Keihin Carb Problem

[jlbonnie2000]

Let me describe the problem I am facing with my 98 Exec 900 Sprint with Keihin Carbs.

The motorcycle starts and runs fine. Then, after maybe 10-15 miles, it will miss-fire at an idle on one of the three carbs. When letting out the clutch, the miss fire can be a real bother.

What I do when this miss-fire happens is two things. First, I rev the motor higher when taking off. Second is to turn the choke on until the miss-fire is gone.

The motorcycle runs absolutely flawless except when letting out the clutch. The problem is I never know when this excessive leaness will happen and cause the idle miss-firing.

I feel certain that I have some type of air leak. On my carbs, I have turned the air adjust idle screw out maybe 2 turns. This made the bike idle much nicer, but it did not get rid of the occasional miss-fire.

I have checked the rubber connectors mounting the carb to the head. I see no cracks in them. I have also checked and replaced the vacuum lines on top of the carbs that go to the fuel vacuum actuator.

I feel I have some type of air leak inside the carbs. I don't know if the choke enrichner lets in air as well as fuel. I know a Mikuni suppies both air and fuel when the choke is actuated.

Anyone experience anything like this with Keihin carbs?? :???:

[ocgeorge]

I know you are asking about carb problems however, I have experienced these same symptoms with a failing coil (Jap bikes). Is your bike a coil over the plug design like the 03 sprint? I know with the remote coils you can just throw a timing light on the plug wires when the problem starts occurring and verify the frequency and strength of the spark.

As far as an actual carb problem.
I believe you can get a chemical coater/sealer and give your carb intake and exhaust boots a coating.
Check your clamps/bolts on exhaust boots.

Has the bike sat for any period of time before you experienced this? Maybe drop some high quality fuel treatment (my mechanic recommended the 12$ Napa stuff) in the tank, run it for awhile to get the treatment in the carbs and let it sit for a few days. This has always helped me when I had an unexpected layup of the bike and the gas in the carbs started to gum up to the point of blocking passages.

If all of that fails, and you are still pretty confident it is a carb issue, buy some rebuild kits and mineral spirits! Rebuilding carbs is not difficult if you have space to stage everything as you take apart and reassemble them. Use new rubber wherever you can (hoses, cross over lines etc.) Mechanically sync them and then sync them with a set of vacuum gages/manometers to fine tune or take it somewhere where you can get this done.

I hope something in all of that helps! Best of luck!
And if all else fails....
:hammer:

[kludge]

The idle air screw controls air flow, not fuel. Therefore opening the screw out leans the idle mixture. If you had an air leak, opening the screw would make things worse, not better.

Yes, the Keihin chokes work the same as the Mikunis. The choke lever opens an air passage and the choke jet passage together.

I think I would start with something like seafoam to be sure the carbs are clean. Then I'd go through the whole procedure for setting the idle mixtures and balancing the carbs according to the workshop manual.

[jlbonnie2000]

A couple of very quick replies. Let me answer some issues and then maybe I can procede with something.

The coils on my 98 Sprint are not like the newer 955 motors. They are seperate coils mounted on the frame with individual spark plug leads. I have added the NGK iredium plugs that replace the standard ones. I have also replaced the spark plug leads (wires) with new ones because when I pushed them down on the plugs, they tended to spring back up.

I do not suspect I have a bad coil itself because if I leave the choke on just a little, the bike runs fine. Once away from any stop, I can turn the choke off and run down the road perfectly.

No, the bike does not spend a lot of time sitting.

I have removed the carbs and cleaned them. They were spotless inside, but I squirted the passageways anyway.

I have had this similar issue with Mikuni and Amal MKII carbs. This was after I determined that the extra air must be leaking around the choke mechanism. In both cases, I plugged the air passageway of the choke and then drastically reduced the size of the cold start jet. Both bikes started fine and ran great.

All I have to do is to locate the air intake hole for the choke. Get the bike to miss-fire, then put my finger over the hole and see if that stops the miss-fire at idle.

Thanks about the air adjust screw controlling air, not fuel. I did get a nicer idle after I went out another 1/2 turn. I don't think this has anything to do with my miss-fire since the bike can run fine for miles on end before it decides to miss-fire.

However, there is a possibility that air is leaking around the adjustment screw upsetting my idle. I can always fill the hole with some silicone and then see if this ends my troubles.

I cannot find rebilding kits for these carbs. Triumph offers individual parts.

