Low rpm bucking problem - Chevelle Tech
Performance Our High Performance area

 10Likes
Reply
 
LinkBack Thread Tools Search this Thread Display Modes
post #1 of 51 (permalink) Old Jul 12th, 19, 12:12 PM Thread Starter
Lifetime Premium Member
Bill
 
Join Date: Apr 2002
Location: Winnipeg MB. Canada
Posts: 7,941
Low rpm bucking problem

Trying to improve off idle low rpm bucking , 475ci bbc 260.050 cam manual trans car pulling away slowly it bucks and you need to get into throttle more to clear it up but there are times on street you just want to go slowly .................... yes big cam is the problem but looking best tuning ideas to reduce issue ....... making the idle circuit richer definitely helps starting to wonder if too much timing is part of the problem ?? I run locked timing 37* with 12* vacuum advance hooked to ported vacuum so no VA at idle but does come in at cruise .......... I can lug car down to 1800 with out bucking which I can live with , its the low rpm take off that I would like better

22lb flywheel dual disc clutch that does not alow much slippage 4.30 rear gear Holley HP 1000

66 CDN Malibu SS A51,L34,M20 Aztec Bronze

Tremec by Liberty[/FONT]

Foxwell 475 BBC 688hp /610 tq
9.5 cr pump gas Brodix heads by Foxwell Straub hyd Roller
Crusin / Track video
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.



To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
CDN SS is offline  
Sponsored Links
Advertisement
 
post #2 of 51 (permalink) Old Jul 12th, 19, 3:13 PM
Tech Team
Harry
 
Join Date: Jan 2011
Location: Los Angeles
Posts: 251
Re: Low rpm bucking problem

Quote:
Originally Posted by CDN SS View Post
Trying to improve off idle low rpm bucking , 475ci bbc 260.050 cam manual trans car pulling away slowly it bucks and you need to get into throttle more to clear it up but there are times on street you just want to go slowly .................... yes big cam is the problem but looking best tuning ideas to reduce issue ....... making the idle circuit richer definitely helps starting to wonder if too much timing is part of the problem ?? I run locked timing 37* with 12* vacuum advance hooked to ported vacuum so no VA at idle but does come in at cruise .......... I can lug car down to 1800 with out bucking which I can live with , its the low rpm take off that I would like better

22lb flywheel dual disc clutch that does not alow much slippage 4.30 rear gear Holley HP 1000
I also have a 3/4 race cam (lol) in my BBC and have fought lots of tuning issues for years.
I run 22 deg of initial timing with 16 deg. of vacuum advance at idle for 38 deg. of total timing at idle. The thing that helped me the most on lean stumbles, bucking, etc... is the "idle-ease". I have a big hole in the middle of my base plate and I turn the idle ease full counter clockwise. This lets a lot of air in and allows me to close my rear and front throttle blades. In addition, you can keep your idle screws 1 full turn out because all that air coming in from the hole in the middle of the throttle plate is leaning out the mixture at idle. So, at hot idle you are sitting there at 14.7 to 1 with the throttle blades barely open and the idle screws set nicely at 1 turn out, then, as soon as you touch the throttle, you immediately go richer because the transition slot fuel pours in with the new air and the idle ease hole becomes inconsequential.

The idle ease is the only way I know of to go from a lean idle to an immediately richer take off.. Without the idle ease, I would go from a lean idle to a lean stumble to a nice richer take off.

Of course, all of this can be cured without the idle ease by just setting every phase of the carb at approx. 12.5 to 1, ie. setting it rich. Holley carbs work great with that rich setting in pretty much all instances with a hot cam. My problem is that I cannot stand a rich idle and I have spoiled myself and my girlfriend to expect no stink and no running eyes with my Chevelle.
427L88 likes this.

Harry
1970 Chevelle SS396 (408), Fathom Blue,
Original Block & Forged Crank, KB Pistons, Edelbrock 6045 Heads,Comp Cam 292H, March Serpentine, RPM AirGap,750HP,100hp Nitrous
A/F Wideband,Tremec TKO 600, 12 bolt,
30 spline Moser Axles, Richmond 3:73's, Hotchkis control arms, swaybars, springs, Wilwood 12" rotors D/S, 4 piston calipers
hjdca is offline  
post #3 of 51 (permalink) Old Jul 12th, 19, 4:53 PM
Senior Tech Team
 
Join Date: Apr 2012
Location: Yerington, Nevada
Posts: 1,576
Re: Low rpm bucking problem

" I run locked timing 37* with 12* vacuum advance hooked to ported vacuum so no VA at idle but does come in at cruise .......... I can lug car down to 1800 with out bucking which I can live with , its the low rpm take off that I would like better "

