Altitude Adjustment- Jet size for lower altitude help please?

[396C-10]

Having moved from Wyoming to Mississippi, my truck runs like crap. I removed the jets and I need some idea of replacement jets and other adjustments I will need to make to make it run like a sewing machine.
Here is what I have going on.
I moved from 8000 ft altitude to 200 ft altitude. I have a 1967 Chevy C-10 with a 70 Chevelle 396 Big Block, bone stock build, no racing cam or anything special. Running a Holley 0-3310S 750CFM 4bbl Carb. I removed the primary jets which were 72's.

Does anyone know what I should replace them with for this lower altitude? Should I also consider replacing the spray nozzles?
Would I need to do the same with the secondaries?

Any advice would be greatly appreciated, and thank you in advance. I must admit, I am not a carburetor guy. I was told buy an old timer once, if you don't know a Holley, leave it the "F" alone... I did not listen.

[GASoline71]

The 3310S is a 4160 style carb (rear metering plate), with vacuum secondaries so there is no pump squirter (spray nozzle) for them. Only a pump squirter for the primaries. Stock is usually a #31 if I remember right.

72 jets came in the carb primaries from the factory. It has a rear #21 metering plate that is non-adjustable (no jets) and equal to about a 75 jet.

Also what is your timing set at? You should always nail down your timing before a whole bunch of carb tuning.

Gary

[396C-10]

I just spent an hour and a half reading your stories on here and WOW! Boy did I get side tracked and derailed..... That is some good, bad and ugly reading material you got going on there. Ha ha ha! So, I am going to try some #68 jets and see how that works, then I am going to check the timing again, though it idles fine and sounds good until I give it throttle. A mechanic at work told me I needed #68 jets and handed me 2 new ones to throw in it... We will see.

[rcazwillis]

You went from high altitude with thin air to low altitude with thick air. You now have more oxygen in the air. Therefore, you need more fuel. Going to a #68 would be the wrong direction. I would start with #74, to match the more air you now have.

Need more details regarding what running like crap means. At idle, at 1/4 throttle, throughout throttle, wide open only?

Also check Holley's web page. They have a method to calculate jet changes based on altitude change.

[James the III]

Originally Posted by rcazwillis

You went from high altitude with thin air to low altitude with thick air. You now have more oxygen in the air. Therefore, you need more fuel. Going to a #68 would be the wrong direction. I would start with #74, to match the more air you now have.

Need more details regarding what running like crap means. At idle, at 1/4 throttle, throughout throttle, wide open only?

Also check Holley's web page. They have a method to calculate jet changes based on altitude change.

THIS
you are lean not rich..
unless you had a lean backfire in the carb and it took out the power valve and it's now rich..

[7tee]

I believe they came from the factory with #65 jets and set up for sea level. Mine was running too lean so I went to 66,67 and settled for #68 and all is good being I'm in the mountains. I would go back to the #65 and start from there... Ed...

[GASoline71]

Originally Posted by James the III

THIS
you are lean not rich..
unless you had a lean backfire in the carb and it took out the power valve and it's now rich..

Only Holley carbs made before 1993 would not have the built in power valve blow-out protection and would need to have a kit installed in the carb. Any Holley after 1993 has no need for the kit since the power valve protection is built into the carb. That being said... a wicked carb backfire can still blow out the PV , so you could be on to something.

Originally Posted by 7tee

I believe they came from the factory with #65 jets and set up for sea level. Mine was running too lean so I went to 66,67 and settled for #68 and all is good being I'm in the mountains. I would go back to the #65 and start from there... Ed...

The 600cfm 1850's came with smaller primary jets, ranging from 62's to 66's depending on the serial number of the carb. Pretty much ALL 3310's came from the factory with 72's in the primaries.


To the OP of the thread. What is your timing set at? What Power valve is in the carb now? Stock PV would be a 6.5

Gary

[396C-10]

Originally Posted by GASoline71

Only Holley carbs made before 1993 would not have the built in power valve blow-out protection and would need to have a kit installed in the carb. Any Holley after 1993 has no need for the kit since the power valve protection is built into the carb. That being said... a wicked carb backfire can still blow out the PV , so you could be on to something.



The 600cfm 1850's came with smaller primary jets, ranging from 62's to 66's depending on the serial number of the carb. Pretty much ALL 3310's came from the factory with 72's in the primaries.


To the OP of the thread. What is your timing set at? What Power valve is in the carb now? Stock PV would be a 6.5



Gary

Gary, you are right on all accounts. The carb is out of the box, brand new with 72's for jets. The timing was set by a friend in Wyoming who knows a bit, but I can not tell you what you want to know because I pulled my carb apart and it is dead in the water until I put it back together. So far Gary, you are right on the nut....

[396C-10]

Man.... She is ALMOST THERE!!!!

[396C-10]

All of this does nothing if it don't run....

[rcazwillis]

If that carb is brand new, did you use the carb in Wyoming? Holley jets the carbs for sea level. Your jetting should be right on target with the factory setup. I would be looking at other tune up items.

[396C-10]

Originally Posted by rcazwillis

If that carb is brand new, did you use the carb in Wyoming? Holley jets the carbs for sea level. Your jetting should be right on target with the factory setup. I would be looking at other tune up items.

I installed the engine and carb in Wyoming, transported the truck to Mississippi, finished it (drive shaft, brakes and exhaust) and now trying to dial it in to drive. It runs really rough in gear. Idles fine. You may very well be right.