blower carb tuning

[Humidi-T]

Read GT's thread below and decided to post my questions.

SBC, 4-71, 12% overdriven, 6lbs at 6000rpm, MSD ignition

Friggin thing is a blast! runs great, no problems with idling, or at speed, no temp issues. I'm almost scared to mess with it at all, it runs so good, but I have a few tuning questions.

It is not rich at open throttle, but is extremely rich at idle, especially once it warms up. Rich enough to be able to see the unburnt fuel at night in the headlights.

BG demon carb Street Demon 650. all four corner idle screws are turned in to about 1/4 turn off closed. Stupid question, does screwing them out lean the idel mixture, or does it richen the mixture. I want to say that screwing them in leans the mixture. BG wasn't much help. basically told me i had the wrong carb for a blower, but the damn thing runs good.

any input?

 

[RPM]

Read GT's thread below and decided to post my questions.

SBC, 4-71, 12% overdriven, 6lbs at 6000rpm, MSD ignition

Friggin thing is a blast! runs great, no problems with idling, or at speed, no temp issues. I'm almost scared to mess with it at all, it runs so good, but I have a few tuning questions.

It is not rich at open throttle, but is extremely rich at idle, especially once it warms up. Rich enough to be able to see the unburnt fuel at night in the headlights.

BG demon carb Street Demon 650. all four corner idle screws are turned in to about 1/4 turn off closed. Stupid question, does screwing them out lean the idel mixture, or does it richen the mixture. I want to say that screwing them in leans the mixture. BG wasn't much help. basically told me i had the wrong carb for a blower, but the damn thing runs good.

any input?

BG is never any help, and DEMON is a good name for their stuff.

Anyway yes screwing in the screws will make it leaner. You don't want it too lean as it might detonate, then good by pistons. So be careful there.

 

[Humidi-T]

BG is never any help, and DEMON is a good name for their stuff.

Anyway yes screwing in the screws will make it leaner. You don't want it too lean as it might detonate, then good by pistons. So be careful there.

The engine is extremely rich at idle, and idle is the only circuit with problems right now. One fellow suggested I change the main jets, but I don't think that is the right answer, because it is only rich at idle. Idle transfer slots look right with just a square showing above the butterflies, it doesn't idle high, about 700 rpm, and all 4 screws are just about a 1/4 turn off the seats. It doesn't affect driveability, just my eyes and sinuses. and the exhaust is knida down heartening to see it smoke, but run so damned good.

 

[RPM]

Sounds like the floats may be a little bit high? You don't want it too rich as it washes out the rings and cyl walls.

 

[Keith Bowman]

Hi,
There are a number of online articles with pictures on how to modify Holley carbs for blower refrencing, the carb companys charge alot for this simple modification-also if the metering block is like a holley it is possible to place a single strand of copper wire(better-piano wire) in the idle feed restriction-there several are online articles on this also-size to use etc. try Carcraft.com Hope this is of some help!
Keith

 

[Humidi-T]

Sounds like the floats may be a little bit high? You don't want it too rich as it washes out the rings and cyl walls.

Floats are just below the middle of the sight glass.

 

[1924 c cab]

with holleys and the like on blower apps tend to run either rich at idle and lean wide open or they run lean at idle and rich at wide open due to power valves not being boost referanced.there is always vacuum under the carbs at all times even under high boost pressure due to the carbs are mounted above the blower and not always opening or erratic opening when under load causing havoc with the power valves as to when to close or open. I modified my hp4150s to boost referance. was easy to do. now they open under load and stay closed at idle and cruise.Hope this helps some what.

 

[railroad]

I normally set the float levels to be at the bottom of the sight hole. I don't think this will make a big difference. The power valve will be key to cleaning up the idle.
Your carb probably has something between a 9.0 and 10.5. This is the vacuum that the power valve opens to enrichen the mains. If your manifold vacuum at idle is less than the rating on the power valve, it is open at idle, enrichening the fuel ratio. Most adjusters start out 1 whole number lower than the idle manifold vacuum reading. If your carb is new, the gaskets are going to a pain, and will require some work to clean up.
You can buy the power valve, the bowl and metering block gaskets separate.
Some of the after market gaskets are designed to be more reusable than others. In the event you do not get the power valve selection right the first time, they are a plus.
I would take the carb apart before buying anything, as there are some differences in
some of the gaskets and power valves. If you decide to address this yourself, post
regular. I look forward to seeing you get it right.

good luck,
railroad

 

[Humidi-T]

I normally set the float levels to be at the bottom of the sight hole. I don't think this will make a big difference. The power valve will be key to cleaning up the idle.
Your carb probably has something between a 9.0 and 10.5. This is the vacuum that the power valve opens to enrichen the mains. If your manifold vacuum at idle is less than the rating on the power valve, it is open at idle, enrichening the fuel ratio. Most adjusters start out 1 whole number lower than the idle manifold vacuum reading. If your carb is new, the gaskets are going to a pain, and will require some work to clean up.
You can buy the power valve, the bowl and metering block gaskets separate.
Some of the after market gaskets are designed to be more reusable than others. In the event you do not get the power valve selection right the first time, they are a plus.
I would take the carb apart before buying anything, as there are some differences in
some of the gaskets and power valves. If you decide to address this yourself, post
regular. I look forward to seeing you get it right.

good luck,
railroad

OK, carb already has re-usable gaskets in it, so disassembly isn't a problem. I'm running 11 inches at idel more or less. I'll have to look into this. Makes sense.
thx

 

[blownt]

power valve should be half what idle reading is. If you have 11lbs at idle you should have a 5.5 or 6 power valve. Take the plug out of the sight glass and adjust float so gas JUST trickles out hole.

