Slow top end

[ninskrillz]

I've been trying to solve this problem for the past couple weeks. I can't seem to get the bike to get past 75mph. I also can't seem to get more than 25-30 mpg. I dunno if those are both intertwined or not but for now I'll just focus on the speed. I'm fairly certain this thing will go faster.

The bike has a PMA, Cornell dubilier 10000uf capacitor, Pamco with e-advance, fishtail ya mama exhaust, vm34 carbs, uni filters, and ngk iridium plugs gapped to 35.

The first thing I did was make sure the cam chain, valves, and timing were set. I got the timing spot on the marks, though I'm skeptical of the accuracy of the marks so I advanced the timing slightly. Marginal if any improvement after that.

I've tried 185, 190 and 200 main jets. It currently has a 200 main, 30 pilot, 6f9 needle in position 2, a p6 needle jet and 24mm float height. The mixture screws are about 1.25 full turns. I think I'm going to shim the needle.

And that's all the info I can think of. I'm sure it'll go faster, I'm just out of ideas! Lead me in the right direction!

 

[grizld1]

Yep, any healthy 650 cc. twin should take you well into 3 digits of speed; your motor is ailing. A few suggestions:
1. You need to locate TDC accurately--with piston stop, degree wheel, and dial indicator, not a stick poked in a spark plug hole.
2. When it's possible to know where it's set, reinspect timing.
4. Check compression and run a leakdown test.
5. When you know that everything else is right, straighten out your jetting. What you're using is way too rich for your setup, from the pilots through the mains, and it surprises me that you can even keep the bike running on that. Use some cheap BP7ES conventional plugs while you get your tuning straightened out.
6.Avoid the temptation to look for a fast one-step fix; do your troubleshooting one step at a time.

 

[ninskrillz]

Ok this video from member I am carbon details how to mark tdc with a piston stop.

Is this a good way to find tdc? Then the degree wheel for the timing marks correct? Any more guidance to do this right would be great.

 

[grizld1]

That will only put you in the ballpark. If you want to be accurate, set up a dial test indicator on the piston stop. Find what looks like the highest point of piston rise and make a temporary mark. Mount the degree wheel and zero it to the temp mark. Finally, select an arbitrary drop off TDC; .050" is enough. Read the degree wheel at that drop before and after TDC. Correct the mark so that it falls at the midpoint of the degree readings.

 

[pamcopete]

Here is a list of possible causes of low top end speed:

Bike goes slow (< 75MPH)

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 75 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.
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

Other possibilities:
1. Which PAMCO do you have?
2. If PMA, then battery or capacitor?
3. Was the bike originally a US TCI engine or points?
4. What octane gas are you using?
5. Which carbs do you have installed?
6. Any mods to the exhaust or air filters?
7. What gear and road speed does the problem occur?
8. What was the most recent work done to the engine besides the PAMCO?
9. Have you tried going up one size for the main jets in the carbs?
10. What is the battery / capacitor voltage at 5,000 RPM?
11. Have the ignition switch and kill switch contacts been cleaned? Ever?
12. What is the altitude where you are?
13. New spark plugs?

If you installed the PMA at the same time as the PAMCO, then several issues could come into focus:

1. How did you establish the timing marks?
2. Does the rotor of the PMA have a key to the crankshaft?
3. Are you using a battery or a capacitor?
4. What is the battery / capacitor Voltage at 5,000 RPM?
5. If a capacitor, then what is the capacity and brand of capacitor?
There can be issues with the PMA that manifest themselves as ignition problems. One way to split the difference is to temporarily disconnect the PMA regulator and run the ignition from a fully charged battery.

 

[ninskrillz]

That will only put you in the ballpark. If you want to be accurate, set up a dial test indicator on the piston stop. Find what looks like the highest point of piston rise and make a temporary mark. Mount the degree wheel and zero it to the temp mark. Finally, select an arbitrary drop off TDC; .050" is enough. Read the degree wheel at that drop before and after TDC. Correct the mark so that it falls at the midpoint of the degree readings.

Ok great I'll try that first.

 

[ninskrillz]

Here is a list of possible causes of low top end speed:

Bike goes slow (< 75MPH)

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?

Other possibilities:
1. Which PAMCO do you have?
90 degree with e advance
2. If PMA, then battery or capacitor?
Cornell dubilier 10000uf cap
3. Was the bike originally a US TCi or points?
Points
4. What octane gas are you using?
93
5. Which carbs do you have installed?
Vm34
6. Any mods to the exhaust or air filters?
Baffled ya mama pipes with uni filters
7. What gear and road speed does the problem occur?
5th gear 75mph~ and about 5k rpm
8. What was the most recent work done to the engine besides the PAMCO?
Stock engine
9. Have you tried going up one size for the main jets in the carbs?
Tried 185-200
10. What is the battery / capacitor voltage at 5,000 RPM?
11. Have the ignition switch and kill switch contacts been cleaned? Ever?
Brand new switch kill only
12. What is the altitude where you are?
Sea level
13. New spark plugs?
Yes new bpr7eix gapped to 35

If you installed the PMA at the same time as the PAMCO, then several issues could come into focus:

1. How did you establish the timing marks?
PO made the marks
2. Does the rotor of the PMA have a key to the crankshaft?
Unsure
4. What is the battery / capacitor Voltage at 5,000 RPM?
Unsure
5. If a capacitor, then what is the capacity and brand of capacitor?
Cornell dubilier 10000uf 75v cap
There can be issues with the PMA that manifest themselves as ignition problems. One way to split the difference is to temporarily disconnect the PMA regulator and run the ignition from a fully charged battery.

