76c xs650 flat spot driving me mad

Byarchie

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England. Essex.
I have a flat spot at 4-4.5 rpm I can't cure. It has std carb and airbox setup. Carbs are clean air filters and rubbers are new. It revs cleanly on the stand but won't under load in the above rev range.You can slip the clutch and go through it and it's ok on the other side. Tried putting the needle on all notches in desperation but it doesn't change the problem. Please help anyone.
 
That RPM range is right about when the slides start lifting on these CV carbs. Maybe yours aren't? I'd check the diaphragms for holes and proper sealing around the edges. You can kinda test for diaphragm seal by lifting the slides with your finger, blocking the oval slot at the top of the intake opening, and letting the slides drop. They should fall slowly. If they drop quick, your diaphragms aren't holding a vacuum.
 
That RPM range is right about when the slides start lifting on these CV carbs. Maybe yours aren't? I'd check the diaphragms for holes and proper sealing around the edges. You can kinda test for diaphragm seal by lifting the slides with your finger, blocking the oval slot at the top of the intake opening, and letting the slides drop. They should fall slowly. If they drop quick, your diaphragms aren't holding a vacuum.
 
Thanks I will try your test. I have checked them for pin holes or tears and they are in good order. The bike is a recent purchase and previously hadn't ran for years. It had pod filters on and was running very weak. I thought changing it back to std would be the easiest route. The airboxes I purchased are from a 78 but look identical as far as my research allows
 
You might also want to confirm float levels are correct. 25mm is the setting for your carb set. If the levels are low, that might cause that flat spot. When the slide lifts, the needle pulls out of the needle jet and increased fuel flow is suddenly demanded. The bowls may not be able to supply that if their fuel levels are too low. Maybe you're momentarily sucking air in until the fuel supply catches up with the demand.

When you check the floats, be sure to measure each side of each float separately. Many times they are twisted or tweaked on their mount bracket and one side sits higher or lower than the other. If you measure and set from one side only, the setting and fuel level you think you have probably isn't what's really present in the bowls.
 
You might also want to confirm float levels are correct. 25mm is the setting for your carb set. If the levels are low, that might cause that flat spot. When the slide lifts, the needle pulls out of the needle jet and increased fuel flow is suddenly demanded. The bowls may not be able to supply that if their fuel levels are too low. Maybe you're momentarily sucking air in until the fuel supply catches up with the demand.

When you check the floats, be sure to measure each side of each float separately. Many times they are twisted or tweaked on their mount bracket and one side sits higher or lower than the other. If you measure and set from one side only, the setting and fuel level you think you have probably isn't what's really present in the bowls.
 
Well, 24mm is actually a MM richer setting, but maybe that's the problem. Maybe you're momentarily flooding out when the slides start lifting. A flat spot is usually a lean indicator, stumbling or sputtering a rich one, but that's not always the case.
 
Well, 24mm is actually a MM richer setting, but maybe that's the problem. Maybe you're momentarily flooding out when the slides start lifting. A flat spot is usually a lean indicator, stumbling or sputtering a rich one, but that's not always the case.
It is sputtering so it could well be rich. I will be having a look after work tomm. Thanks for some direction to try. What is the correct needle height for mine I have read conflicting opinions
 
If you have a '76-'77 carb set, it should have a 4M1 needle and the stock setting is in the middle (3rd from the top) slot.
 
4,000-4,500 rpm is the infamous torque flat spot that's aggravated by improper exhaust.

TX650A02.jpg
 
welcome Archie ...Essex eh ! my old stomping ground as a kid .

it might pay you to mark up your throttle into 4 segments 1/4 1/2 3/4 and WOT . I usually use a bit of masking tape and a felt tip.

Then you can test the throttle response at various parts of the throttle range and pin point which carb component and setting needs attention.

If you get a slow poor throttle response when you snap the throttle from idle to 1/2 throttle then you need to be looking at your pilot jets, air mixture screws settings and slides . You mentioned that your bike had pleated air filter pods.... well the likelyhood is that the pilot jets were increased to cope with the extra air at idle. If this is the case then you will probably need to go back down 1 stop to standard pilot jets.

You may find this jetting range chart helpful. It clearly shows how much the air mixture screw , pilot jets and slide are responsible for throttle response from idle ,right up to half throttle and beyond .

If your throttle response problem is only at around 4000rpm in every gear then mark your throttle position at that rev point and look to see which jets and components are likely to be at fault. My money is on the pilot jets, air mixture screw settings and slides .

Another test that you can do is to hold the throttle steady at 4000rpm in 4th and carefully apply a small bit of choke and see what happens . If the bike picks up it suggests that your fuel mixture is lean at that point ...if the engine bogs then you are too rich at that part of the throttle range .
 

