Hello everyone, I thought I would try to 'purge' a few brains for some simple tips. I just put my 671 weiand on my basically STOCK .030 over 350 chevy with a "RV" cam. I have never done a blower before so Im pretty cautious. I have the pulley set to build less than 8 psi at 6000, which in my 1931 model A coupe with a 700R4 and 4.10 posi Im betting it wont ever build that much boost anyway... The tires will fry or the car will GO first Im pretty sure. So here is the question, What would a good starting point be for jetting my 750 edelbrock manual choke carbs. I am running 2 of them. It seems to run fine and pulls around 20 inches of vacuum at a little higher idle than I want. But going down the road it stumbles alot.. as I try to SLOWLY excellerate this gets worse. If I mash it a little it comes out of it. So I have found no vaccum leaks, Im guessing its a little lean, I just thought maybe someone else on here would have some "TIPS" or advice on jetting or other things to watch for or play with. Thankyou for your time.
2 750's on a 350 cubic inch motor?? Yeeouch..Your really really really (yes really that much) RICH if your anything. There is allot that playsd into figuring jets, like altitude and motor build, etc etc. But I can tell you that 2 750's is WAY over carbed on a 350. like WAY WAY...1500 CFM when theoretically 650 CFM is about ideal. I would start leaning it a bunch, on the metering rods and jets both. Even on a blower motor even if you have a MASSIVe vacuum leak i can't begin to imagine your lean... Anyway, hope that helps. Heres a quick help, pull your plugs and look at them: Black= Rich Dark brown= Rich Beige/light brown= Really close to ideal White=Lean Take a look and go from there. Scot
Exactly! 2 750's on a "basically" stock 350 w/ a blower is excessive. Try a pair of 500 cfms and drop the needles down a size or two.
I'm going run my blown 354 on ONE 750 Edelbrock...and that's a little large IMHO....your problem is too much carb...
Im kinda stuck using the carbs, since that is all I have right now. But I do have a 'jet kit'. I willl start by leaning it out, I guess. Im just a bit concerned about going too lean. I was hoping to get this thing "cruiseable" for the Goodguys in Des Moines Iowa next week.. I dont want a backfire, hehehe, Im kinda scared of one actually. Never messing with a blower before, makes this a whole new world... Then messing with one with a small budget makes it more interesting. If this little motor goes Ill be done playing untill next year or I would have to sell a different project to build a new motor... I hate that. Anyway, Thanks guys.
I'm gonna disagree with the over-carb idea. I've built a couple blower motors (including a 350 chevy) and I think the 750 carbs will be just fine. As a matter of fact, if you check with Dyers, they recommend dual 750 carbs with any blower motor. With your discription of how the car is running, I'm venturing it's leaning out(high idle, stuble at cruise speed - primary curcuit running lean) then cleans out when the secondarys come in. The Edelbrock carbs need to richen their primary side for blower application according to my "How to Build a Street Supercharger Engine" book. Just my 2 cents.
Try this first... Ajust the float level before doing any jetting, sound silly but I have installed three Edlebrock Carburators in the last month and have had to adjust the floats on all three. You think after spending 280.00 you would not have to work on them!!! I have found Edlebrock Carbs to be the best of carbs and their jetting has always been on the money for the specified engine combo. Hollys Carbs are set up way over jetted and still have to ajust the floats
Key word here is BLOWER MOTOR> His motor is a basically stock 350.Stock heads,RV cam...etc. Just bolting a blower on a stock engine doesn't make it a so called 'blower' motor.
the AFB design has an air valve in the secondary side, so he's running the equivalent of one 750 double pumper under most conditions. for reference, I run a pair of 650 double pumpers on my blown 454, and it works fine. I vote for checking the float levels, and perhaps fattening up the mixture a bit. But first pull the linkage off one carb, and see how it runs with only one carb working, that will give you an idea if the problem is too much carb, or a calibration issue.
