2ndgen
Member
or how I learned to love my vacuum gauge!
Progress continues on my Mach restoration, but I have to admit I was pretty let down by the initial startup of the engine. Frankly it idled like crap and made my eyes sting from unburnt hydrocarbons... I was worrying that I had over cammed my stroker and that my car was not going to be much fun in city traffic.
This is my first stroker build (a 6.2 rod 393w) and although I did a lot of research it is fair to say I was pretty nervous... particularly about that cam. It is a hydraulic roller just under 240 degrees duration at 0.050; by far the largest I have used. All the articles and posts I read said that the extra cubic inches would tame this cam - not to a stock idle but still within the "streetable" limits with a lopey idle. The problem was I could not make it idle worth a damn!
First I checked cranking pressure... and found I was between 185 and 190 on all cylinders - just where I wanted it. My compression ratio is just a bit over 10:1; once again just where I wanted it. And yet the plugs were black and wet with gas...
I had checked TDC with a piston stop and had the initial timing set to 12 degrees with a total mechanical advance of 36 degrees. I am running a vacuum advance too.
But the car just would not idle below 1000 RPM and the idle mix adjustment screws seemed to have no effect. I could close them all the way down - obviously I idling on the main circuit, not the idle circuit...
Back to the internet and a ton more research... was I going to have to swap cams???
First problem - how to get on the idle circuit... Verified with a mirror that the throttle blades were cracked open. Notice that the throttle return spring has barely any tension... hmm. Shortened the throttle return spring and watched the throttle action... hmm... throttle still not closing all the way.
Second problem - unhook the throttle cable and notice the linkage closes firmly and fully. Inspect the throttle cable set up (new) and find slight binding on firewall insulation pad (new). Enlarge the opening on the firewall pad - better but not perfect. Finally figure out that the throttle cable is just about 1/8th inch too short. Adjust and now the linkage and pedal operate correctly and the throttle closes fully when released.
Third problem - now that the throttle closes fully, I am on the idle circuits and the mixture screws begin to work. Adjust the idle mixture screws and my eyes no longer burn... (have not checked the plugs yet; will do after I get some miles on the car. It is currently registered non-operational).
Fourth problem - engine idles very rough/choppy at 900 RPM... Internet research revealed an article on setting initial timing using a vacuum gauge... Using this method I find that I can pick up 4 inches of vacuum (went from 10 to 14) by advancing the initial timing to 18 degrees. Recurved the mechanical advance to a total of 34 degrees all in by 3000 RPM. Engine now idles much better! I can drop the idle down to 700 in gear with my foot on the brakes and get a nice lope. The engine runs slightly cooler too. The basic message was every engine is different, the stock timing is for stock components and merely a starting point, the vacuum gauge measures the true efficiency of your individual engine.
Fifth problem - and sure to cause a controversy. In my research I found an article written by the engineers at GM who invented vacuum advance... They were quite vocal about the ported vs. manifold controversy. They claimed ported/timed vacuum was an abomination created by the smog/emissions restrictions and was solely used to retard timing to increase exhaust gas temperatures - thereby reducing emissions. The stated quite clearly that any performance engine should use manifold vacuum. So I jumped ship and went for it... now at idle my engine pulls close to 30 degrees of timing and loves it. The throttle response is great too...
So after a bunch of research and a couple hours tinkering my engine is dramatically different/better. All my cam concerns are gone! Hope this helps someone else...
Good luck,
-Rory
Progress continues on my Mach restoration, but I have to admit I was pretty let down by the initial startup of the engine. Frankly it idled like crap and made my eyes sting from unburnt hydrocarbons... I was worrying that I had over cammed my stroker and that my car was not going to be much fun in city traffic.
This is my first stroker build (a 6.2 rod 393w) and although I did a lot of research it is fair to say I was pretty nervous... particularly about that cam. It is a hydraulic roller just under 240 degrees duration at 0.050; by far the largest I have used. All the articles and posts I read said that the extra cubic inches would tame this cam - not to a stock idle but still within the "streetable" limits with a lopey idle. The problem was I could not make it idle worth a damn!
First I checked cranking pressure... and found I was between 185 and 190 on all cylinders - just where I wanted it. My compression ratio is just a bit over 10:1; once again just where I wanted it. And yet the plugs were black and wet with gas...
I had checked TDC with a piston stop and had the initial timing set to 12 degrees with a total mechanical advance of 36 degrees. I am running a vacuum advance too.
But the car just would not idle below 1000 RPM and the idle mix adjustment screws seemed to have no effect. I could close them all the way down - obviously I idling on the main circuit, not the idle circuit...
Back to the internet and a ton more research... was I going to have to swap cams???
First problem - how to get on the idle circuit... Verified with a mirror that the throttle blades were cracked open. Notice that the throttle return spring has barely any tension... hmm. Shortened the throttle return spring and watched the throttle action... hmm... throttle still not closing all the way.
Second problem - unhook the throttle cable and notice the linkage closes firmly and fully. Inspect the throttle cable set up (new) and find slight binding on firewall insulation pad (new). Enlarge the opening on the firewall pad - better but not perfect. Finally figure out that the throttle cable is just about 1/8th inch too short. Adjust and now the linkage and pedal operate correctly and the throttle closes fully when released.
Third problem - now that the throttle closes fully, I am on the idle circuits and the mixture screws begin to work. Adjust the idle mixture screws and my eyes no longer burn... (have not checked the plugs yet; will do after I get some miles on the car. It is currently registered non-operational).
Fourth problem - engine idles very rough/choppy at 900 RPM... Internet research revealed an article on setting initial timing using a vacuum gauge... Using this method I find that I can pick up 4 inches of vacuum (went from 10 to 14) by advancing the initial timing to 18 degrees. Recurved the mechanical advance to a total of 34 degrees all in by 3000 RPM. Engine now idles much better! I can drop the idle down to 700 in gear with my foot on the brakes and get a nice lope. The engine runs slightly cooler too. The basic message was every engine is different, the stock timing is for stock components and merely a starting point, the vacuum gauge measures the true efficiency of your individual engine.
Fifth problem - and sure to cause a controversy. In my research I found an article written by the engineers at GM who invented vacuum advance... They were quite vocal about the ported vs. manifold controversy. They claimed ported/timed vacuum was an abomination created by the smog/emissions restrictions and was solely used to retard timing to increase exhaust gas temperatures - thereby reducing emissions. The stated quite clearly that any performance engine should use manifold vacuum. So I jumped ship and went for it... now at idle my engine pulls close to 30 degrees of timing and loves it. The throttle response is great too...
So after a bunch of research and a couple hours tinkering my engine is dramatically different/better. All my cam concerns are gone! Hope this helps someone else...
Good luck,
-Rory
