3.9 won't revv over 4500 rpm

Alley Kat · Post by **Alley Kat** » Sun Aug 12, 2007 8:46 pm

I thought the 500 was a combo of mechanical, to start the secondaries opening, and then vacuum takes over.

Vulture · Post by **Vulture** » Sun Aug 12, 2007 8:57 pm

I'm pretty sure it's not lack of travel, as there was no difference between when I had someone push the accelerator for me and me activating the thrrottle at the side of the carb. Anyway, I'll dismount it tomorrow and have a closer look.

mgbv8 · Post by **mgbv8** » Sun Aug 12, 2007 8:59 pm

Not sure of the variances on the 500 Kat, but mine will open the secondaries mechanically. Full pedal is fully open on mine.

Alley Kat · Post by **Alley Kat** » Sun Aug 12, 2007 9:18 pm

Not got mine any more, just had it in my head that it was mech at first, up to a point, then vacuum took over. Dunno. Quick google only turned up a basic "both mech and vacuum" statement on here with no detail http://www.v8engines.com/carbs-1.htm
Maybe different variants like you say, or I'm on the wrong drugs ...

Vulture · Post by **Vulture** » Sun Aug 12, 2007 9:25 pm

All the owner's manual says on that subject is:

The Secondary Throttles (1) begin to open when the Primaries are about 65% open. The Primary and Secondary Throttles arrive at the WOT stop at the same time.

There's no mention on how they open, though, or I've overlooked it.

Post by **katanaman** » Sun Aug 12, 2007 9:45 pm

They are mechanical any reference to vac operated is the flap inside which is a kinda counterbalanced flap that open with the amount of airflow.

Depending on the manifold used the throttle linkage can hit the manifold stopping it from opening fully. I know the JWR duel port is effected by this but others might be as well. check that this isn't the case and if it is then either sort it or bend it out the way. If my memory is right its the end with the big hole you would normally connect an auto kick down to that hits and you can simply bend it out the way.

Eliot · Post by **Eliot** » Mon Aug 13, 2007 7:06 am

You need to study the link between the throttle shafts. If (as you state) they primaries open fully, then the issue is with the link between the two shafts.
All references to vacuum secondaries is talking about the metal trap door part ABOVE the actual throttle plates.

I have a 600 (same as a 500) on the shelf for photo's if it helps

Vulture · Post by **Vulture** » Mon Aug 13, 2007 11:46 am

Yes, some pictures would be nice to see what it should look like, so I can compare with what it actually does look like.

Eliot · Post by **Eliot** » Mon Aug 13, 2007 12:22 pm

Here's a clip of it - (warning, its 3.5Mb)
http://homepages.nildram.co.uk/~eliotmez/linkage.avi

I've got some stills, but the video shows what should be happening - i suspect the link bar is missing or bent.

mgbv8 · Post by **mgbv8** » Mon Aug 13, 2007 6:10 pm

Yep. My 500 does exactly that too. Good vid.

Eliot · Post by **Eliot** » Mon Aug 13, 2007 6:24 pm

First time I've looked at it in three years - arr the wonders of injection...

Vulture · Post by **Vulture** » Mon Aug 13, 2007 9:54 pm

Thanks for the vid, Eliot. That's what my carb does now, too!

The problem was simple: The secondary butterflies hit the manifold, thus they were stuck at only a fraction of opening. We remedied it very easily by drilling the holes that hold the carb in place larger by a millimeter. That way, we could mount the carb a tiny bit more to the front, enabling the butterflies to operate freely and smoothly!

Thanks @ all for helping!