RX3 Power down at high speed, 40mpg

ChopperCharles · 01-17-2021, 10:20 AM

Just a month ago I was running 80mph indicated on flat and level ground, and easily powering up hills on the interstate without dropping below 75. Yesterday I took the bike out, and I could barely maintain 70 up that hill, and 75 was my max speed on straight and level ground. I have a 13T countershaft sprocket so I used to be able to bury the tach deep into the redline on the highway. Not so much yesterday.

I'm also getting 40mpg, no matter how I ride the bike. My speedometer has been recalibrated so it's actually accurate, which did bring my gas mileage down... but it was getting 55mpg not too long ago.

Now, it was a lot colder recently than it was last month, but generally bikes run better in colder weather, not worse.

Power doesn't seem down anywhere else, but top end is definitely anemic now. It's also started stalling on really cold days until it's fully warmed up. It had stopped doing that after a valve adjustment, but it's doing it again... hard to think I need another valve adjustment in 1200 miles...

Thoughts? I'm going to run a compression test today and check my fuel pressure. Down on power and 40mpg is a real drag.

Charles.

franque · 01-17-2021, 04:52 PM

It could be that you need a new spark plug. I'd put a new one in before spending a lot of time on the other stuff...

ChopperCharles · 01-17-2021, 09:01 PM

I’ve got a iridium plug in there. Put it in at 2000 ish miles.

Charles.

ChopperCharles · 01-17-2021, 09:04 PM

Also I did a compression check. 65psi cold. It was 95 cold (100 warm) after adjusting my valves in November. So it’s looking like tight valves are to blame again. I’ll pull it apart sometime during the week and find out for sure.

Charles.

calvarez · 01-21-2021, 01:07 PM

I've seen weird unpredictable results with specialty spark plugs in engines that don't call for them (Iridium, Platinum, etc). The compression is an issue for sure, but just commenting that "better" plugs aren't always right. I've done best sticking with NGK for small engines unless otherwise specified.

ChopperCharles · 01-21-2021, 02:00 PM

It's an NGK Iridium plug that is recommended for the bike. I've got the valves adjusted again and will give it a ride later today to see if my top speed is back. If this is going to need a valve adjustment every 1200 miles though, the bike is going to have to go.

Charles.

grumpyunk · 01-21-2021, 04:52 PM

Could you have carburetor icing? Started the CG250 yesterday, and it did not want to run w/o the choke. Kinda felt around the carb, and it was cold. Put my hand on it to maybe warm it a bit, felt the fins on the cylinder and the head, both chilly, but warming up. Took a while to get it to run properly.
I have done nothing except swap in some roller cam followers.
I do note that it seems the clearance gets bigger when the thing is finally warmed up. I did not listen close when it was cold, but it seemed louder by the time I got home.
The CB with OHC may not do this, but will be subject to needing the carb warm as they all are vaporizing liquid(fuel) and that takes heat. I have not ridden in temps as cold as this so it is kinda new to me.
tom

franque · 01-21-2021, 06:29 PM

It's an injected NC250, unfortunately nothing you said applies.

ChopperCharles · 03-17-2021, 08:08 PM

Problem is the valves. First adjustment was at 500 miles as per the previous owner.

Bike didn't make it to the second adjustment. It was stalling and unbeknownst to me the exhaust valves were tight. Manual said to check every 2500 miles, so the next one should have been at 3000 miles.

At 2300 miles my fuel filter disintegrated and partially clogged the injector, resulting in a lean condition. My exhaust catalyst ignited from the high temperatures. I replaced the filter and cleaned the injector, then adjusted the valves.

At 3349 miles the bike started stalling again, power was down, and gas mileage was abysmal. I did a compression check and there's my problem. Terrible compression. So I pulled everything off again and yup, both exhaust valves were extremely tight.

Now it's been about 500 miles and I'm feeling like power is down again, and it has stalled once. So I think I'll be needing a new head.

