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Too lean at wot?

Yep, will try larger jets in the sec this weekend.
I have gone through a couple of carbs to learn how to tune and the Holley 650 Vac has been a champ, so I dont want to drill in it.
Ive ordered new parts now to build my own carb, Holley style, with adjustable pvfr and more, also new main body with choke delete and adj air bleeds. Lots more to play with.

Im running 98 pump gas.

carbs.jpg
 
You may have to change the PVCR size but generally only if the carb is very small or very big for the size engine it is on.
I have played on the dyno for years and a fair few carburettors. Only ever changed PVCR size two or three times.
 
steve340 said:
You may have to change the PVCR size but generally only if the carb is very small or very big for the size engine it is on.
I have played on the dyno for years and a fair few carburettors. Only ever changed PVCR size two or three times.
Click to expand...
Probably depends on what kind of carb a guy is dealing with as well. I've been running Quick Fuel Q and SQ mechanical secondaries lately. I have had to mess with the PVFR's on all of them. I would bet that vacuum secondary models would be a lot closer from the factory for street use.
 
The 650 VS worked great with factory settings. (So did the Eddy avs2, that annular booster is nice.)

Though to get that fast aggressive response and then smooth acceleration from the Holley I had to change to:
Primary jet: 63 (Stock: 67)
Secondary jet: ongoing tune
Power valve: 7.5 and even 9.5 worked good. (Stock: 6.5)
Pump cam: Orange (Stock: Green)
Pump nozzle: 25 and could go smaller. (Stock: 28) (Note, did not like "tube type" nozzle.)
 
What is your cruise RPM that you feel the AFR is too rich? What are your cruise AFR at 2500, 3500 and 4500?

If you had to step down 4 jets sizes, maybe your rich condition source at lower cruise RPM is due to the IFR size, not the main jet. Said differently, you need to fix the rich condition at cruise, not the lean condition at WOT
 
BSB67 said:
What is your cruise RPM that you feel the AFR is too rich? What are your cruise AFR at 2500, 3500 and 4500?

If you had to step down 4 jets sizes, maybe your rich condition source at lower cruise RPM is due to the IFR size, not the main jet. Said differently, you need to fix the rich condition at cruise, not the lean condition at WOT
Click to expand...

That is true, also another reason to get a metering block with adjustable IFR/PVCR etc. :thumbsup:
I cruise at 2000rpm with around 13.5-13.8 afr, 2500 almost same, 3-4000 it goes above 14.

Edit: I was down at 60 for the primary jet to reach 14.2afr, though my engine didnt seem to like that.
 
Last edited:
Jonas Nordstrom said:
That is true, also another reason to get a metering block with adjustable IFR/PVCR etc. :thumbsup:
I cruise at 2000rpm with around 3.5-3.8 afr, 2500 almost same,
Click to expand...
Something is way out of whack if those are your actual #'s. I don't think the car would even run, if it did there would be some major black soot coming out of your exhaust. Or did you mean to type 13.5-13.8?
 
hunt2elk said:
Something is way out of whack if those are your actual #'s. I don't think the car would even run, if it did there would be some major black soot coming out of your exhaust. Or did you mean to type 13.5-13.8?
Click to expand...

Oups, sorry. Now fixed, thanks for the heads up. :thumbsup:
 
BSB67 said:
If you had to step down 4 jets sizes, maybe your rich condition source at lower cruise RPM is due to the IFR size, not the main jet. Said differently, you need to fix the rich condition at cruise, not the lean condition at WOT
Click to expand...
^^This.
While it's true there's no set rule for jetting spread, I believe if it needs a spread outside a window of 6-10 sizes then something else is off. Just continuing to add secondary jet to get to a desired mixture is the wrong approach IMO. Some of those carbs have been know to have a too-slow secondary opening rate as well...so make sure they're opening when you need them to.

And something that grabbed my attention was the vacuum readings. 16" idle/in gear and 13" at cruise? What cruise speed was that measured at? I ask because cruise vacuum should be higher than idle.....it has to, up to the point that pressure begins to drive the intake charge but that's not until faster speeds/bigger throttle opening.
You might need to start doing some vacuum leak hunting.
 
beanhead said:
^^This.
While it's true there's no set rule for jetting spread, I believe if it needs a spread outside a window of 6-10 sizes then something else is off. Just continuing to add secondary jet to get to a desired mixture is the wrong approach IMO. Some of those carbs have been know to have a too-slow secondary opening rate as well...so make sure they're opening when you need them to.

And something that grabbed my attention was the vacuum readings. 16" idle/in gear and 13" at cruise? What cruise speed was that measured at? I ask because cruise vacuum should be higher than idle.....it has to, up to the point that pressure begins to drive the intake charge but that's not until faster speeds/bigger throttle opening.
You might need to start doing some vacuum leak hunting.
Click to expand...