 

[railroad]

power valve should be half what idle reading is. If you have 11lbs at idle you should have a 5.5 or 6 power valve. Take the plug out of the sight glass and adjust float so gas JUST trickles out hole.

I cannot say you are not right on this, because I tune mostly normally aspirated engines, but from the time the manifold vacuum drops from 11 inches of mercury to 5 or 6 the engine will be running on the main jets only and without the power valve enrichment circuit. If you punch the gas you may not feel this because the manifold vacuum ( N/A ) will go to ZERO, but while up to speed and go WOT you can feel the hesitation in mild mistunes up to missing and backfiring in leaner situations.
Ideally you want the transition (opening of the PV to be un noticed) sorta like tuning
the opening of vacuum secondaries. Mine and I think Holley's suggested rating is
1 whole number less than the manifold vacuum at idle (normally aspirated). The further the PV rating is from the manifold vacuum at idle, the longer the engine is running without enrichment ( lean ). In a ( N/A ) engine not much risk, but 6# of boost may not be very forgiving. I would err on the safe and rich side. Start no more than 1.5 inches less than idle vacuum reading. Be advised most vacuum gauges are very inaccurate. 2 to 3# difference is the norm.

I sincerely welcome any corrections on my advise as it applies to either N/A or boosted applications.

Thanks,
railroad

 

[blownt]

I'm just going by what Steve Zepeda, a local carb guru here, that has forgotten more about carbs them I will ever know. He just redid both my blower carbs. He told me put a gage on the intake and read the vacuum at idle. If you get a reading of 20 then divide that by two and that is what you need for a power valve. I figure he knows his stuff since MANY known people use his carbs and has him tune them.

Zepeda Carburetors

 

[railroad]

I am sure your reference knows his stuff and I agree, his method will prevent the PV from opening at idle, but my opinion is, he just gets a wide enough spread between the PV rating and the idle vacuum that it will not open until the engine is at load.
If that was the best method for tuning the opening, they would only need about 3 PV ratings. Also his method insures the PV will not open due to mis calibrations on gauges and PV ratings. You may put in more than one PV to get it right with my method, but the opening will maintain a more consistant a/f ratio through the rpm range. Again, I am sure Mr. Z is a knowledgable and competent carb man and provides a good product and service. I looked at his website, very nice. He would not be in business if he wasn't good.

railroad

 

[Screaming Metal]

Hey RR, RPM and BlownT have given you great advice.........sounds like your a little too rich......see if you can use your screws and lean them out,.........just a trickle out the plug holes on the carb and just as the gentleman said on the powervalve.........

Another thing.......look down the carb throats as its idling.......and see if theres big dribbles of fuel coming from your squirter nozzles........if it is, and everything else seems fine........back off on your shot size on the accelerator pumps just a touch..............

With a blower.....your engine "THINKS" its several times its size, and at low RPMs its sucking in so much more than it can normally use.....sometimes you gotta lean it ever so slightly,,,,,,,

Its like the old Vitalis commercials.....a little dab will do ya!!!!!:ciao:

Happy Roddin!

 

[Lee_in_KC]

[quote name='Screamin' Metal']..a little dab will do ya!!!!!:ciao:[/quote]That was actually Brylcreem...

[ame=[media=youtube]jRcRIbExrfg[/media] - Vintage Brylcreem TV advert US[/ame]

 

[Screaming Metal]

That was actually Brylcreem...

YouTube - Vintage Brylcreem TV advert US

Well ya Lee..........I was improvising there.........Vitalis was a little more my style there good buddy!!!!!!! Ya Know.....not quite as heavy!!!!! HeeHee!!!!:ciao::lol:

 

[Screaming Metal]

Hey Lee.....I didn't think you were old enough to remember those Brylcreem commercials!!!!!!!!!! HeeHee..........Back in the day....I used to have on the white tee, a pack of cig's rolled up into the shirt sleeve, and about 1 big dab of that brylcreem in the hair with a good comb in your back pocket..........

Needed a cable lubed....all you needed was to rub your hair.....get some of that creem on your pinks and lube it on up!!!!!!! Then........I switched...........of course........:lol:.

 

[Lee_in_KC]

Yep, I used Brylcreem in high school (1962-1966). I combed my hair like Ricky Nelson... I thought he was a cool dude!

 

[Screaming Metal]

Hey Lee......SSSSHHHIsh......hey.....lets not give our stuff away here.....I liked Ricky, too................yep......those were the good ole days.......you could get a speeding ticket and it wasn't the end of the world, either.......heehee:lol::lol: