MULTI CARB TIPS AND SUGGESTIONS
Home Page

About Page

Photo Page

Contact Page

Guest Book Page

Custom Page

Custom2



 

GREETINGS! 

multicarb tips and suggestions

BY VINTAKES

 

GREETINGS!  First off I’m NOT the barry grant of multiple carb set-ups this article is written from my personal experiences building units, articles read in OLD car magazines and the installation instructions - tuning tips from old speed shop dealer bulletins. This article is directed at STREET driven vehicles

 

SELECTION  sizing and number of carbs very important for good crisp street driving yeh it’s cool to show off your 283 with six Rochester 2 bbls with straight linkage on a log intake auto trans and 2.76 rear end gears, but drivability will be very poor unless of course you “dummy the end carbs”  I fell if ya got them they all should function and will not address dummy carbs

It’s basically all about  air speed in your carbs  turn on a garden hose and watch water flow  then put your thumb over the hose end  water going faster right  the same applies to your carbs.  Your engine is a large “air pump” the more air it can pump the more carb CFM”s you can use and be able to handle  and still keep good air speed sizing also depends on the RPM range you run your motor. there was a dyno test on stock 350 chevy motors and the 2bbl motor made more HP torque up to 4,500 rpm them the Q-Jet one!! Going back to our 283  6x2  if you have a 11,000 rpm 283 drag motor the 6 Rochester’s would be to small you would need more carb cfms I’m basing these suggestions on 350 with 9.1 compression ratio mild cam 110 - 112 lobe centers, 260 dur or less with a 5,500 rpm red line  [bigger motors and motors that operate at higher rpms can use more cam] NOTE here building a fresh car / motor with multicarbs??  Install a stock 2 or 4bbl carb to fire fresh motor and break in. Work out all “bugs” with the stock stuff FIRST then install your multi set-up. You now have a performance base line to judge by when you install the multi unit This also removes any possible excuses from your engine builder, like hey the tri-power cause your cam to fail  ain’t my fault and other misc B.S. sure its cool to have your rod sound like a AA/GAS car at the show but extensive cam duration severely lowers motor vacuum [air speed], hampers street-ability immensely and will cause problems with the power vale circuits. So try to plan first, keeping in mind the TYPE of actual usage, [ive seen guys build a rump-ity  rump motor for there ride and then use 2.76 gears with a overdrive trans saying “in od I’ll be turning 1,200 rpm at 70 mph   great!!”  But in reality there’s no HP or torque at 1,200 because there’s very low engine vacuum  NO AIR SPEED  so they will not be able to use OD on the highway  but they can brag they have OD . My recommendation for SBC street motors 327 “ 350”s is a exact copy of the 64 corvette 327 300 HP hydraulic cam it gives smooth idle power from 600 rpm to 5,000 great torque and decent fuel economy good air speed   I think I’ve seen these “blueprint cams” in JEGS I’m not at all fond of the RV cams [I’ve used one]  they seem to lessen low end power and mileage. So choose carefully

  Next up trans kick down cables I’ve seen many  700 r Tran’s without any cables at all.  These aren’t only kick down down shift but also control valve body pressure [throttle pressure] and as such should not be eliminated installing a shift kit STILL requires a cable [unless you have a full competition manual valve body] yeh it takes some doing to hook to carb especially on 4 carb direct port intakes [manifre style] also if you swap out intakes for different looks etc you have to go thru making new brackets pain?? Yep! Answer: hook trans cable thru firewall to gas pedal. If you don’t want to spend time engineering or want a pattern, go to local salvage yard and look for a 74 chevelle , olds cutlass with 350 trans these have factory kick down cables thru firewall hooked to gas pedal simply remove gas pedal assembly and cables, install in your car. This also works for 350 turbo kickdown and makes for a neater looking intake system without excessive cable brackets  show off your multi carbs  

   Distributors will also need attention. Your advance curve is important to work with your multicarb unit I personally like the spalding flame thrower distributors look cool and work great once set up properly [you can find set-up instructions for these in the near future at this web site] the old Mallory dual point dist are also good units [see upcoming article in my web site]

You can also have your dist advance curve changed by dist shop. Or purchase kits from summit jegs etc. if you’re using 97’s no vacuum advance port is available in carb unless you drill one.  94’s have ported vacuum port but a weaker than gm requires vacuum signal for ease of set-up run full centrifugal advance dist. Rochester’s obviously compatible with gm dist and vacuum advance unit 

