My 73 XLCH 1000 Sporty has an 81 Keihin carb that is a little quirky. I bought the bike in October of last year for a great price, because the bike would not start easy at all. It was a broken accelerator pump diaphram that was causing most of the trouble.
I rebuilt the carb completely and cleaned all of the ports while I had it apart. Now it starts very easily and runs solid, except for at low idle or just barely on the throttle when it has warmed up. The idle screw has to be all the way in, in order to keep it from running too lean at idle. The jets are much larger than the book calls for ( I have 89 low and 165 high; the book called for 65 low and 125 high), yet it seems to run lean at more than 20 points above stock.
It "pops" about every 10 seconds when cruising at lower speeds or at idle, unless I pull the choke on about a 1/4". Then it smooths out and runs great, except now the high idle is on and it runs about 1500 rpms with the high idle backed off as much as possible.
If I go lightly on the throttle with the choke all the way off, it "pops". When cruising down the road, it pulls the hills well (with the throttle on more), but cruising down a hill with just enough throttle to maintain speed; it "pops" even more.
I sprayed carb cleaner on the intake manifold where it meets the jugs; as well as where it meets the carb and found no signs of any vacuum leaks. I inspected the manifold o-rings and sealing surfaces twice, and they were all good.
I would like to get it where I can have the idle screw close to what the book calls for (1 1/4 turns out) and have the idle speed normal without any of the "popping". At idle, it almost stalls the engine when it "pops".
Any suggestions?

 

When you refer to "the book" for jet choice, what book are you referring to? The stock jets for the 1981 Keihin carb in a 1981 engine, according to the factory servive manual, are main 160 and slow 52.

The terminology can be confusing - i prefer main and slow for the jets as does the manual. I use pilot screw for what the manual calls the low speed mixture screw. We usually use idle speed screw for what the manual calls the throttle stop screw.

The low speed mixture screw on that carb was capped at the factory. A previous owner has presumably removed the cap so that you can access it.

What does you bike have for exhaust and air cleaner? Probably not stock, probably free flowing A/C and exhaust? so you would need jets larger than what is call for in the mnanual. However that is juts guessing.

Here is a procedure to determine the correctslow jet and pilot screw setting. Start with the recommnended jets for the carb and follow this procedure. After you have determined the correct slow jet and pilot screw setting there is a procedure to follow to select the main jet. But the slow jet and pilot screw must be set first.

The 165 main is close enough to keep in there for this testing. The 89 low will be way too large.

Setting The Pilot Screw on Your IronHead

If the carb is old and dirty the pilot screw passage may be gummed up such that you will not be able to "gently seat" the pilot screw reliably. If this is the case remove the carb from the bike and clean it up. Some guys try to do carb work with the carb in the bike. IMO this is a very bad idea.

In the pilot screw passage there should be, in this sequence: screw, spring, washer, o-ring. Occasionally POs have installed these parts in the wrong sequence; remember that the purpose of the washer is to protect the o-ring from being damaged by the spring]. The screw usually comes out easily. The other parts may require some work. The best technique is to stick a pipe cleaner in the hole, twist it around, and, like magic, out come the other parts on the end of the pipe cleaner.

EDIT: Some carbs, noteably 1966 to 1978 Sportster carbs, do not have the o-ring and washer in the pilot screw passage.

1. You need to have easy access to the pilot screw, easy enough to reliably judge "screw it in until gently seated". Loosen the front fuel tank mount bolt; remove the rear fuel tank mount bolt; prop the rear of the fuel tank up on a piece of 2X4; on some bikes this will not be necessary.

2. With the engine cold [so you do not burn your fingers] turn the pilot screw in clockwise until it is gently seated. Count the number of 1/4 turns as you do it; write the number down. Back it out to the original setting. You may need to return reliably to this setting after experimenting.

The "normal" starting point for this process is 1,1/4 [according to the 79 - 85 FM] or 1,1/2 [according to usual practice] turns out.

