Thanks to everyone for there replies and great info.  Looks like I need to correct my valve lash first because i'm way out but im going to order a few sets of needles to try to get this thing dialed in.  

 

 

Put the numbers above into //www.mintylamb.co.uk/suneedle/ to compare them. AN is way too lean.

 

#5 works perfect in my almost stock 1275, so I think the Dr. is right on for the 6 and7 for slightly bigger.
Maybe the BG is way too rich

AN needles work in single HS2s, but are far too lean for twin HS4s you have.

I find CP4, #6, and #7 needles are on or near the money for twin HS4s on a 1275 or bigger..

twin HS4s are a lot of carburetor for an A series, so there might be more challenge to getting it to run right

When dealing with an unknown set of carbs (even new ones), it is wise to check several key mechanical things first, just to make sure (because these things can have you going in circles getting nowhere, causing you trouble and confusing things if they are not right).

Here is a link to an excellent guide for setting up a pair of carbs: //www.dbraun99.com/Setting_SU_Carburetors%C2%AD_Rev_C.pdf

And here is my checklist for things to make sure of BEFORE setting them:

Start with making sure the valve adjustment in the engine is right, and then go through this list to make sure the two carburetors are mechanically matched, and then, only after those things have been confirmed, balance and adjust the carbs).

Before touching your carburetors, make sure that your valves are gapped properly, and that your ignition is working perfectly, with no drift (with the engine running, monitor the timing marks with your timing light, any drift of the marks more than a degree or two, back and forth, should be a warning that it is time to rebuild your distributor). Make sure the points, condenser, wires, plugs cap and rotor are all perfect. Use only a red rotor from Advanced Distributors, the black ones have quality control issues and are not trustworthy. Most "carburetor" problems are caused by the ignition, or by distributor wear.

After ruling those (most likely) things out, if it still does not run well, then move on the carbs. Dealing with a set of twin carbs that are not running perfectly, it is wise to start with the basics and work your way through the whole shebang systematically. Otherwise, you can end up chasing your tail unnecessarily.

The key to getting a twin carb set up to idle really smoothly, and to make a really smooth transition during throttle "tip in" (when you first apply the throttle) is first making sure they are mechanically matched, and then, only after that is confirmed, balance their settings. The mechanical match is important to check first because of wear and DPO who mix up the parts, over the years.

- balance mechanically by making sure both pistons fall at the same rate (target 3-5 seconds, with the spring and oil removed, and with the piston vent holes plugged by some clay or tape). The Bentley manual includes instructions for how to do it, and John Twist posted a really good video on youtube where he shows how to do this. If the pistons were swapped, or damaged at some point they might be mismatched to their mating bell, making idle and tip in both less than smooth.

- confirm the float levels are correct by removing the piston and bell, pull out the choke and turn the key on and let the pump raise the fuel to its shut off height. Look down the jet hole and measure and make sure both fuel levels are within 0.010" of each other.

- make sure both throttle shafts are tight in their bores, with no slop and no vacuum leak (check by spraying aerosol all around them while the engine is running, if there is any change, then replace the shafts

- make sure the throttle plates do NOT have the little spring loaded pop-off valves in them. If so, either solder them shut, or buy new ones that are solid

- confirm that both throttle plates are centered in their bores and fully shut off the throat when closed (a mis-aligned throttle plate can drive you crazy until you identify and fix it).

- confirm that both jets are set to the same level (rather than counting flats, it is best to measure it directly, with a caliper (remove the dashpot, and measure straight down with the depth gauge on the other end of your caliper). 0.070" is the correct initial setting for HS carbs.

- make sure both springs are the same (HS2 should be blue, 2.5 oz @ 2.625" compressed length)

- make sure both needles are the same, stick out of the pistons the same distance, and are properly aligned and not bent (HS2 fixed needle starting point = AN, spring biased needle = AAC)

- make sure the vacuum vent (the extra hole near the mouth of the carb) isn't blocked by your air cleaner gasket

- make sure the choke cable is adjusted so that it goes completely off (with about 1/8" free play at the knob) when the knob is pushed in, and that both of the jets return promptly to "off" when it is released.

- make sure there's no vacuum leak, either in the intake manifold (there are a lot of plugged holes in a stock intake manifold that need checking), around the manifold x head gasket, carburetor gaskets, or in the distributor advance tube, or in the crankcase ventilation system (use the aerosol test while idling, to check these all out). A Midget with the charcoal canister crankcase ventilation system should have a sealed gas cap and sealed oil filler cap, and both were intended to be replaced annually, to guarantee their seals don't wear and cause a vacuum leak, harming how your engine runs.

After all of these things are confirmed to be matched, then do your balancing of the two carburetor adjustments. There are several steps in the manual, the key point is this:

- loosen the connectors between the two carbs, and set them so both flow air at exactly the same rate at idle (use the most sensitive balancing tool you can get)

- re-tighten the connectors between the two carbs and double check they both flow the same rate at wider openings (if they are balanced at idle, AND if all of the above mechanical stuff is confirmed, then they will automatically flow the same rate at all speeds).

Here is another link, this one to a guide for how to balance the mixture "the easy way" (it is opposite of how the manual describes it, but the point is it makes it easier to tell when the mixture is good, and is very easy to do):

//www.chicagolandmgclub.com/driveline00/0209/p08.html

Hope this helps,

Norm

Thanks for the info guys!  Looks like i didnt put the right search criteria in to find what I was looking for.

Spitz,the current needles are BG.

What needles are in there right now??

//www.minimania.com/SU_Carbs_-_Quick_reference_needle_selection

There are several other articles just use the search on this site.

Haynes has a SU carburetors manual that covers tune-up, overhaul, has specification tables on type H, HD, HS.

How to Modify you Mini - Vizard ... and other good references are out there. 

 

Hey All,

Just installed a dual HS4 carb setup on a freshly built 1380 and need some tuning help.  The carb setup was purchased used from someone that had them installed on a 1380 also.  I took the chance of just installing them hoping they were dialed in but that wasnt the case.  Engine fired up fairly easily but idle was all over the place.  I had to constantly adjust the idle screws to keep it running for proper break in.  After about 30 min it finally just killed.  You can smell that its running really rich throughout and the plugs confirm this.  I messed with them for about another hour or so and cant get it to idle consistently or not foul the plugs.  So here are my questions:

1) Is there a basic setup I can use so I can even begin to dial them in?  Lets say idle screw 1 1/2 turns out and a start point for idle enrichment nut?

2)Any recommendations for needles and jets for a mild 1380?

 

Thanks,