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The Big Bore Carb Jetting Thread - 325 and 350cc
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As there are a few 325s about, and the 350 kit is selling, I thought I should begin a thread about carb jetting for the big bore engines. Those of us who have been running the 325 for a while may have noticed they run a bit rich if the jetting is left standard. Sort of the opposite of what you might expect (counterintuitive I think they call it!) - you would think more air flowing through per cycle would need more fuel, ie bigger jets, and certainly if you improve the airflow of the 250 with airbox cutouts and free flowing exhausts, bigger jets are required to avoid running too lean

BUT

It seems that when the piston is sucking a greater volume of air, a greater vacuum  sucks too much fuel through the jets, rather than just a proportionate increase.

I'm not going to try to explain it.confuse I have just observed this to be the case on my 325 , and now 350. I bought an air/fuel ratio meter to be as scientific as possible when setting them up, rather than relying on old school plug-chops and guesswork, and all I want to do here is record what setup  my bikes seem happy to run at.

 Here's a link to some information on air/fuel ratios to help you when I start throwing numbers out! I aim for 12 as a compromise between lean enough for economy and no plug fouling, and rich enough for cooling and best torque. Of course it is all a compromise when you just have 3 jets and a needle to play with (pilot, main, and the fuel screw adjustment) - only electronic fuel injection can really be fine tuned for ideal afrs.

 

 

www.safrtool.com/SAFR-AFR-values.asp

 

 

Also this guy explains about fuel screw adjustment, well actually about air screw adjustment, but stick with him. Near the end you can see how hard he has to concentrate to get the explanation the right way round. You have to concentrate to get the carb dialled in right too!

 



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Firstly my 325 Thumper Racing Open Enduro:

This bike is running standard header, Motad end can, which breathes a bit better than a standard pipe. Twinair filter, and an oversized snorkel on the airbox. Open Enduro carb so old size emulsifier tube V95.
I began with Pilot 50, Main 147 and the needle clip in the 2nd slot down from the top, and found on a dyno that it was running a rich air/fuel ratio between 10.5 and 12 through most of the rev range so I fiddled around with jets until ending up with 48/140 and the needle still in position 2. I have done some 8000 miles on this engine, including hot desert and hot mountain riding, she hasn't gone bang, doesn't kill spark plugs, and returns 65-70mpg.

Numbers-wise, AFR at tickover 11ish, 12-13 in the mid range and 11 at wide open throttle (WOT) (which is maybe a bit rich, but aids cooling when she's working hard on the motorway)



-- Edited by mossproof on Sunday 9th of February 2020 10:42:22 PM

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Now TTR350 number 1:

Currently running standard header, Motad end can , standard airbox and snorkle, Twinair filter. Open Enduro spec emulsifier tube.

I spent a huge amount of time going round and round with different setups and settled on 48/140 again, this time with needle in #3 position. AFR values go from 12.5 at tickover, between 11 and 12 through the mid range to just over 12 at WOT. I tried leaning off the mid range with the needle in #2 but could feel a slight hesitation or flat spot so stuck with #3 - she just likes to be a little richer there.

When built, I started with a standard end can, and began with 46/142 jets, needle at mid slot #3. AFRs 13 at idle, 11.5-12 mid range, 12.8 at WOT. Later broke my #46 pilot and haven't found another, but I reckon a 48/140 would suffice, maybe with the needle at#2.

I also experimented with FMF Q4 without the baffle, and the airbox lid off - ie fairly free flowing. 48/147, needle in #2. This ran around the 12s, but leaned off at WOT to 13ish, which would be a bit lean for comfort for me. Was also too loud for me so I gave up the experiments at that point, reverted to the Motad end can and settled in to put some (s)miles on...

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On my 325 Thumper Racing engine I am running a 137 main jet, V95 emulsion tube and a 50 pilot jet.

I am running a standard header pipe (to clear the oil cooler), Motad exhaust and the airbox is as standard with a Twin Air filter.

I had some issues on cold starting so went up from 48 to 50 on the pilot but haven't started it from cold often enough to see if it improved matters.