Yup...that'll cause it.
CDN SS likes this.
Dave Ray is offline  
 
post #4 of 51 (permalink) Old Jul 12th, 19, 5:01 PM
Tech Team
Jack
 
Join Date: Apr 2011
Location: S/E Pa
Posts: 283
Re: Low rpm bucking problem

try a 4 hole spacer under the carb

Matthew 22: 37-39
R.I.P. Mike Romeo

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
DragRac69 is offline  
post #5 of 51 (permalink) Old Jul 12th, 19, 6:56 PM Thread Starter
Lifetime Premium Member
Bill
 
Join Date: Apr 2002
Location: Winnipeg MB. Canada
Posts: 7,941
Re: Low rpm bucking problem

Quote:
Originally Posted by Dave Ray View Post
" I run locked timing 37* with 12* vacuum advance hooked to ported vacuum so no VA at idle but does come in at cruise .......... I can lug car down to 1800 with out bucking which I can live with , its the low rpm take off that I would like better "

Yup...that'll cause it.
Thanks Dave I was hoping you would comment so "too" much timing will cause this when trying to drive slow on street I kinda thought so ........ my carb is sorted out pretty well
so I knew it was not far off and adding more idle fuel helped but made it too rich

I have a Crane digital dist with 8 preprogrammed curves since my car now idles at 1000 rpm 8-9 hg vacuum I was going to try the #8curve that starts at 1200 all in at 2800 20* then will add 16-18 initial

What do you suggest for vacuum advance
427L88 likes this.

66 CDN Malibu SS A51,L34,M20 Aztec Bronze

Tremec by Liberty[/FONT]

Foxwell 475 BBC 688hp /610 tq
9.5 cr pump gas Brodix heads by Foxwell Straub hyd Roller
Crusin / Track video
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.



To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

Last edited by CDN SS; Jul 12th, 19 at 7:13 PM.
CDN SS is offline  
post #6 of 51 (permalink) Old Jul 12th, 19, 6:58 PM
Tech Team
 
Join Date: Aug 2018
Posts: 288
Quote:
Originally Posted by Dave Ray View Post
" I run locked timing 37* with 12* vacuum advance hooked to ported vacuum so no VA at idle but does come in at cruise .......... I can lug car down to 1800 with out bucking which I can live with , its the low rpm take off that I would like better "

Yup...that'll cause it.
X’s 2
CDN SS likes this.
Reelysalty is offline  
post #7 of 51 (permalink) Old Jul 12th, 19, 7:16 PM Thread Starter
Lifetime Premium Member
Bill
 
Join Date: Apr 2002
Location: Winnipeg MB. Canada
Posts: 7,941
Re: Low rpm bucking problem

Thanks to those for confirming this a timing issue ....... I find it easier to tune for the track than for good street maners

My locked out timing was from the drag racing days plus not always having the best carb tune so now racing days are pretty much over want to tune for best street manners I can with this big cam small motor
71350SS likes this.

66 CDN Malibu SS A51,L34,M20 Aztec Bronze

Tremec by Liberty[/FONT]

Foxwell 475 BBC 688hp /610 tq
9.5 cr pump gas Brodix heads by Foxwell Straub hyd Roller
Crusin / Track video
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.



To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
CDN SS is offline  
post #8 of 51 (permalink) Old Jul 12th, 19, 7:51 PM
Tech Team
 
Join Date: Aug 2018
Posts: 288
Is this your dist ?
If so, you might enjoy this

https://m.youtube.com/watch?v=cJhLkuw__eI

Try 25 initial, vacuum plugged,
Total 37 or so,
All in by 1600-1800
Put common vacuum canister on and plumb to full vacuum, bottom port
Reelysalty is offline  
post #9 of 51 (permalink) Old Jul 12th, 19, 9:37 PM Thread Starter
Lifetime Premium Member
Bill
 
Join Date: Apr 2002
Location: Winnipeg MB. Canada
Posts: 7,941
Yes mine is the Pro Curve long discontinued as Crane went thru new ownership the Pro Race has no advance mech all lo

66 CDN Malibu SS A51,L34,M20 Aztec Bronze

Tremec by Liberty[/FONT]

Foxwell 475 BBC 688hp /610 tq
9.5 cr pump gas Brodix heads by Foxwell Straub hyd Roller
Crusin / Track video
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.



To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
CDN SS is offline  
post #10 of 51 (permalink) Old Jul 13th, 19, 2:39 PM
Tech Team
 
Join Date: Aug 2018
Posts: 288
If starter kicks back or drags w/big initial timing, put a separate toggle switch for ignition.

When starter is spinning, throw ignition switch.

Also good anti theft device. You’re 4 speed already protects you from millennials 😁
Reelysalty is offline  
post #11 of 51 (permalink) Old Jul 13th, 19, 3:27 PM
Senior Tech Team
Chris
 
Join Date: Feb 2006
Location: Lincoln pk NJ
Posts: 3,370
So is it the 37* plus the can is too much or just the initial with no curve? My son has my old 406 with a 262/269 @ .050 cam and since we put my old x pipe with the same 2 mufflers and 2 45’s this issue has seemed to disappear. It was noticeable with just the 2 mufflers rt off the collectors.
Runs like an ape as it always has
Just curious.
67 C10 with a m20 and 3.73’s and a 28” tire timing locked at 34*.
Was never an issue with the automatic.

