Please help. Issues tuning XR400 Carb on FXR150

**Damien_Toman** · 10th February 2010, 18:52

I had diaphragm problems with my Suzuki FXR150 standard carb at Taupo (TRRS). I have an XR400 dirt bike, so I decided to try it's carb. To my surprise the XR400 carb (1996) worked quite well and felt pretty powerful on first attempt.

I then bought an XR400 (1998) carb on TradeMe to use permanently on the FXR. It also worked on first attempt. I have two outstanding small niggles that I'm trying to resolve and I would really appreciate any advice anyone can offer (I'd like to have it sorted for Mt Wellington practice on Saturday - 13th Feb).

1. It bogs/hesitates coming off idle but goes great when it picks up the revs.

2. When shutting off the throttle it behaves well until down to between 3500 and 2500 revs when it hesitates a little and then returns to idle. The throttle slide goes back to the bottom immediately and does not stick or hesitate (I took the top off to check that).

What I've found out so far:

1. If I wind on the throttle carefully, it goes alright, but I am inclined to wind it on quickly so the bogging is likely to be an issue on the track. The plug was dry but sooty black. The carb had a 55 slow jet and the needle was in the middle standard slot. I found that the 2nd needle slot works best (one slot leaner). I tried smaller slow jets (42, 48, 52) and the 52 works best and the plug now looks fine. I tried all needle positions and the 2nd and middle slots work best. The pilot screw is 1.5 turns out (std. for the '98 - 2 1/4 for earlier models). I've played with the pilot screw but still cannot remove the bogging on hitting the throttle off idle - the standard setting seems best. Raising the idle a little with the throttle stop screw helps but does not solve the problem. I've dismantled and cleaned the carb passages many times. The main jet is standard - 142. As I'm working on the carb the air box is removed - slipping it back on does not solve the bogging. I have modified the air filter with thin high-flow foam and opened up the air intake at the top. The exhaust is bog standard. The more I think about this the more I realise that I might need to try smaller main jets. My focus to date has been on everything else. My testing has mostly been in the garage (doors open

for air) with the bike in neutral (i.e. not under load).

2. The hesitation when shutting off the throttle happens regardless of needle position or slow jet size. The cable and throttle slide movements are free, with no tight spots, and work perfectly. Why would the revs not immediately return to idle? From 6000 revs to 1500 idle takes 4 seconds. Might this also point to the main jet size?

Any suggestions or advice will be much appreciated.

Regards

Damien

**Pumba** · 10th February 2010, 19:14

Hey mate, my carb tunning experience isint worth talking about, however I do believe that Rick (bike 52) was playing with a carb off a large XR so my best advise would be to get in touch with him and see what he did.

**Damien_Toman** · 10th February 2010, 19:21

Hey Pumba! Yes, Rick has a big carb and as he flew past me at Taupo, in practice, it felt like I was stopped! Hopefully he will respond to this post and also be at the Mt Wellington track at the weekend - he usually is. Hope your wrist/arm is mending well. Regards.

**Buckets4Me** · 10th February 2010, 19:26

Rick is a bissy man at the moment

has my bike in his garage and is hoping it breads

dont think he has much time for the smelly old 4 strokes anymore

**richban** · 10th February 2010, 19:50

Hi Damien

Have you got any room to ride it. Have a read of this might help. http://www.iwt.com.au/mikunicarb.htm

It could be a little big. Is it over 28mm?

**Pumba** · 10th February 2010, 19:51

Yea I know he has turned his back on the way of the FXR. All the more reason he should be happy to give away any secerets

**Damien_Toman** · 10th February 2010, 20:05

Originally Posted by Buckets4Me

Rick is a bissy man at the moment

has my bike in his garage and is hoping it breads

dont think he has much time for the smelly old 4 strokes anymore

Hey Buckets4Me, the FXR is the first 4-stroke I have ever raced. Just seemed to be the right bike for starting in bucket racing. I plan to have my 1987 RD350LC YPVS ready for racing this year. I used to race the original 1983 version

when it first came out. I raced the older RD250 air-cooled before that. All 2-smokes! I'm sure Rick will provide his expert view on FXR carburetion.

**Buckets4Me** · 10th February 2010, 20:19

I'm sure he will to
and the fxr is probably the BEST bike to learn on

pitty I got a 2 stroke as a first bucket (well a second but I sold the cb125t before I got to race it)

the fxr is easy to get to grips on

also I have an rd400 in parts that I was building to race but havent got around to it since racing buckets (buckets is way more fun and cheaper)
I like the old yamaha 2 smokers

**TZ350** · 10th February 2010, 20:42

Originally Posted by Damien_Toman

1. It bogs/hesitates coming off idle but goes great when it picks up the revs.

2. When shutting off the throttle it behaves well until down to between 3500 and 2500 revs when it hesitates a little and then returns to idle. The throttle slide goes back to the bottom immediately and does not stick or hesitate (I took the top off to check that).

These are clasic symptoms of being lean at the bottom. If this was my bike, (a 2-stroke) I would think it was lean on the pilot jet or slide. When they are lean here they just go flat when opening the throttle before picking up and then the engine takes a while to settle back down again to an idle when the throttle is closed. If you were riding it, it would run on for a bit when shutting off and entering a corner.

