Carb backfiring after timing curve change
04-04-2018, 07:28 PM
#41
Dr. Detroit
Member Since: Mar 2012
Location: New Braunfels Texas
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More important - Multi spark boxes can give erratic timing indications at low RPM when using some inexpensive timing lights. By the time you get to 3000 RPM the variation effect on the timing light is much less if not gone because there is not enough time for the multiple sparks to happen. Leave the light dial set to zero and set the timing to 34 at high RPM (the top of the curve).
Do not use dial back lights with MSD box.
Jebby
04-04-2018, 07:38 PM
#42
Dr. Detroit
Member Since: Mar 2012
Location: New Braunfels Texas
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Initial timing on a fairly stock engine is not too critical as long as it is somewhere around 12-16.......mild small blocks seem to like 14.......
It is more important to set the total and see where the initial falls in.
Cars with a large cam such as the OP's should have about 18-21 degrees initial......
The largest bushing pulls 18 degrees so set total to 36 (Not sure why Lars recommended 34...I did not think the Holley heads were fast burn) and your initial will be 18.....perfect. Light blue/silver spring.
Jebby
It is more important to set the total and see where the initial falls in.
Cars with a large cam such as the OP's should have about 18-21 degrees initial......
The largest bushing pulls 18 degrees so set total to 36 (Not sure why Lars recommended 34...I did not think the Holley heads were fast burn) and your initial will be 18.....perfect. Light blue/silver spring.
Jebby
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Big2Bird (04-04-2018)
04-04-2018, 08:54 PM
#43
I use a munti-spark module as well. The theory is that at idle and lower RPMs where cylinder pressure is low due to any number of reasons (late valve events, low CR , high altitude, etc) fuel does not atomize as well due to low turbulence action and therefore does not ignite as easily. Meaning it may not ignite on the first attempt by the plug. So to prevent or reduce misfires it sparks multiple times in an effort to insure ignition.
Kettering ignition fires at idle just fine. HEI idles just fine. Pertronix idles just fine. Millions of cars run without CD just fine.
I am not even sure any manufacturer of autos uses CD.
CD NEEDS multi-spark at low rpm to make up for the shorter spark duration, or it does not idle well. (Gotta love marketing).
04-04-2018, 10:12 PM
#44
Le Mans Master
That is not theory of operation. That is the sales pitch to sell it.
Kettering ignition fires at idle just fine. HEI idles just fine. Pertronix idles just fine. Millions of cars run without CD just fine.
I am not even sure any manufacturer of autos uses CD.
CD NEEDS multi-spark at low rpm to make up for the shorter spark duration, or it does not idle well. (Gotta love marketing).
Kettering ignition fires at idle just fine. HEI idles just fine. Pertronix idles just fine. Millions of cars run without CD just fine.
I am not even sure any manufacturer of autos uses CD.
CD NEEDS multi-spark at low rpm to make up for the shorter spark duration, or it does not idle well. (Gotta love marketing).
04-04-2018, 10:48 PM
#45
Pro
Thread Starter
The blue/silver spring combo is the correct combo. I have installed this in over 100 musclecars over the years and it works great. If you have bouncing going on it is something else that is causing it. Find out what it is. Post a pic of your advance mechanism.
You have a fairly healthy combo that is capable of 400 horsepower. The 600 as mentioned is much too small. I think you are leaving the 20 to 25 on the table as Lars states.
You get the timing sorted and a larger carb on it and you will be shocked on how much better it will run.
Jebby
You have a fairly healthy combo that is capable of 400 horsepower. The 600 as mentioned is much too small. I think you are leaving the 20 to 25 on the table as Lars states.
You get the timing sorted and a larger carb on it and you will be shocked on how much better it will run.
Jebby
None of those combos set the total coming on the 2.8k-3k rpm range.
I guess we are talking about black bushing.
BTW, I found that my harmonic is 6.75" so I have to redone the timing sticker. I always thought that L82 engines had 8” balancer.
Last edited by corvetero; 04-06-2018 at 04:19 PM.
04-05-2018, 08:55 AM
#46
Le Mans Master
On the plus side - The Multispark CD box I am using also has an adjustable rev limiter that can be set up or down with the turn of a screw driver. Makes setting the car in 3000 max RPM "Valet" mode pretty easy. Comes in handy when you are concerned about other drivers getting behind the wheel of your baby. 