correct Idle? merc 4.3 v-6

[Squab]

I had to do a total rebuild on my engine this winter and I am now in the final steps before my seatrial. I took it out last week her first trial and everything was fine till I tried to get up on plane. There was not enough power to go. So I limped home and drained the gas tank and changed the oil. Now I just want to make sure that the engine is idling at the proper RPM. The engine is in time and it is idling at around 600 rpm now. Give or take. It runs rough at idle so I think that I may experiment with the idle set screw. I also rebuilt the Carb this winter. Never touched screw but maybe now that it is all clean it is running different. Where do I go from here?

 

[HT32BSX115]

Re: correct Idle? merc 4.3 v-6

I also rebuilt the Carb this winter. Never touched screw but maybe now that it is all clean it is running different.

Howdy,


If you rebuilt the carb, how did you NOT touch the screw? When you rebuild any carb, you should REMOVE the screw(s) so you can flush/blow out all passages.

You may have debris behind it.

Now having said that, if you're not making full RPM on a rebuilt engine with the "SAME-as-before" drive and prop (that presumably DID make FULL rated RPM?)...............

I would suspect either timing, CARB, or BOTH.

 

[Squab]

Re: correct Idle? merc 4.3 v-6

Timing is right on. Gonna pull carb and check set screw

 

[Squab]

Re: correct Idle? merc 4.3 v-6

Ok so I took the boat out again this morning and it still was not able to get up on plane. Everything else checks out. the timing is on the compression is good. New oil/gas/filters. Had a friend throttle while I looked in the carb and I did not see a steady stream of fuel or really any for that matter shooting into the carb. Did see a good squirt when he first gave it gas, however the stream stopped when it needed more. It is starting to sound like the fuel pump or the high speed needle. It is a mercarb on a 1989 merc 4.3.. Is there any way to test if the pump is giving what the engine needs at high speed?

 

[Prophammer]

Re: correct Idle? merc 4.3 v-6

I would be very reluctant to look down the carb while accelerating, it could blow back a flame, right in your face (if it backfired for some reason). To check fuel pressure you need a tee-fitting on the pressure side of the fuel pump line going to the carb, attach a fuel pressure gauge at the tee (Snap-On Tools makes a fuel pressure gauge with a bleed off valve). MAKE SURE NO GAS IS LEAKING AT TEST CONNECTIONS. Monitor fuel pressure under load. You should be able to maintain 2-6 psi. Keep a fire extinguiser handy (But don't shoot fire extinguiser down carb)
If pressure is good you can test volume by bleeding pressure gauge of into separate container with engine idling, it should pump out about a pint in 30 seconds. To check accelerator pump, shut engine off, work throttle, a 2 barrell carb should shoot out two steady streams. If your accelerator pump is too weak it would most likely back fire out of the carb when under load. If you were looking in there while under way, I would guess your accelerator fuel spray is good!

 

[Squab]

Re: correct Idle? merc 4.3 v-6

Prop Hammer,
All good suggestions. I know the accelerator pump is working because I can see the fuel entering the carb. I did a full rebuild of the carb this past winter so I know everything is clear. I did take the fuel pump off and the rubber bladder that connects the two sections was old looking and there was a ton of resin looking gunk on the lower half of the body. Guess it is time for a new pump. Gonna buy today and test tomorrow.