73 nova,3515#(all steel)Pump gas!(91octane)406ci N/A, best 1/4,1.45 '60,[email protected],10.6 @ 122.38, short shifting,NO tuning!


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
WHT/73 is offline  
post #12 of 51 (permalink) Old Jul 13th, 19, 4:56 PM
Tech Team
 
Join Date: Aug 2018
Posts: 288
Quote:
Originally Posted by WHT/73 View Post
So is it the 37* plus the can is too much or just the initial with no curve? My son has my old 406 with a 262/269 @ .050 cam and since we put my old x pipe with the same 2 mufflers and 2 45’s this issue has seemed to disappear. It was noticeable with just the 2 mufflers rt off the collectors.
Runs like an ape as it always has
Just curious.
67 C10 with a m20 and 3.73’s and a 28” tire timing locked at 34*.
Was never an issue with the automatic.
Automatics mask a lot of problems.

For the moment, forget vacuum canister, disconnect and plug. Try around 36-37 degrees all in by 1600-1800. This is a good starting point for a low geared, lightweight 4 speed car.

Back into idle vacuum. That is set maximum as above and idle timing is what it is.

Idle timing should be IMO 25 degrees or so. If not, I suggest a distributer you can tune. For you, a MSD RTR 8360, about $ 350.

When finished, hook vacuum canister to bottom (full manifold vacuum) of your Holley Double Pumper....you do have a DP ? 😁

Pay no attention to any timing with advance hooked up. Below article explains.

Above assumes you run premium gas and have no vacuum leaks.

http://www.superchevy.com/how-to/add...gnition-timing
Reelysalty is offline  
post #13 of 51 (permalink) Old Jul 13th, 19, 5:11 PM
Tech Team
 
Join Date: Aug 2018
Posts: 288
I forgot to mention a digital or dial back timing light is VERY handy to verify dist mods. Innova makes one for about $ 100.

Link to 8360, fits SB and BB’s

If starter drags or kicks back, simply install a toggle switch to add ignition after starter is spinning engine. Same way NASCAR cars are started before F I. Benefits include never cranking hi dollar motor w/o oil pressure.

Benefits of hi idle timing include cooler running engine, way more off idle torque and plenty of idle hp to run electric radiator fan and (if you insist A/C) without idle relay,

Good luck 👍
Reelysalty is offline  
post #14 of 51 (permalink) Old Jul 13th, 19, 5:11 PM
Tech Team
 
Join Date: Aug 2018
Posts: 288
Reelysalty is offline  
post #15 of 51 (permalink) Old Jul 13th, 19, 5:24 PM
Tech Team
 
Join Date: Aug 2018
Posts: 288
Here’s more good info, just search ebay for him. Also another vendor I buy from, super guy. I’ll try and find contact info.



FBO Ignition Products
Now Shipping to Canada Australia and most European destinations
USPS 1st Class Mail $7.00

The Black bushing supplied by MSD with your distributor will allow 18* mechanical advance, most American V-8 engines require a total of 34-36* under load so with the largest bushing they make you can only get 16-18* of Initial timing which in most performance engines is not enough to achieve a lean clean and crisp idle and get rid of that eye watering Raw fuel fumes caused by inadequate initial timing.

Incorrect timing can also be responsible for not only a rich idle but overheating in traffic, stumble off idle, lazy throttle response, lost HP, burned out header gaskets, discoloration of ceramic coatings and poor fuel economy.
Without enough Initial timing it is impossible to set up the carburetor correctly as it won't respond to adjustments without enough timing to burn the fuel
If your Drag Racing you know that consistency is #1 in Bracket Racing, you will never be consistent with the combustion chamber loading up with fuel and fowling plugs. You want to win?... Get the timing right!

This Bushing kit will solve many of these issues if the distributor is correct for the application.
Our rule of thumb is "If the engine will make 10" of Manifold Vacuum at idle with the idle timing set at 30* then it should have a Vacuum Can type Properly curved distributor and connected to a CONSTANT Manifold vacuum port.
These bushings will fix most rich idle conditions, 80% or more of the rich idle complaints we get are NOT the Carburetors, If your not sure just call our tech-line and discuss your issues with an Ignition/Carburetion expert with over 45 years experience.
We have Email and of course Phone and Text access for tech help.
Now only $7.00 Shipping to Canada, Australia and most of Europe

PLEASE NO Calls regarding Edelbrock Carburetors
Reelysalty is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Chevelle Tech forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address. Note, you will be sent a confirmation request to this address.

Email Address:
OR

Log-in










Thread Tools Search this Thread
Show Printable Version Show Printable Version
Email this Page Email this Page
Search this Thread:

Advanced Search
Display Modes
Linear Mode Linear Mode



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
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome