if thats the case your power valve should be a 4, if i read the holley book correctly.
So anybody try a wideband O2 on a crossrammed Hemi yet?
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Apologies in advance for the long post, and going off topic, but I certainly would have liked to have had the following information available when I "scoured the interwebs" for answers about my dual disc clutch setup and the correct installation of the hydraulic throwout bearing:
I'm not aware of ANY "adjustment curve", but I could be wrong, or I went through that experience and didn't recognize it as you're describing it.
I had a LOT of work done to my 70 Roadrunner at the time I had the new McLeod RXT setup installed. It was the McLeod tech's recommendation for me once I described the function of the HitMaster launch control system, and that I'd be slipping the clutch under power at the launch (via the HitMaster) to control tire spin and reduce my 60 ft. times, and the pending 572 installation. I had originally asked them about the RST.
The "slipping under power at the launch" was the main reason for the RXT, as its friction material is ceramic metallic vs organic on the RST, so heat buildup and tolerance made the RXT the obvious choice.
EVERY post I've read comparing the two for street and track use applications have been in favor of the RXT. Yes, it can chatter a bit when slipped at lower RPMs, but those occasions have been so rare as to be insignificant, and once the organic RST is overheated, it's going to have to be replaced.
The American Powertrain hydraulic throwout bearing installation, in particular the "gap" that usually is a must between the throwout bearing surface and the tips of the diaphragm springs (or "finger tips" on a Borg & Beck '3 finger' setup) is NOT the way the hydraulic T.O. bearing is set up. I asked about this to at least 2, maybe 3 tech support guys at American Powertrain (that's probably everyone in that department) and they insisted that the hydraulic T.O. bearing is designed to lightly contact the pressure plate springs in the "clutch engaged" position. Of course there's a spec for the distance along the input shaft housing where the T.O. bearing is supposed to be set at, but since I was replacing a "regular" mechanical clutch setup, and my throwout bearing wore out WAY too fast on it, and I'd guess the most common reason for premature wear is not enough gap, I was even more insistent in interrogating them, very concerned that I may be misunderstanding what they were saying, or they were wrong, or my mechanic hadn't installed it properly. To exacerbate my concerns, the car barely crept forward (or backward if in reverse) for the first dozen or so times I drove it after I had all that installed, which was making me think that rather than there being a problem with having no gap with the clutch pedal pushed in all the way, I thought it hadn't been set up to push the diaphragm springs in far enough, even though the bearing face was making contact, and that's why I was getting that creep...
Luckily for me, that creeping self-resolved, and I haven't had ANY problems with the setup since then.
I hope that helps...someone.
68 HEMI GTS
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That recommendation from holley never made any sense. Adjust the power valve sizing to enrich where needed in transition from cruise to say passing gear. There’s times I run a higher number PV than I even have for idle vacuum.
68 HEMI GTS
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Getting ready to get after my cross ram tuning. I have two sets of carbs I plan to run on the dyno. Original 770’s and a pair of vacuum 850’s. I’ll jet for WOT. I’ll dial in the low speed circuits once it goes in the car.
finish hooking up my afr gauge today and see what it says..set up my 770's per above suggestions with QFT metering plates.
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I certainly hope it runs great, and I know the AFR system is a great tool for performance tuning, but I have to say, WOW that's some beautiful eye candy!
doesnt that just dump unneeded xtra gas when its not needed due to the power valve opening when not nessacary because it doesnt match engine requirements/vacuum .. setting it up for more street than strip.
68 HEMI GTS
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Generally no. The PV only contributes extra fuel when the boosters are active and vacuum low. Therefore it has no affect on idle, even when the PV is open. Usually if the boosters are active you have high vacuum unless you’re accelerating. I have ran into situations where it may need to be adjusted due to low vacuum and the boosters on during off idle acceleration in really large cammed engines. It all depends on the combo. I run a 10.5 PV in my roadrunner and it only idles at 6” of vacuum.
in gear i'm 4.5-5 inches of vacuum, .675 roller. thats why i installed a 2.5 PV.
68 HEMI GTS
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I look at my vacuum at cruise. Say 45 mph. Lay into the gas like you’re going to pass someone. Where the vacuum drops to I usually set the PV number close to that. It’s an enrichment circuit, and I like to use it. With 2.5 PV the only time they provide extra fuel would be at WOT. I promise you you could put 6.5’s in it right now and see no affect on the idle A/F. I do run my cruise somewhat lean though so I usually need the extra fuel for acceleration. It’s something I always experiment with on every engine I tune.
The 700 lift roller in my dads 512 has very poor vacuum. I settled on 5.5 for it. It tends to pull on the boosters early while the vacuum is still low.
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That is what is published but very inaccurate during real world driving. I found that your cruise vacuum is a more accurate and effective number to use. Mine is 19" hg. and I use a 10.5 PV. This allows my engine's lean conditions, i.e. grades or part throttle acceleration during cruise to recover sooner. If I was using a 4" hg. PV my engine would be in a lean condition much longer before it recovered to acceptable AFR. If there was a 12" hg PV I would use it but 10.5 is the highest I have found.
The power valve does not operate at idle so using the idle vacuum as a reference is not accurate.
i also installed adjustable secondary metering plates, now able to change jetts and the .100 idle "hole" is now a screw in .028 IFR bleed..ready to start.
something i thought was odd on these new repro 4235-36 770 carbs, the 4235 has 8 bolts holding the base plate to the carb body and the 4236 only has 6. the 2 middle holes r there just not threaded, and looking at pics of other units i see the same thing. seems very random.
2blader
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almost ready to get back to it. Yea, it has been more time than expected. Things get in the way and other projects unfortunately. However, I have to replace brake line up front and then back to the carbs.
2blader
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Whelp when I got my carbs (770’s) they had 2.5 power valves in them. Thought that was odd so I checked the vacuum at idle with a guage and it was low maybe 4.5-5 @1200 ish RPM. Yea my cam is beyond snotty. .283 @ .050 .667/.646 105 lobe center. Ray Barton engine. Down the road I replaced the stock metering block with AED’s blocks that are adjustable and were set up like the originals at a start point. So I goofed around but never made that much progress with how the car ran. As it had other issues to be taken care of and now back to this. Before this I put 6.5 power valves in and adjusted idle etc. Then later I went back to the 2.5’s buy not sure why. I know it has zero to do with it. I always have a hard time starting when it hasn’t run in a long time. Yes it has a manual pump but thinking of going electric but that is another story. I got the car running and the idle afr was way off after the power valve change and that made me scratch my head. I read the only time that can happen is if you’re in the transition circuit. I guess that means I have to tinker with the IFR’s? Do any of you have drilled butterfly blades?
u dont have to drill the butterflys,,adjust idle/afr with the secondary throttle blades. there's a screw that u access from the bottom, take it out and replace it with either an allen head or square type. i did the square type wrapped with teflon tape to keep it tite. and i can adjust the idle now with it running and not getting out of the transfer slots.
Think 10-24 thread on the secondary screw. Home depot has them in Stainless 3/8" long. Edit: 10-24 is the wrong size believe it's 8-32 nc!
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2blader
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So do you have to remove the carb every if it needs adjustment? and many thanks.
You put the new hex screw in from the top so you can adjust secondary blade from top maybe with engine running. I confess haven't done it yet. Sure hope you adjust from top engine running or not. Got side tracked with a leaky garage roof.
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