Disconnecting secondaries

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70DartMike

70DartMike

Too many projects
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Is there any advantage to disconnecting my secondaries? I'm looking for better fuel mileage.

My question is actually for a 1975 Chev 350 4bbl, in my camper van. Carmen and I will be taking the van across Canada/US in May/June and I'd like to get the best fuel economy I can.
I'll be changing the timing chain before we go, anything else I should be doing?
As for the carb, should I leave the secondaries hooked up while I drive through BC, since it's so mountainous here? And then disconnect while we're driving through the prairies?

Input please.
 
Short answer is no. I assume you have a Quadrajet on there. You can always adjust the secondary air valve a little tighter if you think the're opening too much/early.
View attachment 1a12.jpg

Be very careful with this adjustment. Tiny weak spring and 1/8th of a turn makes quite a bit of difference.

If you decide to do a complete adjust from scratch.......P.S This is done with the engine off.

Loosen the set screw with a screwdriver in the adjuster so it wont snap back on you.
Then slowly turn the adj counterclockwise while pushing open the air door.
Once it will stay open on it's own, slowly go clockwise until the tention will just shut the door.
From there, go about 3/4 of a turn and lock the set screw. "Caution" don't go more then 1 full turn from the start.
At this point you can road test and see how it feels, and make small adjustments as needed.
One tip....If the door opens just free reving the engine, the adj is a little too loose for what your trying to accomplish.
 
70DartMike said:
Where would the secondary air valve roughly be located?
Click to expand...

See post #2

One other check, and some will look a little different, but you'll get the message.........
View attachment 1a23.jpg

Make sure, with the engine running, that the vacuum break #1 is operational. If it's pulling in properly, make sure the linkage at the secondaries #2 is just shy of tight. This is just as important as the spring to keep the air valve from opening too soon.
 
Not sure exactly what your rig weighs Mike, figuring it's on a G20/G30 chassis. The reason i'm against doing away with the secd operation is, if you have to pull some good size hills or pass someone, your asking a awful lot from a low comp. 350 on just those little primaries. 99% of the time you shouldn't be into the secondaries anyway.

Also, wouldn't hurt to check your vacuum advance canister to make sure it's operational. If not, that could really hurt your cruise mileage.
 
It's a G20. I'll leave them hooked up, but I will check that vaccuum canister. It gets roughly 20 MPG, maybe a little better, which isn't all that bad, but I would really like to get the best possible while travelling across the country.
 
A g20 that gets 20? ????? You better get a chain and padlock and lock down the hood!!
 
67Dart273 said:
A g20 that gets 20? ????? You better get a chain and padlock and lock down the hood!!
Click to expand...

LOL.......I drove a few 1/2 ton cargo vans in the 70s and 10/12 city and 14/15 highway was more the norm. Did someone slip a Auto O/D in there at some point? Maybe a liter/km thing...lol. j/k

That's great mileage Mike. Don't think i can offer much to advance that number.
 
Install a vacuum gauge and drive by it. A smart foot can limit the secondaries, but they will be there for safety if you need the power. :)
 
Sounds good to me. I thought that was good gas mileage, but was wondering if I could get it any better. I'll leave it alone though.

KitCarlson said:
Install a vacuum gauge and drive by it. A smart foot can limit the secondaries, but they will be there for safety if you need the power. :)
Click to expand...

Good idea, thanks.
 
There are also likely conditions where the secondaries provide better volumetric efficiency, aiding in economy.

Back in the early 70's I used a vacuum gauge I installed into the clock gauge socket. It helped to help get 25 MPG at 75 MPH for long trips, on my 66 \6 3spd manual Coronet.
 
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