Problems with Offy 2x4?

[Snegrah]

My new fresh rebuilt 409 is on the dyno. It is a 340hp with a Crane cam, roller rockers, and a Offy 2x4 high rise. At 4k rpm it is having a problem with "reversion" from the carbs. Could this be a problem from the Offy intake. I know all this sounds vague but with my limited knowledge, this is all I picked up on my short visit. I guess what i want to know is, has there been problems with Offy dual quad high rise intakes????

Thanks,
Tom

[mpris]

Tom, there are an awful alot of those Offy intakes being used so I wouldn't think the intake would have a design problem. I assume you are talking about reversion on the intake portion of the combustion cycle. One of the things which can cause this is a mismatch of the intake ports of the head with the intake manifold. If the ports on the intake are larger than the head ports, this will cause a reflective wave back toward the carbs at the beginning of the intake cycle. (When the piston is still coming up). Are you certain reversion is what is causing your problem? Do you see a fuel cloud at the tops of the carbs or down in the carbs when this occurs? What is happening to the engine? Does it sputter and pop or just quit gaining rpm's? When I first put my engine together I had a problem at about 4200 rpm which I thought was my carbs. I am also using a Crane roller and have triple valve springs from Crane, however I was encountering valve float. I checked my spring tension and found I was about 15lbs short of tension on the seats. Corrected and fixed the problem.
Sorry for the long post but there is lot of things which can cause intake reversion. If you know for a fact it is reversion, try putting carb spacers on and see if it helps.
Keep us posted on what you find.

[Snegrah]

mpris:

Thanks for the response. There was a cloud above the carbs. I haven't been back to the shop since to find out if anything has changed. I will post back when I find out what caused it.

Tom

[Snegrah]

The "reversion" has been rectified! On the Dyno, the 409 was set up with a homemade set of Tri-Y's. :scratch Apparently, they affected the pulse of the exhaust sending air back up the exhaust through the carbs. :rolleyes: Built a new set of headers and no more reversion! :clap Now, we gotta' recalibrate the carbs to get more gas....waaayyy tooooo leeeeean. Next week!

Life is getting better! :D