Need Carb advice
 

I have an automotive carb on my ZZ502/502. The carb has the automotive type vent tubes. My engine guy says it's ok to remove the existing vent tubes and put in "J" tubes??

The existing carb is a Holly 850 with vacuum secondaries. The engine runs good as is. So I don't want to change carbs if it's not necessary. But I want to be Coast Guard approved, should anything happen.

I've read that putting J tubes on an atomotive carb will affect performace at higher rpms. Is that right?

Advice would be appreciated - Thanks

Performance will be fine as long at the spider valve bearings do not become oxidized. Oxidation here will cause a loss of atmospheric pressure to the kunuter assembly which can be detrimental to performance.

BT :cool:

I'm really glad you brought that up, Dave. The wife agrees. Especially about the precise amount of atmospheric pressure to the kunuter assembly. And as a bonus, that may remove some of the bend from the "J" tube.

Bill

Put J tubes on it.
If you run a mech fuel pump, make sure it is a marine one and run the diaphragm vent to the flame arrestor.

mcollinstn, it is a marine mech fuel pump. The pump, marine ignition and all bolt on's were removed from the 7.4 originally in the boat, and were bolted on to the ZZ.

I'll check to see where the vent line goes. Think it might be going to the carb. Thanks

Kid,

Marine carbs have a "port" for the pump diaphragm vent. You can just add a fitting on the f/a though. Just be sure that the fitting would spit gas INTO the carb throat in the event of a diaphragm rupture. Same idea as J tubes. Spitting fuel MUST be directed into the carb throats.

mcollinstn,

The boat is still covered for/from winter and not sure where the vent goes to now. But I know the vent tube is not going to the flame arrestor. That's why I think it may be going into the side of the carb body.

I'll check tomorrow and make sure it's properly plumbed to the flame arrestor, if it's not going to the carb.

Thanks for the advise and diagram.

Just another example of why this place rocks!

Rag's :D

Side of the carb body is fine if it is ported above the throttle plates. Just make sure it's not dumping into a vacuum tap, as it's not good for the "atmospheric" side of the diaphragm to be under 30" of vacuum on a hard decel.

Ok, I know what & where the throttle plates are. But not sure where the vacuum tap/s are. {I'm a newbie}. If it's not raining or snowing tomorrow {later this a.m. that is} I'll check to see where the fuel pump vent is going to. If I have doubts or questions I'll post a pic. Thanks again.

Hey Rags, What's up?

not much cant wait to get my resin bucket in the water and test my mod's out. Is it summer yet? :D

rag's

Nope. Not summer here....sucks. What mods did you do? Still have the Nova?

I've never tried it, but have read where marine carbs are factory calibrated differently to account for the increased negative pressure created in the float bowls due to the positioning of the j tubes relative to the airstream. Makes sense to me. Maybe this can be tuned out with different idle screw adjustments or float level changes, I don't know. Anyone?

BT :cool:

Anything you do to a motor, it's exhaust, the carb, the flame arrestor, the engine compartment vents, the air temperature, or the humidity will affect the carb calibration in some manner.

Most tiny changes will go unnoticed.

The J tube deal will indeed create a higher negative pressure in the bowls with increased airflow. What I mean by that is that at idle there will be no change - at WOT there will be a bit of negative pressure in the bowls in comparison to the standard slash cut vents. This will in some cases require a fatter main jet.

Float level change affects calibration all the way across the operating range - keep a normal float level. Possible main jet changes, though.


Like any carb change or engine mod, you gotta check (the plugs) and tweak.

But you need J tubes in a boat. No 2 ways around it.

I would ask them what they planned to do for calibration changes to account for the float bowl pressure change. If it is nickerson you are using, maybe they have a tried and true calibration change that negates the affect. Whatever you do, just make sure you fully lube the kunuter assembly before returning to service :D

BT

Dave, thinking about selling the ZZ carb and buy a marine carb from Eagle as you suggested before. That way I'm out only a couple hundred $. However, I know Nickerson has a good reputation and feel that if I give them all the specs on the ZZ they'll set the carb up properly. Not sure about an "off the shelf" carb out of the box being set up right for the ZZ.

Boating season will be here in a month or so, gotta make a decision. Hate these executive decisions :(

Life was certainly easier back in the day when the kunuter was in charge of all decision making :D :D

BT :cool:

LOL. I got married 8 years ago. The Kunuter stopped making the decisions 7.5 years ago.

mcollinstn brings up another good point re: future exhaust changes. No doubt I'll be dicthing the stock Merc stuff and probably be putiing on the Revolution Marine manifolds. Don't expext a huge gain from that, maybe somewhere in the +100 to 200 rpm range. But that combinded with the J installation will add to the mix. Maybe best to feed all the info to Nickerson and let them have at it. What do you guys think??

Without a doubt if you give Dean your specs you will get a good running carb. When I put a B&M 250 on my 502 I had him put everything that I would need in a box, head gaskets, cam, lifters, rockers etc. Sent him my carb and they all passed each other in shipping. Put my engine together, hooked up the fuel and set the idle. That was it. Perfect plugs all across the range. Money well spent. Yes you could probably fiddle fart around and get it right. I had done that in the past and 72 plugs later it was OK but the money spent was worth it.

Tim T.

Ya still have the Nova. It is cheapest to keep it but I would sell it to the right person :D that really wanted it.
Well I did CAM, full port and polish 502 heads, intake, carb, and exhaust to my 330 hp motor.
Rag's

Very nice, Rags! That ought to give the Nova a little more attitude ;)

Thanks for the input BadDog. I just might go that route. Life {and boating season} is too short to be messing around.

Wouldn't it be easier to forget "J" tubes and just run a hose from vent to vent with a hole carved in the top of the hose secured by the air cleaner stud? That will eliminate a neg pressure in the pri/sec bowls.

Roby

Just putting "J" tubes in the carburetor does not make it USCG legal. There are changes that need to be made to the throttle shafts. As far as the tuning goes, it's not just a jet change. Since the pressure will be different in the float bowls then what the carburetor was designed to work with the Air bleeds, and in some cases the emulsion circuitry needs to be changed to get the proper amount of fuel to the engine at wide open throttle.

Looks like I'll be giving Nickerson a call this week.

Thanks everyone. I appreciate the input.

Just buy the 800 Holley Merc uses should be some used ones around if you look put in a halon to be safe.Car carbs tend to be a little lean at midrange in boats. What cam are you using you may have reverson issues with the one that comes in that eng.

Boxed up the carb and shipped it to Nickersons today.

BadDog, when they sent your carb back, did they include anything indicating what they did to it?


Thanks again everyone for the advice.

Bill

Buy fuel injection.... its easier than f*cking around with carbs

hehe, and those with injection are taking them off and putting carbs on.

Bill,
Mine required a total rework, something about drilling this and plugging that and so on. I have tuned several carbs in the past but for a blower application I decided to leave it to Dean. Your carb will have a unique number and he keeps records on what was done, jetting, PV nozzles etc. He will provide you with that if you ask. I think you will be pleased with the results.

Judging from the good rep they have it seems like the carb job will be a good one. Thanks BadDog.