Yellow Bullet Forums banner
Cancel
Create thread New Garage Forums

Roots Blower Carb Tuning

Jump to Latest
34K views 24 replies 11 participants last post by  Kevin Zaske  
5
#1 ·
505 CI
11:1 Compression
8-71 BDS Supercharger
4-5 lbs Boost (Underdriven)
110 Octane Gas
(2) Holley 750 Boost Referenced Carbs

This is my streetcar that my son and I cruise around in the summer months.

My concern is that while cruising at light throttle the A/F reads 10-10.5:1.

WOT is 12:1, accelerating up to speed is 11.5:1 to 12:1, and idle is around 12:1-12.5:1.

What adjustment do I need to make to my carbs to correct my A/F while lightly cruising down the highway?
 
Y
#5 ·
Cruising at light throttle is mostly the domain of the idle/transition circuit so before you got to the trouble to go inside the carbs, change the primary idle air bleeds to larger sizes, go up in .010” steps so the change is large enough to see what the effect is and to understand what part of the throttle range is affected by the idle/transition circuit. You may be able to accomplish what your want for highway cruising by just tuning the idle circuit.

If the carbs run well beyond about ¼ or 1/3 throttle, try tuning the idle circuit first.

If they are rich between ¼ and ½ throttle and the intake vacuum is above the PV closing point, the jet is too large.

Why make this a scavenger hunt? What is the carb list #? What size jets are in the carbs now? What size are the PVCRs? What size are the main and idle air bleeds? What size is the idle jet?

Blower carbs definitely should not have main air bleeds larger than .026” --- .028” is pushing it. Larger MAB makes it richer at small throttle opening and leaner at WOT.

Does it idle OK now, no surging or trouble with a big RPM drop between neutral and in gear?

The way you describe it, it sounds like the idle system is too rich on the T-slot and the main jet is too large.

The Holley 80576 blower 750 has a 76 primary jet and 87 secondary jet as produced. Usually, the primary jet ends up closer to 72 but the power valve channel will need to be made larger to compensate so WOT still has the same AFR. Keep an eye on the WBO2 and if it is too lean at WOT after changing to smaller primary jets do the area ratio math and whatever area is taken away from the jet, add to the PVCR.

You need the power valves, #65 is OK. The 80576 comes with #105 and that is OK too.

Boost referenced is better than not because at part-throttle it is possible to have vacuum between the carbs and blower and pressure in the engine below the blower, and with boost referenced power valves they will for sure be open any time it is in boost.


A Roots blown engine will usually cruise very well with much leaner AFR than a similar NA engine because the heat and turbulence of the blower more thoroughly vaporizes the fuel and that improves distribution.

With lean cruising AFR vacuum advance is a good thing to keep EGT down.

At WOT sometimes they require richer jetting on the ends than in the middle to get enough fuel in the corner cylinders. Some time on a dyno with 8 EGT or WBO2 is worth it if you are running such an engine in a boat where WOT can go on for several minutes.
 
B
#6 ·
I had a similar issue when I first set my 6-71 up. Yeti was a big help. You may want to read this http://www.yellowbullet.com/forum/showthread.php?t=569845 Also, I first tried non-boost referenced carbs. I couldn't get them to work that well on the street. The motor had an aggrevating surge and it fouled plugs every 50 miles. I boost referenced them myself and haven't looked back. Hope this helps.
 
Y
#8 ·
Blown Crazy said:
I had a similar issue when I first set my 6-71 up. Yeti was a big help. You may want to read this http://www.yellowbullet.com/forum/showthread.php?t=569845 Also, I first tried non-boost referenced carbs. I couldn't get them to work that well on the street. The motor had an aggrevating surge and it fouled plugs every 50 miles. I boost referenced them myself and haven't looked back. Hope this helps.
Click to expand...
Ironic that you posted a link to the thread with that carb recipe in it, my to-do-soon inbox has another pair of 750s for a very similar BBC that will get that treatment and the MSD 6530.
 
3
#9 ·
You can get by with a NA carbed engine using off the shelf carbs, or slightly modified ones, but when you are using them with a supercharger, you really need to get a good grasp on how a carb works. Even if you buy one for your exact application, you will have to make slight tweaks to it over time.

Here is a page I set up that covers the inner workings of a carb, and how the circuits interact with each other.

http://brucerichards.com/tuning.htm

Most of the advise you have received here so far will get you started, and understanding this page will help you to understand how it all works together.
 
Y
#12 ·
C1BAD66 Malibu said:
Damn, I just spent 45 minutes or so putting in my engine specs to ask a question about why my passenger side header primaries run 200-250° hotter than the driver's side at idle.

It's like something timed out and my post disappeared!

Is there a better way?

I really need yeti and/or Mark's help!
Click to expand...
Compose you message in MS Word or some other spreadsheet so you can proofread a few times at your leisure and use the spell check.

Many times if I write something and look at it a half-hour later I see things to edit for clarity. It was a teeny bit painful to re-read the posts in the thread linked above and see my convoluted phrases and a math error (20+10=25 LOL). Writing technical stuff is hard and it isn’t made any easier by decades of schlockmeisters pumping out magazine articles written to a third grade level until that’s all anybody wants to see. If they have to read a four-syllable word it makes some people apoplectic.

I tend to compose in conversational or free-association phrases that sometimes don’t come across with intended meaning in the written word like they would in spoken word, because the meaning conveyed by verbal inflection isn’t present. I got busted for that in grade school English, “Your writing is too conversational.”

If you like, sign up over here. http://racingfuelsystems.myfunforum.org/index.php We’re trying to boost the traffic.