 Fuel pumps. I recommend using stock motor driven pumps 4lbs  7 lbs. you don’t need high pressure pumps for the STREET. Stromberg 94 and 97 style carbs will need fuel pressure regulator set around 1 - 3 lbs. holley makes a good one. Also the “dial face” type will work. Rochesters don’t need reg with stock pumps  Electric pumps only and stromberg carbs don’t work well together. Yes you can crank the pressure regulator down till carbs stop dripping at idle but you’ll run out of fuel mid at top end  if you want maximum performance with 94’s or 97’s I’ve “worked out” a fuel bypass system that will work with a holley 7lb 6 valve performance pump you will need a return line to your gas tank. E-mail me for details                                                                                                    Ok enough of motor stuff lets get to the carbs the basic tri “ power  Use either small base Rochester 2 bbls with progressive linkage or the larger 94’s or holley 2100 series 3 bolt carbs. Smaller 94’s and 97’s will work well but you may run out of steam around 4,800 rpm  here don’t be afraid to mix carb sizes larger centers smaller ends etc. The smaller the carbs the more low end power [street power] you will have. More air speed through the carbs and better fuel atomization As carb size increases so does the useable rpm range but the low end power suffers because the air speed is diminished at low rpm [remember the garden hose test]

Now, you can run the center carb and progressively tip in the end carbs the standard way [1x2]. If you are using fact tri-power carbs or the conversion style carb bases with NO IDLE CIRCUITS you must set-up this way.  With regular carbs you can run the end two carbs and progressively trip in the center [2x1] if you’re using regular carbs I fell this is the better way   [both 1x2 and 2x1 are ok]

It will give more exhaust noise and slight mid range torque / HP improvement especially if your using 97’s or 94’s part of the problem with 1x2 with regular carbs is you really can’t get the throttle plates to completely close, so you always have a “air leak” at each carb [yes the air leak is compensated for with the idle mixture screws] sometimes you may have trouble getting engine to idle slow enough because you have two air leaks which need to be equal [end carbs must be synchronized] the center can be different.

  If using the 2x1 set up [this was common place in the early 50’s I have factory eelco linkage instructions which show how to do this with many different types of carbs]

Simply close the center carb [as far as it will close] and use the balanced ends for idle speed I feel it’s better to have one air leak in the center than 2 closer to the valves you decide

  On all set-ups [except fact 3x2 and conversion bases] you need to keep, use and adjust the idle mixture screws in ALL carbs. On secondary carbs I eliminate the power valve. On 94’s you must put the power valve in the carb [under the base] but bock off vacuum port in base. You can get a old lead wheel weight cut off a piece and gently tap into vacuum hole [note here the valve on the outside of some 94’s is NOT the power valve!! it’s a spark delay valve used in conjunction with the old ford vacuum advance only distributors. Install valve, or plug hole with a holley power valve block off plug also if you using one of these stock old ford distributors, do your self a large favor THROW IT OUT!! And replace at least with a 57 and up vacuum and centrifugal advance unit]

  On Rochester’s you must keep the brass valve located between the main jets in the center section and remove the spring-piston assembly from the top carb section also here block off the vacuum port in the carb base with lead [use piece of old wheel weight]  Be careful not to block off the idle circuit holes

  As a rule of thumb when the power valves are disabled [Rochester and 94’s] you will need to increase the main jets sizes around .006  .008 note on 97’s just leave the pump rod off and increase mains .002 I use a numbered drill set for my jetting increases but purchasing actual numbered jets would be better

     4 carb units.  My favorite for SBC  here is the weiand staggered intake with 4 small 94’s or 97’s  these units look cool and street perform very well with progressive linkage. The old set up instructions say to use the outer pair of carbs for primaries and the in-board pair for secondaries. I’ve run these both ways [inner pri outer sec] and have noticed no difference in setting up or performance. Linkage here is not for the weak at heart but well worth the time. You need to have a machine shop make shaft extensions for the 2 carbs mounted on the pass side of the motor. All linkage is mounted on drivers side  you also need to make a support bar that fits over all 4 carb shafts to support the long extensions here pri carbs need to be synchronized together and sec carbs together  the pri and sec don’t have to be the same

   4 carb direct port man-a-fre style units. Be careful here using these. These must run straight linkage only so low-end power is gonna suffer ESPECAILLY with large Rochester carbs. Even the small rochesters will be touchy small rochesters will work on lighter weight stick shift cars with lower rear end gearing [ higher numerically] and or light cars with 3,000 stall converters and lower gearing  you have to keep the rpm’s up to keep decent carb air speed. 1 or 2” risers will help to increase air speed later in article I’ll give you rochester jetting recommendations from man-a-fre service bulletin. 4 small 94’s or 97’s are best for street use. Here again use risers. On all these units all carbs must be synchronized exactly the same [ I think speedway sell a synchronizing gauge but you must use a gauge on these at idle speed and part throttle. Idle synchronizing is done with idle stop screws and above idle sync done by adjusting the connecting linkage rods  [tip here, if you only use one carb throttle plate stop adjust screw per side, leaving the other two carbs without idle stop screws you can synchronize carbs with the idle stops in two and connecting linkage rods on the other two. This should be all you need but still check at above idle. Un-synchronized carbs can cause burnt valves. Pistons plug washing and the backfire thru carb and maybe a real fire  HINT carry fire extinguisher with you]

  5 carb  this is of course limited to inline chevy gmc motors I have no experience with these but common sense would say don’t run straight linkage on the street and use all carbs only on larger inch motors here you can run 3x2  or 2x3 and or dummy the others  hey they sure look cool!

