Altitude/Jets

[three two barrels]

What altitude do you currently live at, and what size jets are in your carb? The higher the elevation the less dense the air is so shouldn't everyone's carbs be jetted differently?

I am currently at 2805 feet above sea level and am running #70 jets in my 750 quickfuel on my stroker small block. With alot of time off this week I want to pull the center carb off my V code and possibly lean it out some as I know it is 'too fat'.

Interested to hear what you guys say.

[Cuda Cody]

With compression and vacuum being so different on each engine based on the cam specs I've found that each engine has to be jetted based on that particular engine.  IMO, the cam makes the biggest difference in determining what jet is needed.

[ek3]

62  jets on center carb is big... it will run a 500 stroker....

[Chryco Psycho]

I don't guess , I would rather tune with results , so yes maybe it will run better at 61 or 60 but maybe not , I used the dyno to tune for peak gain , provable & repeatable but you can get a long way even with seat of the pants , the car will become more or less responsive / crisp with minor changes , just change 1 thing at a time or you are just wasting time ...
I remember years ago I offered to help you tune the J code car , I got a very unfriendly response ...
It seems I am still willing to help tho .

[Wedg2Go]

Having a new permanent location (home?) at a higher altitude, where the air is thinner, is going to necessitate the need to adjust the carburator(s). That is, if the previous tuning was done at a lower altitude. I know, since I am a walking/talking testimony to that situation. I purchased my "Dream Ride" in Sacramento (30' ASL) and trailered it to our home in the Colorado Rockies (7300' ASL). Running "Fat" was an understatement in my case. I went through each carburator, adjusting each circuit until I had the results I needed, within the area where I will drive it the most (around our hometown). Of course living in the mountains creates a little more of a challenge with a lot of variables with different altitudes (5200 to 11,000).

I did some research, into this subject, resulting with several articles stating to drop a jet size for every 2000' of elevation change. Of course, monitor your tuning – since there will be a "Few" trials – with an A/F reading or go "Old-School" and read the results on your plugs.

BTW – The following is just my opinion, but the Holley 750 Quick Fuel tends to be more track than street. Meaning there are going to a "Few" factors to watch for (Vacuum, jetting, power valve, and air bleeds to name a few). Please don't get me wrong. I've seen many Holley Quick Fuel carburators on a few street machines at many shows...,but if you are running your ride around at 2800' and it ran fine before, the difference is going to be nothing more than a shade darker blip on the end of that spark plug. 2800' is nothing. Verify everything within before you re-jet. Like timing (advance and initial), power valve (blown or stated specs per vacuum at an idle – see Holley's WEBpage about power valves), etc. You might find something changed. Never hurts to look into all that before re-jetting!



   

[three two barrels]

I don't guess , I would rather tune with results , so yes maybe it will run better at 61 or 60 but maybe not , I used the dyno to tune for peak gain , provable & repeatable but you can get a long way even with seat of the pants , the car will become more or less responsive / crisp with minor changes , just change 1 thing at a time or you are just wasting time ...
I remember years ago I offered to help you tune the J code car , I got a very unfriendly response ...
It seems I am still willing to help tho .

You must have me confused with someone else as we have never met.

[Chryco Psycho]

So What cam is the engine running ?
Different cams signal the carbs differently , the stock cam was a relatively low lift with a 115* LSA
Most replacement cams are higher lift in the .475 range with lower LSA around 112* which drops idle vacuum ?
Dropping the jet size 1-2 sizes should help the response to be more crisp but it can if the cam is not stock cause a need for slightly more fuel through the accelerator pump off idle .
If it is 4 spd it is easier to tune but Auto can cause the ilde vacuum to fall too low in gear which may require a lower rated power valve to stay closed at idle , you need to be 1.5 inches below the idle vacuum in gear but often 2-3 inches below responds better .

[one bad fish]

I'm running a 512 stroker with a 6 pack and a 4 speed. 11.2/1 compression, 259@50 intake and 263@50 exhaust, LSA at 108,  .600 lift solid flat tappet. In Calgary it's around 3500 feet above sea level. I installed O2 sensors. At cruise, running on the center carb I'm 13/1 air fuel ratio - WOT at 12...I want to be a little rich at WOT. Car is a monster, scary actually on the street. Definitely a big stress releaver