Initially I thought this was my fault, for the lean condition overheating the valves and damaging the hardening of the valves... but now I'm not so sure. I think it may just be poor metallurgy of the valves or seats. The exhaust valves going to shit at 2300 miles, long before an adjustment was warranted, seems to indicate something else has been going on.

In any case I at the very least need two new valves. If I need new valve seats, then it's cheaper to just buy a new head than to have a machine shop install and cut new seats. AliExpress has all new heads, with valves and springs and everything, for a really good price. Debating whether I should take the bike apart and inspect the head first, or just order all new parts and have it all ready and waiting to go when I pull the engine apart.

Also concerned whether aliexpress heads valves and the like are going to be quality, or if I should just order from CSC.

Charles.

Working_ZS · 03-18-2021, 06:08 PM

My advice is to replace the head, at the very least, and possibly the cylinder and piston as well. This is due to the fact that everything got so hot when you leaned it out WOT. It takes a lot of heat to burn thru a catalytic converter's ceramic honeycomb, so all of the power generating parts of the engine (piston, rings, cylinder, head and valves) are suspect to me now.

Where you source parts from depends upon a few things; how fast you want to get it back on the road, who has it in stock, and how much they want for the parts. Optimally, getting the parts from CSC is your best bet from a quality standpoint, if they have them in stock, which isn't a sure thing right now, given the pandemic and its effects on manufacturers production and shipping chains. They probably won't be the cheapest though. If they don't have one in stock, then AliExpress or eBay will probably have to do, and you will have to take what you get from them, metallurgy-wise. Personally, I wouldn't have a problem with AliExpress and eBay, apart from being in a hurry and having to wait for it to come from China.

As for the original metallurgy being no good, I have to point out that there are literally several hundred, if not over a thousand of these bikes here in the US now, not to mention tens of thousands worldwide, and yours is the only low mileage exhaust valve failure that I am aware of. My own RX3, which has ever only had one valve adjustment, is at over 17,000 miles without any issues, and others have even more on theirs. I think that the valves were OK, they just got way too hot and are now toast.

I would suggest that when you get it back together that you not hammer the crap out of it until after you put 3000 or 4000 miles it; after that you can go more WOT, but you need to always keep in mind that it is only a 250 cc engine and it's pushing a 400 lb bike, plus rider. It's fine to break in hard a new bike that you purchased from a dealer, with a warranty; not so much for one that is used, even if it was extremely low miles. Used means no warranty, putting you are on the hook for repairs if anything lets go. Luckily, you got the bike for $1200, which was a literal steal, so you don't have a lot in it even after repairs are factored in. Also, I would recommend that you use the Lucas Fuel Injection and Upper Cylinder Lubricant that CSC recommends for it. One gallon of it literally lasts me over 10,000 miles, and that is at over three times the recommended dilution rate. At $25 or so from Walmart or Amazon, it is very cheap insurance against what you are dealing with right now.

ChopperCharles · 03-18-2021, 07:53 PM

I've been using lucas stabilizer. I can get a new head with valves in it shipped and delivered by march 29 for $215 if I pay for express shipping from aliexpress. So that's likely the route I'll go. I will also put in the big bore kit at the same time. Those are $190 now I believe. So for $500 and a weekend of my time I'll get a nice power boost along with my repaired valve train.

I have been riding on the highway but I've not been abusing the bike. I will go back to a 14T countershaft sprocket once I get the big bore kit installed though.

Charles.

Working_ZS · 03-18-2021, 09:14 PM

Quote:

Originally Posted by ChopperCharles

I've been using lucas stabilizer. I can get a new head with valves in it shipped and delivered by march 29 for $215 if I pay for express shipping from aliexpress. So that's likely the route I'll go. I will also put in the big bore kit at the same time. Those are $190 now I believe. So for $500 and a weekend of my time I'll get a nice power boost along with my repaired valve train.

I have been riding on the highway but I've not been abusing the bike. I will go back to a 14T countershaft sprocket once I get the big bore kit installed though.

Charles.

Good to hear on the Lucas, and yeah, most big bore kit owners swap back to the 14 tooth countershaft sprocket since the upgraded engine has the torque to pull the bike that the OEM NC250 doesn't.

