Holley 3310 to 1850 parts swapping?

[pparaska]

O.k., so I've finally given in - the 750 vac secondary (list 3310) is probably too big to get a decent idle out of with my 9.7:1 compression and 274/280 (236/240@.050) .501"/.510" Comp solid lifter extreme cam.

 

I'm running a Holley 300-36 dual plane.

 

The idle is VERY rich (idle screws change the idle quality, so the plates aren't opened too much, plus I measured that), and it loads up at idle and needs to be held at 2500+ for 10 seconds or so to clear it out.

 

I've tried an 8.5 power valve (new) and put the primary jets to 70, added a secondary metering block and put in 74 jets there. But that stuff should have little effect on idle mixture.

 

The float levels are set to the bottom of the site plug holes. I have 5 psi of pressure at the firewall mounted regulator, according to a new VDO pressure gage.

 

So I'm thinking the throttle bores are just too big with the OE venturis to get a decent signal with a 1000 rpm idle @ 13 in/HG vacuum.

 

I have a 600 vac sec Holley ( list 1850 ) that I could put on, but it needs a rebuild. I'm thinking I'd love to swap the good parts from the 750 to it, if possible. I think that the secondary metering block, center hung float bowls, screws, fuel line, vac sec spring quick change cover, and electric choke mechanism MIGHT swap over.

 

Anyone have any ideas which of these parts I have on my 750 (3310) will go on my 600 (1850) ?

 

TIA,

[pparaska]

Thanks for that info Larry!

 

I posted this on Chevytalk.com in the Performance forum and someone replied that they'd done what you said might be possible - 600 body on a 750 baseplate. The guy said it worked well for him.

 

This is what I may try doing, but it seems that his suggestions for trying a few things (idle fuel restrictions, powervalve change, lower the float level) are warranted before I start machining carb parts!

 

My next choice after his tuning suggestions would be to move all the good parts from the 3310 to the 1850 baseplate/main body, since I have confirmation that all the parts should move over.

 

After that, I can see that maybe the 600 main body / 750 baseplate might be good for added signal at idle. Any idea where you read about that?

 

Thanks! Pete

[Mike C]

Pete, I'd try to turn your idle down to where it will just barely run, then take the carb off. Underneath on the secondaries is a screw to increase the amount of air flow through their blades. Open it just a tick, then put the carb back on. May have to do this a couple of times to get it right. Sounds like you have partially uncovered the transfer slots in trying to get enough air into the primary side for idle with your big cam. This is the reason you see drilled throttle blades in cars with mega cams sometimes. A switch to a carb with 4 corner idle circuits helps as well. The 1850 is just WAY too small IMO. If not, check out (I think) the new Pop Hot Rodding, where they used a piece of wire to reduce the effective area of the idle circuit in the metering block. My brother's STOCK '76 Camaro 350 in his '68 Camaro idles just fine with a 3310.

[Peternell]

The parts you've mentioned will swap from the 1850 to 3310 & visa versa. The secondary spring may not be right because of the increased vacuum signal the 600 w/the smaller venturi's will provide.

 

I've read (have yet to try it) that a trick was to take a base plate of a large carb and put it on a smaller carb (ie 600 body on 750 baseplate)

This was done with 660 center squirter bodies onto 850 center squirter bases. Helps to increase the vacuum signal or increase the air flow while maintaining acceptable vacuum.