Vacuum advance versus mechanical advance

[Gary Schisler]

vacuum advance versus mechanical advance gary schisler i am trying to eliminate all other possible causes of overheating in my before i spend the big money on a repro radiator i know that bad timing can cause overheating but i am a little confused about the relationship between mechanical and vacuum advance anybody out there who can give an accurate desciption of when each works and how to reliably eliminate each as a possible cause of overheating how much mechanical advance should be built into a stock distributor and what is the most vacuum advance i should see and when should it be seen i have everything i need timing light dwell rpm meter vacuum gauge and a pretty good knowledge of all that is required to do this work i have checked the dwell right at factory specs timing is degrees btdc vacuum line plugged idle is set at rpm this car only gets hot at highway speeds i can drive all day in degree weather around town with a normal temp put it on the highway at or greater and it will slowly creep up any pro tune up persons out there thanks gary

[Rolf Krueger]

Re: Vacuum advance versus mechanical advance

re vacuum advance versus mechanical advance rolf krueger the centrifugal advance advances spark with increasing rpm up to a pre set limit this is necessary to get proper combustion at higher rpm the vacuum advance advances spark to a maximum preset level at no load high vacuum conditions once you apply throttle to increase engine power the vacuum decreases manifold pressure increases and the vacuum advance retards spark from the no load maximum this is done to reduce the tendancy to detonate or ping under engine load each gm distributor number has a specific and unique contrifugal and vacuum advance characteristic or curve this is called out in the specification section in the back of the gm service manual for your year car under the distributor listing it is generally not practical to measure or set these curves with the distributor on the car you must remove your distributor and take it to a shop that has a distributor testing machine designed to do just that while not many places have this type of machine today you can still find places that do by looking around for electrical and ignition service shops you can generally get a yes no indication if the vacuum advance is working by having someone disconnect the vacuum hose from the distributor while you have the engine runnimg at idle and are watching the timing mark with a timing light at idle the vacuum advance should give you an instantaneous degrees or so of advance ie a big number the actual number will vary for different distributors one final point vacuum advance units are not adjustable and they all look alike from the outside if someone inadvertently installed the wrong one that had considerably more or less max advance than it should for your distributor you could have a problem that is hard to diagnose that is why the older gm units had the last three digits of the part number and the two digit maximum advance degrees embossed on the outside of the control unit beginning in the late 's gm introduced the transmission controlled spark tcs system to meet emission limits this locks out the vacuum supply to the vacuum advance unless you are in high gear or reverse gear with tcs the vacuum advance only functions in high or reverse i don't believe tcs was installed in mid year cars but i am not sure unless your vacuum advance is not working at all i doubt that spark advance is the cause for your cooling problem engine temp creep at constant freeway speeds is generally caused by restricted coolant flow this can be the result of restricted radiator coolant passages a bad water pump loose water pump drive belt thermostat that does not fully open or some other coolant flow restriction

[Gary]

WOW! Thanks for the help.

wow thanks for the help gary i figured that someone would be able to help here i do appreciate this considerably your answer was clear concise and easily understood there was only one problem you more or less confirmed my thoughts about my heating that this is a radiator problem i'm still going to check the distributor there is a place not far from me that does this work reliably i might as well eliminate every possible area before i spend on a new radiator thanks rolf

[Bob Patton]

Re: Over heating problem

re over heating problem bob patton gary try removing the license plate it worked for me on the se road tour temp ran about or at least degrees cooler at mph for extended periods bob

[unknown]

Re: Over heating problem

re over heating problem unknown thanks for responding to my posting i would like a little more information if you have a minute before you removed your license plate what temp was your car running at on the highway would it continue to creep up or did it stabilize what year do you have big or small block how does your car run around town mine will run in the range in town all day long and very slowly creep up to at highway speed i ran at mph last summer for over an hour on the interstate it never got above degrees but when i shut the car down i could hear the coolant at or near boiling you could also feel it in the upper radiator hose i am trying anything to avoid replacing this radiator common sense and a fair degree of knowledge tells me that this is hopeless but i still have to eliminate all other areas first i also have a guy near me who owns and runs a radiator shop he has a coupe and i intend to see if he can run a flow check on my radiator to test it

[Gary Schisler]

Re: Over heating problem

re over heating problem gary schisler well i just returned from a nice mile drive in my car all is well until mph and above while i was out i stopped at a radiator shop the owner has a coupe that he has owned for years he diagnosed my problem in seconds the bottom half of the radiator is hot the upper half is very hot verdict a plugged bottom half that is uncurable we talked for about minutes and came to the conclusion that there cannot be any other explanation for the temperature difference all of the other factors add up to a new radiator good by