I started the car for the first time this year. It idles good at around 1500 rpm when cold, around 1200 when warm but when I put it into drive/reverse, RPMs drop to 550 and if the car is cold, it stalls.

What cause the rpm to drop from 1200 to 550? How can I reduce this difference?

Timing is 15 degrees at idle without vaccum, 35 degrees at 2500. Carb is a new out-of-the-box Holley SA. Trying to get it running for whatever little summer is left

 

The stalling in gear is the choke not functioning properly. At operating temprature can you adjust the idle lower? Is it idling on the regular idle speed screw or the fast idle screw?

BigBlockk

Later.....

 

My 76 idles, engine warm, 1200 neutral, 850 in gear, but I will be going up to 900 in gear....just idles better there in gear for my config...

 

how big of a cam did you put in the motor. Do you have a vacuum advance on the distirbutor.

Your problem is that you can't run enough base timing when you are in gear at idel. I had the same issue. I had to run a very high neutral idel speed to not drop down to low cold or warm. I since added an even bigger cam and a megasquirt efi computer to control the timing. Now I just dial in the in gear vacuum/rpm and have a timing spike in the timing map. boosting the timing at that point allows me to have a 1000 rpm idel in neutral and a 750 rpm idel in gear with a 300+ duration cam and only 3 inHg at 750rpm in gear.

Trying to make a big cam driveable can be done with electronics but a mechainical distributor is going to leave you with some limitations. Even a good vacuum advance can only get raise the timing 10 degrees with full vacuum. If there is very little vacuum present then your getting maybe 1 to 2 degrees out of it. So your sitting at the base timing still and then you add load which typically requires either more throttle input or more timing. Since your ideling you don't want more throttle input so you need more timing. With the basically 2-d timing map of a mechanical distributor its next to near impossible to make it do what you need it to do. Electronics are the future. Crane and MSD are starting to produce distributors with electornic timing curves, some may even have 3-d mapping capabilities, this may be your next step to happiness.

Once I locked up the distributor and installed the timing computer I was able to spike the timing at idel to 25 degrees in gear, which lets me idle my big block at 750 rpms in gear. As soon as I hit the loud pedal it moves off that spot in the map to a lower timing number to keep it from pinging, try getting a mechanical distributor to do that with less the 5 in-hg.

 

Yes. With both idle mixture and idle speed.

Not sure. How do I check? But idle speed screw DOES make a difference, so it must on that. Fast idle should be set 1500-1600 rpm according to Holley and that's exactly where mine is.

That would be perfect for me too. Just trying to get there...


Cam is CC magnum 270H. Vaccum advance is connected.

How high of neutral idle speed did you have? I don't want to go over 1200 rpm. Car screams pretty loud at that (I have chambered pipes ). In neutral, I have 15 inHg vaccum, not sure what it drops to in gear, but I will check soon.

 

I just realized that nothing is connected to the negative terminal on the choke. Is that for ground?

Just reading the instructions at Holley, I should check the vaccum in gear to properly size the PV...

 

I believe it was around 1300-1500, it was hard to keep it under control when you have to start opening the throttle plates that wide.

With that cam you shouldn't have a problem getting it to idel around 1000 nuet/750 in gear. Just be sure you have the vacuum adv hooked up to a non-ported source (ie manifold vacuum, and not that port half way up the metering block on the passenger side of the carb). My buddy had a CC Xtreme 274h in a 383 and it idled at about 1100 neut/800 in gear.

Defintely connect the ground wire. Does your choke even open currently?

As for the power valve. If you end up with say a 6.5 and it starts to bog off the line go bigger nuemerically not smaller. it seems backwards given the holley directions but it works. I had to put in a 10.5 to keep from getting a bog while cruising down the highway in overdrive. plopped in the 10.5 and it transistions nice at light throttle.

 

 

Desi -
Set your idle mixture screws with the car in "drive." If you set the screws in "park" with no load, the carb leans out dramatically when you drop it in "D" and get the load on it. Also, check and see what your timing does between "P" and "D": If the timing is retarding when you drop it in gear, you need to put some slightly stiffer springs in the dizzy to keep the timing the same at the two different rpm.

 

Lars, I blame all my carb problems on you. You keep postponing your trip to Toronto

Thanks for the tip to tune it in 'D' instead of park.

I will be taking a full curve of my timing at various speeds and post them soon.

