72 JD 400 WILL NOT IDLE

HoosierDeereMan · Post by **HoosierDeereMan** » Mon Jan 14, 2008 3:14 pm

Flacker,

I have been fighting with my idle as well. My idle either constantly varies from 2200 to 2600 on the tach. If I get it to smooth out and crack the throttle it has a terrible bog. I had tried about everything. After some serious head scratching I hooked up a inductive timing light. My light would work on #2 cylinder when it fired but would not work on #1. I reversed the leads on the external coils to test the coils. Both coils are good. I pulled the inspection plate from the flywheel and the points look fine so I'm assuming it's a bad condenser. I had spark on #1 but it is very very weak. It isn't strong enough for the inductive light to pick up or fire the cylinder.

flacker36 · Post by **flacker36** » Mon Jan 14, 2008 8:09 pm

I am going to try new gas, gas can & new fuel filter tonight.

Hoosier - at least yours will idle, mine will but only if the High speed needle is closed. If you haven't replaced your points, I would start there. I ended up replacing the points, condensors & external ignition coils & still have the problem. The only thing I haven't replaced under the flywheel is the internal generation coils, but I have good spark in both cylinders.

Also after replacing your points & setting them, be sure to time the engine with a dial indicator & test light. I don't have a timing light so my dial indicator has to do.

I will update later.

Joe

wgrdafoe · Post by **wgrdafoe** » Mon Jan 14, 2008 9:34 pm

Does this carb use a check valve on the high speed nozzle?

flacker36 · Post by **flacker36** » Mon Jan 14, 2008 11:37 pm

Update on what I am begaining to lose my temper with.

I went back & took one spring off the muffler (like I had it when it seemed to be running good) & started it. I set the carb settings back to spec. The motor did idle but did osilate up and down. I put the air cleaner on & it seemed to level out.

Haveing the one spring on the muffler seemed to allow the engine to run at higher RPM. As soon as I put the other sping on, it lowered in rpm but did continue to run.

NEW INFORMATION: With the machine idleing, if you raise the back of the sled up & down it wants to start bogging down & eventually out. The gas tank is 3/4 full so it is getting fuel. The bogging starts immediately as you lift & set down.

I have pressure tested this engine 3 different ways & am positive I have no leaks.

ALSO: I just tried new fuel/oil mix, new can & new filter - It did not change a thing, still bogs & then dies out.

One thing I did notice is #1 cylinder plug is cooler to the touch than #2 (when ran & then taken out). I am thinking it is due to the fan blowing right on it.

HELP!!! this is starting to get very depressing!

Joe

Danzig · Post by **Danzig** » Mon Jan 14, 2008 11:56 pm

What kind of hose are you using for the pulse line?

JDT · Post by **JDT** » Tue Jan 15, 2008 12:45 am

I bet you have spark problem on #1. The fan blowing closest to that cylinder is not going to affect the temp of the spark plug.
You may have "no spark under compression" in #1.
Also when you raise the back of the sled at idel and RPM changes may be a bad pick-up tube inside the tank. Either too long so that it loops back up above the fuel level or too short so it is sucking fumes when you raise the rear.

Todd

flacker36 · Post by **flacker36** » Tue Jan 15, 2008 1:34 pm

The pulse hose I am using is a very thick black rubber (much thicker & stiffer than the fuel hose).

As for he spark plug: I thought that at first also (that the fan couldn't cool the plug down that much). What is the cause of "no spark under compression" ?? I have spark when tested on top of engine. Is it a bad set of points or internal coil? I just replaced the external coils, points & condensors.

I would add one think, The new points I installed on cylinder #1 do not close completely flat ( one edge seems to make contact but not all the surface) Could this cause the no spark under compression?

When I have it running & I take the #1 plug wire off it does effect engine performance.

I know the tank is vented through the gas gauge. The little black center of my gas gauge is missing (just an open hole now) so I would guess the tank is well vented.

The tank is 3/4 full so I don't think when I pick up the sled it isn't getting fuel. The fuel hose in the tank is right to the bottom of the tank. I do not have any return hose in the tank.

Joe

Danzig · Post by **Danzig** » Tue Jan 15, 2008 2:32 pm

I would consider getting another carb from JDMatt our forum administrator. He is building carbs and testing them prior to releasing them..

HoosierDeereMan · Post by **HoosierDeereMan** » Tue Jan 15, 2008 4:42 pm

I try to keep from using the black fuel line. I had a bad experience once with it separating and collapsing inside and restricting my fuel flow. The same can happen with the impulse line too. I would also make sure that you use clamps on this line as well.

On the spark issue. If you can pull the plug wire on #1 and it doesn't change anything then you either do not have any spark or you have weak spark that isn't capable of firing the cylinder. Instead of pulling #1, if you pull the #2 off does it die? This is what I ran into. I was working at night and grounding out the plug and pulling the engine over. I had a light blue orange spark on #1. If you see orange, you have weak spark. Good spark will be bright blue and you will hear the "Snap" when the plug fires. Night time is best for this!

