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'78 175E - Dies on Road

12 Jul 2020 09:57 12 Jul 2020 10:17 by rhineman.
rhineman
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'78 175E - Dies on Road #1
Hi all, so I had my '78 DT175E in the garage since March and finally get her back on the road about a week ago. I did quite a bit of work to the bike: exhaust carbon burn, painted exhaust system, top end re-bored, block painted and new wiseco piston put in, carb completely soaked and cleaned, exhaust manifold holes tapped with new helicoils put in with new OEM studs, gas tank soaked w/vinegar a few times and completely cleaned out, new petcock. Believe it or not after all that work the bike still had a hard time starting. From my research on here I had anticipated this and had ordered some new jets. After trying to start the bike after all that work and having her barely come alive, I swapped out the stock 22.5 pilot jet with a 27.5 pliot jet. This did the trick and she fired right up. I took the bike on one short ride and she sounded clean and had power throughout so I felt that that jump up in pilot was all I needed. The next day I decided to take the bike into the city as I wanted to leave it in a vacant retail space I am trying to lease, this is about a 30 mile trip. The bike ran fine until about the last 5 miles she began to bog down and die out at a few red lights... I thought this was odd and perhaps there was something with the jetting. I let the bike sit for a few weeks and went to take it out again the other night and it started up right away after sitting there but then after about 1/4 mile it began to bog out and die when I was decelerating. I ended up still taking to a friends house about a mile away and it died basically at every red light.

I am headed back into the city today with some tools and am going to tinker with the jetting but wanted to see if anyone had any advice?

When I ordered new jets I ordered a #25 pilot, #27.5 pilot, #140 main, and a #150 main jet... I figured making such a big jump up to the #27.5 I would want the main to be larger as well but the OEM main jets I ordered did not fit the stem the OEM mikuni carb so I just went with the #27.5 pilot for now. I kept the needle in the OEM position (I think the clip is one down from the top?)

Anyways, any thoughts on this are appreciated. I am sure the answer is to order a stem to fit the 140 or 150 main jets, perhaps lower the pilot down to #25 and change the position of the needle?

Thanks dudes - here's a few photos of her sitting in the display window... everything came out great except the damn VHT 2000 degree rated paint on the exhaust burned off the exhaust near the manifold on the first ride!!!

Steve

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12 Jul 2020 11:21
MarkT
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'78 175E - Dies on Road #2
Love the photos!

Ummm... yeah some red flags.

Need more info on the main jets not fitting? Pictures of jets you bought? Is carb original and not a Chinese clone? It's very unusual.

Anyway, a little richer on the pilot is usually all you need on that model. But when I hear that header paint burned off and the dying problem I'm think you have a dangerously lean condition.... could be wrong carb... bent slide needle... or very possibly an air leak.

Did you ever change the crank seals?

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

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12 Jul 2020 14:32
rhineman
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'78 175E - Dies on Road #3
Mark, I have not changed the crank seals. I also have been reluctant to admit that I have not been running it with an air filter :S as the previous owner did not have one (very mature excuse) and I could never find confirmation any of the damn UNI ones for this bike came with the plastic fitting piece as there is not one in there. I saw posts on this forum that other model DT UNIs did but nothing for the 175E. I also removed the snorkel during this most recent garage / work session. Also note it was 90 degrees out the day I took her into the city when the damn paint burned off.

Headed into the city now to take the carb off and take a look, can share some photos of the stem x main jets. The carb is a mikuni 26mm (I believe or 28, I can remeasure shortly). It is not a knock off. The previous owner had a chinese knockoff prior to putting the mikuni on because he gave it to me in a box with a bunch of other old bunk extra parts. Below are the part numbers for the jets I ordered.

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12 Jul 2020 15:22 12 Jul 2020 18:48 by Lizeec.
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'78 175E - Dies on Road #4
Your carburetor is not the problem, well it was by running it with no air filtration. My guess is your piston is seizing, when the bike runs for awhile everything is getting hot and expanding and clearances are diminished and the piston is seizing in the bore, it then cools down and everything contracts and frees up and runs till it gets hot again, eventually it will seize completely.

