Multiple Carby's

[Rod]

Hi All,

I have been meaning to do this for some time now and what better opportunity to do so on a rainy Sunday afternoon. I apologise before I begin if this post is too long.

I have twin strombergs on my grey (slight port work, electronic ignition-pertronix, otherwise stock) and haven't been entirely pleased with their performance since their installation. Its come to light really in the last few weeks when I realised how sluggish it can be going up hills and even when pulling a 6 x 4 trailer (often having to drop back to second gear). As mentioned I have a pertronix installed electronic ignition under the cap which I plan on getting regraphed soon and the carbys have 48's for mains (after initially having 51's) and stock Power Bypass Jets. I purchased a carby balancing tool a number of years ago and I am confident that I have them synchronised as per the kind help of CraigA instructions. Fuel economy wise I am relatively happy- 22-24mpg around town and 30mpg on the open road. This post isn't intended for members to comment on why the old girl maybe sluggish as I will work through that, its to find out what set up others may have for their multiple stromberg carby's.

I have been doing a far bit of investigation today and have been presented by atomisation, Power bypass jets, venturi restrictors, drill bit numbers and it goes on. This search has occurred within this site, the FB EK Forum and other general areas.

For twins the first thing I would like to comment on is that the main jets generally have been reduced from 51 thou to somewhere between 46 and 48 thou while the power bypass jets have been reduced from no.67 drill to no.70 (yes reduced as with drill numbers the higher the number the smaller the diameter of the opening http://www.accuratebushing.com/pdf/ENG-FractionalChart.pdf). Another common thing (which I haven't got) is venturi restrictors. From what I understand is that they increase the velocity of the air moving through the carby which increases the atomisation of the fuel hence better combustion. Can anyone comfirm this? I have also found that Holley 350 (7448) Venturi sleeves will also fit. If anyone has these fitted could you please comment where they are installed. Is it above the choke or between the choke and throttle butterfly?

As mentioned at the beginning the real purpose of this post is for members who have multiple carby's to list down their set up. Having looked through this site, questions about multiple carby's come up all the time. If we can condense this into one thread (and maybe make it a sticky) its going to benefit others in the future.

To do so I thought it would be good to do it under the follow headings:
- Number of Carby's
- Main Jet Sizes
- Power Bypass Jet Sizes
- Venturi Restrictors (yes or no)
- Engine Mods other than standard (cam, port work etc...)
- Manifold Make (maybe???)

Photo's of linkages would also benefit others.

I will try and find CraigA's instructions on synchronisation of multiply carby's and attach.

I trust that this info will be benefit to others in the future considering there are many multi carby manifolds been purchased for good money on ebay. That alone indicates that there are others out there looking for such info.

Cheers

Rod

[Rod]

Hi All,

Find below CraigA's tuning approach from way back in July 2002.


  Re: Twin Carby Tuning.
« Reply #1 on: July 12, 2002, 03:46:06 AM » 0   Quote 

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Hey Rod,

After owning a few red motor Holdens with various Twin and Triple carby setups I would recommend the following steps:

1. Check the timing and other electrical parts and set or renew as necessary.

2. Confirm the jet sizes in your carbies are the same and note the size. You may have to change the sizes up or down depending upon your engines requirements. This will be affected by valve size/porting/exhaust etc - you obviously have to be able to burn the fuel you pump in, otherwise there's no use putting it there

3. Buy yourself (or borrow) a carby balancer (Rares Spares has them). This tool will help you to accurately setup the linkages and is a key to ensuring the carbies supply even amounts of fuel, both under accelaration and at idle. Spend lots of time and care here and you will be rewarded with smooth idle and power delivery.

4. Set the idle mixtures using an analog tacho, gas analyser, or vacuum gauge.

TACHO: Adjust each mixture screw in all the way then reverse out a couple of turns and start the car. Now adjust each screw in/out until the maximum idle speed is achieved.

GAS ANALYSER: Not normally available at home  but the most effective method. Mixture should be set so that CO2 levels are at their maximum, about 12 to 15% for most engines - never had a grey on an analyser though so this figure may be lower.

VACUUM GAUGE: Using the vacuum gauge is similar to the tacho except you adjust for maximum vacuum minus approximately 1/2" Hg. This can be difficult if you have a larger than standard cam as the idle vacuum will fluctuate due to the valve overlap. If this is the case use the tacho.

These are general rules but should guide in the right direction.

Good Luck,

Craig.
 
 

[FCwagon]

It's no good trying to push more fuel into the engine if it can't get the exhaust out. Any multiple carby setup must run with headers or extractors. The standard exhaust manifold offers no performance.
Leigh