Main air bleed question

[Jim H]

Hi guys,
I have a '77 Corvette, California car with the original carb 17057504. The engine has been ugraded and the carb rebuilt and modified to Cliff's Recipe 2. The car is running well with good performance but I am wondering about something, 76 jets were installed but the upper and lower main air bleeds were left alone so they are in the neighborhood of .110 - .120" diameter. Was that a mistake? The car is getting poor gas mileage, less than 10 mpg, could that be why?

Thanks,
Jim

[Jim H]

I forgot to mention, 44 rods...

[Kenth]

Factory jets in 17057504 is #72 with 41K rods. Upper and lower main airbleeds are .120".

If economy is the main consern i would use the factory jetting and modify only the idle/low speed circuit. Start with APT at 4.0 turns up from seated.

If #76 jets gives best WOT performance i would use 47K-49K primary rods and use the "tip-in" method for APT adjustment.

[Cliff Ruggles]

Unlikely that is the cause of fuel mileage that poor.  Those carbs are horribly lean as delivered and I NEVER use the larger "K" series rods in them.  They don't offer enough room for adjustment with the APT system for one.  IF you look into the carb with the PP in place you'll see that the larger section of the tapered portion of the rods don't go all the way into the actual metering area of the jets.  With "K" rods you only have .005" of taper right to start with, then they "step" to .026".  You really you have very little rood for adjustment.

With a full tapered rod you'll have a LOT more control with the APT system.  We have full tapered 45 rods for them and that's what I use in all of those carburetors here with perfect results. 

Every single time I've put the stock 76 or 77 jets back in one of those and the larger "K" rods they are too lean on the test engine and we get customer complaints if I let them out of here that way.

I don't leave the idle system stock in them either for the same reason, especially with the L-82 engines. 

A few years ago I had a customer who DEMANDED that I return every single part of the carb to the exact stock settings.  It was a 77 or 78 L-82 engine from a Corvette and factory original carb.  I told him that the idle system was too lean and it needed a slightly larger idle tubes and DCR's to make it happy.  He told me to leave it stock, so I did.  It ran poorly on the test engine and sure enough he didn't have the carb on his engine 10 minutes before he was calling the shop bellyaching about it being too lean at idle and speeding up/smoothing out whey they shot some carb cleaner around the throttle shafts or closed the choke flap some. 

His engine builder got involved, a guru of some sort and he demanded that I install secondary shaft bushings as it was his opinion that it was sucking too much air around them making it lean.

So I get the carb back and opened up the idle tubes .002" and DCR's slightly, and replaced the huge "K" metering rods with our smaller full tapered rods.  I did NOT bush the secondary throttle shaft as they had demanded.

The carb ran flawlessly on the test engine after those mods and guess what, it worked perfect on the customers engine as well.

What folks need to realize here is that these carburetors were emission calibrated and they had better fuel back then.  This new fuel takes about 5-7 percent more of it and if you leave one of these carbs completely stock more times than not the engine is NOT going to be happy about it......FWIW.......Cliff

[Jim H]

Kenth: Fuel economy is not the biggest concern but it needs to be better than it is now.

Cliff: So are you saying replace the rods with full taper 45 rods (your Custom 50M Style rods?), keep the 76 jets and leave the main air bleeds as they are?

The engine was originally an L-48 but now has 9.2:1 CR, ZZ4 intake, AFR 195 heads, Hedman long tube headers, 700R4 tranny with 2400 stall converter.

I also have another carb, 17058504 and all options are on the table with that one.

Thanks guys

[Cliff Ruggles]

Did you change the camshaft during the engine modifications?  If so what are the specs?

What has been done to the distributor?

With the OD transmission and much better heads, plus raised compression you should be getting a LOT better fuel economy, so something or combination of things are way off the mark.

Are you using vacuum advance?

Are you locking up the TCC once the car is up to speed, cruising on the highway, etc?

What exactly have you done to the carburetor?  Did you use one of my rebuild kits in it?

More information/details are better than less, but for sure something is WAY off the mark someplace........

[Jim H]

Oh yeah, the cam. Probably the most important part, it's a Lunati 10120702LK: 219 intake, 227 exhaust at .050". Here's a link https://www.summitracing.com/parts/lun-10120702lk. Sorry 'bout that, not too swift on my part to leave that out.

Distributor is MSD 8362, I am using vacuum advance but for now have the lowest setting on the limiter plate at 5-8 degrees. This is because of a high idle that I am also dealing with (I have a previous post on that) which has improved since I tweaked the linkage but I still think some air is getting past the secondaries. This is actually the main reason I am thinking of bringing the other carb into service. I'm using the lightest springs from he MSD kit, all mechanical advance is in before 3000 rpm. 

The torque converter lockout is confirmed to be working.

The carb was rebuilt and modified to your Recipe #2, although we did not restrict the main air bleeds as I mentioned before. And I am not sure why we went with the 76 jets as they are larger than recommended in the Recipe. The parts came from you.

Compression is good, all cylinders at 160 +/- 5 psi.

To be fair most of the driving done so far has been dawdling around town although there have been some highway miles that didn't seem to make much difference. I have driven the car about 1000 miles since getting it back on the road and overall it is running nicely, smooth with good power. 0-60 is mid to low 6 seconds and it passed California smog easily. Air fuel ratios bounce around a bit on my meter but are in the low 14s at idle, low 15s cruise and 12-13 WOT.

Thanks Cliff

[Jim H]

 One more item...initial timing is at 9 degrees

[77cruiser]

I think it could use more timing. What is the total WO vac adv?

