Okay, some of you may have seen my threads recently regarding the problems I've been having with my 72 L-48 and Holley SA 670. After pulling one of the plugs, I now know I'm running very rich. I also have a fair bit of difficulty starting the car when the engine is still warm (i.e. after only being shut down a couple hours) - it usually takes me three or four tries to get it running, and then I generally have to stay on the accelerator a little bit to prevent it from dying. It also occasionally tries to die the same way on a cold start, but will fire up the first try almost always. If I don't give it enough and the engine does die, it takes me quite a bit of effort to get it restarted.

This past weekend was a bit more severe - I went to start it Saturday afternoon after being shut off for a good 12 hours, got it started, accidentally let it die, and then could not get it running again until Sunday afternoon.

The other problem I've been having is sporadic shuddering at take-off. It sometimes happens when the engine is first running, and sometimes just in the middle of a drive. When it does occur, it begins after I come to a complete stop and start to get moving in 1st gear. The engine will rev up very roughly, and I have to slip the hell out of the clutch to prevent it from bogging. It will generally continue until at least 2nd gear and 2000rpm or so. The thing is, it will come and go like nothing - I won't be driving differently and it will suddenly appear, and then disappear a couple minutes later.

Like I said, I know I'm running rich (see pic). However, is rejetting the carb really going to solve all my problems? What else can be causing it? In any case, how can I know how many jet sizes to go down?

I'll give Holley tech support a call today, but just thought I'd see if anyone here had any advice for me...

 

For what it's worth I had similar problems on a quadrajet until I replaced the float.

 

Sounds like float or power valve to me!
TJ

 

The guy at the shop was helping me get the engine tuned, and he said the floats looked to be at the right level (a little bit of fuel spilled out when rocking the fenders), but I'll check them again more precisely. The power valve is supposed to have a "million mile blowout protection," but the engine has backfired a few times, so I suppose I'll check that out too.

 

Problem with the SA carbs is that they are actually set up extremely lean in order to pass California CARB requirements. When a carb is set up very lean, you often have to crack the throttle blades open too far in order to get it to idle (should have only .020" transition slot exposure). This, then, kills manifold vacuum, and you can get into a situation where you're inducing a rich condition from PV opening or from fuel discharge out the main metering circuit, all because of a lean condition.

On the SA carbs, increase (not decrease) main jet size by 2 sizes. Then, increase secondary jet size so the secs are 8 sizes larger than the pri side. Set idle mixture screws initially to 1 turn out from lightly seated. Crack the secondary idle speed screw in about 1/2 turn so you can back off the primary idle speed screw. This will get you a good baseline setup with some good throttle response and performance.

 

Well, that's certainly counter-intuitive I'm a bit leery of increasing the secondary jets that much - it'd require upping them by 7 sizes, which just seems kinda excessive, especially since Holley states that you shouldn't need to increase much more than 4 sizes in most cases. Do you really think that's necessary?

Also - excuse my ignorance, I'm much more familiar with EFI than carbs - precisely what is considered "lightly seated" with the idle mixture screws? And I take it "idle speed screw" and "idle mixture screw" are synonyms, right? Excuse the dumb questions :o

Thanks a lot for the help!

 

Lightly seated means turn the screw in until it stops. Don't man up on it just lightly seat the needle, then back it out.

Idle mixture and Idle speed are not synonyms. Idle mixture screws adjust the air/fuel mixture at idle.

Idle speed screws set the minimum opening of the butterflies at idle. In other words this screw will not let the butterfly close completely when you release the throttle.

Finally, unless you can prove that lars fell out of a tree this morning, take his advice. I am glad I did.

 

Thanks. I'm aware of what idle mixture screws do, and what I thought the correct definitions of each were before all this was correct. I just got confused by Lars talking about the primary and secondary idle speed screws - I thought the SA only has one...?

I'll definitely take his advice. Perhaps my comment was poorly worded - I wasn't trying to challenge him as much as get a more detailed explanation

 

All Holleys have idle speed screws on both primary and secondary sides. The secondary idle speed screw is located under the baseplate on the passenger side of the carb.

 

I see. Well, thanks a ton for the help. I'll make the changes you recommended and see how it goes I'm learning more about carbs every day...

 

Make sure you have your timing and advance systems dialed in before you start tuning the carb - it's important that you're set up with 36 degrees total timing and a properly matched vacuum advance control unit. This has a big effect on carb performance.
Lars

 

Heh, my distributor doesn't actually have vacuum advance, just centrifugal. The timing was set recently and seems to be fine.

 

Yeah....I remember..."90% of the time poor performance is due to IGNITION not CARBURATION"

I learned that this weekend!!!

 

I learned just last week how hard it is to communitcate with typed words versus spoken words. I found out the one persons 2 was the same as my 4. Just keep listening to lars and he will get you straighted out.

 

And listen carefully...Setting your timing is not the same as setting your curve for 36* @2500rpm
The cars I seen at the beer tour W/Lars almost all had advance springs that were so heavy that they would not allow the centrifugal advance to come in at the proper time.

 

I see. Thanks for the tip. I know the difference between setting initial timing and total @ 2500rpm, but I didn't know centrifugal advances were also that tunable. How can I know if the springs are too heavy or too light?

 

With the vacuum disconnected, if you rev the engine past the 2500 rpm and the timing advance is still comming in then you may need lighter springs. You want it to stop advancing at 34-36 by 2500 rpm. 2500 is not the magical point, if it goes to 2800 and stops you are ok. 2500 I suppose is easier to remember.

 

Yup....Easier for me,but like Kevinator stated Not carved in stone!