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Cadillac 500 DD Airboat

Slidin Gator

Well-known member
Disassemble the distributor (knock pin out of cam drive gear on distributor). Spot weld the mechanical advance pins into place. I have had several summit distributors fail due to the pressed in pins coming loose. Reassemble with proper shimming between case and cam gear to minimize end play, Summit crap is always loose.

I agree on the point of eliminating vacuum advance for now. But vacuum advance will make a frog machine sip fuel.

Describe the hesitation. Does the motor hit a wall for a second and then kick in (vacuum secondary), or do you punch it and the motor falls off for a second and then kicks in (mechanical secondary, not enough accelerator pump).?

I’m guessing mechanical. Make sure accelerator pump linkage is set to max charge. Set secondary linkage to bring them in late, after a good shot from accelerator pump. Try limiting max opening of secondaries, it limits air, adds vacuum which’s adds fuel on secondary. Redneck tune.

That Edelbrock is terrible for your power needs, but I would hesitate to go back to cast iron on the stern. You need to be moving weight forward not back. 1900 to hold plane is not out of line in a 7’ 3” wide hull and the weight you have. Start with 200 lbs of barbells in the nose and see how she runs. If you don’t like, just chunk em.
 

Slidin Gator

Well-known member
Get a stock cam, send it to Powell machine Inc, 412 Wilson Creek Rd
Iva South Carolina 29655
Have them grind it 178 intake duration, 194 exhaust duration, .233 intake lift, .256 exhaust lift.
Have them put 10° advance into it as well.
Stock intake, 390 holley tuned as I stated above, curve kit in the hei dizzy, 32° total advance, no vacuum advance at all.
Only had a fleet of 10 dd Caddys running this configuration with 80" wood props on tour boats 24' long x 12' wide at bottom, twin engine on each tour boat, circa 1989 through mid to late 90s carrying 25/30 people on tours, if you took one out empty, it would try to blow over at 60 mph. That was fun standing on both pedals to drive "fly" it back on to the water, cleaning your shorts back at the park.
Mr. Fenner is absolutely correct, if you want to go all in. None of us get our stuff to perform without modification. If you just want to get on the lake and read a book, then tune what you have. What you have want's more fuel and probably less air.

Here is where I get my Qjet parts from.
Carburetor Parts & Kits for Carter, Ford Motorcraft, Holley & Rochester Carburetors
 

John Fenner

Well-known member
Disassemble the distributor (knock pin out of cam drive gear on distributor). Spot weld the mechanical advance pins into place. I have had several summit distributors fail due to the pressed in pins coming loose. Reassemble with proper shimming between case and cam gear to minimize end play, Summit crap is always loose.

I agree on the point of eliminating vacuum advance for now. But vacuum advance will make a frog machine sip fuel.

Describe the hesitation. Does the motor hit a wall for a second and then kick in (vacuum secondary), or do you punch it and the motor falls off for a second and then kicks in (mechanical secondary, not enough accelerator pump).?

I’m guessing mechanical. Make sure accelerator pump linkage is set to max charge. Set secondary linkage to bring them in late, after a good shot from accelerator pump. Try limiting max opening of secondaries, it limits air, adds vacuum which’s adds fuel on secondary. Redneck tune.

That Edelbrock is terrible for your power needs, but I would hesitate to go back to cast iron on the stern. You need to be moving weight forward not back. 1900 to hold plane is not out of line in a 7’ 3” wide hull and the weight you have. Start with 200 lbs of barbells in the nose and see how she runs. If you don’t like, just chunk em.
7'3" beam, measured at top I'm sure, I bet the measurement at the chine bends at transom are only about 6'4"/6'8" if that.
That cast intake likely weighs less than 30# and will gain the velocity back, plus, you can turn it around to where primary is in rear for a simpler pull on cable.
Heck, boats get a sheet of poly riveted or bolted to the hull, the weight of a grown man on the bottom to get moving, not to forget the waterlog in the voids.
 

SWAMPHUNTER45

Well-known member
Well keep us in the loop for your progress. Don’t expect a miracle with these tweaks but when you take 2 or 3 small improvements it can give you a good boost.

Do not let anyone put more than 32” timing in. If they try to treat it like a Chevy and power time it or set it at a higher advance you will get undetected detonation and damage the engine. Make sure your running fresh Premium fuel as well. A pump fuel with alcohol degrades quickly so we suggest you don’t fuel your boat until your ready to slide if using pump gas.
 
Well keep us in the loop for your progress. Don’t expect a miracle with these tweaks but when you take 2 or 3 small improvements it can give you a good boost.

