Need help with mixture adjustment on Mercarb 2 BBL

I have replaced the 2 BBL Merc

I have replaced the 2 BBL Mercarb on my 1987 4.3L with the Reman. Mercarb recommended by Mercruiser which is the 96-98 model 2 BBL w/single mix. screw. Almost identical except the intake side of the barrels. On the old carb the barrels were separated but used a common passage way for the single mix. screw. The replacemnt carb barrels are open between them at the bottom of the carb. I set the idle speed at 650-700 rpms and tried to adjust the mixture per recommended procedures but found a wide range of mix. screw turns that had little or no effect on idle smoothing out. Any changes were very subtle and I had to have a helper watch the tach for any change in rpm. Not much change with nearly 2 full turns after initial 1.5 turn setup. It idles ok but stalls after quick throttle movements when it should be at idle. Also backfires at different screw settings. Only place to hook up a vacuum gauge is the fuel pump overflow on the bowl top.

I placed it in gear at idle and the rpms dropped to 500 so I adj. to 650. In neutral still not able to find sweet spot. Your help please.

Correct base gasket?

Correct base gasket?

"you might be simply adjusting

"you might be simply adjusting the idle misture screw, in which case you can get it close by warming up and turning the screw clockwise till the idle drops off and then backing out a half turn or so.

make sure you've got accelerator pump adjusted correctly.

Suggest you have a manual as well. Not sure but you may need to replace carb jets to adjust the main jet mixture."

"Thanks. Gasket may be the is

"Thanks. Gasket may be the issue; used the original type and trimmed it. I'll get the style for this model carb. Pump is in the center hole on the lever. It responds quickly, engine just wants to stall when you pull back on the throttle quickly. Pulled all the plugs last night and replaced them--they were new but had black carbon from last carb flooding issue. I never thought about the jets since it is a June 2006 factory reman. from Mercruiser w/1 yr warranty and has a seal on a screw. Thanks again."

"In adjusting the idle mixture

"In adjusting the idle mixture, you need to be sure the choke is FULLY open and the motor is thoroughly warmed up. Then, in gear, adjust the idle--I like to tie the boat up to a dock when doing this.

Next, find a fitting on the carb or intake where you can attach a vacuum gage (and DON'T use the PCV valve line or you will adjust the mixture too lean). Use the vacuum gage to adjust for best idle on one side, then the other. And if you can turn the idle mixture all the way in without the motor stalling, then the carb is flooding.

Okay?

Jeff"

"Great idea, thanks Jeff. The

"Great idea, thanks Jeff. The carb is a single mixture screw Mercarb 2 BBL w/o a vacuum port. There is a plug at the rear base of the carb that I could remove but the last plug I removed for the fuel pump leak line on the bowl top had to be drilled and then it took an easy out to remove it. I'll give it a shot at removing the plug and getting a nipple for it unless I can get a plug out from the 1987 manifold. First I'm checking the timing to exclude it. Thanks again."

"I'm sorry, but marine eng

"I'm sorry, but marine engines do not have PVC systems!
If there is a PVC vacuum system either someone has added it or installed a complete automotive engine and didn't know any better than to remove the PVC system or not to use an automotive engine in a boat for matter."

"Hi Guys: I want to do this c

"Hi Guys: I want to do this carb mixture adjustment right so I am not stranded 5 miles out like the last time.

I found a brass plug on the manifold at the rear carb intake port. It has a square female fitting, about 5/16" that I would replace with a vacuum nipple for future adj. use. Before I booger it up, what is the chance of getting it out in one piece after 19 yrs. w/o pulling the carb?

I can pull the carb AGAIN, drill a pilot hole then use an EASY OUT if necessary. But would rather not do it that way.

Guy"

"Guy:

I am a, right, that i

"Guy:

I am a, right, that is the wrong carb for your application. THe 3.0 liter 4 cylinder uses a Mercarb that has only one idle mixture screw, which is correct in THAT application since all of the air goes the same place. But you have a V-6, and its carb MUST have two adjustments. THere's no way it will idle correctly without it.

Jeff

PS: ALL engines, marine or otherwise, MUST use some type of PCV system. If not, the moisture that collects inside the motor will turn the oil into acid, eating up the bearings."