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495 sounds like a misfire at 4000 rpm

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Jet Boat Maloney

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« on: August 04, 2013, 10:12:14 PM »
If I cruise my boat at 4000rpm or 3500 rpm it sounds like its misfiring. If I run it up to 4500 seems to go away. Could it be just coming into the cam or do I have a different issue? The cam is a 772" lift don't know duration.  Could it be spark scatter or fuel pressure? Dual quad 750 quick fuels. Areromotive elecric pump with built in regulator. If I run at 45rpm seems to clear up. Timing locked at 32 deg. Pump pressure 7 psi but drops at hard a a acceleration max rpm is 6800 with a c impeller.  Any suggestions?
« Last Edit: August 08, 2013, 11:02:29 PM by Jet Boat Maloney »



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« Reply #1 on: August 04, 2013, 10:13:12 PM »
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Jet Boat Maloney

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« Reply #2 on: December 03, 2013, 11:15:32 AM »
I think I figured out the popping noise.  I pretty sure its due to the carbs being sideways on the tunnel ram and at cruising speeds I'm only on the primary's. also think I need to bump the timing up about 8 deg since I don't plan on squeezing it.


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GT Jets

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« Reply #3 on: December 03, 2013, 01:25:19 PM »
If I cruise my boat at 4000rpm or 3500 rpm it sounds like its misfiring. If I run it up to 4500 seems to go away. Could it be just coming into the cam or do I have a different issue? The cam is a 772" lift don't know duration.  Could it be spark scatter or fuel pressure? Dual quad 750 quick fuels. Areromotive elecric pump with built in regulator. If I run at 45rpm seems to clear up. Timing locked at 32 deg. Pump pressure 7 psi but drops at hard a a acceleration max rpm is 6800 with a c impeller.  Any suggestions?

Are your carburetors equipped with power valves?

Do you know what your vacuum numbers are at the point of the miss?

You could have a transition problem if your power valves are the wrong rating.

A little more timing sounds needed, but I would not think 8 degrees, maybe 4 more. Too many variables to say for sure.

When the race boat did something similar, we actually found out that the butterfly linkage was incorrect from the factory, once corrected the problem magically went away. This was a 980 cfm Barry Grant.

GT

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  • Boat #1: 1992 Carrera 20.5 Elite (I/O bitches)
  • Boat #2: 19' Bubble deck Jet BBC Berkeley
If i get some free time tonight at work, ill play with it and post it for everyone to see.

Time to man up and yank it John!  :banghead:
Ray


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