Service

SERVICE

Very-hot-rod Suzuki

I have a 1977 Suzuki RM80 that has been bored to an unknown displacement. It has a Wiseco piston, chrome rings, 30mm Mikuni carb, oiled-foam slip-on air filter (the stock airbox can’t be used) and other suspension and transmission modifications. The problem is loss of power when the engine is hot. When first started (cold), it develops an unbelievable amount of power. It hits a powerband about three-quarters through the rpm range that will pull the front end over in first gear even if you sit on the bars. After the bike is warm, however, there is a noticeable loss of power. The powerband still is there but doesn’t have much kick, and adjustments to jetting and such don’t seem to help. What could be the cause?

A. Cross

Nashville, Tennessee

Heat is the culprit. While the modifications to your RM80's engine allow it to generate much more power; they cause it to generate much more heat, as well. But you have not also increased your air-cooled engine's ability to dissipate enough of that additional heat. So, the engine operates at too high of a temperature. This causes the cylinder to overheat and distort, reducing the sealing efficiency of the piston rings; excess heat also causes the incoming mixture to heat up too rapidly as it first contacts the crankcase surfaces below the piston, which causes the mixture to expand too quickly as it is pumped into the combustion chamber. That means a smaller volume of mixture in the combustion chamber; which translates directly into less power.

There is no easy, good solution, since your motor was not engineered to dissipate as much heat as it is now making. A decade or so ago, some aftermarket firms sold cylinder heads with more finning, but they only dealt with part of the problem. The bike manufacturers ultimately came up with the best solution to the problem: liquid-cooling.

What stopped again ?

To replace one that was wrecked in my youth, I bought another BSA (Bastard Stopped Again), even though I knew I would have nothing to show for it but grief and a footdeep pile of parts receipts. This one is a Hornet model, with the same

two-carb, 650cc engine as the Lightning, but with a strange, all-AC electrical system (no batteries or diodes). The problem is that the bike seriously overheats on hot days or when climbing long hills. It will suddenly sound and act like it’s seizing, but after a few minutes of cooling off, it will start again, run a few miles and cut out again. The advance mechanism on the point cam works, but won’t tolerate any retard/ advance tinkering, and the engine will neither start nor wind up if the advance is turned a hair either way. The service manual calls for Champion N3 plugs, but I’ve been trying hotter, N5 plugs to stop fouling. I’ve had the cylinders bored .010-inch oversize, installed a new crankshaft bushing and have checked everything I could. It has K&N Triumphtype air filters, and I suspect the carbs may have undersize jets. What can I do to make this old clunker run for more than 15 miles without something bizarre happening?

Jerry O’Shaughnessy

Pittsburgh, Pennsylvania

There are numerous possibilities, but it sounds as though the problem most likely is with the breaker cam and advance mechanism. The batteryless ignition on your Hornet is a finicky arrangement called an E. T. (Energy Transfer) system, and it uses a different breaker cam and advance unit than do the battery-powered BSA Twins. The E.T. breaker-cam assembly is Part No. 19-8111, while its battery-ignition counterpart is Part No. 19-8112. The mechanisms look very similar and could easily be mistaken for one another by someone not intimately familiar with BSAs; but the E. T. breaker cam has a much shorter open duration, and its flyweights advance the ignition timing only 12 degrees, as opposed to 25 degrees on the battery-ignition models.

Our advice is first to ensure that your Hornet has the proper breakercam unit, and then to set the ignition timing at 37 degrees BTDC with the breaker cam in its fully advanced position. If the machine still refuses to run properly, we'd suggest you call one of the numerous British-bike parts and service establishments across the country’ (many of which are advertised in this magazine) and solicit their help in tracking down a service manual that outlines the rather complicated process of troubleshooting the quirky E. T. ignition system.

Mix-or-match rubber

After reading your test of the Ducati 851 Eight-Valve in your April issue, I have a question about tires. The 85 1 runs a bias-ply Michelin on the front and a radial Michelin on the rear, but I’ve always been told that you shouldn’t mix bias-ply and radial tires on the same vehicle. What gives?

Steven Kemperer Panorama City, California

Tire companies used to recommend that radial and bias tires not be mixed on automobiles, but these days they seem to think that's acceptable practice so long as the same types of tires are used on the same axle. But they've never made any such recommendations with motorcycle tires. In fact, several motorcycle manufacturers have sold models fitted with a radial rear tire and a bias front, including the first bike ever to have a radial as original equipment: the '84 Honda VF1000R sold in Europe. And just about all modern GP and Superbike racing motorcycles use a radial rear tire and a bias-ply front. With current motorcycle-radial technology, that combination seems to give the best overall handling characteristics for world-class racing competition. 0