HTMSteve

540ci, hydraulic roller, 14-71 10% underdriven w/superchiller , 2x 1050 dominator carbs 8.5lbs fuel pressure, bravo 1, 30P prop, MSD blue marine box with MSD distributor. Previous owner had top end of engine stolen - 10-71 with 1050 carbs and replaced with setup I have listed. Engine was pulled and dynoed (Cale Carder HTM2 Lake Havasu), but I don't have dyno sheet, only pictures of engine out of boat before the dyno run. He said, "It made almost 1000HP and almost 1,200FT LBS of torque." I have done no tuning at all yet, all as purchased.

Was worse on tank of old gas, ran that tank out and switched to tank with fresh gas. This made it quite a bit better, but still a struggle to keep it running on initial shift into gear. If engine is idling and put into either fwd/rev, engine RPM drops and struggles to stay running. If I bump the throttle and pull it back before it dies it's OK, but that's kinda squirrely around the docks. I did notice that if I do a quick rev to 3k or so, and then once at idle I go into either gear at the down side of the blower surge, it's OK. Engine idles 1000-1300 RPM with some lazy blower surge, but will idle all day long and revs quick in neutral with no stumble. Carbs are not boost referenced, and I haven't had the boat long and don't know the timing situation. Lake water was cool yesterday, and I did notice that water and oil temps were low too. Raw water pump feeds engine/exhaust through the stern drive, and a hull pickup feeds oil cooler, superchiller, and out transom through a hose to a drive shower. Oil pressure seemed a bit high too, from the low temps, 50psi at idle and 75psi at 3,000rpm.

Thanks in advance for any responses!

HTMSteve

OK, pulled the carbs today and here's what I have:

8082-3
2-circuit 1050 Doms
IAB = 71
MAB = 37
Pri jets = 89
Sec jets = 94
Pri PV = 6.5
Sec PV = 8.0
Idle vacuum 10-11" Hg
Idle mix screws out 1.5 all 4 each carb
NOT boost referenced...yet
Fuel pressure = 7.5psi
Timing locked at 34º
I use 91 octane pump gas, all I can get in Phoenix area unless I go to race gas.
I noticed that NO idle transfer slot is exposed under the butterflys on the primaries, but yet it idles too high already, so if I open them more won't this be even higher? Secondaries open about 1 turn on the idle speed screws.

If I open throttle to 2000RPM and slowly close, engine will idle smooth at ~1100-1200 RPM, but after a minute or so will start to surge ~800-1400. Also will immediately surge after a quick rev and throttle close. When surging, if I shift on the down side, engine still shudders and half the time will die unless I catch it with throttle, but I don't like that, especially near dock or when loading on trailer.

Engine runs strong with no noticeable flat spots or stumbles, cruising at 3200RPM with 0 vacuum as measured under blower.

My goal is to have a smooth (or smoother) idle, but if nothing else, at least not stumble when going into gear and stay running. I have read SO MANY posts about surging idle, but would like to hear some specific to my issue.

I appreciate any help!

dsmawd350

Not really any help but this is exactly why im going efi

Young Performance

Without O2's in the pipes, everything is just a guess. Get them installed and see where it's at. It's sounds as though it's a little rich since you can clear it out and it will idle decently. Once you have some AFR numbers, let us know what they are.
Eddie

MILD THUNDER

iTrader: (3)

What position are the secondary throttle blades? Is there a chance someone opened them up a bit too far?

I have noticed with roots twin carb deals, if the primary throttle blades are too far closed, the engines will surge. I think what happens, is that the vacuum at the carbs becomes erratic, and you start getting an unstable emulsified mixture, too much, too little, too much, too little, (rpm up, rpm down, rpm up, rpm down).

I would make sure the secondary throttle blades are pretty much closed. Turn the carbs upside down, and you'll see a little screw on the baseplate that allows you to adjust how far the secondary plates open. Back the screw out, until the throttle plates are closed all the way. Then turn the screw in, until it makes contact, than about another 1/4 turn. Then, reset the primary throttle blades, to where the exposed transfer slot, appears to be a perfect square shape.

Then, be sure, that the linkage assembly after everything is put back on, is allowing the throttle blades to close completely. The cable pretty much needs to be pushing on them with some tension to make sure they are closed. A little slop that can cause fits. Good luck and let us know how it turns out.

HTMSteve

Thanks Eddie,

I would like to add O2s but have to figure out the best method and location. I have IMCO Powerflow Plus manifolds with water jacketed pipes from the elbow out the transom. Was thinking of having stainless sandwich plates made for each side to either go between the aluminum elbow and tailpipe or between the elbow and manifold...would prefer between the elbow and tailpipe so the elbows aren't any higher. Any help with sandwich plates or successful bung weld-ins are appreciated.

HTMSteve

MT,
I've read many of your posts and Smiity's too and learned A LOT! Thanks for responding!

I had the engine running with the secondaries closed (they're dominators and have an external idle screw on secondaries) and the primaries open very little...approximately 1 turn on both carbs. I did not touch any of the idle speed screws when removing carbs Sunday so I could flip them over and check the transfer slot. No slot exposure was present, and the secondaries were open slightly open, but not much at all. The linkage is progressive so secondaries don't open until primaries are at about 30% open.