  6 carb   my favorite here is the edelbrock X-1 cross ram with small 94’s or 97’s and progressive linkage. If you have a fairly healthy large motor [NOT CAM] you can run the end 4 carbs for primaries and dump in the center two for secondaries.I have some old linkage installation sheets that show 6x2 units set-up this way. This will give power increase in the mid range. For the rest, use centers for pri ends for sec. Try both ways to see what’s best for your car If using log style intakes, again 1 or 3” risers will help low end power and make it easier to linkage carbs All sec carbs should be synchronized together and all primary carbs separately Primary carb synchronization doesn’t have to match sec carb synchronization 6 deuce set-ups are not a good choice for 283 305 307 and stock 327 motors unless stick shift or high stall converter and 4:11 thru 4:88 gears are used [depending on car weight] you can however dummy the secondary units if you require that “look”

   Linkage ball bearing  swivel type is the easiest to use and set-up. Some carbs will need extended throttle shafts. I usually set arms to move between 10 o’ clock and two o’clock or vice versa. Sec arms should have the same angle at idle on all carbs. Make sure all sec arms are the same length [eye to eye] the same goes for the primaries. On 6x2, secondary tip in point should be the same on both sides. Straight pin style linkage takes more time to set properly but is well worth the extra time spent. Install carbs loosely on intake with ALL accelerator pumps disconnected so carbs flop open and closed [like butter] tip here when I do 97-94 style carbs I just loosely install the bases only [tip on 94 style carbs replace the ¼” base screw with a bolt so you can install centers/tops AFTER the bases are bolted down and linkage is set-up. This will enable you to also change carb jets without disturbing and having to re-set the linkage.  Be careful here not to strip the threads]

 with bases or carbs on start with the two  [pri or sec] that work together install rod /pins in arms. Now open and close linkage smooth??  Maybe. This is the key factor. Twist the loosely installed bases in and out, left and right until there’s absolutely no drag [binding] tighten bases and recheck for drag do the same for the remaining base or bases Secure linkage arm bolts and install centers-tops check for full throttle.  Another major problem with multi-carb units is “gas pedal over-pull”   this will cause your carb linkage to slip causing very frequent linkage adjustments and bending of rods make sure when your gas pedal is “floored” you are not trying to pull carbs PAST full throttle also check for full throttle. If you can’t change overpull condition by adjusting pedal linkage install a hood bumper stop [the adjustable ones used on early 50 60’s cars] thru the floor board under the gas pedal. Now you can adjust the bumper to regulate how close the pedal comes to the floor board and prevent over-pull

   Dual Quads. Cross rams, tunnel rams and inline low risers my pick here is the M/T staggered carb round cornered crossram. It has nice flow and a equalizer tube built in.  I haven’t try the two piece crossram [offy, edelbrock or camaro] so no comment here. If using a tunnel ram for street with front facing carbs you can run progressive linkage, same with inline low risers. I feel the best carbs for 2x4 are two 400 cfm AFB’s or Holley 390 - 450 cfm with vacuum secondaries beware of the 450 holley mechanical sec carbs.  These have no secondary squirters and will cause low end full throttle problems

  Next up jetting. These are only guide lines. Temperature altitude cams compression all have a factor on jetting

 3x2 rochesters 1x2 or 2x1 pri 059 sec 057

       94’s [15/16 bore size] pri 053 sec 057

       97’s pri 046 sec 045

4x2 weiand staggered

       94’s  [15/16] bore pri 052 sec 056

97's pri 045 sec 045

6x2 edelbrock X-1 crossram

97's pri 045  sec 044

 

keep watch for upcoming dealer linkage sheets  other carb info and other Neat Old Stuff  john       


spalding specs

 

  Here you'll create or edit a custom page for your Web site.
Use this template for any additional information you need such as products, pictures, fan clubs, links or just more information It's important to regularly change the content on your site and make updates to the information that you display. Doing this will help you to get more return visitors.


Use this template for any additional information you need such as products, pictures, fan clubs, links or just more information It's important to regularly change the content on your site and make updates to the information that you display. Doing this will help you to get more return visitors.

Add your link here

Here you'll create or edit a custom page for your Web site.
Use this template for any additional information you need such as products, pictures, fan clubs, links or just more information It's important to change the content on your site and make updates to the information that you display. Doing this will help you to get more return visitors.

Add your link here


This is one of my favorite images
This is my good friend Hal. I took this picture on his birthday. I think he likes to be in pictures.


This is one of my favorite images
This is my good friend Hal. I took this picture on his birthday. I think he likes to be in pictures.