So a very good friend of mine just moved his growing performance diesel buisness (Mid Atlantic Diesel) into a new shop, it happens to have a dynojet 248x chassie dyno. Soooo the cool thing is I have unlimited access to it... The first thing I learned is the wideband is key to tunning. I have had some very good racer's and engine builders mentor me through the years on plug coloring to figure lean/rich. So after the first pull it made 291hp and was lean, I looked at the plugs and they looked great. So I really think with todays electronic ignitions like MSD like I have and such, it is hard to get a truely accurate reading from looking at plug's. Next I went up 2 jet sizes in front and back, still lean on top after 5500 it went from 14:1 to 15.5-16:1 slowly climing as rpm increased to 6400 were I ended the run. That netted 304hp. Next run I jetted the back up 2 more that netted 310hp but was still lean on top like the previouse runs. Then I went big and wile changing jets to 76's in the front and 88's in the back I knew in the back of my mind something wasn't right. Sure enough the issue at hand showed its ugly face... She went real lean at 6100 and detinated right befor I could get off the throttle. So with all that yacking said I must have a fuel pressure issue. My fuel log started leaking a few weeks ago and I had some spare line and an fittings and maid up a duel feed with two lines out of the regulater, and did not make a spot for a gauge... Dumb Ill be back down to the shop next week to roll again
The biggest affect of jet size is in the cruise mode A/F ratio, which you want to keep at around 14.5-15.0. If you try to richen up the WOT A/F reading of the engine using only jet changes you might make the car undriveable at part-throttle cruise. For power circuit enrichment you should open up the power valve channel restrictions to get an A/F ratio of around 12.8-13.2 as a start.
after I read were you jetted a 2nd time, I was thinking running out gas. Very nice numbers and looking forward to seeing what it pulls when it doesn't run out of gas.
The power valve determines when you get enrichment, at what vacuum level. The two small holes beneath the power valve determines how much enrichment you get when the power valve opens. Holley typically makes these holes smaller than is sometimes needed because they are more worried that someone will experience an unacceptable bog. That may make for better street manners but won't necessarily provide maximum performance.
To increase the power valve channel valve restriction you will need to drill the passage restrictor. You will need a set of number size drills from 0 to 80 and a pin vice. The pin vice is the tool to hold the drills with - you don't use a drill motor to drill out these restrictors. Get a good diagram of your carb and a book on tuning Holley carbs - there are several that are sold in most auto parts stores.
To be honest guys I have never had an issue with a bog or anything with this carb, Ill take baby steps and address the fuel supply issue befor I go drilling anything. It has a 2.5 PV in there now and has allways worked flawlessly pulling 1.60 60's. The car rarely gets drivin anymore and has more miles on the dyno then its had on the street in months...lol Plan 1- Put a gauge on it and check FP at WOT 2- Fix issue 3- Adjust jets according to A/F results 4- Post results
Oh, and don't forget about the squirters, I would imagine the Accelerator pumps are full on a Dyno. But depending on how you launch, be it two step or foot brake, they could be almost gone at launch.
best thing to do on the PVCRs is to drill & tap for Mikuni motorcycle carb jets. Much easier if you get a little too big, you can just change a jet rather than JB welding up the hole and redrilling.
Not that I can tell 2 right. ...................... 5 So I found the issue, a piece of gasket in the needle and seat of the primary. So I fixed that and back on the dyno today. Left the big jets in (76's and 88's) knowing I would have to jet it back but just wanting to see were I was. 324hp on the first pull and 12:0 a.f. and looking great the whole way through to 6,400 (were I go through the traps and I shift). So I jetted it back to 76 and 86 and also decided to keep running it untill the power dropped of to see were I was there. That yeilded 331hp and a big suprise to see that power didnt fall untill 7,000 and peak power was 6900, yeah a hyd flat tappet 10:1 stock 302 crank and rods with an Air Gap and AFR's maken power to 7k I couldnt belive it. Anyway I ended the day with 73's and 83's and made 337hp and 12.9-13.0 A.F. the whole way through the run. The graph seem's really flat too losing only 10ft lbs of torque from 5150rpm to 6750rpm. I have the print outs and a video or two. I will post them as soon as I can. Thanks for the suggestions guys, think I gotter figured out. A big thanks goes out to Lloyd Mills at "Mid Atlantic Diesel" out of Mt. Airy, MD for the use of the dyno.
Good numbers! So that means your vacuum level has to drop all the way to below 2.5 inches before the power valve opens. Those values are usually for motors with huge cams and very low vacuum. You should be able able to open it way before that.
Thanks !! It has been awile since I changed to that pv, but I do know that my engine carries very low vacuum.... 5 or 6" at idle in gear. In the 8 years Ive owned the car I have yet to find out the maker of the cam (came in the engine with the car), I do have all the specs though via cam card, (came with the car in a bag of reciepts) though there is no company name on the card. I think a different grind cam may help a great deal, though I wasnt expecting the #'s that I did end up making.
If you never saw a change in the plugs you might want to try a different plug. I experienced a lean condition a couple of years ago and couldn't get it to show any more color on the plugs. Went to an AR3910 racing plug, reccommended by a Ford racer and speed shop owner and cured that problem. Been running them ever since. Just a thought.BTW Great numbers. BTW I'm running Edelbrock performer heads
