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Engine rebuild - bent rods

my03

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Hi guys,

I'm in the process of getting my engine rebuilt for the dohc cb750

It already had weisco 823 pistons in there, and the mechanic that was doing the rebuild has discovered three bent rods.

he said the engine may have been hydrolocked or something due to bike sitting with fuel tap on, and fuel building up in the carby, when the ignition is fired, the rods bent.

Is this a common problem? how to I avoid it? he said that a good thing to do is to turn the fuel tap off and run the fuel out of the carbies to turn the bike off. CAn anyone shed some light on this?

Also, I not have a set of bent rods and a high compression engine, he said the compression of the engine at 11:1 or more with those pistons. he recomended high octane fuel with octane booster for this compression. Honestlyu all I wanted was a reliable engine for the street. He said putting a cb900 head on there would lower the compression. is this really necessary to lower the compression? or will my bike be fine for street with the wisco kit and stock 750 head?

Also, now that I have bent rods, should i get replacement stock 750 rods or should I get forged items? anyone have expereince with the cheap ebay ones? or carillo the only way to go? is it necesary to go forged for a street motor?

Thanks guys. any help is greatly appreciated
 
There is no way the cylinders should fill with fuel...the carbs have an overflow to prevent this, unless they are plug somehow. And no its not common. The dohc rods are actually very strong and some use them in the sohc motor while building stroker motors. The only way you can bend the rods is if the motor was running while it got liquid in the motor. Just cranking the motor shouldn't have enough force to bend the rods.

You need to run at least 91 octane with that compression....not sure how your fuel is there. I run 12:1 motors on 91 octane here with no issues.
 
Thanks for that, do you have a weico kit on your 750? What is your opinion on if 12:1 compression suitable for the street? would it be necesary to to use a 900 head to lower the compression? I have the cr29's on the bike, i suppose these carbs would have a overflow too?

We can get 95 or 98 octane fuel here in australia. Thanks
 
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