Once warm or off idle it runs really well on this jetting. It will run smoothly at constant throttle opening across the range with no stumbling. It doesn't bog and accelerates brilliantly. Great fun to ride biggrinbiggrin

Brian



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48/140 or 50/137 I guess do a similar job.
Thanks for adding to the info-mine



-- Edited by mossproof on Wednesday 15th of July 2020 04:49:36 PM

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mossproof wrote:

Is that with shorty header and Motad pipe, standard airbox?
48/140 or 50/137 I guess do a similar job.
Thanks for adding to the info-mine


I am running a standard header pipe (to clear the oil cooler), Motad exhaust and the airbox is as standard with a Twin Air filter. I have edited my post to include that information - thanks for the reminder Simon wink

The needle clip is in the middle notch.

I think I may have found the cause of my cold starting problem (choke circuit not working) and, if I have, I might go back to a 48 pilot jet.



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My 350 carb is running 147/54 think the needle is dropped one clip? but can't fully remember. May be a very tiny bit rich, but pulls like a train through all the rev range with no flat spots smilesmilesmile

 

Edit bit

Standard airbox, shorty header and OSS exhaust

As the OSS exhaust (similar to the extinct Motad) and shorty header were designed not to need re jetting on a standard bike, would it effect the 350 set upconfuseconfuse



-- Edited by ttr steve on Thursday 13th of February 2020 07:54:36 PM

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Fuel flow must sound like Niagara Falls with jets that big Steve
Never seen a 54 before.

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Think my MPG may be a bit on the low side, lucky I have the Acerbis tank fitted smile



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So, I'm starting out setting the carb on the 350. I've done 25km on it and checked the plug. No colour change. I have a feeling it's a bit lean.
I haven't changed anything on the carb. Tomorrow I want to play a bit with the air screw. What do you guys have it set to?

I haven't 'opened' the bike up yet.  I've been riding her gently until I've done a few hundred km.  I heard that it's important to do this.  

 



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Glad it's running Michael. Well done! Built on the floor of your apartment - must be a first?
I'd be suprised if it was running leaner than before unless the movement has dislodged some grot in the carb, but if it's not popping and farting on overrun, or stuttering on acceleration it's not too far out. Definitely run it for a bit to let things settle - until the rings have seated there will be oil burning and lower compression to confuse the issue.
Running in: Definitely be gentle, but not too gentle. Gentle then brisker accelerations, plenty of engine braking (overrun) a mix of running times if possible to create different temperature cycles, and no wide open throttle (WOT) for a hundred or so km and no "lugging" in too high a gear.
What exhaust system and jets are you running?
Fuel screw adjustment is tricky. You are looking for the spot where the engine picks up fastest when you blip the throttle from idle. It will never be as quick as a 2 stroke.
Put a dot of white paint on one lug of the screw so you can see it's position easily. Start at 2 turns out and go from there - there's loads of stuff on youtube - on 2 strokes they call it an air screw, but you will understand what they are trying to acheive.
Good luck number 6!
Simon.

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I really should have opened the carb and checked the jet numbers. I really can't remember. But it should be stock. I have replaced the standard exhaust with an aftermarket one. No idea the brand or name. I guess kind of a no name!

Alrighty. I shall follow instructions:)


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Ok. Opened up the carb today. It had a 47 pilot jet and a 145 main jet. The main get went into a V05 case (dont know what to call it).

I put in a 50 pilot jet and a 148 main jet. The case is a V95. It was running lean before. I think quite lean. also, there seemed to be a flat spot at wide open throttle. I'll take it out tomorrow and see how it behaves. Also, Simon, how do you get the carb in and out with out taking half the bike apart!!?

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No shortcuts to pulling the carb out I'm afraid, but you'll be suprised how fast you get with practice!!
I just about managed needle adjustments without carb removal - tank and seat off, slacken hoseclamp screws and twist carb over to get access to the top, but it's tight and fiddly.
Have fun

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Hummmm I will have to try that! The new needles arent working out so well. Feels odd. Only had it our for a 30 min spin. We are in our rainy season here. Hopefully Ill get it on a longer run soon.

