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Discussion Starter · #1 ·
Here is a good one for you blower guys. I have a 8.5 to 1 468 BBC w/hyd roller cam. Motor is on a run up stand and did run very good. Other than the cam this motor is pretty much stock. I have installed a USED Weiand 6-71 on it. It came off a SBC so I had to buy the Intake, pully parts, and belt to make it fit the BBC. The intake is new. The carbs are matching 600 Holleys. I am running 39 tooth on the blower and 35 tooth on the crank, 9 percent. Here is the problem: As the motor turns over I can feel the vacuum pull through the top of the carbs but as soon as the engine starts, that vacuum pull goes away and it dies. The RPM is very high which I am sure is due to a lean condition. It is just burning the fuel I pumped in it to get it started. Carbs are fresh but not mine so I changed to a single 850 that I took off a running race car to eliminate the carbs as the problem. No change to the running condition. It acts like a vacuum leak so I siliconed the blower to the intake and the carb adapter to the blower. NO CHANGE AND GETTING FRUSTRATED. Got any ideas??????? Sarge
 

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AKA Lee in KC
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Running a 6-71 on a 468 at 9% under-driven you are making absolutely no boost. You need to run 1:1 just to make 2 pounds. In my opinion, a 6-71 is too small for a BBC. To make any appreciable boost you're going to have to over drive the blower by at least 10% (5 pounds). The faster you spin the blower, the more heat you add to the charge. Heat lowers your detonation margin.

Also, the interface between the blower case and the manifold is critical. You should use a gasket made for that purpose and carefully torque the mounting bolts. You do not want to distort the blower case as you can cause the rotors to be misaligned, put odd loads on the bearings, and have leaks.

I'm guessing that you are actually restricting the motor's breathing with that small, under-driven blower.
 

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Discussion Starter · #4 ·
I'm not trying to build any boost really. This is going to be for show and sound. I have 3 go fast cars so don't really need another one. What is the blower supposed to be torqued at??? I will try changing pullies but don't really understand what that has to do with loosing the vacuum signal after the engine starts!!!
 

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AKA Lee in KC
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msggator said:
...don't really understand what that has to do with loosing the vacuum signal after the engine starts!!!
Probably has nothing to do with it... just an old hot-rodder making an observation... I'll go back to my room now. :rolleyes:

Oh yeah, here are the torque specs from BDS that I used with my 6-71.
 

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Get in, sit down, hang on
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My 454 is naturally aspirated.
Holley 750 (3310) on an RPM Air Gap intake.
My son and his buddies stopped by yesterday and wanted me to fire it up.
It's got an electric fuel pump, so I'm reasonably sure that the bowls were full.

Usually it's two pumps, and kit the key ... and it fires right up. It will die if you let off without giving it a minute to warm up a little.

Well, it's been quite cool here lately, and the temp in the garage was around 50° F. Two pumps, crank ... nothing.
4 pumps, crank ... nothing. WTH?

So I snapped the choke closed, cranked breifly, and away she went.

The moral of the story?
454's like fuel ... we all know that.

I know ... it's probably MUCH warmer than 50 in Texas, but you've got a lot more distance (and volume) between the carb(s) and the intake valves than I do.

Try the choke, and see if it helps.
 

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Discussion Starter · #8 ·
Thanks for the torque specs. Any idea I get from anyone is worth looking into. This motor runs but only for a couple of seconds then looses its suck from the carb so the idle circuit is not receiving a signal which means no fuel. Sarge
 

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Discussion Starter · #10 ·
Thanks for the message. A lack of fuel is why it won't stay running, I'm sure, but the lack of fuel is because there is no vacuum through the top of the carbs to signal for fuel. Sarge
 

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AKA Lee in KC
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msggator said:
...there is no vacuum through the top of the carbs to signal for fuel.
Not that I doubt you, but I just don't see how that is possible if the blower is turning. On mine, I can produce "suck" at the top of the carb just by turning the blower input pulley by hand (with the belt off, of course). Roots blowers are positive displacement, meaning if they are turning air is being moved. There has to be a huge leak somewhere between the carbs and the blower. Do you have all the vacuum ports in the carb bases plugged? Did you check the gaskets between the carbs and the adaptor plate? Is there a gasket between the adaptor plate and the blower case? (Ooops, just re-read your original post about the silicone... probably not the best solution.) Does the adaptor plate have any vacuum taps and are they plugged? Could the adaptor plate be warped, or even the blower case? Maybe a crack in the blower case or one of the end plates not sealed?

