Got questions?
We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. Last week, we tackled a rich air/fuel condition. This week, we’re revisiting the popular subject and offering up a few more possible solutions as well as some general tuning tips.
Q: I have an old Nova that I built from scratch. I put in a 355-cubic-inch small block, with 9.75:1 TRW flat top pistons, 64cc heads, 280-degree duration/.486-lift COMP Cams camshaft, a Holley vacuum secondary carburetor on an Edelbrock Torker Intake, and an HEI distributor. The car also has a Turbo 350 transmission, and a 10-bolt rear-end with 3.73 gears and a posi.
The problem is, the engine is running too rich! I have rejetted the carb several times with no improvement. I have checked the primary power valve weekly, and it is good. I am now running a #65 jet in the secondaries, and the thing is still too rich. I gap the spark plugs at .050-inches. What gives?
J.R.
A: The possibilities are almost endless when talking about a rich condition. Running on the rich side can be caused by a spark plug with too hot a heat range, carburetor float level set too high, and dozens of other things. I think you have a carburetor that is too large cfm-wise. Here is a rule of thumb on Holley carburetor sizing: “If you have to go more than four jet sizes up or down from the jets Holley installed at the factory to make it go rich or lean, then you need to go to a different size carburetor.”
In your case, you made all the adjustments you could make, and you’re still running too rich. That means you should go to a smaller cfm carburetor. If you shift gears manually, try a 650 cfm carb with mechanical secondaries. This should also yield a crisper, cleaner throttle response thanks to the higher air/fuel velocity.
I agree with the answer as the possibilities are endless. I have had similar issues on close to the same package, mine was a dodge 408 smallblock stroker, single plane intake not unlike the torquer you list, and a little more duration and lift than listed, I went through several carbs, first was holley 650 vacuum secondaries, worked great, and the throttle response was very crisp with only a slight hesitation. The next in a quest for more power, was a 770cfm street avenger holley, vacuum secondaries again. This carb always ran rich with multiple adjustments similar to your checking and tweaking. My conclusion is uneven fuel distribution with the single plane intake, they are very prone to bad fuel distribution at idle and low speed due to intake pulses. My fix was a holley double pumper with 4 corner idle adjustment and my problem was solved. I installed an air/fuel meter to verify and assist with tuning. I should say that the carb is a holley 850 hp race, but it also has the adjustable secondary butterfly plates. It now idles wonderfully and no more running rich. I may not recommend an 850 for you but if you can try a fully adjustable double pumper 650 or 750 maybe. also a small vacuum secondary carb could be the ticket. Just my 2 cents worth, hope it helps. it helped me.