Hello, Im tuning a SA 770 according to Lars on a earlier thread you need to up the primary jet size by 2 then up secondary 8 from that. My problem is that I live at 8000 ft above sea level woodland park colorado.
According to holley I need to lower the jet size by 4 each but according to Lars which I have read and learned from all his papers I need to increase the jet size but does this apply to my area and also what should my timing be. Ive always run all my cars with a high amount of initial time due to the altitude that I live in.

My primary jet size is 72 and my secondary is 75 and plugs look like ive been running rich but right now I only have 860 miles on my 427 horse 383 stroker with a wild cam in it hope ive given enough info for someone to give me a starting point. Like how the motor runs right now but it sputters a bit off idle at take off.

 

You might want to get a manifold vacuum reading on that engine - there is a good possibility that your 6.5 stock power valve is cracking open too soon, causing that rich condition. Also, make sure you check to see how much of the transition slot you are exposing at idle: Set the idle speed, then remove the carb and flip it up-side down and see if you're showing more than .020" transition slot exposure below the throttle plate. If you are, then you are pulling extra fuel out of the transition circuit at idle. Fix this by adjusting the secondary idle speed screw.

 

Thankyou Lars for your response. Yes I have check manifold vacuum and surpising at idle Im running 11 I do have to run at 1100 rpm at idle due to no stall converter and the size cam I have. I just wonder if your rule for SA carbs applys for my altitude and at this time im running 15 initial time due to the altitude and the fact the I need a lighter spring on my advance because it doesnt advance within your recommended 2500-2800 rpm. I will change the springs for advance before changing jets and will also check the throttle opening to be sure of the .20.If not will try to adjust the secondary 1/2 turn so that I can reduce the primary. My question is does you rule for SA carb jet change apply for my altitude or am I running rich due to the altitude that I live.

Thankyou again for your help. Wayne

 

Lars has a lot more time in tuning for altitude than i will ever have but my experiance in tuning a race engine to run at 6000 was i jet size down for every 1800-2000 ft of air density you go up in. we also had to make the pump squirt stronger by putting 2 or 3 3/32" flat washers on the pump arm spring (to make the duration spring stronger) to cure a hesitation coming out of the hole. the initial timing we used was 18 degrees BTDC but the cam we used was 250 plus duration @ 0.050 valve lift.

i would suggest you buy or borrow a wide band oxygen sensor based air/fuel meter such as the Innovate LM-1 it will save you a lot of time and you will know what the air/fuel mixture really is.

henry @ oles carb

 

I agree with Henry on the jet size change for altitude - 1 jet size down for every 2000 ft altitude is a good rule of thumb on a standard Holley and/or BG carb. The only thing you have to be aware of on your S/A carb is that it is jetted really lean to start with - normally about 2 sizes too lean for a performance sea level application. That means that your S/A carb is set up about right for a 4000-foot application out of the box, not counting that the secondary side is extremely lean, so you might consider only dropping main jet size by about 1 size, if any. I also agree with Henry on initial timing - 18 degrees initial with a good cam and high altitude is just about perfect. Then limit total to 36, although you can sometimes run up to 38 at this altitude.

I have an LM-1 if you want to bring the car over sometime and run an A/F check on it.

 

Thankyou for the help, When I set my time to 18 btdc I get 42 total so im going to order a adjustable vac advance so I can set it right on 36 or 38 which ever works best. Thankyou also Lars for the offer to measure the a/f I hope it doesnt come to that maybe I can resolve this with the other suggestions you two have given me. I will work on it this weekend and try some of your suggestions.

Best Regards,Wayne

 

i think you need to recurve the distributor to give you 18 degrees of mechanical advance for a total of 36 degrees (maybe 38 degrees as per lars @ high altitude) mechanical @ 2800-3000 rpm plus the vacuum advance. as for the vacuum advance i would not use a adjustable unit because we have seen them change adjustment on thier own as the vacuum advance moves or customers that do not have to tool to adjust them are under the hood adjusting them when they are sleep walking.

henry @ oles carb

 

Thankyou Henry, I didnt know those adjustable vac can were that flaky. Ill work on getting A can that applys less advance. When I set Initial to 18 without vac advance then hook vac up I jump 24-26 and puts over the 36-38 then I rpm up to 3000 and im at about 54 so my spings are pretty tight its a new HEI distributor. So Ill look at the parts list from Lars Papers and come up with a vac advance that is lower. If im off track please let me know and thankyou for the help.

