Big Fat Mid-throttle Dead Spot

Got a new beast (63 Rivi..401)...has the original "4 Jet". Prior owner included receipts of recent rebuild and another receipt for trouble shooting the flat spot.I never had a Rochester before, and I'de can it in a minute but the choke parts are all intact and work great. Starts immediately EVERY time, idles great. Problem occurs at part to mid throttle if I am less than ginger with the throttle. Loses it's breath and all but konks out before I back off the gas...any thoughts come to mind? Thanks, Steve

 

hi Larry,
Thanks for the reply. I did a quick tune up on her. The idle mixture screws are about a half turn out...I thought 4jets were 3 turns out as a preliminary setting? I adjusted to highestRPM but i will use the vacuum gauge this time. I only checked that I was getting advance, period, using the timing light. I will look at mech and vacuum separately..Also It's a pertronix ignition and the rotor is not stock. Has studs with nuts for a holddown as opposed to the phillips screws. waiting on a pal to tell me what a p-tronix is made up of as installed. The dist body and vacuum can are obviously new...I am thinking it's a generic body without the proper weights and springs for a dynaflow. Whole engine rebuilt.Seems like a good job, about 4K on her. Quiet and powerful other than a low speed shudder in the driveline (center bearing/driveline phasing??...course it's hard to tell for me withn a dynaflow, only hitting the rpm range once) and this flat spot. Also has a se through gas filter, always half full. The best thing about this carb (at least I attribute it to the carb) is the immediate starting as opposed to my carters...like a fuel injected newbie. I am guessing it's more of a closed design and slower evaporation of the fuel in the bowls..I'll post back with the results of my "white glove" tune up.. Steve

 

Have similar dead spot on 65 Riv 401

Any luck yet fixing your problem? I have a similar problem on my 65 Riv GS, 125K miles original engine. I have 401, single Rochester 4-jet, TH400, A/C, California PCV system. The dead spot occurs anytime I go from throttle closed to about 1/3 throttle. The engine stops firing completely until I lift. If I floor it, it's slightly better. The problem is much worse when the engine is cold, and it is not as bad when the vehicle is moving. Plenty of power at full throttle, starts and idles nicely, drives steady state nicely at all throttle openings. Here's what I tried so far and summary of results:

1) New plugs AC 44S, had 2 badly carbon fouled, so this eliminated a miss. Also put on a set of Autozone wires and checked compression. All cylinders 145 to 150psi except #7 at 125 psi.
2) Rebuilt original carb with Carburetor shop kit. Found shrunken accel pump seal and mis-adjusted secondary air valve. Set air valve with 1/2 turn preload on spring retainer screw, then tightened set screw and staked. This improved the dead spot (stopped stalling), but did not fix completely. This completely fixed any rough idle issues and made it start extremely easily at all temperatures. I adjusted floats, choke, accel pump linkage, and fast idle all to factory specs. 550 rpm idle, adjusted idle mixture in gear. I'm about 1 turn out on the idle mixture screws for best vacuum.
3) New points, condenser, cap and rotor. --Minor improvement from new points, but stumble still there.
4) Set dwell to 28 deg idle (increases to about 30 at 4000 r's). Set timing to factory setting, 2.5 deg BTDC at idle with vacuum advance disconnected. I'm getting about 24 deg timing at idle with vacuum advance in. Centrifugal advance brings total timing to about 38 degrees at 4500+ RPM with vacuum disconnected. This made a considerable improvment again, but dead spot is still there. Some light spark knock on 93 pump gas at high throttle mid rpm.
5) Tried advanced and retarded timing 5 degrees in either direction. Both were a degradation.
6) Tried disconnecting and plugging all vacuum ports with no change. Also disconnected and plugged PCV valve to no effect. Checked the exhaust crossover and valve. Operates correctly with valve opening when engine is warm.
7) Replaced coil with Borg Warner unit from Pep Boys. Ran ever so slightly worse with new coil vs. original Delco Remy coil.


Any insight would be greatly appreciated!!

 

Sorry for the slow response...In my case the problem was low float settings from the prior rebuild. Apparantly was done according to carb kit specs, which were a million miles from the factory settings. The accelerator pump rod was not set properly either. Off about a ". Between these two changes and a soak in some carb cleaner I am back in business . Running like a bear.. the neat thing is the new style needle and seat...a one piece ball bearing deal...seems to work GREAT! learned a few things about Rochester 4 barrels, too. Some neat features in them..I like the self leveling chambers between primary and secondary side.. Enjoy the rest of the summer, and take care Larry, Thanks for the post..Steve

 

Hey Guys

The problems that you are experiencing is normal for a Q-jet that's warped. All Q-jets are warped and have loose throttle shafts. If you go to www.mrqjet.com you will see how to check your Q-jet for warpage, and what you will need to fix it where it will work better than new. When the front 2 bolts are tightened down on every Q-jet it pulls the float bowl assembly down on the outside edges into that soft thick gasket between the base plate and the float bowl assembly, and after enough time and heat it warps the carb. and in some cases up to 1/4 of an inch on each side. The carb. is about 200% more warped when tightened down than when you take it off and check it. Let me know if I can help.