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Another test that you can do is to hold the throttle steady at 4000rpm in 4th and carefully apply a small bit of choke and see what happens . If the bike picks up it suggests that your fuel mixture is lean at that point ...if the engine bogs then you are too rich at that part of the throttle range .

So on my 82 Xs650 with short exhaust and stock air box, it cuts out in 5th gear around 3/4 throttle but runs strong other times. The PO had a large main jet (I believe a 147.5?) with everything stock but bike didn't run strong. I redid the carbs and put in 1 step above stock on the main and runs strong until 5th gear. I've moved up and down on the main jet and still hesitates and cut out around 3/4 throttle. Suggestions?
Screenshot_2015-11-29-02-52-52~2.jpg
 
well i would say that its possibly due to your short exhaust pipes. its a well established fact that short pipes muck up the power delivery around the 4000rpm band. Minton of Minton Mods fame did some extensive testing of headers and exhausts decades ago and came to the conclusion that short pipes muck up the backpressure balance with the inlets .
have a read of Minton Mods there is a link somewhere on the forum.
http://www.650central.com/tech/mintonmods.htm
 
If you have a '76-'77 carb set, it should have a 4M1 needle and the stock setting is in the middle (3rd from the top) slot.
It is sputtering so it could well be rich. I will be having a look after work tomm. Thanks for some direction to try. What is the correct needle height for mine I have read conflicting opinions
Hi. The slide test was fine. Have adjusted the floats just tested it and it's still sputtering. It sputters from 4 up to about 5.5/6. It improves slightly if I feather out the throttle. It has 4mi needles on number 3, 25 pilots and 122.5 mains. The filters are dry yellow course foam. Any other suggestions please its not going to beat me
 
welcome Archie ...Essex eh ! my old stomping ground as a kid .

it might pay you to mark up your throttle into 4 segments 1/4 1/2 3/4 and WOT . I usually use a bit of masking tape and a felt tip.

Then you can test the throttle response at various parts of the throttle range and pin point which carb component and setting needs attention.

If you get a slow poor throttle response when you snap the throttle from idle to 1/2 throttle then you need to be looking at your pilot jets, air mixture screws settings and slides . You mentioned that your bike had pleated air filter pods.... well the likelyhood is that the pilot jets were increased to cope with the extra air at idle. If this is the case then you will probably need to go back down 1 stop to standard pilot jets.

You may find this jetting range chart helpful. It clearly shows how much the air mixture screw , pilot jets and slide are responsible for throttle response from idle ,right up to half throttle and beyond .

If your throttle response problem is only at around 4000rpm in every gear then mark your throttle position at that rev point and look to see which jets and components are likely to be at fault. My money is on the pilot jets, air mixture screw settings and slides .

Another test that you can do is to hold the throttle steady at 4000rpm in 4th and carefully apply a small bit of choke and see what happens . If the bike picks up it suggests that your fuel mixture is lean at that point ...if the engine bogs then you are too rich at that part of the throttle range .
Hi. It runs fine up until 4. I just pulled the plugs and they are sooty around the base so it would appear to be running rich above 4 cos the electrode was brown which was the last half mile home. Don't know where to go from here.
 
have you done a plug chop at 4000rpm yet ?
Inspecting your plugs when you get home is a waste of time . All you'll just see is your slow running mixture .It would be more instructive to inspect the colour of the ceramic insulator rather than the electrodes

I made several suggestions earlier including exhaust pipe length and applying a little choke at 4000rpm to test your mixture which you've not replied to, or acknowledged, so I presume you think my suggestions are not helpful.
 
have you done a plug chop at 4000rpm yet ?
Inspecting your plugs when you get home is a waste of time . All you'll just see is your slow running mixture .It would be more instructive to inspect the colour of the ceramic insulator rather than the electrodes

I made several suggestions earlier including exhaust pipe length and applying a little choke at 4000rpm to test your mixture which you've not replied to, or acknowledged, so I presume you think my suggestions are not helpful.
Hi. I will have to wait another day now to try your tests. Sorry didn't mean to appear ungrateful
 
Didn't appear ungrateful at all.

Hard when your following advice from one person and another poster expects you to start following their advice and they have got confused about who their advice is directed at, a hijacked post our yours.

That being said...............A detailed description of your bike and/or pics help people give the right advice. As you can see most posters have some info in their signature at the bottom of each post.

If your exhausts are aftermarket that will affect the jetting as well, again no info to go on
 
You say in your first post that you have new filter elements. I assume they are aftermarket. Maybe they're of poor quality and are too restrictive. A simple test would be to remove the elements and take a test run without them. I did encounter bad aftermarket airbox filter element replacements on another bike once. The bike ran like crap. The replacement element didn't flow enough air.
 
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