502 Mag plug reading
 

32 Sunsation Dominator, twin 502 Mag MPIs.
Ran it today for basically the first time, started and idled great, good power, but kind of fell flat after 42-4300 rpm. Saw 72mph and then nothing more. Could not gain RPM on either motor past that point.

Got home and pulled the plugs out, most of them look lean to me, but I'd welcome additional opinions.

I've suspected the fuel pressure is low on both motors, despite new pump on Stbd motor, possibly bad regulators on both (?) motors since pinching the return lines results in a spike to 60+ but normally they are running at barely 32psi with the vacuum lines removed from the regulators. Fuel filter/water separators are both new. I'm going to compression test tomorrow, and I'm probably going to order 2 new regulators and send out the injectors for cleaning/flow testing unless someone else has a better idea.

Opinions on these plugs?

Thanks!

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X2 on the compression test then get back and repost. You might have a few down but unless you do a proper plug check it doesn’t really tell you much. You under propped?

And X2 on the injectors also!

Curiosity kept me from waiting til tomorrow. Here's the comp test results:

Port

1 160 2 171
3 161 4 175
5 166 6 171
7 161 8 179

Stbd

1 175 2 175
3 179 4 180
5 159 6 165
7 166 8 172

Your fine there.
i figured you’d ck sooner. 👍

Are these vst or cool fuel engines ?
How old are those plugs ? Some look old and some oil fouled.

Getting the injectors gone thru is smart. They weren’t the most dependable. Arcticfriends/smitty knows these intimately.

Is this a new running condition or is boat new to you ? If new to you, there where a ton of these boats with these engines do i’d see what props you have snd what owners of these boats found where best props.

On the upper set of eight plugs, #5 and #6 are heavily fouled with Ash. 8 is wet.
On the lower set of eight, #1 has ash building up.
In any case new spark plugs in both engines will probably fix you right up. Temporarily.
My plug insulators were white like yours. Living in Florida I dropped the heat range. Now they have a Florida tan.

Thanks. No clue on age, I just bought the boat last month, these were in it and I got no records. This is the first time I attempted to find its top speed.

These engines have the older injectors and the MEFI 1 style ECMs, but the later cool fuel modules (no VST).

I know that stock, Sunsation determined that Bravo 1 4-blade 28P were the best prop, but this boat has 4- blade Hydromotive 28Ps on it.

Thanks. I have a set of NGK BPR6FS waiting to go in today after I get done pulling injectors.

Doc, If you need any help, please let me know.
Tx,
SD

Take a look at inside of dist cap for fuzzy posts too.

I wonder how the plug wire resistance is on the boogered up ones. Would be something to check also that doesn’t take much time.

Plenums and fuel rails off, injectors out. They look pretty crusty. Several visibly distorted or damaged o-rings, and many of the spacers (?) at the top of the injectors are badly distorted or compressed. This pic is after a brief soak / rinse in gasoline. They were significantly worse beforehand.

Think it's worth sending these out or should I plan on finding new?


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Send them to Smitty ( Articfriends ) on here he's rebuilt and flowed several sets for me and others.

X2 that’s the route I’d go so when you reinstall once you get them back you know first hand their gonna be right. Heck even ordering new ones you still don’t know what your getting or if your gonna have failures.

Plug wires all checked out OK. 14K-20K ohms, proportionate to length.
Caps and rotors weren't terrible but weren't new either. Port was worse than Stbd, fuzzy terminals inside, Stbd rotor wasn't seated fully on the dist shaft so the rotor plastic was rubbing the cap plastic.
Both distributors a little crusty inside.
Stbd distributor cap mounting screw holes were drilled out at some point (maybe secondary to cross threading?), so there's locknuts on the screws holding the Stbd cap down, and that was fun.

Sent you a message on injector service. The "32" psi you seen, was that at wot when it wouldnt pull at 4000 ish? Bad regulators usually allow fuel pressure to drop to about 26/28 psi at wot, with your cool fuel pumps, with no vacuum, you should be around 38/39 psi , 35 MININUMUM, Smitty

Well, I wasn't able to monitor FP at WOT because my gauges are in the engine room on the end of the fuel rail. But at idle with vacuum line removed they are at 32 psi or so. With vacuum line connected they are more like 26 psi at idle.