Filled in the blanks. Removed what I've already tested or wasn't applicable. Got some mor stuff to test!

 

[ninskrillz]

That will only put you in the ballpark. If you want to be accurate, set up a dial test indicator on the piston stop. Find what looks like the highest point of piston rise and make a temporary mark. Mount the degree wheel and zero it to the temp mark. Finally, select an arbitrary drop off TDC; .050" is enough. Read the degree wheel at that drop before and after TDC. Correct the mark so that it falls at the midpoint of the degree readings.

How do I attach the indicator to the piston stop? I'm under the impression you need the piston to contact the stop and the needle of an indicator? To find it precisely I note degree position when the piston has dropped .050" in both directions and find the mean of the two?

 

[grizld1]

Re. mounting an indicator, you can either use an indicator with a long stem and set it up it the plug hole directly, or you can mount an indicator so that the tip is on the piston stop with the stem running parallel to the stop, or you can mount a dial test indicator similarly and not worry about the stem angle. You don't attach the indicator to the piston stop, you mount it on a holder and find a way to attach the holder to the frame or motor; to check valve lash, for example, I mount a piece of flat stock to a top mount hole on the motor and bolt a rod from a dial indicator mount to it. Re. correcting the mark you're right, if your degree wheel reads, for example, 12* BTDC and 8* ATDC, you need to retard the mark 2* so that you get the same timing on each side of TDC.

 

[cruzin]

Cant help with the top end but wanted to ask if you found the e-advance holder/box you got from me usable? If so where did you mount it. good luck on the tuning.

 

[ninskrillz]

Re. mounting an indicator, you can either use an indicator with a long stem and set it up it the plug hole directly, or you can mount an indicator so that the tip is on the piston stop with the stem running parallel to the stop, or you can mount a dial test indicator similarly and not worry about the stem angle. You don't attach the indicator to the piston stop, you mount it on a holder and find a way to attach the holder to the frame or motor; to check valve lash, for example, I mount a piece of flat stock to a top mount hole on the motor and bolt a rod from a dial indicator mount to it. Re. correcting the mark you're right, if your degree wheel reads, for example, 12* BTDC and 8* ATDC, you need to retard the mark 2* so that you get the same timing on each side of TDC.

Awesome thanks for breaking it down for me

 

[ninskrillz]

Ok I was able to correctly mark tdc. It was just a few degrees off. My timing (a bit advanced from the original mark) was pretty much spot on. Adjusted it it perfectly and took her out for a spin. Still maxing out at about 5k and was able to get up to about 78.

I checked voltage of the capacitor at idle and came up with 14v. Checked it at 5k and it dropped to about 2-3v. I checked voltage at the kill switch and it stayed at ~14v from idle up to 5k rpm. Is that normal?

 

[retiredgentleman]

I don't know why some lads waste their time and money with capacitors. Install a high quality battery and your bike will run much, much better.

 

[ninskrillz]

Small for chopper #chooplyfe

 

[grizld1]

"Just a few degrees?" That's enough to do more than just a little damage. Anyway, you've found the trouble source; at 5K you don't have enough power going to the ignition for a healthy spark.

 

[ninskrillz]

Probably the addition of the cap... I'll add another or a battery and see if that solves the problem.

 

[ninskrillz]

"Just a few degrees?" That's enough to do more than just a little damage. Anyway, you've found the trouble source; at 5K you don't have enough power going to the ignition for a healthy spark.

What should I be testing next? Charging system?

 

[ninskrillz]

So I tested the capacitor again and it was idling at about 12v and went to and 14.5v at 5k rpm. I guess my first test I wasn't holding the leads of the meter fully. Checked a couple times and it was reading correctly all times. More advice appreciated.

 

[grizld1]

You already got more advice than you've taken, mah man--you're only on the first step! Start with Pete's suggestion to take a run with a fully charged battery and alternator unplugged to see if your PM system is making enough electronic noise to interfere with your ignition. Then, to review--basic adjustments (cam chain tension and valve clearance), compression check, leak down check, jetting correction (start by reducing PJ size 1 step and raising the needle to #3 position). Don't bother to fool with the carbies until you know that everything else is right. On VM34 and VM36 carbs near sea level with either stock or hopped up XS650 motors, I usually wind up with 22.5 PJ's, needle at #3, and mix screws open 1/2 to 3/4 turn. Don't take that as a recipe, just an indication of how very much too fat your jetting is. You shouldn't need mains larger than 185, at the extreme; 175 or 180 would be more likely.

 

[ninskrillz]

I realized after I posted that I still needed to do the compression and leakdown tests. I missed the tidbit in Petes post at the end derp. I'll give that a try in between the two tests. Can leakdown testers be rented?

And tons of brass incoming to lean out the jetting.