Jetting advice - VM29ss on a FXR150?

[Pumba]

Well I tried posting this in the mechanical portion of the site but apears no one knows shit, so thought I might chance my luck here; below is what I posted last night.

Ok I have taken a VM29ss with an accelerator pump and intend on using it on my FXR150. It is the first time I have really started from scratch with a carb on a buke so just looking for a bit of advice.

Put it on the bike and put in a 130 jet based on not really to much.

The bike started fine and idled not to bad, when I jumped on the throttle the engine would bog heavely and basicaly die. If I slowly creapt past that point and then rolled on it would rev strong through the remainder of the rev range.

Now what would be causing this bogging? I suspect either the piolet jet or the neddle position but I am only really guessing.

I did play a bit with the fuel mixture screw, but it didnt seem to make any difference to the problem.

Another observation I made was when I pulled the carb off there seemed to be very wet in the inlet manifold with fuel. To bigger main jet maybe?

Advice welcolmed.

Further to this I have had a bit more of a play tonight and have come to the conclusion that I am getting a heap of spit back through the carb so really need to have a look at my float levels, also went down to a 120 main jet and it seemed to make bugger all difference (actually the smallest I tried was a 90 main jet and it was well lean but but still spitting fuel)

Anyone got anything to comment on?

[Henk]

Bit of guesswork here but I'd try a smaller pilot in the first instance and try and turn down the pumper to start with. Possibly turn it off if you can, one less variable to worry about.

[woodyracer]

if you had any common sense you'd get fuel injection :laugh::laugh::laugh:

[thealmightytaco]

...and try and turn down the pumper to start with. Possibly turn it off if you can, one less variable to worry about.

Yeah if problem is only when you "jump" on the throttle as you say, it's probably the pumper, that's the only time pump comes into play, delivering a big fat squirt because the fuel flow has slowed and won't get back up to speed for a little while. I've only ever been told about them in their 4 wheeled application, where the fuel flow is higher, but I imagine it applies in a similar fashion, and the old 150 aint exactly a guzzler, doesn't need that big squirt to compensate for having to wait for the fuel to start flowing again, so it's just drowning the engine.

[richban]

Bit of guesswork here but I'd try a smaller pilot in the first instance and try and turn down the pumper to start with. Possibly turn it off if you can, one less variable to worry about.

What he said. Also mark your throttle so you know where exactly the problem starts and finishes on throttle position. People use tape, I use paint as I am always playing with the carb. If its not the pump it may be needle as it sounds like you were about 1/2 throttle. I would tune back from the main jet. Get that sorted first then look at the needle. This may help http://www.mikuni.com/pdf/vmmanual.pdf. Also what sort of filter are you running not running? If it revs strong above 1/2 throttle you are in a good place.

[hmurphy]

Not sure if this carb will work the same as mine but I am pretty positive it will.... the pumper only works when you wind the throttle on quickly - mostly from no throttle to full pin. The faster you wind the throttle, the more aggressive the squirt will be. It will only send 1 quick squirt of fuel. If you are being careful with winding the throttle on slowly, it's unlikely to do anything. But turn it off while testing for now just in case it's a tad faulty and leaks (this could be causing the dampness). Once your carb is jetted properly all the pumper will help do is give you a quicker throttle response, so tune that to work later as it should be a relatively easy job (.....in theory).

[Pumba]

Cheers guys, some more things to have a look at tonight.

Rich I havent been running a filter while I have been playing around because I have wanted to see what was happening at the mouth of the carb.

[Brian d marge]

you might find that carb is too big and the signal strength is weak

I use a 28 on my CR250 as the tracks are tight

disable the pumper , go lean and try if if still shows signs of bogging , try smaller carb

Stephen

[Pumba]

Thanks Stephen, it was a 29mm that came off there so figure I cant be too wrong puting a 29mm back on

[Damien_Toman]

Hi Pumba. I have exactly the same issue with an XR400 carb on my little FXR150. I tried changing all the jets and settings but found no solution. As Richban says "If it revs strong above 1/2 throttle you are in a good place". I therefore chose to control my right hand and ease the throttle at low revs but try to keep the revs up where possible. I suspect it is the slide cut-away that needs to change or else the carb may be too big (someone suggested to me). I can't easily experiment with slide cut-aways so chose to leave it and live with it for the time being. If you find a solution, I'm all ears. The pump is therefore probably not the problem - my XR400 carb does not have a pump.

[richban]

Cheers guys, some more things to have a look at tonight.

Rich I havent been running a filter while I have been playing around because I have wanted to see what was happening at the mouth of the carb.

Yeh thats a good idea. I have found that any filter be it Pod, Mesh or Airbox element, strangles my engine. Still airbox works good for me, and others. Also should give you really reliable jet readings.

[Pumba]

Well spent some more time playing in the shed tonight, I disabled the pump and there was no change so I think we can eliminate that for the moment. Changed the needle valve for a new one, and checked the float level, no change in performace. Droped the neddle and it ran worse, well at least somthing changed. Ifter talking with a few people the posiblity of having to much air, so I covered up half the intake and it seemed to improve things a little. It is still spitting a shit load of fuel back out the carb. Time to have a think about the implications of all this and go again tomorrow night.

