explain this to me

i changed from a super mag 4 to a msd 7al on my blown bbc, the motor sounds and revs way better as im sure my mag had problems, anyway last time out with the mag my motor made 27lbs boost in the burnout, and since the ignition change the boost went up to 35lbs and then it broke the blower belt does this make any since to you guys?

 

explain

ok mike im gonna check that out and i will let you know what i find.

 

finding

i pulled the vcovers off and everything checks out fine, then pulled the plugs and all the cylinders have good comp, so all seems well, however this is the first time any of the plugs have the cadmium burned so it did get lean, im guessing that my old mag was weak and since swapping to the 7al there is more fire going in there. as for the boost reading im still ?? on that

 

Ditto to all posted, another thought might be that you reached a higher RPM with the newest ignition , hence the higher boost reading-- do you have a data logger or just gauges with tattle tales ?

What do the plugs look like ? Do you change them often ? What gap are you running with the mag and now ?

I assume the plug wires are new now too ? Or the same ones ran with the mag ?

If you were to drop 2 or three cylinders while running you could see the increase you stated for all the reasons Mike posted.

Do you know what RPM's you peak on the burnout ? 27lbs seems like alot for a 6000 or so burnout though------------ is this a 14-71 ?

 

hello

yes i did roll the motor over and check all the valvetrain movement and all is well, its possible that maybe it popped back a little and broke the belt and that pop is the reason for the high boost reading. thats the conclusion ive came to since all the internals seem fine.

 

explain

running A 990 pump had a 125 in the pump loop and 100 in the main blower usually makes 27lbs boost but im thinking the new ignition is what is throwing me for a loop. the plugs do have some color to em so it was getting leaner than it ever has

 

The next question is , have you been slowly changing pill sizes to larger and larger ones chasing the bad mag ???

What's the smallest main jet you have ran well with vrs what was in it ???

 

explain

im running 18* gap, and yes im afraid ive been chasing a bad ignition the whole time im not for sure yet as i havent really made any good passes since i started this project but maybe i can get it tuned up now

 

plug gap

remember that it takes volts to jump a gap and current to maintain the spark. The MSD 7's, 8's and 10's have high voltage but less current meaning they should be able to fire a larger gap where they are not at their current limit. eg: a MSD-10+ will comfortably jump a large gap (.020+) at say 30-40psi where a promag 44 may not as it does not have the high voltage of the 10+. The 44 will however maintain the spark (at a smaller gap) at much higher boost pressures than the 7's, 8's and the 10+. Hope that makes sense, personally unless you are running at the limit and have to use a 44 I would prefer the MSD boxes as they start easier and keep the plugs cleaner due to the multiple sparks at lower revs. Cheers Graham

 

I agree with Mike. Plugs should be set at .014. I'd put some fuel in it until you decide it's safe if you haven't hurt it. Boost doesn't just jump up at the same rpm. You want to make sure the port checks are clean and seating too. You may have fixed a bad ignition but you need to get the fuel back to safe.

 

SM IV mags do not "go away" like the SM II or IIIs. We have made approx. 475 runs on SM IVs and I have dug into this exact topic. Previous to that we ran IIs, then IIIs and yes they go away, we experienced that.

Did you check the valve clearences, is there an intake that has no lash? As in it leaned out, tuliped and now when that cyl fires the intake manifold fires too, breaking the belt, etc.

Did the timing change when you went to the 7al to account for the quicker electronics vs the SM IV points? This will make more heat

 

Do a leakdown

35lbs in the burnout with no load on motor seems really high. How much boost do you normally make under a load at 8000? I would do a leakdown to see if you have a bent or burnt valve. We have popped burst panels due to lean conditions but we have also popped them due to bent valves. Tattletale would read 50+lbs when we popped them.

 

hey

the 35lb reading was from the actual pop back it self the guage was just reading from the intake pressure, i fired the engine up today and its ready to rock!! thanks guys for the help