Engine only runs at full or partial choke

[gary gerber]

I had a similar problem at the beginning of this season in my late model A4 in my 1970 Morgan which I posted and was helped thru the difficulty. The problem has appeared again. I had found a water leak into the cylinders from the head gasket, I pulled the head, cleaned the ports, cleaned the block, replaced accessable hoses, installed a new thermostat, new gaskets, retorqued the head to the proper settings, checked tightness of the exhaust manifold and carb bolts. The engine starts right up and runs smooth but only with the choke out either full or half way. I adjusted the carb idle screw and adjusted the throttle screw over and over again trying to find the right balance all to no avail. The fuel and the fuel filter were all new this season.

The engine running smoothly under full choke in her slip, is exhausting water in a normal manner. I get good spark to the plugs. I did notice that if I cut back on the choke the engine will "search alittle" but she will rev up strong if I give it throttle, I suspected an air leak but everything is tight and there are no fuel leaks. The oil looks good no appearance of water contamination.

The carb is essentially a new carb from Moyer. I welcome some experienced advice.

[Mo]

Leave her tied to the dock and put the power to her...does she rev up; does she sputter and rev; or does she just not have it...let me know when you get a chance.

Best
Mo

[Mo]

If you have to choke it she could be starved for fuel as well..fuel pump could be getting weak. If it will run with choke in try to throttle and see how it is; if you need to keep choke pulled you may have an air leak or starving for fuel. Just because you changed fuel filters this year does no mean they are not fowled; this may very well be your problem. Another thing to look at is the pump itself - mechanical or electric.. and try and determine if you are getting enough fuel; if all this fails look for a small leak in your exhaust manifold that is messing with your mixture...You sound like you have A4 knowledge and capability so doublecheck the obvious, choke connection and it's range of motion; air leaks at carb; hang with it there and more members will jump in here and help you out...some of these guys have the links to all this stuff.

Best
Mo

[hanleyclifford]

It sounds like insufficient fuel is getting to the main jet and discharge nozzle. The smallest piece of material in the main jet will cause your symptoms. Most members now have a second, final, so-called "polishing filter" installed just before the carb typically downstream of the fuel pump. The large scale introduction of ethanol and other pollutants into gasoline mean that we have to assume that years of accumulated impurities in fuel tanks and systems will find their way downstream to the carb unless opposed by quality filters. The MM online catalogue lists two types of filters - you need to have both. It is pointless to keep cleaning the carb unless you take steps to keep it that way. Once you have good filters in place, remove the bowl plug and drain the contents into a clean container. You may catch the culprit.

[gary gerber]

Thanks for the tips. Always good to get fresh ideas. I plan on checking the carb today for sediment and will replace the water separator filter also.

The carb is , as I said, essentially new and I had pumped out the old gas and replaced it with fresh 92 octane fuel.

I welcome any other suggestions, this forum is truly a lifesaver.

[Marian Claire]

First I would set the idle mixture to the factory setting. I think it is 1 ½ turns out. Then remove the flame arrester and visually check for proper choke operation. I would then check for flow to the carb. Disconnect the line at the carb and see if you have good flow. If yes then the pump and filters are OK. Then reattach and open bowl drain and check. If yes the the needle valve is OK. As stated above it takes only a tiny piece of crud to mess things up. How did you check for air leaks? Did you check the carb to manifold connection? Do you have a scavenge tube? Is the connection at the spacer/manifold tight? Keep us posted. Dan S/V Marian Claire

[gary gerber]

Hi guys, well I pulled off the carb and blew thru the various openings and orifices, no blockage. I removed the fuel bowl and there was some sediment particles in the bottom. I remounted the carb and again, she starts right up under full choke.

Leaving her running at full choke, very smoothly initially then after about five minutes begins to decrease rpms, then regain speed, decrease engine speed, then regain rpms again then decrease rpms until she stops, some minor backfiring, just a few "pops". I have never had a backfire situation with this engine in the thirty five years I have owned her. It is actually running worse now then a few days ago when I first experienced the full choke problem.

