stinger390

New carbs are bolted on but I have not had a chance to take it out since it started raining on me. Carbs are 9022 Holley's, 800 CFM, they are jetted 82 front / 90 rear,1 power valve, electric automatic chokes. I think stock from Holley is 72f / 87 rear jets so these might be a little rich, but I will soon find out. Richer is better at first anyway right? They do start and run fine. One choke is not wanting to open fully, but I figure that is just a quick adjustment somewhere.

Even at the dock I can tell a definite difference in throttle response, even in neutral. I can't wait to get it out and see what it is like under way.

Where is the best place to wire the choke power? The Holley manual says not to wire it to the coil due to a reduction in voltage to the ignition system. The place that sold them to me said to wire it to the coil! Every diagram I can find in the Mercruiser manual shows the choke power going to the coil. Holley's reasoning makes sense but there really arent that many more ignition switched 12 volt sources right at the engine to wire to are there? Any ideas? I have it wired to the coil right now and it seems to work but if there is a better way, I'm all ears.

blue thunder

I find it is best to maintain the out of the box split in jetting front to back. Stock is 72/87. That is a split of 15 jet sizes and volumetrically represents the amount of enrichment that can be expected from the power valve channels. For instance, my 800 holleys are jetted at 77, 92 a 15 size split. You are plenty good with what you have 82/92, but may want to work back to the 15 size split.

BT

MILD THUNDER

I think your ok to wire it to the coil. They probably dont want you to hook it to the coil if you had a ballast resistor, like in a points distributor, but you have the Thunderbolt IV, so your coil should get a full 12v.

stinger390

New carbs are set up and ignition timed at the dock and ready for a ride to see what will happen. They seem to be singing at the dock. Hopefully it will stay that way.

The right engine still gives a bad stumble for the first few hundred RPM. Almost sounds like 2 or 3 cylinders are dropping off line. Sounds like a nasty chugging until 1500 RPM or so where it clears out and runs smooth. It does not sound like the engine wants to die, just a real struggle to accelerate.

Could that be a symptom of bad valve timing? Intake leak? Bad plug wires? Bad ignition module? (advance works fine and advances 20 degrees perfectly like it is supposed to (V8-HP module)) Total advance is set at 30 degrees as recommended by the local performance shop with the fuel and setup I am running. (89) Different base timing also seemed to make no difference. Runs great at idle, and after 1500 RPM or so. Everything in between sucks.

When this problem starts the tach drops a few hunderd RPM, but it does not sound like the engine is slowing down. Really hard to describe accurately but sounds like a constant chug...chug...chug, until I advance the power more or bring it back to idle. Carb change made no difference to this problem. Right engine only, and really wierd. Replaced coil with no difference either.

MILD THUNDER

I would pull the cap, and check for moisture or carbon tracks. When it gets dark out, start the engine and spray all the plugs with a windex bottle full of water. See if you see any sparks jumping around. How old are the wires?

stinger390

I'm really thinking about the props now. I am starting to think that the power I have will never spin up 15 1/4" X 22P 4 blades on this boat. I don't think it is making the power I thought it was. I'm still setting up the engines and will take it out tomorrow for a try with the new carbs, but after reading Mild Thunders reply a few back about a nearly identical boat with about the same power turning 4 blade 19P revolutions, I think I'm way off the mark on props.

I bought a used set of 14 3/4" X 21P Mirage 3 blades to try from ebay for a song. I figured even if they don't work I can always sell them again. The $265.00 I paid with shipping seems like a no brainer.

I read a 1983 and 1984 test report on this boat last night, and it seems the 390 Stinger came from the factory set up with 15 X 19P cleavers with 400 cyclones and would do 62 mph at 5100 RPM. I figure with difference in power that I have, and MT running 19P 4 blades successfully, that 21P 3 blades with a slightly smaller diameter than what I have now may make the engines wake up all the way, not work as hard, and be easier on the drives. I'll see what happens I guess.

PatriYacht

Do some plug checks. After going up 10 jet sizes, you are probably quite rich on the primaries.

PatriYacht

I ran some numbers on the Bam website prop slip calculator. www.go-fast.com A 19 pitch prop with 12% slip at 5200 rpm equals 55 mph. A 22 pitch prop with 14% slip at 5200rpm equals 62 mph. The numbers in that report may have come from a speedo. GPS wasn't in use then. My old 40 Scarab with 400's would go 60 at 5000 rpms with 23 pitch 4 blades. It will be interesting to try the 21 Mirages, but my experience is that they slip a lot when used on big,heavy boats. All around I think you will be happier with the 22 4 blades. If your compression is 9 to 1 or less, run 34 to 36 degrees total advance with the 89 octane gas. Then work on getting the jetting right. Running rich kills power.

MILD THUNDER

I think you should be able to turn the 22's. My ran 5200 on the starboard, 4600 on the port with the 19p revs. I think with both motors turning 5200 I should be able to step up to a 21-22 rev. I actually like the hydromotives on our hulls, it seems as if they need stern lift, not bow lift. Heres a running shot of my boat with 22p bravo props

crmax

you said those carbs are auto carbs? and auto fuel pumps?? thats a fire waiting to happen and your insurance will not cover it. safety should be your first concern.

I see you went with the 800 marine carbs , did you change to a marine fuel pump?
and the edelbrock rpm intakes youll be in your power band more toque where you need it . you may have a intake leak try some carb cleaner around the intake and see if the rpm changes personaly Id just change the intakes a single plane is not good till over 5000 rpms