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TOPIC: Mikuni VM28 on a DT175

Mikuni VM28 on a DT175 16 Apr 2018 16:10 #1

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Hello!

Not my first post here but my first as this profile. (Last time I tried to register with my e-mail I had issues, and had to resurrect my facebook account...)

So I've been doing a refresh on my '78 DT175 engine, just rings and bottom end seals. Previous owner didn't take great care of her, got a hotspot in the cylinder but nothing that can't be honed out I don't think. Piston clearances were mint, but rings were waaaaaay out of spec so I got some new ones.

Getting to the point....the goof put a Mikuni VM28 on this baby. I know when a carb is too big but unfortunately the 28 seems like a maybe/maybe not situation for the stock setup. Needless to say I am going to try and make it work 'cause ain't nobody got money for a new carb. The only issue I seem to have is bogging/starvation at WOT. Just hits a wall. If I let off the throttle a bit and hold it there, she races through the powerband like a champ. I'm thinking I'll start messing with the main jets and see where that gets me, and if that doesn't help I'll just find me a Big One pipe lol.

My question is, has anyone fitted a 28mm on a stock 175, and if so, what's your tune if you don't mind sharing <3

Thanks guys!

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Mikuni VM28 on a DT175 16 Apr 2018 16:28 #2

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Hi, welcome, or welcome back if that's the case. My ideas--28 is the perfect size for a DT175 in my opinion. If i think of a std RD350 or the earlier R5 350 twin road bike so 175 cylinders they run 28's. My kids yz80 had a 28 std in 1978. So go for it. Can't realy advise on jetting at the mo but keep the pipe in mind incase its restricted with carbon. Just remembered i have a 28 on my yamy 100 which is a 1980 AG100 & loves it. Std was 22 but carb was same size externaly as a 28. Must admit i've lost the sharp bottom end but doubled the top end which was the aim of the swapping. Hope ya get it sorted.
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Mikuni VM28 on a DT175 16 Apr 2018 18:14 #3

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Not that a pipe might not help... not the answer though... RT325 is correct (as usual!) and I have in fact run a VM28 off of, guess what? An RD350.

Might be too lean is my first guess... which is usually what it means if it bogs at WOT and then "speeds up" when you close the throttle a bit... that symptom is right out of the Mikuni tuning manual I think...

Hope you get it sorted out and it's just jetting!

P.S. Went back to stock carb... VM28 really didn't help much on stock engine... hurt throttle response a bit though... with stock carb it would pull the front wheel off the ground a few inches if cruising slow in first... 2000 rpm or so... and then quickly giving it full throttle... with the 28 it took off okay and throttle repsonse was good but not as strong as with the stock carb.
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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

Mikuni VM28 on a DT175 16 Apr 2018 19:40 #4

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Thank you for the responses! Looks like I'll order a few jet sizes and go from there :)

Surprisingly enough I didn't notice too much low end struggle riding on the trails last summer, but then again I don't know what a 175 with a stock carb feels like, but thinking now it could use a little more peppiness on the low end. The way I see it is when I have the money I might drop down to a 26 or even back to a 24. I'm not looking for crazy power on this one. Oh and it definitely gets the wheel up for me with the 28 under the same conditions that you described!

Just curious, Mark, you wouldn't happen to remember the main jet size on the 28mm that came off the RD would you? If you never changed it I might be able to look up the stock jetting. I'm also thinking the RD had more aggressive porting and wasn't necessarily aimed for low end power like a trail bike, which made the 28s that much more acceptable for it.

Thanks again!

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Mikuni VM28 on a DT175 16 Apr 2018 20:34 #5

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Sorry... don't remember the size and it was a few years ago... not likely I'll find the exact carb I tried in my pile of used carbs. :Ugh

I'll look and see though... maybe I'll get lucky.. not sure I have more than one RD carb.

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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

Mikuni VM28 on a DT175 17 Apr 2018 00:34 #6

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Stock Main jet for an RD 350 is # 140 .

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Mikuni VM28 on a DT175 17 Apr 2018 09:59 #7

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28 is definitely not to big it just needs to be calibrated correctly , I have a 38 on mine but nothing is stock and has great bottom end with 6 venturi dividers in it

Rich
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racerclam

Mikuni VM28 on a DT175 17 Apr 2018 16:45 #8

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38mm, hell, that'd suck in low flying ducks. Well it'd shred them through the dividers--just saying.
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Mikuni VM28 on a DT175 17 Apr 2018 19:22 #9

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well if I had a K&N air filter the ducks might get through but they dont make it through the UNI 8o)

Rich

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racerclam

Mikuni VM28 on a DT175 18 Apr 2018 16:00 #10

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wow! a 38! Is there a thread about your bike?

hmm I guess knowing the stock jetting on the RD carb doesn't help me too much. I've currently got a 190 in VM28, and I'm getting fuel starvation at WOT, so going smaller doesn't make sense. Thank you for the information!!

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