QuestionMy car was running fine last year with the stock engine. This summer I installed twin European spec HS4's from Quantum Mechanics and 4-2-1 headers. I did a lot of research and I have everything plumbed correctly and kept the carbon canister and anti-run on valve connected. The carbs were completely torn down and cleaned. The needle valves and all gaskets have been replaced. It starts right up and is idling fine below 1000 rpm. It revs smoothly but when I drive it, you can tell right away the acceleration just isn't right. I checked the timing and it is BTDC but the mark is just past the end of the scale. This is how it has always been even before the mods. Adjusting it a little either way causes the idle to go down and die so it seems like this is best setting. I still have points in the distributor.
I am sure all my connections for the carbs are correct. The carbs themselves do not appear worn at the shafts. I adjusted the jets with a Colortune and it is blue with splashes of yellow. When you lift the carb pistons 1/16" the idle goes way down meaning it is lean but no matter how rich I make them, the idle still goes down, not up or stays the same. Engine temp is normal. Vacuum line from rear carb to distributor is good as is the vacuum advance diaphram.
Not sure what else to check or try.
AnswerHi Pete,
any time you modify an engine you change what is needed for fuel mixture and the timing full advance and the advance curve itself.
When you lift the side pin on the font carb you are checking the rear carb. If it drops off and runs like an old tractor it is OK. If it drops off and dies it is too lean and if it speeds up and stays fast the other carb is too rich.
It is difficult to tune a modified engine without any equipment but can be done.
You should set the timing first before the carbs. You can do it roughly by advancing the timing until you get some pinging on a quick throttle opening then retard it a few degrees at a time until it stops. Then see if you can make it idle with carb adjustments. I have never used the Color tune unit but have heard some good mechanics say it works.
sometimes it is necessary to set timing and then set the carburetors and go back and reset the timing again and thus reset the carbs.
High RPM detonation is hard to hear so on some engines you need to purchase a knock sensor system like the MSD unit. This way you can tune for total advance without the worry of destroying a piston just tuning the modified engine.
The old method of advancing the timing a little at a time until you get a power drop off at high RPM, then back the timing off a little works but it is dangerous to cast pistons. Forged pistons can take a lot more abuse when tuning.
Howard