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Well, that 50 and 148 we way too rich for the bike. I put the 47 145 back in. Runs much better but I still have the flat spot at wde open throttle.... Any ideas?

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Just want to clarify the problem. When I get to wide open throttle the engine seems to keep reving but there is no more power.

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I would check your carb float height. (Clear tube on the drain, looped up the side of the carb. When you open the drain screw the fuel level inside will be the same as that in the tube. With bike upright not on sidestand)

Could try running with airbox lid off - if it goes better it was running too rich before.

Have you done a plug chop at wot? Ride flat-out, then pull in the clutch and simultaneously cut the engine then check colour of plug. Should give you an idea of what's going on inside at wot.



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OK. I'll try those. Thanks Simon.

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TTR250 keihin pwk.jpgTTR250 keihin.jpg

Well my TTR is only a 263cc 2mm over does that qualify as a big bore? biggrinbiggrin

Anyway I may have found a compatible 80 mm piston so hopefully 300 cc soon. Anyway just bought this Keihin PWK carb.
Will see how it all goes, was only about $60 AUD.  I'll let you all know if it works.



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Forgorin wrote:

Just want to clarify the problem. When I get to wide open throttle the engine seems to keep reving but there is no more power.


 Clutch slip ?



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Kaos wrote:

 

Well my TTR is only a 263cc 2mm over does that qualify as a big bore? biggrinbiggrin

Anyway I may have found a compatible 80 mm piston so hopefully 300 cc soon. Anyway just bought this Keihin PWK carb.
Will see how it all goes, was only about $60 AUD.  I'll let you all know if it works.


 Qualifies as a big bore, yes, but the thread was aimed at the TK carb jetting setupsbiggrin

I'm interested to hear how it goes though, and awaiting Gadget's Mikuni results too...

If you're resleeving the barrel take care to get the liner positioned just right - otherwise the cracked barrel issues of the Thumper Racing 325 will occur. To mitigate the chances of leaks, get your machinist to slop plenty of green loctite round the sleeve before pressing it in. Worked for me on my 325.



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I can now usefully join in the conversation now I have my 350 up and running wink

The carb I am going to use on the 350 ran really well on the 325 engine (albeit a tad rich) and the jets are 137 main, 48 pilot, with needle in middle notch. TwinAir filter, no airbox mods and a free-flowing Motad exhaust.

I took the 350 for a run this morning and, although it seemed to run nicely I was only using part throttle as it is running in. However, when I got back to base, the header pipe had really coloured up making me think it was running lean. I took out the plug and it is white rather than brown which sort of confirms my suspicion.

Also, the choke isn't working so I had to take the carb off to fix that. I took the opportunity to change the jets to 147 main and 50 pilot. I had a 145 main jet I was going to try but I think it had been badly treated in the past as it broke off under only slight pressure when I was fitting it no

I went right through the choke circuit with cleaner and the airline and can see nothing wrong except maybe some distortion on the float bowl gasket in that area. I have fitted a new gasket so will now need to check the rebuilt carb out - tomorrow biggrin

EDIT  The choke now works but, once warm, the TTR ran like a pig - really really rich and bogging down badly. I was lucky to get home on it. I pulled the plug and it was wet with petrol. I will leave in the 50 pilot but replace the 147 main with a 140 and see how that goes. Got distracted by the  BSA C15 project again today but I gotta make use of my brother-in-law when he visits biggrin

2ND EDIT - No wonder the TTR ran badly. "Someone" had filled up with diesel disbeliefdisbeliefdisbeliefdisbeliefdisbeliefdisbeliefdisbelief  Having spent most of the day with Keith trying everything to get the 350 to start (swapping jets, carbs, etc) Trish asked what was going on and, after telling her the whole sorry story of our efforts, she asked if I could possibly have filled up with diesel! Swapped to a clean supply of petrol and all is now well - albeit incredibly smoky! Clever girl eh? biggrin 

 



-- Edited by TTRfan on Tuesday 3rd of November 2020 05:47:14 PM

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OMG!

Lucky you have Trish around, you defo need to get an AFR reading on your bike ASAP, it must be super lean.



