Afterfire

[70 RR]

I own a 1970 Roadrunner 440 + 6, 4 speed. I just installed an MSD distributor, 6AL Ignition Box and new MSD wires. When l am coming to a stop, l get a lot of afterfire through the exhaust. If l gear down it gets worse. If this is unburned fuel, what more can l do to minimize this problem. A little bit is ok but this is silly. My timing is 13 degrees initial 38 total. Easy starting, no pinging. Car runs great otherwise. The only thing l have not done is gap the plugs to 50-60 thousandths which is MSDs suggestion for motors up to 10.5-1 compression. Plugs are currently 35 thousandths. Any ideas?

 

[benno440]

up the initial timing until it pings then back it off 2* this should be spot on initial, 38 is a tad high is it not? i thought they are better at 36*
have you retuned the carbs since the install?
i have a 505 10.1 compression and my initial is at 22* 36* total, i bought custom bushes from 4seconds flat to give me that curve, i run the silver and blue springs in the msd dizzy to bring total advance in around 2700rpm.
when i was at 15* i was backfiring like you are saying.

i reckon try that first see what happens,

 

[70 RR]

I have not messed with the carbs but when l tried to advance the timing, l was getting a bad backfire through the middle carb. I was reading that 35-38 on the total was fine, l tried it at 35 and had the same results. I run the 2 light silver springs and my total advance is 2900. I use the red bushing.

 

[benno440]

mine did not like the 2 light silver springs, it threw the timing aorund too much, it seems to enjoy 1 light silver and 1 blue or 2 blues, try advancing the initial timing some more, which means you might need to put the black bushing in, you want the highest initial timing the engine will accept before detonation begins.

the middle carb is the main one isnt it? if it is backfiring then you are usually too retarded (no offense intended haha) i would definetly try advancing. if it is still doing it then i am not sure after that.

 

[70 RR]

I will experiment for sure, thanks for the input, l will keep you posted. Your semi-retarded Roadrunner buddy! Oh, and yes the middle carb is the main one.

 

[70 RR]

I was able to solve my problem by gapping my plugs to 60 thousandths. That took advantage of the MSD and pretty much took care of things. I am still running the two light silver springs and the tining at 13 and 38 but I may tweak it when I am off next week. Thanks for your help!

 

[67 coronet]

if you solved a run on issue by increasing gap,then you are dumping a good amount of fuel.def take a look at some fine tuning there.were the plugs blackened when you removed them?

 

[70 RR]

No, the plugs were a nice tan color. MSD suggested the gap of 55-60. I also adjusted the timing both initial and total as well as adjusting the curve. The idle mixture was tuned too.