(adsbygoogle = window.adsbygoogle || []).push({}); Hi here is a tutorial on how to do exactly what it say's in the title :black:

You will need the following:
An iPod Touch or iPhone 3G with iOS 4.1 already installed and shsh blobs are not required for this(I’m not sure if this works on 3GS)
download iOS 4.0 and save it as a ZIP file.
The latest version of iTunes
A plist editor. I like to use one called iPodRobot. You can download it here: You must login or register to view this content. download the freeware version.

If you saved your ipsw file as a zip file, just simply open it. If you haven’t, don’t worry. Just open any explorer window, click on ‘Tools’ on the toolbar, and click ‘Folder options’. Click the tab that says ‘View’ and look for ‘Hide extensions for known file types’ and uncheck that. Click ‘OK’ and go to the location where you saved your ipsw file. Change the “.ipsw” extension to “.zip” So “iPod2,1_4.0_8A293_Restore.ipsw” should become “iPod2,1_4.0_8A293_Restore.zip” (Note: the iPod 2G uses the ipsw labeled as “iPod2,1″ so download the ipsw that applies to your iPod model.)
Install iPodRobot plist editor if you haven’t already. If you installed it already, then just go to step 3.
Open “iPod2,1_4.0_8A293_Restore.zip” and copy “BuildManifest.plist” into your desktop and open it using the plist editor. Look for 8A293 entries, and replace it with “8B117″ no quotations. Save it, and open that same zip folder again, and delete the old “BuildManifest.plist” Don’t worry if explorer freezes. Its gonna take a while for it to get deleted. When its done, just simply copy the “BuildManifest.plist” that you edited.
Wait for it to get copied.
Now that its done, copy the “Restore.plist” into your desktop and open it in the plist editor. Look for “8A293″ and replace it with “8B117″ and save it then delete the old “Restore.plist” in the zip file, and when its done, simply the copy the new “Restore.plist”.
Now change the zip folder’s extension from “.zip” to “.ipsw”, so “iPod2,1_4.0_8A293_Restore.zip” should now be “iPod2,1_4.0_8A293_Restore.ipsw”.
To hide your file extensions, just go back to “Folder options”, look for “View” and check “Hide extensions for known file types” and click OK.
I recommend editing the hosts file just in case. To do this, just simply go to “C:\WINDOWS\system32\drivers\etc” and open “hosts” file using notepad. Not the “host.ics” file.
add “74.208.10.249 gs.apple.com” without quotes, and save it. (Saving it works if you are the administrator of the computer.)
Open iTunes, and set your iPod into DFU mode. Just press the home and sleep/wake buttons for 10 seconds, and iTunes should tell you that your iPod is in recovery mode.
Now, shift+click the restore button. (Hold the shift key and press the restore button in iTunes at the same time.)
Now a dialog window will show up asking you where is the ipsw file you want to use to restore your iPod. Choose the one that you downloaded and modded.
Now iTunes will downgrade your iPod to iOS 4.0 WITHOUT using shsh blobs.
During the downgrade process, you will notice that the screen on your iPod has turned white. Once again, this is normal.
Please be patient, this will take a while.
If everything goes alright, you shouldn’t see any errors pop up in iTunes.
When its done restoring, you will notice that in the iPod version info on iTunes now says “4.0″ if it says that, then congratulations! You have successfully downgraded your iPod!!!
Ok, now here comes the easy part. (You need a wifi connection to do this!) in your iPod, go to Safari, and in the address bar, type in “[url]https://www.jailbreakme.com/“[/url]
When it loads, you will see a slider on the screen that says “Slide to jailbreak”. So do what it says, slide the slider to jailbreak it. It will now download the files to jailbreak it, including Cydia.
When its done, it will reboot your iPod. (Don’t worry, its untethered!!!)
Now Cydia is installed!!!

Hope this helped

hey bbuddy not really checked this post but new tutorial up
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I'll try this, but it really shouldn't worked, I did try it before (maybe I screwed up) but it needs the blobs for it to be 100% sure to downgrade.

hey buddy this is old now, I've put a new one up follow link in post above yours.

It dosent matter just use tinyumbrella and load ssh's it worked for me and I never had saved ssh blobs

Can anyone confirm if this works for iphone 3gs?