4.3 lt Mercriser Cranks But will not start on idle and will not idle.

SearayFranko · Wednesday at 9:09 AM

So I have been working on this boat for about a month now becuase it was running really rough. New fuel pump, rebuilt mercarb. good compression, new plugs and wires, new rotor and cap. Was then hard to start but got it running ok on the trailer for a while a couple of weeks ago. Now it will not start at idle. It will start at idle if I pour gas down the carb and run for a few seconds. Oherwise it will not not start at idle. If I place throttle at 1/4 to 1/3 it will start and race but cuts off when I back off to idle. Ive adjusted the idle mixture from 1-1/4 turns out to two turns out. no difference. Lately have heard some popping out the carb as I crank it to start.
Any Ideas?

ste80 · Wednesday at 9:30 AM

Wich 4.3 you have? Lx?
Carbs normally has more difficult to start just at idle, you need to give gas a bit.
I just rebuilt all my motor and yes it start at idlle but with low low rpm so if i give some gas it is much better.
Normally you shoud pump a couple of time, kepp a litle bit the gas open 1/3-1/4 wait for pump to fully fill carbs and it should start.
If you have issue it could be the registration of the timming or some issue with choke. In hot condition does it get better?

SearayFranko · Wednesday at 1:59 PM

ste80 said:
Wich 4.3 you have? Lx?
Carbs normally has more difficult to start just at idle, you need to give gas a bit.
I just rebuilt all my motor and yes it start at idlle but with low low rpm so if i give some gas it is much better.
Normally you shoud pump a couple of time, kepp a litle bit the gas open 1/3-1/4 wait for pump to fully fill carbs and it should start.
If you have issue it could be the registration of the timming or some issue with choke. In hot condition does it get better?

Thank you for you responce. Here in Florida now its 80 -90 all day long. Not sure if the choke is doing anything. The choke blades are always full open. I'm starting to think it may be a timing at idle issue. I dont understand how to check timing on these engines. I have the thunderbolt V ignition system.

SearayFranko · Wednesday at 2:04 PM

SearayFranko said:
Thank you for you responce. Here in Florida now its 80 -90 all day long. Not sure if the choke is doing anything. The choke blades are always full open. I'm starting to think it may be a timing at idle issue. I dont understand how to check timing on these engines. I have the thunderbolt V ignition system.

alldodge · Wednesday at 2:52 PM

Using MR43LTS plugs and TBV should be 1995 or later
Pump the carb and do you see fuel squirt down throat of carb?

Is the fuel filter full of fuel?

beruken · Wednesday at 3:09 PM

Are you adjusting the jet or the idle set screw? Also, review the procedure for setting up your throttle cable too.

SearayFranko · Wednesday at 3:10 PM

alldodge said:
Using MR43LTS plugs and TBV should be 1995 or later
Pump the carb and do you see fuel squirt down throat of carb?

Is the fuel filter full of fuel?

Thank you for your responce.
I have those plugs, Fuel water seperator filter is new and full. I disconected the fuel line from the carb and energized the fuel pump, and it is pumping gas. Its a new fuel pump. When I push the accelerator pump lever I do see gas pump out the jets.

Does anyone have a wiring diagram for this engine to check my wiring.?

SearayFranko · Wednesday at 3:13 PM

beruken said:
Are you adjusting the jet or the idle set screw? Also, review the procedure for setting up your throttle cable too.

Thank you for your responce.
I adjusted the Idle Mixture needle to 1-1/4 turns out from bottom initially. then to 1-3/4 and then to 2 turns out. No change in start. I adjusted the idle RPM screw to 2 turns after contact with the cam at full throttle.

alldodge · Wednesday at 3:17 PM

Did I read you correctly? It will start with pouring gas down the carb but not when fuel squirts from the jets?

If that's the case you have bad fuel in the tank

SearayFranko · Wednesday at 3:52 PM

alldodge said:
Did I read you correctly? It will start with pouring gas down the carb but not when fuel squirts from the jets?

If that's the case you have bad fuel in the tank

Clarification;
I have only seen fuel come out the jets when I push the acelerator pump on the carb with my hand.
I have not looked to see if fuel comes out the jets when I am cranking with throttle at idle since I am working by myself.

I guess I should get someone to help and see if I can see fuel in the carb when cranking.
If I pour fuel in the carb , then go and try to start, it will start and run for a few secounds then shut down if i do not accelerate it.

During this troubleshooting period, I am not running from the main tank to eliminate the bad fuel possibility. I am running from a 5 gallon jug with fresh fuel.

Eric 3.7LX Alpha 1 · Wednesday at 4:08 PM

I agree, it sounds like bad gas. Collect a sample.. a half teaspoon of it (out of the boat) and see if it will light on fire. If you can't light it the engine won't either.

alldodge · Wednesday at 4:22 PM

SearayFranko said:
Clarification;
I have only seen fuel come out the jets when I push the acelerator pump on the carb with my hand.

If I pour fuel in the carb , then go and try to start, it will start and run for a few secounds then shut down if i do not accelerate it.

