If I remember right, I had 80s in my 850 on the 427. 70 sounds small I would bump them up to like 72-74 in the front and start there.

 

well im pretty sure the 3310 came with 72 frt 76 rear , and i think the average carb should only be jetted up or down 6 sizes .
with that in mind ,i had a 3310 on a 440hp 406 , and i used 74&80s .. optimally.
So i figure to be safe start with 78 and 84 . this will most likely be on the rich side.
oh and thats with blocked rear powervalve

 

Rich,

IIRC that carb you have is a new factory replacement. It's like a 3310 or similar 780CFM?

I wouldn't change the jets at all. First because they're probably right and don't warrant change unless it can be shown that it's running lean or rich on a wideband O2.

Second because it's unlikely your engine will be running on the mains at all during break-in. It will be running on the transfer circuit which is plenty rich. if you get any main system activity at all it will just be a little dripping.

What you really need to do is ensure enough ignition timing during break-in. have a timing light handy and make sure it has at least 40° at 2500 or whatever RPM you have it at. More is better. That usually isn't enough RPM to to fully extend the mechanical advance.

Have a big external fan on the radiator, have a garden hose handy to give a light spray on the radiator core if the temp goes over 220° or so. Burp the air out of the block and heads unless you've drilled a hole in the t-stat. Leave the rad cap off so you see the water when it starts to circulate, sometimes the level drops even if you've burped it. After you see hot coolant circulating nail the cap down.

If it starts getting hot on the temp gauge or the headers get so hot you can't stand near it that's a sign it needs more timing.

When are you going to do this? I'll PM you my cell phone in case issues arise as they often do.

 

I've already got a large box fan in the shop. I have my tuning tach and timing light at the ready. I also have one of those infrared thermometer things to keep an eye on overall temps and header tube temps. I'm picking up a new fire extinguisher tonight or tomorrow. I plan to keep a close eye on fuel line leaks as it is ALL new with stainless steel lines they tend to need more torque than regular steel lines.

Good tips, I'll be sure to keep an eye on the coolant flow. It has a high volume water pump and an Alumitech radiator so I'm thinking I'll be fine on temperature control. All I have is a factory gauge though, maybe I should at least temporarily install a "real" numbered gauge too. I've had cooling issues in the past on the L79 so I've become very familiar with how the factory gauge reads and the sending unit is an authentic original '65 unit so should be very compatible with the gauge. I plan to disconnect the power steering belt for the initial firing, one less thing to worry about. I also plan to run the heater on full during the break in to pull out even more heat.

Oh, and the digital camera is all charged up and I dug out the tripod so I can share my success with my TC friends!

 

I agree stay with the factor jets , it pbly has a fat PV in it ....timing is critical and assume you have vacuum advance .... my highly mod 427 big cam uses 76/86 ( no sec pv ) and its slightly rich for start up you will be fine you dont want too rich either so 34* total timing and 70's will be fine to get the carb / idle mix set .you may know this but an easy way to set timing for first time start ( as long as you have a electronic triggered ignition msd type box etc ) is with key on crank engine with starter with timing lite connected and set centrifical where you need it ie 10* or whatever you need to get 34toal ....makes for an easy start up .starter rpm is fine for this

 

Rich, check out this previous thread. I asked the same question on my nearly exact 427 but i have the next step down 272/282 solid FT cam.

http://www.chevelles.com/forums/showthread.php?t=236523

I went with 72's in the front, and 82's in the rear.

-Daniel

 

If it helps,I have 80's in my 830 Holley on my 427,they're running a nice sandy tan.

 

If it is a factory Holley with the original jets run it as is. You should not need to change the jets more than 2 sizes normally, any more than 4 and you have other issues. When you are plugging a PV, you will need to go up 6-8 as an average, otherwise fire it as is.

 

You should not need to change the jets more than 2 sizes normally, any more than 4 and you have other issues. When you are plugging a PV, you will need to go up 6-8 as an average, otherwise fire it as is.

That is not a factual statement if you are gearing for acceleration at the strip. Also,if you plug a powervalve,you go up to 10 sizes. My greatest and simplest ET gain other than swapping manifolds for headers was taking my stock 3310 w/72 jets/rear metering plate and simply upgrading to 74 primary/84 secondary with no secondary powervalve.

Also,read this thread:http://www.chevelles.com/forums/showthread.php?t=180558&highlight=454+dyno+test
the 454 that was dyno tested picked up 30 HP with the appropriate jetting vs. stock and the combo just recently proved itself at the track :thumbsup:

FYI Rich,you can't go wrong with what Keith mentioned for what you are doing.

 

The 780 Holleys on our 396 and 427 engines have 6.5 power valves front and rear. I run #73 jets in the front and #76 in the rear. I also run headers. You should have no problems with this setup.

