Holley carb tuning - Page 3
Have an account? Register

Forgot your password?

Forgot your username?

 
Likes Likes:  0
+ Reply to Thread
Page 3 of 7 FirstFirst 1 2 3 4 5 ... LastLast
Results 21 to 30 of 65
  1. #21
    Rods 'n' Sods Junkie SpannerPete's Avatar
    Join Date
    Nov 2009
    Location
    Bedfordshire
    Posts
    301
    Images
    19
    Post Thanks / Like
    SpannerPete is offline
    Quote Originally Posted by Roscobbc View Post
    I am using a 'bracket race' cam, power band 2500 to 6500 - I specifically chose the Edelbrock RPM Air Gap inlet which is a dual plane - the reason? with a street use BB Chevy its the best choice across the range and looses out only to Victor single plane at 6000 rpm plus (and it meant that port velocity would be high at low rpm = low rpm streetability). A single plane inlet will work fine on a big cube engine at lower rpm where you have bundles of torque - but less so on smaller cubes, heavy vehicle and cars with low numerical axle ratios.
    I did wonder if the dual plane was a magic 'cure all' but obviously they still need setting up as well.

    Lots of good advice here, I will have to do some playing about and see what happens.

  2. Remove Advertisements
    RodsnSods.co.uk
    Advertisements
     

  3. #22
    Carburation 'sucks' Roscobbc's Avatar
    Join Date
    Apr 2009
    Location
    South West Essex
    Posts
    4,093
    Images
    14
    Post Thanks / Like
    Roscobbc is offline
    With your lightweight 'T' - timing, carb size, single plane inlet are all OK imho - you could have even used a double pumper with that combo - you really could do with finding-out the cam spec' - do you know anyone with a similar sized engine/carb combo? - see if you can borrow their carb - see how it runs on your car?

  4. #23
    Official RnS Addict WB54's Avatar
    Join Date
    Apr 2009
    Location
    Herts
    Posts
    4,599
    Images
    198
    Post Thanks / Like
    WB54 is online now
    Quote Originally Posted by Roscobbc View Post
    WB54 - I'm using a ProForm 950 DP - good call about opening-out the 'slots' - makes sense - is that opening-out vertically? - would you need to mill the slot or is it do-able with a 'swiss' file?
    measure then first but they can be lengthened with a junior hacksaw blade ground down and fashioned to fit, fiddly but acheiveable

  5. Remove Advertisements
    RodsnSods.co.uk
    Advertisements
     

  6. #24
    Carburation 'sucks' Roscobbc's Avatar
    Join Date
    Apr 2009
    Location
    South West Essex
    Posts
    4,093
    Images
    14
    Post Thanks / Like
    Roscobbc is offline
    Quote Originally Posted by WB54 View Post
    measure then first but they can be lengthened with a junior hacksaw blade ground down and fashioned to fit, fiddly but acheiveable
    Sounds like a good old fashioned British engineering solution !!!!!!!!!

  7. #25
    Rods 'n' Sods Junkie SpannerPete's Avatar
    Join Date
    Nov 2009
    Location
    Bedfordshire
    Posts
    301
    Images
    19
    Post Thanks / Like
    SpannerPete is offline
    I've just been re-reading this article

    Carburetor Tuning the Scientific Way

    Is it worth fitting a couple of Lamda sensors as he describes? Could it give me a better idea of what the problem is?

    The Cheap Way

    Buy a single wire O2 sensor at your local parts house, make your own bung, and read the voltage with a digital voltmeter. The sensor that I bought is a Standard brand, number SG-12. The threads on this sensor are the same as a small-block Chevy gasket-style spark plug, so the bung can be made from one of those spark plug anti-foul adapters. Other O2 sensors use the large diameter threads of 18mm big Ford spark plugs. Just cut and fishmouth the adapter so that the sensor sticks into the exhaust flow. You need to put the sensor as close as possible to the engine so it gets hot and stays hot. Just make sure you route the wire so it doesn't get burned by the hot exhaust pipe. Weld the bung to the pipe, then drill and file the hole to clear the sensor.

    Since the purpose of this sensor is just a guide to help you tune your carb, not run a fuel injection computer, if you can't get the sensor really close to the engine, don't worry, because it will still work for your purpose. All that will happen is that your reading may go away during periods of idling. On the same subject, don't worry about using a heated sensor, as the expense and complications involved are not worth it for carburetor tuning. Remember, your eyes are using this data, and if it stops for a while, no harm is done!

    Sensor installed in Pinto exhaust.

    O2 sensor in Pinto exhaust

    Sensor and welded bung:

    Outside view of sensor installation

    Once you have the sensor installed and wiring run up to the inside of the car, attach a digital voltmeter (you really should have one of your own, but you can sometimes borrow these from friends if you don't have one) to the sensor and a good body ground. The sensor is positive. The readings you'll get once the sensor has heated up will be from 1.1 volts (1100 millivolts, or mv) down to about 100 mv. The high readings are rich, the low readings are lean. The perfect mixture for cruise is 400 mv. I have found my car to run well at about 700-800 mv. Once it gets below that, it tends to get into a lean misfire. Your results may vary.

    Here is a general idea at what the O2 sensor voltage output looks like. As you can see, the slope around 400mv, which is 14.7:1, or perfect combustion, is very steep. This is why only computerized fuel injection systems can really hold anything close to 400mv. If you're wondering about how a sensor can read oxygen content in rich mixtures where there is no extra oxygen, the sensor begins to act as a temperature sensor above 400mv.