ChopperCharles · 04-05-2021, 03:40 PM

So, at around 3800 miles I checked the valves again and they were within spec, but my compression was down to 85psi cold. Should be 100psi cold. Since I have a big bore kit on the way, and hopefully a new head as soon as CSC gets back to me... I decided to take the motor out and see what I'm working with.

It was actually pretty easy. The whole motor came out of the frame with little drama, and it was light enough to carry to the workbench with my arms outstretched - so as not to stain my shirt with oil and grease.

I pulled the head off and took the exhaust valves out, and... well, there's not a lot of damage, but there is damage. One of the seats is showing scoring and lines, and it's not a bright and shiny ring all the way around like the other exhaust valve is.

The valve itself looks fine though.

If you look closely at the seat in the head, it's not perfectly round. There are areas where the seat is wider and where it's narrower. And the damage I'm seeing is at the narrowest part of the circle.

What are your thoughts on this? I mean, it's not obvious catastrophic damage, but then, the bike still ran fine, it was just a little low on compression, and the exhaust valves kept tightening up. Unless the seat is in there crooked (which it very well may be), I think I might have been able to get away with lapping the valves and reinstalling the head. (But with a new one on the way and all the effort to get the engine out of the frame, I'm not wanting to take that risk)

Charles.

franque · 04-05-2021, 04:18 PM

If you've got a new head, just use that. My guess is that the head wasn't properly cut for the seat, or else there is something wrong with the casting. It could also be that the seat wasn't cut properly, but my guess is that the uneven wear is due to the aforementioned reasons.

Rustybolts7 · 04-05-2021, 11:00 PM

That damage appears to be due to overheating of the valve and seat. You can see were blow by has passed by head seat. The valve literally hammered the over heated seat. Just my opinion.

01-17-2021, 10:20 AM	#1
ChopperCharles Join Date: Feb 2018 Location: RDU, NC Posts: 683	RX3 Power down at high speed, 40mpg Just a month ago I was running 80mph indicated on flat and level ground, and easily powering up hills on the interstate without dropping below 75. Yesterday I took the bike out, and I could barely maintain 70 up that hill, and 75 was my max speed on straight and level ground. I have a 13T countershaft sprocket so I used to be able to bury the tach deep into the redline on the highway. Not so much yesterday. I'm also getting 40mpg, no matter how I ride the bike. My speedometer has been recalibrated so it's actually accurate, which did bring my gas mileage down... but it was getting 55mpg not too long ago. Now, it was a lot colder recently than it was last month, but generally bikes run better in colder weather, not worse. Power doesn't seem down anywhere else, but top end is definitely anemic now. It's also started stalling on really cold days until it's fully warmed up. It had stopped doing that after a valve adjustment, but it's doing it again... hard to think I need another valve adjustment in 1200 miles... Thoughts? I'm going to run a compression test today and check my fuel pressure. Down on power and 40mpg is a real drag. Charles.
	0

01-21-2021, 01:07 PM	#5
calvarez Join Date: Sep 2018 Location: Peoria, AZ Posts: 334	I've seen weird unpredictable results with specialty spark plugs in engines that don't call for them (Iridium, Platinum, etc). The compression is an issue for sure, but just commenting that "better" plugs aren't always right. I've done best sticking with NGK for small engines unless otherwise specified. __________________ 2016 RX3 2002 Honda CBR1100XX
	0

01-21-2021, 04:52 PM	#7
grumpyunk Join Date: Jul 2018 Location: nw of atlanta Posts: 169	Could you have carburetor icing? Started the CG250 yesterday, and it did not want to run w/o the choke. Kinda felt around the carb, and it was cold. Put my hand on it to maybe warm it a bit, felt the fins on the cylinder and the head, both chilly, but warming up. Took a while to get it to run properly. I have done nothing except swap in some roller cam followers. I do note that it seems the clearance gets bigger when the thing is finally warmed up. I did not listen close when it was cold, but it seemed louder by the time I got home. The CB with OHC may not do this, but will be subject to needing the carb warm as they all are vaporizing liquid(fuel) and that takes heat. I have not ridden in temps as cold as this so it is kinda new to me. tom __________________ vertical and above ground - my daily goal
	0

