backfire above and near 3000 rpm's?
 

I have a bad backfire more like firecrackers going off on the port engine when I get up near 3000 rpm's. it will do it at even lower R's if I just hammer the throttle. when it starts it will go away if i back off just a little. I have changed the cap and rotor, pulled off all the wires and reinstalled. replaced the coil wire. the only thing that helped was: I found out while pulling the wires that the coil wire was rusted. I cleaned it out with wire brush and steel wool and have since replaced the wire all together. The first time I went out after just cleaning the wire it went all the way to 4000 rpm. I was about to jump for joy and then it started again. now I have the same 3000 rpm cap. what do you all think? where should i check first as far as wiring related to the ignition?

Re: backfire above and near 3000 rpm's?
 

what mtr / ign........ do you have twins ? try swaping the coil if so, If that doesnt do it try swaping out the rotor,check for play in the dist shaft

it could be anything from an ign problem to a bad valve spring,could even be carb related its just a matter of figuring out what its not......

Re: backfire above and near 3000 rpm's?
 

Ignition timing may not be advancing. If its too low or isnt advancing it will start to back fire above a certain rpm (loaded). Check the static timing and timing at 4000 with the good advance reading timing light or a standard timing light and timing tape on the balancer.

Re: backfire above and near 3000 rpm's?
 

I would try replacing the plug wires, or just swap cap and wires from your other motor.

Re: backfire above and near 3000 rpm's?
 

Could be starving for fuel, filter, anti syphon valve ect. This causes a lean pop which sounds like little firecrackers.

Re: backfire above and near 3000 rpm's?
 

I have twin sbc 260 hp mercruisers. there is no play in the distributer. thanks for all the info. I changed the carb, so its not that. I think it sounds like what bowtie is saying a lean pop. starving for fuel. but I can feel abump or miss at idle which could also be my old a$$ alpha clutches and what not, or a u joint. crap who knows. I will start swapping stuff out coil etc.
thanks again guys

Re: backfire above and near 3000 rpm's?
 

chk for lobe going away on camshaft

Re: backfire above and near 3000 rpm's?
 

Yup, had this problem. Found a little snot of silicon blocking one of the jets.
My pop was pretty constant once I got to 3000, however if you pumped the throttle a few times the pop went away for a few seconds from all the extra fuel the accl pumps dumped into the motor. Then the pop would start again.

Re: backfire above and near 3000 rpm's?
 

Definitely sounds like fuel starvation as have been said. Check fuel pressure all the way up to the rpm where it starts to pop. Could be filters, pump, antisiphone valve, carb, etc

Re: backfire above and near 3000 rpm's?
 

I've had silicone get into the fuel bowl once in the past and create the exact same problem you are referring also. It went through a BG in line fuel filter as well...I was surprised because the piece was about 7" long and had to come from the tank.