Ok, we did a head change on a roller Fcam motor with RPM airgap and 600cfm bg carb. Long tube headers. Before the change the car ran fine. Now, it runs better until it hits 2500-3000RPM and then cuts off completely. It has the MSD ignition with a mallory distributor. Rev limiter is 6K. At idle and under no load it will rev up. As soon as a load is applied it cuts out at 2500-3000? We recently replaced the module in the dist. It cuts out like someone pulled the plug. It has to be ignition, right? I don't think a vacuum leak would do this. Also, the car runs great up to that point. A bad vacuum leak would cause a very poor idle! What about a malfunctioning PCV? Any ideas. I know not much info but a few guesses would be great.
Do you loose power to the coil when this happens???Try it and see.I cant help but think you have a problem with the MSD box or a wire is loosing its connection somewhere when the engine torques up causing something to break contact.Your sure every thing is wired up correctly right??Could also be the coil failing when a demand is put on it.
be nice if you could find a spare msd to see if it does the same thing. i had a hard time finding an electrical problem 2 years ago. i didn't know it was an electrical problem at the time. changed a number of parts before changing out the msd 6al. that cured it. sent the original back for repair. cost about $60 to get it fixed.
I had a similar problem with mine. Mine turned out to be the 2 pin trigger connector (violet and green) on the MSD. When the connector was snapped together the pins pushed out and barely made connection. I pulled the pins out and bent the retainer tab out further so it would stay in the connector body. Start the car and move the wires around to verify if it is a connection issue.
Don, don't have the spare MSD. Maverick1970 I tried that with the three wires leading to the distributor. Let me add more details. I wasn't present when this happened but two days ago the car was at idle and died as though someone turned it off. It would not restart. So, I tried to repeat the process today so I could test spark from the coil to the dist. and then to the plugs. Well, it didn't do it. But, I did learn that it only cuts in and out under partial throttle. In a moment of frustration I nailed it and no hestitation and a little less rubber on the rear tires! So, it only cuts off under partial throttle. I can give it 1/3 throttle and hold it there and it will cut in and out. I doesn't appear to be a fuel problem as I would think it would spit and sputter a little. It literally acts as though someone is turning the key on and off. When it cuts out and starts again under 1/3 throttle it sounds fine. No spitting or sputtering of any kind under any circumstances. It has to be electrical, right?
It may be carb. problem. That is a very good carb you are using but you may want to give Barry Grant a call with your symtoms and see what they have to say... It may just need a little tweeking.
I'll give that a try. After the full throttle tire roasting I'm beginning to believe it may not be electrical.
efi I had the same problem then my msd 6a box just quit working all together. first it would cut out intermitten at different rpms then it just quit altogether I had to replace the msd box.
It HAS to be electrical...if the carb wasn't keeping up, I would think it would sputter and die, not just cut off like someone hit the key to OFF. You are losing spark making it immediately die.
i just had a similar problem with mine. it would fire and run fine until it reached appx 170-180 degrees then cut out like someone threw a switch. after checking the box,coil, etc it ended up being the magnetic pickup on the msd distributer. it was randomly cutting out when it got hot. $40 part was leaving me on the side of the road for over a month. at least i found it. i am dumping ALL my msd parts and replacing them with mallory. i have had nothing but trouble with msd parts.
I had a 76 T-bird w/351-M that did that as well.Took 3 tries to get a good(new) pickup for the stock dist.I hate it when that happens.
j miller, it seems to be temp related. The only confusing part is it does it under partial throttle. Though it has died once while at idle. I still haven't been able to get it to cut off so I can check spark to the coil and plugs! Is there any way to check the module, pickup etc?
