carb/idle adjustment update

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Jul 19, 2003
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I received some excellent information last week on a hot idle problem I'm having with my Commando. Special thanks to Norbsa and L.A.B. Based on these inputs, this weekend I decided to mechanically baseline the carbs. Here's what I did and what I found:

1) I reset the float levels on both carbs as both were low. The procedure of heating the bowl and then tapping on the needle seat with a 3/8" punch worked well. I got them both the same. I noticed that when I checked the float level with the bowl upside down I got a different reading from checking with the bowl right-side-up because of the slop in the contact between the float forks and the needle collar. The difference in the float level setting between the methods was around .20". I chose the right-side-up method because when I set one using upside down, there was very little movement on the float needle and I suspect that the bike is running lean at idle. I may have to revisit this later.

2) I cleared the air pilot jets using a #78 drill bit epoxied into a plastic tube used for spray carb cleaners. Working very slowly and carefully, and rotating the plastic tube while applying very little pressure, I was able to fully insert the bit into the jets and clear any blockage that might be there. The left carb seemed fine, there may have been a small obstuction in the right one, but it was hard to tell. Both are clear now.

3) I tried to adjust the throttle slides at full throttle. From the procedure I have it says that the bottom of the slides should be right at the top of the carb throat at full throttle. Using the throttle cable adjusters, I dropped the slides as much as possible, but ran out of adjustment. The slides are probably still 1/8" up past the top of the carb throat at full throttle, but there's no more adjustment. It seems that this might make the bike run rich at full throttle because the needle is pulled farther out of the jet, but the carb isn't flowing any more air.

4) Then I tried to adjust the slide idle position using a 3/16" drill bit as a reference. I was surprised to see that both slides were at about 1/4" with the current setting that gives me the best idle. An 1/8" of a turn either way on the throttle stop screws would have the bike either idling at 2K rpm or it would die. Clearly these slide heights were necessary to make the bike idle with the previous carb settings. Anyway, I was just barely able to get the left carb to drop to 3/16" and contact the drill bit when I backed out all the way on the stop screw. However the right carb slide still is at least 1/32" above the drill bit and there's no more adjustments.

I think there may be a problem with the length of the throttle cable, but before I do anything else I wanted to get some advice from knowledge base in this forum. Clearly some things are wrong with the setup of these carbs because I can't get them to a nominal setting.

Also, one other possible related issue is that I've seen today that the slides don't always immediately snap closed when the trottle is closed from full open. The action of the cable seems very stiff.

Any suggestions are appreciated.

Some practice now in the shop will come in handy later on a road far away. Lets start with some concepts about why you are doing what you are doing. This will be useful later as there are different approaches to do the same things. Why would I suggest to use the pair of 3/16 drill bits? There are other ways to get there but what is important? I could have said, without any cables or springs working only with the slides in the bodies and the idle setting screw find the exact spot that the idle screw begins to touch the slide as it rests on the bottom of the body. Now be very careful to note the position of the screw and run it in one and one half turns exactly.
You see the reason to use the 3/16 as a feeler gage is to get the slide resting near the idle range and exactly the same mechanically and to create a reference point on both idle screws to do future adjustments from. You see once you are using vacuum gages that tell the truth for fine tuning you still need this reference point to be sure that you haven't strayed too far to get them set. You see that if you have built a pair of carbs the settings will be very close and a big difference would indicate that something is wrong elsewhere.
Similarly if you had the carbs fully assembled and pulled the throttle all the way till one slide was dead flush with the top of the throat and than adjusted the outer cable housing to bring the other(lower) slide even to the top as well without moving the throttle you might be very close to having the slides in synchronism.
Using the pair of drill bits does both things at once because the the bits when set to a slip fit give you a reference point for the idle screws and when first moving the throttle show very clearly if the cables need adjusting to lift the slides at the same time.
When you have these settings working down low with the bits when you pull to full throttle both slides should more than clear the top of the body hole but what is important is that they clear it together at the same time.
Now if you have discovered that your cables are made badly you will not be the first by a long shot. I always make my own cables for this and other reasons. But before you do something that you might regret be sure that all the cable ferrules are seated in their abutments and that the ferrules are set firm on the ends of the cable housings. The free play lengh should be the same and give ampule room to get the slide to the bottom of the body. That's enough for now maybe next on to gages?
Once again Norbsa, thanks for the information. I understand the purpose for the 3/16 feeler gauge to set the slide height and as the idle baseline for future fine tuning. The big problem I ran into today is that I couldn't get the slide for one of the carbs to drop to the level where it would touch the drill bit. I removed the stop screw completely and adjusted the cable to as low as it would go and I still have about 1/32" to go before the slide touches the 3/16" bit. The other carb just barely would make contact with the bit with the stop screw out of the carb and the cable at it's lowest level.

After reading some past posts on this, I think I need to trim the outer cover on the throttle cables. Currently they look fine, the cover is nicely seated in the ferrule. If I trim the cover a certain amount and seat the cover in the ferrule, will this drop the slides approximately the same amount? Any tips on how to trim the cover and not screw up the cable?

This has been so enlightening for me. With the throttle slides being set so high to get a decent idle, the air pilot screw was only 1/2 to 3/4 turn from full stop. Because the slide is too high at idle, the bike is getting too much air and running lean, then the pilot screw has to be turned in way past a normal setting to compensate. Throw in uneven and wrongly set float levels and a possible obstruction in at least on of the pilot jets and I can see why I was having some issues. Full throttle and crusing it runs strong, but idle has always been a problem.
Nipping a bit off the outer cable housing is tricky.Go slow take off small amounts hard to put back on. It's best achieved buy using small sharp dyke's and trimming one ring at a time from the end of the steel coil. Remove the wrapping just the amount you need with a razor than just nip off a piece at a time file smooth and re -fit the ferrule.
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