Rejetting a carb for elevation changes

[Dan859]

Hi everyone,
I recently bought a 66 Fairlane with a stroked 351W engine, from Nevada, which is 4400 feet in elevation.  The engine has a Summit Racing 750 cfm double pumper carb on it.  My question is about rejetting it for use at about 300 feet elevation.  What I read says to change the jets one number up for every 2000 foot drop.  Does anyone have any real life experience with this, and do I need to change both the primary and secondary jets?  Is there anything else I should be aware of?
Thanks,
Dan

[thatdarncat]

My question would be - was your engine dyno'd when it was built in Nevada, so you know that starting point was close? Was the air-fuel ratio checked & adjusted when it was in Nevada? If not you may just be taking a wild guess here. The recommendation you cited sounds pretty close. I'm not home right now so I can't reference it, but the Ford parts book shows recommended jet calibrations on stock applications at 5,000 foot intervals, and they are at least a couple steps each. I've bracket raced with my friends at Bandimere Speedway in Denver a few times ( 6,000 ft elevation ) and I tried going 4 steps leaner on both the primary and secondary sides a couple times and I think once on just the secondary side. That was from running usually at Brainerd Int'l Raceway in Minnesota at 1,200 feet.  That seemed to work fine and a check of the plugs looked good. Some friends changed as much as 8 steps leaner and others didn't re-jet at all and to be honest we all ran about the same amount slower at Denver. I think most carbs come stock with a base jet setting good for sea level through a couple thousand feet elevation, you could see too if someone knows the original jetting of your carb.

[Dan859]

Thanks!  The seller has had the car for several years, and it was run on a chassis dyno after the engine was installed.  He said it runs very well and he hasn't had any problems with it.  I don't want to just assume everything is OK and run the risk of it running lean when the elevation gets that much lower.  If I put it on a diagnostic machine after I get it home, would that be able to tell me the air-fuel ratio?

[thatdarncat]

I've always just done my jet and timing changes at the dragstrip periodically on test day to find the "happy" spot. When we were at Bandimere we never really had a chance to test so it was always just a best guess kind of thing, but based on knowing the car was close before going. I've never run a chassis dyno test, but my understanding is you can check air-fuel ratio and make changes. Hopefully someone who has done it will chime in. I would ask the shop ahead of time too. Do they need O2 sensors in the exhaust? Or do they use a tail pipe probe? Although most people hate it, if you have smog testing in your area that can even be a tuning aid.

[Falcon67]

I would get the list number off the carb and compare to the installed jetting - as a start, put it back to stock as noted by Holley.  We go from 1719' MSL, usually 3500~4000 DA to Dallas where it's 700' and 2000ish DA, I never re-jet.  Our effective altitude can change 2000' here in a day, we'd go nuts chasing some "optimal" jetting.   No time for that during a bracket race.  I jet a new carb in the spring if needed and after that, never touch it unless it needs service.

[My427stang]

I agree to either just try it as is, or put it to stock if it's been "super tuned" already.  I'd bet it's stock as is, which tends to be a bit rich anyway

The main jets are critical at WOT, especially after the PV closes, but before that, the non racers likely won't even feel a 2-4 number jet change. especially on the secondaries, and moreover, the other two circuits (fourif you count the opening and closing PV and accel pump shot) are going to contribute to plug color so much that it's impossible to read plugs on a street car anyway.

If it is a race car, shutting down immediately and towing it back to the pits, or jetting for trap speed is good, but you may as well start with what you have and see

[Dan859]

Thanks everybody, I appreciate the help!  There's shops here that can do the smog testing.  I'll take it there and have the A/F ratio checked.   

[Falcon67]

If this is your carb

http://www.summitracing.com/parts/sum-m08750ms/overview/

These are the old Holley 8401x series designs, that incorporate boosters like the old Ford 4100s and use Holley parts (jets, pumps, choke, etc).  Click on Repair Parts and you'll see they are all Holley.  Not a bad all around carb and should respond well to tuning.  With those big annular boosters, don't expect jetting to be similar to a regular Holley 750 double pumper.  The only downside to these units that I've found is that there is no accel pump shooter adjustment, except to drill out the shooter passages.  If you mess that up, you'll have to source a replacement booster assembly or plug the holes with brass and re-drill.

I ran a 750 vacuum List 84011 (now Summit MO8750VS) on a 351C-2V, single plane, 11" converter, 3.73 gear setup and it ran really well.  Actually still have the carb.

[Dan859]

I think that's the carb, but I don't have the car yet to actually check it.  If the A/F ratio checks and the car runs well, I'm not going to touch the carb.  I don't plan on racing the car, so I don't see any need to have on the cutting edge.

[My427stang]

I wouldn't even mess with the A/F checker.

The carb will get fuel from all circuits at low RPM, I doubt if they will allow a 6000 rpm blast on the emissions machine, so no way to see which circuit is lean or rich.  If it runs well, run it and watch for peppering on the plugs.  However I seriously doubt it'll happen

[Dan859]

Sounds like a plan  .  After I get the car, I'll post some pictures.