Last night I took the car out for a long drive (120 miles of mixed city/highway). It idled choppy, but I increased the idle speed to 900 under load (that meant 1600 in neutral!!!). In downtown the car shut off while at stoplight twice. That caught me completely off gaurd. But started right away, so it wasn't too embarassing

I checked the negative terminal and it IS grounded.

Also, I was thinking that all these issues could be because the gas is about 11 months old (from last October ) so I decided to run the tank dry and fill it with 94 octane Sunoco gas. This morning the car had same symptoms so I don't think the gas has anything to do with it.

I will try tuning in drive and post results later.

 

Amir -
Have you richened that SA carb up a bit? It was during the first Toronto TFB Tour that we ran into the SA lean-run problem: One of the guys had a 770 SA carb, and we richened up the primary side 2 to 3 jet sizes and did an 8-size split to the secondary side - it really fixed the drivability issues - not sure if you remember who that was. Since then, I've been duplicating that "tweak" on other SA carbs with identical results. Are you running stock jetting? If so, bump that jetting up a bit and then set the idle mixture in "D" for the final tuning.

But do check that timing and see if it's changing between neutral and "D" - a change in timing will have a huge effect on that rpm drop.

Lars

 

Lars, the carb is out-of-the-box literally. As you know, I suck at carb tuning My plan is to get it to a dyno for a proper A/F ratio tuning. At this point I am just trying to do the basics (idle mixture, idle speed, pump shot, PV, choke, etc). The jetting, I will leave for the shop.

During your second visit, I learned this from you (yes, I was paying attention ) :

Timing at idle - 15-18 degrees (Vaccum disconnected)
Total timing of 36 degrees, all in by 2500 -3000 rpm
Total with vaccum connected not to exceed 52 degrees

This is CARVED in my mind now. One of these days, I WILL learn to tune a carb too

 

Amir -
If you're running vacuum advance hooked up to a manifold vacuum source, check and make sure that the vacuum advance unit is "soft" enough that it stays fully retracted and "pegged out" up against its stop when you drop the car in "drive." If you loose enough vacuum going into "D", the vacuum advance will drop out timing, and rpm will change dramatically. You can test for this by pulling and plugging the vacuum hose and setting up rpm and mixture without the vacuum. Drop it in drive and see if the rpm change is different. If it's better, you either need a "softer" vacuum advance control unit or you can switch over to ported vacuum to eliminate the vacuum signal at idle.

 

Is this safe??? My emergency brake ain't the best!!!!

 

use a friend

 

Apparently these are automatic cars, is anyone using a stall speed converter? 1200 RPM`s when warm is way too much. It must also be hard on U-joins too. With a proper converter the idle from neutral to any gear should not drop over 50-100 rpms with a solid lifter cam and idle at 8-850. Similarly with a hydraulic but at 700.

 

I guess someone you can trust. Or a friend who's wife you didn't sleep with??

 

Lars, here's what my timing curve was set at before we (myself and 75Stingray!) started playing:

16 deg at 1000 rpm
20 deg at 1250 rpm
23 deg at 1550 rpm
27 deg at 1950 rpm
30 deg at 2200 rpm
36 deg at 2500 rpm
37 deg at 3000 and more rpm

All of the above was done with vaccum advance disconnected. With vaccum advance connected total timing was 53/54 degrees.

Vaccum was at 18 inHg at idle (1000 rpm) in park, but dropped to 9 inHg in Reverse (load). Also the timing dropped to 11 deg at 600 rpm.

Then we played with a whole bunch of spring combo's. Initial timing would range from 16 -22 degrees and max would be in 33 - 45 deg. Regardless of the combo, the drop in rpm was around 800 in load vs no-load. And the timing always dropped by 5 degrees.

In the end, our original springs were put back in since these gave us the best timing curve.

Then we played with the idle mixture screw. We set it in 'D' to get max vaccum and RPM. The car kept stalling however. So we had to set the idle SPEED higher to prevent stalling.

At the moment, the idle is set at 800 rpm in 'D' and yields 1550 rpm in 'P'. Anything lower than 800 in 'D', the car just stalls.

How the hell do I reduce that drop in rpm? We tried all sorts of timing curves (both with advance and without) but nothing changes that drop of 800 or so RPM and 5 degrees in timing...

 

Anyone

 

Amir -
Try this:
Disconnect and plug the vacuum advance hose. Set the carb up for best idle mixture and idle rpm without the vac advance. Then see how much rpm drop you get going from P to D. That 5 degree change in timing will make a bit of difference in the rpm, so we need to find out whether the timing change is from the vacuum advance unit dropping out timing in D or if it's the centrifugal curve dropping out the timing.