Dist/carb tuning question |
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BuffaloBFN
Grand Poobah Joined: June-24-2007 Location: Gainesville,GA Status: Offline Points: 6094 |
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Posted: July-14-2008 at 7:22pm |
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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. |
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BuffaloBFN
Grand Poobah Joined: June-24-2007 Location: Gainesville,GA Status: Offline Points: 6094 |
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I meant to include this too; the secondaries start to open around 2100-2200 under hard acceleration and about 2800 on average acceleration.
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TRBenj
Grand Poobah Joined: June-29-2005 Location: NWCT Status: Offline Points: 21169 |
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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. |
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BuffaloBFN
Grand Poobah Joined: June-24-2007 Location: Gainesville,GA Status: Offline Points: 6094 |
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Thanks for the response Tim. When you say 'higher', do you mean a larger # before TDC or the opposite? Does advancing the timing mean farther or closer to TDC? I thought I understood this, but I feel turned around?!!?
I'm baffled at the moment but do expect to get a book tomorrow. The throttle has the travel to go where it should, but it stumbles. How much does timing affect when the secondaries open if at all? |
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TRBenj
Grand Poobah Joined: June-29-2005 Location: NWCT Status: Offline Points: 21169 |
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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. |
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backfoot100
Platinum Member Joined: January-03-2007 Location: United States Status: Offline Points: 1897 |
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I gotta agree with Tim. The secondaries should open a lot higher than that. It's based soley on the vacuum from the engine. Almost sounds like the carb is way undersized if they're opening that soon. Another thing you can do is get different size springs to put in the vacuum diaphram to adjust when the secondaries come in at. I've just never heard of the secondaries coming in that soon.
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When people run down to the lake to see what's making that noise, you've succeeded.
Eddie |
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BuffaloBFN
Grand Poobah Joined: June-24-2007 Location: Gainesville,GA Status: Offline Points: 6094 |
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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 |
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BuffaloBFN
Grand Poobah Joined: June-24-2007 Location: Gainesville,GA Status: Offline Points: 6094 |
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It's definately running rich. I pulled the plugs yesterday and both electrodes are clean and all of the plugs are burning evenly, but the surrounding area is sooty. I haven't tried the 600 yet...kinda hard to kick a new carb to the curb.
I have another small issue as well...I'll have to spend a few minutes cleaning up the jawbones that are scattered all over the boat?!!? LOL I pulled a few of my new neighbors last night. They're mostly new to wakeboarding and are used to a 6 cylinder I/O. A few had never been behind an inboard and none had ever been behind a BFN...maybe it was the noise they weren't used to?!!? |
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eric lavine
Grand Poobah Joined: August-13-2006 Location: United States Status: Offline Points: 13413 |
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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 |
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BuffaloBFN
Grand Poobah Joined: June-24-2007 Location: Gainesville,GA Status: Offline Points: 6094 |
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I have about 7 hours on it now. They are moly rings from hastings, and I'll check for blow by. I do have the stock broomstick...that was a $2500 decision. The only mods were the performer intake, carb, dist., and it's .030 over. DUI calibrated the dist to have 24 degrees @ 3000, so I'll check that today.
