Tuning strombergs

[wayne taylor]

OK:

 

Exhaust is a little on the sooty side. Lumpy tickover and 25mpg. All of this, to me says badly tuned carbs?

 

How do I tune the twin Stromberg 175CD's please?

 

Thanks

 

Wayne

[ChrisR-4A]

Hi Wayne, assuming your engine is standard or near to and the ignition & tappers are set correctly then firstly , check new or clean air filters, check that the diaphragms are in good condition, with oil damper caps off and engine running open throttle slightly and check pistons are rising together if not reballance carbs if in doubt replace Diaphrams.Next check there is none to minimal play on the throttle spindles and that the choke bar and return springs, especially the one connecting choke to throttle cam are working correctly and choke bar is fully off when choke cable is in.

Don't worry to much about the exhaust, this can go black on start up or with a bit of oil blow by , use the plug colour. When mine came out white and a bit too weak the exhaust still looked black.

If car starts and runs without choke nearly straight away it is rich, with temp at 12 deg C mine needs some choke for the first mile or so.

Try turning jet screws up (clockwise viewed from below) a couple of flats and

take car for a brisk drive and pull into layby and do a plug chop, say no1 and no 3, from rich to too weak can only take a few flats so if close go one at a time. Repeat plug chops next time 2 & 4 this saves time but when about right do all 4.

For more detail refer to the manual.

There is a procedure using the piston lifting pins but this has never helped me to get it right through the range.

[wayne taylor]

I strobed it yesterday as I have just fitted E.I. It runs very well but I wanted to ensure the timing was spot on. Having painted the pulley tdc in white I fired the strobe gun at it and couldn't see the damn mark no matter what i did with the distributer movement??? Why might that be?

 

I have also received this advice:

 

"...Take off the air filter and lift the piston 1/32 in. (1 mm) with a thin screwdriver. Listen to the engine note while you do so. If the mixture is correct the engine speed should rise slightly for a moment, then return again to normal.

If it rises and stays fast the mixture is too rich.

If the engine dies when the pin is lifted, it is too weak...."

Would you agree?

[peejay4A]

Wayne, are you triggering the strobe from no 1 spark plug? If so where is the rotor arm pointing (to which plug lead) when your TDC mark is in line with the pointer?

 

That sounds like the correct check for mixture, at idle of course. The needle will determine what happens at other speeds.

Edited May 27, 2013 by peejay4A

[bob-menhennett]

Wayne

The advice..as to engine response,, is correct.You can use a thin screwdriver or....

Use the piston lift pins to raise the pistons.Disconnect the carbs from each other by loosening the shaft clamps.You can "manually" accelerate both carbs and set each carb correctly.Then you can "join " the carbs back together again and adjust idle speed.Blip the carbs every now and again to clear unburnt fuel so you don't get false responses.

Bob

Edited May 27, 2013 by bob-menhennett

[keith1948]

Hello Wayne

Tuning Strombergs is a bit fiddly but once set up they are fine.

1. Ensure they are balanced using a carb balancer. Listening to air inlet with a tube is an alternative but not so accurate. Also ensure both carbs move exactly together. Sometimes there is a bit of play on the connecting spindle that allows one carb to move before the other.

Raising lifting pins fully and allowing to drop back you should hear carb pistons fall (remove damper to do this). Pistons should rise & fall easily. If not then either they need the sides of the pistons and the bore cleaning and/or they are misaligned.

To align then you need to loosen upper nut (bush retaining screw) at base and wind piston right up using lower mixture adjusting screw. Raise & allow piston to fall a few times as bush retaining nut is tightened and then wind adjusting screw (lower one) back down. Piston should rise & fall easily.

To set mixture strength raise piston lifting pin or use screwdriver to lift piston (after removing air filter). At correct strength engine speed should rise slightly and fall back. If it cuts out it is too weak if it speeds up it is too strong. Once set plugs should be light tan in colour.

I also use fast road plugs which run cooler NGK BP7ES to prevent run on.

Regarding strobe ensure at TDC the rotor arm is pointing to No 1 cylinder and not no 4 which would indicate distributor is 180 degrees out.

Turn distributor body clockwise to advance and anticlockwise to retard. Pulley is approx 427mm in circumference so 4 degrees BTDC is about 4.7mm clockwise from timing hole on pulley.

Keith

[wayne taylor]

Wow. Don't you just love the enthusiasm and support on this forum!

Thanks guys.