Mercruiser 3.0 - Engine wont idle, hunts, and then stalls below 1200rpm

mrsaundep

Seaman
Joined
Jun 9, 2009
Messages
55
Mercruiser 3.0, MCM 140, No. 5568367, 1982, Alpha one (Boat maker Sealine)

Sorry for the long post but I’m at the end of my knowledge/tether and want to give as much info as possible in the hope that some of you knowledgeable guys can help me to solve this.

The existing Carb (Rochester 2BBL) has the following markings: GM 2 Jet, Bay City, 7041110, 23 1-CF (2 idle jets screws), Distributor is electronic DELCO EST distributor kit.

I have owned this boat 11 years. I performed a full overhaul myself some 6 years ago with the engine and leg fully serviced. The main Engine work included: engine out of boat, pistons out, liners honed, new rings, head stripped cleaned and lapped in valves, new engine mounts fitted (2 rear 1 forward). Rebuilt carb with overhaul kit. New throttle and shift cables. At this time I also fitted an electronic distributor - a new 3.0L DELCO EST distributor kit 107-A and coil, new HT leads and plugs. I also took out the automatic choke and fitted a manual conversion. Engine has run ok since then, the odd tweak now and again to keep it idling / starting o.k. The boat is not used a lot, some 300 running hours accrued since this big overhaul.

Earlier last year (2019) the engine would not start. After some trouble shooting found a lot of fresh water in the fuel tank. Assumed it was rain through the fuel filler screw cap, it was found loose (never had this problem in 11 years). Drained, cleaned and dried, new petrol, new filters x 3 (water separator cartridge, filter in the Carter (low volume) fuel pump, and the small one in the carb filter nut fuel connector. New spark plugs (gapped correctly). After flushing through the fuel line and reconnecting it, the engine would start and run ok at high revs but would run rough below 1200 rpm and stall around 800. Tired adjusting idle stop and idle mixture screws, no success. I decided to strip and rebuild the carb, and done this with a genuine rebuild kit. Float adjustment set as per specs. Engine run o.k for half an hour. Took the boat out to set the idle mix up in gear at low RPM, done this, then suddenly found I had no drive (engine would run and rev but prop not turning). Rowed back to the staithe. Found the drive coupler was shot. Hauled the boat, removed engine, ordered and fitted a new coupler, aligned engine, relaunched boat. Run the engine up and drove the boat from the launch ramp to the staithe, about 20 mins – all well. I came back about 2 weeks later, engine run ok for a few mins then stalled. Long story cut short – fuel in the tank again! Turned out some local was going around trying to syphon out fuel and swap it with water. So I fitted a lockable fuel filling cap and to date (some 3 months now) not a drop of water in the fuel tank – so that one is solved.

Anyway, over the next few weeks I repeated the previous exercise (fuel/water out, clean and dry tank, fresh fuel, new fuel filters x 3. Engine starts OK and runs fine above 1200 RPM, as soon as I drop below that it hunts – it revs up between 900-1400 over about 30 seconds, then down again, and wont run steady below 800, and then stalls. This happens both with the throttle remote control cable fitted, and with it disconnected and operating the throttle by hand.
So I decided to strip the carb again. I got the 2 main metering jets and the power valve assembly out and cleaned the lot in carb cleaner and also in an ultrasonic cleaning tank. Note I could not (and never have been able to) remove the power piston! Rebuilt with new carb kit including needle and seat, accelerator pump, float weight was fine (10.9g) and set the float drop etc all as per the manual. Refitted the carb, same symptoms, hunts and runs rough below 1200rpm, stalls below around 900-800. What is also odd is I can sometimes get the engine stable at around 1000 rpm and it will run for several minutes and seems like it will run forever but, the revs drop and the engine dies if someone (my or my assistant) move transversally in the boat and the boat heels over – what could be the cause of this?