Sorry about the bad news that is just my guess, if you remove the carburetor or exhaust manifold you can inspect the piston and bore, another issue is possibly the clearances were incorrect after the bore job and install of a Wiseco piston, Wiseco pistons have their own specific clearances that differ from factory Yamaha pistons, also from your description you did not follow the correct top end break in procedures, also stated the paint burning off that quickly on the exhaust manifold are tell tale signs of running way too hot (lean condition) or cheap paint or poor application of paint, once again I hope I am completely wrong and you find the problem to be something minor.

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12 Jul 2020 16:05
MarkT
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'78 175E - Dies on Road #5
Yamaha still sells the filter... I'm sure I have a new one and a cage I could part with.

I've run in over 100 degree heat and the only time I burned paint off head pipe was when I slightly bent the slide needle without knowing it and it was running lean.

Impossible to say exactly what has happened but it does sound lean... and it could have been trying to seize. Maybe just the air filter missing was enough. But I'd replace at least the ignition side crank seal. Not that hard to do. Use the 77-79 IT175 seal.

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
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12 Jul 2020 16:15
MarkT
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'78 175E - Dies on Road #6
Oh... on the main jets the pictures that come up are the right type.

It's possible the previous owner swapped the Chinese needle jet (the tube the main threads into) into the original carb.

I highly recommend removing the needle jet and slide needle and checking them for the original Mikuni markings and the correct sizes... those parts have sizes and are at least as or more important than the pilot and main jets as most riding is done at 1/4 to 3/4 throttle. That throttle range is where the needle and needle jet are calibrating the fuel delivery.

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
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14 Jul 2020 05:50
rhineman
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'78 175E - Dies on Road #7
Mark - thank you for the feedback, super helpful. So I did notice a few times when putting the carb back on the slide needle would get stuck and not go into its hole. The last few times I really had to mess with it to get it in. I will get this carb back apart hopefully today and inspect the slide needle and needle jet and compare to what Mikuni shows as OEM. I will also start to dig into the process of replacing the ignition side crank seal.

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14 Jul 2020 06:04
MarkT
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'78 175E - Dies on Road #8
I think I bent mine trying to get the slide into the top of the carb... not much room... had slide at angle and was a little too heavy handed trying to get it to drop in. :Ugh

Keep in mind that sometimes it can be a pain to get the needle to drop in the jet even if it's not bent.

Mine wasn't too obviously bent... rolling it on a flat surface showed it was though.

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
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24 Jul 2020 11:57 24 Jul 2020 12:04 by rhineman.
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'78 175E - Dies on Road #9
So I trailed the bike back from the city and have the carb apart. Here are photos and few notes:

www.dropbox.com/sh/ma75fgwpfcxwr22/AAAgD...5SZyp91We80AWfa?dl=0

SLIDE + SLIDE NEEDLE: The slide needle does not appear to be bent enough to notice or have an adverse impact on the way it should sit. I did grab the needle and slide from the old carb (cheap unbranded carb) the guy I bought it from gave me when I purchased it. It may be possible that I swapped out the slide / needle on the Mikuni I am using with the ones from the old carb? Photo 3779 shows the needle + slide I've been using labeled as "NEW".

JETTING: as I mentioned, I swapped out the pilot jet with a #27.5 and purchased a main jets 140 + 150, neither of the jets fit on the stem... they will screw in 1/4 of a full turn maybe a little more and get stuck... (photo 3769) they appear to be too large threads as well compared to the jet that was on there. Any thoughts on this? The OEM stem for these carbs I have not been able to find online.

CARB ITSELF: labeled as a mikuni but have not measured it yet and has no other markings on it besides the mikuni logo

AIRBOX CONNECTION: Another thought is perhaps there was a leak where the airbox connected to the carb. When I was reassembling the bike, I had a hell of a time fitting the carb into the the connector to the air box (shown in 3777) I had a hell of a time getting it back on and I ended up zip-tying this piece to the carb (photos 3778-3779 show the connector piece on the carb) and jamming it onto the air box fitting. I had a hard time even knowing I got it on there flush but it felt tight... I am wondering if this have not been fully air-tight and perhaps came loose into my ride as I made it about 30 miles on the bike before having this bogged out dying issue.

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