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Question 1977 DT400 Fuel Delivery Issue

  • Bsimpson
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1977 DT400 Fuel Delivery Issue was created by Bsimpson

I've posted once on here about an electrical problem, but I seem to have a bigger problem now that I need help with before I can fix lights.

My 77 DT400D has a Mikuni VM34 carb in it, and the carb runs just fine through all throttle openings, idles, everything it should.

I started to notice a problem where fuel would not flow from the petcock into the carburetor on reserve or on. There is fuel in the gas tank, the fuel line is unobstructed, and the petcock does not leak.

When you disconnect the fuel line from the petcock, it functions as it should. Also, when you disconnect the fuel line from the carb inlet, fuel flows through the line normally. I have disassembled the carb, cleaned it, verified the float functioned normally, and the float height is fine. Removing the gas cap makes no difference. It is not a venting problem with the tank. I am at a loss because it is like some wizardry mess where fuel will not flow into the carb but will flow AS SOON AS I remove the line from the top of the carb. It wont even leak out of the petcock into the line, I didn't know that a float problem could cause that but as I said the float seems fine and is at the correct height.
1977 DT400D Brick Red
1978 DT125E Leaves Green
04 Dec 2021 17:49 #1

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Replied by MarkT on topic 1977 DT400 Fuel Delivery Issue

Check the float bowl vent lines.  A friend once had the same problem.  Drove us crazy...  finally noticed he had pinched the vent tubes off.by stuffing them between the engine and frame mount under the bike. 
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
The following user(s) Liked this Post: Ht1kid
04 Dec 2021 19:52 #2

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Replied by Lizeec on topic 1977 DT400 Fuel Delivery Issue

Float needle sticking? Doesn’t make since that you say it runs and operates as it should, obviously fuel is getting into the float chamber for it to run as you say properly?
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04 Dec 2021 20:18 #3

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Replied by Ht1kid on topic 1977 DT400 Fuel Delivery Issue

I agree with MarkT on the vent it’s just like your house plumbing must be vented
Last edit: 04 Dec 2021 20:50 by Ht1kid.
04 Dec 2021 20:48 #4

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Replied by MarkT on topic 1977 DT400 Fuel Delivery Issue

Lizeec made some very good points!  Does the bike run fine still?

If it does, let me guess...  you have a clear fuel line?

Cure is simple, get rid of the clear fuel line!  You'll never fix it any other way.  It's an optical illusion.  Fuel is flowing just fine.  
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
04 Dec 2021 21:18 #5

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Replied by RT325 on topic 1977 DT400 Fuel Delivery Issue

My guess. Sounds like you're using a clear fuel line so its probably operating normally with air in the line & a full bowl so cant go anywhere. Use a black line & stop worrying about it. Haha, now i read the post above so there ya go, we both think alike. Stop worrying.
04 Dec 2021 23:34 #6

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Replied by Bsimpson on topic 1977 DT400 Fuel Delivery Issue

Actually no it does not run right now. It did run fine, now it is starved of gas. There is no gas in the line and it will not crank at all. I promise I would not be that dumb. Oh and also there is no vent tubing on this carb for some reason. Just a bunch of connectors, and all of them seem clear when I spray carb clean through them. But no I should have been more clear the bike ran fine now it will not crank.
1977 DT400D Brick Red
1978 DT125E Leaves Green
05 Dec 2021 10:17 #7

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Replied by MarkT on topic 1977 DT400 Fuel Delivery Issue

It wouldn't be "Dumb".  A lot of people have been concerned about clear fuel line looking like it had a problem...  messed with me the first bike I had with clear line. 

So how do you know fuel isn't flowing into the carb?  Or that it's not full already? 

Maybe spark plug went bad or something else?
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
05 Dec 2021 10:30 #8

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Replied by Bsimpson on topic 1977 DT400 Fuel Delivery Issue

The carburetor was completely dry when I removed it to clean it. When I put it back completely empty no fuel flowed into it, and the bike wouldn't start. That is how I know it is empty. And the bike has spark and the plug looks fine. I know it is not a spark problem I can tell no fuel is flowing into the carb.
1977 DT400D Brick Red
1978 DT125E Leaves Green
05 Dec 2021 10:42 #9

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Replied by Sneezles61 on topic 1977 DT400 Fuel Delivery Issue

Open the bowl drain plug.. apply a modest amount of pressure to the fuel tank.. Watch the fuel lines... expanding.. then I'd suspect they are collapsing when the carb is "sucking" gas from the tank... A rubber 2" plug a plumber uses to cap a vent line with a small hole for the blow gun from the compressor..
Sneezles61
05 Dec 2021 11:50 #10

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