bad stumble off idle, help!

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coolhandluke

coolhandluke

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So, I bought a rebuilt carburetor off a guy on here to replace my 2 barrel. My car's a bone stock '72 dart, 318 auto. The carburetor is a 600 cfm vacuum secondary holley with electric choke. It has electronic ignition, if that makes any difference. Anyway, I got it all hooked up and it has a pretty bad stumble off idle before it picks up. The floats are set correctly and the timing is set at 10 btdc. It's running too rich, the plugs are black, which I know is one problem. I checked and the jets that are in it are 62's, while from what I've read online stock size jets in this carburetor are 69. I originally thought the secondaries might be kicking in too soon causing the stumble, but it does it in idle as well as drive. I took the diaphragm apart and it already has the black spring in it, so I don't think that's the problem. The accelerator pump diaphragm is green, which means it's a 50cc?
So, what could my problem be? Should I just try to jet it down and size and see what that does, or could the 50cc accelerator pump diaphragm be causing the stumble?

Sorry if I missed any information anyone needs to help me out
Thanks!
Luke
 
My guess would be accelerator pump. Off idle vacuum drops so you need that extra squirt of gas until the primary circuit can catch up. I take it you are not punching it. Pretty easy to test. Look down the barrel on the primaries. Engine off. If you acutate the throttle do you see jets of gas shooting down? Should look like a squirt gun stream, not a dribble.
 
How far out are your idle mixture screws? If there out more then about 1 1/2 turns it's probably covering for that lean primary jet.

Mdl. 80457 shows #69 jet and Mdl 1850 shows #66 jet. Also your pump squirter should be stamped #25 iirc.

I'll post a few videos that might help also.


[ame="http://www.youtube.com/watch?v=w3F3ssOb8lg"]Accelerator Pump - YouTube[/ame]

[ame="http://www.youtube.com/watch?v=4xOiJQj6CSU&feature=related"]Vacuum Secondaries - YouTube[/ame]

[ame="http://www.youtube.com/watch?v=UPGLUsW5ZeE&feature=related"]Idle Mixture Screws - YouTube[/ame]
 
Power valve ruptured? Would cause pig rich as raw fuel is sucking through perforation from bowl to intake. You replace a 2bbl with a 600, adapter, new intake? Any noticeable fuel visible down throat at idle is a float level issue. The pump arm has an adjustment on it, it should be off the pump lever by about .005, NOT touching as this causes idle issues. Jets dont cause idle issues. Your supposed to adjust the primary butterflys to expose the idle slot by about as much as they are wide, so the exposed port looks like a perfect square (per Don at FBO), then adjust idle by mixture screws and timing. IF the screws are out more than 2 then there are other issues. The screws are very precise, just a 1/2 turn either way from baseline 1.5 out should get you where you want to be, otherwise there are other issues. Timing, vacuum leak, etc.
 
Lean out the pump shot. (Smaller squirters.) Then maybe re jet for cleaner burn. Also, you could change the cam that determines the pump shot duration.
 
pishta said:
Power valve ruptured? Would cause pig rich as raw fuel is sucking through perforation from bowl to intake. You replace a 2bbl with a 600, adapter, new intake? Any noticeable fuel visible down throat at idle is a float level issue. The pump arm has an adjustment on it, it should be off the pump lever by about .005, NOT touching as this causes idle issues. Jets dont cause idle issues. Your supposed to adjust the primary butterflys to expose the idle slot by about as much as they are wide, so the exposed port looks like a perfect square (per Don at FBO), then adjust idle by mixture screws and timing. IF the screws are out more than 2 then there are other issues. The screws are very precise, just a 1/2 turn either way from baseline 1.5 out should get you where you want to be, otherwise there are other issues. Timing, vacuum leak, etc.
Click to expand...

