Dist/carb tuning question

I'm slowly getting my engine dialed in and I'd like to bounce this off of the pros. My DUI dist instructions say to set my initial timing at 12 BTDC and go as far as 14 depending on emission requirements and whether or not a ping is heard. I think the emissions thing is only for CA and I haven't heard a ping at either setting. The difference is that at 12 degrees it stumbles around 3700 rpm's and at 14 it runs on up to 4000. I realize my prop may be a bit oversized and that I probably need to raise the secondary floats a little more...but why the difference based on timing?

I put the timing back to 12 because it popped through the carb once at 4000 with the timing at 14.

The jets are 76 and the PV is 7.5

13-14" of vacuum at 700 rpm's

Thanks guys.

Greg, thats strange. Are you sure youre timing the boat properly? I know next to nothing about the BBC's, but from what I know about the SBF's here are my comments:

- Stock (initial) timing in the 6-14 deg range should give you reliable performance, though the higher the better (without pinging). I only got some popping and hesitation when my 302 was set to 0 deg.

- Are you only getting 4k RPM at WOT? That seems awfully low. What prop are you running? What did you spin for RPM before the rebuild?

- Ive never seen vaccuum secondaries open that early. All of my boats open around 3500-4000 RPM, regardless of load.

All timing is BTDC. Initial will be in the 6-14 deg BTDC, final will advance somewhere on the order of 24 deg to a final of 30+ deg BTDC.

Timing wont have an affect on when the secondaries come in- thats based on vaccuum. For whatever reason, I never see the secondaries come in until 3500+ RPM, regardless of load.

Thanks Eddie and Tim. To clarify, that's when I saw them start to open.

I don't think the carb is undersized...it's a 750 DP. I think I'm going to hook the old 600 edelbrock up just to see how it acts. I haven't been back through my bowtie thread, but I'm pretty sure I remember Chris(79) saying I was going to have probs with my holley 750 because the floats are center hung rather than side hung.

As it is, the 750 runs beautifully until it gets around 3600 rpm's, but I do think it's burning too much gas. Maybe the early secondaries are flooding it?...but it acts like it's starving for fuel on top.

I used to have a TV that required a smack on the side...guess that won't work on this?!!?   LOL

Greg, how many hours and what type of rings did you use? it takes a while for mollys to break in and as they break in will create more vacuum at the lower end, i dont think the 750 is to big, if the rings have'nt quite seated completely you will experience some of the problems your having such as running rich and possibly throwing off when the secondaries are opening,
are you getting any blow by out of the valve covers? if so that is a sure sign that the rings are not quite broke in completely, your better off running rich right now than lean untill you get the problem figured out,
the vacuum readings you are seeing is what is throwing me off, i think you should be seeing at touch more vacuum at idle unless your running a high cam, but i believe you said you were going to run the stock broomstick

BuffaloBFN wrote: I don't think the carb is undersized...it's a 750 DP.

Double pumper = mechanical secondaries? Or do you have vaccuum secondaries?

What is your advance curve like. It sounds like it is not getting enough advance as you go higher in the rev range this will create a rich condition as you wont be getting spark when it is required resulting in unburnt fuel.

First check that the TDC mark on the balancer is actually tdc. If you have a graduated balancer you can check the advance curve at different rev ranges. If not just add a couple of more notches on your balancer at say 25, 30 and 35. Not sure of the total advance a BBC can handle but the SBC was around 38-40 degrees before detonation. I used to set my warm 350 chev at 36 degrees at 4200rpm.

Possibly a simple way of proving this is adding more static advance in 2 degree increments and then running the boat at the higher rpms. I used to run 18 degress static advance on my SBC.
A 750cfm vac secondaries carby should be fine a 454 cu in motor 600cfm would be a bit small at higher rpms but would give good throttle response out of the hole and good economy.

Hope this helps might be worth a try before swapping carby's.

lewy2001 wrote: What is your advance curve like. It sounds like it is not getting enough advance as you go higher in the rev range this will create a rich condition as you wont be getting spark when it is required resulting in unburnt fuel.

Thanks Lewy. I talked to a friend last night who agrees with you on the advance. He also thinks the PV may be too large.

I still have the mallory dist that was in it, maybe I'll switch that back?

eric lavine wrote: the engine is in the recovery room after the operation and it takes some time to seat the molly's

And the surgeon was a butcher?!!?   LOL

I found my bonehead mistake. I've been running it 'cold' because I forgot to run the dedicated 12 gauge wire to the dist. There may still be more to it, but I'll fix that first.

running larger wire to the dist won't change anything. might give little more spark energy but nothing to the advance

BuffaloBFN wrote: I'm sure I could come up with a list of excuses, and I read the instruction sheet several times; but the fact is I forgot to do it.

if it didn't have 12v all ready at the dist then it wouldn't run at all your starting to chase your tail and it's not looking pretty.

how are you stepping up the voltage your not going to get 14.5 volts out of a 12 volt battery and your just going to burn up the electronics if you do anyway, another great bit of advice along with the 24 degrees of total advance. The more you guys talk about those DUI units and what they are telling you to do just shows me they are full of it.

pv 6.5-5.5 but you need to get that rich carb fixed before you burn off all of the honing marks in the cylinders leaving power on the table big time.

BuffaloBFN wrote: DUI calibrated the dist to have 24 degrees @ 3000,

this is your biggest issue not enough timing you should have at least 30 degrees at this rpm 24 is what you should have just after 2K oh well the DUI guys know how to get people to buy into BS if nothing else. Easy to sell parts to the unknowing uninformed folks out there especially with all of the free PR they get here.

79nautique wrote: BuffaloBFN wrote: It's a 12 gauge wire from the ignition post to the dist. from what ign post alternator? ign switch? did you keep the ballast resistor in the loop or something on the orginal wire? Sounds a little squirlly to me.

It runs from the ignition swith straight to the dist. The ballast resistor is bypassed.

79nautique wrote: BuffaloBFN wrote: DUI calibrated the dist to have 24 degrees @ 3000, this is your biggest issue not enough timing you should have at least 30 degrees at this rpm 24 is what you should have just after 2K oh well the DUI guys know how to get people to buy into BS if nothing else. Easy to sell parts to the unknowing uninformed folks out there especially with all of the free PR they get here.

Chris, he means 24 degrees of advance is built into the dizzy. You have to add the base timing to that 24 degrees to get the total advance. No idea if thats a good set up for a BBC, but 24 degrees of advance works great on my SBF.

Greg, your old carb was an edelbrock, correct? It would be interesting to know what the stock jet sizes would have been with a Holley. Its just a WAG, but Id say 76 on the primaries would be quite rich. Maybe try dropping it a few (maybe a 74 or 72), run it briefly to see if performance improves and do a plug chop.