Dellorto PHBH28 Jetting questions: Yamaha TZR125 2RK engine

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

Spent the past hour or so trying to jet my Pit bike ready for its track day on the 31st.

Now I started off with a 110 main jet, running quite a skinny carb needle.
It seemed to be running a little too lean, The next size jet I had was a 120.
I've got:

102 : 105 : 110 : 120 : 125 : 135

Now I've also got a slightly fatter carb needle, with more of a taper to it.

I switched to this needle, And fitted the 120 main jet. Thinking the bigger jet will compensate the thicker needle.

It was boggy, and running poor. So I went up to the 125 jet. Still boggy, but ran better.

Upped it to the 135 main jet. And it ran OK. Little bit slow on the throttle response. So I raised the needle to the highest setting (Bottom notch).

It seems to run okay now. Was a bit hard to start at first, But I've screwed the Pilot screw out about 5 or 6 complete turns. (I think on the Delly carb, out = Richer)

It starts with no revs now, Just on the kick start. And revs right through the range without any stutters.

My only problem is. A Stock TZR runs a 26mm carb, with a 180 main Jet.

I am running an open Air filter, A Race exhaust, a 28mm carb. And only a 135 main jet.

The plug has started to brown/black up on the electrode only. But I can really ride it too far as it's not road registered any more.

How obvious will the plug colour be after only maybe 30 or 40 seconds of full throttle riding. I can get it right up through the gearbox into 5th or 6th gear easily in this time.

Will this be giving me a good indication to how it's running?

Also wanted to mention, I'm also running it on 30:1 premix. But once it's run in a bit, I'm going to up it to 40:1 premix.

Ben

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

Okay, a quick browse on ebay shows the biggest jet available is a 145 main jet.
Going on the assumption that the jets for sale are sold with the RS125 in mind. Then the 135 main should be plentiful for the TZR engine.

The Rotax RS engine has got much bigger ports than the TZR engine.

Hmmm *thinks*.

Might just buy a 140 and a 145, and go from there...

Any RS125 owners out there fancy confirming this for me?

Cheers

Ben

stirlinggaz · World Chat Champion Joined: 22 Jul 2007 Karma :

having used this carb on both the rs & the mito (before going for 34/35mm's) i reckon your jetting sounds about right.
biggest i ever went on the rs was 128 & that was with a good few mods (vectorcone,stage 2 tune,jl exhaust etc).
tbh, i would have thought 135 would have been a bit on the big side & went for 132. (i know the tzr engine is a completely different engine)
but suppose its always best to work your way down.
what needle is it your using? & have you looked at the dellorto book, which gives dimensions of the needles?
curious to see what effect using a big main jet will have (like 145) in conjunction with your current needle.
oh, just out of curiousity, have you tried the 32mm dellorto fitted to some tzr's?
one of them would give you the power increase without having to resort to using different needles etc. worth a thought.
cheers,
GAZ

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

I'm on a bit of a budget at the moment. And I've got a couple of PBHB carbs in my garage. So I thought I'd make use of them.

Not sure on the needle size, It does have a number on it. I'll check it and let you know.

The reason I wanted to change to the bigger needle is because the smaller needle I had was bent. God knows how!
I straitened it out best I could. But still, Wasn't so happy about it.

So the thicker needle, Along with the bigger jets seems like my best option right now.

I've just ordered a 145 main jet. That'll be here tomorrow or Monday. I can try it there, and work my way down.
I've just been out for another quick blast, and the bike started to bog down when I opened it up from Low RPM.
It only did it once though.
Wonder if this could be pilot jet related.

Ben

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

Pictures make it look a little blacker than it actually is.

https://i902.photobucket.com/albums/ac227/pitbikeben/Photo1534.jpg

https://i902.photobucket.com/albums/ac227/pitbikeben/Photo1533.jpg

stirlinggaz · World Chat Champion Joined: 22 Jul 2007 Karma :

hi,
the problem with using a random needle, is it causes all sorts of havoc with the fueling.
have a look for the number on it & compare it to the needle that was originally in it, & look at the dimensions on the dellorto site, you'll be surprised at how much of a difference there is.
cheers,
GAZ

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

Okay, will have a look.

The needle came out of another RS125 carb.

One is a Dellorto PHBH28RD?
And the other is a Dellorto PHBH28BD?