I had checked the carb screws and the exhaust manifold nuts for tightness earlier.

Truthfully, I am at wits end on this one.

[tenders]

Backfiring -- are you sure it's really backfiring? not just dying loudly? -- makes me think:

• Misconnected wires (triple check 1-2-4-3, NOT 1-2-3-4!!)
• Sticking valves
• Profoundly mistimed

Varying engine speed makes me think:

• Dying fuel pump
• Air leak in fuel system
• Bad condenser
• A return of your water problem

[Marian Claire]

Sorry if I am repeating but did you check for good fuel flow to the carb? What kind of fuel pump do you have? Did you use WD-40 to check for air leaks? Do you have a primer bulb? Dan S/V Marian Claire

[gary gerber]

One more test before giving up for the day, I retorgued the head after the engine was warm and ran a compression check. 90 pounds across the board for all four cylinders.

[gary gerber]

Sorry, forgot to mention I had good fuel delivery to the carb from the mechanical pump which was rebuilt within the past few years. I had checked the spark to the plugs on the first day I had a problem.

Hand cranking the engine when the head was off, after cleaning the block
( also cleaning the head of carbon build up ) each valve opened and closed smoothly.

When just running the compression check, the plugs looked good were new
the beginning of this season.

I did not get to spray anything around the exhaust manifold to look for air leaks, I plan to do this tomorrow morning. Needless to say I already have burns working around the hot manifold in the confined space, something you can appreciate.

Thanks guys for taking the time to help me with this, it's becoming like a soao opera.

[sastanley]

gary...since you have a mechanical pump, you have TWO bowls to check....the fuel PUMP bowl, and the bowl in the carb. There is a drain plug on the bottom of the carb bowl (to confuse you even more, some have two!!), the one on the side of the bowl gives you access to the main jet...this is often a trouble spot...you need to spray some carb cleaner directly in that jet to ensure it is free & clear...sometimes you can successfully flush out a piece of crud by opening that access port (be careful not to lose the fiber washer) and pumping some fuel thru the carb bowl with the mech fuel pump lever....but if it was me, while I had that plug out, I'd spray the heck out of the jet itself. If THAT doesn't do it, then you'll need to crack the carb open and get all the other jets.

I think that was what Dan was alluding to...did you get some carb cleaner in the main jet in the bottom of the carb bowl??

If you do a little digging around the forum, there are lots of diagrams of the carb and pictures of them apart, etc... I had this problem last fall, and it turned out to be a piece of teflon i used in sealing threads upstream in the fuel system.

Ok - where is Jerry with a picture of the carb with the jet showing?

check this - http://www.moyermarine.com/forums/showthread.php?t=3697

[gary gerber]

Hi, Yes there are two fuel bowls. I did remove the pump bowl and clean it and I removed the plug on the bottom of the carb housing and fuel flowed out into a cup. I then removed the carb completely from the engine to check orifices, excess fuel flowed out of the openings however I did not disassemble the carb.

I plan to attack the problem tomorrow using WD40 looking for air leaks and spray carb cleaner as you suggest.

I appreciate the guidance.

[Marian Claire]

Good deal. The carb is not that hard to work on. Getting the halves apart without tearing the gasket takes the most finesse. I assume that your main jet has not been modified into an adjustable. Make sure you clean out the two small holes in the throat. A small piece of SS wire and cleaner works well. As you remove the parts sometimes the gasket/O ring will stay in the body and then come flying out when you blast it with cleaner so have a nice clean area to work on and wear safety glasses. I check, look thru each piece, as I take it off before I clean it. I usually can see the offending crude. Each jet/part should have a nice round opening when held up to a light. Good luck. Dan S/V Marian Claire

[ArtJ]

Sorry if I am repeating but did you check for good fuel flow to the carb? What kind of fuel pump do you have? Did you use WD-40 to check for air leaks? Do you have a primer bulb? Dan S/V Marian Claire

How do you use WD 40 to check for air leaks? Are you spraying in on
fuel hose connections?

Regards,

Art