Why lean under part throttle at low rpms? secondary diaphram?

[Shawn MacAnanny]

I just noticed today that my car is running very very lean under part throttle. It wasnt like this before. I did just change my vacuum secondary spring but it was fine afterwards as far as i could tell. I removed the spring to see if maybe the diaphram was torn and around one of the screws i did notice it looked a little wrinkled up (i have a quick change kit). My question is would this be the cause of it running lean? It'll go to 18:1 or leaner if i keep pushing the pedal down at low rpms. At WOT it runs a nice 13.6 which is perfect for me.

[gearbanger]

I think the diaphragm leaking could cause it to be lean at part throttle. I think 13.6 is too lean at WOT.

[Mommy Wagon]

Sounds more like a power valve issue.

Could also be a jet issue.

When you say part throttle, do you mean accelleration at part throttle? or, do you me crusing at 45 mph part thottle?

Most "Holley" type vacuum secondaries have a "wrinkled" diaphram. The easiest way to check it is pop it off and suck on the vacuum hole on the side. If your scondary actuation rod pulls in and holds, then your diaprham is fine.

What is your ratio at idle, and a wide open throtle? If you get to 18:1 PLEASE STOP. You shouldn't ever be that lean. If this occurs at WOT, in the upper range then your high speed air bleeds are too small. This could also be a jetting issue.

Again, main isses: Engine size, carburator size, jetting, spring color, timing at idle, timing at full advance and when, if available, header temp at idle and immediately after a spirited run.

[Shawn MacAnanny]

I am running holley adjust a jet metering plates on a vacuum secondary holley 750 carbureter.

The stumble seems to be below 2500 rpm, in gear, a light load. Like say i got to acclerate from 55-60mph in 6th gear it will go lean for about 3 second just barely pushing on the throttle.

The acclerater pump is a 50cc and i have since swapped from a 25 to a 35 squirter. This has made the lean stumble only go to 15.5-16. Do you think i really need a bigger squirter?

The engine is fine at WOT and through the powerband. IT's about 11:1 right now at WOT since i installed the new adjust a jets, and through the rpm band just free revving its like 12:1. I'm taking it to the dyno to tune in the next couple weeks.

Engine is a 346 LS1, roughly 500-550HP right now.

I really noticed the stumble when installed my exhaust come to think of it. I put on some 2" true duals that make the exhaust just about inaudible to pass inspection. I'm going to go ahead and install my 3" x pipe setup and then start tuning with that.

[Mommy Wagon]

What size of power valve do you have?

What is your vacuum at idle?

Since it is at part throttle, or when your carb is at a transition state between circuts this could be the problem.

[Shawn MacAnanny]

Idle vacuum is roughjly 11-12"Hg. IT's a 6.5 Power valve. I think you may be onto something as ive tried adjusting my jets every way and swapped out 3 squirter sizes and went up to a 50cc accelerator pump.

[Mommy Wagon]

Alright. Here goes, as there is a misconception of what a 50cc accelerator pump is.

The 50cc pump is only the "reserve" of fuel for the accelerator pump circuit. This will not increase your volume of the squirt. You must adjust (replace) your pump cam to change the volume of the fuel squirt on each pump cycle. Try a larger cam, and make sure it is adjusted tight without any freeplay.

I would drop your power valve to a 3.5 (or a 4.5 if it is around), just to try. Your engine must have a fairly substantial camshaft to only have 11" at idle. What is your timing set to at idle, what is your total advance and when is it all in?

The plan is to make your idle circuit, power valve enrichment, accelerator pump, primary and secondary jets, high speed air bleeds, and ignition timing all work as seemless as possible. As I had mentioned, I think that your problem is between your pump cam and power valve (maybe a combination). The assumed fault of these circuits are always when you have a sudden drop in vacuum, slighly after tip in, though midrange when vacuum increases as fuel begins to be pulled from the main circuits.

[Shawn MacAnanny]

The cam i have is a red cam I've tried both settings on it. I have it set as coming in the quickets right now which seems to work the best. I have the linkage just barely not touching like maybe .015" or so.

The camshaft is 236/236 (285/[email protected]) .651/.651 110+4LSA its ground on LSK lobes and its a hydraulic camshaft. It is a pretty substantial cam for a 346.

Timing i have dropped to 17 degrees at idle with advance but i have the advance lowered for around idle. Cruising part throttle i have about 36 degrees (keepin it on the safe side) WOT it is 28 degrees. I unly run 93 octane.

[Mommy Wagon]

Have you tuned your secondaries yet?

I would replace your spring in the secondary diaphram with the lighest one possible, and go to a 4.5 power valve.

