weak part throttle acceleration

[bob69]

Looking for some advice to solve a weak part throttle acceleration. Light throttle acceleration is strong and heavy acceleration is strong. But part throttle acceleration (around half throttle opening) is weak. Looking at jets, power piston spring, or vacuum advance as possible causes.

Engine is a 1969 396 with a mild performance cam. 700r4 with 3.31 rear gear.

Qjet is 7043215, jet is 74, rod is 38, stock power piston spring. Cruz afrs are in low 14s to low 15s. Idles very smooth at 600 rpm in gear. Only change to carb are idle screws at .093, idle bypass at .106. All other measurements are stock. Vacuum at idle is 16 hg and stable.

Distributor is a 1111955 (date code 8m17). Stock mechanical advance is 22°, initial is 12°. Total comes in around 3000 rpm. Has a B1 vacuum advance can.

At light acceleration,  like pulling from stop in traffic,  the car pulls very strong. Assume due to ported vacuum opening and additional timing coming in.

The light throttle acceleration is noticeably stronger than what seat of the pants feels like for half throttle acceleration. Moderate acceleration power is low when ever the throttle is opened quickly. Accelerating from idle or from Cruz.

So, are the jets a little small. Is the power piston closing to soon. Is the drop in vacuum on acceleration causing the advance to drop too much. Any suggestions?

[Cliff Ruggles]

The B1 advance can is WAY too late and slow for optimum performance in the "normal" driving range for what you are doing.

I've tested them and they start really late and too much vacuum to get them all-in. 

I'd start there, and consider larger jets, different primary metering rods, and one of my external APT screws for those carburetors.....

[bob69]

Thanks for the info. Any suggestions on which vacume can to pick up.

[Shiny]

I hope Cliff can guide you on a preferred "curve" for your advance.

Lars Grimsrud published a useful list of advance unit specs for points distributors ("Distributor Vacuum Advance Control Units Specs and Facts for GM Point-Style Distributors").  You may find it on the net, but best to email him and request a copy.  I just went through this and he responded quickly.  Send me a PM and I'll give you his email address.

His table compares starting vacuum levels, vac for "max" advance, and the amount of advance at max.  It includes the "B" labels.

For reference, B1 in Lars' summary is spec'd to start at 8-11" with max of 8 distributor degrees (16 at crank) @ 16-18"

I just bought a Standard Products VC24 that is labeled B1 and it measured 7" start, 9.5 max @ 21".  On the advice of many, I limited the max to 6 (12 degrees at crank).  I attached what I measured.

There are several "stock" options that start at lower vacuum levels, like B28 that starts at 3-5" but not sure if it is easy to buy.  NAPA seems to carry a good selection...  Crane also sells an adjustable unit but I don't know the range of adjustment.

Whatever you buy, measure it and good luck!

[Cliff Ruggles]

Many of the VA cans listed are NLA, so be aware of that during your research.

I can supply the correct one which is one that I purchase and modify to supply less timing.  I have to do that these days because very few of the short-slot VA cans are still available that have lighter springs in them. 

You can also search and locate the correct VA for start/stop and purchase one of Lars "limiters", size of one half dozen of the other, same end result.

In any case the B1 cans I've tested that are currently being sold do NOT make the grade and WAY too late to be "all-in". 

I'd also add here that if things are find at very light throttle but not so great heavy throttle but not enough to get into the secondaries it simply may want more main jet.  With few exceptions the 1970-up big block carbs came with 77-78 main jets and 49B rods. 

I'm not recommending that jet/rod relationship but larger main jets would be in order here in addition to a different VA.......

[bob69]

Thanks for the info. I have the Lars write up. Downloaded some time ago, along with his other articles. Good info, I've been using for all my cars. Also remember reading an article recommending the B1 vac can which is why I used it. But after getting  everything else sorted out, it just was not as good as some I have. Really like the delco 355 -15 can, but its unobtainium.

I think I'll try changing the vac can first. Some of the high performance one are still available. I found a standard Motors VC181 that's marked B26. Very similar to to B28 (which to looks to be discontinued). Also found an echin VC1765 marked as B20 which has the same specs as B26. Just a different price.

If I have to go with jet changes, I'll make sure to keep the same jet/rod relationship. The cruze performance is right on. I'll provide an update when the changes are made.

[Cliff Ruggles]

The B26 is one that I use but they add too much timing for most well thought out engine builds so need limited.

When I set those carb up here I employ the APT system by installing one of my external APT screws.  Then I can get the jets size right, and fine tune part throttle A/F w/o having to pull the carb apart. 

Since you didn't buy your rebuild parts from me plan on putting an acc pump in it for sure.  Greg bought a lot of parts from me, including my accl pump springs, but kept using the "soft" blue seals on his pumps that swell up in this new fuel.  He didn't use the high flow non-windowed N/S assemblies either, which are usually required for big block applications.  You usually don't find that out till you get a bloody nose when the car noses over at the top of 1st gear on a hard run and you bounce it off the steering wheel!.......

[bob69]

Update. Finally installed a B26 adv can. That did the trick. Part throttle acceleration is strong now. However,  B26 gives too much advance as Cliff noted. The new can was giving 18° total adv when fully employed. The B1 was around 16°. I could hear a slight ping at times. Not constant and barely noticeable, but there. The exhaust is kinda loud.

Looking through the internet for ideas and decided to make a stop from brass shim stock. Shaped like a T. Bolted on with the front screw and going back to the advance slot. Advance now limited to about 13°.

Not only did this stop the ping but overall performance was noticeably improved everywhere. Obviously the over advance was hurting power.

Not sure how well the advance stop will last as it is thin shim stock. I saw a few posts talking about stops that were available in the past. Does anyone know if/where the are available.

Some measurements for the two adv cans. B1 adv started at 10 hg and fully in at 20 hg. The B26 started at 8 hg and fully in at 14 hg. B26 is the only one I found that was very close to the HP can GM used back in the day.

Fyi,  I bought two B26 cans. One checked out at 8 hg/14 hg. The other checked out a 7 hg/15 hg.

[bob69]

Found a Crane 99619-1 vac adv stop in my dist spare parts box. This stop attaches to the rear vac can screw to limit the vac cans overall travel. The stop provides adj in 2° increments. More tunable than a fixed stop attached to the front vac can screw. Set the max adv to 12°. 

Forgot I had it. Did not use it at first because it will increase the amount of vacuum needed to start the advance. Using it changed the vacuums starting point to 9° from the 8° without it. Seems to be working good. Too bad they are not available any more.