RumRunner

Increasing the timing should make the engines run better, but shouldn't solve this problem. The simplest thing to do at this point would be to swap the metering blocks from one carburetor to the other. You should be able to do this in five or ten minutes. The problem will either stay with the port engine, or travel back to the starbord engine. Without knowing what has been done to these carburetors in their past it's impossibly to say specifically what is causing this problem.

850 Demons are going to be extremely big for this type of combination, which will lead to rich idle, and part throttle drivability, and potentially lean at wide open throttle. Also, there are other changes besides just installing or making "J" tubes. The jetting, and air bleeds are different as well as the baseplate on the carburetor. Just installing "J" vent tubes would not be recommended.

Liberator21

If you're going to take the carb apart, I would check for damaged parts, warped body components, drilled out passages etc. Back in the "old days", guys used to buy a hot rod magazine, and think they could modify their Holley, or Quadrajet. After drilling everything out, and screwing everything up, they would end up at a swap meet for some other sucker to get stuck with. You should see some of the mutilated abortions I've seen. Honestly though, it seems like you've done almost as much as you can do with this carb. I think time and money would be better spent on a set of more appropriately sized, and known good carbs.

formula31

Theres more to marine carbs than just j tubes. Those are easy to make. Sealing the throttle shafts is another story. Not to mention calibrations to handle the sustained loads at all speeds.

But, I am not familiar with the BG's either. Well, at least not the carbs.

stinger390

She finally woke up!!!!!!!

The problem motor loves the 800 CFM Holley I tried today. It is a blue mercruiser 13549T21 carb tuned for the HP 500, which means it is most likely a little fat for my engines but she finally turned over 4000 RPM's with nice response. I did notice some things today I did not like now that I had her running better.

After about 3700 - 3900 RPM on the left motor (the one with the test carb) I would notice a little like a ping when I tried to go higher on the power and pulled back right away. As soon as I pulled back it went away. The right engine would do this only at nearly max power, since it is an 850 it is most likely running richer and I really did not notice that sound as much. I'm thinking the timing is too far advanced for 89 octane at around 34 degrees total. I figure backing that off may solve that problem. Perhaps that test carb is too lean. (I did wire tie the choke plate wide open since I did not have electric choke wireing in place to set it up.) I don't know what is in that carb, but most likely stock jetting for the HP 500 Mercruiser. I could reach a top RPM of about 4100 - 4300 or so on the left side (test carb), A little more on the right engine with the 850 carb at around 4500 - 4600 RPM +/- . Seemingly a little low. I did not think I was over propped but perhaps I am. I am turning 22P Bravo 1 - 4 blades. Perhaps these engines are making less HP than I thought and 22P is too big. I guess I have to finish fine tuning everything and figure out how it runs when everything is dialed in.

The 850 carb (right side) gives a bad stumble out of 1000 RPM through about 1400 - 1500 or so. Almost like the engine is chugging to life. It almost sounds like it is not fireing on all cylinders. If the throttle is opened just a little too much is starts sounding like that until I get it above 1400 or so where it abruptly clears out, or if I pull it back even just a little below where it starts, it clears out and idles perfectly. When this happens, even though the RPM's do increase the Tach falls off to 800 - 900 RPM or so while it is chugging along, like I am drowning the ignition or something, and reads normal when the engine gets cleared out. It also did this with the other 850 carb when I had the sides switched. I was figuring that it is a big carb on that single plane intake with a low / mid range cam causing that. It also seems to happen on a lesser scale when in neutral and reving up slowly. It has done this since day one but I have had other concerns with this thing. I mentioned it to the mechanic the day we had our oil pressure problem (day 2) and he did not seem worried about it.

At least I am making steps in the right direction now. No more fuel in the water behind the boat and the transom is cleaner, less smoke etc. And finally goes fast (er)! I saw only about 55 or so for the second or two I had the handles that far forward. If I can get these RPM's up I think she might do 65 - 70, which is where is was from the factory with 400 cyclones in 1985.

MT what does your Eagle do with your 420's? What props?

blue thunder

Yeah, glad to hear the progress!! Persistance wear resistance my Mom always used to say.

Make sure your floats are now set at the bottom of the site glass. Maybe the bowls are running dry? It is imperative that if you are hearing pinging you fix that post haste or pop the engine. Definatley get base timing in line at around 8* before going out again. Also do plug readings to check for lean.

BT

MILD THUNDER

What eagle? I have a Excalibur hawk!

Its actually identical to the stinger 390 hull. My port engine was down on Rpm this summer some. But I was turning 19p Revoltion 4 blades. Starboard spinning 5200, port 4600, at 50-53mph. I plan on tearing into the port engine soon to find the problem. I also have the crappy 3" log manifolds and dual plane intakes I think with some 450Hp 454's, running right, should be able to turn 22p 4 blades and the boat should be doing around 60-65.

Im glad to hear you got it running better. I still suggest getting your hands on some 9022 holley 800 double pumpers, get your timing dead on, fresh plugs, wires and cap, and I think your problems should be solved.

What intakes are you running? I think the weiand team G manifolds were stock on the 420/425 models.

stinger390

sorry...sorry....Hawk / Eagle (both winged, sleek, powerful critters though, and both Excaliburs right? ).

The carbs I have available to me are from the HP 500 Mercruiser. They are 800 CFM with automatic electric chokes and mech. secondaries. They are rebuilt be a Mercruiser dealer and performance boat specialist here in CT.

I think I definitely need to look into intakes in the future and get some good Edelbrock air gaps or something. These Aluminum single planes are just a mismatch to these engines.

Hopefully I'll get everything dialed in on the engines and get the power to spin up the 22P props and not have to get new ones.

Now the right engine seems to be spitting oil around the base of the distributor, running down the back of the block, and is making a mess in the bilge. One more thing to take care of!

MILD THUNDER

Probably just the o-ring under the distributor flange. if you can get a good deal, I say go for those carbs. Those are what they put on the 420's. Just get them jetted correctly and you should be fine. Then buy a full rebuild kit, go thru the 850's thoroughly, and put them on ebay. I hear the air gaps are good, Im not sure about what intake would be ideal for you.

Liberator21

Make sure the carb builder gives you some type of warranty with the carbs, and make sure he tells you exactly how he sets them up (jets, power valves, etc).

PatriYacht

Wieand X-celerators were used on the 420's but the Air Gap may be even better. Just because I don't know anything about your intakes,I would be inclined to change them. You couldn't go wrong imitating the old 420. They were strong, smooth running engines. With a little more compression ( stock advertised 8.8 to 1) and better exhaust they were an easy 450 hp.