470 mercruiser backfires flapper noise

"Mercruiser 470 4 cylinder wit

"Mercruiser 470 4 cylinder with freshwater cooling (has heat exchanger).
History, former owner rebuilt engine 2 years ago. Noted that carb may have had a problem on last time in the water. Boat setup for 2 years. I have just now placed new outboard impeller pump and motor cooling fine. Engine idles great
but when you try to accelerate it intermittently backfires. Seem to do it less after it warms up. Have not put in the water yet, just using muffs to test run.

I noticed NO pulse of gas out of accelerator pump so pulled the top off the carb. Someone had apparently been in the carb and somehow left the internal linkage off the accelerator pump. Eventhough I thought I had found the problem
I went ahead and pulled the two main jets(they were not all that bad) and cleaned. I pulled the power jet and it was blocked so I cleaned it out good.
I blow carb cleaner in the main jets and see small stream out some holes in the top of the ventura ( I guess this is what I'm looking for). I re-assembled and have good accelerator pump stream now. Strangely enough my problem is stil
there maybe not as bad when engine is warmed up.

So my question...which of the following should I persue?
1)I did not pull the ventura off and am now worried that there could be a blockage in one of them eventhough I could spray carb cleaner thru main jets and get spary out of the small holes in the TOP of the ventura.

2)Since the previous owner noted a carb problem of some sort after the engine had been rebuilt, Could it be that my problem is solely timing? I don't have timing light so didn't want to bother unless I eliminated all else.

3)Lastly, I have a very strange rattling noise in the exhaust in the outdrive. It sounds to be on the left side mostly. The small water spray exhaust hole on the left side is were it sounds like it is coming from. I can put my thumb over the hole and the noise goes away, in fact if I cover the right side hole the noise almost goes away too. I also have a very small (only a drip) water leak on top edge of exhaust rubber pipe eventhough clamps are now tight. I have read some threads about flappers breaking off and such.

If I can get the boat to not hesitate (in most cases it backfires) it should be a good engine because it idles very smooth. Please help."

"Opps, double post. I didn&#39

"Opps, double post. I didn't see first.
Still have backfire trouble."

"R.Thomas, I sent you this ema

"R.Thomas, I sent you this email when I
couldn't find your post. Not sure you got it so have posted resposne
. My problem turned out to be a dirty carburator and bad accelerator pump. We pretty much did what you did, took the carburator down and cleaned it out. Besure to check the float level since adding a new needle valve can change the setting; or so I was told. In my case the float was set wrong and was starving the carburator for gas causing the lack of pwer and backfiring . We reset the float and after running out the old gas, the problem cleared up.
There is a big difference between running the engine with ear muffs and running it under load. My boat purred at the dock and would accelerate fine, at the dock. When I put it under load it would get to 8-10mph and it would back fire.
I should mention that a couple of good mechanics couldn't find the real problem; the guy in the next slip helped me out. as well as some good advice on this forum.
I hope that helps a little,
Paul"

Choke opening to soon?

Choke opening to soon?

Charli:
I have noticed that t

Charli:
I have noticed that the choke is always open after I crank up. Even if it was closed just prior to initiating a crank up. Is it suppose to stay somewhat closed for a period of time? I see it has an electronic device (maybe solenoid) near it.

"Paul:
Got your message. Sti

"Paul:
Got your message. Still trying to decide what to do. I had purchased a accelerator pump and as I said I found the internal linkage not hooked up properly when I pulled the top off the carb. After that I pushed down on the acc-pump I got good streams into throat. Went ahead and cleaned carb (except for ventura holes) and still backfiring. So I'm thinking about carrying the new acc-pump back and getting a full kit. Full kit only $20 more.

One thing that maybe my problem. Previous owner said he drained tank and filled up with new fuel. I'm wondering if he really drained it. Also, I see a water catch (or filter) on the bottom of the fuel pump. Have you ever pulled that off before? Thinking about doing that and draining the tank myself and then starting over.

I have also been told to check if the advance timing is working properly by using a timing light. I don't have one so going to have to borrow to check.

So you see I have several variables I need to eliminate. Thanks for your comments and any other suggestions are appreciated."

".."So you see I have seve

".."So you see I have several variables I need to eliminate"

You said it! Your semi-carb rebuilding method leaves lots to be desired. I'd pull it and do it right--soak the casting in carb cleaner (NAPA) overnight, then blow out everything with compressed air. New gaskets everywhere will also help.

On timing...I'd get the carb right first. And see if there's multiple locations for the accelerator pump linkage (to improve the shot). The pump should move as much as possible for the amount of throttle opening.

Jeff"

"Fastjeff:
Your right. I just

"Fastjeff:
Your right. I just hated to remove the ventura since I didn't havef a kit at the time. I actually thought I had fixed the problem since the linkage was unhooked inside.

I think I will get full kit now. On the acellerator pump shot do you mean make sure there is no play it the linkage? That may be a little bit of the issue because you do have to move the accelerator about 1/16th of inch before fuel comes out. I think if I put a small washer on one end of the linkage that it will be a little more tight and maybe fix that.

I still wonder though because I can move the accelerator ever so slightly (really slow) and it will sometimes backfire. You know, like it the curb idle is very lean. If I'm not mistaken after I get out several turns on the curb-idle it doesn't seem to make any difference like the curb port might be restricted. Anyway we are going to pull the carb and not just clean but put a kit in it now."

"By "shot" i meant the

"By "shot" i meant the quantity of fuel it shoots out per stroke. Most carbs have a way to adjust the flow, and rarely are they set on maximum.

Jeff"

"Update for information on Bac

"Update for information on Backfiring.
Put a full carb kit in and then hooked up a temp small gas can to the fuel pump inlet (known good fuel)
Fired up and absolutely no more backfiring.
Now I need to drain the fuel tank.
Still noies in the stern, might still be a broke flapper lodged there."