size vote & most defiantly plug off any of the vacuum ports that are not being used.

I plugged up the unused ports and no change. One thing that I didn't mention is that the mixture screw isn't original. The original got bent in shipping. The replacement is different by about 2mm. Not a big difference, but then again, I'm not the expert. I'll check to see if the fast idle cam is working correctly.

I think I got the fast idle cam working ok. It doesn't die when I let off the gas. It does want to die once I try and get it moving.

Also on a side note, there are sparks in the exhaust smoke. But that could be from any number of things I did to the manifolds.

When its screwed all the way down the engine slowly dies. The smoke is isn't that thick and more lightly colored. Definitely not black. I'm wondering if some of the extra gasket material might be burning off. I'll try the carb cleaner and see how it works.

The original needle was bent during shipping. So much so, that it broke in half while I was unscrewing it. Below is a pic of the two screws. The bent/original one is on the left.


The guy I bought it from sent me a replacement that isn't exactly the same but very close.

I am going to put the old Rochester B on soon to measure the throat and just to see if any of the other modifications I did are causing the problems.

Did some quick research on my monojet. I saw somewhere that is is originally for a auto transmission, mine is manual. Does this make a difference?

Take old needle- cut in 1/2 where break is - (making it shorter) then saw a small slot in the end for a small screwdriver to fit in. Reinstall it-try that.see if that fixes your problem??

The idle screw might be your problem. It is a lot different than the replacement screw that the seller sent you. There are a couple of used Rochester 7040014 carbs on eBay that you can buy to get the right idle mixture screw out of if it turns out to be your problem. I think that the cheapest one is $29.99 with free shipping. Here's a link for it.


http://www.ebay.com/itm/1970-Pontiac-Checker-Buick-6cyl-250-Carburetor-Core-/350530741255?pt=Motors_Car_Truck_Parts_Accessories&hash=item519d42a407&vxp=mtr#ht_902wt_689

I actually did take the broken piece, cut the notch in it and try it out. But that was before I capped off the ports on the front side. I'll try it again now.

What model/numbers of Monojet are you guys running on your 1st and 2nd gen L6 motors? I'm questioning whether or not I got the correct carb.

cculella, I have a box full of Monojets if you need spares...


Dan

I am running model number 7029007 on my 69 Chevy van. It is the one that came from the factory on it.

Donivan - When I first started trying to diagnose I did try the vacuum hose on all the ports. It died on anything that wasn't the base port. I will have more time tomorrow to do some more diagnosing.

Dan - I may contact you for one of your spares. Do they still work or are they for parts only?

cculella,
There are complete. I bought a box of 4 a while back from a guy for $100 and have collected a few more since then. I've cleaned them up a bit and put them on the shelf. I would recommend rebuilding any carb before installing it since the kits are about $12. I do have one parts carb if that's what your looking for.

Dan

Ok, so I messed around a bit more.

- Sprayed some carb cleaner down the throat while it was running and it revved it up and smoothed it out for a second.

- Unplugged the distributor vacuum hose while it was running and it died.

- Put in the old mixture screw/needle cut down with the notch for adjustment(this time with all ports capped). That seemed to make it run the best. I'm guessing some of my changes to the fast idle cam got rid of the high revving at the startup, then the original needle and capped ports evened out the mixture and kept it from dying. It made it around the block a couple times, so that's a good sign. I feel there is still a lot of tweaking to get it running perfect, but at least it is running now. Thanks for the help. I'm sure more questions will come up as I get into tuning it just right.

Don,
You have so many pictures of parts & knowlege of our vans you should write a repair manual for us.

Alright, got the carb linkage idle screw set to the photo above. Still no luck. Then I realized that the intake that I used had a hot idle compensator attached.


So I plugged that up, restarted the engine and now it idles without the vacuum advance attached. Doesn't run the best, but it doesn't die right off like before. So I should dial in all the settings while the vacuum isn't attached and it will run even better attached?