EDIT: An old HotXL magazine article recommends for Keihin butterfly carbs between 1/4 and 1,1/4 turns out. My experience is that this works best. If you are more than 1,1/4 turns out your pilot jet is too small.

3. The engine must be at full warm up. It will have very hot parts; to avoid burned fingers have a well lighted, comfy place to work.

4. Set the engine idling at about 1000 RPM. You want it to be idling at the slowest speed that is consistent with a smooth idle so that you can hear or feel slight changes.

5. Turn the pilot screw in clockwise until the engine idle becomes worse; tending to stall. Count the number of 1/4 turns as you do this. Then turn the pilot screw out counter clockwise until the engine idle gets good, then becomes worse, tending to stall. Count the number of 1/4 turns as you do this.

6. The best setting for your bike will be somewhere between these two settings. The FM says to use the leanest setting [most screwed in] consistent with a good idle quality. Some guys say to go between the two settings.

It should be between 1/2 and 1,1/2 turns out from gently seated. If it is not within this range you should change the slow jet.

EDIT: I follow the advice from the old HotXL mag article - set it between 1/2 and 1,1/4 turns out.

7. You may have to adjust the idle speed.

8. Make sure that the spark plugs are clean, then ride the bike. I usually go for a half hour or so ride outside the city, then ride home with the last 10 minutes at city riding speeds [so that it is on the "idle port", not the "idle transfer ports", the "mid range port", or the "main jet" [see carb diagrams in FM]]. Hopefully the plugs will come out a nice medium gray or tan color. If they are too dark you can screw it in another 1/4 turn; too light screw it out 1/4 turn, and try the ride again.

9. If they are really light or really dark the problem is not with the pilot screw setting. For example, too light might mean an intake or exhaust leak, and too dark might mean the pilot jet is too large [among other possibilities].

10. My experience with this process is that after making a change i have to clean the plugs and go for a good ride [say, an hour or so] before i can trust that the new results are reliable.

 

I appreciate your input on this Mick.

It is the low speed mixture screw that I was referring to. Someone before me has removed the cap. It had to be closed completely for the smoothest idle.
The air cleaner is an S&S and the exhaust is 1.5" drag pipes with bolts in the pipes near the ends to create a slight back pressure. I did that because the low end torque after I got it running the first time was nonexistant. It has tons of low end torque now and still has plenty of noise too. Made a big difference in the performance in good way!
The high idle screw is all the way in to keep the rpms down (2000 rpms) when running with the choke on.
I have the idle set to 1000 rpms when warmed up and with no choke on.
I drove it all day yesterday, a mixture of city, country, and highway driving. It ran really good with the choke cable pulled out 1/4", but I had to push it in when stopping to keep the idle down. It started on the first kick every time.
It averaged 35 mpg running with the choke on as described above.

Today, I pulled the carb and completely disasssembled and cleaned it (including both jets). I found the low speed mixture screw had all of the components that you mentioned, in the proper order, except that it has no spring.
When I disassembled the carb, I found lots of trash in the carb bowl. I have installed a fuel filter in the fuel line to prevent any more trash from getting into the carb. The joys of old iron!
I have reassembled the carb with the exception of the low speed mixture screw. I need to come up with a spring for the low speed mixture screw, and then reinstall the carb on the engine.

Once I have it running, I will adjust the carb as you have described. I have a stubby screwdriver that I made to fit the space under the tank that works very well without having to lift the tank.

I am hoping that the trash in the carb was the bulk of the problem. I made sure that all ports were clear and clean before I reassembled the carb.

I received my new clear timing plug today, so I plan to check the ignition timing on the engine after I get it running again. I am thinking that this could be a small part of my problem.
I have some other maintenance that needs to be done also, and I am adding some new stuff (hand controls, switches, and grips). I figure this is a good time to get that done too.

I hope that this coming weekend I can have Ole Yeller running better, and ready to ride again!

Thanks again for your input!

