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Author Topic:   ? for 4412 Carb Experts
Dirt Freak

Total posts: 279
posted April 07, 2005 05:56 PM  
OK, I'm getting kinda frustrated with this new 4412 2bbl carb. I've done my best to tune this carb and it hasn't worked as good as my beat up old one. The new one seems to have a stumble when I get on the throttle on exit. For this, I'm installing the 50cc pump, arm and cam from the old carb. Plus, I think I'll step up the sqirters from the stock 28 to a 35. I'm hoping that this will fill the hole that I think exists there.

I have another doubt, however. I put a vacuum gage on the intake manifold right behind the carb and I found that the car has 6" of vacuum at idle, so I put in a new 4.5 powervalve. The spark plugs and water temp have dictated the use of main jets between 83 and 86 which seems to be higher than most folks' and certainly higher than the stock 73s. I was thinking that if I carefully enlarged the powervalve channel restrictions (PVCR) about .010" (I haven't checked what size they are now) that I would be able to run a smaller main jet. Then I ran into the question I have: If the vacuum on a 2bbl engine acually rises when you step on it (I've been told), then it won't fall below the 4.5" setting to activate fuel enrichment from the powervalve, would it? This tells me that the powervalve circuit doesn't really come into play with a 2bbl and thus it's all up to the main jets while racing. I bet I'm wrong, but why? When I hit the throttle in the parked car, I saw the vacuum go up, not drop out. I haven't seen the gage while on the track with a load on the engine. Is this the difference? I do run an automatic with a low stall converter.

Help out dudes.

El Santo
Dirt Full Roller

Total posts: 36
posted April 14, 2005 12:52 AM  
Under a load on the track the vacuum will drop to 0 at WOT. However in some high rpm 2 barrel applications I have seen the vacuum creep back up at high rpm (7500 rpm+). As far as your stumbling goes it just takes time and a little experimenting. Knowing your engine setup will probably help in knowing which direction to send you on your carb dilemma.

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