I have checked the following and have found nothing wrong: Compression check - (140/135/134/140), borescoped all cylinders - tops of pistons all look good (can’t see valves), removed rocker cover and observed push rods and valve operation whilst cranking – all look good, check the timing with timing light (as it does not idle its not that accurate but looks advanced enough to be just about right for 1000 rpm. I have also tried adjusting the timing by moving the electronic distributor back to the point where the engine wont even start, and then round and through the range to the next point where the engine won’t start, but nothing changes throughout that range. By the way, I have bypassed the boat fuel tank and water separator / filter by placing the fuel lift pump suction hose direct into a 5 litre can of fresh petrol, and has the same issue.

My thoughts are the carb is knackered. Research shows that , “Bay City” was a company used as an ‘overflow’ manufacturer by Rochester when they needed extra capacity, and that this carb number was cast in 1971 - noting the engine is 1982! With hindsight, when I overhauled the carb I noted the following, which at the time I did not think significant, but now feel these may be the issue:
  • Both idle mixture needle screws have slight radial scores about 4mm from the tip (as if they were probably screwed into the seat too hard at some time in the past),
  • I could not get the power piston out when cleaning it,
  • The beading on the float bowl gasket surface is worn away in some places,
  • Power valve has no check ball fitted - the book says only later models have them and mine is apparently 1971, (so should have one?),
  • Accelerator pump shaft lever bush is worn and the shaft is slack in the bush (maybe allowing to suck air in?)
  • The baffle that fits around the float needle and seat is missing (this was not there when I Overhauled the carb in 2013 and the engine run ok for several years thereafter).
I have sourced a replacement carb but it is GBP 450 so reluctant to buy it until I have exhausted all other potential causes.

Thanks in advance for any advice,
Paul
 
Last edited:

Prophammer

Chief Petty Officer
Joined
Jul 2, 2010
Messages
466
I feel your torment with all you've gone through, and with worn throttle shaft bushings, makes me wonder if correct idle adjustment is unreachable, you might be able to call these guys and tell em' what you got :

Carburetor Exchange , 3207 Peck Rd, El Monte, CA 91731 phone (800) 491-2633

their specialty is hot rod carbs, but they work on marine applications also

https://www.carb-x.com/specialty-cars-a-boats

hope this helps
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
The power valve is very lightly staked in place. Depress the actuator against spring pressure and let it snap back. Repeat several times and the piston should pop free. If you feel smooth movement of the piston as you compress the spring and the piston returns smoothly chances are there is nothing wrong internally but it is a good idea to pop it out. Shouldn't affect idle quality anyway if it is closed when it should be. Loose shaft bushings can indeed let excess air in and would affect idle quality and adjustability. Although you are adjusting the float properly, are you certain it is actually floating in the fuel and not sinking? High float level would cause poor idle quality as the boat lists one direction or the other.
 

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,093
Turned out some local was going around trying to syphon out fuel and swap it with water. So I fitted a lockable fuel filling cap

Next time try a piece of chain and a cinder block around the ankles
 

mrsaundep

Seaman
Joined
Jun 9, 2009
Messages
55
. Depress the actuator against spring pressure and let it snap back. Repeat several times and the piston should pop free.

Thanks for the reply - It says that in the repair manual and I tried this numerous (dozens) of times each time I have stripped the carb, it wont budge. Tried soaking in release agent and leaving for days. I can't really see where it is obviously staked over or I would remove the burr. The actuator does move nice and springs back freely so I was not too bothered about it.
 

mrsaundep

Seaman
Joined
Jun 9, 2009
Messages
55
I have received the following from my parts supplier: “The existing carb was never suitable or intended for use on your engine. According to Rochester, that number indicates it is a R2-2GV and was used on 1971 Chevy/GMC/Pontiac with V8s, it does not mention marine applications or an inline 4 application. And looking at Mercruiser’s OE listings for it, by serial number it uses carb 1351-7355A 1. Which supersedes to 1347-818620R02 (No Longer available from Mercruiser) and that would be our 18-7608-1".

If this is the case, I assume I have done O.K in managing to keep it going for the last few years? I have ordered a new carb and will post an update on here once I have fitted and tested it.
 

