Honda XRV Forum banner

1 - 20 of 24 Posts

·
Registered
Joined
·
15 Posts
Discussion Starter #1
Hello,
Help needed please!!!... ;-)
I've spent 5 years off and on trying to get an old 1996 Dommie working that I got off eBay.
I soon realised that the original engine was fried, low compression, sludge oil, so replaced it with a 2005 FMX engine.
Removed and blanked off the emission system.
On inspection, the carb looks to have been modded with 'Daves mods', its the original RD08 Keihin carb, in fairly good nick.
The guy I bought it from (eBay) said he heard from the original owner (long ago) that it had been modded.
The bike has an original airbox..
Pilot screw is in unknown position.
I'm new to tuning, have done a bit of research, but don't know where to start to fix this.

OK, symptoms, I did a first start..
It requires little or no choke to start and idles but pretty rough, it occasionally stalls.
If it stalls it hard starts, and i can smell gas through the exhaust, its flooded.
Any choke whatsoever leads to very rough idling and some sputtering/small backfires.
Any throttle whatsoever leads to some massive backfires, I cant add much throttle as it cuts out.

I'm thinking its way too rich.. but I may be wrong.

Can anyone suggest something for me to try?

Ideally I'd like, I think, to cut off the airbox restrictor, and re-jet/re-tune with 'Daves mods' (I think I'm half way there) for better, smoother and cooler running. Shall I just cut that off before I do anything else?
 

·
Registered
Joined
·
198 Posts
Don’t cut anything off.

First thing is to screw in the mixture screw all the way in and then turn it 2.25 to 2.5 turns out. That will give you a rough start on the mixture. The only adjustment is the mixture screw which will only affect the tick over. The mixture adjustments at revs is sorted by the main jet but you need the bike running to give an ideas of which way to jet (up or down).

Fire it up if it will and have a good feel under the rubber intake manifold to see if it’s split. Spray a bit of brake cleaner in that area and if the revs rise you’ve got a split manifold. If so that needs fixing first.
I was guilty, as are so many people of rejeting, cutting stuff off as a first point of call. These carbs work best with the full air box.

Hope this helps.

Ian.
 

·
Registered
Joined
·
741 Posts
BoBN had a great post on here about the VE carbs, you have to make sure that the 4 little holes under the butterfly are clear and working, check the slide lifter cam on the side of the carb that it actually lifts the slide, the deceleration air-cut diaphragm on the side of the carb is OK connected to the front of the carb where it mounts to the manifold rubber.
Check main diaphragm in the top of the carb is good, not torn or punctured.
Main jet is 155 and pilot 58 IIRC.
Rubber hose attached to the side of the big nipple on the side of the slide so side wind won't affect the running.
 

·
Registered
Joined
·
15 Posts
Discussion Starter #4 (Edited)
OK, thanks for the advice chaps. (Sorry for the slow response, I've been skiing)

I've subsequently pulled the carb off to give it a good clean.. its totally stripped down now, and there were a few holes in the lower part of the main jet that were blocked, so there may have been a few other blocked bits, especially in the fuel screw bits which i'd never taken off.. so I'm glad I took it all apart. Its a VE Keihin carb.

I've also identified that the main jet it has is a 145, and the pilot jet is a 50. And that the fuel screw was a minimum of 3 turns out.

And that the two lifter holes have been drilled out to the recommended to 'Daves Mods' 5/32 holes.

The slide lifter cam was also a bit gummed up, now nice and free!, and the deceleration diaphragm and hose all look OK.

I've also seen some awesome vids on YouTube on motorcycle carb tuning, so fully understand what I'm doing now, and what everything does. (Which I consider mandatory now if you're messing with your carb newbies!!!)

So for this week, the plan is this:

  • Leave the stock airbox intact.
  • Get some brake cleaner and clean the hell out of the whole thing, check that all the little holes are clean, especially the 4 on the inside.
  • Get the 'Daves Mods' recommended 158 main jet and put that in. This will be good as its got an aftermarket Arrow exhaust, it will also make the bike run cooler, and avoid the factory lean condition.
  • Get the 'Daves Mods' recommended 55 pilot jet and put that in. I think a bigger pilot jet was required anyway as the fuel mixture screw was over 3 turns out.. meaning the previous owner was battling with a tuning issue and was too lean, which makes sense as it has the aftermarket Arrow exhaust, which would need bigger jets because its more free flowing.
  • Shim the needle with a small washer, as per 'Daves Mods'
  • Put back on the bike, with 2.25 turns out on the fuel screw.
  • Tune fuel screw at low idle from there, to try and get maximum idle without rpm dropoff.
Hopefully all of this will fix it. And the HUGE backfires it produces with any throttle whatsoever!! LOL. Can someone let me know, what causes this? too rich a pilot jet/mixture screw?