SearayFranko said:
I am running from a 5 gallon jug with fresh fuel.

ok, than your rebuilt carb idle jets are clogged up.

You get fuel when you pump the carb, so you should be able to keep it running if you pump the throttle a little (back and forth) to keep gas going in.

SearayFranko · Wednesday at 4:36 PM

alldodge said:
ok, than your rebuilt carb idle jets are clogged up.

You get fuel when you pump the carb, so you should be able to keep it running if you pump the throttle a little (back and forth) to keep gas going in.

OK thank a bunch.
Makes sense to me. I will get some help to be able to look and verify if there is some fuel coming out the jets or not as I crank it at low throttle.

nola mike · Thursday at 6:58 AM

You may not need need your choke at those temps, but maybe it would help.

Scott06 · Thursday at 7:12 AM

SearayFranko said:
So I have been working on this boat for about a month now becuase it was running really rough. New fuel pump, rebuilt mercarb. good compression, new plugs and wires, new rotor and cap. Was then hard to start but got it running ok on the trailer for a while a couple of weeks ago. Now it will not start at idle. It will start at idle if I pour gas down the carb and run for a few seconds. Oherwise it will not not start at idle. If I place throttle at 1/4 to 1/3 it will start and race but cuts off when I back off to idle. Ive adjusted the idle mixture from 1-1/4 turns out to two turns out. no difference. Lately have heard some popping out the carb as I crank it to start.
Any Ideas?

either fuel is not getting to fuel bowl or idle passages in carb are not clean. Either way when you dump gas down you are providing fuel until it burns off. When you advance the throttle you are coming off the idle to transition and main circuits that must be flowing fuel.

My carb rebuilding improved dramatically when I got an ultrasonic cleaner from Harbor Freight. If you open idle mix screws up and it does nothing, means those passages are plugged

Is the idle speed screw out enough to keep throttle blades from fully closing ?

SearayFranko · 2025-07-12T07:37:35-0600

Update on my troubleshooting!!

This morning I adjusted the electric choke per the instructions on "Mike Carburators" rebuild video on you tube.
Engine started up cold instantly, no problems. I was amazed!.
About a minute later or more, its stumbles and dies.
Started up again, a little rough but ran for 30 seconds or so, then died.
Adjusted the idle mixture needle again to 1-1/2 turns from bottom. Started up rough, then died.
Gave the idle mixture screw another 1/2 turn towards open (Now at 2 turns total from botton), started up rough then died.
By now the engine is warm, but will crank but not start , or run at idle, when I manage to start it pumping on the throttle.

I'm convinced its the carburator, either adjustment, or something I did wrong in the rebuild. But then , why did it run fine right after I bolted it on a few weeks ago?

Any words of wisdom from those who have more experience?

would appreciate it.

I think I have done all I know, or dont know, how to do. So if i do not get any more ideas or suggestions, I believe my next step may be heading towards dropping it off at marine engine mechanic, and have them figure it out and fix it. .

Eric 3.7LX Alpha 1 · 2025-07-12T08:17:01-0600

Crap must have gotten in there after you bolted it on. So it won't run off choke? Means a possible lean fuel mixture.. too much air and not enough fuel. Does it begin to run poorly as the choke plate opens?
If you manually hold the choke plate partially closed does it stay running? I wouldn't use my fingers in case of a backfire.. use a stick or long screwdriver.

nola mike · 2025-07-12T08:51:49-0600

Eric 3.7LX Alpha 1 said:
If you manually hold the choke plate partially closed does it stay running? I wouldn't use my fingers in case of a backfire.. use a stick or long screwdriver.

You're smarter than me. I was going to suggest holding your hand over the intake to see if it improves things

cyclops222 · 2025-07-12T09:19:39-0600

Ignition is great.
Cold engine now is great & normal.
Engine dies when choke system is wide open.
What is the problem ?

ESGWheel · 2025-07-12T10:20:58-0600

As folks are suggesting all indications are clogged idle circuits. As AD always suggests > run fishing line thru all the passages to ensure are free and clear. If does not go, use small gauge safety wire.
Here is my suggestion:

Get yourself another rebuild kit and do again. This time with the ultrasonic cleaner if desired (great excuse to buy one!) One thing I do is to use a can of carb cleaner with the straw attached and use that to squirt thru all the passages. And then do the fishing line. Take before pics. Make sure everything is really clean and cleared out. Reassemble, reinstall and follow the process for the throttle cable and setting the choke. I’ll bet it will work.

Prior to doing the rebuild, a shade tree mechanic method to unclog passages in the carb would be to pull all the plugs, short out the high-tension lead fm the distributor, back out idle screws (but still installed) and then put your hand over the throat of the carb (sealing it off). Crank the engine for 5 or so seconds. Remove hand and crank a little more to clear out the gas fm cylinders and put all back together and see if that works.

Also, for clarification, the squirt of fuel you see when advancing the throttle is from the accelerator pump which is what you are seeing. There is no fuel squirting out of these jets when at idle or running, only when advancing the throttle.

Good luck! And let us know how it goes.

4.3 lt Mercriser Cranks But will not start on idle and will not idle.

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