Sundial Moto Sports

GT185 jetting advice
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Author:  sbaugz [ Wed Jan 08, 2020 4:41 pm ]
Post subject:  GT185 jetting advice

looking for jetting advice.
GT185 with higgspeed chambers and UNI pod filters. Otherwise engine is stock.

Author:  Vintageman [ Sun Jan 19, 2020 1:40 pm ]
Post subject:  Re: GT185 jetting advice

My advice is to put stock airbox back on. Sort out pipes first.

It would be wise to start 2 sizes larger and try that. I believe that is what I did (Maybe 3 at the end for cooling all my mods (see below). 2 size up on main, clip on needle at 4th.

If Higgs Pipe only, they prove to be mild, stock air box you may be OK with jetting as is but still would start with main jet one size higher, then check.

If you run WOT long time, mostly how that bike needs to be driven (at least when modded), if you can run larger main, it well help cool it. Also drop down a plug heat range at least. Check plug color. Also I think if too lean or too hot it you will see it start to hole the top of piston (Look into EX port by removing muffler or remove head now and again and see if all is well. When not right listen for pinging (If not drowned out by noise pods make :wink: )

I had a gt185 1975 with pods, then added jemco pipe (wicked boost top end. exciting). then bored carb ~2mm over, bored cyl 2mm over, widened (2-3mm) and raised (very little). then 0.020" head gasket vs 0.040". I may have modded/removed airbox snorkel on air box too. Not sure of exact order of all these mods now.

I think Higgspeed pipes much milder then Jemcos (was a kit and more race pipes). Jemcos, I had to change some trans gears that I pulled from a Suz tc100 or tc125 (forget.. maybe posted here somewhere what I did) which allowed the higher gears ratios to be spaced closer. Else, stock if i didn't over rev a lot before shifting , drop out of power band. Fast little Gt185 when done, but not tuned best street or touring. Put stock pipes back on at the end only, ran very nice that way!


Author:  sbaugz [ Mon Jan 20, 2020 11:39 am ]
Post subject:  Re: GT185 jetting advice

thanks for the advice. I also have an e-mail out to David Higgs too. Just waiting to hear back.
I understand it would be easier to use the stock airbox, but I don't really have that as an option. The bike came as a "parts bike", and it was missing alot of items- including the stock airbox. I have already cut off most of the excess frame tabs anyway, so I have to use PODs. From what I can gather, the stock main jet was 72.5, so I suppose I will go ahead and get some 80 and 85 Mikuni 4/042 main jets and start with that. When you say the "clip on needle on the 4th", is that forth up from the bottom of the needle tip, or forth down from the flat top of needle?

Author:  Vintageman [ Mon Jan 20, 2020 5:53 pm ]
Post subject:  Re: GT185 jetting advice ... SwGrBeHyOC
I know, pods are cool 8) . If I recall pods were not much of challenge to tune for me vs other bikes I have tried (Again makes me claim decent stock air box)

Bike with just pods I think (hurts my ears bad with my Full Face helmets).
Bike all stock carbs but Jemco chambers + silencers
doesn't have all the other mods yet -

Nice custom Bike with short pipes and no silencer

I long length tune and thus mild

There are others now with higgs

Edit, This guy made me buy the Jemcos :up:

Author:  sbaugz [ Tue Jan 21, 2020 7:05 am ]
Post subject:  Re: GT185 jetting advice

I got in touch with higgspeed and asked them about jetting. They were no help at all. Basically said they could not advise me on jet sizes (probably for liability reasons).

someone on FB suggested that I use the formula stock jetting x 1.2 as a starting place for pods and chambers. so 72.5 x 1.2 equals 87 main jet.

Author:  Vintageman [ Tue Jan 21, 2020 7:51 pm ]
Post subject:  Re: GT185 jetting advice

Engines can seam to run OK for they can run on a wide range of AFRs, be it too lean or too rich. I have used wide band AFR meters on cars with turbos... never tried on 2 strokes bikes. I am sure others have.

Too rich can be of concern also, especially on oil injection only bikes. Running WOT, too much fuel, can result in not enough oil remaining on piston skirts (fuel wash). May be wise to mix some oil to fuel... If you don't thrash it, while testing, or run long time wide open, lean or rich you should be OK as you verify and sort it out. Empirical tuning, you should try a range of jets until you are properly jetted. Since you are not changing porting but removing air box unlikely go below stock. If I am wrong and those Higgs pipes due boost significantly (raise compression) you may need a extra fuel to cool things down. run cooler plug.

Good luck

Author:  Jaguar [ Sun Apr 26, 2020 6:46 pm ]
Post subject:  Re: GT185 jetting advice

For the best information on how to perfectly jet a carb please go to

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