Interesting that you are having the same problems Damien, if I find any solutions will defently share. I know what you are saying about the cutaway, but is the amount of fuel mine seems to be spitting at full throttle that makes think although it may be contributing that somthing else aint right.

[Damien_Toman]

Yes, I agree, I have my airbox still in place with a thin open foam piece to minimise restrictions and still keep rough dirt out.

[Brian d marge]

Jeez i checked and it is indeed a 29 carb , for a 150 engine !

What are your plugs like at differing rpm , black sooty , cold? or lean white ?

is the Airbox in place ?

All valves sealing and ignition working ok?

if it ran better when you put hand over inlet , suggest lean , try using the choke as a make it richer device , if it does throughout the rev range then thee is your answer

Stephen

the spitting back , making me think

[Pumba]

Havent pulled the plug (probally should though and see how it looks)

Valves should be all good the engine has just gone back togeather with all the clearances done.

I have been using the choke, helps to bring it above the bogging point before reving through to full throttle.

[Damien_Toman]

I also had spitting back but I don't notice it now with the airbox on :).

[Henk]

Is that because it's not doing it or because you can't see it through the airbox?

[richban]

after talking with a few people the posiblity of having to much air, so I covered up half the intake and it seemed to improve things a little..

29mm is not to big. If anything you will get better top end and a little less bottom end performance. I would say there is no such thing as to much air just the wrong air fuel mix. Lean or Rich. Have a read of this. Its 2 stroke but the principles are all the same. http://justkdx.dirtrider.net/printcarbtuning.html

[Ray LeCheminant]

Havent pulled the plug (probally should though and see how it looks)

Valves should be all good the engine has just gone back togeather with all the clearances done.

I have been using the choke, helps to bring it above the bogging point before reving through to full throttle.

I am by no means the most experienced , but from a previous life in TQ racing, we ran the same carbs (4) on a GSXR750
Excess fuel could be created by emulsion tube firing backwards, does it run better when you choke it with your hand over the inlet?

We would by rotating the emulsion tubes 90, then 180 degrees we could improve it.....

...but as I said not that much of an expert , and not REALLY sure your carb has the same style emulsion tube setup!

Ray

[Pumba]

Thanks Ray (just got your message to by the way). Yes it is the same carb and I suspect in a previous life that these carbs may have been on a TQ or purchased as spares.

I will have a look at the emulsion tubes, its not somthing I had considered and I have had the carb right apart so it wouldnt surprise me if I put the dam thing in backwards.

[Pumba]

Right to some progress tonight. I have concluded that the poilet jet is getting to much air, so I will need to go richer there. That will solve the hesitation problem at the the bottom.

The real issuew that I am still concerned about is the amount of fuel that is comming back out the carb mouth. Maybe I should just use damians theory and put the filter back on so I cant see it any more

[koba]

Right to some progress tonight. I have concluded that the poilet jet is getting to much air, so I will need to go richer there. That will solve the hesitation problem at the the bottom.

The real issuew that I am still concerned about is the amount of fuel that is comming back out the carb mouth. Maybe I should just use damians theory and put the filter back on so I cant see it any more

I know old Volkswagens run like shit unless you have a trumpet to contain the fuel fog that hangs out there. Maybe try an open bellmouth...

[Damien_Toman]

Is that because it's not doing it or because you can't see it through the airbox?

Mine didn't spit back much and I changed the jets so many times that I'm not sure if it happens much now with my current settings and the airbox on. Certainly the airbox is not dribbling fuel but I should check it out just after a ride to see if it is wet.

[Pumba]

Well I am a happy camper. Had a think about it all day at work, come home, covered up the pilot air jet with a bit of tape, threw on the factory velocity stacks that went into the air box that were sitting in the shed and away she went.

Stoked, just need to track test it now. Bit of a shame I cant get out there for testing on Saturday (free V8 tickets, with pit passes cant really be passed up), so will have to give it a go Sunday morning.

[Rick 52]

Good stuff see you Sunday

[timg]

Bit of a shame I cant get out there for testing on Saturday (free V8 tickets, with pit passes cant really be passed up), so will have to give it a go Sunday morning. Not a problem - I'll take care of those V8 tickets for you :lol: Good to see u got it sorted.

[Dutchee]

Stoked, just need to track test it now. Bit of a shame I cant get out there for testing on Saturday (free V8 tickets, with pit passes cant really be passed up), so will have to give it a go Sunday morning.
So, there must be alcohol involved too, to make it worthwhile being bored watching things with 4 wheels go round/over streets? I'd be taking Rick up on the offer of looking after the tickets for me if I were you (but I ain't, so you have fun and try not to get too bored - take a book).

[Pumba]

Well the conclusion is that the carb has potential. But I really need to sort out the pilot circuit by the more conventinal means because just puting some tape over it only lasted a half doesn laps or so.

Or I just need sticker tape. Not sure what the answer is yet:laugh:

[Henk]

Half a dozen laps is just fine.