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Discussion Starter · #1 ·
Hi all. I recently purchased my dream car. It has a 383 stroker with a 6-71 (maybe 8-71? It was listed with an 871 but I think it's a 671) blower.
Never owned a roots blown motor. It's a beast, runs great most of the time, but I have a couple issues so I want to make sure it's tuned right and might as well sanity check things while I'm at it. I would describe it as a 100% well built engine that has maybe fallen to 90% in hands of next owner and needs to be checked over. Ya'll helped me so much tune a carb and timing on another vehicle I'm hoping I can learn even more about supercharged motors from ya'll!

First things first I believe would be to check timing and idle speed (idle seems high, jumps into gear too when I put it in drive). What should those be? Idle seems high but maybe that's a supercharged motor thing?

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1923 T Bucket equipped with a B&M blown Chevy 383 Stroker
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Blowers are different from turbochargers in that even at lower rpms, they are still producing boost, being belt driven and all. This could be your issue, just lower rpm boost kicking in, but this happened to me for a period after I had to take my carb off and put it back on. It would jump forward when I put it into drive and idle higher. What it ended up being was the cable connecting my pedal to the throttle assembly on the carb was too tight, causing the throttle to always be open a little bit. Not sure if your pedal is even connected with a cable, my VW's isn't, but it's worth checking out.
 

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Timing is going to be based on cam/compression/gas and ignition system (boost retard?). At idle and cruise, there is no boost. Idle should not need to be anything too high and should idle smooth with no surging. Unitl you know what is in that engine, it is all guessing and could be a disaster right quick under load when boost comes into play.

From the picture, it looks like a single 4 on the blower. I cant see the side of the blower, so not sure which blower (size) you have. Idle speed is only for idle, get it down and see what it likes. The engine will like all the initial advance it can get.
 

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Discussion Starter · #5 ·
Thanks. I have learned a little about timing with boost and was mostly concerned about it being too high. There's no boost retard system from what I can tell. Has an aftermarket distributor with no vacuum advance (assume it's mechanical) and a flamethrower coil. It idles smooth, no surging. I will check and record current timing asap and get back. I was under the impression you NEED a timing retard device with boost, or does it just let you go higher on your initial timing?
I tried getting more info on the engine and I can't. :( The guy I bought from doesn't know. I can see the heads are aluminum, carbs are 80457 Street Warrior 600cfm, Headmen headers, basically what I can see visually. Sounds like a mild cam, no big lope or anything. Is there a way to "measure" what's in the motor without tearing it down?

Side note, working on non-motor stuff too. I want to add power brakes. The weiend manual has a blurb about attaching the power brake booster to the base of the carb. It doesn't state which port. Other forums I've read on it say you can't use vacuum from a roots motor and have to have an electric pump. Why would the weiend say that then? Can anyone verify I can run it from the larger port that's just rubber plugged at the base of the carb? I can get a pic of what I mean, there's one that's got a larger diameter, at an angle, between the carbs, coming right from the actual base that mounts to the blower.
 

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You dont need a timing retard device, but I always found it better as I could have higher advance at idle and cruise to have the engine running clean. Any idea of how much boost is being created? I would keep total timing under 28 degrees until you know more about the engine. Are the carbs boost referrenced? A blower can also calm a cam sound. You can kind of figure the cam lift at the lifter and get close, but make sure the lifter (if hydraulic) is not squishing and throwing off the measurement. I never messed with power brakes. Above the blower may cause too much pull and the booster may not like that. Can put below the blower, but under boost, you will get pressure. There are one way valves that can take care of this issue as well. Learn how to ready plugs and watch them to see what is happening.
 

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Discussion Starter · #8 ·
No power steering yet either. That's down the line, deciding if I want to do a front clip first. Those electric pumps are expensive! Electric booster I have not seen, I'll look that up.
I'll get boost (told it's based on pulley so I can just look that up?) and current initial timing later today. Totally makes sense then, I may need a retard device. There is a hesitation sometimes in the throttle which I ran into on a friends car and that was because it only had half the timing it needed so I'm guessing that could be this too?
The carbs are referenced from what I can tell, based on my currently limited knowledge. I'm still learning!
Here is a pic of what I think is the vacuum referenced setup. The line comes from the manifold past the blower and connects to what are the vacuum reference ports I on the carbs I believe?
This pic also shows the port I was talking about for the booster. Behind the vacuum connection on the left carb in the pic, middle of the base between the carbs, there is a port about twice as large as those with a rubber cap on it.
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Discussion Starter · #10 ·
Haven't heard a back fire. Just sometimes hesitates. If I get on it slowly, or get on it when I'm already going say 30-40, she get's right on it. If I get on it too quick from an idle or just cruising say 10-20, there's a blip then it takes off, but no back fire or anything that sounds off.

Another side note, anybody in any groups I could join around Sacramento CA area? Be great to meet some other hot rodder/classic people, probably also could learn a lot fast that way. I'm planning on some shows (not to enter just to meet and have fun) soon when weather is better.
 

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Discussion Starter · #11 ·
Ok, so far I have looked at the blower info I can find. No identifying marks on the blower itself, but it had BDS pulleys so I looked up a BDS blower. It looks just like the BDS 671, although the seller stated it as an 871, odd.
Bottom pulley is 33, top is 39, looking at charts that's -15%
Looking at more charts, and it's a 383, it would be 8.6lbs if it's a 671 and 10lbs if it's an 871. I'm saying ~9lbs until I can find out more.
 

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You can use the big port between the carbs for vacuum to the brakes.
it is good the carbs are boost referenced, but because they are vacuum secondary the secondary pods also need lids with a hose nipple, so that both pods can be connected so they are syncronized and open at the same time and rate.

