Backfiring intermittently

[Jsiples]

Have some questions on a backfire I am getting. When starting initially I always let it set until it is at operating temp as it says to. I know this is probably not imperative all the time but I do it anyways just to be safe. Well when taking off for the first two or so times I get a backfire instantly when trying to go WOT. I can always baby the throttle and get it full out eventually. The strange part is that as the day goes on this problem seems to go away and I can 9 out of 10 times, just go full boar no problem. To reiterate problem, first couple of takeoffs then basically no issues from then on. Seems like a very strange problem. I was thinking fuel pump? I have to think its with fuel I just am at a loss on this one. This has always been a fussy motor since I have owned it but has always been reliable as far as starting and running.... almost always. I got this boat two years ago and had been sitting outside for two years prior, so I have a lot of faith in Merc (GM) they just seem tempermental kinda like my lady Any ideas appreciated.... not a big problem really, but seems like it needs attention

FYI- points, condenser, rotor, cap, plugs wires, timing, fuel filter at pump all done this year and carb rebuild last summer.....am I missing anything?
84 Merc I4 140 hp

 

[Jsiples]

Re: Backfiring intermittently

Anyone have any ideas?

 

[Don S]

Re: Backfiring intermittently

Did you set the points by dwell, then reset the timing? Does the advance work?
You may have a lean condition. Are you sure the mixture is adjusted correctly? and I don't mean it's set to the turns out the book says. That is just preliminary adjustments.

 

[Jsiples]

Re: Backfiring intermittently

Yes dwell on points was set, advance is working correctly, and timing redone. I adjusted mixture out as needed to most smooth idle in forward gear in water. I may redo these items again to check for mistakes and see if the problem lies there, thanks Don.

 

[bruceb58]

Re: Backfiring intermittently

Accelerator pump working?

 

[Jsiples]

Re: Backfiring intermittently

Im still a novice how would I be able to determine that? The tube from the carb to the pump does not have anything in it?

 

[Jsiples]

Re: Backfiring intermittently

Also wanted to ask isn't there a screen or filter up near the carb also? If so, I have not checked that either I wasn't sure if I had one present on my model.

 

[Haut Medoc]

Re: Backfiring intermittently

Remove the flame arrestor, & have an assistant pump the throttle....
You should see a small stream of fuel.....
If you don't have an assistant, just listen.....
You should be able to hear it......
Yes, there is a screen at the base of the carb where the fuel ine enters.....​

 

[Jsiples]

Re: Backfiring intermittently

Cool, thank you I know I am getting fuel I have had someone pump the throttle and see the jets putting fuel in. I am going to check that screen and see if it is blocked up just enough to cause this problem.