Running rich and shuddering at 5K rpm

[nwforester]

Hello fellow XSives. Looking for some advice here.

I've got a '79SF that I bought last year. I tore the carbs down several times to clean and restore. All of the sudden, #4 has stopped firing. After a 5 minute ride I can still hold my hand on the pipe.

The carbs are spotless and in near-perfect sync, so I can't imagine fuel not getting in. However, I've got a Colortune and can see that there seems to be plenty of spark. But still no ignition. So I'm stumped.

Even when I pop the throttle in neutral with the Colortune, I don't get a real ignition, only a few bright yellow flashes.

Any immediate thoughts? I was definitely going to swap ignition wires, just to double check the spark. But if that doesn't help that means fuel, right? So what would prevent fuel from getting in, across a wide throttle range? I'm pretty sure the float bowl is getting gas, although I'll check that too. #3 obviously is. I just put brand spanking new air, main and pilot jets. New needles, too. It was stopped firing before before I put these in, though, so the installation is un-related.

Ahh! Frustrating. Any thoughts are appreciated.

Thanks!!!

[egsols]

Is the plug wet?

[GrandPrix46]

My #3 cylinder wasn't firing (plug was wet), and I found it was because of the gasket from the carb boot to the head. The gasket wasn't installed correctly by the PO (sitting crooked). Fixed that, put the carbs back on, now it fires just like the others.

Moral of the story: maybe a vac leak somewhere near that cylinder?

[randy]

Have you opened the fuel drain tap to see if there is fuel in the #4 bowl?
If there's no fuel, the float may be hanging up on the inside of the bowl, try tapping the bowl with the handle end of a screwdriver. That MAY free the float. If not, you're going to have to remove the carb to find out what's keeping the float stuck.

[newc4]

No fire across throttle settings suggests a very extreme fuel/air mixture problem and usually means no fuel getting to the carb. Your colortune shows that you have fire. I’d first check to make sure the intake boot is tight and that you have no vacuum leaks. Then, as Randy said, check the fuel drain tap to see if there is fuel. The fact that it has quite running abruptly points to a fuel problem.

[nwforester]

This is awesome. Thank you to all of you. I had not considered a vacuum leak nor a stuck float.

Considering that the issue arose after I last adjusted the float heights, I'm betting I torqued one and that it's getting hung up. Makes perfect sense.

Since it's on the outside carb, I could theoretically get to it with out tearing the whole bank out, right? I'm mean I'm definitely gonna lose a little gas ($2 at today's prices) but it's possible right? I'd rather a 15 minute job over a 90 minute job and dealing with those #%$ing fuel lines.

[boyat68]

nwforester:

I tried that one time on my cleaning effort and did not solve the problem. You cannot see what causes the float to stick. Also, it is almost impossible to set float height correctly upside down. What I found on mine was the new gaskets were squeezing out inside and forming a lip as well as the drop setting on the floats were either too much or too little. A combination of maladjustment (lousy eye hand coordination) and gaskets that needed trimming had to be solved one by one. Off the bike and on the bench and test, test, test before re-installing.

[birch]

I bought my standard for 1200 dollars because the previous owner could not get gas in 3 and 4 cylinders. He took the carbs off numerous times and there was plenty of fuel in the bowls. Enough to keep the float and needle valves closed. He simply gave up.

I got the bike and began tracking down vacuum. I found a slight crack in the vacuum line coming off the boot to the petcock. It was getting enough vacuum to send a trickle of gas into carbs 3 and 4, but would not give it enough to run. I got a new nylon enforced vacuum hose and have not had a problem since. You might want to check that as well

[nwforester]

Just to update everyone, I toyed around a bit with the floats & float heights and then checked all vaccuum lines, all with no success. I started shooting compressed air through all the ports as a sort of diagnostic check. It was then I noticed that on #4, the air which came out of the main jet when I blew air in the air jet was signficantly weaker than the others. I traced the airways pack and found that one of the jets wasn't screwed all the way in, maybe 1/2 a turn from seated. Can you believe it????

100% my fault but a great lesson learned. Now she's running on all 4, albeit rich. I even went back in and adjusted needle height one notch, but still blackened plugs. Do y'all think one more notch or go after the float heights? If the latter, do I want more millimeters or less to lean it out.

Also, although she accelerates like a banshee, at about 5K rpm she starts to fade in and out, even shudder. It does it with the gas cap vented and the petcocks on prime, so I think we're looking at a fuel/air mixture issue. This rpm range is a transition point in the jets, right? Does anyone know if this behavior indicates a rich or lean condition? I have a K&N filter but left the factory jets. Should I increase the mains by one? But if the plugs are still reading rich, why add more fuel? Could she possibly want more air? I could drill the airbox, but with factory jets and float height, this just seems the wrong direction.

As always, your thoughts and opinions are greatly appreciated. They always lead me to a solution and more enjoyable riding.

Cheers,
nwforester

[petejw]

hey nwforester,
good work in finding ur problem,
if ur bike is pretty much stock,
put the clip back in the middle position
on the needle,
tho it works primarily for the mid range,
it also affects off idle and the main,
if the plugs are fouled, if u can put new ones in.

wat have u set the float heights at?
just curious wat ur mixture screws are set out also.

[nwforester]

Expertise from across the sea, I love it.

Float heights are pretty close to spec, about 26 mm. Idle mixture screws are backed out quite a ways (closer to all the way out than all the way in, maybe 3/4 of the way) and were set per Colortune.

All jets are new, needles are also new.

[nwforester]

Running rich and shuddering at 5K rpm

Please look to this thread for a full history. http://www.xs11.com/forum/showthread...560#post160560

[petejw]

5000-6000 thats ur mains kicking in,
will it rev past the splatter say to 7000 or 8000?
if not and its bogging down, sounds like the mains are 2 big.
wat jets size and brand did u put in /
can u put ur old main jets back in?
still need 2 kno a bit of history about the problem
did u change the jets cause of this problem
is it playing up when both cold and warmed up

from personal experience i find if the idle mixtures are screwed
to far out that is rich, i would tend to lean the fuel levels out,
if your mixture screws are out around 1 - 2 turns 3 turns max, then i reckon your fuel levels are pretty close.

[nwforester]

Stock jets, 137.5. Don't know the brand, bought carb rebuilding kit of eBay. Click Here

I put 140's in it when I added the K&N air filter. I had also intended to drill the airbox and add aftermarket pipes, the combination of which indicated I should increase one jet size according to the jetting recommendations section of the Forum. But when I decided to delay on the pipes, I dropped back down to factory after only a few days. If I remember, the shudder occurred when I had the 140's in, as well.

Problem does seem to exist when cold and warm, although I'm not really in the habit of revving it too high until it's warm. I'll check again today.

Okay, so you recommend leaning out the fuel levels. That means float height adjustment, correct? Should I increase the measurement, say 27mm, or decrease it to 25mm in order to achieve this?

[petejw]

im not familiar with the carb kits
whether there any good or not,
but if its running rich and bogging down at 5000
id put the stock original jets back in, make sure there
clear, set the clip in the middle position on the needle,
carbs upside down increasing the float level to 27ml will
lean it out, also make sure all air passages are clear,
air filter cleaned. and new plugs.
let us kno how u go