I fully agree. Blowers are VERY intake restriction sensitive. The two eddies should be fine. I really doubt you are over carbed. It very difficult to overcarb a 6-71 unless you have dominators. Call edelbrock. They are very helpful when it comes to their products and should be able to give some recomdations. The 750's are jetted lean from the factory, so you will definately have to richen them up. You can start by using the thinnest rods you can in the carbs to check if its a lean issue. This will richen up everything but the idle and will tell you if its a lean issue. If it runs better but still is not great, you will have to change the jets in the carb to a larger size and start playing with the rods again. On any blown or turbocharged motor, start rich and lean it out, reading the plugs or A/F guage all the time. Bad things happen fast when you go lean under boost. Expensive bad things. Always check the float level on an eddie carb. They are off quite on a lot of carbs. What do the plugs look like?
does 8 psi on a stock motor seem high to you guys? a stock motor propblably has 8.5 - 1 static - that gets way up there on the compresion ratio chart. I am really just wondering - I am trying to stay under 5 psi with a 671 and 8:1 ratio. also - did you change your valve springs? you are about to pressurize the intake side - your blower motor could open the intake valves that are supposed to be closed and cause KABLAM
I've seen 1150 CFM Dominators run great on small blocks too for what its worth, my poijnt was strictly HIS combo. Basically stock 350 with an RV cam.. We are all just guessing, me included. PLEASE do this and tell us what you find then we can more accurately help you on this combo. So...take a quick look at a couple plugs. if you are concerned with cruising RPM, here is a really simple way to check that too. head to a place you can pull over wherever you want. Take it up to cruising rpm and hold it there for a minute or so then shut the key off. Coast off to the side of the road and pull a plug from each side of the engine. Use the mini guide I quoted again here. Rejet...rinse repeat. You can actually check full throttle the same way, hammer it and right before you would normally shift, shut the key off and coast off the road. If you just pull them, you are reading whatever the last throttle position was, usually you are reading the idle circuit. so best way is to do it like I just explained. Good luck and I hope this helps! Scot
actually - the best thing to do is install and oxygen sensor and intall and a/f ratio meter to monitor. They aren't that expensive anymore. You could also run it on a dyno to get it set up properly. Engine shit is just too expensive to ball park it.
that's why I run holleys, you can actually know what the carb will do just by what number jets are in it.
Put in the stock jets, it'll be just a tad rich, drop a few numbers to lean it out, or go up a few sizes to really fatten it up if needed. none of that funky metering rod crap.
Metering rods are sooooo easy to figure out. 5 minutes to change. And you don't leak fuel all over the place. Both brands of carbs have their strengths/weaknesses. It all comes down to preferences. I like 'em both!
The one big advantage of the AFB design is that they naturally don't leak. Holleys need periodic gasket and accelerator pump replacement.
I appreciate ALL of your inputs, Im gonna go rich FIRST to see what happens. Ive got all friday to play with it. I had not thought about the 'plug-reading' bit. Sometimes the most obvious is the first thing over-looked. Im not going for MAX power, more for drivability. So I chose the eddies instead of Holleys. Not trying to start an arguement, I just believe the eddies to be a set-it-and-forget-it type of carb. No spontainious leakage or Jet changes.. besides, I put the eddies on forward and saved myself a hundred bucks by makin my own 2/4 throttle linkage out of a piece of strap. On the 8 lbs boost question, YES according to Wieand and Dyers I should not go over 5 psi. Which, in my VERY LIGHT and low geared car I doubt it will. So far, just playing around my boost gauge has only went to to boost side about 1 lb. And it was 'cookin' . Dyers says you cant run a "cast" crank either, because it will break and send the front thru the radiator... I have driven a buddy's NOVA with the same set up at the drag strip shifting at 6 grand and no breakage yet... its been together for 3 years.. BUT he runs HOLLEYS and has no knowledge of eddies' so thats why Im askin you fellows. I have not changed the valve springs yet, For this season I was just wanting the LOOK and to make it drivable. Kinda LEARN as I go. I doubt I will ever pull the engine hard enuff to worry about it. A couple of small town parades, a few cruise ins and shows and this thing is going back into hibernation untill next year. HOPING to build a better motor with blower pistons etc for next year.. Untill then I just wanted the LOOK. But its hard to look COOL when the damn thing is stumbling all over itself. Im off to the shop. Thankyou for all your suggestions, they are noted. Its good to hear things I had not of thought of or never knew... Very helpfull. Thankyou
