General Tuning Advice
Quite often we get people emailing us for advice on tuning their engines. Of course it's VERY hard to do without actually being able to see the car or hear the engine run to do our own trouble shooting and testing, but if we have time to respond, we try to give it our best. Here is a question we got from a guy from Australia with our response that may help some of you with similar issues.
"I have a 393ci stroker. Been trying to get it to run properly for 4 years. Its hard to start when cold and it fouls plugs all the time. Tried colder and hotter plugs, same deal. Have had "expert" tuners set up timing etc but to no avail. Can you please advise on where to start. It has a Crane HI-6 ignition and MSD Dist."
Well, if it’s fouling plugs, (assuming the fouling is from gasoline and not oil, such as caused by bad rings or guides), it’s telling you either the mixture is too rich, or it ain’t firing cleanly, or both.
First; check to make SURE your timing marks are actually reading true TDC at “0”. It is VERY common for engines that have been re-built with aftermarket stuff to have the marks NOT read correct at TDC. If it is 4 degrees off, you’ll THINK you are at your target timing when in fact, you are off by 4 degrees, or 5, 6, 7, 8, degrees. That can make BIG difference in how it runs. To check this, remove spark plug #1. Stick a piece or welding rod or stiff wire into the hole. Rotate the engine slowly by hand until you feel the piston come all the way up, then very carefully try to find its highest point by going back and forth a few times 10 or 20 degrees until you are pretty confident that you know where TDC truly is. Now look at your timing mark and see what it reads. It “should” have the 0 marks lined-up. If it doesn’t, then however far off it is, is how far off your timing is when you THINK you have set it. You’ll need to put a new mark on it where TRUE TDC really is.
Timing should be at 36 degrees total, and your MSD distributor should have the large (black) bushing with either the lightest (silver) springs, or the middle (blue) springs. You want ALL of your advance to be in by 2,000 - 2,500 RPM, and you want to run a lot of initial
Plug gaps should be set at .034” - .035”. FORGET what MSD says about .045” gaps. It won’t fire as good with wider gaps REGARDLESS of how much “voltage” the ignition has. A large weak spark isn't as good as a small intense spark. The more power you make, the SMALLER the plug gaps should be, period. So make sure they are at .034” - .035”.
Heat range on the plugs should be a stock heat range. The idea of running colder plugs on performance engines is “mostly” a bunch of BS unless you’re running a blown, alcohol, nitro engine, or unless you have 13:1 compression or higher. The idea is to run a plug as warm as you can that doesn’t burn-off either of the electrodes. Carbon, oil, etc sticks to cold plugs, just like you can’t clean-off greasy dishes with cold water. You need hot water to melt-off the grease. Plugs are no different. If they are too cold, crud sticks to them and fouls them out. If they are too hot, the electrodes will melt and/or round-off.
Some people think that running hotter plugs will cause the engine to run hotter. It’s an absurd thought being that the combustion temperature heats the plug... the plugs don't heat the engine. Plugs are inert to heat. Hold one in your hand... is it hot? No. So how can an inert item heat-up your engine? It can’t. All a hotter plug means is how hot the core temperature of the PLUG will be to help it to “self clean”. What do you do to a modern oven to clean it? You turn it to the “clean mode” which super heats the inside and melts-off & incinerates any crumbs, etc into a lightweight black carbon powder. It’s the heat that causes it, just like the core heat of a spark plug dictates how well it self cleans itself. Obviously if the oven got too hot it would melt the metal inside, so it too needs to be within it's safe range to prevent this from happening. If you are unsure of what heat range to run, start with a typical stock heat range. If you are choosing between two heat ranges, start with the hotter plug first and go from there. The worst that can happen with too hot of a plug is it will melt away the electrode and cause the plug to stop firing. It will not cause any engine damage.
Check the manifold vacuum signal with a gauge. Make sure it reads steady and isn’t wiggling back and forth like a tight or bad valve would cause. A tight or bad valve will cause a drop in compression and will cause the cylinder to not fire, thus causing the plug to foul.
Find out WHEN it fouls the plugs, ie; when idling or when driving. If it’s when it’s idling, then you need to lean-out the idle circuit by increasing the size of the low speed air bleeds, or decreasing the size of the idle feed well. If it is when driving, then it probably needs to be re-jetted leaner.
Go back to checking the manifold vacuum again and NOTE where it idles at. If you have a lumpy cam and it idles at around 8 inches of vacuum, you’ll need to check what power valves you have in the carb. If they are 8.5’s, then they are opening and dumping fuel while you are idling which WILL make it run WAY too rich and WILL foul out the plugs, no matter WHAT heat range they are. You want about 5 inches of vacuum difference between what your idle vacuum is and what size power valves you have in the carb. If you idle at 12 inches of vacuum, then you’ll want 6.5 power valves. Most Holleys come from the factory with 6.5's in them. So, if your idle vacuum is only like 8 or 9 inches, then you would need to drop the power valves down to something like a 4.5s to stop this.
Just remember; power valves are normally open. Engine idle vacuum closes them off and holds them closed until you mash the gas. When you mash the gas, your vacuum signal drops, which causes the power valves to open-up and dump more fuel. People make the mistake of believing that ALL of the gas comes through the main jets, and that simply isn’t true. When you are on the throttle, you are pulling off the main jets AND off the power valves. Power valves are equivalent to “about” 10 jet sizes!! In other words, on carbs where the power valves are removed (a bad idea but some people do it, such as on drag cars), you MUST step-up the main jet sizes by 10, so if you had 72’s in it, you will need to step them up to 82’s. So aside from the main jets, the power valves DO play a BIG role in delivering fuel. If they opening too soon, or are opening at an idle, you WILL foul-out plugs and it’ll run like crap.
I hope some of that helps.
If you want to learn more about this kind of stuff, in much greater detail, be sure to check out our Auto Shop Videosseries by clicking HERE. We have one titled "Performance Carburetors" that covers all of this kind of stuff and MUCH more. These make great Christmas, Father's Day and Birthday gifts!
