Tech Q&A
Richard Simonds
Replacing classic tires
Q
I have two classic Mercedes-Benz cars that originally came with 185HR-14 Firestone Phoenix or Continental radial tires. At the time, the aspect ratio was typically 80 (European) or 78 (American). Over the years I have used S-rated 205/70-14 or 195/75-14 tires as replacements, but have had clearance problems with the larger tires rubbing on fender panels and also being too big for the spare-tire space in the trunk. Another problem with the replacement tires that will fit in the spare-tire space is that they are not the same diameter as the original 185HR-14 tires, which leads to speedometer error (showing faster than my actual speed) and slightly higher engine rpm at the actual speed. There are fewer and fewer sources for tires to fit 14-inch wheels – virtually none from the major manufacturers. Further, I prefer whitewall tires that were typically supplied in the United States in the 1950s, ’60s and ’70s. What can I do to get my two classics back to original-spec tires?
A
The good news is that there are companies supplying tires for all ages of classic, vintage and antique vehicles. The bad news is that they are not made in large volumes, so the price is higher and the less popular sizes may be at the end of their date-code life when you purchase them. The date code is molded into the sidewall of tires to show when they were manufactured; current recommendations are that tires should be checked frequently after five years, and replaced after 10 years of manufacture, regardless of mileage or tread wear. Having a classic that is not driven many miles each year has maintenance costs that are not related to usage. For that matter, so are annual oil changes, lubrication, brake- and clutch-fluid flushes, antifreeze replacement and batteries, etc., that need to be performed regardless of usage.
To answer the basic question, many sizes and types of tires from older cars can be found by doing an internet search: Companies I have used to find these tires include Coker Tire, www.cokertire.com; Universal Vintage Tire, www.universaltire.com; Performance Plus Tire, www.performanceplustire.com; and Tire Rack, www.tirerack.com. Regardless of a tire’s size or style desired, one of these sources will likely have what you want.
Each of these sites has its own pricing for the same tire brand. Some will ship directly to your tire installer – or their recommended tire installer for mounting, balancing and warranty. Some will ship only directly to you to find your own installer. Be sure that your tire installer uses road-force balancing to ensure the best ride and best tire wear on your new investment.
Specific 185-14 size tires include:
Vredestein Sprint Classic 185HR-14 (blackwall, 25.6” diameter)
Michelin MXV-P 185HR-14 (blackwall, 25.6” diameter)
Phoenix 185R-14 (S-speed rated, 3/4” whitewall, 25” diameter)
Maxxis 185/75-14 (S-speed rated, 3/4” whitewall, 25” diameter)
This advice was specifically provided for the 185HR-14 radial tire, but the basic ideas are applicable if your classic, vintage or antique Mercedes-Benz has 13-inch or larger wheels that need tires.
Starter solenoids
Q
I have a 1972 280SEL 4.5 that does not want to start when it is hot. The car starts immediately when it’s cold, or even in very hot weather when the engine is relatively cool (well below normal operating temperature). However, after driving for a while, especially in hot weather, if we let the car sit as we run errands, it will not start. This happened most recently when I passed the Virginia Safety Inspection, but couldn’t start the car to back out of the inspection stall. This wasn’t a safety issue, but it was very embarrassing. Upon turning the key, there is no sound from the engine compartment; no click, click, click – nothing. A jumper cable? Still nothing. We let it sit overnight and it started immediately.
A
This concern has starter solenoid written all over it. The Bosch SR59X starters were notorious for this symptom. Move the shift selector from park to neutral – foot on the brake, of course – when the no-start issue occurs to rule out the neutral safety switch. Be sure you have tight, clean electrical connections, particularly the ground connections, as these are paramount as always. Because you have already replaced the ignition switch and checked the other wiring and ground connections, replacing the Bosch SR59X starter should solve your problem.
Ignition issues
Q
My 1965 230SL started misfiring under full acceleration above 4,000 rpm, but is smooth as can be under 4,000 rpm and while cruising along without full acceleration. I recently had my annual tune-up (valves, timing, oil change, etc.) The fuel-injection system and the engine were both rebuilt a few years ago and it had been running flawlessly. Any ideas?
A
In response, we recommended a visit to a good mechanic who could test various components. The mechanic found that the problem was ignition-related with misfiring between 4,300 and 4,500 rpm, but not above and not below that range. Removing the distributor cap, the mechanic found “dust and debris that seemed to be coming from the distributor cap and rotor.” Cleaning it out with an air nozzle and wiping everything clean eliminated the problem. NOTE: This car has a 123ignition system and, like many other electronic ignition systems, it has an optical pickup to trigger the ignition pulses to the coil and on to the sparkplugs. The debris was affecting the optical pickup function. If you’re using a 123ignition system, you should add distributor-cap cleaning to your periodic maintenance routine.
Ignition and fuel distributors
Q
My 1987 560SL had developed an engine stumble under acceleration, but once I got going, everything was fine. I put new injectors in it last fall and things were pretty smooth until recently. Doing my own repair was leading me down the path of lots of expensive parts that did not fix the root cause of the problem. I finally took the car to an experienced mechanic and he found two defective spark plug caps. With the ignition sorted, cylinder No. 8 was still misfiring. The problem was in the fuel distributor, so I sent it to CIS Flowtech for a rebuild; it turned out the replacement injector for that cylinder I had installed last year was bad. At least my car is back and I drove it to work today. What fun! [First published on mbca.com Technical Forum]
A
This story has two morals: There is just no substitute for having a skilled mechanic equipped with the proper tools and diagnostic equipment assess the problem(s) without just throwing parts at it in the hope that something will work – and – even new replacement parts can be defective. Through my years with a Mercedes-Benz SL that has mechanical fuel injection, I have had problems with check valves in the injection pump and with new fuel injectors that were defective, both of which contributed to misfiring at startup and at low rpm, but that ran fine starting around 2,000 rpm.