Also, I've noticed that a carb rebuild kit (which I don't think I need as all the bits look OK), has an air mixture screw in it .. is there one of these on the carb, maybe under a welch plug? that needs to be checked?

Anyway, does all of this sound like a good plan? Any additional steps?

Once again, I appreciate all the help! ;-)
 

·
Registered
Joined
·
741 Posts
Generally when tuning a carb I follow the rule of one change at a time, and document it in a little book.
If you change many things at once it can lead you down several levels of misery when trying to get something running correctly.
All engines are different and may need slightly different settings.
Backfires are caused by a lean condition it could be the aircut valve not working or the emissions system pulling in air depending on what has been removed/blanked.

I'd start with a clean carb stock jetting or close to it, 55/158 sounds pretty close and no more.
Once you have it running and idling correctly then move on.
 

·
Ridden for years
Joined
·
2,805 Posts
Whilst I love my brake cleaner and go through tons of it, you'll need carb cleaner; very different.
Eats your nitrile gloves, but hey.... it can also bugger your o rings, but hopehully you're replacing those.... aren't you... :)
 

·
Registered
Joined
·
15 Posts
Discussion Starter #8
Hi Chaps,

I hear what you're saying, make one change at a time, but I've never seen this engine run, never seen the bike run, and it didn't work with the carb that it came with (the shipped one), so I've made the jetting changes above.

Anyway, so the jets arrived and I cleaned the carb fully and stuck the new jets in.

It starts and idles, and works better than it did last time..

- last time: 145 main jet, 50 pilot jet - wouldn't idle without the choke on a bit, idled really rough, backfired loud and many times with any application of throttle, mixture screw was more than 3 turns out.

- this time: 158 main jet, 55 pilot jet - seems to be idling fine, starts easy, idle drop procedure now completed and it seems OK, can sometimes open the throttle but it misses lots, and does backfire occasionally, mixture screw 2.5 turns out.

I think the smaller jet is OK as I've played with the fuel / mixture screw and the bike does the idle drop at around 2.5 turns out.

Here's a video of what it was doing yesterday before I completed the idle drop procedure.

https://youtu.be/HXZ0f6wYsOk

What do you think I need to do?

My thoughts are. It requires a larger main jet, as the mixture is too lean when the throttle is opened, and so the fuel mixture is not burning in the cylinder, but causing backfires in the exhaust.

Can anyone confirm my thoughts?
 

·
Registered
Joined
·
15 Posts
Discussion Starter #9 (Edited)
OK, just keeping the thread up to date...

I've decided after reading some of the other FMX650 threads here, that my engine (although much improved) is still running very lean. The thread I read has many guys jetting these engines in the UK to 175 main and as high as 220 main!

So I've purchased a 175 main, and just in case that's a bit too much, a 162 main.. lets see how that goes.

And I think I'll mess with the pilot jet as well (probably not at the same time), but I've got a 58 for that. The 55 is almost 3 turns out on the mixture screw, and its a bit loose out there.. so wouldn't mind getting that down to 1-2 turns out.

Getting there slowly!!! - I'll report back.

Oh yea, I'm also going to make sure the float is adjusted properly as well.

NX650 float adjustment.JPG
 

·
Registered
Joined
·
15 Posts
Discussion Starter #10
OK quick update..

I have taken out the carb about 10 times now.. i still cant get the bike to run at over 1/4 throttle.

The slow jet is now a 55 and everything slow works great.

But over 1/4 throttle, the bike does exactly what it did above.. I went from lean 145 main jet, with backfires over 1/4 throttle, extreme misfiring and almost undriveable.. all the way to rich 180 jetting which blackened the plug..

I have gone back to the 158 main jet and think this is ok, but the bike wont run over 1/4 throttle.. sounds almost the same as above.

Any suggestions?

Anyone living near Woking willing to come and have a look?

I'm thinking it may be another carb issue, or a coil or HT or plug issue.. as its totally baffled me.
 

