[69-CHVL]

Ok, got my 770 setup pretty close where it needs to be - 72/84 (stock is 72/75). According to my AFR meter, I can still go down on my primary jetting - perhaps alot. This engine seems to like lean idle/cruise mixtures, but much fatter on the top-end. I was just reviewing the data from my last 2 outings at the track, and car is still lean (slightly) but otherwise runs superb.

Question is, by going down on the primaries, I would imagine I would have to compensate by increasing the secondaries by a like amount, correct?
If thats the case, with careful PV tuning I think I can get my primaries down to 68 or 70 (maybe lower who knows) but I would have to get the secondaries to 86 or 88. According to some, this is too much jet for a 770 carb in the back (I'd be 11 to 13 sizes bigger than stock back there).

What do you guys think?

 

[greasefire]

I have had good luck with disconnecting (for tuning primaries) the secondarys while jetting for the primaries for cruising, then using the wideband and gradually drill out the Power Valve Channel Resrictions (PVCR) until you get the wide open throttle primary mixture you desire. Then reconnect the secondarys and jet the secondaries for WOT with the wide band.

This will get you in the ball park (for WOT) until you get to the track again and can see what the engine likes as far as mixture goes for peak power by you MPH. Doing it this way allows you to keep a lean cruise while still having even fuel distribution at WOT.

be careful when drilling the PVCR because alittle goes along way. btw use numbered drill (and a pin vise) bits not fractional.

Also remeber at light throttle the transfer and idle circuit provide a significant amount of fuel for your mixture, so if your jetting the primary down and not seeing a difference (with the wideband) at light throttle this it why.

This mixture is changed with the idle feed restrictions (and of course your idle mixture screws), dont get fooled by the name it provides fuel for the transfer circuit as well. Too lean drill out, too rich you can use small strands of wire to restrict the oriface.

 

[GRN69CHV]

I think you should raise the PV to about a 8.5 first before I did any drilling.. It probably comes with 6.5, you may be into the PV more than you think.

PS - I think that's how you do it, but then your motor has real high vacuum, maybe you need to go higher . What are you pulling 18" at idle right? So at low speed cruise, you may have 18 - 20" vac at part throttle? Isn't it 1/2 of cruise vacuum?

Vince, I edited this response. Confused at to whether to lower to 4.5 or raise to 8.5. At any rate I would try that first.

 

[69-CHVL]

I dont plan on drilling anything - I'm not qualified for that sort of thing. What I meant by PV tuning was more or less just picking what # to use. Whats nice about these Street Avengers is that the idle/cruise AFRs seem to be in the 13 to 14 range, but then when I'm on the throttle moderately, the afr dips down and stays around 12.5 till I back off and cruise. I'd like to get that # up somehow.

 

[69-CHVL]

Good point Joe,I have a 4.5 I can try.These HO motors make so much vacuum though. Had the vac gauge taped to my window, I was making 15-16" at idle, 10" during moderate/heavy throttle. I'd have to be pretty into it to be down around 6.5, but it is possible. The 850's for the ZZ502 come w/4.5 front, 3.5 rear so there must be something to it.