off idle stumble

[clumsum]

Hi, I'm new to this forum but have heard Cliff is the man when it comes to rochesters. I have a stock '70 pontiac firebird that I recently got a rochester 7040573 for and it has a dead spot when I crack the throttle open under load. I really have to give it gas and ease the clutch out slowly to keep it from dying. I also feel this dead spot in every gear as I first apply throttle. Once past that dead spot it goes like a raped ape. I checked the accelerator pump by looking down the throat of the carb with the engine off and seeing gas squirting when I worked the throttle and I tried moving the rod to the pump to give it a bigger shot but it didn't help. The guy I got the carb from had it on a gto and said it ran fine for him and that he had it rebuilt a few years ago. Anyway I am not really familiar rochesters so I hope someone on here will be able to point me in the right direction as to what to try to fix this. I guess I'll go out and try to find Cliff's book at the book store first. Thanks

[Frank400]

Was that car running before you put that carb on ?  If so, was it running good and what carb was on it ?  Your timing curve may have something to do with that as well.  What is the motor in that car ?  Is it stock or modified ?  More details will help.  Frank.

[clumsum]

It was running good before I swapped the rochester that was on it for this one. I only wanted to put this one on because it's the right number carb for my engine. It's an unmodified ram air III 400 engine.

[Cliff Ruggles]

I would install one of our SR kits, and primary jets and metering rods while it's apart.  The 7040573 is the California emissions version, and pretty lean on the primary side.  It will respond well to giving it a tad more fuel, we can supply the correct items with the rebuild kit.....Cliff

[clumsum]

Cliff, I'm in the middle of reading your book right now and so far it is very helpful in understanding how these work. I'll take your advise and get your kit and the new rods and jets. Can I buy them on line or do I have to call?

[clumsum]

Well I put in the 72 jets and 39 rods and rebuilt the carb with the kit you sent but I'm still getting the stumble. Do you think I still need to richen it up? Should I go with different metering rods? Thanks

[Cliff Ruggles]

Remove the triangular shaped cover on top of the airhorn, and gently seat the adjustment screw under it.  This will be full rich on the primaries.

72/39 is PLENTY of part throttle fuel for that carburetor number.  Make sure the float level isn't too low, seat the upper airbleed screw, and it should be fine....Cliff

[clumsum]

Remove the triangular shaped cover on top of the airhorn, and gently seat the adjustment screw under it.  This will be full rich on the primaries.

72/39 is PLENTY of part throttle fuel for that carburetor number.  Make sure the float level isn't too low, seat the upper airbleed screw, and it should be fine....Cliff

Yes, I did that already like we talked about. It still stumbles just as I crack the throttle open no matter what gear I'm in and seems like it runs out of steam at around 4000 rpm. I set the float to 9/32 when I had it apart and replaced the k&n filter with a fram. Could it be a fuel pump issue? What do you think?

[Cliff Ruggles]

That's the first place I'd go.  With the high flow seat assembly, it shouldn't be sucking the bowl dry at 4000rpm's in that application.....Cliff

[73 Z28]

Just out of curiousity, what size engine are you running 350 , 400 or 455?
Are you getting black smoke out of the tailpipe? Do you know what the engine timing is set at?
This info will help to diagnose the problem your having.

Paul

[clumsum]

Just out of curiousity, what size engine are you running 350 , 400 or 455?
Are you getting black smoke out of the tailpipe? Do you know what the engine timing is set at?
This info will help to diagnose the problem your having.

Paul

It is a 400 ram air III. No black smoke out the tailpipe. The timing is set at 9 degrees before tdc at idle, I've never checked the total. Thing is, it doesn't stumble when the choke is on and had pretty much the same problems before the rebuild.

[73 Z28]

The key piece of info to solve the problem you just provided. That being that with the choke on there is no stumble. This indicates that the carb is too lean on the idle circuit. During the rebuild, did you remove the idle tubes?  If you did remove them, did you increase the size of the hole in the idle tubes and the down channel restriction? If not, I would recommend increasing the idle tube hole to .038-.039 and the down channel restriction to .062 With a 400 cubic inch or larger engine, the idle circuit needs are much greater than for a 350. The other thing I would do is to increase the jet size to a minumum 73 jet and test drive the new calibration. This should improve your vehicles performance and eliminate the stumble.

Paul

[73 Z28]

One more thing when you have the carb apart, remove the idle mixture screws and check the hole size using small drill bits as a go-nogo gauge. On a 400 I would use a minimum hole diameter of .095 to .099 then reinstall the screws and set them to 4 turns out from fully seated to start with.

Paul

[clumsum]

I didn't remove the idle tubes (was too scared that I might screw them up). I didn't really want to start drilling on this carb since it's the stock one for the car but I guess I could. Being that this is the stock carb for the car and the engine hasn't been modified shouldn't it run ok without drilling passages out?

[73 Z28]

The reason for pulling the idle tubes is that they have a small opening that can get clogged over time reducing the idle fuel flow to the engine. Pulling them allows you to clean them out and by opening up the passageway allows for better fuel flow to the engine, the mixture screws will still be the final control over how much idle fuel the engine gets. If you aren't sure about pulling the tubes, I would find a practice carb to try it on first, but it really is easy to do.  I believe that in an earlier post you said that you had another carb that was doing the same thing, if you still have it, I would use it as a practice carb first.

Paul