More Carb help - Demon, Holley or Q-Jet

[fastcat]

I know everyone is tired of talking about carbs, but.....
Just when I thought we had decided on the new carb for the boat……more questions.
Facts: ’68 Classic 18; V-Drive; SBC 350; with we’ve been told, a Chevy high performance intake.
We’ve had good recommendations from board members for the BG Demon 650or another Holley. Now a friend of my husbands that builds race cars said, we don’t need 4 bl or double pump. Throw a 2 bl Q-jet on there with vacuum advance and forget about it.

I like the BG Demon 650- heard it works good, (and it looks good) I think we need 650 cfm.
Husband likes the Holley - because it’s cheaper- but it's 600 not 650cfm.
We have no opinion on Quadrajet.

I want to know if anyone out there has experience comparing 2 or all 3 of these carbs with a SBC . We need recommendations and what’s up with 2 barrel vs. 4 barrel; Vacuum secondary vs. mechanical.
What’s important is reliability and ease of use first, then performance. We’ve had too many one-way outings recently because of carb failure. We say that speed and performance isn’t important, but I know that once we get the kinks out- it really is.

[mrfixxall]

I know everyone is tired of talking about carbs, but.....
Just when I thought we had decided on the new carb for the boat……more questions.
Facts: ’68 Classic 18; V-Drive; SBC 350; with we’ve been told, a Chevy high performance intake.
We’ve had good recommendations from board members for the BG Demon 650or another Holley. Now a friend of my husbands that builds race cars said, we don’t need 4 bl or double pump. Throw a 2 bl Q-jet on there with vacuum advance and forget about it.
I like the BG Demon 650- heard it works good, (and it looks good) I think we need 650 cfm.
Husband likes the Holley - because it’s cheaper- but it's 600 not 650cfm.
We have no opinion on Quadrajet.
I want to know if anyone out there has experience comparing 2 or all 3 of these carbs with a SBC . We need recommendations and what’s up with 2 barrel vs. 4 barrel; Vacuum secondary vs. mechanical.
What’s important is reliability and ease of use first, then performance. We’ve had too many one-way outings recently because of carb failure. We say that speed and performance isn’t important, but I know that once we get the kinks out- it really is.

a two bbl is mainly for torque and you will loose top end speed.

If the engine has a 4bbl intake on it stick with a 4 bbl carb.

A 650cfm with mecanical secondarys should work,the bg carb your looking at will flo up to 830 cfm..

dont put a 600 cfm on the boat,not enuff fuel and you may run into a lein condition and pound the intake valves into the valve seats and distroy the engine..

vacume secondarys are intended for cars with automatic transnissions to eliminate the boug at full throttle acceleration.

on a vacumeecondary carb,the secondarys usually come in at 3500 rpm but you can buy a adjustable secondary spring kit for 50.00 bucks,may as well put that money into a better carb...

[mrfixxall]

that makes absolutely no sense what-so-ever... airflow ability and fuel volume delivery capability are two entirely different and unrelated things... just because a carburetor cannot deliver all of the air that an engine may want to ingest does not mean it will go lean (which is a lack of fuel delivery for the amount of air thru-put)??? as long as there is enough fuel to achieve the proper a/f ratio you are just peachy... :boggled:
if this was the case.. all of those 390 cfm equipped sbc's in roundy round wouldn't make it half a lap...

Rootsy,im just sharing imformation...one of my customers has a formula with sbc in it...the guy he used to work on it put a set of 600 cfm holly carbs on it and thats what it did,pounded the valves out of the heads on two engines..so all i did was replace the heads and installed holly 750 cfm with mechanical secondarys on it and the boats been running great for the last 3 seasons...

as for a roundy round cars their limited to a certain valve lift and only run short distances not coutunious full throttle runs.:shades:

[Carl C]

I vote for a 650 Holley double pumper. Remove the choke plate and cut off the choke housing to improve airflow a bit. Many vacuum secondaries never open. I have proven this on cars by removing the hood and attaching a small piece of mechanics wire to the linkage. When the secs open you will see the wire go up. Often you must put a MUCH lighter spring in the vacuum diaphragm to get them to open.

[gcarter]

This is the problem I'm running into with the Minx and a 600 ( but not pounding valves) just running out of air. Plugs look good, mixture seems good, just won't go over 4400 RPM no matter what prop I'm running.:confused: I'm about to install a Q-jet as soon as I can finish it.

[Carl C]

This is the problem I'm running into with the Minx and a 600 ( but not pounding valves) just running out of air. Plugs look good, mixture seems good, just won't go over 4400 RPM no matter what prop I'm running.:confused: I'm about to install a Q-jet as soon as I can finish it.George, I wouldn't go with a Q-jet. If you have vacuum secs you can test them as I mentioned above. It'd just be a bit different in a boat. Remove the hatch and flame arrester and secure a wire to the sec throttle plate and have a passenger watch it. As mentioned in an earlier post you can buy a spring kit. 600 cfm should not starve your motor.

[mrfixxall]

fastcat,i have a friend that works for bg carb,,he said to give him a call and he will go over what engine specs you have and he will build the carb for that engine....706-864-8544 ext 123 his name is mike...

[Carl C]

Here is what the sec springs look like. George, I'll probably never use these and if this is your problem you can have them cheap. Send a PM.

[fastcat]

So I think I'm hearing...
- 4 barrel over 2
- At least 650cfm
- double pumper?
Still not sure about Vaccum or mech secondary

Carl C- - you said cut off the choke plate and housing- my old carb had no choke attached but figured that was not a good thin. So we don't need a choke?