[Jim H]

<< I think it could use more timing. What is the total WO vac adv?>>

Thanks Jim, yes I think you are right. I do want to add more vacuum advance but I have to solve the idle problem before I can do that. The MSD 8428 kit provides 24 degrees mechanical advance so that plus the 9 initial gives me 33 degrees. I could add one more degree as California smog rules allow +/- 2 degrees from the factory setting of 8. I would prefer to keep the car legal if I can, that's less I have to undo come smog time and I already will have to swap out the electric choke and put the hot air back in to pass visual.

I have a thread from a few days ago on the idle, since then as I mentioned it is better since I tweaked the secondary linkage. It's down to around 900 now, I suppose I could try going to the next position on the limiter plate and see what happens. That would add 8-11 degrees. I think some guys get away with 15 initial and 15 vacuum, not an option for me.

 

[Cliff Ruggles]

If adding manifold vacuum to the advance at idle causes you to loose control of idle speed then move it over to a ported source instead.  The ONLY difference between ported and manifold vacuum to the advance is that ported will NOT add timing at idle and coasting.  It does EXACTLY the same thing otherwise providing FULL manifold vacuum to the advance right off idle, and light load/normal driving scenarios.

Well thought out engine builds do NOT want, need, like, or respond well to butt-tons of timing anyplace, especially at idle speed.

For most engines I get involved with here I shoot for 10-12 initial, 10-11 mechanical (20-21 at the crank) and 10-15 from the vacuum unit, usually using a well located ported source, not manifold vacuum.  This provides around 30-34 total mechanical and mid to high 40's at light throttle cruising. 

Of course with that said those numbers are for efficient engines with optimum compression, tight quench, efficient combustion chambers and well chosen camshafts for the application.  Engine with a lot of quench, for example, or too much cam (overlap) may want quite a bit more timing to be happy.

I do NOT like use or recommend light distributor springs or super-quick timing curves for most of these engines, unless it falls into the category mentioned at the end of the last paragraph.  At that point you are just "crutching" things as inefficient engines like, want, need and respond well to a LOT of timing and fuel.

Anyhow, the timing should start advancing right off idle or 900-1000rpm's and all in by 2800-3200rpm's is early enough.

Some folks are still stuck-in-the-60's and believe all the crap you read on the Forums and install light springs with super quick timing curves which cause nothing but issues with idle tuning as more times than not some of the curve is in at idle speed.  Not necessarily the case here but needs to be mentioned as lots of folks with troubled engine combos will read these threads.

Anyhow, your fuel mileage is WAY off the mark for the combination of parts.  Not sure exactly why since the A/F ratios aren't horrible, but for sure some additional tuning is needed here to get it where it needs to be.......Cliff

[Jim H]

Ok, thanks for the insight guys. Cliff, I am monitoring the timing and could very well end up with heavier springs when all the dust is settled.
Jim

[Cliff Ruggles]

I'm going to add here Jim is that for the past 10 years or so I have opened up a couple of Saturdays a month to "custom" tuning troubled set-ups. 

Often I get the entire vehicle here and sometimes from great distances.  The record to date is 1800 miles one way, but typically they come from less than 200 miles or so.

Of course they are only coming here because the owner, all his friends, beer drinking buddies, and local shops and "gurus" have been unsuccessful in curing the issues they are having.

Most think it is carburetor related so they contract with me to show up early Saturday morning so I can sort things out with their engine combo.

95 out of 100 times distributor tuning is part of the equation.  I've come to LOVE those cheap POS spring/weight kits MrGasket and others sell to folks for OEM distributors.  I yank that junk out of them and put the factory parts back in which sometimes is most of or all of the cure.  For sure I take the carb apart and correct any and all issues with it and dial it in exactly for the application.

Between those two things I get 99.9 percent of them running like they should be.

What I see with nearly all of these troubled combos is that some of the advance curve is coming in at idle speed making idle tuning difficult if not near impossible.  Before you can proceed to carb tuning you absolutely MUST correct any issues with the distributors mechanical advance curve.  No timing whatsoever can be "in" at idle speed, but it needs to start advancing right off idle (900-1000rpms). 

There is seldom if ever a need for a "super-quick" mechanical advance curve and it is more times than not detrimental to a street driven engine. 

Vacuum is advance is NOT an option either, even though 2/3rd if not 3/4's of the troubled combos brought here are not using it.

Another BIG problem I've been seeing is defective "modules" that replace the points.  Don't think for a second that installing some bug-zapping points replacement unit is working like it's supposed to.  In recent years I suspect many of those companies are "outsourcing" them offshore someplace because I've had to toss out a good many of them and put points back in these distributors.

....continued below

[Cliff Ruggles]

Anyhow, timing and fuel curves need to be spot on IF you want the best results from your engine.  When the engine is well thought out it's relatively easy.  It will want about 8-12 degrees initial timing, 10-11 mechanical (20-22 a the crank) and another 10-15 degrees vacuum advance.

When you find yourself going outside those numbers, like having to run 20-30 degrees initial timing just to get the engine to think about idling decent below 900-1000rpms you have a fundamental problem someplace else.  This could include the addition of a camshaft with really tight LSA, too much duration and overlap for the CID/static compression ratio, or the carbs idle system simply isn't up to par for the lower than "normal" engine vacuum at idles speed.

With these things folks need to realize that the better we do with engine choices, compression, duration, overlap, intake closing point, quench distance, combustion chamber shape(s), etc, etc, the LESS timing and fuel the engine will want to make best power and be most efficient on the street, on the dyno and at the track if you are racing it.

The days of flooding these engines down with gas-sucking "pig" rich carbs and distributors with super quick timing curves and no vacuum advance should be long gone......but from what I've seen here folks are still doing that sort of thing on a broad scale.

Not complaining at all as it keeps me busier than I ever want to be and my wallet full at the same time!........Cliff

[Jim H]

That is all great advice Cliff, lots to sort out for sure.
Thanks again.