Posts
17
Joined
4/21/2019
Location
GB
Morning guys (from a surprisingly sunny England),
Really hoping someone could shed some light as to what's going on with my little zuk. I'll try to document everything and then hopefully someone could chime in with some 'next steps' as to her ailment. I have 2 x 80's RMs - The 'T' I believe. My race bike as such (the one I favour) seems to suffer horribly from power loss as the motos go on. Case in point this weekend where the motos were 2:10 Lap times, fairly hilly but with long straights in between - meaning she was on the pipe the whole time. Lap 1 to 3, no issues, but then come lap 4, 5, and 6 the power starts to drop rapidly. To put into perspective, hills I was running up in 3rd gear come lap one, by lap 6 I could barely make it up in 1st gear. Once to the top of the hill, from a stand still the bike on full throttle can barely pull away. I weigh 180lbs too, so not toooo bad a strain on the little zinger.
The condition of the bike:
Last weekend she had a new bottom end (bearing, seals, rod, crank machined to take a 22mm rod over standard) and new Wossner piston (0.5+) the cylinder was checked by the same guy who pressed the crank and he confirmed that she seems good and no need for a re-bore. She's running some very light port work, what appears to be an aftermarket twin reed cage with good condition petals (as in reeds either side of the cage, not a dual stage single reed cage), 34mm carb, 3rd needle clip, 150 main and standard pipe. Fuel petcock is off a later 250 for beter fuel flow to the carb. Checking the condition of the plug - she's absolutely on the money and I'm running Sunnoco supreme mixed with ESSO super unleaded here in the UK at a ratio of 32:1. Gearing is 12 / 51 as per standard, ignition is in good condition with strong spark.
Symptoms:
Any race where the track is fairly level or short races, the bike seems to run perfectly. However, point her at a track with long power draining hills or longer motos then as per the opening statement - she goes 'off' pretty quickly. Before I start to tear her down again for what feels like the millionth time, I thought I would turn to the wise world of vital for some guidance. My intial thoughts are that I throw the 32mm carb back on as per standard and have the ignition re-wound for peace of mind, but that's more for maintenance rather than curing this issue. Perhaps the igntion is degrading as the motor gets really hot? Maybe she's just getting so so hot that the expansion means I'm losing compression on the rings?! Thing is, although fairly new to twinshock MX, I don't believe I should be looking at doing a new top end on her after EVERY event?
I'd really welcome some pointers guys and thanks in advance for any assistance you can provide. Bart
Really hoping someone could shed some light as to what's going on with my little zuk. I'll try to document everything and then hopefully someone could chime in with some 'next steps' as to her ailment. I have 2 x 80's RMs - The 'T' I believe. My race bike as such (the one I favour) seems to suffer horribly from power loss as the motos go on. Case in point this weekend where the motos were 2:10 Lap times, fairly hilly but with long straights in between - meaning she was on the pipe the whole time. Lap 1 to 3, no issues, but then come lap 4, 5, and 6 the power starts to drop rapidly. To put into perspective, hills I was running up in 3rd gear come lap one, by lap 6 I could barely make it up in 1st gear. Once to the top of the hill, from a stand still the bike on full throttle can barely pull away. I weigh 180lbs too, so not toooo bad a strain on the little zinger.
The condition of the bike:
Last weekend she had a new bottom end (bearing, seals, rod, crank machined to take a 22mm rod over standard) and new Wossner piston (0.5+) the cylinder was checked by the same guy who pressed the crank and he confirmed that she seems good and no need for a re-bore. She's running some very light port work, what appears to be an aftermarket twin reed cage with good condition petals (as in reeds either side of the cage, not a dual stage single reed cage), 34mm carb, 3rd needle clip, 150 main and standard pipe. Fuel petcock is off a later 250 for beter fuel flow to the carb. Checking the condition of the plug - she's absolutely on the money and I'm running Sunnoco supreme mixed with ESSO super unleaded here in the UK at a ratio of 32:1. Gearing is 12 / 51 as per standard, ignition is in good condition with strong spark.
Symptoms:
Any race where the track is fairly level or short races, the bike seems to run perfectly. However, point her at a track with long power draining hills or longer motos then as per the opening statement - she goes 'off' pretty quickly. Before I start to tear her down again for what feels like the millionth time, I thought I would turn to the wise world of vital for some guidance. My intial thoughts are that I throw the 32mm carb back on as per standard and have the ignition re-wound for peace of mind, but that's more for maintenance rather than curing this issue. Perhaps the igntion is degrading as the motor gets really hot? Maybe she's just getting so so hot that the expansion means I'm losing compression on the rings?! Thing is, although fairly new to twinshock MX, I don't believe I should be looking at doing a new top end on her after EVERY event?
I'd really welcome some pointers guys and thanks in advance for any assistance you can provide. Bart
If it revs slow, maby the crank is to wide?
Spinns free when cold but expand when hot, and too tight in cases.
You put in C3 bearings?
But does it sound the same and rev quick when you loose power?
If it does, i would look if the clutch is slipping.
If it dont, try to explain how it feels and sound, maybe a record with your mobile?
The Shop
There are 2 coils on stator,,,high speed and low speed. If you check the spark it may look fine...low speed coil isn't the issue...but high load and rpms cause the high speed coil to short or build resistance.
I used Custom Rewind https://www.facebook.com/pages/Custom-Rewind/165645350129391 to do the RMs. YZ I located stock NOS coils. It may take a week or so to have your redone....but it's done by a skilled american and not a asian child.
Paw Paw
In the event that it isn't your stator you will be needing it. Every old RM I've owned needed a stator at some point.
One bike ran but was only made 75% power.
Taken on board what you have said and I agree that it's likely the high speed coil winding breaking down. Stator is off to be rewound as we speak by a renowned specialist here in the UK. I've also picked up new rings to gap correctly and then will go give her some hell and see if that's made a difference.
Up-next: Likely have the cylinder bored and go to a 0.8 oversized Wossner piston.
Now, as a closing question - there seems to be some regular advice to inhibit or cut the rev limiter wire coming out of the CDI? Some say to cut the single black wire, others say the blue. Is there any truth to that on the 'T' cdi unit?
Cheers as always from drizzly Blighty
Post a reply to: 1980 RM125 Loss of power over time, help!