I'm running the RD body. With the needle, slider and cap from the BD. Everything is identical apart from the Needle.

Body wise, identical, apart from the Pilot jets. They are different fitment. Main jets are identical though.

Ben

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

By the way, The needles are identical in length. Just a different thickness.

Surely as long as it's jetted to suite, The needle thickness wont make any odds?

stirlinggaz · World Chat Champion Joined: 22 Jul 2007 Karma :

hi,
i dont think i understood you properly....
you have a phbh carb, with pretty much standard jets,right?
but you have swapped the needle for a visibly thicker one, yes?
where did this needle come from, as the needles in rs125's carbs are all very similar to the naked eye.
so if its bigger/thicker its not standard & the jetting wont suit it, if im getting this right.
its not just the thickness thats different, iirc there are about 6 different dimensions on a needle, the taper, tip diameter,shank lenth & diameter etc.
seriously have a look at how many different needles are available from dellorto,lol & you'll see what i mean.
have you had a chance to get a magnifying glass on the needle? is it a K type? X type?
i think the needle makes a big difference to the whole set up.
EDIT : have a look at this & you will see what i mean https://www.dellorto.co.uk/editor/uploads/images/X-W%20Needle%20charts.pdf
cheers,
GAZ

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

Yes, the needle came from another RS125 carb.

I've got 2 complete Dellorto carbs.

Ones is a PHBH28RD, And the other is a PHBH28BD.

The thicker needle came from the BD. I was running the RD on the bike.
I've now changed to the other carb, The BD. It's got a slightly different pilot jet arrangement.

But this carb came off a 125cc RS125, Which was bog standard. So it's the stock carb, Stock needle, Stock jet.
I cant remember what jet was in it when I got it, As I've changed it now.

I'm currently running the PHBH28BD, with the needle on one notch from the top.
135 Main jet. Pilot screw (Which is not in the same place on both carbs), set to about 2 turns out.

Seems to run OK now. Although it is a bit boggey low RPM.
When I open it up fully, Then close the throttle, On over run, it sounds/feels like it's cutting out. Then when I get back on the revs, I can be a bit snatchy.

But it will rev right out almost, without bogging down. Very high RPM it splutters a bit.

Strange...

I hate carbs! Laughing

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

Just a note.

The needles are identical in length. The thickest points on both needles (The top) are both the same. They both slot into the carb sliders perfectly etc.

Just one tapers down thinner than the other.

Other than that, Both needles are the same length and size etc. Just one is not as thin at the tip.

stirlinggaz · World Chat Champion Joined: 22 Jul 2007 Karma :

i editid my post, not realising you had replied,
take a look at the needle charts & you will see there are other characteristics to be taken into consideration when changing needles, not just lenth & diameter https://www.dellorto.co.uk/editor/uploads/images/X-W%20Needle%20charts.pdf
i'd try & identify which series of needle it is 1st,
the BD is off rs125, iirc & the RD is off a mito? or is it the other way about? lol. anyway, could you list the other jets & emulsion tube numbers, that would help a great deal.
cheers,
GAZ

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

There are loads of different needles used in Dellorto carbs.

The needle will make next to no difference to full throttle mixture, while the main jet will make next to no difference to anything other that full throttle.

The slimmer needle will give a richer mixture from about 1/4 to about 3/4 throttle. Below that the mixture screw and the shape of the carb slide control the mixture. Above it the main jet is the important factor.

The mixture screw can be either an idle air screw (undo to allow more air through, so anticlockwise = leaner) or an idle mixture screw (undo to allow more fuel through, so anticlockwise = richer). Looks like you have one carb of each type. If the mixture screw is nearest the engine then it is a mixture screw, if it is on the airbox side of the carb then it is an air adjustment screw.

Mikuni and Dellorto jet sizes are not comparable (and Mikuni have used different methods to size them).

Personally, ignore the needle for now. Find a steep hill and do a few plug chops, making sure you are on full throttle, to get the main jet right. Once that is done play around with the needle(s) to get it to work on normal throttle openings, and then play with the mixture screw.

Basic guide to dellorto carbs here:-

https://www.bikechatforums.com/viewtopic.php?t=17124

All the best

Keith
____________________
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binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

2 Very helpful posts there. Thanks Chaps.

I'm just getting ready to go to the coast with some mates, When I get back, I'll pull the carb apart, and list exactly what I've got in there.