 

I have a spare set of 74s, I might pick up some 72s so I have some room to play. I should check to see what I have in the back before I gas up the carb. It's a vacuum secondaries carb so no PV in back as far as I know. This would be a good opportunity to also check the PV size up front.

I dropped nearly 5 gallons of coolant in it tonight, never had a clue it would hold that much! I gapped the plugs and torqued them in. It's been sitting a while so I gave each cylinder a small shot of motor oil before I put the plugs back in. It'll probably burn a bit of oil off the tops of the cylinders at first, but better to have the rings plenty lubed I'd say. Of course I broke one of the plugs so I have to pick up a replacement tomorrow. I spent more time looking for a heater hose clamp I dropped than just about anything.

 

I have built quite a few 780 Holley's for BB's. The jetting that has worked out the best is 74 primary and 82 secondary with no secondary PV.

Dave

 

Intersting reading!

http://www.nova-ss.com/information.htm

Also scroll down to "carb jets -L78"


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:thumbsup: this is exactly what was done to the carb that was used in the dyno I posted the link to...the stagger jetting was worth 30 HP over the stock jetting

 

But the stagger jetting is moot since I have a dual plane manifold with a cut in the plenum, right?

I'll need to buy myself a wideband at some point.

Also, when talking about staggered jets and saying "left front" or "right front" is that from a perspective of looking at the carb from the front of the car (left = passenger side, right = driver's side) or from the perspective of sitting in the driver's seat (left = driver's side, right = passenger side)?

 

Rich,in the dyno link I posted,stagger jetting picked up 30 HP with an RPM intake which has a notched plenum.

 

We did run the staggered jetting on my father's car when he ran the GM manifold in the mid 80's. The car went 9.90's with it.

 

cool rides Mark,thanks for sharing :thumbsup:
What did the Chevelle run?
My ride responded very well with the jetting I mentioned...I raced it for 5 years and believe me,I had to make sure every test and tune session counted,as I drive my car 270 miles round trip to the track.

 

The Chevelle ran 11.60 best with a pump gas 10 to 1 406 with a hydraulic cam and 461 heads. I sold the car some time ago, it was destroyed in a staging lane accident a Speed World last year. One thing you have to remember, you can take the same carb on two different engines and have different jetting needs. That is why I suggest starting with the factory jets. If it has a metering plate in the rear a metering block conversion is a nice addition to make secondary changes easier.

 

Was that the 55 that was trying the trans brake in the lanes and plowed into a couple of other cars?

 

Yep, didn't own it but still made me sick.

 

More info about staggered jetting !
http://www.cartechbooks.com/vstore/...store/showdetl.cfm?st=0&st2=0&st3=0&CATID=23&Product_ID=3209&DID=6&chapter=7179

http://www.superchevy.com/technical.../technical/engines_drivetrain/induction_poweradders/0406sc_airgap/photo_12.html
http://www.superchevy.com/technical...com/technical/engines_drivetrain/induction_poweradders/0406sc_airgap/index.html

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Stagger jetting is fine if you have O2 or EGT data and reading the plugs shows one of the corners rich or lean. Different combinations of heads and intakes can have all 4 jets different for optimum performance. This is why you can't jet according to someone else's combo, even when things are similar a different manifold or timing curve is all it takes to make a difference. Tune the carb for your engine not what someone else did. It will thank you and hopefully reward you with improvements.

 

I agree but you are kind of missing the point of the thread. I'm just trying to find a good starting point to get the engine started for the first time. The carb came with front #70 jets which seems kind of on the lean side. The only reference manual I have shows that for the 425hp/396&427 the fronts are supposed to be #74. I certainly don't want to fire this thing for the first time on a too-lean setting. My engine consists of a lot more than the factory 425hp engines did as well.

As the carb sits right now:
Front - #70, 6.5 PV
Rear - #76, 8.5 PV

 

Rich;

Your 3310 is jetted fine to get everything up & running, but if you're really concerned, you could go to 72 or 74 on the primaries, and I would replace the 6.5 primary power valve with an 8.5. That should get you very close to ideal.

Save the stagger jetting for later.

Eric

 

Like I said in an earlier post, and Tom repeated, unless you're driving it around it won't be on the main jets anyway. Getting the timing right and the cooling fan is more important.

 

Rich, Holley changes their metering on carbs even though they have the same part #. Changes in boosters, metering blocks and the emulsion setup may allow it to flow a similar amount of fuel even though the jets are smaller. The 3310 with a rear metering block is listed from the factory as using 72/76, and all the newer ones with a metering plate list 72 primary. You won't hurt anything with 70's, but if it makes you feel better put the 72's in it. You will have more issues with high temps from incomplete combustion due to insufficient timing than a couple jets sizes small. Once you get it started and ready to tune, start pushing the initial timing by limiting the advance in the distributor after you figure what total timing it likes best. Also try to bring the advance in as soon as possible, your engine will tell you when you go too much, it will start pinging under low speed loads. Good luck and keep us up on how it runs.