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carb tuning

2K views 15 replies 6 participants last post by  vinniekq2 
#1 ·
Having a dilemma on which way to go with tuning my carb.
Ive recently posted about intake manifold choice ( am running an old holley street dominator low rise single plane ) and 9/10 recommend I go to a dual plane which very likely I will. Performer rpm probably.
Its a 427 bbc , headers , 4 speed stick , 3.43 on a 73 camaro , 10:1 compression , ported iron heads , timing 38 all in 2600 and 18 at idle roughly 750rpm.Engine gets about 15" vac at idle Cam is hydraulic lifter 227/235@50.114lsa.
Before I decide to change the intake I want to make sure car is tuned nice and optimised.Most driving idle to 4500 with bursts for fun up to 6000.Holley 750 vac seconds.

The issues are this- when Im at a standstill and bring up the clutch to get moving the engine judders a bit and needs more throttle than Id typically feel is needed. AFR says Its leaning out so I need a bit more initial fuel.
Holley recommended changing the power valve from 6.5 to 7.5 to help this. All well and good but: If Im cruising at 1000-2000 and go WOT i get an initial second of power ( wheelspin) then the car bogs right down. My AFR gauge shows a very rich reading. So If I change the power valve to a 7.5 won't this make the WOT bog situation worse? Better to play with squirter or pump cams?
Holley also recommended changing to a heavier vacuum secondary spring to stop the bogging down at wot.
I asked about when the secondaries are running and how to road tune and was told that at 2500-3000 steady cruising Im on the main jets but Im also only on the main jets at 5000rpm steady cruising . Im only on the vacuum secondaries when I'm WOT accelerating but not cruising , at any rpm.Which Im fairly sure isn't true.
Which has made me question the advisors knowledge and advice....
 
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#4 ·
Need to know the cam timing and more about the ignition system. Falling way rich is as likely to be a failure of the ignition to light the mixture as it is the carb supplying excess fuel. The cam timing gets into the idle to transition phase, larger cams are often a problem in getting this to blend properly leading up to main metering. It would also be helpful to know which head? Not all heads react the same way.

The 750 is typically a pretty flexible carb that doesn't cause problems. The vacuum the secondary's come in is adjustable by changing the opposition spring. It is also necessary to know that the secondary's are not operating on manifold vacuum but rather venture vacuum which is opposite to the manifold. while manifold vacuum can be used as an indirect indication it is not the working element of actuation.

Bogie
 
#6 ·
Sounds more like an IFR(idle feed restrictor) issue, although there are many factors to look over before any modifications are to be done. Here is a link to some good info using a wideband to help tune a Holley:
Tech | Burton Machine


I had a bit of a stumble with a Holley 770 and I had to open the primary IFR's from .032" to .034", doing so cleared up the off-idle stumble(just before main system start-up) and my AFR's in that area richened just a bit as a result. Hope this info helps.
 
#8 ·
If you have the 781 iron heads you have a very good set of large oval port heads. But I see you have the small valves still. The best thing that could have been done is upgrade to the 2.19/1.88 valves when all the porting work was being done. If you have a 4 speed trans why are you using a vacuum secondary carburetor? The camshaft specs you have listed is it a custom grind? If not what is the brand and part number? I would like to see more info on that camshaft.
 
#9 ·
Below is a quick fuel vacuum secondary pod you can use to really fine tune your secondary side of things just by turning a screw. Only drawback is you have to take the pod off to try a different color spring but other then that you can adjust how fast the spring rate responds or delay it. I use it on my carb and it does help with out having to put in too stiff of a spring and keeping the secondary's from opening up all the way.

Quick Fuel Holley Carburetor Adjustable Vacuum Secondary Kit Quick | eBay
 
#10 ·
hi , can't remember the cam - digging up the info. Its a herbert cam either 225/235 or 230/235@50 hyd lifter.114 lsa. On the post i averaged the intake to 227 to give an idea on the cam.
The porting on the heads was done nicely but there was concern we'd hit the water passages with the bigger valves and didn't want to ruin a decent set of heads.theyre 215's if i remember right.
Im under the impression the bigger valves aid higher rpm and my goal is more idle to 6000 if I have that right. so the bigger valves weren't critical vs the risk of hitting water.
vac seconds as its still a street car and they still operate under engine demand irrespective of manual or auto.Again , if i have that correct
 
#11 ·
at 5500 rpm in a rat you could easily surpass 500 hp. To make that type of power the heads need to flow 300 CFM or more and a 750 cfm carb does not flow enough air to go very far past 500 hp. Your trips to 6000 rpm is not any where near performing to its potential
 
#12 ·
The more I look at the compromise intake I'm using , a holley low rise single plane street dominator 300-3 the more I think this is the weak link in my combo. Its the same height as the Torker 2 BUT the runners are straighter and the flange is about an inch high ( the worker is only a 1/4 inch odd) so the runners are again shorter.
Bearing in mind generally a performer rpm could give 20-30hp and ftlbs over the torker surely this would be even greater over the holley?
Th holley is no taller than my gm low rise intake with the 3/4 carb adaptor. Yet its got shorter runners and is single plane. Wondering if even my old iron intake would be better? Again idle to 6000.

thanks all
 
#13 ·
Your car is not making power to 6,000 rpm with a 750 on any intake. I took a performer off my 454 Camaro and put on a victor oval port and picked up 4/10s. Biggest single improvement made on the engine ,then slicks,after a few power increases.
Not sure you are even reading my posts?
 
#14 ·
Hi Vinnie

Yes 500hp would be nice but am stuck with the heads Ive got. which i think have 260cc intake runners and have been ported. Really just trying to optimise the engine with whats on it. Only considering if I can make any seat of pants improvement with intakes and carbs. That said all the carb size calculators steer me to a 750.....
Just looking at the street dominator next to a performer I can't help thinking up to 6000 this is the issue.
Maybe hard to see ,the plain performer here next to the holley is already a lot bigger , taller too by an easy inch plus the one inch flange on the top of the holley. The holley runners are comparatively tiny.Hurting low end? and the tiny plenum can't be good at the top..?
How was the holley intake rated at idle to 5500?
 
#16 ·
already mentioned but will re iterate ,,,INTAKE Victor Oval port. 750 cfm carb will flow enough air for just over 500 hp.
carb calculators, use them for a guide only,,,

how much horse power do you think is required to flow the full 750 cfm of air through the carb?
now use that same formula and see how much hp you can save by flowing 750 cfm through an 850 cfm carb. How much hp did you save that can now go to the rear wheels?
 
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