The carb currently fitted has a Fuel mixture screw. It's right by the inlet.
The carb I've just taken off had the Air bypass screw, Which was by the air filter.

So basically this thicker needle is just giving me a leaner mid range. The thinner needle will give me a richer mid range.

Ben

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

Okay, I think I may have made a little fuck up too.
The two spouts on either side of the carb. What do these do?

I thought they were some sort of overflow spouts! Laughing

So I've piped them into an overflow catch bottle, Which could well be restricting air getting into them if they are what I think they are.... Neutral

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

Inspecting both carbs now.
Carb bodies differ slightly.

One is a BD carb, Which has a Fuel Mixture screw. Inside the float bowl area, It has only one jet in the top left hand corner as you look at it upside down.

The other is an RD Carb. It's got an Air Bypass screw. and it has two jets in the top left hand corner when you look at it from the underside with the float bowl off.

Carb sliders, I have two. they both fit in both carbs. One has 50 written on the underside of it, The other has 55 written on the underside of it.

I have 2 Carb needles. The thicker of the two has X1 25 written on it.
The thinner of the two has X56 written on it.

2 Jet holders which appear to be identical.

Now the jets in the bottom of the carb. I am not sure which are called which.

Both carbs have a small jet just beneath the Main jet. I cant get one of them out, It's stuck in situ.
The other has a 5.5 in there.

Jets in the top left hand corner. Both have the same bigger jet, Which are 70.

Only one of the carbs have a smaller one in the corner, The other has nothing there.
The smaller one has 90 written on it.

All confusing stuff ay. Laughing

I'm going to assemble a carb using the better of the two bodies. And go from there. If it comes to it, I think I'll just drop it off at a 2t tuners place and let them do it all for me.

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

They are probably breather pipes for the float bowl. Allows air pressure into the float bowl so the carb works.

Not sure about an X1 25, but assuming it is an X1 then that is 2.48mm thick at the top, 1.2mm thick at the bottom with the taper being 26mm long.

X56 is 2.5mm thick at the top (marginally thicker), 0.8mm thick at the bottom with the taper being 26mm long. However it has a 2 stage taper, with the 2nd stage starting 22mm from the bottom, starting at 2.36mm thick.

Details are here:-

https://www.dellorto.co.uk/editor/uploads/images/X-W%20Needle%20charts.pdf

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

I feel like im getting somewhere now.

I've gone with:

The PHBH28RD carb again. The one with the Air bypass screw. Simply because it's in better, cleaner condition.

I've gone with the shallower slider (50)
Thicker needle (X1)
And dropped back down to a 125 main Jet.

Full throttle, It seems to pull fine once it's in the powerband. Anything below about 6000rpm is tempremental. Sometimes it'll rev up fine, Sometimes it'll bog.

Plug is started to brown off, Maybe a little black, But far from oily.
I can live with that.

It's the jetting/adjusting of the midrange which baffles me. I have no idea how to tell which way round it is running. Lean or Rich.

I whack the throttle open, And it'll burble/bog, Close thottle up a bit, and open it slower, and it'll rev right out freely.

Cheers

Ben

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

Okay, Just reading through your post a little further up Keith.

Assuming i've cracked the main jet now. And low idle isnt such an important factor right now.

What do I change to adjust this boggy midrange?

Reading your post it seems like only the carb needle thickness can change the running of mid range revs.
I've only got 2 needles...

Would the clip position adjust anything?

Cheers

Ben

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

It is a slide carb. It won't work properly if you just give it full throttle at low revs. You are opening it up to run on the main jet yet there is stuff all pressure drop through the venturi to draw fuel from the float bowl.

Coping with full throttle from low rpm is why CV carbs became popular. They could ignore rider abuse like that Wink

Moving the clip on the needle makes it richer / leaner. Clip closer to the sharp end makes it richer. You choose a needle with the right shape to move from lean to rich by the amount you need, and then use the clip to make it richer or leaner overall.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

Bollocks. Just loaded the bike into the car. Taken it to some private land.
Blasted it up and down a few times, And once it was fully warm, It completely died. Just bogged out as if it was out of petrol.

Let the bike cool down, Started up almost straight away.