When I start my 6-71 SBC cold it takes a lot of gas to get it started and keep it running. It won't idle on its own for probably five minutes until it gets some heat built up. Until then I have to keep jazzing the accelerator pumps.
 

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EFI Rules and Carbs Drool
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msggator said:
As the motor turns over I can feel the vacuum pull through the top of the carbs but as soon as the engine starts, that vacuum pull goes away and it dies. The RPM is very high which I am sure is due to a lean condition.
How can the RPM be too high if it dies when it starts?

My guess is your starving it, swap the pullies top to bottom so you will be at least 9% over.

According to the BDS charts, they don't even reccomend a small bore blower at that ratio underdriven. Wonder why?

http://www.blowerdriveservice.com/techcharts.php
 

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Discussion Starter · #15 ·
Thanks a ton for all of the info. I will try to address the messages I received one at a time. XNTRCI-T I wasn't really fond of the silicone idea either but I had to be certain there was no vacuum leak. All gaskets are in place and sealed, and all carb vacuum ports are plugged. Like yours it does have vacuum when I turn it over. I have built every race motor I have ever run so I understand how a motor works. If the piston is on the down stroke with the intake valve open it HAS to be sucking.... This situation makes no sense. I checked for a warp condition on all surfaces, including the new intake, and found nothing. At least nothing a gasket wouldn't do it's job on plus the silicone. Also looked for cracks, nothing I can see. Keep thinking!!! Sarge

DAWG: I am going to give them a call again. On hold last time for about 20 minutes. Guess they have one guy for the entire world. Did not do a leak down. This motor runs great naturally aspirated.
Sarge

Arrowhead: It starts at about 3000 RPMS not at a normal idle. I am going to change to pullies around. XNTRCI-T has already chewed my but over that. You guys win. LOL

F-BIRD88: This motor runs about 2 seconds. How would I get a timing light on that??????
 

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AKA Lee in KC
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How are you determining that there is no vacuum through the carbs after the motor fires? By the way, what is the current carb configuration?

BTW, I would not expect the Weiand people to be very helpful. You have a very unusual problem (or you're leaving out some critical information). If anyone can figure it out, it's F-BIRD. The boy knows his motors.
 

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Discussion Starter · #17 ·
F-BIRD'88 said:
Again what cam is in it? (ya it matters) What are you setting the timing at and what rpm are you warming up the engine at? or attempting to warm up the engine at?
Have you verified that you do have compression in all 8 cylinders? (Compression test)
I am not sure of the cam specs anymore but it is a hyd roller. I built this motor 4 yrs ago and it has been bagged ever since. I finally decided to start the motor, so put it on a run up stand and started it. After tuning, it ran and sounded great. An opportunity for a 6-71 came along and I jumped. I've always wanted to try one and I have a '68 CST orig big block chevy truck, w/air, that is screaming for me to make a change. I dropped the dist. in so I could start the motor but I can not tell you what the timing is set at because the motor has never (since the blower) run more than a couple of seconds!!! I can't tell you any temps because of the couple of second thing. I have not done a compression check because the motor ran great naturally aspirated. I know the 6-71 is a little small for a BBC, if you're looking for all out performance, but I'm just going to putt around town and go to car shows. I want it to be mild, dependable, sound good, and look cool.
Sarge
 

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Discussion Starter · #18 ·
XNTRCI-T said:
How are you determining that there is no vacuum through the carbs after the motor fires? By the way, what is the current carb configuration?

BTW, I would not expect the Weiand people to be very helpful. You have a very unusual problem (or you're leaving out some critical information). If anyone can figure it out, it's F-BIRD. The boy knows his motors.
I can't feel any when I put hands over the carbs. 2 600 holleys inline. I'm am leaning on the carbs being the problem though. They are not blower carbs and I think the motor is starving. Accelerator pumps will keep it running if I really work the accelerator hard but RPM really gets out of hand. I hate running a cold motor that hard.
 

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Livin in ChevHelle
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msggator said:
2 600 holleys inline. I'm am leaning on the carbs being the problem though. They are not blower carbs and I think the motor is starving.
Ive never built/ran a blower engine..what are the diferences between the two styles of carbs? Normally aspirated vs forced(blown)? I understand the intake signal is diferent, but is the change in metering? Idle circuitry?..
 
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