I think a 4-Stroke is probably much the same and its lean at 1/8 throttle..............the pilot jet..

**Rick 52** · 10th February 2010, 20:49

Hi my carb is a xr 500 I was running a 137 main but Tim got me a 135 when he got one for himself and it runs really good now Tim has smaller carb but that also runs great with the 135 . They say get the main good first then work down to pilot I did plug chops to get it close a good colour on the plug.

**timg** · 10th February 2010, 20:57

Could it be that the XR400 carb is simply too big? From what I can understand the quick FXR's (& wannabe's like me) down here are using GN250, XL/XR250 or DR250 carbs. The DR250 flat slide is supposed to be the business

**CookMySock** · 10th February 2010, 21:09

Originally Posted by TZ350

These are clasic symptoms of being lean at the bottom..

Yeah, but he's also got a sooty plug.. Is there ever any backfire with the hesitation? If there is, then it's lean for sure, and maybe the sooty plug is indicating rich on the (midrange) needle.

Can you try a good long cruise on the open road with a new plug? Stay well away from full power and thereby well off the main jet, and see if you still have a sooty plug. Also vigorously try and provoke it to hesitate, and see if you can make it do a lean backfire.

Does it start easily with no enrichener?

Steve

**SS90** · 11th February 2010, 00:34

Originally Posted by Damien_Toman

I had diaphragm problems with my Suzuki FXR150 standard carb at Taupo (TRRS). I have an XR400 dirt bike, so I decided to try it's carb. To my surprise the XR400 carb (1996) worked quite well and felt pretty powerful on first attempt.

I then bought an XR400 (1998) carb on TradeMe to use permanently on the FXR. It also worked on first attempt. I have two outstanding small niggles that I'm trying to resolve and I would really appreciate any advice anyone can offer (I'd like to have it sorted for Mt Wellington practice on Saturday - 13th Feb).

1. It bogs/hesitates coming off idle but goes great when it picks up the revs.

2. When shutting off the throttle it behaves well until down to between 3500 and 2500 revs when it hesitates a little and then returns to idle. The throttle slide goes back to the bottom immediately and does not stick or hesitate (I took the top off to check that).

What I've found out so far:

1. If I wind on the throttle carefully, it goes alright, but I am inclined to wind it on quickly so the bogging is likely to be an issue on the track. The plug was dry but sooty black. The carb had a 55 slow jet and the needle was in the middle standard slot. I found that the 2nd needle slot works best (one slot leaner). I tried smaller slow jets (42, 48, 52) and the 52 works best and the plug now looks fine. I tried all needle positions and the 2nd and middle slots work best. The pilot screw is 1.5 turns out (std. for the '98 - 2 1/4 for earlier models). I've played with the pilot screw but still cannot remove the bogging on hitting the throttle off idle - the standard setting seems best. Raising the idle a little with the throttle stop screw helps but does not solve the problem. I've dismantled and cleaned the carb passages many times. The main jet is standard - 142. As I'm working on the carb the air box is removed - slipping it back on does not solve the bogging. I have modified the air filter with thin high-flow foam and opened up the air intake at the top. The exhaust is bog standard. The more I think about this the more I realise that I might need to try smaller main jets. My focus to date has been on everything else. My testing has mostly been in the garage (doors open

for air) with the bike in neutral (i.e. not under load).

2. The hesitation when shutting off the throttle happens regardless of needle position or slow jet size. The cable and throttle slide movements are free, with no tight spots, and work perfectly. Why would the revs not immediately return to idle? From 6000 revs to 1500 idle takes 4 seconds. Might this also point to the main jet size?

Any suggestions or advice will be much appreciated.

Regards

Damien

I can't help but wonder if there is another problem, giving you these symptoms......

While it may well be that the carb is at fault, the fact that you have substituted pilot jets (bigger and smaller) needle settings etc, combined with the fact that you have had trouble with the original carb (what was that trouble by the way?), I just wonder if there is another possibility....

Can you 100% be sure that the inlet manifold is not leaking?, an easy test is simply to spray something like breakcleaner or easy start around the inlet manifold rubber when it is idling, if you find that the idle increases immediately, there is every chance that there is a manifold leak.

Another option is to check you valve clearances.

On a hotted up four stroke, there is a good chance that the valves are getting a hard time, simply because of the increased RPM.

If this is the case, and the valves, particularly the inlet, can, quite often stretch, closing up the clearance.

And, if so, this will cause the bike to run lean, and quite often "spitback" off idle.

Of course, there are other symptoms that will cause this to happen (carb problems mostly), but it is something I would check before spending any more time on the carb.......it only takes an hour at the most to check your valve clearances, and at the very least eliminates this as a cause!

**koba** · 11th February 2010, 05:32

It looks like heaps of people have different opinions...

Stuff like this is really hard to solve without being there, you best bet would be to strongarm someone with 'the touch' into having a look in person.

I'm going to say play with that pilot screw a bit more and run it up and down the road to test.
After that I woiuld agree with TeeZee, the slide cutaway is next on the list.
I have convinced myself the cutaway height is the main reason Helens FXR aint right but I also think the CB250 carb on hers is on the large side for any track smaller than Taupo.

Does it have a pumper?