4.3lt Mercruiser problems.. HELP

[hamish24]

Hi, i have a 96 4.3lt V6 mercruiser. recently had carb rebuilt as it had a lot of **** in it from sitting over winter.. ever since put back on boat it has had a problem when running engine between 3900 & 4600rpm, over and below these rpm boats runs great. But in this range it has like a popping/backfire sound coming up through carb?? im sure its not a backfire but it sounds similiar and it runs a bit rough.

we have timed the engine at these revs and has not fixed it, put new plugs in it still no better.

Carb man has had another look and is adament that it is all set up exactly right and is working perfectly.

any suggestions as to what my problem could be?? it has the thunderbolt ignition....

am getting really frustrated with this engine as we have had all sorts of problems in the past and finally got it going good last season and then this came along.....

thanks in advance

 

[Warpa]

Sounds like over fueling to me which is deffinately a carb problem, can the carb bloke come with you on the boat to let him hear for himself? If not i would insist on another carb, money back, new fitter etc.

 

[Bassplayer]

If it is spitting back thru' the carb it is running weak. Try partially covering the air intake at these revs and see if it improves / increases the revs. Or push the choke flap partially closed assuming it is autochoke and has a strangler flap.
Otherwise it could be a bad electrical connection - you will sometimes get these symptoms if a connection is a bit loose and the vibration at certain revs will cause it to "spark" thus upsetting the ignition.
Is there an inline filter for the fuel? Has that been changed and is the fuel old?
Good luck!

 

[dingdongs]

is fuel fresh?is there any water in it?try running on a seperate outboard tank with fresh fuel to see if it eliminates problem.if it persists it must be carb or have you had your compression checked lately?

 

[AndieMac]

am getting really frustrated with this engine as we have had all sorts of problems in the past and finally got it going good last season and then this came along.....

What were the other problems? Just in case they are vaguely related.

 

[hamish24]

fixed problem

hi all, thanks for your suggestions, finally after a lot of money i have fixed the problems with it, the timing had actually never been set despite being told it had been, the guy that fixed it said the timing mark on the pulley was all rusted over and could have been checked as it was miles out, so he has retimed it, found a faulty spark lead and fixed the settings in the carb and it is now running like a charm

 

[AndieMac]

The crankshaft pulley seems to attract a fair amount of rust being so low to the bilge.
After a number of years in the salty environment, we find the older carbs need a full birthday, being bead blasting, re-anodising and re-kitted, for great results.