MrFixxall - thanks for the BG contact. What did you man about "build" a carb? My instincts are to go with a brand name- are you referring to build a generic one?

I haven't heard much here about Qudrajet...is that "old school"?

[BUIZILLA]

. Throw a 2 bl Q-jet on there with vacuum advance and forget about it.no such animal...

JH

[gcarter]

George,
you had a vacuum gauge on that thing at WOT yet?
Not yet, I've been busy w/all the other hull issues.
Some day soon.

[mrfixxall]

So I think I'm hearing...
- 4 barrel over 2
- At least 650cfm
- double pumper?
Still not sure about Vaccum or mech secondary
Carl C- - you said cut off the choke plate and housing- my old carb had no choke attached but figured that was not a good thin. So we don't need a choke?
MrFixxall - thanks for the BG contact. What did you man about "build" a carb? My instincts are to go with a brand name- are you referring to build a generic one?
I haven't heard much here about Qudrajet...is that "old school"?


In other words build a bg>demon carb to the engine specs...theirs all differaant power valve,idle jets,high speed jets and the main jets in the bg & demon carbs that will need to be addressed...you can just put it on and it will probably run good but me being tuner freek i like to get the most out of what i build...

[Cuda]

I haven't heard much here about Qudrajet...is that "old school"?
I'm guessing 90% of the marine engines originally came with a Q jet. I have a couple of them.

[fastcat]

I vote for a 650 Holley double pumper. Remove the choke plate and cut off the choke housing to improve airflow a bit. Many vacuum secondaries never open. I have proven this on cars by removing the hood and attaching a small piece of mechanics wire to the linkage. When the secs open you will see the wire go up. Often you must put a MUCH lighter spring in the vacuum diaphragm to get them to open.


Carl- Are you saying that we do not need an operational choke? The demon says it comes with an electric choke- do we just not hook it up instead of cutting the part off?

[mrfixxall]

[QUOTE=fastcat]Carl- Are you saying that we do not need an operational choke? The demon says it comes with an electric choke- do we just not hook it up instead of cutting the part off?[/QUO

no you stil have to hook up the choke wires or the choke will stay closed..

most of the racing carbs have no choke or choke horn,they machine them off for an extra 25cfm..

the carb im running is a proform which i like vary well,it has a bunch of extra goodies on it that the others dont and it too has no choke(you dint really need a choke in the summer usually one pump will do when i first start the engine and never have to pump it when its warm..just a little tap on the starter and its runnung..

[Carl C]

Admittedly this is a car engine. It starts fine without a choke and I drive it on days when you wouldn't think about boating. At worst you have to feather the throttle a little at first. It's just a little trick to help airflow which the choke housing hurts. The double pumper has a mech choke and if you can start the motor without it then cut it off.:) edit: I just noticed a rotten vacuum plug from that pic!

[fastcat]

At worst you have to feather the throttle a little at first. It's just a little trick to help airflow which the choke housing hurts. The double pumper has a mech choke and if you can start the motor without it then cut it off.:)
The carb we took off, which was a non-marine holley dbl pumper, did not have a choke that we could see. We had to feather the throttle quite a bit at first start to keep it running till it warmed up. I was thinking an electric choke might help this?

[Carl C]

The carb we took off, which was a non-marine holley dbl pumper, did not have a choke that we could see. We had to feather the throttle quite a bit at first start to keep it running till it warmed up. I was thinking an electric choke might help this? It may have been modified like mine. The dbl pr should have a big choke housing and choke plate on top. If it's gone there's not much you can do except let it warm up at the dock which shouldn't take long.

[fastcat]

And the winner is......
BG Speed Demon 650; Mechanical Secondary.
Thank you to everyone for your input and technical help.
I'll let you know how it runs with the carb change from the Holley.
But first- we have to drain our gas tank and flush it before we run the new carb as we have sediment in it--not rust luckily- just old stuff getting sucked up.

[fastcat]

Yes, it's a spin-on. I think the one we had been using was a Sierra.

[fastcat]

Sierra P/N 18-7846

[Donzigo]

I'll tip toe into the fray. . . . . I feel sure of myself on issues on which I post; but, in the land of carbs, I am a stranger in a strange land. I love this post and I wish I understood all of what I am reading about.

I have owned several 350s and 454s. All of my 454s came with either a Holly 600 or an Edlebrock (it was also a Holly). The 454s all topped out at around 4,400 rpms.

I haven't ever had fuel problems or valve burning problems. Am I just lucky?

And, if the engine is rated at 4,400 rpms and I am turning 4,400 rpms, what's not to love?

I have rebuilt the Holly 600. It's a pretty simple piece of equipment, power valve, etc. etc. But, when I go to the Holly website, there are many different Hollys from small to big. I just don't understand the difference between the sizes or the need for so many.

Gentlemen . . . . . very interesting discussion . . . . I'm learning a lot . . . . more, please!

[Lenny]

Carl- Are you saying that we do not need an operational choke? The demon says it comes with an electric choke- do we just not hook it up instead of cutting the part off?

On my BG 650 Mechanical, electric choke, we did not hook up the choke wires, simply adjusted it by dialing the assembly back so that it is always open.

[fastcat]

[QUOTE=Donzigo]..... in the land of carbs, I am a stranger in a strange land. I love this post and I wish I understood all of what I am reading about.
Donzigo- I'm with you. There are too many choices and variables. Give me a set of Triple SU carbs and I'm good to go.