The lop-sided header temps can be because of the way the fan moves air around the engine. If the carbs are sideways, the amount of fuel going down each side of the blower is probably not the same on each side. The engine doesn’t just run on the fuel coming out of the carb, it runs on the puddle in the manifold and the film moving on all the interior walls, blower case and manifold.
 
#15 · (Edited)
'Morning, yeti.

Perceived problem: Pass.-side EGT's 200-250° hotter than driver's side @ ~1000-1100 RPM idle. ~10" vacuum.

GMPP ZZ-502 with port matched GMPP rect.-port heads.

BDS "large-bore" 6-71 street blower @ 9% underdrive.

2 sideways-mounted 4777 650's, boost referenced, t-slots all squared, 0.094"-dia. [properly placed] holes in all 8 plates so I could breathe around running car in garage. 'Car's on jack stands.

NGK -11's @ 0.025" gap. HF laser-aimed temp. gun.

Your thoughts are welcomed!
 
Y
#17 ·
It is likely there is uneven fuel distribution because the carbs are mounted sideways and the 4777 only has Curb Idle Discharge on the primary side unless they have been modified.

The base plate has a channel connecting the primary curb idle discharge to the secondary where the CID would be if the port were drilled into the throttle bore. If your carbs do not have the discharge port on the secondary you can drill them as shown in the pictures in this thread.

http://racingfuelsystems.myfunforum.org/viewtopic.php?f=25&t=364

Start with a sharp center punch to make a start for the drill bit. Use a 1/16” drill bit to start and finish the hole with a drill the same size as the hole in the primary side which is probably 5/64” (#47=.078”).

Some OE Chevy carbs for the engines with sporty cams had these discharge ports and that is why the throttle plates have this passage. The purpose is to add idle fuel from the adjusting screws to the air coming through the throttles on the secondary end to improve fuel distribution.

If you have original 4777 carbs the secondary idle air bleed is much smaller than the primary, perhaps only .028”. This will make the secondary T-slot very rich if it is exposed at all at idle. If this is the case, and the carbs are mounted so the primary is on the passenger side, the secondary side is too rich and the primary side is lean to compensate and so the distribution is lopsided.

Measure the voltage from each O2 sensor with a digital volt meter. The higher voltage is richer however the sensor voltage is affected by sensor temperature so it cannot be taken as any more than an approximation.

It only requires a tiny exhaust leak to make O2 sensors read leaner than the actual AFR. Uneven valve lash will have an effect the amount of air passing through the cylinders on overlap and that will skew the sensor output.
 
#18 · (Edited)
yeti, thanks for your reply!

Yeah, my problem is likely an unbalanced [idle] fuel problem.

It's a bit overwhelming for me to understand, but I'll keep re-reading it.

Yes, my 4777's are not modified for adjustable secondary idle fuel metering. My thought was to square up the secondary t-slots and adjust the idle mixture at the primaries in addition to the throttle plate holes that fixed my major rich condition. I'd have to run outa' the garage after a few minutes of idling to get fresh air, while crying like a baby!

Of course, up jumped the EGT imbalance phenomena.

Are you suggesting I shot myself in the foot by not opting for purpose-built, boost referenced, 4-corner carbs?

If I attempt to modify my current carbs for 4-corner idle adjustment, I'm worried about IAB's. ¡No se habla idle bleed convention! Oh, of course, my AB's are non screw-ins.

I havta take a step back to formulate a plan to rectify the problem. ~$1200+ for new carbs is tough for me to go for.

If you were having dinner here, I'd make sure you got an extra piece o' pie for desert!

Oh, I read your post on that other site and saw what you had to say about fuel distribution in a blower manifold plenum on this site. 'Makes sense. No one else (NA guys) has been able to give me insight.

Thanks! Am I allowed to ask any more questions if need be?
 
Y
#20 ·
I don’t mean to make your carbs into 4-corner idle, though you can. I mean to merely add the discharge holes on the secondary.

Connecting the primary idle discharge to discharge holes in the secondary by that passage in the base provides adjustable idle fuel to the secondary from the primary idle adjusting screws. Adding the holes on the secondary gives the carb adjustable idle fuel mixture with the airflow past the throttles in all four bores.

If your secondary T-slots are exposed at idle now they are dumping fuel into the engine and you have no control over how much. Close the secondary throttles to close off the T-slots and drill those holes to let fuel from the adjustable primary curb idle port also discharge in the secondary and you have control of the fuel entering the engine on both sides of the blower.
 
Y
#21 ·
506RR said:
505 CI
11:1 Compression
8-71 BDS Supercharger
4-5 lbs Boost (Underdriven)
110 Octane Gas
(2) Holley 750 Boost Referenced Carbs

This is my streetcar that my son and I cruise around in the summer months.

My concern is that while cruising at light throttle the A/F reads 10-10.5:1.

WOT is 12:1, accelerating up to speed is 11.5:1 to 12:1, and idle is around 12:1-12.5:1.

What adjustment do I need to make to my carbs to correct my A/F while lightly cruising down the highway?
Click to expand...
506RR, did you get anything out of this thread?
 
Yellow Bullet Forums
posts
24M
members
152K
Since
2005
A forum community dedicated to drag racing drivers and enthusiasts. Come join the discussion about racing, builds, pro mods, hot rods, events, turbos, nitrous, superchargers, and more!
Full Forum Listing
Explore Our Forums
Trash Or Be Trashed Naturally Aspirated Let's Talk Drag Racing Turbo Charged Nitrous - Induction Solutions