Is the timing causing my idle problems?
 

I am still working on my idle. I have the mid-range and top end AF ratio pretty close with the 1150 Dominators using 3.5 PV's and 87 X 88 jets. The AF ratio is steady @ 13.5-13.6 from 3000-5200 RPM. At 5300 RPM, the PV's open and the AF ratio drops to 12.5-12.6. I am good with this...but I am still working on the idle spread in and out of gear.
Looking at some other comments on other threads, it occurred to me that the MSD ignition (mechanical advance)may be part of the issue. On previous engines, I didn't have these idle variances with the Merc Thunderbolt IV and the V-6 box. My total timing is at 40 degrees.
Looking at the MSD website and the various distributor spring and bushing combinations, it appears the MSD timing curve advances approx 3-4 degrees from 700-1200 RPM. Does this indicate that locking out the timing advance could be a good method to reduce the RPM spread going from in-gear to neutral?
On my next trip, I will see what the actual timing difference is between 700 & 1200 RPM.
Does anybody have any positive experience with locking out the MSD advance?

I have mine locked out at 34 deg. But i also have a msd start retard box ,that retards the timing back 20 deg. when you start it . The starter likes it alot.I havei not had any problems with it .

The timming swing in that rpm range can cause the idle to suck. I had same problem until I was told to lock out timing. I had max at 30. Solved all of my problems. Max of 40 might be a problem being locked out. I would try stiffer springs so timing starts coming in later.

what initial is the timing now ? 40 total is a lot for a marine engine..what compression are you running ? but yes locking the timing at 34-36 really cleans the idle up a lot, I run a locked MSD dist in my race car and it starts fine, but only a 9-1 comp motor, locking the msd dist is a easy, but you have to remove and take apart, Rob

Rob,
I appreciate your comments. I talked to MSD today and in the end, I felt like they do not have a very good understanding of the issues in a marine application with their limited offering of springs and bushings. Yes, you can lock out the advance but now you might need an electronic start/retard to have reliable starting. Also, for me this would have to be winter re-rig project.
One OSO'er PM'ed me that he replaced his MSD springs with an small o-ring...full advance quickly and fixed his idle issues!
I suggested to MSD that they might consider including a "extra light" spring and bushing combination that would allow for retarded timing at cranking RPM and then full advance at 500-600 RPM...not by relying on electronic bolt on devices.
At then end of the day, I'm starting to think a mechanical advance distributor is marginal for marine use unless it is for a full time race set-up.
I wish I never would have never sold my Mercury Thunderbolt IV and V-6 boxes....
Oh well, just another challenge.

Wow 40 degress of timing seems kinda high. Might want to back that down to 36 or 38. But if you look your Dist. you just need to have a set-up to crank the engine over with the ignition off. Once its cranking flip ignition on. This will solve the lock out issue/starting issue.

Butz,
40 degrees may seem like a lot, but it is the right timing for this combination. Iron heads, efficient chamber, 9.3 CR. This timing makes the most power with no detonation. I do use 93 octane gas.
I will either re-rig with auxiliary cranking switches this winter or find the right light springs. These solutions all seem like patches for what MSD should offer with their marine ignition system.

Good deal just did not want you to hurt the engine.

Please take this constructively -

I'm with Vincent, I have never seen a big block Chevy marine engine that wanted more than 36 degrees total. I will say it has been a few years since I built one, but I did my share back in the day. The newer heads actually need less.

Why do you think the PV's are opening at 5300? Antyhing more than cruising speed and your manifold vac is going to be very close to 0 - hence open PV.

Yes - eliminate the timing moving around at idle, will help a lot.

Marc and Vincent,
I understand the scepticism on 40 degrees. I have raced most of this current combination since the late 90's. The 40 degree timing has been confirmed as optimal on the dyno at Tyler Crocketts, Keith Eickert's, Sterling Performance, and MRE Power Products. Tyler, Keith, Mike D and Mike R were all either watching, participating, or wrenching. Tyler and Mike R build engines for me. An MSD knock sensor was used while running on each dyno. The max HP (no detonation) on pump gas was at 40 degrees timing from 3500 to 7000 RPM. (we ran race gas at Sterlings as that is how his shop is set up) What about 42 degrees? 0-3 HP...not worth the stress.
Will the engine last? We finished every APBA race over 13 years (2-6 races yr) and never had an engine failure racing. I started with 468 CI, GM Sq port heads and cast iron exhaust in 1988...with 34 degrees timing.
Bottom line...I am old school, but 36-38 is really old school if you have the heads, cam, combustion chamber, piston shape, etc. These APBA 1997-2002 A/B Class legal engines made 750-765 HP @ 6800 RPM (depending on the dyno) on pump gas with CMI race E-tops...820 HP with 2.25 Big Tubes in 2007.
Power valves: I tested 6/08 in the boat with an Innovate Motor Sports A/F ratio gauge. From 3000-5300 RPM the A/F ratio is steady at 13.5-13.6. At approx 5300 RPM, the A/F ratio drops from 13.5 to 12.5. We were testing with steady acceleration, not WOT.

Results are what matters, if 40 works then I'll certainly re-think it.

As to the ratio I don't doubt it's changing, but not as a result of the power valves. Was there any manifold vaccum during the test? Have you visually looked at the carb at those RPM's? I had a pair of tunnel ram motors that were pulling fuel out of the accellerator pumps at high RPM. Wound up adding tiny sprngs to the check valve.

Marc,
I have tested with a vacuum gauge but it was last year.... can't find my notes. I am pretty sure the PV's are opening because when I tried 2.5 PV's (back to back test same day) the 2.5's would not open....at least at 6000 RPM and the A/F ratio had climbed to 14.1.
I agree that an internal carb leak can cause confusion, last year I had a PV gasket installed incorrectly and he took me a day to figure out why I lost idle adjustibility.
Good thought though...when I get it all dialed-in, I will confirm the PV opening RPM with both a vacuum gauge and the A/F ratio meter.
Here is some HP results same day dyno runs at KE's with just a timing change:
38 degrees
5000 RPM 589 HP
6800 RPM 753 HP

40 degrees
5000 RPM 595 HP
6800 RPM 760 HP

I have played with msd light springs and ended up with them bouncing around at idle and cause erratic idle...I don't think with your compression running it locked would cause any problems, I am noe running a mallory unilite, and it has a great advance adj you can change from lock to 20 in a minute, so far I like it...Rob

on your power valves, holley says ck idle vacume in gear, even boat, divide that # to get the correct PV, Rob

ez,
The plan is to lock-out the MSD distributor and see what issues if any arise...40 degrees is a tall number. At 700 RPM in gear, the tested vacuum is 5+ and I have already confirmed that the idle is good with the 3.5 PV. The 1400+ RPM (700 RPM spread) in neutral is the issue. Locking the distributor (permanently or temporally) hopefully will close the gap.

locking it will clean up the idle imensly as long as you have no carb issues, etc..