I am contemplating a new choke assy. Triumph sells these at $15 each. Between sealing off the air thru the choke and sealing off the air that may leak around the idle adjust, I think I could solve my problem.

Last, this miss-fire is not a big issue. At idle, you can't really hear or feel the miss-fire. You only see a slight "waver" to the tach needle.

When letting out the clutch, all I have to do is give it a little more gas. It's when I ride double or have to go uphill after a stop that these miss-fires (or basic leaness) becomes a real pain.

It just seems with all these Triumphs having carbs, someone would have experienced this exact same thing and solved it.

[jlbonnie2000]

I have solved my problem. In case this happens to someone you know, this info may be helpful.

To start with, my Exec Sprint is a 1998 with 40,000 miles. With this kind of mileage, some tweeking is necessary at times.

I had an off and on "leaness" to my carbs. What this caused was a slight miss-fire on the idle. Also, a very annoying stumble as I let the clutch out to go. If I took off fast, no problem at all. But, taking off fast (to me) is not a lot of fun.

What I did was to remove the air filters, carbs, gas tank and plugs. I cleaned everything just for the heck of it. Then, I started checking things.

I found one of the chokes screwed into my Kiehins only finger tight. I removed all three chokes, cleaned them, then re-installed.

I found the rubber manifild that holds the carbs to the head would spin even though the screw clamp securing them was tight. I added a shim under the clamp by wrapping a couple of wraps of electrical tape. This then secured the rubber manifold tightly.

My carbs came with the idle air adjust screw out 1.5 turns. 2 turns out gave a very smooth idle and also allowed me to ride at slow speeds in high gear. In the past, the engine would stumble when trying to go slow.

I also sealed the bottom of the screw hole for these adjusters just in case there was air leaking around them. I do not expect to have to adjust them in the future. If I do, I simply remove the silicone sealer.

My bike now runs perfect. I rode it 50 miles with no stumble at all. In the past 2-3 months, the longest I rode without it stumbling was about 5 miles.

[seumas]

Hey how about joining us on the T300 forum? It's for the carbureted models and you'll find some people who have a lot of experience with these bikes.

[jlbonnie2000]

Last comment is the evaluation of the idle adjust. My carbs came with the adjust screw slightly rich at 1.5 turns out. When I increased this to 2 turns out, I recevied both a nice benefit as well as a slight issue.

There were two very nice benefits. The first benefit is a nicer and smoother idle. The second benefit is that I can go pretty slow without any stumble.

The one negative issue is when I let out the clutch. I get a very slight leanness that can cause the bike to stumble. It is ever so slight and only happens if I don't give it any gas. If I give the bike a little gas, everything is fine.

Would I turn the adjuster back from 2.0 to 1.5? No, I like 2.0 a lot better. I will just teach myself to give it a little gas before letting out the clutch. ;-)

[Cuyose]

Can you shim the carbs? I know this does wonders richening up the idle circuit a tad, especially when you add an after market can and don't want to opt for an entire jet kit.

You can simply go to your local ace hardware and grab 3 tiny washers a shim your carb needles. This basically sets the needle up a tiny higher in the jet allowing for a slightly richer mixture until you get off the idle circuit.

It is a super quick swap, and if you don't like the results you can always go back to original. I have found this makes for much nicer low throttle response. It wont effect midrange or high end, as that is limited by the jet size, but you should have much nicer low speed operation.

[jlbonnie2000]

Back in my 650-750 Bonneville days, there was very little I did not try to do. Especially adding the Amal MKII's to 650 Bonnies.

My goal back then was to develope a Triumph into an awesome sport touring maching. This is back in the 80's when nothing existed in this catagory.

I managed to get a 750 Bonnie to about 55-58 Mpg on average. This then allowed me to build the engine to entirely different specs. I built two such Bonnies. My son has both of them with over 100,000 per machine still running today. He presently lives in Germany.

I was able to "tune" the needles like you suggested. I found that on the Amal MKII especially, I could obtain very good gas mileage, but I had to sometimes set the needle between notches using shims.

I would often shim a needle by only .015 to .020 inch. It was .040 inches between notches for the circlip of the needle.

I am not considering shimming for my Exec for one big reason. That is all the trouble just to get to the carbs. If I could screw the cap off like on the old Bonnies, I would experiment, but I have to actually remove the carbs to do anything effective. Takes several hours to get the carbs off and back on the bike.

In my case, I have very good fuel economy and good low speed engine running right now. I can average about 55 Mpg. With gas costing over $3/gallon, I tend to ride the bike very efficiently!!

Thanks for the tip.........John