    O2 Sensor output


  8. #26
    Carburation 'sucks' Roscobbc's Avatar
    Join Date
    Apr 2009
    Location
    South West Essex
    Posts
    4,093
    Images
    14
    Post Thanks / Like
    Roscobbc is offline
    Good idea - however if you have a 'radical' cam with loads of overlap and the consequentially required rich idle mixture to get it to idle the Lamda sensor may not tell you too much at lower rpm and idle - it should give you plenty of help at cruise etc though. 'Old school' way to check mixture is to find a bit of quiet 'A' road, cruise steadily at say 50 mph - knock the transmission in to neutral and cut the engine at the same time - pull over to one side - pull ALL the plugs and note the colour - do exactly the same at wide open throttle (a 'kin good trash, say in intermediate up to high revs for 15, 20 seconds) and note the result. If the colour of the plugs is the same all through, fine - if it differs you'll know if there is a carb imbalance - (although a single plane inlet may fudge the results at lower rpm) - you should be looking for 'coffee' colour plug tips with perhaps darker brown further away from the tip. Repeating the same thing for WOT will help give you an idea whether the secondaries are lean or rich. Its unlikely with a modern inlet manifold that you will need staggered jetting. If you have a 'cam' the plug colour at idle will likely be black and sooty - it should not be like that at any other speed. Changing jets on Holleys is simple but messy - the steps in jet sizes are quite small so if changing go-up 3 or 4 sizes rather than on or two. Let us know the can spec - it will help determine things.
    Last edited by Roscobbc; 21-05-2011 at 10:57.

  9. #27
    Rods 'n' Sods Junkie
    Join Date
    Feb 2010
    Location
    london
    Posts
    458
    Images
    0
    Post Thanks / Like
    zephyrc is offline
    it would be good to know the cam spec and type, also the compression ratio..sounds like a fairly large cam in there judging by the low idle vac, but a cam large enough to have fairly low idle vac would normally go way further than 6000 rpm,, i have a large roller in my sbc and it idles at 9 inch vac at 1000 rpm, but it makes power round past 7000 rpm... what is the total timing on this motor..i would always use a double pumper on a fairly healthy motor, but one with as much adjustments as possible,, does your carb have adjustable airbleeds etc

  10. #28
    Rods 'n' Sods Junkie SpannerPete's Avatar
    Join Date
    Nov 2009
    Location
    Bedfordshire
    Posts
    301
    Images
    19
    Post Thanks / Like
    SpannerPete is offline
    Quote Originally Posted by zephyrc View Post
    it would be good to know the cam spec and type, also the compression ratio..sounds like a fairly large cam in there judging by the low idle vac, but a cam large enough to have fairly low idle vac would normally go way further than 6000 rpm,, i have a large roller in my sbc and it idles at 9 inch vac at 1000 rpm, but it makes power round past 7000 rpm... what is the total timing on this motor..i would always use a double pumper on a fairly healthy motor, but one with as much adjustments as possible,, does your carb have adjustable airbleeds etc
    I think the cam was a custom grind, I'll have to check with the engine builder. I did ask for a street engine so I'm not expecting anything radicle and it made 429hp on the dyno so don't think it's anything very exotic. It does have Rhodes lifters which I was told would help.

    It hasn't got adjustable air bleeds which means I'll have to order up and parts to try things out which why I thought the lamda sensor might be a wise move to get a bit more info.

  11. #29
    Rods 'n' Sods Junkie
    Join Date
    Feb 2010
    Location
    london
    Posts
    458
    Images
    0
    Post Thanks / Like
    zephyrc is offline
    so the cam is hydraulic flat tappet,, do you know the total timing on the motor.. incorrect timing does cause very low vac,, in my experience 38 degrees is a fairly safe ballpark for total timing and tune from there

  12. #30
    Rods 'n' Sods Junkie SpannerPete's Avatar
    Join Date
    Nov 2009
    Location
    Bedfordshire
    Posts
    301
    Images
    19
    Post Thanks / Like
    SpannerPete is offline
    Quote Originally Posted by zephyrc View Post
    so the cam is hydraulic flat tappet,, do you know the total timing on the motor.. incorrect timing does cause very low vac,, in my experience 38 degrees is a fairly safe ballpark for total timing and tune from there
    Ok that's interesting I didn't know that. I've been told it should be 14-16 degrees at idle which it is, I have the total timing written down at work and when I checked it did seem about right (hard to look at the rev counter & timing light at the same time)

+ Reply to Thread
Page 3 of 7 FirstFirst 1 2 3 4 5 ... LastLast

Quick Reply Quick Reply

Register Now

Please enter the name by which you would like to log-in and be known on this site.

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

Please enter a valid email address for yourself.

Where you live, Town\City\County etc.

Please enter your full name
This field cannot be edited once it has been completed other than via contacting the site admin. Please make sure the information is correct first time.

Log-in

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

    Similar Threads

    1. Marc 55 Chev Carb tuning
      By ozzie in forum Chat
      Replies: 11
      Last Post: 04-07-2010, 23:51
    2. holley carb plus rover v8 bits
      By harris66 in forum Stuff - WANTED
      Replies: 1
      Last Post: 31-01-2010, 19:58
    3. holley carb id ?
      By 100ev8 in forum Tech Discussion
      Replies: 1
      Last Post: 01-10-2009, 20:59
    4. holley carb question
      By vw2v8 in forum Tech Discussion
      Replies: 4
      Last Post: 29-08-2009, 13:56
    5. Holley carb
      By TATFINK in forum Stuff - FOR SALE
      Replies: 0
      Last Post: 03-06-2009, 11:09

    Bookmarks

    Posting Permissions

    • You may post new threads
    • You may post replies
    • You may not post attachments
    • You may not edit your posts
    •