Rods 'n' Sods - UK Hot Rod & Street Rod Forums banner

Holley carb tuning

8K views 64 replies 8 participants last post by  SpannerPete 
#1 ·
Hi,

I've got a 383 SBC with a holley 750cfm (0-3310S) and a single plane high rise manifold.

It idles ok has loads of go when on the gas but at very light throttle is coughs/misses/judders (1400 rpm/ 30 mph just when you need it!). :(

It has about 9" of vacuum at idle and a 5" power valve. Vacuum when the problem occurs is about 15", a little throttle and the vacuum drops to below 10" and the problem goes.

I've tried upping the spring rate in the vacuum secondary and this didn't help but just gives a slight hesitation when I go on the gas.

Not sure if it makes much difference but it's in a light car (fibreglass T) and has a lightweight flywheel.

I'm getting to the pulling my hair out stage so any suggestions welcome.

Thanks

Pete
 
See less See more
#6 ·
Will this effect it at steady throttle position which is where I have my problem? I'm still trying to understand all the different parts and what they all do.

What sort of cam in there to get only 9 " at idle ? Sound radical ? What's the quoted power band?
Not sure on the exact cam but it shouldn't be too radical. It should be good from idle to 6000, but I've only got a dyno sheet from 3000 to 6000
 
#7 ·
Sounds as though you may have a 'transision' problem where the carb is going from off-idle (where it runs OK I presume) to main jet where I guess you have done a plug colour test at wide open throttle on a run? (to be sure its not main jetted too weak). This 'stumble' can be a really difficult thing to tune-out of the carb - typically you MAY need to change idle bleed jetting if it has removable jets or restrict/drill-out passages if a low-spec carb. It sounds typical of a 'stiff' cam/low vacumn scenario where the carb isn't pulling enough vacumn through the venturis to atomise the fuel just off-idle. With non-adjustable idle bleeds an old trick was to drill a couple of 1/8" holes in primary butterfly to allow enough air to flow off-idle.
 
#9 ·
Antons T is running a 1500-6000 cam and pulls about 18" at idle . My Pinto had a 1000-6000 cam and was pulling 21 at idle . His used to read around 16 until we sorted the vacuum secondary spring which initially had a blocked vacuum port to the vacuum secondaries .It was brand new carb out the box as well. I always tune for max vacuum at tickover by balancing timing as well as mixture and tickover screws.

If your vacuum is too low at that 1400 cruise it could be causing the secondaries to be opening too early and making it stumble. Higher tickover vacuum should move the effect elsewhere in the rev range which can be tuned with secondary vacuum springs then.

Wiht Antons, also a T bucket we had to go several springs lighter to get the best performance.

Just looked back at your initial post and I'm not understanding why you start at 9 " of vacuum and it's increasing as you accelerate ? Where is the vacuum gauge reading being taken from?
 
#14 ·
It's got a Engine Works USA, Outlaw series billet alum. race distributor with Megafire iginition. Timing set to about 14-16 deg at about 800 rpm. not sure about the springs

So it runs uneven when you're cruising at a steady 30?
And not when you boot it when you're doing 30!

Dunno much meself but trying to pin down problem :D

I've followed this guide and sat and watched the subtleties of how the gauge needle moves and what it's telling you.
Vacuum gauge, the wonder tool

Or, if it's been dyno'd and was fine mebbes the dizzy's loose/moved.

cheers
Jimbo
Yep you got it. Your the first one to mention about single plane manifolds doing that, I did wonder about it too. There is some bad reports on the web but also some good ones as well.

Interesting link :tup:

sorry read you wrong, is it a stumble when you floor it from idle or stumbling at low rpm cruise speeds ? Does it foul plugs, smell rich, backfire through carb or exhaust, is it a misfire dropping cylinders etc.
Doesn't foul plugs but they do look rich (black), doesn't backfire but it is almost like a misfire/cough

Click on scenario 10 on this link, may be the answer and would certainly give the sympthoms you describe?

How to Use and Interpret a Vacuum Gauge
Wow, never seen that before. :tup: I think it's more like scenario 3 but the vacuum does seem low but maybe it's just the cam.

Thanks for all the suggestions.:bigsmile:
 
#11 ·
Single planes do that don't they?

So it runs uneven when you're cruising at a steady 30?
And not when you boot it when you're doing 30!

Dunno much meself but trying to pin down problem :D

I've followed this guide and sat and watched the subtleties of how the gauge needle moves and what it's telling you.
Vacuum gauge, the wonder tool

Or, if it's been dyno'd and was fine mebbes the dizzy's loose/moved.

cheers
Jimbo
 
#22 ·
With your lightweight 'T' - timing, carb size, single plane inlet are all OK imho - you could have even used a double pumper with that combo - you really could do with finding-out the cam spec' - do you know anyone with a similar sized engine/carb combo? - see if you can borrow their carb - see how it runs on your car?
 