·
Registered
Joined
·
198 Posts
Just out of curiosity, have you tried it with the air box side panel off? I see in your video that you have the snorkel top on the box, but with the carb mods and aftermarket exhaust the box may be restricting the air flow too much.

Also checking the basics, does the fuel run from the tank freely and have you got a good seal in the carb from the air box?

A tap tap with the handle of a screwdriver on the cdi unit and a waggle of the wires never goes a miss. Those cdi’s are prone to bad connections.

Good luck
 

·
Registered
Joined
·
15 Posts
Discussion Starter #12
Much thanks for the response Ian, I will try those things next..

I also had a long chat with a mechanic who said it sounds like its not being restricted enough.. he said it may need a better vaccum for the slide to rise in the carb, and to really try restricting it to test..

Ive been trying to find a mechanic available in the next 2 weeks, they are all busy.. seems to be a good thing to do, be a bike mechanic!
 

·
Registered
Joined
·
147 Posts
No idea if this s related to your problem, but, did you put the NX pulse sender in on this FMX engine ?
IIRC, someone has previously said they are different and you need the NX one for it to run right
 

·
Registered
Joined
·
15 Posts
Discussion Starter #14 (Edited)
No idea if this s related to your problem, but, did you put the NX pulse sender in on this FMX engine ?
IIRC, someone has previously said they are different and you need the NX one for it to run right
AWESOME!! SIMPLY AWESOME!!

Now that is interesting.. I thought the engines were the same.. can you remember where you heard this from?

I will have to look it up.. it may well be the problem..

No.. I haven't swapped that.. it may explain why the tachometer doesn't work either.. as the pulses may be different.. I'm pretty sure I saw the tachometer worked (twitching slightly) when tumbling the old engine, which never started.. it doesn't do that now.

WOW.. GREAT INFO!!! THANKS..

Oh.. before I go.. and I think you got this right.. but just to say again, Its an NX650 im restoring, and I have a replacement FMX engine in it now.. NX650 carb and starter .. all other engine bits are FMX.. including pulse generator.. smog system has been removed and blanked off.
 

·
Registered
Joined
·
15 Posts
Discussion Starter #17
Ah yes you're right!! and they are different!! hooray!!!

When I originally observed the spark when testing the ignition, I noticed that there were 'lots' of sparks, where they should have been just 1 or 2.. this is almost certainly A problem, if not THE problem that I am having.. and again it would explain why the tachometer isn't working anymore ..

I will swap these bits out ASAP!! HOORAY!! .. at the very least I have a lead now.. I cant say thanks enough..

And I found the thread that explained all of this.. its here..

https://advrider.com/f/threads/any-honda-dominator-nx650-owners-out-there.252960/page-132
 

·
Registered
Joined
·
15 Posts
Discussion Starter #20 (Edited)
OK, thats it.. all working now!

So the issue I had, was that the replacement FMX engine that I transplanted into my 1996 NX650, has a different Pulse Boss on the Primary drive.. the Boss on the FMX650 is very different to the Boss on the NX650 and provides 5 sparks per rotation instead of just 1. Symptoms included, irregular idle, not being able to run over 1/4 throttle, and a spark plug that clearly is sparking too many times per revolution.

The bike passed its MOT today and I have subsequently done 20 miles..

It does cut out at idle, and this is a lean mixture setting.. I will give it a 1/2 turn out later to richen it up, but that's all minor now.. the bike runs..

I cant thank you chaps enough for the help, especially 'Bungle', who knew what it was!!... I literally rebuilt the bike in 2 months, and then spent 2 months taking out and rejetting the carb like 10 times thinking it was a carb issue, when all the time it was a Pulse Generator issue..

I have performed Daves mods, and left the pilot jet richer at 55, and the main jet richer at 158. Originally was a 50 idle and a 145 main.

I've got an aftermarket oil cooler that I may install by splicing into the oil return pipe, as it hit 100 degrees celsius on the road, and the engine is HOT, if I'm off road at higher revs it will probably need additional cooling..

Happy riding!

Here she is.. in all her glory..

MOT history..
2008 38,441 miles
2009 40,090 miles
2011 40,990 miles
purchased June 2014 - and rebuilt - engine replaced with running FMX with 20k on the clock, original engine had sludge oil and low compression.
8th May 2019 42,758 miles
And shes back.. to live another life! .. I love saving old bikes!! ;-)

IMG_8532.jpg
 
1 - 20 of 24 Posts
Top