Hey thanks for answering the boost question - so you're already running the motor and haven't seen more than 1 lb of boost? that is cool. My car is light but I'll have a 3:80 gear. I'd like to hear how your start up went. I am picking up my big CC heads from the shop today. One guy I was talking to said you only see boost when the car is under the most load. I was sort of confused by that - I thought just spinning the blower would creat the boost - like it was proportional to the rpm. he said the load created the boost - I am not sure how it works. I have cast crank and pistons too. Like you - I am not trying to max out the boost and run the thing as hard as I can. Its more or less - just to add a serious kick in the pants. I think the info they put on those sites is the safest most gauranteed to work info they can provide so they don't have guys building combos that need a ton of tweaking or will end up causing them to bust parts.
A stock low compression motor with an RV cam is a perfect mild blower motor. The wide LSA and low overlap of the RV style cams are great for blowers, plus they are almost all dual pattern with more exhaust duration - anotehr plus for a blown motor. The only tricks to keeping a stock bottom end blower motor alive is keeping it out of detonation, and making sure the tune is good. Rev it as high as you would rev the STOCK motor. 5000rpm or so regularly is safe. If you are going to actually be racing it regularly, I would say pull the engine and build it as a blower motor with a forged crank (roots blowers put a lot of strain on the crank) and forged pistons at the least. Harder bearings, opening up the clearances some, and a good oil pump are also needed if you are racing. Then you might as well pick up a set of rods too... Roots blowers are POSITIVE DISPLACEMENT devices. They pump so much air per revolution. How you make boost is that the air "stacks up" in the intake tract because the supercharger is forcing more air into the engine than it can process normally and the density increases. Turbochargers need load to make boost since they are exhaust driven compressors and you need lots of exhaust velocity and energy to make the turbine spin. For instance - free reving my turbocharged camaro, I get no boost until 4000rpm. Driving it, I am under full boost by 2500rpm.
A blower motor will only make boost when the throttle is open...because boost is air pressure, and you need to get a LOT of air into the blower to be able to get positive pressure in the intake (the engine is working hard to pump whatever air there is out the exhaust). So if you can hold the throttle open for a little while, you'll make boost, and the only time you can hold the throttle open is under load, othewise the engine revs too high and you have to close the throttle before any boost is made. My explanation sucks, sorry. Anyways, a street blower motor needs to have kind of strong pistons, and a kind of strong crank, a mild cam, good flowing heads, and a redline that matches the cam's characteristics. Keep rpm low, build lots of torque, and drive the piss out of it.
I agree - especially if you're new to all of this and don't have much experience. You don't know . . . what you don't know. Also - with todays fuel blends, the traditional methods of reading plugs is not always accurate - in many cases they won't color like they used to . . . which means you can be off quite a bit. Using the above method will give you the accurate information you need to understand what is really happening from a rich/lean perspective. Let us know how this works out . . . Dale
Squirrel is correct in what he says. You need to do several things. Do more research. I recommend Corky Bell's book called Supercharged. build it strong. Wanna cheap out on stuff? Good luck. Like Dyers says, you can build it on a budget & w/ stock heads, cast crank etc...but it gets risky. Things break. Build it right & make 8 lbs. boost instead of 4lbs. Still want to play guessing games w/o doing your homework? Don't. Go get the book! It shall reveal all.
And another thing. Blower motors like blower carbs. Mighty Demon 650's flow like 750's & the power valve has a boost reference. They are $$$ but hey
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