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Post Info TOPIC: The Big Bore Carb Jetting Thread - 325 and 350cc


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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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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

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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 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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