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Hey Brian, you're not the only one who's filled with diesel!

I filled my 600cc Suzuki up to the brim in Gateshead in June 2002 and it got me to Newcastle before it conked out going over the Newcastle Bridge.

I free-wheeled down to the hill to beside the river Tyne before I realised what I'd done.

Dscf2593.jpg

I had to take the tank off and dispose of 4.19 gallons of diesel down a shady hole.

I locked everything up to a road sign and walked a mile and a half, borrowed a gallon can and bought the petrol, then back to the bike, fit the tank, put the gallon in and tidied everything up.

DSCF2595.JPG

Eversince then for about a week it smoked like heck and was difficult to start on all four cylinders!

Never again I said - and lucky was my middle name for ages afterwards.

History now, but you joined the club and admitted it.

Good on yer - don't make a habit of it.

Martyn



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Can't say you're not thorough in the 350 testing Brian. Proved it won't run on paper towels, won't run on diesel. Coal next? Anthracite/smokeless only...

Definitely need to sort the afrs out before you burn out one of those expensive special pistons!

Crikey Martyn, you're lucky the bike was still there - back streets by the quayside not ideal, and that sign would easily pull out of the ground if the cable wouldn't cut with a pair of pliers! Close indeed!

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Simon,

Back in the old good days (18 years ago) people were working hard all day and didn't have time to go nicking things tied to notices. biggrin

Nowadays they'd nick your Granny if you left her unattended. hmm

 

Yes, I was very lucky, but needs must at that time. It spoiled my day but tought me a lesson.

Martyn



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They love free flow muffles lets her rev out fast and open air box if u have bike take tail pipe off go ride her c wht I found out rev like yzf lol 

 

 



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what

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I've been out on 350 no.3 this weekend and my girlfriend rode her 325 raid briefly today. We should have some feedback in the next few weeks!

Great bikes!

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Glad you got the bike sorted smile Thankfully there are people like Simon around, be lost without them.



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Steve / Simon
Are you doing another run of 350 kits???

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Yes. Machinist started prepping the castings on Friday. Run of 6.

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I just realised that the TM33 on TTR350 info from me never made it to this page... So here it is:

I use this one: www.mikunioz.com/shop/yamaha-ttr-250-mikuni-tm33-8012-pumper-carb-kit/ (the red frame bike in the pics is actually my bike)

There is a photo of a head bracket that has white X's on it.... That's what needs to be cut off. But I had spares, so I just used two of the left sided brackets.

The airbox boot- not as tight as the TK carb, but not sloppy. It clamps in nicely. The carb came with a 127.5 main fitted- I put it up to the 137.5 that was supplied in the kit.... But I'm in Australia where the kit comes from.

Whatever pilot came in the TM33, I left there. The main that it was installed with was a 127.5. The way that I originally read the ad for the TM33, was that it came with a few jets either way of what is in it..... The jets provided as extras were 137.5, 140, 142.5. I opted for the 137.5 which got the WOT AFR at about 12.5 (near enough for me). Then looking through the rest of the rev range, I found that it was pretty good everywhere but 3/4 throttle, where it took a dive into being too rich for me at 10.5. There was a distinct stumble at that range when I stopped looking too much at the meter. I lowered the needle 1 clip position (clip now at second from top) which seemed to smooth that out.


Kaos had teed up to get my follow up dyno run done yesterday (and fine tuning)... but unfortunately **** was happening and I couldn't make it- I even had the bike ready to go.

Kaos was still able to get his 263 done. What was interesting to note, was that his 263cc TTR had a 2.5hp gain changing from Teiki to Keihin 36mm carb.



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Hi Mark - it's on the forum with your kind permission - copied from FaceBook. See https://ttr250.activeboard.com/t53141894/fitting-a-mikuni-tm32-and-a-tm33/?page=2#lastPostAnchor

Very tempting!

Brian 



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Open muffler's make bike killer fast try your bike just with header pipe they flow well I built a full s/s muffler as lost old one new muffler is 50mm from header pipe to end of tail she barks hard and revs her tits off it only 325 she all talk but still getting 135km out 250 gear box is petty good for old girl



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