Next time : Cell phone on record. :)

I decided to see if my fuel pressure gauges are actually even accurate at all... just to see if I'm chasing a red herring there. With my air compressor regulator set to 40 psi, one fuel pressure gauge reads 38-39 psi (close enough). The other reads 30 psi! Not close at all.

I'm sending both gauges back to the manufacturer tomorrow, so they can address the issue. Not going to touch the regulators at this point.

I'll bet the injector service and ignition tune up will restore the lost oomph. A cooler plug might help a little too as mentioned but I'd try to look at it again after a few hours of running with the injectors done.

Fuel pressure is very important, I get alot of phone calls from people wanting to "buy a replacement mefi ecm", after I talk to them a good part of time their issue goes back to low fuel pressure, idle fuel pressure with a accurate gauge will at least tell you IF your close BUT you REALLY need to see the fuel pressure at wot as at idle your injectors are open like 1.2 milliseconds at idle and accounting for dead band, more like .7 ms , at wot more like 10 MS (10 times more) so extend gauges temporarily to near helm with hoses or something as you could chase things in wrong direction without that data! those two ashy plugs in first pic look like they have saw quite alot of oil, does that motor use alot of oil, more than the other? does it ever idle to high (bad pcv) which will draw alot more oil in and maybe it gets to those two cylinders

Even a cheap bluetooth wifi boroscope / endoscope :)

Appreciate the info. The boat is new to me so I don't have any data on oil consumption yet. PO told me he added a quart to each, fairly consistently, but I don't recall how often he said that was.
No high idle noted thus far. PCV valves replaced last week.

Do yourself a favor and just ditch the tiny PCV system and go with a catch can breather setup. I got some cheap Chinese ones for ~$50 each and plumbed them straight to each valve cover and capped off vacuum port on plenum. My 960+ hour 502MPI's went from burning a quart every weekend to burning a whole quart over the entire season just by letting them breathe. Mount the cans near the arrestors/TB to help suck in some of the fumes.

I also recommend having Smitty (Articfriends) flow your injectors. I learned the hard way with a melted piston from bad injector. These things are still original (sans a used piston/ring set to replace melted one) and run like swiss watches with Smitty's injectors and tuning and I actually picked up several MPH afterwards.
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How are you limiting the vacuum draw?

*f’ing autocorrect

Tested my Stbd fuel pressure regulator today, with compressed air. Blew off at 33-34 psi, far short of the 43 psi spec. Replaced with new Merc unit.
Also determined my distributors are in rough shape, significant corrosion, modules questionable, probably will replace both.

I put the Davis Unified distributor modules and coils on my motors. Been running on them several years now after Smitty (Articfriends) recommendation from his dyno escapades. I also included the part number for a very reasonably priced spark plug wire set that fits these perfectly and has great quality for the price. Both of my fuel regulators were bypassing with low fuel pressure however are basically inaccessible without pulling engines or doing serious disassembly to access. Put a secondary universal regulator on the return line which has worked very well.

Summit Racing part #'s:
DUI-000444 Davis Unified Ignition Ignition Dyna-Module
DUI-31723 Davis Unified Ignition Screamin Demon Coil
FST-255-2416 FAST FireWire Spark Plug Wire Set

Anyone know if the DUI modules have a built-in advance curve and what it is? I'd like to update mine - I'm carbureted now but using the Delco distributors. The modules appear to be factory original though their characteristics seem to be a little different from one another. Sorry for hijacking a little.

Sb has told me multiple times that different variations of 8 pin small cap efi/hei modules have different advance curves BUT I have never found them to have ANY advance curve. IE, you set base timing in a mefi ecm at 8 degrees, the wot timing at 34 degrees at 50 KPA/5000 rpms and the factory default setting in the MEFI " FALLING REFERENCE ANGLE IN REGARDS TO NEXT PHYSICAL TDC" TO 8.1 degrees and aside from timing chain stretch, cam deflection/twist, when you rev it up to 5000 at 15" of vacuum you will pretty much see that 34, if you change it in the programming to 20, you will see 20, if you change it to 40, you will see 40. Now, the saturation time, dwell time, storage time then realease, whatever you want to call it when module converts the signal of rotor passing and sends it out , is ever so slightly different on some modules. What I mean by THAT is say a merc module is in dist and set to 8 base, you put a dui module in, base might change to 9, 9.5, put a weak standard module in, might change to 7 or 10, but again at that point the cap was just off and disturbed too.
so after changing module, I have had to adjust base 1, 2 degrees before to get it back to the correct base number (8 degrees on most mefis) that makes the tables correct, Smitty

8 pin GM modules . Can’t vouch for the DUI replacement.
When you remove the 4 wires attached to right side of dist, as we do for cabureted app’s, you lose the 5 volt ref and ecm ‘timing’ wire connection. When those aren’t there, the module uses it’s own advance curve.

