After I recently recommissioned my RD125 after sitting for a few years I've been trying to figure out why it's lacking top speed and power. :umm:
It has a very hard time even pulling 50mph when it used to do 60-65 before it was parked for a long period of time. And if I hit a small hill or rise in the road it starts to slow down quite a bit. And it does the same thing into the wind, it just starts slowing down. It just has no power any more (not that a 125 twin is a high powered beast).
Timing is 1.8mm (stock)
New points
New UNI drop-in filters in the stock airbox (no performance change from the old foam ones that were in there)
Running premium fuel (91 octane)
All new fuel lines
New Visu-filters
Pump is intact and running Motul 710
Jetting is stock
Pipes are stock
Today I checked compression.
Right: 122psi cold
Left: 116psi cold
It runs great otherwise. I'm starting to think it may be caused by a rich condition, although it doesn't four-stroke. I've noticed that it hardly needs the choke to start. Plugs are dark but not sooty or fouled.
I assume you confirmed baffles aren't clogged.
Quote from: RDFL on October 09, 2021, 08:20:36 PM
I assume you confirmed baffles aren't clogged.
Yep, pipes are NOS with about 100-125 miles on them. Same pipes before and after recommissioning.
Carb slides opening all the way?
NOS points or aftermarket? Could be point float?
Dirty carbs?? My rd almost always needs the carbs soaked if its been sitting for any length of time. It runs great low end but falls off on the top end.
If it is too rich it would certainly slow down.
Quote from: jradnich on October 10, 2021, 02:38:59 PM
Carb slides opening all the way?
Quote from: Striker1423 on October 10, 2021, 07:43:08 PM
NOS points or aftermarket?
Quote from: klaird on October 10, 2021, 08:17:55 PM
Dirty carbs??
Carbs slides are opening all the way. That was verified when I synced the slides and adjusted cables.
Points are aftermarket.
Carbs are spotless clean. They were compeletely torn down and run through 2 20-min cycles in my ultrasonic.
Also the enrichener was rebuilt with new viton o-rings and the float height was reset as per the factory manual spec.
Aside from the top speed and top end power issue it runs great. It revs out cleanly, take off is good and throttle is crisp and returns to idle quickly.
Quote from: 1976RD400C on October 11, 2021, 06:24:30 AM
If it is too rich it would certainly slow down.
I'm starting to think that it has to be jetting. Just a little too rich, but enough to cause power loss without causing four-stroking. However the loss in power is quite dramatic, much more than I would expect. For example, a slight rise in a flat road or a 5-10mph headwind causes it to slow down to 40mph and requires a down shift. It never did that before.
Unfortunately, TeiKei jets are unobtanium. And I'm not really too eager to try to rejet with Keihin brass, which can be substituted, as it's not a straight correlation for the mains (and I don't believe there is any pilot jet cross ref).
see if its better with headlight off.
Quote from: m in sc on October 11, 2021, 12:20:45 PM
see if its better with headlight off.
This is with the headlight off. I've been running with Ignition switch in #1 position (headlight and tail light off)
see if its worse with headlight on. :whistle:
Quote from: m in sc on October 11, 2021, 12:25:39 PM
see if its worse with headlight on. :whistle:
Roger that. Next time I'm out on it I'll run a few comparison tests on the small hill near me
Related to that check for voltage drop to coils.
Any sign that the clutch is slipping?
Quote from: John Ritter on October 14, 2021, 10:20:25 AM
Any sign that the clutch is slipping?
Don't think the clutch is slipping. It's more of a loss of oomph. Revs drop into a very light breeze or a tiny hill.
So many potential issues. You have to cross your t's and dot your lower case j's.
Does it run better if you back off the throttle to like 70%?
A compression test and then a pressure test might point to something.
I would have to look but I think I have engine parts laying around, I was going to sleeve a 125 twin to 100 for land speed because the LS1 cylinders are impossible but like so many projects oh well so if you need cylinders or bottom end stuff let me know.
Quote from: John Ritter on October 14, 2021, 11:18:42 AM
A compression test and then a pressure test might point to something.
I did a compression test last week.
Right: 122psi cold
Left: 116psi cold
For a pressure test I'll need to fab up some block-off plates for the intake ports, and see if my leakdown kit has small enough exhaust plugs or source some new ones. This has been on my to-do list for a while. Too many other things going on lately. :wink:
Quote from: sav0r on October 14, 2021, 11:11:26 AM
Does it run better if you back off the throttle to like 70%?
No change really at just off WOT when running at max speed, after that it starts to slow down with less throttle input.
A lot of jetting issues turn out to be ignition issues. Id recommend to double check timing and change plugs, or maybe even put the old points back in there. The set in there may be doing something weird when they are opening and closing at warp speed but seem fine at lower speeds.
Stock jetting should really be lean with modern fuel.
Quote from: SoCal250 on October 11, 2021, 12:24:21 PM
Quote from: m in sc on October 11, 2021, 12:20:45 PM
see if its better with headlight off.
This is with the headlight off. I've been running with Ignition switch in #1 position (headlight and tail light off)
Quote from: SoCal250 on October 11, 2021, 12:52:04 PM
Quote from: m in sc on October 11, 2021, 12:25:39 PM
see if its worse with headlight on. :whistle:
Roger that. Next time I'm out on it I'll run a few comparison tests on the small hill near me
I was able to do a little comparison testing with the ignition switch in both positions (#1 lights off & #2 lights on). Also on position #2 I compared with light on (high-beam) and light off (burned out low-beam). There doesn't seem to be any discernible difference with any of the settings, but I'm going to do some more testing just to be sure.
What about vent in gas cap, not venting properly causing poor fuel flow. ( starving )
Quote from: RustyRD on October 17, 2021, 09:36:58 PM
What about vent in gas cap, not venting properly causing poor fuel flow. ( starving )
Don't think that's an issue because it's a brand new OEM cap and it seems to vent pretty well. In fact it doesn't seal very well and kind of makes my garage smell like fuel. However, I can try twisting off the cap next time I'm out ripping around and see if there's any change.
I was also considering replacing the inline fuel filters with straight hose to see if there's any difference.
I plan to swap in some new plugs just for giggles even though the "old" ones have less than 500 miles on them.
And I'm probably going to swap in some new OEM points when I get the time to test if there's any change there.
Another thought, resistors in the plug caps degrade causing weak spark. Sounds like you are about to cover it all, my next thought would be leak down test.
being as nobody has mentioned it, i will. just one thing to cross off your list is binding brakes :sad: when a bike is slower for whatever reason i always check for brake binding first - if it's out on the road.
cheers, gil.
Quote from: Gil Gallad on October 19, 2021, 08:46:37 AM
being as nobody has mentioned it, i will. just one thing to cross off your list is binding brakes :sad: when a bike is slower for whatever reason i always check for brake binding first - if it's out on the road.
cheers, gil.
Yup!