Re: backfire above and near 3000 rpm's?
 

If you ignition has a magnetic pickup point, make sure there is no rust. I spent weeks figuring this one out on my MSD dist. Same problems ... over 3,000 and pop, pop, pop. Change the starwheel mag and bingo .. runs great. Swapping carbs is a great idea, check plugs and wires by pulling one at a time while in gear at idle, check valve springs ... broken or weak. Good luck.... you'll get it!!!!

Re: backfire above and near 3000 rpm's?
 

the thunderbolt system uses a hall effect pick up in the dist

Re: backfire above and near 3000 rpm's?
 

Is it backfiring through exhaust or the carb?

if exhaust what bank? Starboard or port?

Re: backfire above and near 3000 rpm's?
 

Thats a good question, the back fire is through the carb.

Attack if its the magnet and piece of metal the the wheel on the bottom of the rotor travels between it does have a small amount of rust on it. I actually cleaned it off a little when I did the cap and rotor. I will have to inspect it closer.

As far as the plugs and wires goes I can't get at the plugs when its idling but I can pull wires one at a time off the distributor.

thanks again guys all good stuff

Re: backfire above and near 3000 rpm's?
 

ok so its poping through the carb.

Try pulling a wire at the cap off one at a time to determin what cylinder it is.

This indicates It is firing when the intake valve is slightly open.
Possible causes

ignition timing
Bad cap
lean condition
intake valve not closing.
large vacuum leak.


By isolating the cylinder or cylinders makes it
easier.

Just another approach.

Gerry

Re: backfire above and near 3000 rpm's?
 

KNOT RIGHT it even left a small carbon deposit on the old carb I pulled off.

Re: backfire above and near 3000 rpm's?
 

try the pulling the wire deal

Re: backfire above and near 3000 rpm's?
 

You have an 85 that had engines rebuilt before you purchased the boat.

Let's step back and do some basic diagnostics.

1) Check compression - no exterior replacement part is going to fix a wounded motor.

2) Check fuel psi as stated.

3) Make sure timing is spot on and wires are routed correctly. Make sure proper spark plugs are in motor. Make sure proper voltage is getting to coil and distributor. Make sure your grounds are 100%. Make sure batteries and connections are 100%.

3) Check for foreign particles in carb. ie: all passages ! Not just jets + needle/seat, although these are most common area to get plugged. If all looks okay replace needle and seat anyway. Your there and they are cheap. Seen a few NEW carbs with faulty needle/seats.

Speaking of which, you mention you replaced the carbs. Was the reason because of this condition ? If not, why.

4) Once you check every conceivable option you may want to check for broken valvesprings and such. Rebuilt motors only a few years ago does NOT mean they are definately 100% now.

=============
We are guessing/instructing because that's all we can do on a keyboard.

Start from the basics, and work your way up the food chain, one step at a time....not many steps at a time.

Good luck. Keep us posted.

Re: backfire above and near 3000 rpm's?
 

Flatout. Backfires through the carb are as far as I know related to a lack of fuel. Also considering the two situations when it happens, when trying to pass 3000 rpm or hammering the throttle at lower rpms, this further points to a restriction in the fuel delivery. There is simply not enough fuel when demand is past a certain level. Carb or elsewhere.
Good luck.

Re: backfire above and near 3000 rpm's?
 

a exhaust lobe going flat will also cause the exact symptoms you describe,a compression test or leak down won't show problem,may not be your problem but don't overlook,if your backfire is very fast(like firecrackers)rather than a slow ocasional pop look there