Best Regards Wayne

 

i limit the vacuum advance to 10-12 degrees that is a little low when compaired to other tuners but it works very good with the gas we have here in california. if you have a HEI distributor we use a D1370a from ac-Delco. you may want to find a shop with a distributor machine and have them check the curve or if you get some really light springs you can rev the motor up and see what total you see, most performance (non msd) HEI distributors have 22 degrees of mechanical advance so 16 initial would give you 38 degrees plus the vacuum advance. i do not like a total with vac advance over 48 degrees but i always tune on the safe side. i would rather lose 5 horsepower than melt a piston.

you are on the right track and as i said buy or borrow a LM-1 and see how your jetting really is, it is a great tuning tool and if you decide to try the spacers on the pump duration spring let me know how the engine responds. our race motors (a 30' offshore race boat with 2-468 chevys) really liked the more active pump squirt and at high altitude (6200 ft) we went from a 0.033 squirter to a 0.031 squirter

 

Thankyou for the great info, I will look for the d1370a and try that and also try the spacers on the pump gives me some fun stuff to try this weekend. It may be worth it to just buy the lm-1 there only 350.00 and could use it on other cars I have like my jeep cj that Im working on also.
I dont mind timing at lower but it has a more negative effect at my altitude as you well know. This give me some good things to think about and try. I also want to try some of the suggestions Lars had on my primary and secondary carb adjustments. Thankyou again for the great info.

Best Regards Wayne

 

good read

 

Here is a update on the tuning this weekend on my timing. I set initial at 18 as suggested and found a vacuum can to get me to 36 advanced when vacuum connected and then changed to the med springs on my advance on distibutor and got 50 at 2500 rpm and 52 total when reved all the way up. also added the 3 washers to tightened the pump spring and like that very much. The engine runs great and have lots of throttle response and power.

I still have a stumbling when taking off and I know its not my timing now for sure. I then got out Lars paper on holley basic tuning and found that I didnt check a very important part of the holley tuning. At idle I get 11 on the gauge but when I put it in drive it drops to just over 5. That means that my power valve is adding fuel when in gear at Idle so Im going to change my power valve to a 3.5 and should take care of the problem. Thankyou again Henry and Lars for the help If you have any more suggestions for me please let me know.

Best Regards, Wayne

 

Make sure that I am reading this correctly? Are you saying that you have 36deg advance with the vacuum can connected? That is what you should have without it connected. The 50deg you talk about after the spring change is ok but what is your total timing without the can connected? It sounds to me that something is still a little off. Have you tried setting you timing without any springs (springs removed from car) and the vacuum can not hooked up and the hose plugged to prevent vacuum leaks? Now set the timing to 36deg and lock down the distributor. Then replace the springs and see what your initial timing is and verify that all 36deg timing is in around 2500rpm still without the vacuum can hooked up. After that is done connect the vacuum can and see what your total timing is now with the can connected. Try that then post your results. I think this will help both you and us to better quantify what steps need to be taken if any more are needed.

W/o vacuum hooked up
Idle rpm ___________
Idle timing ___________
1500rpm timing ___________
2000rpm timing ___________
2500rpm timing ___________
3000rpm timing ___________

Now with vacuum hooked up
Idle rpm ___________
Idle timing ___________
1500rpm timing ___________
2000rpm timing ___________
2500rpm timing ___________
3000rpm timing ___________

And yes it sounds like a new lower P/V may be in order.
Lars and Hennery let me know if I am off base with my thoughts.
Dave

 

Thankyou Dave for your response, Due to the higher altitude I have my initial set to 18 then I found a can that gave me the other 18 which set me to 36. Then I 1st tried the light springs and the timing shot way passed 36 at idle and passed my balancer chart of 60 so I went with the med springs which gave me 50 at 2500-2800 and 52 above 3000 so those springs are limiting my full advance which is good so I dont go over 52 from what I can tell and the motor is responding very well to that. So based on that if I was to take springs out based on what lars said a mechnical advance on a distibutor is around 22-24 then I would be 40-42 max. I can check all those numbers on that chart but Im sure that I would find close to the numbers I just stated. My other can that I had added like 24 which was to high also. Hope Im not missing something I was fealing like I found what my problem.

Best Regards,Wayne