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Discussion Starter #1
Apologies for yet another possible carb issue / tickover problem post but before I go and take the bike for some dyno time and tweaking I've been trying to make sure everything else is ok.

Work so far including recent work by the previous owner -

new CDI
carb clean and rejet to 60 pilot, 170 main and 0.5mm needle shim
new coil pack
new RR
new fuel lines and filter
Ram air pod filter
2into1 Stubby Scorpion exhaust (baffle removed)
oil and filter replaced
Iridium NGK plug
Valve clearances set to 0.1 inlet and 0.12 exhaust at TDC
Compression tested by PO


It starts easily whether hot or cold (needs choke when cold) yet the tickover is still erratic and sometimes hangs which should be a sign of a lean mixture yet on occasion you can smell fuel and the exhaust is quite black which is also a sign of a rich mixture so I'm getting confused and seriously considering just fitting an FCR carb and have done with it. There is no real popping on deceleration and it isn't jerky while cruising at 30/50mph like it was originally. sometimes it feels like the carb has stuck as the revs will hang with no sign of dropping after releasing the throttle when on a run.

If anything it probably ticks over better when cold, when hot it stumbles at times and applying choke makes it worse. Usual operating temp is 90-100 degrees C

Can anyone identify this noise? If you listen there is a stumble at around 6 seconds and then again at around 10 seconds before the bike dies completely.

https://youtu.be/HavsCzMW5oE
 

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Its a misfire look at the carb being blown back, is there a clip on the inlet manifold around the carb i cant see it in the vid
 

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Discussion Starter #3
There is a clip around the inlet manifold from the carb, I've sprayed it with wd40 while the engine is running and there is no change. The pod filter is clamped directly to the other side of the carb.

The bikes running great otherwise and dare I say its more fun than most, if not all of the V2 and IL4 engines I've had before but this tickover stumble is annoying the life out of me.

Cheers.
 

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Is your airbox in place or did you connect your pod filter straight to the carb?
Airbox helps to stabilise airflow into the carb. I've heard recommendation to install a short extension tube between the carb and pod filter in order to smooth the airflow and enable more stable running. Could be worth investigating . . .
 

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CV Carbs are notoriously finicky about how they breathe, considering that the slide is lifted through pressure differential having still clean air at this point is very important, the airbox provides that still clean airflow and a pod filter does not, it may be worth trying the extended tube on the back of the carb to the filter to see if that helps, if not you may have to go to a mechanical carb like Keihin FCR or Mikuni TM 40/42
 

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Putting a length of pipe between the carb and the pod filter works great on mine.

I attached the pipe to the standard kink that runs in to the airbox and attach the pod to that.

CD6AE531-A7D9-401D-A39D-476C0D51F524.jpeg
 

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Discussion Starter #7
CV Carbs are notoriously finicky about how they breathe, considering that the slide is lifted through pressure differential having still clean air at this point is very important, the airbox provides that still clean airflow and a pod filter does not, it may be worth trying the extended tube on the back of the carb to the filter to see if that helps, if not you may have to go to a mechanical carb like Keihin FCR or Mikuni TM 40/42
Is your airbox in place or did you connect your pod filter straight to the carb?
Airbox helps to stabilise airflow into the carb. I've heard recommendation to install a short extension tube between the carb and pod filter in order to smooth the airflow and enable more stable running. Could be worth investigating . . .
Thanks for the replies, the pod filter is currently connected directly to the carb unfortunately. I have been making a stainless tube at work with a branch connection that will allow me to connect the crank breather tube back to the airbox though so hopefully that will help.

I also noticed today that there is a small length of blanked off tube that was connected to the inlet manifold but it didn't seem to be a very snug fit so this is going to be replaced and i'll replace the carb to inlet clip while i'm there.

Thanks
 

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Discussion Starter #8
Decided to crack on with having a little free time earlier. I've got a new vaccum tube so that will be another box ticked off the to do list and I started finishing the air box link pipe idea.

I couldn't get hold of any 62mm O/D tube, the best I could do was a 60.3mm O/D SS tube with 1.77mm wall (hopefully large enough to fit without restricting) and I polished the inside a little to help air flow. A 21.3 mm O/D SS tube (turned down to around 16.5mm O/D) with a 3.73mm wall will be used for the crank breather branch connection.

air box tube 1.jpg air box tube 2.jpg air box tube 3.jpg

I'll do a trial fit before welding and cutting down the tube but fingers crossed this will help with the air flow issue.
 

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Are you connecting the crankcase breather directly into the extension to the pod filter?
You can just fit a small filter into an extended crank breather hose, and keep it up high.
41jKELI6-LL.jpg
 

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Discussion Starter #10 (Edited)
I had looked at the breathers on ebay before but I had also thought about directing the breather tube back into the air flow like the original design would have been. I tried a test fit of the SS tube tonight but it seems the branch connection is too big for the breather hose to slip on so I'll try to remove a bit more material but I'm not overly confident it's going to fit.

I'll probably order a breather filter in the morning and mount it next to the pod filter, do you know what size filter fits as there seems to be a few different sizes?

Cheers.
 

