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what course of action is indicated?

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bikerfox View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bikerfox Quote  Post ReplyReply Direct Link To This Post Topic: what course of action is indicated?
    Posted: Apr/01/2021 at 1:45am
I brought the Jav to a dyno shop (chassis dyno) to check on the engine's performance.
The result is that the A/F ratio is quite rich (401 w/crossram and Holley 4160s,
mech. secondaries) and needs to be leaned out.  Primary jets were already replaced
by a smaller size to have a leaner idle, as the initial idle was too rich.

My mechanical engineer friend says that the first thing to do is to enlarge
the main/high speed air bleeds to introduce more air into the system. Then,
he says, I can change the secondary metering plates to a smaller "jet" opening.
He feels that a valve might be burned in the process of dyno-ing the engine if I don't
modify the air bleeds first.

The dyno shop and another carb buddy say no, first thing to do is to change the plates
and that any main/high speed air bleed changes will be inconsequential compared
to the plate changes.

Which direction is "more correct?"  Is there a more effective direction to take here?

See graph below.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote WesternRed Quote  Post ReplyReply Direct Link To This Post Posted: Apr/01/2021 at 1:55am
The chart gives absolutely no indication of what is going on below 3,000 rpm, which is actually going to be more important for running around town.

My solution would be to install EFI, but hopefully somebody smarter than me will be along soon.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bikerfox Quote  Post ReplyReply Direct Link To This Post Posted: Apr/01/2021 at 2:01am
Originally posted by WesternRed WesternRed wrote:

The chart gives absolutely no indication of what is going on below 3,000 rpm

Full throttle began @ approx. 3k rpm.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Boris Badanov Quote  Post ReplyReply Direct Link To This Post Posted: Apr/01/2021 at 4:40am
IMO do nothing.
It seems to be running good.
A little rich is preferred to detonation and
worse, back fires.

IMO don't waste your time unless you spend
most of your time at 4000 rpm +

A back fire with a plenum that big
is a scary thing.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote iapexl8r Quote  Post ReplyReply Direct Link To This Post Posted: Apr/01/2021 at 5:05am
the best way to fix this rich at WOT condition is to invest in a wide band o2 gauge and sensor, get a base line then start with the simplest adjustment and re road test and just move forward from there. or take a bunch of carb parts  to the dyno . its best to take baby steps with the adjustments. get a note book and write everything down so you can keep track of all changes.
-chris
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Post Options Post Options   Thanks (0) Thanks(0)   Quote WesternRed Quote  Post ReplyReply Direct Link To This Post Posted: Apr/01/2021 at 5:52am
I’d second that regards the wideband, most so called “dyno tuning centres” have absolutely no idea how to tune a car for road use. They will take your money and give you a peak power number that means nothing.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mbwicz Quote  Post ReplyReply Direct Link To This Post Posted: Apr/01/2021 at 7:39am
Do you have room on that manifold to install secondary blocks that allow jet changes, rather than the plates? This would allow it to be easier to make changes and go back.

Having your own AFR meter would be useful.

The big question is: How does it run? If you are happy with the throttle response and there aren't any stumbles or flat spots, leave it. Chasing the AFR 'just because' may be quite a bit of work for little benefit. If the cruise and idle are running well, then you may just leave it.

If you made the front jets smaller to improve the idle, did that give any surge while cruising (which would indicate you are lean on cruise)?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Apr/01/2021 at 8:08am
For my pennies worth, vacuum secondary's in parallel with a cross ram intake solves these issues.
But I've only worked on Chryslers way back when, so take that for what it's worth - maybe nothing!
The thing is big and the variables of air pressure, temperature, humidity ... all effect your A/F mixture. Mechanical secondary's may be good one day and terrible the next. Let the engine tell the carburetors what it needs.
Did the dyno sheet also give you the vacuum readings? 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 6PakBee Quote  Post ReplyReply Direct Link To This Post Posted: Apr/01/2021 at 8:17am
What does your brake specific fuel consumption (BSFC) look like?
Roger Gazur
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bikerfox Quote  Post ReplyReply Direct Link To This Post Posted: Apr/01/2021 at 9:43am
Thanks for your replies.

There's no vacuum info given (although it was 11" @ idle) and no BSFC that 
I can see, although when the car was being dynoed I vaguely recall that acronym
on the screen.  Yes, there's room for secondary blocks and I've thought about 
going that route.  There is no surging going on and a stumble was fixed.


Edited by bikerfox - Apr/01/2021 at 9:45am
1969 Rebel SST (1970-1987)--SOLD
1968 AMX (2005-2011)--SOLD
1969 SC/Rambler (owned 2011-2019)--SOLD
1970 Javelin (2019 to ?)--"Jane"
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