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428 breaking up at high rpm
Hello Everyone,
I have spent the last few weeks getting the Cobra set to run around this summer and I almost have it nailed. However, I have a tuning problem that I cannot seem to solve. The motor runs great up to about 5800 RPM and then suddenly breaks up (kind of revved popping). This eems to happen more often when its cold out or the car is not at full operating temp. Other than that it runs perfect and idles perfect. I'm not sure if this is a timing thing or a carb thing or even a MSD thing. I have a 6200 RPM limit on my MSD box so I don't think that is the issue. Any ideas? Thanks, :D --Mike |
If you trust that you're at 5800 rpm and not up against the rev limiter, I would try another coil. Are you sure you're not at the rev limit? The MSD is fairly accurate while a tach may not be.
If your coil a cheap round, chrome covered type mounted lying down like I saw in your gallery, that could be your problem. BTW, I don't mean to insult your coil, it's just that sometimes they get overlooked during a build. |
Mike,
Right you are, it is a cheap coil. You didn't insult me. :mad: :D :D :D :D I never thought of the coil being the issue, but now that I think about it, you may be right. It is the only piece left over from the test running of the motor that was not up graded. I will purchase a new MSD coil and give that a shot. I'm pretty sure I'm not hitting the rev limit, however I will check that next. I am running the tach off of the MSD box, so I hope the gauge reads exactly what the box is putting out. :D --Mike |
Mike,
Spend the extra $5 or so and buy the black MSD Vibration Proof coil. I went through two of the Red Blaster Coils before someone suggested I buy the Vibration Proof Coil. Clois |
Sounds like valve float to me...
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MJ, I THINKS CLOIS MAY HAVE IT RIGHT. HOWEVER, TIMING 2 DEGREES TOO ADVANCED OR RETARTED CAN CAUSE THE SAME THING. HAVE HAD IT HAPPEN. WHOS VALVE SPRINGS ARE YOU USING? HOW DO YOU LIKE THE CAM? AM BUILDING THE SAME COMBO OF TRANS AND REAR GEARS AND WITH A 060+ 390 AND AM THINKING OF USING THAT CAM. HOW DOES IT WORK WITH THAT COMBO. HOW IS THE IDLE.? GOOD LUCK AND THANKS!
BYRON W. |
Byron,
I love the cam, makes tons of power, but your idle will be rough. It is streetable if you tune the timing and carb right. The gear set is perfect for driving around as well. I think I'm going to change out my coil for a MSD Vibration Proof coil and see what happens. It could be valve float as well, but I will have to check my build sheet to see what springs were used. If the springs and coil checkout I will start messing with my timing. This issue only really creeps up when the motor is still running cooler than 180 degrees. :D Thanks, Mike |
Ever experiance carbon fouling? Had a friend with a 69 GTO.When it would appear it would start out missing at higher rpms.The higher the gear the lower rpm the miss.Like 5,500 in 1st 5,200 in 2nd and 4,900 in 3rd and 4,500 in4th.As it got worse the miss would start at lower rpms. New plugs would make it all better. Timing ,plug temperture,ignition etc all affect .
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Been there...done that. If you are using hydraulic lifters then I agree with Bob....you have valve float. Cure? Heavier valve springs. When you remove your rocker assembly look for a star pattern on the top of the stem. This would indicate valve float. A circle pattern would be the correct pattern without float.
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Joe,
I'm on it. I'll let you know. Thanks for telling me what to look for. The carb checked out and so did the coil. I'm going to pull the covers tonight and take a look at the valves. Michael, I am also going to look into what you said. The problem does vary by RPM and with differnet carb tuning. Thanks to everyone. This is why I love CC. :D --Mike |
SLOW TACH
Mike,
you have a slow moving tach, they are dampened so the needle doesn't bouce. I've found the shift light comes on 400 rpm before the autometer tach reaches that rpm. So I think you are at the rev limiter, put a higher chip in or a shift light. Perry.:cool: |
Sounds like valve float to me too.
You may want to reconsider revving the heck out of it when it is not up to temperature too..... |
I had the same thing with my 514. We thought it might be running out of gas, so I put a black Holley pump in it. I didn't think much more about it till I did the rebuild on the engine and started messing with the timming. My advance was right at 32deg at 3000rpm, but it did not stop there. It was advancing way past that. The dist had the smallest bushing on it for the advance stop. I am not sure if that is what it was or not, since I have not rev the engine that high yet.
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Only when "cold", under 180. I wonder if your running the carbs a little to lean!
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