Engine runs poorly at speeds over 45/50 (4th and 5th gear)

[Yamaha Bobber]

Hello Guys,

I have a freshly rebuilt 1975 XS650, with Mike's XS PAMCO high output electronic ignition/PMA package. It was easy enough to install and get running good. Also upgraded to a flat slide carb set up (again from MIke's XS). Bike ran fine for about 2 weeks, but now I am having issues with the motor popping and sputtering when I get into higher speeds, like 45 MPH and up (in 3rd - 5th gears). Bike idels fine and runs strong through 1st and 2nd gear giving full throttel, but once I hit 3500 RPM in 3rd, 4th, and 5th gear, the sputtering and popping starts. So bad that the bike will only go about 55-60 MPH on the freeway in 5th gear.

Here is what I have done so far with no results:

1. Checked the timing with a timing light, timing seems fine and advance looks good (38 degrees at 3K rpm)
2. carbs are getting gas (checked the float bowls)
3. bumped up the main jet size up one size (carbs came with 3 sizes and i am now running the largest size)
4. put in a new battery cus the guy at auto zone did a battery check and said it was bad. Put in the new battery and had the exact same issue. returned that battery and put the old one back in.
5. checked the spark plug wires, spark plugs, gap in the spark plugs, and put in new spark plugs. still nothing
6. checked the gas cap vent... it is working fine. Even rode the bike with the gas cap off (very carefully), and no change.
7. Opened up my wiring box and checked for shorts, blown fuses, disconected wires, and looked at the auto advance unit to see if it had burnt out. nothing looked broken or disturbed
8. checked and adjusted the cam chain tensioner and valves... in that order

I have been checking and testing for the last 2 weeks. Please give me some suggestions on what to check next... at this point I am just going in circles.

My primary suspicions are the auto advance is not working at the higher speeds or the engine is not getting enough fuel. Let's hope the auto advance is fine (cuz it is just a month old), so then I am stuck thinking it is somethins with the carbs/fuel. Help

 

[jd750ace]

You "upgraded" to Mikes flat slides, huh? your pics show BS38 stock carbs. I suspect this "upgrade" is the source of your troubles.
The E-advancer does not just quit at a certain RPM. It's either working or it's not.
If your battery failed the load test, you STILL need another battery, even if it did not fix your current problem. You can ruin a lot of your new stuff in various ways with a crappy battery. Higher RPM is higher load on the coil and the battery, btw.
I suspect your problem is your "upgraded" carbs are not tuned anywhere near correctly, and the biggest main in the kit is probably drowning it.

 

[pamcopete]

Here is a list of possible problems:

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 rear wheel.
11. Enricher plunger stuck partially on.
12. Enricher (choke) left on or 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.
13a. Tight advance springs. Timing never gets to full advance.
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
34. Weak TCI magnet in rotor. Unplug Reg/Rect to test
35. Cracked carb boots.
36. Hole in carb diaphragm(s)
37. Cheap pods covering air hole in carb inlet

 

[Yamaha Bobber]

jd750ace: The pic in the post is pre Flat Slide Carbs... good eye. I will move to the smaller jets that came with the carbs. The carbs came with 3 main jets and 3 pilot jets (jets in the carbs included).

Carbs came with the mid sized jets installed, but I put in the bigger ones because I thought the the issue was not enough fuel at the higher speeds. But your saying it could be too much fuel??? I'll test and see what happens.

Regarding the battery, I was not convinced that the test at Auto Zone was correct. He did not tell me why the battery failed the test. It showed volts at 12.6 if I recall. I think he was just looking to sell me a battery. Plus the old battery is only less than 6 months old. I use to work at Pep Boys back in the day, and those battery testers there are notorious for giving false results. I'll be doing my own test to confirm.

pamcopete: I will be checking and double check all of your suggestions in the next 2 days and report back with the progress. thanks!