Interesting engine power loss over 7000rpm solved

[LC Cnd]

Put on the Microns to start the ride season with the bike at all stock settings last night. Noticed throttle response a touch sluggish during warm up in neutral thought idle was good at 1200rpm. Thought well its a warm up issue as the throttle response to 6000 rpm was crisp before tacking off. Initially the power band was ok but 10 minutes into the ride I was hearing the right side exhaust periodically cutting out between 7000-8000 rpm then rev up. Returned home to investigate: no air leaks on engine seals or carbs, cleared all carb passages (good), ok'd carb float levels and proper carb slide direction, balanced carbs, checked carb caps and cables - good. Yet when I rev'd it up to past 7000 rpm the right side pipe would cut out just like it was under load. Ck'd clearance of air box/carb boot sealing - good. Found left side spark plug cap a little loose and decided to replace both to match the new cap's resistance. Warmed up the bike again and found the right side exhaust again cut out above 7000 rpm while parked. Could the original CDI or right spark plug wire be the issue? The Microns were clear and repacked before installation. Mechanically I don't know what else to investigate.

[kpke]

I know you said you replaced the plug caps, but did you swap left and right plug wires and check it?

Put the pipes you took off back on to rule out the microns???

Seems carb related to me.

[silverstrom]

It sounds more electrical than fuel related, but it can be difficult to nail it down as electrical faults often come across as fuel faults.

As mentioned, swap the plug wires and see if the fault moves to the left side.

It's worth removing the plug wires and making sure you have solid connections on both ends. Cut 5 mm from each end then screw into the coil and then screw the caps on.

Did you meter your ignition coil? A failing ignition coil can work well at low RPM but under high RPM load it can fail.

You can meter the stator to rule out any high speed issues, although that should typically affect both sides, but strange things happen.

A CDI fault would typically affect both sides, but it's worth investigating further.

Have you pulled the choke and tried running at high RPM to eliminate lack of fuel as a cause?

Have you tried with the airbox lid off to supply more air and see what happens?

If you're not making progress you can do a compression test on the right cylinder.

I assume you've made sure the right Micron isn't plugged?

Keep at it. You'll get there.

[LC Cnd]

Yep your suggestions that its likely an electrical issue is what I'm going to focus on. Tried the choke - bogged out when throttling. I think I've got a spare new one interested purchased from Cech Republic that I was going to use on another build - I'll dig that out. If the new CDI works out, should a Yamaha banshee CDI be a proper replacement in a LC? Just looking Ebay pre-owned one seen expensive and a bit chancy. The Cech one was a cheaper price hence the purchase. Dig time.

[LC Cnd]

Double entry - pls remove. Thx

[silverstrom]

If the new CDI works out, should a Yamaha banshee CDI be a proper replacement in a LC?

I would expect the Banshee CDI to have a different ignition curve.

[LC Cnd]

Checked the primary coil which read .33 ohms (plug caps pull off) and secondary coil 3.7k ohms - so, happy with the stock coil performance considering its age. I'm aware after market are better and will probably invest in a Ignitech. Interestingly the Cech coil read .37 and secondary 5.8 (bit high) although that is was the company recommended. Had to remove the top half of the air box to remove the coil - all clear. Going to remove the right pipe next for possible blockage.

[Zedixe13]

The Banshee / RZ CDI is a different beast , the LC CDI is a simple fixed curve advance ignition , I suspect it's the same as the RD400 F and G , they look the same save for a pair of switched wires in the connector . I've seen a YZ125 1980 and the CDI look also very simple and could be compatible .

[logierob]

Have you tried removing your fuel cap when this happens? Perhaps its a venting issue. This happened to me and would affect the right carb first as the fuel line is longer getting to the carb. It was just enough to starve the right side of fuel. I could pull over let it sit and the right side would sputter back to life. I shut the bike off and heard the hissing through the cap. Had me stumped for a while though.
Cheers
Rob

[LC Cnd]

Rob what did you do to clear the cap or did you just purchase a new one?

[logierob]

Hey sorry for the late reply . My fuel cap was in bad shape i bought a new one from Norbo. Was that what the problem was ?

[logierob]

I did manage to clear the cap by taking it apart cleaning out the rust removing the rubber seal soaking it in metalmedic rust remover but opted to replace it because it leaked from the top after that.

[CDN_RD350LC]

Is there a rebuild kit available? Or is it not worth the time and to buy a new one? Is there a exploded diagram of the gas cap? I have feeling I could be having a venting issue as well. When I get on it hard, sometimes it has symptoms of gas starvation which could be a blocked cap.

Cheers

[logierob]

Hey Sorry again for the late reply. I didn't even check for a rebuild kit i just bought one from rdlc crazy AKA Norbo. The way i confirmed it was on my way home from the third ride that it happened on. I heard the hissing and then opened the cap which confirmed the issue via vacuum release then road home with the cap in my pocket and the problem was gone. So could just get on the bike with the cap in your pocket and see if it goes away. Not sue if there is a rebuild kit or not.

[LC Cnd]

Got a "4L0" new fuel cap and attempted to fit it on my tank: could twist it on but could not lock it. Compared the original to the new and one can note the difference. Disappointing but perhaps I can grind it to fit - don't know. Did note when putting on the old one it did produce a sucking noise for approximately a minute and a half. Perhaps its still functioning - not sure if pressure release is immediate. Probably slide on the stock exhausts next.