I have changed to the purple spring and the secondaries opens all the way with a strong smooth acceleration.

Cruise vacuum is a different story though as I hunted for a lower number thinking it had to be lower than idle.
Now when I know better I will get out this weekend and do a proper reading. :thumbsup:
 
If I were you I would start from scratch. Sounds like you were a little confused what you were looking for in the first place. One thing I learned when tuning with a wideband is don't always chase the number on the gauge. See what the motor likes. You don't have to see 14.7 on that gauge at idle and cruise. What do the plugs look like.

A little point of view from my own experience . Tuning my current 440, thought I had it running pretty good, idle around 13.8 cruise was a bit rich at 13.2-13.4 and I had a super rich WOT at around 11.0. Adding a secondary metering block I was able to get the WOT down closer to 12.0. It wasn't until I recurved my distributor that the car REALLY woke up . And all my carb setting just sort of seem to fall perfectly into place, the car idles, and even runs cooler now that the timing is better suited to the engine/car setup.

I was able to drop the primary jets down 1 more size to 71. Now the car cruises at 14.5 AFR everthing else was left the same , but it sure does work better now !!

YOu can throw air and fuel at a motor all day long but if you don't tell it how to spark it... your pissing in the wind!!

IF your happy with the igntion setup then move on and I would Lock out the secondaries , and go threw the carb circuit by circuit.

Really we don't know the rest of the combo either. What is your ignition setup ? Base timing , total timing and when??? Could be chasing a carb problem when it really ignition is the problem.
 
Canadian1968 said:
If I were you I would start from scratch. Sounds like you were a little confused what you were looking for in the first place. One thing I learned when tuning with a wideband is don't always chase the number on the gauge. See what the motor likes. You don't have to see 14.7 on that gauge at idle and cruise. What do the plugs look like.

IF your happy with the igntion setup then move on and I would Lock out the secondaries , and go threw the carb circuit by circuit.

Really we don't know the rest of the combo either. What is your ignition setup ? Base timing , total timing and when??? Could be chasing a carb problem when it really ignition is the problem.
Click to expand...

I was not confused, I knew what i was looking for, but it is exactly what you say here: "don't always chase the number on the gauge. See what the motor likes".
While hunting for the 14+ afr I passed a moment where the engine ran really well across the band, it just felt right at 13.5. Though I continued on for that perfect gauge number. Now I am going backwards towards where the engine ran ats its best.

I am happy with the ignition set up at 14 btdc and 35 total mech though recurving the dist is something I am also going to try, got some springs in the mail.
I did lock out the secondaries, heck i spent lots of gas just testing out pump cams and nozzles.
 
Allright, weekend is over and I got some readings.

First the vacuum:
Idle park: 20Hg
Idle gear: 16Hg
Cruise 2000rpm: 17Hg

Now, my afr for cruise went up a little to 14 (No primary jet change) ...could this be because the weather suddenly turned warmer here?

Just for the heck of it I changed the secondary jets three steps up to 76 and reached a WOT afr of 13.

Now for the interesting part, as beanhead said: "Some of those carbs have been know to have a too-slow secondary opening rate as well".
I did the zip tie test again on the VS and this time really carefully went on one WOT runs and checked the zip tie directly afterwards.
I found out that the secondary does not open fully until 4000rpms, this is also where I got the 13 afr reading.

So for next test Im going to try one of the lightest springs (I dont have a screw-adjustable VS yet.).

Quick question: I do get the same results when testing VS opening rpms at first gear, right?
(When going 4000rpm 3rd gear with a 3.23 rear I am nervous about any cops being around.) :D

ps. It has electronic ignition with the little black box from Mancini.

engine_1.jpg
 
Its very odd that your cruise vac is basically equal to your idle . I don't really know what to make of that.
 
Canadian1968 said:
Its very odd that your cruise vac is basically equal to your idle . I don't really know what to make of that.
Click to expand...
Yeah, something is not right with that.
 
I don't think it is a worry. Probably a sign of a mild camshaft grind and a small carburettor.
 
Who knows - every engine/carburettor/camshaft combination would be different.
Even a worn out engine will have different vacuum readings.
For instance if you bolted a 950 cfm carb on your engine it would have a different reading.
But around 14 -17 inches of mercury I suppose. I would only be concerned about a really low reading.
What you posted earlier is fine in my opinion.
 
steve340 said:
Who knows - every engine/carburettor/camshaft combination would be different.
Even a worn out engine will have different vacuum readings.
For instance if you bolted a 950 cfm carb on your engine it would have a different reading.
But around 14 -17 inches of mercury I suppose. I would only be concerned about a really low reading.
What you posted earlier is fine in my opinion.
Click to expand...

Ok, thanks.
I dont know much about the cam as it came with the motor, it could very well be stock.
 
I think you are right it is stock or very mild. 20 in Hg at idle is pretty high.
 
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