40 running rough. Help!

W

Wmgmitchell

Junior Member
I have an original 40 coupe with about 73,000 miles own it. The car ran great until yesterday when it mysteriously began to run very rough. It happened while I was on the freeway and it's rough running from the time I start until I turn it off. Doesn't matter whether it's cold or hot.

It has a rebuilt coil from Skip; I changed the condenser; it has spark to all cylinders; the plugs are good and almost new; the carb was rebuilt recently using what is probably a Chinese kit; I've poured MMO down both sides of the carb, added some to the fuel and accelerated in third from about 7 or 8 MPH; the gas cap is vented and in spite of all this, the problem persists.

My mechanical abilities are limited, but I'd be eternally grateful for any suggestions on how I might solve this problem.
 
It almost sounds like the gas tank may have some "crud" in it that got sucked up into the fuel line and its interrupting the flow of fuel through the fuel pump to the carb.
 
Vic, the car has a new gas tank. There's no fuel filter but there's nothing in the bowl on the fuel pump. Where else can I check for this condition?
 
I would check if pump is putting our sufficent gas and if so send out the carb for a USA parts one.Get rid of the China made garbage in there. Could be wrong type of accel. pump,wrong power valve,wrong inlet needle [rubber] generally a pile of junk. ken ct. BUT their cheap. you got what you paid for/
 
Ken, stop trying to sell carb overhauls, and read what the man said.

WMG,There are many possibilities so we start with some basics.
When you say it does it all the time. Is one (or two) cylinder(s) not firing at all?
Have you got a cross fire? Does the engine drop revs on every cylinder, if you short out each plug lead with a screwdriver? If one does not, then it is time for a compression check.
 
Ken, when I had the carb rebuilt I was a bit younger and far more stupid. I didn't realize that there was an issue with the quality of these kits nor did I know there were alternatives. I intend to send the carb to you for a rebuild, but is there a way to determine if the things you mentioned are at the root of my problem? BTW, it does have a new repop fuel pump.
 
Bluebell, I checked the spark on each cylinder and they all seem to be okay. I don't think there is a cross fire; I haven't messed with the plug wires at all.

I'll short out each wire and see if the revs are affected. Will you tell me how I should do this test, please?
 
Was this a rebuilt pump done in the USA or a new China made one.Rubber in pumps is just as bad as whats used in carbs from you know where.Pump output into a can should come out in spurts not a steady stream. Use a wood or plastic handle touch metal shaft from the plug top metal and end of blade to the head surface. Motor should slow down when you do this as you are shorting out that plug.Im not trying to sell you arebuild job,there are other guys on here that rebuild also if needed.Charlie,NY,52 henry take your pick,it may be in the FP so try to rule that out. ken ct.
 
Just run her at idle. select an insulated screwdriver. Put the business end of the screwdriver onto the cyl. head, and lean the shank over to the top of the plug.
Make sure you keep your hand away from the shank, or she will boot you.
If the engine revs drop off, then the cylinder is firing. If you get an even result from all cylinders, then she is likely in reasonable mechanical condition.
That means you have compression, which means rings are ok as well as the valves. This then leads you on to the other two things you need:
1. fuel in the correct quantity (this is where Ken comes in)
2. Spark at the right time.
See how you go.
 
How old is the flex line that goes into the fuel pump? these can look good from the outside but can suck air and cause missing condition.
also the gasket on the fuel pump glass bowl (if it has a glass bowl) can be bad and suck air in.
a compression test is always a good idea.
 
I'll see what happens when I short out the plugs and ill check the fuel flow and the rubber fuel line and bowl gasket. Thanks very much for this information.

The car does have an electric fuel pump in addition to the mechanical unit. I haven't been using it but I did switch it on after this problem developed. Normally, there is a clicking noise when I turn it in but that's now gone so it may not be working. If it is working, wouldn't it mitigate any problems with the mechanical pump, and if it isn't functioning and is in the off position, could this have something to do with this problem?

I really appreciate all this wisdom and experience!
 
What about the gas itself? It is possible you got a bad load of gas. A group of V8ers coming home from Tahoe all stopped and got gas at the same place, a few miles up the road they were all stopped on the shoulder with crap in their carbs....IF everything else is fine I would try draining the tank and getting new fuel from a different source then the last tank. Have you looked into the bowl of the carb to see if it is clean in there? Let us all know when you find the problem.
 
I have looked at the fuel in the bowl and it looks fine. I had filled up earlier in the day before the problem surfaced, but it ran fine for quite a while before the problem began. The fuel was Shell, so one would think it would be okay. I did add some Stabil ethanol treatment after the fill up.
 
If your elac. pump isnt working and its the non pull through type,the mech one will not pull gas through it. Hense like running out of fuel. ken ct.
 
air cleaner off??...look down carb(with choke open) and see if you can see fuel squirting when you operate the carb, accelerator pump.....sounds like you have fuel problem, so maybe start at carb and work back to tank??...good luck....Mike
 
I grounded out each of the plugs and with each plug on the left side, there was no noticeable difference in the engine revs. On the right, each test resulted in a noticeable stumbling of the engine. There seems to be a good spark all the way around.

What can one deduce from this?
 
Wgm, I don't know now.
The different result from one bank to the other (shorting the plugs out at idle) does not point to a carb issue, because if there was a difference from one side of the carb to the other, it would effect 2 cylinders on each bank.
This doesn't mean you don't have a fuel problem that is common to all cylinders.(or a part of he carb that is common to all)
Try slowing your idle down,so that it will just run, then check it all again.

JDL and Ken, have made very good points. Sometimes an air leak on the suction side can be very hard to pin down, as is a poor pump delivery volume.

My line of thought from the beginning was to have you fault find the problem in a logical manner. So many people just dive in and change things without establishing the cause of the problem.
What I am trying to establish is that you don't have a mechanical problem.

The problem I have now with one bank of cylinders doing what it should(dropping revs when one plug is shorted out) is fine. Those cylinders are working for a living. I'm not sure what to make of the other four.
I am wondering if you have blown the head gasket between two cylinders, but dont want you pulling a head on my say so.
Can you describe in more detail what the rough running is like. Does it sound like 1 or 2 cylinders (or 4) are not working as they should, or is it more of a general sort of a thing across all cylinders? Is it worse at idle?
 
Ok away from it being a carb problem,A bad rotor blade on a 40 type rotor would affect 2 cyl's on each side,might it be a bum rotor,or those 4 wires not making good contact in the inner cap or plate contacts are coroded or worn down. Looking at all possibles. ken ct.
 
The engine sounds like it's running on 7 or 8 cylinders. It doesn't stall at idle but just keeps chugging in a very lumpy fashion.

I removed all the plugs and can't see any evidence of a broken head gasket. Some of them are sooty, but I think this has always been the case.

I looked at the rotor. To me it looks okay, but I'm no expert. I will look more closely at the two caps and the individual wires.

I'm going to look further at the electric pump this morning. There may be a filter in front of the pump; not sure but ill check it out. I haven't been using the electric pump and haven't had any problems in the past, but maybe the filter is clogged (assuming it has one).

I've checked the rubber fuel line going into the mechanical pump and the bowl gasket. These seem okay.

This is driving me nuts!!
 
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