Thanks to you and your team at Lincoln Land we have been enjoying our 1971 Mark III. One issue though is a hesitation at about 1/4 throttle. When accelerating normally the engine looses power at about 1/4 throttle – no rough running or missing or coughing – just no power. When you give it more throttle it wakes up and supplies great power. This condition is consistent. I have not been able to find a vacuum diagram that shows where the vacuum advance connects but I have it connected directly to the carb base port – drivers side of the carb. This is where my carb rebuilder said it should connect. I’ve seen other Mark III’s that have the vac advance connected to the fitting at the thermostat housing. (I don’t know how this fitting should be connected to vacuum as currently nothing is connected to it. ) I may be incorrect but Im thinking the vac advance function is the causing the hesitation. The vac advance appears to function properly but I’m not sure that its connect to vacuum properly. Any ideas. I have all the manuals and vacuum diagrams but cannot find the vacuum circuit for the advance at the distributor. Thanks!
Hello John –
Not knowing the tune up history of your Mark regarding ignition and carburetor I need to say that the points, condenser, distr. and rotor must be in good shape as well as all of their respective adjustments etc.
The 1969 manual has a better diagram of the vacuum valve hose routing and I will have George send you a copy of that page. Not only should it be connected but it must be functioning as well. It can be tested with a hand held vacuum pump or other vacuum source. Newer style vacuum advance replacements are easily adjusted externally with an Allen key through the vacuum port as per the chart that is supplied with the replacement. This adjustment is very important! The initial ignition timing adjustment is also important to avoid a hesitation on acceleration. Problems arise with correct timing adjustment on these engines if the damper rotates on its rubber and shifts the timing mark. At this point the correct timing cannot be attained with a timing light. A new or rebuilt damper will be necessary or the timing will need to be set by ear and a road test. The road test is to advance the timing in steps slowly until good power is noticed to the point of some pinging on acceleration and then retarding the timing until the pinging is only very slight on heavy acceleration.
If all of the above is in good order, the fuel quality and a possible internal carburetor problem may need to be considered. Did this issue surface after a tune up or the carburetor rebuild or?