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Discussion Starter #1
I had a pair of 502's built for my cat. Engines are .020 over 502's with 10.5:1 comp. Brodix BB2+ heads and matching intake, cams are hyd roller .615 lift 310 dur n/a cams. Carbs are Q950 quick-fuel. jesel lifters, shaft mount rockers etc, also msd ignition, pro billet distributors. All parts are brand new.
This is my 1st experience tuning a carb'd n/a motor, I'm a turbo/efi guy and I'm still looking for the data port on those carbs! Lol
Here's the problem, they wouldnt even pull themselves with the factory jetting. It had 75's in front with power valve, and 86's in back. I swapped to 86 front and 94 rear and it was better. I still get a lot of popping out the exaust below 2000 rpm. Remember a boat is under load off idle.. the engines run fine over 2000 rpm with this jetting. I tried 90 front and 95 rear because I was told the spread was too big. Now the back of the boat is black, still had popping just off idle. I'm going to go back to the 86 front and try 90's rear, but I know it's still going to pop. What's causing this? I know nothing about carbs or air bleeds so help me out!
Also what kind of power should these make? I was hoping for 650
 

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Factory jetting on the HP500 which is where I would start is 81 port primary, 75 starbord primary, 89 port secondary, 93 starboard secondary.

What exhaust do you have on the engines. Wet exhaust and big cams dont' get along to well. The heads you have are quite large and as you said it is loaded at idle and you have no gear change to help it.

Also on Q950 are these marine carbs? If not and this is a closed engine hatch boat you are gambling with your life. I have seen first hand what non marine carbs and distributors can do. That man never got to see his kids grow up, the parts cost his wife and children their lives.
 

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EFI/N2O JUNKIE
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Hell yeah Chris, the explosion and fire can be quite spectacular, not something you want with your butt a few feet away!
 

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Marine carbs with “J” bowl vents use different jetting than the same carb with common straight tube bowl vents. Depending on flame arrestor configuration, larger jets are usually required with the “J” vents to have the same AFR as straight tube vents.
 

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Long Live The King
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install a 3.5 or 4.5" PV

make sure corrent PV's arent blown.


Jon
 

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If it idles OK and the problem is only in the “just off idle” area, the idle circuit may be too rich. Can you post the calibration of the idle circuit, idle jet and idle air bleed sizes? Can you measure the length and width of the T-slots and describe the bottom end of the slot location vs. the butterfly position at the adjusted idle speed

When you are in the problem throttle position, is the boat riding bow-high such that the carbs are tilted with the primary bowls high, therefore spilling fuel through the mains?
 

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Discussion Starter #7
Ok. Idle screws are 1 full turn out.
And FYI I'm not a fool. It's not a closed hatch boat, but they also aren't marine carbs.
Front bowls have 45 powervalve none in rear. They are good.

YETI- you seem to be on to something, but I have no idea where to look for what your talking about. Help please
 

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Are the air bleeds the screw-in type? What size are the idle air bleeds? Do you have a spec sheet that came with the carbs or can you get the info from the carb builder?
 

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Discussion Starter #10
It's full water jacketed dry exhaust, the boat runs fine once it's going. Been 110 gps first time out, but sounds terrible getting to 15mph
 

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Discussion Starter #11
I'll take a look and post the air bleed sizes as soon as I get home. Yes they are screw in. No build sheets just off the shelf Q950 street and strip carbs.
Timing is standard msd mech advance with a msd marine ignition box. Full advance at 3500 set at 36 deg
 

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I'll take a look and post the air bleed sizes as soon as I get home. Yes they are screw in. No build sheets just off the shelf Q950 street and strip carbs.
Timing is standard msd mech advance with a msd marine ignition box. Full advance at 3500 set at 36 deg
so you most likely have the blue or red bushing in there

@ 36 total you only got 15-17 degrees off idle till 3500+++.....even then it isn't steady till 4500 or so....... either lock out the dist
for full 36* timing or use the black bushing with a weld ball on it smoothed and rounded off so the highest (biggest) part of the bushing measures .485, that'll give you 10* total advance and get you 26 @ an idle...also try and get it all in by 2-2500 rpm. bet it fixes your issues

I do that bushing deal on all my street motor customers and it works very well........... hope this helps

RCM
 

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Yes…^^...what RCM said about timing.

However, I don’t think making the limit bushing out-of-round is a good idea. If the bushing isn’t round and it turns a little it will jamb the mechanism.

I think in the case of this boat, the black bushing and the two softest springs would be the best, unless they don’t retard fully at the desired idle speed. If that is the case, use two blue springs.

Do not mix springs on a GM style advance mechanism. Spring tension must be balanced or the mechanism will be sticky and not retard correctly. The stronger spring will pull the advance parts off-center and squeeze the grease from the tight side of the main shaft and then the mechanism will drag and stick. There will be a several hundred RPM lag when the engine slows down which will make the idle erratic because the timing will not be uniform every time the engine slows down.

If you want to reduce the travel of the MSD GM style advance mechanism the best way is to bend the weights so as to close up the angle between the weight and the lever that rolls on the advance cam on the shaft. Put a piece of 3/16” stainless tubing (nitrous fogger line) in the holes of two weights, grasp the short levers with a pair of vice grips and the weight with a pair of vice grips at the far end from the pivot hole (with the tube in it) and with a torch heat the narrow part of the weight near the pivot hole to a red heat and apply gentle force with the vice grips to close the angle between the lever and the weight. Watch closely to keep weights and levers parallel.

I like to use a red or smaller bushing with weights closed up this way because it makes it so the mechanism runs out to the toe of the weight, furthest from the pivot, and so minimizes the timing scatter caused by torsional oscillation from driving the oil pump.
 

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Another advantage in bending the weight angle is at the fully retarded position it offsets the spring pins to an angle more tangent to (more offset from) the center of the shaft. The offset is such that the leverage is better to pull the mechanism to retard and so makes the retard RPM more consistent.
 

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Discussion Starter #15
Ok this is making ALOT of sense. I don't think the carbs are the issue at all now. The thing idles ad runs great over 2000 when it's got some timing! I'll try this first, and let you know if the takeoff popping is gone. Since it's exhaust popping the retarded timing makes sense. Once it gets some rpm, it rips!!
 

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Ask yourself this; how much load is really on the engines below 3000rpm? If you peg the throttle at 3000rpm, what happens to the rpm?
 

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You will have to retune your idle circuits once you get the timing sorted out. I agree, minimum of 22~24 at idle.
 

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Discussion Starter #20
Should have 24 deg at idle. There's a TON of load on them at idle to 3000!! That's when the MOST load is on em. Over that the boat lifts and the props are partially surfaced.
 
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