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carb jetting

 
WScott
V-6 Camaros rule!
Posts: 40
Joined: 03/12
Posted: 03/22/13 09:03 AM

will going from a 72 primary jet down to a 70 affect the cold idle
quality of an engine, when i jet my carb with 72 prim. it seems to run
a bit rich ( sooty ex pipes)in the cruise rpms  2200 - 2600 so i jetted
down to 70 primary jets. now when i start the engine in cooler weather (no choke)
i get back firing out the ex. pipes until the engine warms a bit, does this
sound normal or does it sound like i have other carb problems, thanks Wayne  

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waynep71222
I have an SS396 tatoo
Posts: 448
Joined: 03/12
Posted: 03/22/13 02:13 PM

i take it that you are running the 670cfm street avenger carb ...

do you happen to have an air fuel ratio display and a oxygen sensor installed in your collector to know what the air fuel ratio is as you idle and then as you pass thru idle transition to the main circuits..

Holley 4150 4160 Idlecircuit2 Zps1ffa8cfd


look at the right side of this image .. the primary... since at fast idle.. even with no choke.. the engine might be running just on the idle circuits and the idle transition circuits..

Holley4150bothidlecircutis Zpsa4655b11

this is an important tuning step that gets left out of a lot of builds....

this allows you to transition from idle.. thru the idle transition to the main circuits without either going too rich or too lean...    since at cruise speed, 1400 to 2200 rpms or so. ... the holley will be operating partially on the idle transition circuits..  its critical to know what's going on..

will reducing jet size effect primary air fuel ratio .. yep.. but the need to know where you are actually at right now..

more opinions and help to come from other forum members...

could your accelerator pump discharge nozzle size be too small or the pump lever cam be the wrong shape for your application.. ???  

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pepsi1
I mow my lawn and find Chevys
Posts: 1718
Joined: 09/11
Posted: 03/22/13 03:16 PM

Wscott
Try stagger jetting. .072 and .070 in your primary's.

The backfirering is because it is too lean. Those avengers are set up for MPG and performance. If it's not fouling your spark plugs leave it alone.

If you saw the way my 271/289 HiPo. 1964.5 Mustang was it had a black cloud behind it when I had the manual choke set. If I tried to drop the choke before it actually warmed up it would stall. I put a warmer plug in it for the winter months. It was not a cold weather engine at all.

Bob  

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WScott
V-6 Camaros rule!
Posts: 40
Joined: 03/12
Posted: 03/22/13 04:48 PM

sorry i should of stated which carb i have, it is a holley 750 vac sec
i do not know how old the carb is or how many times it has been rebuilt,
it had the stock jetting in it, the rear metering plate has been changed to
a metering block, the engine is a 406 sb and the air/fuel needles give me the
best idle quality at about 1 turn out at 800 rpm in neutral, i do not have a
air/fuel sensor installed in the collector, would i have to have one in each collector ?  

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waynep71222
I have an SS396 tatoo
Posts: 448
Joined: 03/12
Posted: 03/22/13 11:12 PM

you can usually get by with a single oxygen sensor in one pipe...

if you are going to install one in the collector... use a 3 wire sensor...

Bosch/Oxygen Sensor
For your 1995 Chevrolet Truck C2500 3/4ton P/U 2WD 5.7L TBI 8cyl
Part Number: 15700


the proper wire to the air fuel ratio display... one wire to the coil positive supply for the heater element.. the other for the ground for the heater circuit..  usually to the back of the head...

almost every muffler shop will have oxygen sensor bungs they can weld in quickly.. and cheeply.




you can install the air fuel ratio gauge in a temporary bracket and wire it with a lighter plug.. so you can remove it easily..  

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pepsi1
I mow my lawn and find Chevys
Posts: 1718
Joined: 09/11
Posted: 03/23/13 07:23 AM

WScott
What are you working with. Car/Truck what year. Engine size.

The engines cam will really have an effect on the idle quality hot or cold.

I would go back and check all the base settings, of the carburetor itself.

Float level, needle and seats, could be leaking also.

I show .075 primary jets, .076 secondary jets. an 85 Power Valve.

Adjust the air/fuel with a vacuum gauge for the highest vacumm setting, not for idle speed. Use the curb idle for that. You may have to go back and forth a few times until it all settles in. This could be part of the lean miss.

Just a little FYI:
Get a fuel sample from your tank. I have had dirt particles so small actually hold a part of the diapharm of the power valve open to leak raw fuel into the system. The engine ran great at 2000 RPM and above. The part thats funny but not funny, is this. Everytime I took the front bowl off and Not knowingly I would disturbed the power valve and the little dirt particles. I'd reinstall it and go around the block it was fine. Drive it awhile and it would act up...I was really chasing my tail, or a catch 22...Just some of the years of experience and the heart breaks that go with the learning curve.

Bob  

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pepsi1
I mow my lawn and find Chevys
Posts: 1718
Joined: 09/11
Posted: 03/23/13 09:47 PM

WScott
I went looking for this P/N, and got side tracked and I'm back  I lost the post I had. I hope I answered some of your question.
  The Power Valve that you really want for any competition engine is this old number. 25BP-400A-65 Its a 6 hole outlet thats an old number. But it may still be good..They can cross reference it.  
  Its a #65 Power Valve. I use to get a lot of my Holley Parts at NAPA. They referenced Echlin carb parts.
  "Never run your engine without a Power Valve". IE. a plugged Power Valve circut.
You will need to run jet sizes in the triple digits. IE if you needed a .080 NOw you will need a .120 jet this is just an example.

Bob  

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pepsi1
I mow my lawn and find Chevys
Posts: 1718
Joined: 09/11
Posted: 03/23/13 09:47 PM

WScott
I went looking for this P/N, and got side tracked and I'm back  I lost the post I had. I hope I answered some of your question.
  The Power Valve that you really want for any competition engine is this old number. 25BP-400A-65 Its a 6 hole outlet thats an old number. But it may still be good..They can cross reference it.  
  Its a #65 Power Valve. I use to get a lot of my Holley Parts at NAPA. They referenced Echlin carb parts.
  "Never run your engine without a Power Valve". IE. a plugged Power Valve circut.
You will need to run jet sizes in the triple digits. IE if you needed a .080 NOw you will need a .120 jet this is just an example.

Bob  

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