#25 ·
I've just been re-reading this article

Carburetor Tuning the Scientific Way

Is it worth fitting a couple of Lamda sensors as he describes? Could it give me a better idea of what the problem is?

The Cheap Way

Buy a single wire O2 sensor at your local parts house, make your own bung, and read the voltage with a digital voltmeter. The sensor that I bought is a Standard brand, number SG-12. The threads on this sensor are the same as a small-block Chevy gasket-style spark plug, so the bung can be made from one of those spark plug anti-foul adapters. Other O2 sensors use the large diameter threads of 18mm big Ford spark plugs. Just cut and fishmouth the adapter so that the sensor sticks into the exhaust flow. You need to put the sensor as close as possible to the engine so it gets hot and stays hot. Just make sure you route the wire so it doesn't get burned by the hot exhaust pipe. Weld the bung to the pipe, then drill and file the hole to clear the sensor.

Since the purpose of this sensor is just a guide to help you tune your carb, not run a fuel injection computer, if you can't get the sensor really close to the engine, don't worry, because it will still work for your purpose. All that will happen is that your reading may go away during periods of idling. On the same subject, don't worry about using a heated sensor, as the expense and complications involved are not worth it for carburetor tuning. Remember, your eyes are using this data, and if it stops for a while, no harm is done!

Sensor installed in Pinto exhaust.

O2 sensor in Pinto exhaust

Sensor and welded bung:

Outside view of sensor installation

Once you have the sensor installed and wiring run up to the inside of the car, attach a digital voltmeter (you really should have one of your own, but you can sometimes borrow these from friends if you don't have one) to the sensor and a good body ground. The sensor is positive. The readings you'll get once the sensor has heated up will be from 1.1 volts (1100 millivolts, or mv) down to about 100 mv. The high readings are rich, the low readings are lean. The perfect mixture for cruise is 400 mv. I have found my car to run well at about 700-800 mv. Once it gets below that, it tends to get into a lean misfire. Your results may vary.

Here is a general idea at what the O2 sensor voltage output looks like. As you can see, the slope around 400mv, which is 14.7:1, or perfect combustion, is very steep. This is why only computerized fuel injection systems can really hold anything close to 400mv. If you're wondering about how a sensor can read oxygen content in rich mixtures where there is no extra oxygen, the sensor begins to act as a temperature sensor above 400mv.

O2 Sensor output

 
#26 · (Edited)
Good idea - however if you have a 'radical' cam with loads of overlap and the consequentially required rich idle mixture to get it to idle the Lamda sensor may not tell you too much at lower rpm and idle - it should give you plenty of help at cruise etc though. 'Old school' way to check mixture is to find a bit of quiet 'A' road, cruise steadily at say 50 mph - knock the transmission in to neutral and cut the engine at the same time - pull over to one side - pull ALL the plugs and note the colour - do exactly the same at wide open throttle (a 'kin good trash, say in intermediate up to high revs for 15, 20 seconds) and note the result. If the colour of the plugs is the same all through, fine - if it differs you'll know if there is a carb imbalance - (although a single plane inlet may fudge the results at lower rpm) - you should be looking for 'coffee' colour plug tips with perhaps darker brown further away from the tip. Repeating the same thing for WOT will help give you an idea whether the secondaries are lean or rich. Its unlikely with a modern inlet manifold that you will need staggered jetting. If you have a 'cam' the plug colour at idle will likely be black and sooty - it should not be like that at any other speed. Changing jets on Holleys is simple but messy - the steps in jet sizes are quite small so if changing go-up 3 or 4 sizes rather than on or two. Let us know the can spec - it will help determine things.
 
#27 ·
it would be good to know the cam spec and type, also the compression ratio..sounds like a fairly large cam in there judging by the low idle vac, but a cam large enough to have fairly low idle vac would normally go way further than 6000 rpm,, i have a large roller in my sbc and it idles at 9 inch vac at 1000 rpm, but it makes power round past 7000 rpm... what is the total timing on this motor..i would always use a double pumper on a fairly healthy motor, but one with as much adjustments as possible,, does your carb have adjustable airbleeds etc
 
#28 ·
I think the cam was a custom grind, I'll have to check with the engine builder. I did ask for a street engine so I'm not expecting anything radicle and it made 429hp on the dyno so don't think it's anything very exotic. It does have Rhodes lifters which I was told would help.

It hasn't got adjustable air bleeds which means I'll have to order up and parts to try things out which why I thought the lamda sensor might be a wise move to get a bit more info.
 
