marine quadrajet 17082515 on gm 4.3

[Mudsport96]

If i  remember you had DG rods in it. They are a bit richer than the DRs. Could always try them again

[Zyen]

i dont have a 'nice' assortment;  DG, DR, DP and V, N, P, L.  currently running richest i have.

the problem i see currently is i *think* the mains arent tuned properly yet.  still working on that.
however, as this 4.3 will never open the 2nd door past ~40% and all the secondary rods are +/- 0.002" at that setting, i don't know how much difference that will make.

[Mudsport96]

I would use the DP rods and tweak the hanger a little to lift them higher.

[Zyen]

I would use the DP rods and tweak the hanger a little to lift them higher.

while i appreciate the advice, i don't see how going to a much leaner rod can fix a lean bog on the secondaries.   at 40% the DP is 0.1157" whereas the DG is 0.1075".  even bending the hangar to get the rod to 60%, the DP is still 0.100" fatter.
am i missing something?

[Mudsport96]

My mistake. I was lookin at the specs for CP... not DP.    ill be over in the corner....

[Zyen]

LOL.
i understand...get cross-eyed at times looking at all these things.

rebuild kit came in yesterday for 'new' carb.  i wound up stripping couple air horn screws (which were very close to gone already), so gonna clean/install/tune new carb.  may very well be 'starting over', but this core is in much better shape than what's on there now.
gonna put it back how i received it with 71/41 and DG/P.
will update soon.

[Kenth]

P is about the leanest hanger there is.
Most V6 marine Q-jets i have seen has an E-H secondary rods hanger (with DH secondary rods).
If you want richer use a B-H hanger with your DG rods.

[Zyen]

so, i got off the internet, pulled my head outta my ass, and pretended to be the tuner i say i am.   i *thought* previous 70 jets were rich because (as it turns out) some PO had the choke wired half-closed.  i remember removing ties and adding divorced choke linkage.  the 66 jets from merc specs are for the merc carb, which has smaller primaries than qjet.  the change to 70/39 didnt help much so did some math and found that the 67 combo was running an estimated 17 afr and 70s were around 15.7 afr.  the 70/39 and the 'new' 71/41 were both jetted for a 3.8v6.  changed to 73/43 for an estimated 13.2 afr @ 4k, and 8° base using a nib/nos cobra dizzy.  today's run on a 19p ss prop gave me 4600 wot @ 41mph on gps. idle to 3600 was crisp and responsive.  4200-4600 was slow and lazy.  this gives me a good baseline to work on fine-tuning. 
i cant advance timing much more because of the advance 'hump' at 1600 in the timing curve; anything above ~8° base puts my LPCP around a dangerous 10-11° atdc at 1500 - 1800 rpm and not much better till 2500+, but the LPCP at 4800 on 8° base is a lazy 25+ atdc, as seen by the 1.5-2 second climb from 4200 to hitting a wall at 4600.
however, did a good 2-3 minute run at wot with no problems.  hard to see <1.5" hg with the gauge im using, so estimating < ~0.5" hg at wot.
so, unless anyone wants details, i'm gonna say this is closed as fixed.

thanks for the help all.

[Cadman-iac]

  I've got a question if you don't mind my asking.  How did you calculate the LPCP for your engine? And since yours is a marine engine, is it any different than one in a vehicle?
 You're not using a vacuum advance if I remember correctly, so maybe the calculations would be different on an engine with vacuum advance?
 Thanks for any help you can offer.

 Rick

[Mudsport96]

If Mercruiser calls for 8 degrees that is where I would put it. You aren't trying to win races ya know. The problem with trying to get street curved ignition timing on a boat application is that load varies sooo much o. The water vs pavement. That is why the ignition curves are "lazy" from the factory. The odds of losing traction while cruising down the street is exponentially lower than spinning a hub or ventilating the prop. At which point revs go wild obviously.
You could play with weights and springs to get every last bit of timing. But, if you need every last bit of power the 4.3 has... I'd drop in a 305 or 350. As for fuel usage. I have two close friends with almost identical boats. One is a 4.3 16.5 footer and one is a 350 18 footer. And cruising together on the same route, the get within .5 gallon per hour difference.

[Zyen]

mudsport,  your's is a bit easier, so i'll respond to it first.  yeah, most 4.3 stock is 8° base to keep the timing peak around 1600 under control while giving the most power/torque to get on plane. 
you're right...it's not a race boat, i don't time it, i don't run anyone in it, and im not trying to get a street curve in the ignition...that's a sure-fire recipe for disaster.  the curves are lazy to account for the multiple variables in boats and their respective owner/operator.  since i can control some of those variables, i can account for them and reset accordingly.   i dont have room for a larger engine, nor am i looking for an outrageous output from this one.  the mileage is negligible.  dont' really care, but want it reasonably efficient and somewhat close to the 205hp it's rated for.

[Zyen]

cadman, no marine engine should have a vacuum advance.  that's way too much timing for marine loads.    LPCP calc's are the same for any gas engine.  the advance doesn't really matter, cause advance is advance in the math.  im about to head to bed, but will give a breakdown on how im figuring it tomorrow.  and, i may be wrong.

[Zyen]

How did you calculate the LPCP for your engine?
 Rick

Here's how i did it, using my current wot and ignition timing as example:

crank rotation:  60,000 ms / (RPM * 360°) = ms/deg
4600 rpm = 0.0362 ms/deg

~12.8 afr using 87/E0 and ~90% VE (~0.5" hg) = ~1.3ms burn time to peak pressure
~0.3 ms from spark to flame propagation. = 1.6ms
Total burn from start to peak pressure = ~44.2° of rotation.

timing is set to 8° base and hard stop of 10.5° advance @ 3200 = 18.5° TA > 3200 rpm

burn time from ignition to peak pressure - spark advance = lpcp
44.2° burn - 18.5° btdc start = 25.7 atdc lpcp


-Allan