Carburetor and Fuel System
Title: M74-B
carb adjustment
, Cotten's float, been running like a dream. Last night when cruising along at around 40, it felt like it was surging. Seemed to accelerate fine, but when I tried to maintain an even speed (30-40-50 didn't matter), I swear it felt like it was going ever so slightly off and on with the throttle. I don't remember feeling this before. I tried to listen to the motor while maintaining speed, but we all know how well you can hear while trying this. Motor was not sputtering or anything else audible, sounded ok. It just felt like a slight off and on surging feeling. I tried the high speed needle a click either way, but it didn't seem to do much, so I stopped screwing around with that any more. Does anyone have any ideas? I'm wondering if my last gas fill was the shits? I always buy the same premium fuel (94oct) at the same stop that I know is good. The weather is hot here... last night it must have been 70. I won't be able to tell if this was the cause until we get some cooler days. There's more than likely a simple reason for this, but I prefer a little insight before I start fiddling with everything. I should mention that the plugs where dry, black, slightly sooty, the front one appearing a little leaner than the rear.
Footnote: Ever since my front tire scare, I'm a little more attentive to tiny noises and vibrations that I might have dismissed a while ago. You guys ever go through this after a close call or am I just gun shy?
Post by kell on Jul 20, 2005, 1:36am
Human nature, dude.
Post by Cotten on Jul 20, 2005, 2:36am
Your manifold might have moved. A pressuretest will tell.
Post by jarhead on Jul 20, 2005, 10:24am
While we on the subject of carb adjustment, is it possible that over a extended amount of time adjustments in the idle, idle mixture,or accelerator pump, change due to vibration of the motor? Not that this would be panfreak"s problem. Just curious.
Post by Cotten on Jul 20, 2005, 11:44am
Since Linkerts have no need of an accellerator pump, we can eliminate that one.
The needles themselves are held in place by detente balls to prevent movement from vibration.
The idle setting screw has a tension spring on later models, and a pinch screw on earlier.
Internally, a venturi that has shrank with age will vibrate and eventually wear grooves into the carb bore, allowing an increasing amount of air around it, however this would be a very long process.
Vibration and the cyclic thermal expansion play havoc with the manifold, however. It eventually saws into o-rings, and work-hardens brass ferrules.
Any sudden change in carburetion makes it the prime suspect.
Also be aware that various fuels require varied volumes to be metered for optimum power. Alcohol blends must have the needles opened. If you tune to straight petrol, and then must fill up with a blend by necessity, you will lose accelleration and your idle will change. And vice versa.
It must be added that changing one needle affects the other, so naturally a rider may "trim" his needles often as fuel, road, and service conditions change. Its a fun part of operating a piece of machinery made before everything became "consumer friendly".
Post by panfreak on Jul 20, 2005, 3:29pm
No cause for alarm.... got it dialed in again.
I hadn't really changed the settings on my carb since motor was rebuilt, and now that I have almost 1800 miles of break in time, combined maybe with the temperature around here lately, all it needed was readjustment. Two clicks on the high speed made a difference for sure.
Cotten,
I am a believer in what you say about monitoring manifold seal, I have a block with air fitting that I use to test. Since it starts beautifully and front plug was only slightly leaner looking (still a little rich though I think) I did not pressure test this time when carb adjustment solved my problem. My question to you... doesn't the front cylinder naturally run slightly cooler, and if so wouldn't this explain the plugs? Do all leaks, even small ones, show up on the spark plugs, or should a person who suspects a leak dismiss this if plugs look fine? I'm trying to digest how seasonal temperature changes correlate to rich/lean conditions. Thanks for your input, you have been an extremely helpful tutor for us all.
Post by Cotten on Jul 21, 2005, 3:03am
I have found timing to give a slight variance between plugs, or wires and coil of course. But otherwise we must assume the plugs should burn the same. Too many machines run perfect.
Everyone should please remember when pressuretesting that even the tiniest leak that may take minutes to produce visible foam still can affect performance.
(And always check the pancover screw over the ports!)
I have pleased many who brought me their machines for service that ran "good", but ran "like brand new" after sealing a very minor leak.
Post by Panzerama on Jul 21, 2005, 3:53am
Everyone,
let's not forget the obvious, simple solutions/causes of poor engine performance. Yoday's gasoline is obviously not what it used to be in the good old leaded days. One of the most overlooked sources of poor performance is plainly and simply water in the fuel.
If you ride in areas of the country where the humidity levels are consistently above 70% chances are very good that you have condensate moisture accumulateling in the fuel supply. I live in central Arkansas where the temperature and humidity usually run together: if its 90 degrees the humidity is 90%, it stays right at the dew point, day and night. Today's fuels, by nature, are hydroscopic which means that they will draw moisture from the air. Adding alcohol to the fuel increases this problem, if you buy higher octane rating fuels in the grain belt they all have as much as 10% alcohol added to to the petroleum based fuel.
I have 3 bikes with Linkerts and I ride them as often as I can. All Linkerts have "traps", it is quite simple to see how much water you have accumulated by unscrewing the trap and dumping the contents into a glass container.
It is still a mystery to me where the water is coming from, I keep the bikes in a dry garage, I buy the best fuel I can from reliable sources. I don't hose the bikes down excessively when washing them, but every time any of the bikes start to run rough I can dump the water out of the trap on the Linkert and get the bike to run smoothly again.
All I'm trying to say here is that, before you try a lot of other costly or time consuming solutions, make sure that you have dumped the trap on the Linkert and that you don't have water in the gas.
Post by rigidpanman on Jul 21, 2005, 8:11pm
ummm,im not a scientist by no stretch of the imagination,but i thought alchohol dried up water.hence the alchohol in gas drier products.
Post by Mbskeam on Jul 21, 2005, 11:23pm
hello,
I thought it joined with the water to get it thru the fuel system. the gas drier in a can is like 90-95% alchohol,
by this I mean that it makes the water mol. break down into smaller bodys, remember how water will fill up and higher than the top of a cup, water surface tension , or something like that. But im probably wrong.
mbskeam
Post by Red55FL on Jul 22, 2005, 12:09am
No Mike, you are not wrong. Alcohol mixes with both gas and water.
Therefore, fuel that is an alcohol blend will absorb a certain amount of water and will carry it through the whole system. Of course there are limits to how much water can be absorbed.
Years ago adding alcohol to the gas tank was a cheap and easy way to cure a slight water in the gas problem. If there was a lot of water in the tank, you were better off draining the tank, then add alcohol when you refilled the tank to take care of the residual water.
Red