Very good advise.
 
as soon as the throttle moves..the pump needs to move...never seen an acelerator pump cause an idle problem...

most stumbles are either timing ..or the lack of timing...or carb problems with the acelerator pump causing most of the problems....check to make sure the floats are adjusted correctly as a low float leverl will delay the main circuit from starting to flow fuel to the carb...

what distributor and where is the timing set? always get the timing set correctly before messing with carb....
 
I'd start over with stock jetting. What's the idle RPM. A stock 318 should idle easily at 700rpm.

As long as the total timing number doesn't get out of hand, I'd up the initial to the 14 range, see if it stills starts OK hot and see if that helps.

Like Tony says, Timing before carb.
 
Yeah, it idles fine, it's when you hit the gas that the stumble occurs. I found best idle with the idle screws at 1/2 turn out from bottom (I don't have a vacuum gauge) I also found the accelerator pump arm was hitting, so I adjusted it off the thickness of a sheet of notebook paper (can't find my feeler gauges). It actually took a couple turns of adjustment to get it that far off, so I think it was hitting it pretty hard.
I ran it down the block and back, and the stumble was still there, but MUCH less. It didn't seem to have a lot of power above 2500rpm or so.
Now I'm thinking I should change the secondary spring to the stock one (the heaviest black one is in it now)

As to people's questions, the distributor is the stock electronic distributor, with the timing set at 10 btdc. I'm using an Edelbrock performer intake.
 
Are you getting the stumble after the initial pump shot or off idle?
 
I guess I'm not REAL sure how to tell, but i think it's after the initial pump shot. It revs for a second, stumbles, then picks back up
 
with the idle mixture screws turned so far in to get the best idle, that would mean I'm running rich, right? I can see some black smoke coming from the back while accelerating. I could be completely wrong, but I'm thinking going down to 60 jets, and changing out the secondary spring back to the stock one
 
I can't see any way you can be jetted too rich being at #62 when the carb came with #69's. You've got to be pulling fuel from somewhere else. If i'm correct you do not have external float adjustments on that model. I would check for a sunk float or a really high setting. If that carb did indeed come with #69's it would be the newer model #80457 iirc and should have power valve blowout protection so that shouldn't be a issue. Also, make sure as i said earlier that your pump nozzle is a #25 and not something bigger.

Here is a jet # to size chart if you decide to go that route.



http://www.junkyardgenius.com/holley/jetsize.html


P.S. i found the pictures from the sale and you do have ext. float adj.....sorry.

After further review the adj. float 600's i found were mdl. 80450 & 80453 and both come with 10.5/5.5in. dual power valves. They seemed to use them in smog era type carbs and i imo there junk. might want to put in a straight 10.5 to make sure it's not malfunctioning.

Also. if you happen to pull it apart, make sure the secondary metering plate is not warped and seated well.
 
I would check the o ring in the Seat Assembly. It could be damaged or worn causing fuel to get by and not through the needle making it run rich.
 
Could be a bunch of things.

If the check ball/needle in your accelerator pump system is gone that could cause pullover.

Float level correct? Especially if the carb had fuel in it then sat, as mentioned, check the needle and seat O-rings because they shrink pretty bad.
 
Well, I believe I pretty much got it fixed. With the idle screws so far in, I couldn't get it to crank very well, so I moved them back out to 1 full turn out, and it's back to cranking good. The front float was pretty low, so I adjusted it back right. I drove to hell (Walmart) to try it out, and the stumble is gone and there's no smoke coming from the back now:D It'll spin the tires pretty good from a stop, but doesn't seem to have a whole lot of power on the upper end, so I'm thinking about changing back to the stock spring on the secondaries.

On a positive note, I had lost the c-clip from the vacuum secondary and one of the o rings on the cross-over tube was marred, so I went to advance auto to try to get replacements, and they said all they had was the rebuild kits, so they opened one and gave me the parts I needed from it, for free. I've had problems with some of their parts, but my local store has awesome service:toothy8:
 
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