mercruiser 4.3 idle problem

[cdoggy81]

I have a 1990 Celebrity with a 4.3 Mercruier & a 2bl carb.
My problem was it would not run WOT or anything over 3500rpm for more than 5 seconds before dieing down for a few seconds then surging back. I figured it was time for a tune up... I just had the carb rebuilt, put on a new water separator filter, rotor, cap & plugs. I got the carb back this weekend. It is the type that only has 1 adjustment screw on the front so I started it at 1.5 turns out when I put the new rebuilt carb back on. It started ok & it ran fine for about 10 minutes then I guess once it really warmed up it started to idle like crap... It will not idle very well at all or want to come off idle. Here are the symptoms;
Above idle (1500 - wot) runs great, no problems. The WOT problem went away & it runs a lot smoother now. There is a slight bog when I go from idle to WOT to pull skier up but overall no problems once over 1500rpms. The main problem is anything less (say 700 - 1300 rpms) the engine sucks air & usually dies or barely limps up to the 1500 safe rpm point.
I messed with the carb once I got back to the dock and If I hold the butterfly to almost closed it will idle pretty darn good. My thoughts were the engine was starving for gas right? I started to open the idle mixture 1/2 turn & it got a little better, another 1/2 turn & a little better.. long story short I'm at almost 3.5 turns out before the problem seemed to go away. By the time I got it working better it was dark so no time to test again.

I guess my question is that many turns out ok or seem normal? I thought the idle mixture should be somewhere between 1 to 2 turns out max or you have a problem somewhere along the way.
Thanks & LMK what you guys think.

 

[cr2k]

Re: mercruiser 4.3 idle problem

Did you check/change the water seperator/filter? Is there fuel in the tank? Fuel pump pumping enough pressure/volume?

 

[achris]

Re: mercruiser 4.3 idle problem

Primary mixture setting (starting point) for those carbs is 1-1/4 turns. Sounds like you have an air leak in the manifold system somewhere. Mechanical fuel pump or electric?

Chris...

 

[cdoggy81]

Re: mercruiser 4.3 idle problem

OK, so I took 2 quick videos before I started messing with the idle mixture.

http://www.youtube.com/watch?v=njJe9Eso5Tk&feature=context-cha

http://www.youtube.com/watch?v=-KxcnVYBnSA&feature=plcp

you can hear how it runs bad at idle as well as when I just start to give it gas. Once you hit the 1500 rpm mark she is good to go. I'll take one tomorrow of it running good now that I adjusted the mixture screw out.


CR2K - yes to all 3 of your questions.

ACHRIS - are there differences between the Rodchester 2bl & the Mercarb 2bl? I thought I read somewhere the Rodchester was 1.5 & the Mercury ones were 3 or something along those lines. Maybe that's urban legend & I'm totally off here too...
I think you may be on to something with that leaky manifold... reason being is I cleaned the manifold off completely when I put the carb on but the other day when I was adjusting it I noticed little pools of oil under a few of the manifold bolts. Not much but enough for me to notice they weren't there the day before... I'll tighten down those as well. What I really need to do is get a vacuum gauge & she what she is doing. As for the fuel pump it is a mechanical unit.

Thanks guys

 

[Stamey]

Re: mercruiser 4.3 idle problem

Get some carb cleaner and, with the engine running at the lowest RPMs it will keep running, squirt the carb cleaner in short bursts al around gasket mating surfaces and the carb itself. Take care to shield the actual intake of the carb from getting carb cleaner into it, as it will affect the results of the test. When the engine RPM changes, while spraying carb cleaner, that is a place where there is a vacuum leak and you'll want to look closer at that spot.
Next, from your videos, it seems like maybe the accelerator pump needs to be checked. If it's not pumping immediately when you bump up the throttle, you'll have stumble when bringing up the RPMs rapidly. It seems like it picks up pretty quickly after 1500 RPM, to it is working, but may be slightly out of adjustment as to when it starts pumping. That can happen when a carb is rebuilt. A vacuum leak is the prime suspect though, so that should be checked first.


Chris