No power in 4th and 5th gear

sk650

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Hello everybody.

I purchased a 1978 xs650 this Friday. I rode it home, it was about a 130km or 80 miles from home.... I was told the carbs were recently cleaned, and a new battery was also recently replaced... After I took the bike out for a bit, money was exchanged, the bike plated and I hit the highway. First, second, third no problem. Then I hit fourth and and it started to bog, hesitate and kinda have a pull back sorta feeling. If I tried to keep it in 4th (going a max 80kmh 50mph) It would make loud poping noises or backfire type noises. So I nurse it home on a 110 kmh going maybe 60kmh at most... Fast forward three or more hours I get home replace the spark plugs and the bike seems to be running better though 4th/5th are still untouchable.

Where do I start? Or what might be the cause. Any help or suggestions will be greaty appreciated.

Thanks!
 
Another note of maybe interest. The spark plug on the left side that I replaced was very black'ish, while the spark plug on the right was more of a tan... After I replaced the plugs, I went around the block pulled the plugs out and sure enough on the new plugs the one on the left was starting to go black and the one on the right was on its was to being tan..... Is that normal? Or signs of a problem?....
 
Here is a list of possible problems that keep a bike from going fast:

Bike goes slow (< 60MPH)

1. Low battery / charging system. A low battery will produce a low ignition voltage at the plugs which is most evident at higher RPM's.
2. Timing. Did you use a timing light to set the timing?
3. Stuck or no advance. When checking the timing with the timing light, did you check for maximum advance at 3,000 RPM.
4. Binding advance rod. Check out this video:


5. Fouled plugs. Did you install new plugs? Check them after you got it running?
6. Plug wire not making contact in the coil. Measure the resistance from plug cap to plug cap to ensure that both plug wires are making contact in the coil.
7. Brakes dragging.
8. Speedo out of calibration. You are actually going faster than 60 MPH! Use your GPS to check the speedo.
9. Tire pressure. Pump up to 32 PSI for high speed.
10. Drive chain too tight. That will rob you of power to the wheels.
11. Enricher plunger stuck partially on.
12. Enricher (choke) left on on partially on.
13. Weak or loose advance springs. Weak or loose advance springs cause the advance to use up some of the available movement because you have to essentially retard the advance to get it on the idle advance mark, so there is less movement available at higher RPM's.
14. Timing chain needs adjustment.
15. Cheap or bad gas.
16. Low octane gas.
17. Water in the gas.
18. Partially blocked pet cock or fuel filter.
19. Paper filter that has low flow when the tank is not full.
20. Try a run with a full tank of high octane, fresh, expensive gas.
21. Blocked fuel cap vent.
22. Incorrect sprockets. What RPM are you getting at 60 MPH.
23. Rear tire rubbing on brake rod or swing arm.
24. High wind resistance. Are you a large person?
25. Throttle cable not fully opening the throttles.
26. Blockage in the muffler(s)
27. Dirty or blocked air filters
28. High altitude
29. Very hot air, like 95+F
30. Very cold air, like 32F
31. Low oil
32. Really dirty oil
33. Gas in the oil
 
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