Carb decision time - mech or vacuum

[JHuck]

My 383 build is progressing along and now it's time to select a carb. Based on Chris Straub's recommendation, I am going to use a 750 cfm carb. My combination consists of 195 AFR's, Edel RPM intake, GLM manifolds, thru hulls and a custom roller cam in a '95 Baja Islander.

For overall performance, Chris recommended mech secondaries. I have seen similar recommendations on other sites. I have also read some posts that state vacuum secondaries are more user friendly at various speeds through out the rpm range. Since I will be using it part time to tow skiers, this matters to me. What doesn't matter is fuel use, I figure it will be poor or poorer with either set up.

So what is your experience? Anyone gone both ways...from a carb standpoint?

 

[ScottRogers]

Re: Carb decision time - mech or vacuum

Unfortunately a few questions come to mind. First, why so big on the carb? second what kind of prop will you be using? and are you in salt or fresh water? I am asking about the water because of the intake manifold. Aluminum + Iron + Salt = Bad corrosion. The carb. might be a tad to big unless the heads are 2.02's. If you are towing skiers, my opinion would go with vac. secondaries and get the Holley trick kit (if you are running a Holley) and use the lighter vac. spring so the secondaries open a little sooner. Mechanical is fine if the boat is light and it's proped right, but if the weight of the boat is heavy, go with vac. secondaries. I have tried both on my 1988 Liberator 201 with a 350. and I am using a 650 vac. Holley. (some work done to it) I found that the boat responded better out of the hole with the vac. secondaries I am also using a 3 blade SS Viper 14 3/4 X 17 prop.

 

[CharlieB]

Re: Carb decision time - mech or vacuum

If I remember correctly, Edelbrock makes a MARINE version of the RPM manifold AND a Marine Quadrajet, this combination would work very well on your 383.

Careful on your cam selection, maintain lobe seperation at least 114 to prevent water reversion from the exhaust.

 

[JHuck]

Re: Carb decision time - mech or vacuum

Thanks for the thoughts everyone.

As far as using a 750...I had that question answered on an older post of mine. Chris Straub replied,

"Velocity rates in these "stroker" engines at mid to low rpms increase greatly as stroke is added. Piston speed plays a part on the pull on the carb signal. A little larger carb on these engines can actually give you better fuel atomization and in turn make for a more efficeient engine. . .well as efficeient as a boat can be. For what you want to do and the rpm range we spoke about on the phone yes 750 would be better suited."

My engine machinist also recommended a 750. Based on the new intake, great flowing AFR's and decent lift of the custom cam, this thing should really breathe. Thus, I feel good about the 750.

I live in Indiana so nothing but fresh water surrounding me.

Reversion concerns were taken into account with the design of the cam.

I have looked at EFI information before. I know there are benefits to be had but at this point, I've got to stop spending (my wife will be appreciative).

 

[ScottRogers]

Re: Carb decision time - mech or vacuum

Good deal ! I'll tell you, if I lived in fresh water, I would have the RPM intake on my boat. Why not go for the RPM Airgap? Also check out a "Super Sucker" spacer plate.

 

[Tail_Gunner]

Re: Carb decision time - mech or vacuum

My 383 build is progressing along and now it's time to select a carb. Based on Chris Straub's recommendation, I am going to use a 750 cfm carb. My combination consists of 195 AFR's, Edel RPM intake, GLM manifolds, thru hulls and a custom roller cam in a '95 Baja Islander.

For overall performance, Chris recommended mech secondaries. I have seen similar recommendations on other sites. I have also read some posts that state vacuum secondaries are more user friendly at various speeds through out the rpm range. Since I will be using it part time to tow skiers, this matters to me. What doesn't matter is fuel use, I figure it will be poor or poorer with either set up.

So what is your experience? Anyone gone both ways...from a carb standpoint?

I wish i had a option for carbs...mech all the way ..edelbrock ..no brainer.. PSSST...marine enigne's dont work off vacum in any form it's just there....750 thats a lot of carb for a 350..errrr 383

 

[JHuck]

Re: Carb decision time - mech or vacuum

Yep, my machinist recommended I go with an Air Gap if possible. I measured the space below the sundeck and I have plenty of room.

 

[wca_tim]

Re: Carb decision time - mech or vacuum

Your choices are right on the mark for edelbrock or hollet 750 marine carb. Good air cleaner, goor exhuast and you're ready to rock. The only thing I would seriously do differently is exhaust manifolds.

I just came in from checking things over and adjusting the valves... I'm ure you'll be pleased with the route you went. I was thinking about swapping to a set of afr's but just tooo worried about the brackish (and downright salty) water I wind up spending a fair amount of time in. If I dissolve an aluminum intake, that's one thing... a 1500 set of heads is quite another. Am thinking a bigger cam and port job may be the way to go for this year. Look forward to seeing how your toy once it is up and running...

 

[CharlieB]

Re: Carb decision time - mech or vacuum

"worried about the brackish (and downright salty) water "?

You always have the option of installing fresh water cooling, and run those nice alum parts anywhere.

 

[JHuck]

Re: Carb decision time - mech or vacuum

I debated long and hard about the manifolds but once again, I had to cut off the spending somewhere. I figure the GLMs will be a decent compromise. Chris went ahead and ground the cam for stock risers. He said that if I would have chosen full length jackets, I could have gone as high as I wanted with the duration (within user friendly reason of course). Oh well, like I said earlier, this thing should be a lot more fun than last year's set up. Heck, the original carb was out of tune so it didn't even run that great to begin with.

Well, I've got my entire interior in the back of my truck and I'm headed to a talented upholsterer. I laid fresh carpet this weekend and will start buffing the gel coat when my Trewax comes in. The boat itself is headed to the south side of Indy next weekend for a bimini, mooring cover and to have a couple gel coat chips repaired. In June it'll get a new stereo and hopefully a new Mirage prop.

Do you guys here a loud sucking sound?

 

[ScottRogers]

Re: Carb decision time - mech or vacuum

When I repowered my Liberator, I installed GLM manifolds and risers, this will be the 4th season on them, but only 30 hours worth of run time. They installed great, and still look new. I do however remove them every spring. I inspect them and touch them up with paint. Good product for the price.

I was also wondering, will you be running roller rockers? If not, I highly recommend them. Post some pics when you are done !

 

[JHuck]

Re: Carb decision time - mech or vacuum

Yes, full rollers have been ordered. They are Scorpion 1.6 rockers. The cam was ground based on the 1.6 ratio.

I need to decide on valve covers next. Sounds like tall covers won't fit between the stock risers so I'll have to go with stock type. Probably AFR polished short covers.