It's not as if it's a compression issue or anything, The engine is mechanically sound. It's all brand spanking new.

Could it be running a hell of a lot too rich?

Plug has started to black over, But it's not had a decent run to do a proper plug chop.

Ben

stirlinggaz · World Chat Champion Joined: 22 Jul 2007 Karma :

hi,
you sure its not the floats?
if the float height isnt set correctly or the whole float arm is bent or needle jet is blocked, it could be running until float bowl is empty, then conking out.
then it will re-start after the float bowl has slowly filled back up.
just a thought.
cheers,
GAZ

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

Certainly something worth checking, Not got time right now though.
Thinking about it, The carb does sit at quite a steep angle. Meaning the float bowl is probably always only 1/3 full.

May have to poke around a bit and re-angle the carb.

Ben

stevo as b4 · World Chat Champion Joined: 17 Jul 2003 Karma :

Feel your pain Binge!
I've had enough trouble myself trying to set up bigger or different carbs on tuned 2stroke scooter's/bikes before.

I do agree that with a fresh plug and some common sense guess work that it's possible to get the main jet somewhere close at the least. The idle can be sorted the next easiest, but playing with th pilot screm is only a very fine adjustment, and normally the pilot jet needs changing for a given carb to suit another engine.

The midrange is where it's really hard to get a decent fueling slope throughout the rev range under all load conditions. The other thing to remember is the fact that there's alot of overlap between jetting circuits, and if you get say the main jet about right, but that the midrange is way too rich, then you can end up correcting the midrange fueling, but then you go too lean again at the top.

TBH with a brand new or re-built engine i would get it running okayish on the main jet if a bit rich, and then take it to an experience dyno tuning centre, one of the ones that is still experienced with carb tuning. This way at least they can run the bike in a controlled enviroment and log the AFR at different loads and through the rev range. It costs a bit to have a bike dyno set up, but im too worried about blowing up or seizing engine's to experiment with jetting a bike properly which unless you have a lot of experience with, is very hard to get your head around and get a decent result quickly.

Crafty · Derestricted Danger Joined: 24 May 2010 Karma :

Hi there,
I've had similar issues with a 21mm PBHG on my sons Suzuki TS50X. With hind sight I should have just bought a brand new standard 12mm mikuni Embarassed

but it was a lot more to buy Rolling Eyes

Heres what I found AFTER FINALLY using a dynamometer
Air jet & air screw control 0-1/4 openings (any carb)
Needle controls 1/4-3/4 openings (fatter = less fuel, thinner = more fuel) circlip raised to top leans mixture & lowered richens it
The needle restricts flow from the main jet thats all it does (as in any carb)
Main jet controls 3/4- full openings (also any carb)

I have spent weeks making calls & sending emails to various top tuning experts over in the States as well as here at home (karting as well as bikes) & the general talk is that these are excellent when set up correctly, but to set them is just about impossible without using either a dyno, or an air fuel analyser due to the MILLIONS of combinations there are for them. (so thats why the top guys use them then me thinks) Embarassed

These carbs also seem to work completely opposite to a Mikuni in there readings using logic, even the plug chop gave same sort of results your getting ie black.
My air screw is set at 3 & 3/4 out for instance, not 1-1 1/4 as a mikuni

First of all you need to check the atomizer (Main jet screws into this) is an "AU" & NOT an "AN" TYPE
AU is for 2 strokes, AN is for 4 strokes
AN types are full of holes & mess up low & mid range on 2 strokes so you'll never get your setup right if your running an AN.

GETTING CORRECT MAIN JET ONLY GIVES CORRECT MIX @ 3/4+ OPENINGS
Its a bar room myth that holed pistons ONLY HAPPEN @ FULL THROTTLE.
Holed pistons can happen at tick over if run long enough with weak mixture, likewise with any throttle position/weak mixture. Thats why,
IT'S VITAL TO GET THEM SET UP @ ALL THROTTLE OPENINGS
When it's sorted you will get INSTANT response from any throttle opening i.e. No lag or hesitancy

I hope this helps you out but I can say thay £50# a session for dyno was money well spent in the long run. Rolling Eyes

____________________
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Fastest is the 1ST to CROSS the FINNISH LINE.
2ND, IS the 1ST of all the LOSERS.