new Sq 4 Mk II rebuild is running lean

[pauldridge]

Just finally got my MK II back together again after a long and expensive 2.5 year rebuild.

Mine is a '53 with Solex carb.

The engine runs very nice.. smooth, no dead spots in acceleration, and ticks over at 500-600 rpm. However, it seems to be running hot, confirmed by brown coloration on the exhaust pipes, and beyond toasty spark plugs.

I am wondering if the idle mixture screw/jet on the Solex could be contributing to this? I disassembled and cleaned the carb and jets during the rebuild, but didn't take note of what position the idle mixture screw should be (number of turns out from fully closed).

If anyone has a suggested starting point for the mixture screw, or any other suggestions on what may be contributing to my lean running, I'd appreciate your opinion.

Tomorrow I will more carefully check the ignition timing.

Phil
1953 Sq 4 Mk II

[paul.jameson]

If you solder a length of brass rod into the screwdriver slot of the idle mixture screw you can turn it using a screwdriver from the side without burning your fingers when the engine is hot. This should give you a good chance of getting the mixture right at minimal throttle openings. If the butterfly spindle is worn it will let air in and weaken the mixture but I would not expect this to be a problem by the time you get to say 1/4 throttle open. Do check the jet sizes though. I have a feeling that one of the jets has a different size between the Mk I engine and the (1953) Mk II.

[pauldridge]

Thanks Paul.. I was trying to determine a scheme to get in there without permanently branding myself!

[simon.holyfield]

Phil, I start with the mixture screw 1.5 turns out - I'm pretty sure mine is currently further out than that, but it should get you started.

Have you seen Mark Walsh's thread about float height? I have one fibre washer under my float valve and my bike is running rich.

I recorded my jet sizes in this post:

http://ariel-square-four.blogspot.com/2 ... ecord.html

Paul's right about jetting - Waller says 1953 bikes have a 170 air correction jet and a 060 pilot jet - earlier bikes are 150 and 070.

[pauldridge]

Thanks Simon and Paul.. This carburetor functioned flawlessly on the previous engine, before my dreaded thrown rod episode, and I haven't changed jets in the interim. Tomorrow I am going to pull the the carb and re-check that all jets are clear. I did disassemble and run it through the ultrasonic washer, but, who knows, maybe some trash got in there. And, I'll look at float height as well.

Phil

[nevhunter]

Make sure your float isn't perforated. See/test how it floats in petrol. If you pack it up don't use a washer that has any real weight/mass. Nev

[pauldridge]

Thanks to Paul Jameson for the suggestion to solder a piece of brass rod to the idle mixture adjust screw. I found a rod at my shop the perfect diameter to hammer into the screw slot, and successfully soldered it into place. With the carburetor off, I learned that, during my carb "overhaul" I must have never set the idle screw, as I found it to be a good 3+ turns out. Back on the engine, with mixture set at 1.5 turns, the old gal started up without protest. Once warm, I was able to experiment with screw adjustments and found the sweet spot to be somewhere south of the 1.5 turn suggestion.

prior, I had been having to open the throttle some good 3/4 of the way to start, which I knew wasn't right, and even then, took a good 10-12 kicks to get her going. This time, with proper adjustment, the following day, I started the cold engine with a single kick.

Life is good! Amazing how a little screw jet can affect the operation of the engine so much!
Phil

[simon.holyfield]

Good stuff Phil.

By 'south', you mean slightly more than a 1.5 turns?

[pauldridge]

Guess that's a Texas term!
No a bit LESS than 1.5 turns out