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Zylinder Head question

  • Wessi
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Zylinder Head question was created by Wessi

Hi all, could you please have a look at attached pictures. My 1975 DT250 still has loud "klong" noises at warm motor when running constant speed. Maybe the PO did a tuning and removed some mm from the head for more compression?
My head is looking like pic No1, but lots of others i found on Fleabay, all with the Fins cut for Muffler on the right side, have a step at the bore, where the gasket sits. Like in pictures 2-4. Or are those heads from a DT400 with bigger bore?
If this step normal, how deep is it?
Is there any mark on a zylinder head to identify it´s a 498-11111-01 , which i need?
I have this shitty "Klong" now for more than one year. Meanwhile changed Zylinder, Piston, Crankshaft and switched to VAPE ignition, but this noise still is there. Ignition is 3.2mm advanced, so should be ok..Carb was several times cleaned and all parts are new orig Mikuni. Zylinder head now is the only thing whats left to be suspicious..
Please help


  
 
Last edit: 27 Feb 2021 10:11 by Enduronut.
25 Feb 2021 04:30 #1

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Replied by RT325 on topic Zylinder Head question

Welcome from NZ downunder. Maybe CC the head to compare to original specs. Pic 2 looks a bit like piston has been touching but not sure if that's your head or if someone has marked it with something scraping it. Sounds like you know your motor very well though so not something like a baffle plate popped the spot welds in the exhaust main chamber. I'm sort of thinking if its been doing it before & after rebuilding then she's a good old motor. Had one myself which i think was "77 monoshock but much the same & gave it heaps in the forest in enduro's & never faltered.
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25 Feb 2021 04:50 #2

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Replied by MarkT on topic Zylinder Head question

All the stock heads I remember have a "step".

Found this picture of a 75 250 head.

 



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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
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25 Feb 2021 06:52 #3

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Replied by Wessi on topic Zylinder Head question

Dear both, thank you very much for you help.
Today, i pulled my Zylinder Head and found , as i had in memory, there is no step in the dome bore. See picture
  
Taking a beer and thinking about that, then again a beer, i could imagine the following: in my country in the 70ies, a 17 HP bikes insurance was much cheaper, than > 17 HP. In the title of my Yamaha, there is added the sentence: "Vehicle is equipped with 2 Cylinderhead gaskets to reduce to 17HP". This as a result of lower compression. Having this title, the insurance is about a third in price

Just guessing the PO wants to fool the Police in case of being controled. He removed about 1mm Material from the head to equalize the additional gasket to get full HP even so having 2 gaskets.
Now i came along and drive ths head with only one gasket and have much too high conpression....
Could it fit to the symptoms?
But how o proove?
Isn´t there anybody in this Forum with a  spare Zylinder head in his Garage, who could measure the step-bore ? Please, could you measure depth and diameter of the step and mybe take a good photo?
My stamping on my Zylider head is 113. Does this have any meaning related to the parts number?
Any help is appreciated.
Cheers, Guido
Last edit: 27 Feb 2021 10:14 by Enduronut.
27 Feb 2021 09:55 #4

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Replied by RT325 on topic Zylinder Head question

Wondering if it's an MX250A head modified as well as having two gaskets. I say that because i think the combustion area in the middle looks shallower than a the trail bike head but i guess it depends how much has been removed from the head although if the squish band hasn't been machined then the center dome won't be changed so could compare easily with the two models DT & MX.
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27 Feb 2021 14:31 #5

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Replied by Wessi on topic Zylinder Head question

Case closed, i purchased a used Zylinder Head and now can answer my own questions.
Yes, the PO machined down about 1mm from the head, but no machining marks visible
Yes, originally the heads have a step of about 1mm. Gasket thikness is 1mm and my compression was too high with only one gasket.
Yes, original head is stamped 113 .
And yes, i thought about strangle PO because it took me over one year to figure out, why bike doesn´t run  smooth..
Did a small ride yesterday, seems to be fine at least at cold motor.
 
 
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03 Mar 2021 23:21 #6

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Replied by RT325 on topic Zylinder Head question

Could add a second base gasket to lower the com a bit more or cut a double thickness one out of proper gasket material if available.I had two rolls of 'i think' 0.8 & 1.6mm, actually that doesn't sound enough but whatever the thickest was i used it on my YZ490 as used to suck in base gaskets so i made a double thick one & left it slightly overhanging so it bit in & left the bit sticking out uncompressed. Whatever, it worked haha. Slight port height difference did nothing & had more power than i could handle.
04 Mar 2021 02:20 #7

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