Rick Stephens

Admiral
Joined
Aug 13, 2013
Messages
6,118
I read your original post a couple times and didn't have anything to contribute. I kind of wanted to see a picture of the carburetor you are using since I didn't recognize the one you noted. Pictures are always good! If you would, satisfy my curiosity and post a pic of your carb. My bet is it isn't even a Marine carb - which is potentially dangerous. I realize this is just curiosity, and moot since you are replacing it.

Thanks!

Rick
 

mrsaundep

Seaman
Joined
Jun 9, 2009
Messages
55
[No message]
 

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mrsaundep

Seaman
Joined
Jun 9, 2009
Messages
55
Yes Rick. I was surprised at the reply. I have overhauled the carb 3 times, it looks the same as all the photos in the various steps in the overhaul section of Mercruiser manual 3.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Jul 23, 2011
Messages
47,297
Unless skippy at your parts department is well over 70 years old and going off original shop books from 40 years ago, I would question the validity of their claim that your carb is off of Poncho/GMC. as more and more information is put into databases, more and more data mistakes are occurring.
 

fishrdan

Admiral
Joined
Jan 25, 2008
Messages
6,989
I have a 72 140 HP and that carb looks like a 2GV for a 140HP, internally vented float bowl, no "additional" fittings etc that may be found on an automotive carb. If that carb has run on the engine well, I wouldn't worry about ti being the wrong model.

I have to rebuild my carb every 2-3 years as I don't use the boat enough and ethanol evaporates, fouling the carb. I'd check over the idle passages with a fine tooth comb, squirt carb cleaner into the idle meter circuits and make sure all the holes are clear. I've yanked a wire out of my steel brush, bent it 90*, then probed the idle circuit holes, just make sure they are clear (not try to distort the metal and open them up). As I remember, several of the passages under the venturi cluster feed into the idle ports, so not only the idle mixture screw holes, but the 6-8(?) other metering holes. Make sure the all look like they are flowing he same.

Also, the engine might have a vacuum leak, causing the idle circuit not being able to overcome the additional leaked air. Manifold to head gasket, carb to manifold gasket. Spray carb cleaner around the carb base and manifold gasket to see if the RPM changes, it shouldn't.

Last time I rebuilt my carb I soaked the metal parts for 12 (24?) hours in PineSol, came out clean as a whistle, but it stripped off the original "varnish" (or whatever yellowish coating they use from the factory) off the aluminum casting, left it with a powdery film that was easily removed with a light brushing. I'm freshwater, so the raw aluminum wasn't a huge issue for me. Not sure I would leave it bare like it come out in a saltwater environment.

FWIW, I have never removed the power valve, just made sure it's operating. Though, the power valve shouldn't be causing your idle issues.
 

mrsaundep

Seaman
Joined
Jun 9, 2009
Messages
55
Update ** New refurbed carb ( with external vacuum break) fitted. Had to adjust throttle linkage a fair bit. Engine now starts and idles great, even at 600 rpm. Adjusted nicely to 650 in ahead gear. Best it has ever run. Success.

New problem. Ahead gear and neutral fine but would not engage astern gear (clunking). Adjusted the position on the slotted connection on the shift cable a little at a time until it engaged astern (about hslf way up the slot). It went into astern ik and ryn for a couple of seconds then the engine stopped suddenly and I now have no electric power to ignition. Put shift back to neutral - no ignition power so starter wont turn (when I turn key to run the tacho normally shifts to read zero- that now does not happen). Checked battery ok at 12.4V. all other electrics on boat work fine. Stripped out the limit switch in the gear shift (as I know that can cause this issue) but switch is fine. Astern cut out switch also works ok on a test meter (but that would not shut down all power including to the starter motor- would it?). Negative (ground) cable is connected ok. There is a large red 50A "click to rest" fuse at the front of the engine - pressing this has no effect. Presently reviewing old posts on similar subjects. Will raise a new thread if I cannot resolve this.
 
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