I'm wondering if the secondary PVs are causing me issues. They are stamped with E 8 5 0 (starting with the E and going CCW about every 90º), so I guess these are 8.5 PVs. When the surge starts, the vacuum is bouncing between roughly 5/6 and 10" Hg, so would guess both sec PVs are dumping fuel, and front 6.5 PVs are probably adding some too which whacks things out.

I bought fittings to boost reference pri PVs and plan to try the 6.5 PVs that are there now and then block the sec PVs. Will start with jets as is at 89/94 and play with idle mix screws and maybe a little less timing to 30º or 32º if necessary.

Plan to try this first, and then report back with results. Also read about IABs, which are at 71 on both carbs, and may look there next.

Does this sound like a reasonable plan?

MILD THUNDER

iTrader: (3)

Hey Steve, thanks for the kind words.

In regards to your powervalves being the problem, I would say no. Reason being, the power valve opening rate, has no effect on the idle circuitry. When a power valve opens, it simply add's fuel to the main well, and is fed to the boosters, as well as the main jets. For explanation purposes, lets say you had a carb with 80 jets in the primary. Installing 90 jets in the primary, will not effect the idle mixture. Entirely different circuit of the carb. Even if the power valve is opening/closing at idle, without the main circuit being activated, it should have no effect. A simply check, watch for fuel dripping from boosters. IF no fuel drip is present at idle, you'd be chasing a ghost. PV referencing is nice, but not "needed" so to speak on a marine engine.

Now, back to your problem, or what I think may be your problem. It sounds to me, as if you are on the lean side of things. Like Eddie said, putting a wideband on it will do wonders to find that out. I'm gonna guess, that if you turn the idle mixture screws in, the stalling problem gets worse, and the surge probably gets more violent? With a bravo drive back there, that can be a real issue.

I've touched base on this before, but I'll mention it again here. In your metering blocks, you have what they call an "idle feed restrictor". What it simply is, is a idle jet. The air bleed, is your air jet so to speak. Now, reducing the air bleed diameter, will give you a richer emulsified mixture from the idle feed ports where air/fuel enters the engines. A larger bleed, will lean that premixed amount out.

But, getting back to the IFR (idle feed restriction). Most carb's have them in the upper location of the metering block, where it is above the level of fuel in the bowl. What happens is the idle mixture needs engine vacuum, to draw that fuel thru that IFR jet. When the vacuum starts fluctuating, you get a large amount of ups and downs so to speak, on the fuel delivery part of things. Now the surge starts getting more violent, and and quantity of air/fuel just wont stabilize. Its like if you had a little cup of fuel, and dumped it in the engine, it rev's up, burns the fuel off, then idles down, until you dump a little more fuel in, and does it again.

Now, when the IFR, is located at the lower portion of the metering block, the jet is submerged in fuel in the bowl. What you get there, is a more consistent flow of fuel to the idle feed port. The air from the bleed, is pretty constant no matter what.

Icdedppl's twin 850 carbs on top of his 10-71's, have the IFR's in the lower location. On his engines, I can pretty much can richen the idle up as much as I want, with no surging. On my engines, where the IFR is at the upper location, I can richen it up (back the mixture screws out) until the thing surges 500-600RPM.

These big roots blown big cammed engines, need to be rich so they dont stall when shifting. If the AFR is 13, its probably gonna want to stall. If the AFR is a steady 11.5 or 12, it will probably be just fine. However, sometimes to get to 11.5 AFR, you end up getting such a violent surge, and you start swinging from 10.5 AFR to 13.5 AFR, as the fuel delivered into the engine, is all over the place.

Now, heres a problem when you have the throttle blades completely closed, blocking the transition slots. The transition slot, is now no longer allowed engine vacuum signal, and, is simply seeing high pressure. Again, the signal to the entire idle circuitry is now affected. The transfer slot is now acting like an additional air bleed, rather than a air/fuel delivery source. Remember when I mentioned having the throttle blades almost completely closed can exaggerate the surging?

It is really important to have those primary transfer slots exposed to engine vacuum. I would start there. Then try all other tuning, without touching that primary idle speed screw. Close secondary blades fully, , turn mixture screws, possibly play with idle bleeds, etc. I would probably lean towards going down in Idle air bleed diameter, and see how that does.

when i mentioned the linkage, what i was meaning was the actual cable adjustment from the helm. I have seen many times where the cable at the engine side, where it connects to the carb linkage, is loose/sloppy while at rest. Sometimes the throttle blades get hung open, sometimes they dont, etc. Just be sure the cable is literally pushing the primary throttle blades fully closed at rest/idle position, so the throttle blade stops are hitting the idle screws.

fbc25el

Also if you take the carb off and look from the bottom side and close the throttle plates all the way to see if they seal all the way around. Sometimes you have to adjust them to get them right. This may let you to open them enough to expose the transfer slot with out gaining too much rpm.

sutphen 30

sounds like you need to play w/ the idle mixture screws.the extra fuel is helping out from the accelerator pump.
get it to idle as best as possible(may never get rid of the blower surge w/ that big blower) then turn 1 screw per carb up a touch to help it go in and out of gear.
I usually do this tied to the dock.