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Dt400 pinging and slap

  • Finnocitta
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Dt400 pinging and slap was created by Finnocitta

I got a 78 dt400 that has been a whole lot of headaches since I picked it up. It’s been mostly gutless with lots of pinging when the rpms are high. I just had the cylinder rebored and the head refinished and fit with a new piston and rings by a qualified tech. It’s on its third overbore.

I got it out for a ride today and it seems to just fall apart when I get the rpms over 4500. Lots of subtle pinging and even power that turn into loud metallic stumbles that lose power for quick moments.  

Also, coming out of turns and pushing hard on the throttle it almost sounds like loud cracking coming from the engine, kind of like piston slap.

There isn’t any play in the crank, I’m running the highest octane pump gas I can find, I changed the left crank seal, and the timing is retarded just a touch, I changed the CDI for the box from Rex’s speed shop, I also think I’m running a bit rich to try to avoid the pinging.

Is this normal for this bike? Is it outside of the realm of possibility for this bike to be ridden kind of hard and hold it together?

thanks!
01 Jun 2021 21:08 #1

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Replied by MarkT on topic Dt400 pinging and slap

Have you ever marked the flywheel and confirmed the timing?  I've seen more than one that's WAY off when you check with a timing light.

One member here had to have a shop re-drill the stator plate about 30 degrees retarded to get the timing right.
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
01 Jun 2021 21:25 #2

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  • Thebassguitarist
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Replied by Thebassguitarist on topic Dt400 pinging and slap

I second the timing idea. 

Another thing to check would be for air leaks, I've experienced similar symptoms when the bike was running very hot and very lean with a leaking base gasket. 

Report back with what you find, I still deal with small similar symptoms with my 400. 
 
01 Jun 2021 21:44 #3

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  • RT325
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Replied by RT325 on topic Dt400 pinging and slap

My first port of call for lack of performance is exhaust tail section. Also crank flywheel key comes equal first depending on a few things. If its a rex's system i don't know if they run a key or just time it, do it up Tight, & hope it holds. Blocked up exhausts can make horrible noises within the chamber from trying to get out but cant. Try it minus the complete tail pipe muffler & don't get arrested doing it, 'noise'.
01 Jun 2021 22:42 #4

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  • Finnocitta
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Replied by Finnocitta on topic Dt400 pinging and slap

That blows my mind! If the timing was way out, wouldn’t the rest of the rpm range also be a bit off? I know the CDI advances as it turns faster, but is it progressive as it speeds up or does it jump ahead when it gets to a certain speed?

I’ve noticed a good amount of carbon build up in the pipe, I’ll also try removing it and see if that makes any difference. Throw it in a fire and cook it for a bit. Has anyone had any success with an FMF silencer on this thing? I have one laying around.

I definitely had an air leak before that I was able to fix with new gaskets, new crank seal, and cleaning up the intake manifold. I’ll double check with the tech to make sure he did a leak down test. 


Thanks for all your feedback!
Last edit: 02 Jun 2021 05:59 by Finnocitta.
02 Jun 2021 05:55 #5

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Replied by MarkT on topic Dt400 pinging and slap

CDI retards for starting but once running should be pretty much fixed timing at the spec in the manual.  I've seen all sorts of strange behavior.
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
02 Jun 2021 06:20 #6

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  • Finnocitta
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Replied by Finnocitta on topic Dt400 pinging and slap

Heroic update!

I cooked the pipe until is just about stopped smoking and then ran the bike without the silencer and it was absolutely wild. Lots of power and when it gets above 4500 just comes alive. I’m at 9500 ft of elevation and it was breathing really clean, lots of power up top and super fast response on the throttle. Even got it up to 85 which wasn’t even possible before.  

so I’d love to run it like this all the time but I’m sure I’ll be getting some attention from the law if I get down into town.. Any tips on cleaning the silencer / installing an FMF turbine core ?
04 Jun 2021 12:37 #7

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  • RT325
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Replied by RT325 on topic Dt400 pinging and slap

Cooking the pipe would have helped for sure but i'm guessing the main problem still lies in the tail section so pull the baffle out of the end & burn out 'that' muffler. Has a spark arrestor if you look way down in the end you'll see a twirly shaped thingy which blocks up. So burn it out & if fully clean you'll have your performance & quietness as well.
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04 Jun 2021 16:43 #8

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Replied by MarkT on topic Dt400 pinging and slap

Congrats!

Some have used the FMF...  you need to make the connector pipe though. 

Original mufflers are hard to find and can be expensive so you might think twice if thinking about cutting it up.  Aftermarket muffler will often hurt value a lot if you ever decide to sell 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
04 Jun 2021 17:23 #9

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  • Finnocitta
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Replied by Finnocitta on topic Dt400 pinging and slap

I hear that! I’m intent on keeping all stock bits I have for this bike and don’t intend on cutting up the original silencer. If I can get that silencer clean and still maintain performance, I’ll for sure keep it on. I love the way the stock set up looks and I’m grateful to have a totally stock set up.

I’ll report back after cleaning that silencer. Anything I need to be aware of when trying to clean it?
04 Jun 2021 21:23 #10

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