Hot Rod Forum : Hotrodders Bulletin Board - Reply to Topic -- Hot Rod Forum

Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine> Lean backfire
User Name
lost password?   |   register now

Thread: Lean backfire Reply to Thread
Send Trackbacks to (Separate multiple URLs with spaces) :

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:
Please select your insurance company (Optional)


Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

Additional Options
Miscellaneous Options

Topic Review (Newest First)
04-15-2003 05:45 PM
1BAD80 You say it runs rich until you step on the throttle then goes lean. It sounds like you have 2 condition's. Do you have any holes drilled in the throttle plates (butterflys)?
One is drop the front jets down 2 numbers, and when you hit it the pedel that's when it goes lean ,that's the accelerator pump system. Put on a red pump cam on the #1 hole, and a 3.5 power valve. Sounds like a larger pump nozzle size needed
I Copied from Holly a little to make it easer to understand and write faster.

The accelerator pump system consists of three main components: the pump diaphragm, the pump cam and the pump nozzle. This is the carburetor system that is most responsible for having good, crisp, off-idle throttle response. Its purpose is to inject a certain amount of fuel down the throttle bores when the throttle is opened. By accomplishing this purpose it acts to smooth the transition between the idle and main circuits so that no stumble, hesitation or sluggishness will be evident during this transition phase.

The first adjustment to check is the clearance between the pump operating lever and the pump diaphragm cover's arm, at wide open throttle. This clearance should be around .015". The purpose for this clearance is to assure that the pump diaphragm is never stretched to its maximum limit
at wide open throttle. This will cause premature pump failure. Once this clearance has been set take a good look at the pump linkage and work the throttle. Make sure that the accelerator pump arm is being activated the moment that the throttle begins to move. This will assure that pump response will be instantaneous to the movement of the throttle. These adjustments can be made by turning the accelerator pump adjusting screw that is located on the accelerator pump arm together with the
pump override spring and lock nut.

The amount of fuel that can be delivered by one accelerator pump stroke is determined by the pump's capacity and the profile of the pump cam. The period of time that it will take for this pre-determined amount of fuel to be delivered is affected by the pump nozzle size.

A larger pump nozzle will allow this fuel to be delivered much sooner than a smaller pump nozzle. If you need more pump shot sooner, then a larger pump nozzle size is required. During acceleration tests, if you notice that the car first hesitates and then picks up, it's a sure bet that the pump nozzle size should be increased. A backfire (lean condition) on acceleration also calls for a step up in pump nozzle size. Conversely, if off-idle acceleration does not feel crisp or clean, then the pump nozzle size may already be too large. In this case a smaller size is required.

Holley accelerator pump nozzles are stamped with a number which indicates the drilled pump hole size. For example, a pump nozzle stamped "35" is drilled .035". Pump nozzle sizes are available from .025" to .052". Please note that whenever a .040" or larger accelerator pump nozzle is installed the "hollow" pump nozzle screw, P/N 26-12, should also be used. This screw will allow more fuel to flow to the pump nozzle, assuring that the pump nozzle itself will be the limiting restriction in the accelerator pump fuel supply system.

NOTE: When changing the pump nozzle it's best to jump three sizes. For
example if there's currently a off-line hesitation with #28 (.028")
pump nozzle, try a #31(.031") pump nozzle. If you must use a #37 (.037")
or larger pump nozzle, then also use a 50cc pump. The same applies to the accelerator pump cams. Once a pump nozzle size selection has been made the accelerator pump system can be further
tailored with the pump cam. Holley offers an assortment of different pump cams, each with uniquely different lift and duration profiles, that are available under Holley P/N 20-12. Switching cams will directly affect the movement of the accelerator pump lever and, subsequently, the amount of fuel available at the pump nozzle. Lay out the pump cams side by side and note the profile differences. This little exercise may help to better explain the differences between the cams and their effect on pump action.

If you still need any help please email me. IM getting some annular boosters to put on my 2 650 carbs on my cross ram to make it twice of responcive, 9 holes compaired to 3 in a regular one. Getting themfromCNCMotorsports <a href="" target="_blank"></a> a good place for rebuild kits and other parts.

<img src="graemlins/sweat.gif" border="0" alt="[sweat]" />

[ April 15, 2003: Message edited by: 1BAD80 ]</p>
04-15-2003 02:19 PM
6-71 sbc i have a question about the carb thing for you bad80. you have given me some good adive in the past with my blower carbs i have the new proform main bodys and billet metering blocks. it works killer. and i put an air/fuel ratio meter. it still runs rich unless i give it a quick reve on the gas and reeeeeely leans out for a second. no backfire. i have #63front#71 back. i have a 30cc pump. 6.5 power valve. is the power valve wrong or the pumps. what is your thought. and i got my boost back when i changed the intake gaskets. and now i am back to terrorize the euro trash. i still need to bring down the jetting some more. <img src="graemlins/evil.gif" border="0" alt="[evil]" /> <img src="graemlins/evil.gif" border="0" alt="[evil]" /> thanks
04-15-2003 12:49 PM
1BAD80 I understand the question, I thought by reading it yourself, you would get a better understanding how the fuel system will cause a backfire from being lean and how to fix it. If you timed it correctly it shouldnt backfire. Advance causes spark knock, retard will cause a miss in the exhaust. Carb causes backfire.
04-14-2003 07:11 PM
Camaro76 thx for the info, but i think u ment it for a post i already made and understand(getting the stuff for it friday)

what i want to know is how does it backfire when lean, what couses what?

for example, to much advance will couse detonation becouse the spark happons so early that the fuel is burnt most the way, way befor the piston reaches TDC

i hope this clears my question up abit

thx again
04-14-2003 05:16 PM
1BAD80 Here's Hollys site, it has all the info you need.
It also has a free downloadable manual and trouble shooters guide.
<a href="" target="_blank"></a>
04-14-2003 06:05 AM
Lean backfire

with my new engine i haven't had this problem yet but, i want to know how going lean will couse it to backfire. we all know it will, but how/why?

educate me alittle


Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

All times are GMT -6. The time now is 06:52 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999 - 2012. All Rights Reserved.