Does the taller intake affect vacuum? |
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TRBenj
Grand Poobah Joined: June-29-2005 Location: NWCT Status: Offline Points: 21169 |
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Double pumper = mechanical secondaries? Or do you have vaccuum secondaries? |
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BuffaloBFN
Grand Poobah Joined: June-24-2007 Location: Gainesville,GA Status: Offline Points: 6094 |
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Vacuum |
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lewy2001
Grand Poobah Joined: March-19-2008 Location: NSW Australia Status: Offline Points: 2234 |
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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. |
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If you're going through hell, keep going
89 Ski <a href="http://www.correctcraftfan.com/diaries/details.asp?ID=5685" ta |
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79nautique
Grand Poobah Joined: January-27-2004 Location: United States Status: Offline Points: 7872 |
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well the curve was set by the wonderfull masters of distributors at DUI that believe 24 max advance is great for these beast of a motor we use PR BS way too many have bought into and left perfomance on the table because of. And the rich fuel washed down cyclinders are real good for blow by but horrible for seating rings |
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BuffaloBFN
Grand Poobah Joined: June-24-2007 Location: Gainesville,GA Status: Offline Points: 6094 |
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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? |
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eric lavine
Grand Poobah Joined: August-13-2006 Location: United States Status: Offline Points: 13413 |
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way to much fuel will wash the cylinders but if you pull the dipstick and smell unburnt fuel or raw fuel you got problems, i have scattered a motor because of too much carb, it washed the cylinders and didnt take long. its a fine line and the statement was in regards to no back fires because of too much fuel...it just sounds like its running a touch rich and will straighten out with time and then dial her in. the engine is in the recovery room after the operation and it takes some time to seat the molly's
the vacuum guage is the true source of what is going on and it seemed low due to the rings not seating yet, with a stock cam i would think you should be pulling 16 to 18 inches as a good running engine should |
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BuffaloBFN
Grand Poobah Joined: June-24-2007 Location: Gainesville,GA Status: Offline Points: 6094 |
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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. |
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eric lavine
Grand Poobah Joined: August-13-2006 Location: United States Status: Offline Points: 13413 |
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Jitters again my friend?
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79nautique
Grand Poobah Joined: January-27-2004 Location: United States Status: Offline Points: 7872 |
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running larger wire to the dist won't change anything. might give little more spark energy but nothing to the advance
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BuffaloBFN
Grand Poobah Joined: June-24-2007 Location: Gainesville,GA Status: Offline Points: 6094 |
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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.
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79nautique
Grand Poobah Joined: January-27-2004 Location: United States Status: Offline Points: 7872 |
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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. |
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BuffaloBFN
Grand Poobah Joined: June-24-2007 Location: Gainesville,GA Status: Offline Points: 6094 |
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I did have 12 volts before, but DUI wants 14.5 and I now have 14.47 with the dedicated wire. I pulled 1 skier after the switch last night and it did burn a little cleaner and smoother, but it's still rich. I also verified that TDC is TDC and checked the advance. The initial timing is much more stable than it was and the advance seems to be right on. I still haven't swithed the carb or dist. I'm trying to do 1 thing at a time so I don't get dizzy from chasing my tail! Next I was thinking of dropping the jet and PV sizes. The jets are 76's now, and the PV is a 7.5 Thoughts? |
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79nautique
Grand Poobah Joined: January-27-2004 Location: United States Status: Offline Points: 7872 |
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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. |
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BuffaloBFN
Grand Poobah Joined: June-24-2007 Location: Gainesville,GA Status: Offline Points: 6094 |
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As I understand it, it's full alternator voltage. It's a 12 gauge wire from the ignition post to the dist.
As far as fixing the rich carb, I thought smaller jets and PV would work towards that end. |
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79nautique
Grand Poobah Joined: January-27-2004 Location: United States Status: Offline Points: 7872 |
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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. |
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79nautique
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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. |
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BuffaloBFN
Grand Poobah Joined: June-24-2007 Location: Gainesville,GA Status: Offline Points: 6094 |
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It runs from the ignition swith straight to the dist. The ballast resistor is bypassed. |
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79nautique
Grand Poobah Joined: January-27-2004 Location: United States Status: Offline Points: 7872 |
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would have been much easier to tap in the battery lead at the back of the motor, still don't understand the need, existing wire should have been fine, oh enjoy the hassles of not having it curved right. |
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TRBenj
Grand Poobah Joined: June-29-2005 Location: NWCT Status: Offline Points: 21169 |
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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. |
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BuffaloBFN
Grand Poobah Joined: June-24-2007 Location: Gainesville,GA Status: Offline Points: 6094 |
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Thanks Tim, I just picked up some 71's and a 2.5 and 4.5 PV along with the necessary gaskets. I'll get it surrounded soon! Chris, FWIW you didn't have a problem with the DUI when I bought it. |
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