03-18-2021, 06:08 PM	#10
Working_ZS Join Date: Aug 2016 Posts: 365	Replace The Head My advice is to replace the head, at the very least, and possibly the cylinder and piston as well. This is due to the fact that everything got so hot when you leaned it out WOT. It takes a lot of heat to burn thru a catalytic converter's ceramic honeycomb, so all of the power generating parts of the engine (piston, rings, cylinder, head and valves) are suspect to me now. Where you source parts from depends upon a few things; how fast you want to get it back on the road, who has it in stock, and how much they want for the parts. Optimally, getting the parts from CSC is your best bet from a quality standpoint, if they have them in stock, which isn't a sure thing right now, given the pandemic and its effects on manufacturers production and shipping chains. They probably won't be the cheapest though. If they don't have one in stock, then AliExpress or eBay will probably have to do, and you will have to take what you get from them, metallurgy-wise. Personally, I wouldn't have a problem with AliExpress and eBay, apart from being in a hurry and having to wait for it to come from China. As for the original metallurgy being no good, I have to point out that there are literally several hundred, if not over a thousand of these bikes here in the US now, not to mention tens of thousands worldwide, and yours is the only low mileage exhaust valve failure that I am aware of. My own RX3, which has ever only had one valve adjustment, is at over 17,000 miles without any issues, and others have even more on theirs. I think that the valves were OK, they just got way too hot and are now toast. I would suggest that when you get it back together that you not hammer the crap out of it until after you put 3000 or 4000 miles it; after that you can go more WOT, but you need to always keep in mind that it is only a 250 cc engine and it's pushing a 400 lb bike, plus rider. It's fine to break in hard a new bike that you purchased from a dealer, with a warranty; not so much for one that is used, even if it was extremely low miles. Used means no warranty, putting you are on the hook for repairs if anything lets go. Luckily, you got the bike for $1200, which was a literal steal, so you don't have a lot in it even after repairs are factored in. Also, I would recommend that you use the Lucas Fuel Injection and Upper Cylinder Lubricant that CSC recommends for it. One gallon of it literally lasts me over 10,000 miles, and that is at over three times the recommended dilution rate. At $25 or so from Walmart or Amazon, it is very cheap insurance against what you are dealing with right now.
	1

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01-17-2021, 04:52 PM	#2
franque Join Date: Nov 2017 Location: Marseille, France -> Conakry, Guinea Posts: 1,481	It could be that you need a new spark plug. I'd put a new one in before spending a lot of time on the other stuff...
	0

01-17-2021, 09:01 PM	#3
ChopperCharles Join Date: Feb 2018 Location: RDU, NC Posts: 683	I’ve got a iridium plug in there. Put it in at 2000 ish miles. Charles.
	0

01-17-2021, 09:04 PM	#4
ChopperCharles Join Date: Feb 2018 Location: RDU, NC Posts: 683	Also I did a compression check. 65psi cold. It was 95 cold (100 warm) after adjusting my valves in November. So it’s looking like tight valves are to blame again. I’ll pull it apart sometime during the week and find out for sure. Charles.
	0

01-21-2021, 02:00 PM	#6
ChopperCharles Join Date: Feb 2018 Location: RDU, NC Posts: 683	It's an NGK Iridium plug that is recommended for the bike. I've got the valves adjusted again and will give it a ride later today to see if my top speed is back. If this is going to need a valve adjustment every 1200 miles though, the bike is going to have to go. Charles.
	0

01-21-2021, 06:29 PM	#8
franque Join Date: Nov 2017 Location: Marseille, France -> Conakry, Guinea Posts: 1,481	It's an injected NC250, unfortunately nothing you said applies.
	0