#30 ·
Ok that's interesting I didn't know that. I've been told it should be 14-16 degrees at idle which it is, I have the total timing written down at work and when I checked it did seem about right (hard to look at the rev counter & timing light at the same time:lol:)
 
#31 ·
disconnect the vacuum on the dizzy if you have it ,,, connect timing light ,,, if the damper is fully degree marked find the 38 degree and mark it with tippex or white paint.... if the damper is not fully marked beg borrow or steal a dial back timing light......get a mate or the missus to watch the tach .... rev to about 3500 and hold it there so all the mechanical advance is in and see what it is,,,, adjust to around 38 degrees ...
sbc can run up to 20 degrees initial as long as you limit the total to about 38 degrees... what ignition system do you use, msd.crane etc
 
#34 ·
With that compression and that cam I would expect the vacuum to be about 15 at idle. I would check timing and if that's ok look carb. Take it off. Spin it over and check the idle slots.. should look like squares when correct. WHen cruising what vacuum does the motor have. LAmba sensors are good but you can just as easily sort it by checking the plugs
 
#35 ·
Not had time to check the timing yet but from previous experiments the following mayor may not be relevant but relate to the questions above: Please note I have always been trying to cure the same problem.

When I first tried to set the idle I found that it would still run with the idle jets wound right in so I supected a faulty power valve. New power valve, same problem. I then found the secondary butterflys were slightly open so I closed them off with the adjusting screw. Idle can now be set correctly.

I did take the carb off to check the slots. Secondary ones can't be seen, primary ones are square. I must admit from what I had read I would have expect to have to open the butterflys a lot more to get it to idle with low vacuum.

Vacuum when cruising is about 15" (light throttle 1400rpm) , and I have the problem at this vacuum and rpm.
 
#41 ·
I'm confused now :S The Holley book says 1 is a timed spark source for distributor vacuum advance so I thought it was correct (I haven't a clue what a timed spark source is?)

What is the difference between the two ports and what are the advantages of connecting to 2?
 
#42 ·
1 is timed(ported) 2 is full vacuum. 1 will not give any vacuum with the throttle blades at slightly open position. 2 will give vacuum at that same position. Put your gauge on to see the difference at idle.
Ported was introduced in the early days of emissions rather than full. In very basic terms they were trying to get engines to run hotter at low speed to burn off emissions and discovered that a more retarded timing was a way to achieve that. I tend to run full vacuum as do many others but some disagree and prefer to use the ported connection. There was a great link on here the other day to the debate written by a GM engineer.
It's always a controversial subject!
 
#46 ·
It's always a controversial subject!
So true, potentially more controversy then.... Why use the vacuum advance at all with a tuned motor, especially in a light car. Seems to be agreement in this thread that with a non standard cam, you get a non-standard vacuum profile. So most tuners don't connect the vac advance.

I've removed the vac advance off my truck altogether, never had a problem with pinking overheating or whatever as a result, even when towing a loaded car transporter trailer.
 
#43 ·
Try this:

Quote"



As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
 
#44 ·
Thanks for posting that is very interesting and I can see a potential problem with low vacuum.

I've now got my cam specs, I think these are correct as the engine was built a while ago. This means absolutely nothing to me but hopefully some could tell me if the idle vacuum (about 9") is correct for this cam or could I have a leak?

Comp Cams 292° Magnum hydraulic, with 292/292° of duration, 244/244° of duration @ 0.050 in. lift, and 0.501/0.501 in. lift
Rockers are Warhammer 1.6:1 ratio

With Rhodes lifters!
 
#47 ·
What I've found is that extra bit of advance at idle can help a cammed engine to run when in gear and stationary. Not having it will not induce overheating or pinking it's just that you may gain something by having it. I've not run a 292 so can't comment but I would suspect there's a potential for lower than average reading there but the Rhoads lifters should help not hinder? I think it's generally accepted that a 292 is fairly poor in low speed manners?
 
#50 · (Edited)
I think it's generally accepted that a 292 is fairly poor in low speed manners?
I'm not trying to start a technical p*ssing contest here, apologies if my post read that way.

If the cam is the same one I've just googled the 292 cam has a 110 degree lobe separation angle. So it must have loads of overlap; and if the cam is timed "down the middle" i.e with the points of maximum lift @ 110 ATDC inlet and 110 BTDC exhaust, I wouldm't expect there to be much vacuum at tickover because bth the inlet and exhaust will be well open at TDC, hence my comment about the vacuum advance not being much use, one of its main uses on standard vehicles is as a "lack of vacuum retard" for cars/trucks being slogged on hills in lhigh gear by drivers who don't know better; then of course it stops or at least reduces the motor's tendency to pink.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top