Arnolt tuning specs

[Kevin H]

Jim McQuay sent the following by email ...

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According the Arnolt Bristol owners manual, the static ignition timing is set at 5 degrees before top dead center with the ignition fully retarded. Contact breaker gap is specified at .010 to .012 (A bit narrower than I am used to). Spark plugs were originally KLG P10L80 single piece. Substitutes are Champion Z9 or Z10, or NGK C6E or C7E (six is hotter than 7). These all are 10 MM plugs with 5/8 head size and 3/4 inch reach.

Again, the manual calls for a plug gap of .018 to .020. Also much narrower than I am used to. Even my 74 Norton has a recommended gap of .024 to .028

As for the carburetor adjustments, these should be minimal when operating below 3000 feet of elevation IF the original jetting is retained. There are two idle screws. One controls the engine speed at idle and should be set at 750 to 900 RPM. All three carburetors should be adjusted the same and draw the same amount of air. This can be determined with a "unisyn" or possibly a manometer. Another method is to listen to each carb intake with a hose up to your ear.

The book calls for inserting a piece of paper under each slow running adjustment speed screw, adjusting them until a slight resistance is felt removing the paper, then turning in the screw one turn. (this screw is to the left, and works against the throttle stop. Start with the center carb.

The idle mixture is controlled by the mixture control screw. It should be turned out one full turn to start. This screw is just to the right of the slow running screw. Turning it clockwise (in) leans the mixture. Tuning it out richens the mixture. (opposite of Amal motorcycle screws)

All other adjustments are made by changing jets. Since I didn't operate my car at high altitude, I never messed with the jetting. I am sure that Bill Watkins or some of the other race car drivers can guide you better in this regard. I go by the theory that if it isn't broke, don't fix it!!.

If you live in Denver, you will need leaner main jets and corresponding air bleed jets. As a rule of thumb, reduce the main jet by one size for each 3000 feet of altitude. It is important to make sure the engine is up to 70 degrees C before adjusting the carburetors. Also you need to be sure that all of your valves are seating properly and you don't have excessive wear in the valve guides.

From a whole lot of years of experience, I can deduce that if the engine is running poorly, the odds are that it is an ignition problem. While not ALWAYS true, it is the first place to start unless you KNOW that the problem is elsewhere.

Regards,

Jim McQuay
West Linn, Oregon
A/B 3015 and 3016
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[John Simmons]

The odd thing is: on page 8 of section 1, the ignition time is given as 23 degrees before TDC static advance. I've been unable to reconcile this with the 5 degrees given on page 80. Could the 5 degrees be for one of the lower tuned engines? Of course, since I don't have the Bristol engine in my car, I haven't had to resolve this myself.