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TOPIC: 1978 yamaha dt250 carb question

1978 yamaha dt250 carb question 01 Sep 2018 23:43 #1

  • kakon864
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I recently acquired a 1978 Yamaha dt250 and I've never worked on a bike before, but have worked on car engines and outboards. So what I'm wondering is how the oil injector pump hooks up to the carb. I don't have the original carb, but I think it's a dt175 one the guy gave me. So in image 1, I assume A is the inlet for the oil pump from the tank and that B hooks to the carb to inject oil(still need to install the cable from the throttle). In image 2, I'm not sure what 1 is, 2 and 4 I assume are overflows (not sure) and 3 is the gas in from the tank. In image 3, I want to guess this is where the old oil pump inlet barb used to be, but not sure, if that was true it would come out of the blue circle in image 4, my other guess would be the image 4 red circle, but that barb seems too big. This is all based off guessing and I'm not sure where oil is injected. I wasn't able to find a diagram labeling the barbs. Worst case I can run premix till I get the proper carb, but I'd prefer to use the oil injection system.

P.S this is not on the carb question topic, but why do I need a CDI box if I got points, condenser and a coil to provide a spark. The reason I'm asking is I was able to get a spark out of the plug and then it stopped and then came back and then left. tried a bunch of different plugs and none work now and also tried redoing connector. The coil wire will spark if I hold it near the frame though (consistently). I gapped the points and nothing changed. I've ordered and new coil and condenser to see if those maybe went bad/got weak from sitting won't be here for a few days. I hear the cdi box might cause weak spark thought, but not sure why it has one on a points system.
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1978 yamaha dt250 carb question 02 Sep 2018 01:44 #2

  • Gr8uncleal
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Welcome Kakon864.

Hopefully I can help answer some of your questions, but probably not all.

First up, it looks to me like you have been given a carb off a twin, not a single. The reason I think this is that the part that you have circled in red on image 4 is where a small length of hose connects the 2 carbs on a twin (possibly an RD, or maybe an earlier model). It might, therefore, be mission impossible to try and even use this carb!

As far as I am aware, the 2 stroke oil is fed directly from the oil pump into the base of the cylinder/barrel. On earlier models the narrow pipe was connected to the cylinder with a banjo bolt, but later bikes might use a hollow pin, with a clamp used to secure the hose to it. I'm pretty sure that the two stroke oil doesn't go through the carb unless pre-mix is used.

If you can get access to a parts manual (Tech Library on this forum, CMSNL and Partzilla are good sources for this), the this will help you.

There should be numbers stamped on the carb that will help identify which bike(s) it is from, using an internet search. You should at least be able to recoup some of your costs by selling it.

I'll leave your ignition query to more knowledgeable members of this forum, but I'd be very surprised if you needed a CDI box when the bike uses points...........!! Photos might be useful here and is there any evidence that someone has tried to change the system from points to CDI or vice versa? I'm not familiar enough with bikes of that era to say which system was fitted as standard.

Good luck and I hope my rambling has helped!

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1978 yamaha dt250 carb question 02 Sep 2018 04:00 #3

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Just lost an essay bugger it--phone rings & thought i'd pushed submit GRRRR.
Guts was most of what was said above by' Gr8uncleal'. Ok, you have a high top right side RD350 28mm Carb. Number 2 [red] is a breather pipe & has another in the base of the bowl. Both lead to the bowl in the end, one from above & other has a tube finishing above the fuel level. Number 1 [blue] is the choke pipe from the left carb & that 'left carb' has the choke plunger & mechanism so block off 'that' tube--bit of hose with a bolt or a cap of some sort. Number 3 [orange] is the fuel into the bowl. Blue Circle is the oil pump into the motor via 'that' fitting which is missing. Just a right angle push in fitting. Apart from the choke, it's possibly in with a chance if you're desperate, as the specs show a 28mm carb. If you're ordering jets i'm not sure if the RD carb runs short main jets & pilot jets & maybe the DT carb runs long jets in both pilot& main but if you use it then stick to the right jet type for that carb you have. Link here to the partzilla listing. www.partzilla.com/catalog/yamaha/motorcy...0e/carburetor-dt250e Cold starts will be a pain, or squirt something volatile in wherever's easiest. I just use CRC 5-56 which is penetrating oil down here & they fire up straight away, even just fire some in the high breather pipe could get enough volatility to go bang. Keep the pilot air screw on the rich side might help cold starting--in for rich, out for lean, screw on the angle by number 3. I use one of these on my 100 road racer & also on my Yamy 100 farm bike, except the left side RD carb with the choke in that case. Cheers.
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1978 yamaha dt250 carb question 02 Sep 2018 05:19 #4

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after my rambling on & a cuppa tea i read the story & everything you say is right & was only that choke linking pipe that had you wondering. You're onto it. Interestingly i see the partzilla link doesn't show the little fitting which would be on the other side of hows it's drawn but should be there anyway. Doesn't show it with the pump either but should really be with the pump maybe but no matter. "75 RD350B carb shows it drawn.

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1978 yamaha dt250 carb question 02 Sep 2018 05:39 #5

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Yep, no cdi box required. Just clean points set about 14 thou. Good plug cap or non resistor type as they can fail in the resistor. I assume the new coil is the plug coil for up top & condenser is coming so only leaves points not contacting or the wire to the points is earthed somehow. Must have the little piece where the wire locating bolt passes through the fiber washers to stop the bolt touching earth & keeps it central, plus wire goes on top of the fiber not under it. Anyway it sparks a bit so won't be any of that. After fitting all those it almost only leaves the source coil in the mag. Not unheard of but not a very common one to fail on points mags. Run the black from the points/source coil/condenser [just one wire exiting up to the loom--run it straight to the top coil--probably an orange going into the coil but only one wire even if an aftermarket coil. Just no kill switch that way just to see if she runs. Make sure carb shuts off fully & good luck. Put a fitting on the choke pipe so you can easily give it a squirt via that & she'll be away.

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1978 yamaha dt250 carb question 02 Sep 2018 11:54 #6

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Thanks for the replies. In all the pictures i see the oil tubes goes to the carb. There is a cdi box, but it seems to be stock. need to look at a parts manual.
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1978 yamaha dt250 carb question 02 Sep 2018 12:00 #7

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Found a parts manual. That box is a voltage regulator.

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1978 yamaha dt250 carb question 02 Sep 2018 16:51 #8

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I think you have a 79 dt250 by looking at the louvered front fender. They may have a CDI not sure

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1978 yamaha dt250 carb question 02 Sep 2018 20:47 #9

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Yep, that's ya bulb saving regulator. Not critically important but if there's a hiccup in the electric you risk the chance of blowing bulbs if the battery is empty of water or disconnected. Anyway--someone said it looks like a "79, [FrankC above], so just say it is for a moment then it might have an unused cdi box somewhere & had the mag replaced with an earlier points system, therefore not required. No problem with points ignition--Or is there, just looking at my woffling story further up lol.

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1978 yamaha dt250 carb question 03 Sep 2018 02:58 #10

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I've just had a look on our 1977 Yamaha RD200DX and on that the hose marked 'B' on your first photograph does, indeed, link the carb to the oil pump.

I'd still try and get hold of a standard carb!

Did you get a conclusive answer on your 'mystery' box? From what I can see from your photograph, it looks like it is bolted to the battery box and, from my reading of the Electrical section of the parts manual, this is where the voltage regulator is sited. Also, I think that a regulator would only have 1 (yellow?) wire coming from it, whereas a CDI unit would have 2 bunches of wires with multiple connectors. I suppose that conclusive evidence would be if any data is printed/stamped on the item.

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