Jet selection?
Thread Starter
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Senior User
Joined: Nov 2003
Posts: 348
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From: Alberta
Jet selection?
From your experience, what jets would you recomend for a 1.21 Motorcraft carb that came off a early 80s van with a non-emission M block engine. The original jet size was 60.
This carb is now on a:
1986 302 (originally fuel injected)
dual exhaust
NO emission controls
4500 pound Bronco
manual transmission
3.55 gears
31 inch tires
Duraspark II ignition
Altitude of location: 2100 feet above sea level
What do you think? I was considering 59 or 57 to start with and to tune from there.
This carb had 60 jets in it with no EGR. The other cars I have seen here with the same carb tend to have 55 or 54 jets but they all had EGR. I think non-emission (EGR) cars needed larger jets. Is this true?
Thanks a bunch,
Clayton
This carb is now on a:
1986 302 (originally fuel injected)
dual exhaust
NO emission controls
4500 pound Bronco
manual transmission
3.55 gears
31 inch tires
Duraspark II ignition
Altitude of location: 2100 feet above sea level
What do you think? I was considering 59 or 57 to start with and to tune from there.
This carb had 60 jets in it with no EGR. The other cars I have seen here with the same carb tend to have 55 or 54 jets but they all had EGR. I think non-emission (EGR) cars needed larger jets. Is this true?
Thanks a bunch,
Clayton
Posting Guru
Joined: Aug 2000
Posts: 1,127
Likes: 3
From: (near) Denver USA
EGR was installed by the factory on all 400s starting in MY1973. When M-blocks (351M/400) were used in trucks and vans starting in MY1977, even the HD-certified engines had EGR.
One function of EGR is to allow the engine to run a leaner mixture or more ignition timing advance without pinging or excessive combustion temps. Other factors being equal, a richer mixture is required without EGR.
Since you've already got the 60 jets, start with those and see what happens.
Here's a thread in the 335-series forum where I described some factors in determining the right jet size:
https://www.ford-trucks.com/forums/s...hreadid=189345
Bottom line is: unless you have a gas analyzer, it's a trial-and-error process.
One function of EGR is to allow the engine to run a leaner mixture or more ignition timing advance without pinging or excessive combustion temps. Other factors being equal, a richer mixture is required without EGR.
Since you've already got the 60 jets, start with those and see what happens.
Here's a thread in the 335-series forum where I described some factors in determining the right jet size:
https://www.ford-trucks.com/forums/s...hreadid=189345
Bottom line is: unless you have a gas analyzer, it's a trial-and-error process.
Thread Starter
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Senior User
Joined: Nov 2003
Posts: 348
Likes: 0
From: Alberta
FYI: Perhaps US engines were differnt but I have seen a few non-EGR M-Block engines up here in Western Canada. None of these engines were altered. They had no provision for EGR at all. All of the non-EGR M-Block 351/400 engines I have seen were on trucks and vans. The carb I am using came off one of these engines. The jetting seems to be ritcher than the typical emission controlled M-Block.
