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1977 Merc 500 Not getting enough Gas on top two cylinders at lower speeds.

F

Fuddyduddy

Regular Contributor
My old Merc runs well at WOT and higher speeds, but doesn't idle down and quits when it gets too slow. Tried adjustments but just will not idle. Seems like the top 2 cylinders are starved at low range. I pull the top 2 plug wires and it makes no difference. The top two are getting spark (boy! Howdy! with a woohoo thrown in) WAGs are welcome. kinda at a loss. Ran well last fall and did all the necessaries. Starts up good.
 
I would start with removing the 2 carburetors together and cleaning them.----Not much coin involved in doing that.
 
racerone said:
I would start with removing the 2 carburetors together and cleaning them.----Not much coin involved in doing that.
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Both carbs had new kits last summer and I just cleaned them again a few weeks ago. Didn't seem to have any crud and the bowls had no sediment. The fuel pumps checked out fine. Almost seems electrical but the spark is there. Going to try a timing light and check the timing but it really hasn't been changed since I have owned it. Put new plugs in it today so I will get a chance Friday to take it out to tinker some more.
 
Is the tube item 37 clean in the carburetor? ​
.......................I don't run a filter from the bulb to the motor so it may have something in the carbs again. I will find out if it is another "opportunity" to get aquainted with taking them off. Answer to the question is: I don't know but the carbs were cleaned (removed) just a couple weeks ago and the main gas tube was clean on both. We are taking the boat out to run it so I will get back later this afternoon with the report.
 
not sure if yours has the two pumps like mine on the side of the block with tubes going everywhere? might be a pump diaphragm? Install inline psi gauge and check while running? if one pump not applicable id look at carbs yes.. also check the thin vac tubes that run between cylinders (if u have them)
 
racerone said:
You are aware that a plugged carburetor can be a risk ?---No fuel going in means no oil either !
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Yes, I am aware. Almost nuts from overthinking all that can go wrong. We ran it up until it gained some rpms by itself on the cold lever then it gain some more and then I brought it back down to idle and it didn't quit. Then I took it and checked the hole shot and it was very quick and ran like it would pull two skiers. Still a bit rough on idle then so just decided to redo the top carb and the fuel pump on it. I have the kit but I need a good float just in case. The Ol' Merc Mechanic I had with me thought it would be a good idea since I am going to take the carbs off again. He was wondering about the jets that are Lock-Tited on. Never have had those off. Going to also get a fuel pump kit and needle and seat for the bottom carb.
 
FstaRockr Burns said:
not sure if yours has the two pumps like mine on the side of the block with tubes going everywhere? might be a pump diaphragm? Install inline psi gauge and check while running? if one pump not applicable id look at carbs yes.. also check the thin vac tubes that run between cylinders (if u have them)
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I pumped the ball to get it to stay on idle smoothly and it changed nothing. I am still going to install a kit for the pump. Read the post above for complete update and thank you. BTW- there are two pumps each on the side. I will look into the vacuum tube thing. Never heard that mentioned before.
 
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Installing the pump kit and I have two pieces that are identical in form but one is a fine rubber gasket the other is a fiber gasket. Which goes on closest to the carb body? My manual says just one of those two check valve diaphragms gets installed. So which do I use?
 
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I had both together and that was one too many when I rebuilt that pump last year. The rubber one is the same material and thickness of the main diaphragm.
 
FstaRockr Burns said:
im sure either wd work on a carb but id prob opt for the fiber one - not sure what the "rubber" is made of..
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I took off the bottom carb and checked how the pump was configured on it. It had both so I built the upper the same since the lower carb works very well.. Need to run it at a lake.
 
fastjeff said:
Dying to hear how you make out!

Jeff
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I am going to take it out to my "old standby test-track" and run about a gallon out to see if it will straighten out. It starts right up and idles great on muffs for about two minutes and then starts to idle rough when it is warm. The cold start lever needed to be raised at first and then about a minute into warm up it was put down and the motor didn't quit this time. Just idled rough. Glad I planned a bucket list trip to Ontario in June with my older Brother. I am not getting much fishing done around here. Maybe I'll stop by and bug Racerone. He will probably move before I get there.:cool: Thanks Jeff. I'll post results.
 
Finally got it out for a test. Started easily and idles off the trailer nice. Putted out past the speed zone and canned it. Ran great and had a great hole shot. Came back in and slowed down to troll and it quit. Started right back up but wouldn't stay running unless I raised the warm up lever a bit. Then it would idle just fine and troll nicely. Tried that for several minutes and then gave it throttle to plane and again ran fine. I really haven't got a clue why it doesn't want to idle, now. Might be that it is, like its owner, old.
 
These motors in good condition will idle like a dream.----Have you checked compression ?---Lower crank seals ?--Reed valves checked ?----Strong spark on all 4 cylinders ?----Carburetors opening too early perhaps ?
 
Sounds like either your idle mixture is a bit off, or the low speed throttle stop needs adjustment.

Hang in there!

Jeff
 
Racerone- It idles like a dream but I have to have the cold start lever up a little to do that.. I don't really know about the other things except the spark and compression are very good. I really don't want to open up the "upper" at this stage. Old girl runs like a new one on all but the idle. Not too sure my health will hold up for more than a couple more years. COPD.

Fastjeff- I will try to get back out next week with my "Ol Merc mechanic" and see if we can't get it adjusted a bit better. Runs real well in all ranges just needs to up the idle. Last summer I found the "economizer jet" missing on the problematic upper carb. I just screwed in the new one nice and tight. By any chance that is causing the problem?
 
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