Chuck

 

Here's my notes on setting ignition timing ...

Setting The Ignition Timing on Your IronHead

1. Checking vs Setting the Ignition Timing

You can check the timing without changing anything. Changing the timing can be done by one person but is easier with two. The ignition module or points is on the right side of the bike, the timing hole is on the left side. Easier with one person on each side than to go back and forth. Standard advance for the 1980 XL is 40`. I have mine set at about 35`. At 38` i get pinging.

2. Equipment That You Need

You need an induction timing light which you can buy at any auto supply store. Best to get the "dial back" type. These allow you to set a number [like 35`] into the timing light and then adjust the ignition module/points backplate until you are there.

If you have very well insulated spark plug wires the timing light may not sense the signal thru the wire. If this is the case then open the gap on the spark plug to 050 or more. Reset it to your normal gap [030 for points or 040 for electronic] when done with this procedure.

It also helps to have a throttle lock as the timing should be checked/set with the engine at 2500 to 3000 RPM.

You will need a piece of rubber oil line hose about 1 inch long. Press one end up against the side of a stone grinding wheel [or whatever] to make sure it is perfectly flat.

3. Front Cylinder: TDC Mark vs Advance Timing Mark

The TDC [Top Dead Center] mark is used with a dial back timing light to check the timing, or to set the timing at a specific degree, such as 37`. The advance timing mark is used to set the timing to the factory setting, such as 40`. Best is to use a dial back timing light to check or set timing.

4. To Identify Your Front Cylinder TDC Timing Mark

Remove spark plugs. Rear wheel off the ground, shift to 2nd gear, rotate wheel until it clicks, repeat until you get to top gear. Now by rotating the rear wheel you are rotating the engine. Almost impossible with the plugs in; very difficult in lower gears.

You want the front cyl to be at TDC. You can see in thru the spark plug hole, especially with a flash light; you can try inserting something soft like a pencil and watch it rise and fall as you rotate the engine with the back wheel but be careful it does not break or get stuck! You will have to hang on to the pencil with left hand while rotating the rear wheel with the right. Helps to have a good wingspan, or an extra person.

With the piston at precisely TDC look in thru the timing hole. You should see a drilled dot or a line. That is your front cylinder TDC mark. Once you know for certain what the TDC mark looks like you can check the timing.

5. Procedure, Checking The Timing

Begin with the engine at operating temperature. Remove the timing plug from the left side of the engine. Insert the 1" hose smooth end first, right tight up against the flywheel. I use a needle nose plier and twist it in there as firmly as i can. If it is not smooth and tight against the flywheel oil will spray out while you are working.

Start the engine, have it at about 2500 to 3000 RPM [it will be extra noisy with the timing plug out], set the timing light at 40`, aim it into the timing hole thru the hose. Click it down one degree at a time while you look for the timing mark. If you don't find it you may have to work up from 40`.

If it is in the 35` to 40` range and there is no pinging then you may wish to leave it as is. With pinging on acceleration you may want to retard it by about 1/16 of an inch or less [see Math below].

6. Procedure, Setting the Timing

Remove the points cover from the right side. You will be rotating the back plate so first mark it so you can get back to the current position if needed. Make a scratch mark or use a permanent magic marker, or better still a small bright red or whatever color paint mark.

To change the timing loosen the two standoffs holding the points/ignition module and rotate the backplate. Clockwise advances [larger degree number] counter-clockwise retards [smaller degree number].

7. Some Math, How Much to Rotate

The backplate has a 3" diameter = 1.5" radius.
The circumference = 2 * pi * radius = 2 * 3.14 * 1.5 inches = 9.42 inches
The circumference = 9.42 * 16 ~= 151 sixteenths of an inch.

One rotation of the backplate corresponds to two rotations of the engine, so it is like a circle that has 2 * 360 = 720 degrees. 720 / 151 ~= 5

Therefore a rotation of the backplate by about 1/16 of an inch results in a timing change of almost 5 degrees!