speedtigger

iTrader: (16)

Keep reducing primary jet sizes until you are in the 14s at a cruise on the main jets. Then drill out your PVCR until your WOT A/F is back to where it belongs. How big is your PVCR right now?

Do not fatten up the secondaries to compensate for lean primaries. This will cause potentially disastrous cylinder to cylinder variations. The front cylinders will go very lean. Especially on a single plane manifold.

GC99TA

iTrader: (30)

Thanks man.....that makes sense and is what I was afraid of. It really didn't seem right to jet up the secondaries that far to compensate for the leaner primaries. But for now I'm kinda scared to drill anything "permanent" on this carb until I get back down to sea level because I'm sure it will all change.......but I might get brave and do it anyway. Man I wish these carbs had the adjustable/changeable PVCRs and IFRs in the metering blocks, but from what I've read they don't. I didn't have a gasket so i didn't pull the primary metering block off to check yesterday. Oh, but to answer your question......my PVCRs are 60 on the primary side. Were yours the same on your 950?

So that brings me to a few more questions. From what I gather, the airbleed size numbers are referenced to their diameter in thousandths, correct? Like my idle air bleeds are 76, so they are .076 diameter.....right? And my hi speed bleeds are 36, so they are .036? Are the PVCRs and Idle Feeds referenced the same way? Again, my PVCRs are 60 and the idle feed says 35P. What is the "P" for?

Also, I've been looking at numbered drill bits. I see most sets come in #80-61 (.0135-.0390) and the others are #60-1 (.0400-.2280), with the latter being significantly more expensive. So which size range set did you get? It seems like a guy would need both sets if he wanted to mess with/resize all the different bleeds and fuel restrictions on these carbs.

For example......If I wanted to be able to drill new high speed bleeds and idle feed restrictors, I would need the #80-61 set. But if I wanted to mess with idle air bleeds and PVCRs I would need the #60-1 set, right?

speedtigger

iTrader: (16)

I just ordered a pair of Quick Fuel metering blocks for my carb. They have replaceable PVCR.

I really wish I would have just bought a Quick Fuel Carb to start with.

GC99TA

iTrader: (30)

Cool. I'll be curious to hear what you think of them. I looked at those on Summit's site the other day. I think they were like $58 a piece or something....right? Do you know what size PVCRs and IFRs they come with? Not that it matters since they're replaceable but I'm just curious what their baseline is. EDIT: Just saw on their site their baseline IFR is .033.

Can anyone confirm or counter what I asked above about the actual diameter of carburetor air bleeds and fuel restrictors such as PVCR's and IFR's? Are the numbers listed also their respective diameters in thousandths? I don't have a good way to measure them without appropriately sized drill bits to use as gauges.

GC99TA

iTrader: (30)

So I messed around with my main jetting a little this weekend. At my current elevation and DA (4500ft actual/6600ft DA) it seemed to like a number 70 jet in the mains to bring the cruise AFR into the low 14s. With the original 76's it was in the mid-12s under the same cruising RPM conditions.

I got brave and also took it up to WOT a few times just to see how lean this jet reduction had made my WOT AFR. To my surprise, it was now only leaned out 13.2-13.3 with 70/86 jetting as opposed to 12.8 or so with the original 76/86 jetting. I expected it to get leaner but then I considered the fact that my six jet number reduction on the mains was only a small portion of the total fueling at WOT since I had done nothing to reduce or change the fueling from the PVCRs or secondaries. I guess the bottom line is that it really shouldn't take too much of an increase in my PVCRs now to bring my WOT back in line. I did some calculations to determine how much total area I had reduced in my jetting, and the results support this idea......it was marginal when compared to the total area of all four jets and the PVCRs.

I picked up a set of numbered drill bits (#1-60) at Harbor Freight on Saturday. Now I just have to work up the nerve to actually use them. I'm sure it won't be a big deal once I make the first minor change and measure the AFR change, but for now I'm just a little hesitant to take a drill to my $700.00 custom carb....LOL. The other factor is that I'm afraid to make such a "permanent" change to my set up when I will be moving to a drastically lower elevation in less than six months. I guess the key there though, is it will only want more fuel at the lower elevation and it's easier to go bigger later rather needing to go smaller. That would require drilling and tapping for replaceable jets in the PVCR......which is probably not a bad idea either, but not something I'm mentally up to just yet....LOL.

speedtigger

iTrader: (16)

Bok Bok LOL

GC99TA

iTrader: (30)

LOL......yeah guilty as charged. Maybe one of these weekends I'll grow a pair and go for it.....LOL. I guess if I screw it up it'll give me an excuse to buy some Quick Fuel metering blocks to start over with.

speedtigger

iTrader: (16)

It is a very simple process. Just go a little at a time until you get there. You can drill a bigger hole but not a smaller one.

You can probably start out by going .010" bigger.

GC99TA

iTrader: (30)

Well, I had a great weather day to be at the track today and the car was deadly consistant. In fact, it was so consistant that I couldn't change the 60' times no matter what I did. The worst DA I ran in today was better than the best DA I had run in before, so that was a nice change. I had the chance to try a lot of things today and although I didn't set any new personal best ETs or MPH, I did set some new bests ETs through the full exhaust. Here's how the day played out:

The first pass was my baseline run so I ran the car exactly the way I've been running it (same launch RPM and shift points) up until now. Along with the actual values, I'll also list the corrected slips for comparison sake only. I like to do this to check for consistency between runs on a single day and between runs made on previous dates:

First Pass
DA: 3235ft
Launch: 3800RPM
60’: 1.67
1/8: 7.37
MPH: 94.55
1/4: 11.56
MPH: 116.99
This pass corrects to 11.11@121.83 at a DA of 0ft

Next I wanted to try a higher launch RPM off the 2-step, so I went straight for it and set it at 4400 RPM. To my suprise, this resulted in an identical 60' time and only slightly better ET and MPH, which can probably be attributed to there now being a little more temperature in the transmission.