For carbs, we just run + and - to coil, then attach two other terminals on coil to two wire connector on dist. Again, nothing plugged into 4 wire connection on dist.

Now, so much of the advance is under 1000rpm and alot under 600 rpm, most with a timing light don’t see this and think the module has no advance.

Diagram shows hook up. Diff fist but same deal. GM est/voyager dist just plugs in with two wire connector.


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Here is what volvo shows with running this module/distributor on their carbed 5.7.

Again, looking at this shows why many timing their engine thinks these modules have no advance. Most all of it is in at normal idle speed.

Another reason these dist’s/modules good on bigger cammed performance engines compared to tbolt iv dist’s /modules especially

You can also hook the Daytona Sensors box too it, of course, like you can with other type dist’s…if you want programmable timing. Simple, just bypass module and use dist as magnetic pickup.


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So, turns out one gauge was in fact 8psi low, and the manufacturer (Marshall instruments) is sending me a free replacement (bench tested prior to shipment).

Injectors are in process of cleaning then testing tomorrow probably.

^^^M instruments, i knew (somewhat) the old man. Recently passed. He liked to come out here from out west , to my neck of the woods in nh, skiing and climb mt washington. He was a character and tough as nails !
His son and daughter that now run it, are nice peeps.

Just some fun facts. :)

Btw: if gauge is engine mounted it should be dry type, not liquid filled. The glycerin expands when hot , and if too hot it pushes against gauges diaphragm which makes it read lower.

I may have experience in this. Just few hundred times a year. Lol.

These gauges are in fact liquid filled -- with silicone. Not sure if silicone behaves the same way as glycerin under heat. But if I see lower pressures when hot I'll know why!

Yep, still not sure where cuve or advance would be coming from as I only know the module to convert the pulse to something ecm can use.
Now heres my "test" for sake of discussion : If I hook my MSD Ignition 8998 to those 2 pins coming from dist pickup to input side of module (I often do this trouble shooting) using it as a signal generator, I can simulate the engine going anywhere from 10 rpm to 10,000 rpm, a standard brand module will smolder after 30 seconds much above 5000 if the rest of mefi is plugged in, a merc or dui will take 5600 briefly in air, more if mounted to something to pull heat from it i ended up mounting one on aluminum as a heat sink .
Anyways, IF I go in the mefi ecm program and I set timing to say 15 degrees in every map vs rpm table, top to bottom, plug my lqaptop in, set it to scanner pro or plug a merc scan tool in, crank the signal generator between idle and 5000, it will show it commanding 15 degrees of timing top to bottom.
Now, if a mefi motors on my dyno, I again set those timing tables to 15 degrees from top to bottom, I set my actual jumpered timing at 8.1 to correlate those tables (FALLING REFERENCE ANGLE IN REGARDS TO NEXT PHYSICAL TDC" TO 8.1 degrees) rev motor, it will show that 15 degrees thruout the rpm range (it may vary a tiny bit lower from cam deflection, chain stretch as you rev it higher) but in general. So if the ecm dictates when to fire the coil based on timing tables, if the module started doing its own version, where would timing actually be and what would be controlling it?

how do you know the compressor gauge is accurate?
I have a tire gauge that has the best tolerance out there,forget the brand off my head,I use that digital gauge to test w/.but the spread on your gauges is enough send them back.

the last half of what your talking about is how I've been setting timing,seems I'm always doing blower motors,I have a program thats just for setting timing.the whole map is 25°top to bottom,left to right.makes timing a snap.

IF you re-read my post more carefully, you'll see that I DID send them back. Then the manufacturer DID test and confirm that one was faulty.

i know what you wrote,thats why I send you were right to send them back,,just on the spread of the readings.