The points are supposed to be flat. If they are rounded at the edges this may be your problem. What does the exhaust look like when running? Is the exhaust wet and blowing 2 cycle oil out the exhaust or around the ball joint before the muffler?

If you have weak fire in #1 and you do get it fixed I'd replace the spark plug in #1. It may be weak or bad from fuel fouling caused by the lack of juice to fire it.

flacker36 · Post by **flacker36** » Tue Jan 15, 2008 6:00 pm

When I pull either plug wire off while it is running, you can tell it is only running on one cylinder but the engine does NOT die out.

The points are new, but the one set just does't close with the two halves completely touching. The points on #2 close completely.

Could this cause the bog & how would I fix it? New set of points?

Now that I think about it I think the spark in #1 has the orange/blue look. I have tryed several different plugs, with the same outcome.

How about the tank venting? anything there I should look at?

In the meanwhile I will look into a different carb.

Joe

flacker36 · Post by **flacker36** » Tue Jan 15, 2008 6:03 pm

Forgot the muffler issue - when running at higher speed, it seems to work fine. But yes the muffler joints are spitting out black liquide the belly pan was nice and clean but now black as coal. Is this a sign of lack of spark in #1?

Joe

wgrdafoe · Post by **wgrdafoe** » Tue Jan 15, 2008 9:14 pm

How about a condenser, they can be bad even if new.

HoosierDeereMan · Post by **HoosierDeereMan** » Tue Jan 15, 2008 9:51 pm

I would try this at night or in a dark garage. I would pull both plugs. Replace them both with new plugs properly gapped at .020. Do not install them in the engine. Ground #1 and note the color and the snap of the spark. Do the same with #2. Are both cylinders weak or just one? If just one I would reverse the external coil leads. And try the spark test again. Did it stay the same or move to the other cylinder. If it moved its the coil. If things remain the same or both had weak spark you need to dig deeper. Don't forget to swap the coil leads back. Which ever cylinder is weak you need to dig into the points and condenser for that cylinder. #1 is white and #2 is red. If it leads back to the points set that isn't flat I would file them until they are. Are you sure the timing is set correctly? .023+/-.005 BTDC. Reset the points gap at .014+/-.002. I would do the spark test again on the weak cylinder. If it didn't change it might be either the points or the condenser for that cylinder. I would also check all the wiring leading from the engine plug to the stator points and condenser. Look for any signs of a short or pinched wire that might be shorting through the insulation. Pay close attention to where the wiring passes through the case halves. Is the protection grommet good? Sometimes when soldering in new condensers it is possible to get them too hot and damage them. Always try to use just enough heat to get them stuck securely.

If you get the ignition problem resolved I bet you can reset the carb and get it to idle.

flacker36 · Post by **flacker36** » Wed Jan 16, 2008 12:25 am

I will get some new plugs & do what you recommend on Wed. The points, condensors & external coils are new for both cylinders. The only thing I did not replace under the flywheel are the two ignition generating coils. Do these go bad often & if so I haven't seen them for sale anywhere.

One thing to remember is that I can get it to idle most of the time if I close the high speed mixture needle all the way. If it will idle that way most of the time, wouldn't that eliminate a spark strength issue?

As for the black oil leaking from the muffler joints: When I got this thing, you could tell it was run at 20:1 all its life & then blew the connecting rod bearing & seal. The inside of the sled was covered with black oil. The muffer & pipes had this caked inside them also. I cleaned them as well as I could with brake cleaner. There is still some in there that I couldn't get.

I am taking the carb to a guy that works on them locally. He seems to think it has something to do with the fuel pump. He said something about triming the fuel pump gasket to allow the fuel pump spring to work correctly. He wanted to know if I bought a Walbro kit or aftermarket. I did buy the Walbro kit. It will be interesting to hear what he will say, because I have been through the carb a billion times.

Update to come!

Joe

HoosierDeereMan · Post by **HoosierDeereMan** » Wed Jan 16, 2008 2:09 am

With the advancements in oil from '72 to today I would run 50:1. I'm starting to wonder if the muffler is clogged too. You said it idles until you put the spring back on which sealed the system and made back pressure. Don't laugh at this but I've heard of guys throwing mufflers into a hot fire to burn it out for this very reason.

As far as the high speed needle, this is just a guess. You may have the idle stop adjusted too high(throttle butterfly opened too far). It could have the butterfly opened to what the book calls the intermediate position. If this is the case fuel flows though the high speed needle in this position. This would cause a rich condition and cause the idle to surge depending on the fuel flow. When you seat the high speed needle it is shutting the fuel off which in turn leans the carb and allows it to idle. If you have service manual look at the principles of operation for the carburetor and you will see what I'm talking about.

I don't think you have fuel pump problem with this carb. If it runs the pump is working. If you have black automotive gas line for fuel supply lines I would change it to clear. This way you can see what is happening with the fuel flowing in the system. There should be a solid flow of fuel in the line, no air bubbles. JDT mention the fuel pick up possibly sucking air. This would help verify that.