help with BB distributor

[d4a4r0t]

hey there
i have a 440 in a 72 dart. i am using a MP distributor. this is the problem as well as my friends problem with his 383. if i have my vacuum line hooked up to a vacuum port on the carb(not the full vacuum) the car will backfire. the car's timing is set at 10deg.BTDC. total timing was at 38 deg. the funny thing is on both of our cars the cars run fine with the vacuum line pulled and plugged. any one else have this problem?
thanks in advance
bill

[451boy]

My guess is that the reluctor is out of phase. This is just another defective part which the idiots who run Mopar Performance provide to their paying customers. You have to remove the reluctor and have it remachined in order to clean up the problem. What is going on is that the rotor is swinging over under full vacuum conditions far enough for the spark to jump to the wrong tower. If you drill a hole in the cap near the #1 tower and shine a timing light down in there you'll be able to watch it happen.

[340king]

Well 451boy I've just read two of your posts and your answers do not look too boyish. You are right on with the reluctor phasing(MSD tech bulletin covers this) maybe being the culprit. As for MP parts, I reserve 1 hour for each part when it comes in for inspection, correction and trial fit. I haven't had a single part work or fit correctly for some time, without some rebuilding.

As for reluctor/rotor phasing you can use a timing light through the hole 451boy describes to check your rotor phasing. Vacuum advance moves the whole plate while mech advances just the reluctor and rotor together, in phase.

[451boy]

Thanks for the kind words 340. I had this same problem with my 499 motor so I remachined the reluctor by adding + and - 5 and 10 degree slots. (Having access to CNC milling machines makes working on cars a whole lot easier!) Rotating the reluctor 10 degrees solved the backfiring problem. Wish I knew how to inform someone at MP that their parts are crap but unfortunately they do not have any method of contacting them. Seems they aren't too interested in feedback.

[PRO]

You could have a ruptured vaccum advance diaphram(both of you)which will cause the timing to jump around.Just put your timing light on at idle and apply vac. to the vac. advance if it jumps around its bad,it should hold steady adding 9-13*....PRO..

[340king]

Well PRO I am not running any vacuum advance on my distributor at all. I needed to phase the rotor and reluctor when I removed the vac advance. I have the breaker plates welded in my distributors to keep the phasing between the cap and rotor aligned. This is a must when running alchohol and MSD. This is for a race only application requiring some modification of the stock ditributor. I also have a MP race distributor that is sans vac canister and has mechanical tach drive built-in.

[This message has been edited by 340king (edited December 06, 1999).]