Carburetor Adjust Screw Does Nothing?

[RuthlessINXS]

For all you more experienced folks, I just finished doing a rebuild on the carbs for my '81 Merc 80HP 2-stroke 4-cyl outboard. They didn't appear to be all that dirty but the gaskets got replaced and everything was scrubbed down and degunked. After reinstalling the carbs I noticed that the lower carburetor was flooding. I removed the drain plug and shot canned air down through the bowl just in case i was lucky and the pin was just stuck. No more flooding now and she seemed to idle fine at 2 turned from seated. I took her out to the lake to fine-tune when I noticed that I can adjust the top carburetor from next to no gas to bubbling gas. The bottom carb, however, never appears to actually change to amount of gas.

So, long story short is, to make the bottom carb actually adjust the gas flow is it going to be a float and pin issue or more likely a dirty fuel line/carburetor issue?

Not sure how applicable this is but this motor has been run w/ Seafoam additive through 10 gallons as well as shot straight through the throttle plates as the previous owner was a bit...sketchy w/ maintenance.

Thanks again for any help y'all can offer.

 

[Chris1956]

Re: Carburetor Adjust Screw Does Nothing?

First of all if the motor is running faster than 1000RPM at idle, the idle circuit has little control over it. Hopefully that is your issue.

If the carb is flooding, you messed up the carb rebuild, and I would recommend you recheck the float adjustment. Did you install new floats, inlet needle and seats? If not, you need to do that.

 

[RuthlessINXS]

Re: Carburetor Adjust Screw Does Nothing?

Thank you for the reply. I have no way the gauge the RPM that I am aware of. I did not replace the floats, needle, or seats as they appeared to be in good shape but I guess I'll get on that and reclean the carb. I take it that these issues would effect the ability of the idle screw to let gas through?

 

[Chris1956]

Re: Carburetor Adjust Screw Does Nothing?

Flooding will alter the ability of the idle mixture screw to control anything....

 

[RuthlessINXS]

Re: Carburetor Adjust Screw Does Nothing?

Right, but it is no longer flooding. That issue was resolved by draining the bowl and running air through the overflow outlet...

 

[RuthlessINXS]

Re: Carburetor Adjust Screw Does Nothing?

So I replaced the floats, seat, and pin and made sure everything was clean and even replaced the fuel lines to make sure nothing else got clogged it. Ran up the boat on water muffs and while there is a small difference in tone now when you adjust the lower carb screw it is still pulling far less gas than the top carb (which is capable of bubbling gas). I passed my hand in front of both carbs, the lower carb seems to have less suction. Both carbs were cleaned and all gaskets replaced although only the lower has had floats, seats, and needle replaced. At this point I'm passed my knowledge level.

So question number one, are both carbs supposed to be capable of the same output of gas?

2. could this be the fuel pump failing and not supplying enough gas for both carbs?

3. Could this be due to the top carb not having the float, needle, and seat replaced? if so, why would it not flood?

 

[oldman570]

Re: Carburetor Adjust Screw Does Nothing?

When rebulding carbs you need to use an air compressor to blow out any dirt and gunk in the passageways and jets before reassembly. If not done you still could have a pluged idle jet. That would cause the trouble. You should be able to see thru the hi speed jet and in the holes on the sides of it. The idle mix should be set on the lower carb before setting it on the top carb to get the best setting. A link and sync should be done anytime the carbs are worked on. A OME manual will have all the steps to rebuild the carbs and do the link & sync and is a must have item if working on the motor yourself. Manuals are cheap investment and will save time an money in the long run.JMO
Oldman570

 

[Silvertip]

Re: Carburetor Adjust Screw Does Nothing?

You obviously worked on the carbs for a reason. Since that did not apparently resolve the situation, did it occur to you that there may be something else wrong with the engine -- such as an air leak at the carb/manifold or a cylinder that is down on compression. I would also question the term adjusting the idle screw so it "bubbles" gas. Where are you seeing this bubbling? The idle mixture screw should not bubble gas anywhere. You still haven't answered the question about adjusting the float level or if the engine idle SPEED is proper. As was pointed out the mixture screw will have little to no effect if the engine is running too fast.

 

[RuthlessINXS]

Re: Carburetor Adjust Screw Does Nothing?

Oldman570, The carbs were cleaned first to remove large deposits o' gunk. Then they were submerged in carb cleaner for 2 days, hosed down with canned carb cleaner. After that "safety wire" was run through all jets before more canned carb cleaner was run through them and checked to see that the cleaner was coming out all exits. I do not have the OEM manual yet however that will be fast coming. The link between the two carbs is a keyed plastic bar that controls the throttle plate for both carbs. I'm not sure what else you would do sync them up but I guess I'll find out when the manual arrives.

Silvertip,

The gasket at the manifold-carb mating surface has been replaced twice both times the area has been prep'd. I will have to get back to you on the compression check as I have only been able to get at the top 3 cylinders so far and they are spot-on (within 3psi of each other). In regards to the "bubbling gas", it is not the idle screw that is bubbling gas but the primary jet. I adjusted the float level when i re-rebuilt the lower carb but I have not done it on the top carb. Idle speed-wise, I do not know of a method for determining the RPM without a gauge which this model does not have.

I appreciate the input. It sounds like the best bet is to simply get the OEM manual and run through the carb, cylinder, ignition troubleshooting.