The stumble/blip when you hit the throttle is most likely lack of a big enough accelerator pump shot....may need either bigger pump cams/faster rate pump cams or larger pump nozzles("squirters or "shooters") or possibly both.
The easy ability to adjust pump shot is why Holley carbs are the most popular.

What color pump cams are on the carbs now? White, Orange, Pink, Blue, Green, Red, Brown, Black, Tan ?
What size nozzles are on the carbs now? #28, 31, 32, 35, 37 ?

Your blower case length is too short for it to be an 8-71....it is a 6-71.

Beware, there is a lot of exaggeration in the vintage performance car sellers market....too many guys who don't really know what they are talking about buying and selling .....that is probably where the 8-71 descriptor got thrown into the mix
"it looks big....well then, it got's to be big" mentality.
 

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Discussion Starter · #13 ·
Got a little more info, timing coming later. Love the scoop but man it's a PITA taking it on and off, hope I don't have to do that often!
Both nozzles are 31's, both cams are orange on position 2.
This normal to not have chokes or secondary nozzles on these setups/carbs? (The latter must be, there's no threads to have a nozzle on these carbs in that position).
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Common for blower use to have all the choke components removed.

After you've got the timing sorted out
You may need to try either Blue or Green cams, position #1.....they are faster rate cams than the Orange.
If that doesn't fix your hesitation then it may also need bigger nozzles, #35's maybe.

There are no secondary shooters because they are vacuum secondary carbs. Only mechanical secondary double accelerator pump carbs have the secondary nozzles.

You need a pair of secondary pod lids that have a hose nipple, so you can connect the two secondary pods together so they move together when they open the secondarys.
 

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Discussion Starter · #16 · (Edited)
Hi again! I'm back after upgrading to power brakes. Took some time but it's awesome!
Back to motor, still not tuned. I'm learning more about the beast as I go.
Timing is still a mystery, I'll check again today, maybe it's "locked out"? I've been learning about that option.
It's definitely running rich in addition to the hesitation when you get on it too quick. You can smell it, it floods after a run, and I pulled all the plugs and all 8 look the same but indicate rich running.
I read somewhere that I should start at the bottom, fuel pressure, maybe too high? So, I have a regulator but no gauges. Current questions:
  • I'm looking at helpful gauges, either a double or triple under the dash, and a fuel pressure gauge at the regulator. Which of these, or all, should be a necessity on this motor and would help diagnosing? If you added one at a time what order would you add them? I've been looking at (besides that fuel pressure at the regulator) tach, boost, wideband air/fuel.
  • What should the fuel pressure be on a roots motor? I thought I saw pretty low somewhere, like 5-6, but I can't find the thread now.
  • Seems to me a hesitation is opposite of running rich, why am I getting both? Isn't a hesitation a lack of fuel?
  • Would it be normal for someone to use a marine distributor for this application? In my goal to learn as much as possible about the motor, especially whats happening with timing, I took apart as much of the distributor as I could without messing up current timing. That means just cap and rotor off and as many pics as I could get. The model # happens to be straight towards the firewall which in this little car gives me no space to even squeeze a phone in for a pic. It's MSD, mechanical advance, aluminum, and 4 wires, red/orange/black and a separate gray tach wire. I looked and looked and the only thing that matches up is this marine option: MSD 83606 Chevy V8 Marine Ready-to-Run Distributor
 

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I've got a baby blower B&M Powercharger 250 (4-71) on my 406 with dual 600 Eddy's that are boost referenced as well. I run an HEI triggered by a MSD 6AL with a boost timing master, and pull one degree per pound of boost, 28 degrees locked out with an ignition start retard box that pulls 10 degree of timing to start. Any more and detonation becomes an issue for me, can't complain, getting 16 mpg cruising my toy around to shows. Currently set up for 5lbs of boost which for the street is perfect for my needs, idles around 800rpm has instant throttle response with no hesitation backed with a 700R4 with a 2200 stall. One of the best things you can install is a wide band O2 gauge to help dial in your tune. There is a lot going on in a blower motor, it's easy to melt down, so the tuning is so important. Not bragging about the combo, just letting you know there is a lot going on with both timing and fuel metering. It's trial and error and I had my fair share parts not working together.


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Discussion Starter · #18 ·
Thanks, yeah, I feel like I'm going down a rabbit hole, lol. I finally found a pro I'm the area but appointment is April 25th. Now I'm contemplating. Spend $3-400 in gauges and MAYBE figure it out on my own? Bite the bullet and get the Fitech efi? Supposedly fixes all this, self tuning, timing control, air fuel readings, fan control... But so expensive!
Then there's boost. If I want to change that at all do I do it first before tuning? Will that change richness? He kept mentioning I could swap the pulleys for more boost, could it be he swapped to underdrive for sale and that's why it's running rich? Because it was tuned for more?
 

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With FiTech make sure you get the 2 bar map sensor for boosted applications. But with anything there will be more tuning tweeks when you start adding more boost to the equation. Are you hooking up with HoleShot Perfomance tuner in Sac? I hear he's good and very busy. No affiliation what so ever.
I have looked into fuel injection for my car but yikes, it's another rather large chunk of change. But yes, it would greatly enhance the engines efficiency.

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Discussion Starter · #20 ·
Talked with Holeshot yesterday, apparently only tune EFI motors. He did say go Holley over Fitech so that's good info, if I ever have the money for such a thing! My appointment in April is with Mandich Automotive which my buddy was very happy with, tuned up a carb and fixed some electrical in a day for $200. But that's a lot of missed driving opportunities between now and then. ;) I ordered most of the gauges, I figure for now, I'll keep learning and do what I can on my own one piece at a time. I just wish I knew what was really going on with timing so I'd have some sort of starting point there.
 
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