I assume that the engine is in your Firebird (Gorgeous car, I love that style), the car is fairly light, and with a single plane intake you should have a slightly weaker vacuum signal.

The suggestions above will have your secondaries actuate faster, and offer a better 3/4 throttle response. The problem with a power valve that is too close to your idle vacuum is it comes in too early and will cause a stumble. Too late and it can also cause a stumble.

I have had another thought, last night I was thinking about this. What are you using to determine your mixture? If you are using a typical O2 sensor with a meter attached, you may wish to try another method or manufacturer. Some, not all, react too slow to be used as a tuning method under acceleration.

How does the drive feel? Is it still a hard linear acceleration that feels and sounds right? Or do you mash the pedal and it leaps, then slightly bogs after the accelerator squirt is finished and picks up again as the mains are under vaccuum?

[Shawn MacAnanny]

The engine is in my 68 Firebird, 74 Nova rear end with 4.56s. Car is around 2800-2900lbs. Mini tubbed etc. I was finishing up my exhaust today and my welder spool decided to unwind and go all crazy so i gotta go buy some more wire to finishing welding up my 3" x pipe exhaust with side exits, no mufflers. I think this may help with some of the stumble as i never really noticed the stumble when i was running the open headers. The 2" exhaust i put on to quiet the engine down was a huge restriction i know.

I was running the white spring but thats also when i noticed two cracked intake runners on my car so i ended up taking the carb out and putting my new metering plates on and installed a long yellow spring in it. So i should install the white spring (lightest) and tune it using that?

I am using a Fast Dual Wideband system. 1 sensor per header collector and it logs and replays readings.

[Mommy Wagon]

I personally always start with the lightest spring and work backwards. Some of the heavier springs will not allow full opening on some engine combinations.

Good sensor arrangement, you should be fine.

Still say power valve and spring adjustment.

Since you are re-doing the exhaust, have you considered using a collector extention? In many cases this will afford a better low-mid torque band. The cost is almost insignifigant too.

[Shawn MacAnanny]

I ran 14" of 3" pipe before the X pipe, is that what you mean by that?

[Mommy Wagon]

That all depends, what is the diameter if your collector?

If you have a 3" collector, and 3" exaust then there is very little you can do. Just try not to have an X-pipe any closer than the 14" you have stated.

The smoother the transition into the X-pipe the better. You do need length of straight pipe before the bend, as you want as much velocity out of your headers as possible, a bend will slow it.

[Shawn MacAnanny]

Yeah there is a 3" collector and 3" x pipe setup. They are slip fit connectors, not bolt on flanges. I havent measuered it but there is probably 6" of straight on the header collector, a 12" piece i welded in then about another 6" piece before the 45* bend then about 12" into the x pipe then it comes out of the x pipe into about a 36" piece before the 45* bend then about a 6" piece which exits right between the front of the traction bar and the subframe connector.

I figure with no mufflers the bends and such cant hurt performance too bad. I know the exhaust i had on there was robbing me of a good 40hp or so. 2" is way too small especially when it quiets down your engine so the loudest thing is your rockers.

[Shawn MacAnanny]

I got it all welded just havent isntalled it since its raining but heres a pic of it.

I installed it had to loosen the mixture screws one full turn to get it to idle at 14:1. The new exhaust DEFINATELY helps it breathe better. I can actually feel when the secondaries are fully open now as i loose all traction at about 55mph in 3rd gear. I think i need a lighter spring as they are not fully opening until about then it seems. I know it might be where my peak power is as well but i htink the long yellow is just too stiff of a spring for a 2800lb-ish car.

[Mommy Wagon]

I would agree, for a car under 3000lbs you need a lighter spring.

Start with the lightest available. You will be happy.

Great looking exhaust!

[Shawn MacAnanny]

Thank you! I am going to mount my MSD box closer to the firewal and run my serial cable inside the car to read engine vacuum at cruise and such next so what my vacuum is when i get the stumble.

[Shawn MacAnanny]

I weighed the car today. It weighs 3120 with me and a full tank so It's not as light as i would liek it to be. It's still all steel pannels even the hood but the added weigh of the sfcs, rear shock frame rail, and traction bars are probably another 150lbs. Oh well 2940 is light enough with a full tank for me i suppose.

I put in a 5.0 Power Valve today and WOW what a difference. It still has a slight stumble but the part throttle is soooo much smoother. I think i am going to go down a 4.0 power valve next and that should cure it. Once i swapped out the tall yellow spring for the white one i noticed the stumble was more prevalent. My AFR is still good though throughout the band so i think its just a power valve issue.