Do not let anyone put more than 32” timing in. If they try to treat it like a Chevy and power time it or set it at a higher advance you will get undetected detonation and damage the engine. Make sure your running fresh Premium fuel as well. A pump fuel with alcohol degrades quickly so we suggest you don’t fuel your boat until your ready to slide if using pump gas.
Update. Reduced blade pitch by 3 degrees. Remember, I had added about 2 degrees earlier trying to gain speed, but lost due to lack of HP. Now with the blades repitched, I can finally hit the 3000 rpm max mark. Had three Carbs to play with. Started with the original Q jet that I had the Carb guy tweak for optimal performance. It worked OK at the bottom end, but when the secondaries started to open, hit a big lag ( I assume this would be caused by too much cfm as the secondaries look to be about twice the size as the primaries). Then mounted the borrowed Demon Carb and just after startup, I began to throttle up quickly and got a little backfire (first time ever) which caused a little fire in the carb so stopped at that point and pulled it back off (don't want to hurt something I don't own). Finally installed the borrowed Holley (new)
which is a 650 cfm, and it ran up just fine right out of the box. Primaries will get her up to 2700, and when I push it a little harder, the secondaries open just a tad and I'm up to 3000. Yes, it sounds pretty mean too! As for getting up on a plane, what used to take 250 feet is now under 100. Tried some quick turns from an idle speed, and seems to be doing just fine now, at least enough to make me nervous of allowing water over the side (and I have 20" freeboard). Since this is a new experience, I will have to play a little with both my and the boats limits as to what I feel comfortable with when it comes to sharp turns. I'll be doing this in the shallow areas (which by the way, is plentiful in our little 25 square mile reservoir, where a majority of the water area is less than two feet deep. This area was pretty much flat farmland 80 years ago and when local flooding downriver got to be too much too often, the Corps of Engineers damned up the river and created this flood control reservoir, thus it might be 20 feet deep right by the dam, but once you pull away from it, it begins to shallow out pretty fast).
I left the Edelbrock performance intake on, and left the new Distributor in place, and with the new shiny Holley in place, the boat looks updated, and seems to be performing well enough for me at this time. Have only taken it out once, and even with small swells from distant boats slowing me down a little, I was able to attain 39 mph, a personal best. Good enough for now. Keeping an eye on the temp, but after two hours on the water, never broke 195.
Attaching a couple pics. I did measure the chine and it came in at roughly 6' 11" (Mr. Fenner was wondering)
Also, I have a power tab that helps tremendously when a little breeze adds some chop to the surface and I need to force the bow down a little to smooth things out.
A side picture for Slidin Gator to show that the majority of my weight IS in the last 1/3 of the Hull. With a 25 gallon tank, two batteries, engine, and two passengers puts my rear weight around 1420 pounds, not counting the cage or hull. (I'm allowing 800 pounds for the engine, but just a guess). Once on plane, it does take about 2100 rpm to maintain it, depending on the surface conditions.
Thanks for everyone's advice and comments!
Ps - would like to discuss radiator options in the future. The way my hoses have to cross great distances seems a bit weird and looks dumb,
and I wonder how much air intake is blocked from the props with a 36" X 20" radiator sitting 5" away from them.
 

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John Fenner

Well-known member
Ok, 6'11" on bottom at chine bends not too bad, reviewed all pictures of the boat and setup, not too shabby,,, ok, my advice is this, at 48" forward from transom edge,, the bottom must be the exact same profile as the transom laterally to the length of the hull, this will fix planing issues in which it seems to be the issue at hand, the hull is plowing,,, the 650 might feel good but way too much CFM for response on a DD
 

hdsadey

Well-known member
Mr Fenner what would you think about a Holley 2 barrel 500 for this application? I think throttle response would be a world of difference on a 2 barrel. Maybe combined with a tall spacer on top of the adapter plate? Velocity surely would increase. I made this one out of maple to test, it's 2 inches tall.
 

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John Fenner

Well-known member
The velocity needed is at the boosters to provide a crisp response, meaning,,, once the throttle plates are cracked open the pilot circuits are active, transferring to the main jet feed to atomize the fuel efficiently, if tuned right you won't need accelerator pump action at all.
 

John Fenner

Well-known member
Took a 4.3 V6 from a truck, TBI intake, built a plenum to fit a monojet, with sub stacks into the intake to build velocity, would spin a 72" prop to 3000 in an instant, no hesitation.
 

hdsadey

Well-known member
Are you in agreement on the 2 barrel option? I'm confused lol. I agree on the accel pump in a slow opening but upon a rapid opening the accel pump is there to take up the lag in vacuum response no?
 

John Fenner

Well-known member
CFM is just that, tuning these things is a science, as per your question, the picture on the left shows 500 CI at 3000 RPM,,, the recommended CFM is 368.92, round that up to 390 CFM, proper tuning with jetting etc will give best performance and efficiency, think about it this way, you won't be opening all 4 barrels constantly. So no, 500 cfm is too much air.
 

SWAMPHUNTER45

Well-known member
Real brief

You want a radiator that does not have a cap.

The radiator should be isolated to guard against vibration.

If you start trying to rethink the wheel and move the radiator out of the flow of air your engine will run hot. Cadillacs don’t live long hot and lose power. A big oversized radiator keeping your engine around 155-165’ is where we try to keep them.

You will get 10 different opinions on buying your radiator from this guy or buy it from that guy. I have run custom units and off the shelf as well. If mounted properly and sized to meet the need your usually good. Use a coolant / antifreeze and don’t over fill it. An expansion / overflow type set up is best.

As far as carbs go if it’s running good with a 650 Holley leave it alone. Just put a good filter in the line.

Enjoy your summer
 

hdsadey

Well-known member
CFM is just that, tuning these things is a science, as per your question, the picture on the left shows 500 CI at 3000 RPM,,, the recommended CFM is 368.92, round that up to 390 CFM, proper tuning with jetting etc will give best performance and efficiency, think about it this way, you won't be opening all 4 barrels constantly. So no, 500 cfm is too much air.
Roger that. So your suggesting the 390 4 barrel? Never got to play with one of those.
 

SWAMPHUNTER45

Well-known member
I was able to view the pics of the radiator and lines. For a Cadillac the way it is set up is fairly common. The unit is mounted inboard of the rigging and appears to have been in place for many years. If you can find an exact replacement radiator I would probably go with that. (Check Northern Radiator)
In the pic you can see that debris blowing around had impacted a large percentage of the cooling fins. This bends them and restricts air flow. You would probably run a few degrees cooler with a fresh radiator.

Short of making sure it is isolated to protect against vibration I would not move it.

Bring it staying cool and running good I would put quality fuel in it and ride.
 
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