Second Pass
DA: 3235ft
Launch: 4400RPM
60’: 1.67
1/8: 7.35
MPH: 94.47
1/4: 11.54
MPH: 117.40
This pass corrects to 11.09@122.25 at a DA of 0ft

Since the higher launch RPM had no effect, I set the 2-step back down to my proven leave RPM of 3800 and decided to look for 60' elsewhere. I have been running my Caltrac bars in the upper holes as I believe this to be the best position for my car to hit the tires hardest. I've got a buddy that's been bugging me to try the lower holes to see if it would pick up in the 60', so I tried that next. And guess what......no change.

Third Pass
DA: 3451ft
Launch: 3800RPM
60’: 1.67
1/8: 7.37
MPH: 94.22
1/4: 11.57
MPH: 116.45
This pass corrects to 11.08@121.62 at a DA of 0ft

At this point I determined that my car probably isn't going to 60' too much better until I make more power. The overall chassis and suspension seem to work well and it doesn't seem like power is sufficent enough at this point for it to become responsive to suspension changes just yet. I left the bars in the lower hole for now, but may move them back up to the top until it becomes a problem later on.

So I decided to move on to testing some different shift points to see what affect I could have there. First up was trying 7200 shifts as opposed to my normal 7000ish shifts.

Fourth Pass
DA: 3451ft
Launch: 3800RPM
60’: 1.68
1/8: 7.40
MPH: 94.53
1/4: 11.59
MPH: 116.99
This pass corrects to 11.10@122.18 at a DA of 0ft

As you can see, the car was kind of indifferent to being shifted this high. It really didn't gain or lose much. Maybe just .5 mph in the 1/4. So probably not worth the abuse of the extra couple hundred RPM.

Next I decided to try short shifting it to see if maybe 7000 was excessive too. So I dropped my shift RPM down to 6800.

Fifth Pass
DA: 3530ft
Launch: 3800RPM
60’: 1.65
1/8: 7.37
MPH: 93.32
1/4: 11.60
MPH: 115.74
This pass corrects to 11.10@121.00 at a DA of 0ft

You can see that, despite being the best 60' of the day, this was still the worst ET and MPH of the day to this point. Granted, the DA had gone up again since the 7200 pass, but the 1 MPH loss even shows up in the corrected MPH. Corrected ET was mostly unchanged.

I was pretty satisfied with my launch RPM and shift points by now. So the last thing I was curious about was running with the air filter. I have a 3 inch K&N element but it's on a drop base set up that puts the lid pretty close to the carburetor air horn. Shift point for this pass was moved back to 7000. Here's the result.

Sixth Pass
DA: 3667ft
Launch: 3800RPM
60’: 1.68
1/8: 7.43
MPH: 93.55
1/4: 11.66
MPH: 115.43
This pass corrects to 11.13@120.90 at a DA of 0ft

As you can see, ET and MPH dropped again, with the MPH being the most significant. Again the DA had gone up, but the corrected time shows close to a full MPH loss over ealier corrected times.

So all in all it was an interesting day. Everything came home in one piece and I was back home and unloaded by 3pm. Based on what I've learned about this car so far, I think I could have gotten some 11.4X's out of it in today's weather if I had dropped the exhaust. I guess thats one more track outing in the books for me.

toxic99

Whats the actual altitude at your track?

GC99TA

iTrader: (30)

3075ft.

Outlaw666

iTrader: (3)

awesome! I cant wait to get my heap up and running again

Prorac1

iTrader: (4)

I just got light headed READING that elevation. WOW. I think here in Martin MI we are around 800ft give or take. When you move down a couple thousand feet that thing will deffinately be in the 10s. Congrats on bringing it home in one piece, I know what a challenge that can be, lol. Eric L

GC99TA

iTrader: (30)

Thanks man. Good luck getting it going!

LOL.....yeah, it's just a little thin up here. Trust me.....you can feel it in your lungs when you exert yourself physically. Where I live right now (not where I race) we're at 4500+ actual. One interesting thing I've found when calculating DA is that, the higher your actual starting elevation point, the faster the DA climbs with subtle weather changes. For example, if a track were at 0ft actual elevation and the weather conditions were 70 degrees, 50% humidity, and barometric pressure was 30.00inHg, the DA would be 785ft........so 785ft more than actual. If the same exact weather conditions exists at a track that is at 3000ft actual, the DA would be 4477ft, or 1477ft more than actual.

If my time slip corrections are correct, it would take a DA of 0ft to put me in the 11.0-11.1 range through the full exhaust, and 10.90-11.0 range with open headers. When I get moved back east, I'll likely do most of my 1/4 mile racing at MIR (Maryland International Raceway). The actual elevation there is 80ft above sea level. So getting a DA at or below 0ft will just take nice cool spring or fall evening (say 50 degrees and 50% humidity).

But of course this is all just speculation with math. I don't know if they are accurate, but hopefully they are at least consistent. I use it mostly to eliminate the variable of weather in order to more accuarately measure changes I make to the car. Only time will tell how accurate this is. Once I can run the car in that kind of DA I'll be able to see how the calculations measure up against reality.....