Kidnova

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

blue thunder

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

Kidnova

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

mcollinstn

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.

Kidnova

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

mcollinstn

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.

Kidnova

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.

ragtop409

Just another example of why this place rocks!

Rag's

mcollinstn

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.

Kidnova

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.