This was originally for GTE guys, but read down and it applies to 1968 Holley 390 carbs. I have done this to one member of the site (PacificaXR7GT) and his car ran much better! 1968 was a “lean” year for Holley carbs due to emission control. This applies to List 3765 and 3796 Holley carbs on 390 Cougars and Mustangs.
So, how many GTE guys are running the original List 4088 Holley carb? The 600 CFM carb that amazes me that Ford would put on a 427 even in the smog era. We are running one on Bart’s GTE.
I have struggled a bit with the idle characteristics on this carb. Not liking to idle in drive with the torque convertor pulling. I am an experience rebuilder of all muscle car carbs and ship around the US with fully wet tested carbs if I cannot have the car in my shop. Pulled it of yesterday and went after the secondary throttle plate alignment and how they closed. My bad on the first session on this carb a year ago not noticing the small misalignment of the secondary throttle plates. This allowed the secondary’s to hang open a bit forcing over closing of the primaries. Closing the primaries to much affects the air mixture screws due to misalignment on the primary transfer slots. The fun of working on an item that many people have had their hands on.
Sorry about all the tech crap.
So I backed off the secondary stop screw to allow me to loosen the throttle plates and achieve full closure of the secondary plates by loosening the throttle plates on the shaft and retightening. Bingo better closure. This allowed me to open up the primaries and get a better alignment on the transfer slots and it idled better, but not good enough.
Attached is a PDF of a Ford Technical Service Bulletin from 9 February, 1968. I have performed the enlargement of the secondary discharge ports on about 5 1968 GT 390 Carbs with poor idle. The 4088 Holley for these cars are nearly identical to the Holley on the 1968 GT-390 cars. I opened the discharge ports on the 4088 to .030 for even a bit more fuel at idle. Upon reinstallation today the GTE was idling almost a 1,000 rpm faster than before the enlargement. I backed of the primaries to bring it back down and it idles even better that yesterday in gear.
All and all a nice fix to a smog era carb to make it run better. Transition to moving forward is much smoother and overall acceleration is way better.
Original 4088 on mine. It runs consistent 14.10’s at about 100 MPH at the Pure Stock Muscle Car Drags with the smog system functional. It’s dipped in the high 13’s if I shift it and launch hard but it’s not as consistent so I don’t race it that way at PSMCD.
I used to have a C9AF-U carb on it and no smog. It was maybe a tenth faster that way but technically not legal for PSMCD if they checked.
Mine runs fantastic at idle or WOT. It’s been rebuilt once by Mike Ulrey (RIP) and twice by Joe Bunetic over the twenty seven years of my ownership. Since I switched to race gas about 7 years ago it has run much better.
Royce,
Glad to hear that your car runs great. My thread was to give other owners a bit of insight to official Ford documentation as to problems with carbs in this smog era. I will post tomorrow about another anomaly about these carbs that also attribute to poor idle conditions.
So many things can attribute to poor idle such as timing and poor electronics. Trying to help the rest of the GTE Cougar guys.
That TSB is quite valid for the GT-E carburetor. Joe did that to my carb the first time around, probably 1994 - ish. I never ran it any other way so I have no idea about how that would have been but likely not as good. Initially (1994 - 2002) I had the factory MR intake and C3AF carbs, headers, wild cam etc. It was fun but the PSMCD is more fun so it’s stock now.
Royce,
Joe was a smart man and must have been in the TSB loop through Ford Dealers! This small modification seems to work wonders on these Holley 600 CFM carbs in the 1968 era!
Joe worked for Holley when they were in Kentucky and learned his trade there. Then he spent 30 years as a mail carrier for the USPS. He has been retired from the USPS for a couple years so all he does is restore Holley carbs these days. He is an old master of the highest order.
Soooo, now that we have digest the above, here is another anomaly that may exist in any Holley carb in the 1967 (late) 1968 era. I have seen this mostly on 3795 and 3796 Holley carbs on 390 Mustangs and Cougars. I have also seen it on a couple 4174 and 4168 carbs for 428 Cobra Jet engines. The 4088 is cast with the same area for these extra needle valves in the throttle plate. This hole was not drilled on the throttle plate in the 4088, but has been on about 5 carbs I have rebuilt. This hole contained a small needle jet that I will try to send another picture of. My guess is Ford requested Holley to assist with emission control and this was a second idle mixture screw to lean out the idle. After installed a thick paste was put over the jet hole to prevent air leakage.
The Holley carbs that I have seen on the 390 and 428 were so lean at idle that they performed very poorly. I have removed the paste and screwed that idle jet out enough to take it out of play. Reinstalled with some lock tight. They are very fragile and I have had to use heat and persuasion to get them out the first time. Once out of play the idle mixture screws in the metering block react more and I can smooth out idle and improve the transition away from a stop sign.
I called the Holley tech line, but they are too young to have any knowledge of this.
I also should have said that I would not be surprised to see the throttle plate mixture screws on a 4088 carb. If so they would affect the idle a whole bunch. I am beginning to feel that Royce and Bart are the only ones with a GTE that runs!!
I’ve learned a lot reading this thread. While I’ve played with Boss a 302 carburetor (what would that have been, 4511?), I came along a bit late and never owned one of the 600 CFM ones or 428CJ.
I checked all the Holleys I have here. None have those holes drilled. Hard to see on carbs that are installed but I could see the outside with a mirror and flashlight.
Two List 4088’s (one 7A date code on my car, and a 1969 dated service replacement on the shelf)
Two 4174’s both dated in April 1968
One 4609 with a 1974 date code.
Two 3310-1 carbs with mid 1970’s date codes
One Street Avenger 785 that was new from Summit Racing about a month ago. Didn’t write down the List number but did look for those holes.
Royce,
Were the castings similar, but the hole not drilled of course. Bart’s 4088 had an identical casting to the ones I have seen drilled, but again his was not drilled. After a bit (a year or so) this casting configuration disappeared in my experience.
