FLAT SPOT AT 4000 REVS

I have an annoying flat spot at 4000revs i can power through it then bike runs very well..the carbs have been cleaned and everything looks as it should ..only thing i didnt check was float hight ..could this be the problem ?

 

I have the same problem I'm still trying to diagnose. I just put new plugs in mine and will pull those tonight to see how my mixture is set. Let me know what you figure out. I'm in the EXACT same boat.

 

Is the flat spot more apparent when the bike is hot or cold?

is everything stock, or do you have aftermarket exhaust/intake/jets/needles?

what bike year/model?

questions, questions... :mrgreen:

 

I can only speak for mine here. The flat spot is almost non existant when cold and gets much worse when hot. Everthing I'm running is stock. I'm riding an 82 550 maxim.

 

SaltyCity, one possible reason could be float level. If it's too high, it's much easier for the gas to reach the top of the emulsion tube when the bike's warm. You might try dropping it a bit (1-1.5mm or so) and see if the flat spot goes away. You may have to turn the idle mixture screws a couple of degrees out, since the idle mixture will get leaner, too.

 

A "Flat Spot" that gets better ... - or - one that you can work through is a classic symptom of the Diaphragm Pistons being hindered in their rise by oxidation on the Diaphragm Piston Bores.

Search: Clunk Test

Your Diaphragm Pistons need to go CLUNK ... or, you aren't getting the most out of that bike!

 

Stereo, the bike is a stock 1981 xj 550 apart from motad 4 into one pipes which it had when i bought it .Like Salty the problem is much worse when the bike is hot.The intake and filter for the carbs are stock also. Since my last post ive replaced the air filter and re checked the boots breather hose, intakes etc for damage and tightness, but this didnt improve anything I put new plugs in a while back also but color looks fine on all four, having said that the bike does seem to be running too hot.. mmmm i wonder...off for a look at the manual ..:O0

 

Guess what Rick ... on close inspection found a small tear in the diaphram on carb number 2 ,so looks like thats it ..now to sort out the overheating problem..many thanx for everyones input really appriciate it..

 

Rick- I clunked all those pistons. They are super smooth as I used the dremel and the polishing wheel to clean and polish. The engine brakes really well when I let off the throttle. I'm wondering about those intake boots. Mine are cracked on the outside, but look to be okay on the inside. I'm wondering if when it gets hot that rubber is expanding a little and allowing an air leak. I'm going to use the liquid tape other have used and see if that fixes it. I'll loo into the float height as well. That could be a real possibility as I didn't adjust those when I did the carbs. Shame on me!!! As for overheating, how do you know a bike is overheating? What temp is too high?

 

I don't know the figure.
I know the symptoms.

Plugs that are clean, too light or show signs of erosion right at the Center Electrode indicate you are running too Lean and that's bad news.

Also, tiny stress fractures at the end of the Ceramic surrounding the Center Electrode ... that, too. Heavy heat causes that.

Some people say that looking at the Header Pipes will also tell you if you are too lean and running it too Hot.
If the Stock Pipes are getting Blued beyond the first bend.

Another indicator is needing to add Oil to keep it topped-up.
Running way to Lean and Hot causes the bike to burn the Oil sheen on the Cylinder Wall as Fuel.

If you are doing that; it won't be too long before you start having an issue with the Pistons and Rings.

My greatest fear, now that there are so many Members getting accustomed to doing Fine-tuning, is having somebody get hooked on how very well the bike will run -- Lean!

The Mixture explodes in a powerful and searing hot burn, generating a Power Stroke that makes the Bike really fast!
If the Mixture it critically lean the Cylinder Heads Combustion Chamber and the Piston Crowns can't possibly shed enough heat to withstand that kind of heat and begin to deteriorate.

Inevitably, the engine self destructs.
That's why I have been emphasizing the NEED for some Color on the Plugs.
I don't want somebody to have gone through a lengthy restoration; rebuilt their Carbs and get the Mixture too damn Lean to cause all that work to go down the drain!

 

I definately don't want to ruin my engine after all I've gone through. I replaced the plugs just the other day, and still have some riding to do before I pull them and look at the color. The ones I removed are a light chocolate color and have none of the cracking of the ceramics. I've owned PWC for years and have done all my own wrenching on them. I know it's not running lean. Sounds like I'm running in temp range as my pipes aren't blueing and the plugs look to be okay condition wise.