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Discussion Starter #11
So a little more progress today, I got the connecting tube finished and installed with the crank breather in place (minus a couple of jubilee clips).

The bike started with very little choke so I leaned out the mixture screw down to 1.5 turns but then the revs started hanging again. It's currently set to 3 turns out and would tick over from start but will still die when out on the test ride and slowing to a halt.

https://youtu.be/NotOt6f6fKg

I think it's coming up to decision time, either a couple of hours on the dyno or look into a flatside replacement as I'm running out of ideas.
 

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hang in there bobbler, you'll solve this.
Are you sure the carb internals are completely clean?
Is there any chance of a clogged pilot jet? or crud in the little holes in the venturi inlet below the butterfly valve?
Is the pilot screw assembly put together correctly? Is the tip of the pilot screw straight?
Any chance of a tear in the slide diaphragm?

Did you mention in an earlier post that the previous owner told you he'd cleaned the carb? If so he may have missed something or put something together wrongly.
 

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Discussion Starter #14
Try a longer piece of pipe, put more space between the carb and the filter. It will help stabilise the pressure.
That's probably the next move as I had thought it looked a little squeezed in, when I removed the old air filter box I made up a replacement battery mount with a small tool tray since there is so little storage on the bike and I don't always want to travel with the top box fitted. The next move will be to either redesign the mount or I may make another connecting pipe but with a 45 degree elbow to bring the filter down by the rear shock.

Batt tub.jpg batt box 1.jpg excuse the scruffy wiring that's next on the list.

hang in there bobbler, you'll solve this.
Are you sure the carb internals are completely clean?
Is there any chance of a clogged pilot jet? or crud in the little holes in the venturi inlet below the butterfly valve?
Is the pilot screw assembly put together correctly? Is the tip of the pilot screw straight?
Any chance of a tear in the slide diaphragm?

Did you mention in an earlier post that the previous owner told you he'd cleaned the carb? If so he may have missed something or put something together wrongly.
I went through the internals again at the weekend and although I can't see anything it may be worth arranging to have it cleaned just for peace of mind.
All jets are brand new and the little holes didn't have any visible blockages but they are so small I can't guarantee it.
Pilot screw was poorly put together by the PO so when the carb was stripped it was replaced with new.
The slide diaphragm looked fine with no tears or crush marks, the slide moves freely by hand too.

The only thing I have noticed that other carbs have but this doesn't is the balancing hoses on the outside are missing. I'm starting to wonder if this could be contributing to the issue?

Thanks for all the help so far guys. I will beat this (or burn it lol :blob6:)
 

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Discussion Starter #16
When you get tired of the CV tuning, you might investigate these.
https://www.abp-racing.de/vergaser/modelle/
I did some more work at the weekend, reinstalled the original airbox with snorkel removed and standard filter knowing it would be rich but I wanted to try to get a base line to change one part at a time. The bike started, would tick over slightly erratically (rich mixture to be expected) and revved without hanging so I decided to order some new jets...

It was at this point I found a reasonably priced FCR40 from a 2008 CRF250R :D:D

Back to square one in a way but it's due this week and getting cleaned and rejetted to basic settings before fitting. From what I can find a decent setup can be had with:

42/45 pilot jet
160 main jet
35 leak jet
NCVT needle (2nd clip).

Once I have stripped the carb I will know what needs ordering and feed back on here. Thanks for all the help :thumbup:
 

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Discussion Starter #17
Getting very close to washing my hands of this bike now.

The FCR carb arrived only to turn out to be 37mm so that's going back to the seller, I had considered giving it a go but if i had modified it to find it didn't work it could have been a costly mistake so I went back to the original carb. Starting at the beginning I repaired the broken air box, fitted a standard filter and rejetted the carb back to standard (50 pilot, 148 main).

The bike started straight away with a little choke and even ticked over once warm. Then it had a fit and started cutting out and then it would be ok for a bit and so on and so on.

I made 2 videos today within around a couple of minutes of each other to show the difference in case anyone can recognise the symptoms and maybe point out something I haven't yet looked at?

https://youtu.be/FA9MyBT4nto

https://youtu.be/MpS1715BFCM


Any help appreciated as I just want to ride the damn thing now :(
 

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I wonder if it’s more electronic than fuel related? Coil breaking down under load or cdi?


Sent from my iPhone using Tapatalk
 

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Discussion Starter #20
Sticking float valve?
Possibly. The float was replaced and internals inspected but it won't hurt to do it again. I think am getting to the point where I could strip the bike and remove the carb with my eyes shut haha.

I wonder if it’s more electronic than fuel related? Coil breaking down under load or cdi?


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This has crossed my mind as well since it appears to happen more when the engine is up to temp. I have a spare cdi and coil that came with the bike so it won't hurt to try changing them over again. The other thought was to start unwrapping the loom and look for any breaks or poor connections as well.

I took the bike for a spin locally yesterday and I'm almost certain I felt it trying to cut out a couple of times while riding along at 30mph, it felt as if there was a miss and I swear I heard that hiss noise through my helmet but then it ran fine again other than cutting out at a junction.
 
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