03-17-2021, 08:08 PM	#9
ChopperCharles Join Date: Feb 2018 Location: RDU, NC Posts: 683	Problem is the valves. First adjustment was at 500 miles as per the previous owner. Bike didn't make it to the second adjustment. It was stalling and unbeknownst to me the exhaust valves were tight. Manual said to check every 2500 miles, so the next one should have been at 3000 miles. At 2300 miles my fuel filter disintegrated and partially clogged the injector, resulting in a lean condition. My exhaust catalyst ignited from the high temperatures. I replaced the filter and cleaned the injector, then adjusted the valves. At 3349 miles the bike started stalling again, power was down, and gas mileage was abysmal. I did a compression check and there's my problem. Terrible compression. So I pulled everything off again and yup, both exhaust valves were extremely tight. Now it's been about 500 miles and I'm feeling like power is down again, and it has stalled once. So I think I'll be needing a new head. Initially I thought this was my fault, for the lean condition overheating the valves and damaging the hardening of the valves... but now I'm not so sure. I think it may just be poor metallurgy of the valves or seats. The exhaust valves going to shit at 2300 miles, long before an adjustment was warranted, seems to indicate something else has been going on. In any case I at the very least need two new valves. If I need new valve seats, then it's cheaper to just buy a new head than to have a machine shop install and cut new seats. AliExpress has all new heads, with valves and springs and everything, for a really good price. Debating whether I should take the bike apart and inspect the head first, or just order all new parts and have it all ready and waiting to go when I pull the engine apart. Also concerned whether aliexpress heads valves and the like are going to be quality, or if I should just order from CSC. Charles.
	0

03-18-2021, 07:53 PM	#11
ChopperCharles Join Date: Feb 2018 Location: RDU, NC Posts: 683	I've been using lucas stabilizer. I can get a new head with valves in it shipped and delivered by march 29 for $215 if I pay for express shipping from aliexpress. So that's likely the route I'll go. I will also put in the big bore kit at the same time. Those are $190 now I believe. So for $500 and a weekend of my time I'll get a nice power boost along with my repaired valve train. I have been riding on the highway but I've not been abusing the bike. I will go back to a 14T countershaft sprocket once I get the big bore kit installed though. Charles.
	0

04-05-2021, 03:40 PM	#13
ChopperCharles Join Date: Feb 2018 Location: RDU, NC Posts: 683	So, at around 3800 miles I checked the valves again and they were within spec, but my compression was down to 85psi cold. Should be 100psi cold. Since I have a big bore kit on the way, and hopefully a new head as soon as CSC gets back to me... I decided to take the motor out and see what I'm working with. It was actually pretty easy. The whole motor came out of the frame with little drama, and it was light enough to carry to the workbench with my arms outstretched - so as not to stain my shirt with oil and grease. I pulled the head off and took the exhaust valves out, and... well, there's not a lot of damage, but there is damage. One of the seats is showing scoring and lines, and it's not a bright and shiny ring all the way around like the other exhaust valve is. The valve itself looks fine though. If you look closely at the seat in the head, it's not perfectly round. There are areas where the seat is wider and where it's narrower. And the damage I'm seeing is at the narrowest part of the circle. What are your thoughts on this? I mean, it's not obvious catastrophic damage, but then, the bike still ran fine, it was just a little low on compression, and the exhaust valves kept tightening up. Unless the seat is in there crooked (which it very well may be), I think I might have been able to get away with lapping the valves and reinstalling the head. (But with a new one on the way and all the effort to get the engine out of the frame, I'm not wanting to take that risk) Charles.
	1

04-05-2021, 04:18 PM	#14
franque Join Date: Nov 2017 Location: Marseille, France -> Conakry, Guinea Posts: 1,481	If you've got a new head, just use that. My guess is that the head wasn't properly cut for the seat, or else there is something wrong with the casting. It could also be that the seat wasn't cut properly, but my guess is that the uneven wear is due to the aforementioned reasons.
	0

04-05-2021, 11:00 PM	#15
Rustybolts7 Join Date: Dec 2019 Location: Virginia Posts: 2	That damage appears to be due to overheating of the valve and seat. You can see were blow by has passed by head seat. The valve literally hammered the over heated seat. Just my opinion.
	0