I have a 73 Maverick with a reman 302. Everything is new. It starts up well, Once warmed up it idles perfect. I'm having a problem with the engine stumbling and acting like it's starving for gas when punching it. (kicking the 4bbl in.) It's not the carb because I've swapped out the carb twice with known good carbs and it does the same thing. Fuel pump and pressure is good. Timing is correct. I have electronic ignition to replace the points. When driving normal, It doesn't stumble and runs great. Just happens when I try to punch it or hit passing gear. What am I overlooking?
How about a blown diaphragm on the vacuum advance of the distributor or leaking vacuum hoses. Does it still have an EGR valve? Possible busted diaphragm on the EGR valve if you have one. MD
Checked the vacuum advance diaphragm. (New distributor) It's good. No EGR valve. Strictly old school. It has a JEGS 4bbl intake and a new Edelbrock 600. The correct metering rod springs are installed for the manifold vacuum pressure also. I had a new Holley 600 prior to the carb now and it was doing the same thing. All new vacuum hoses and lines. Everything is new. Doesn't seem to be a carb issue. Changed out the carb three times with new or known great functioning carbs and same issue.
The timing isn't an issue. It's been addressed on several occasions by different mechanics on what they believe the correct timing should be. Each time the issue is still there. I'm thinking about swapping the electronic ignition module back to the old points system just to see if there's a difference.
It runs very well and performs great at about half throttle. It stumbles like crap only when kicking in the 4bbl. It continues this until I level off. It's not like a hesitation at first and then kicks in. It stumbles throughout the 4bbl thing.
Sounds like it is leaning out when you open up the four bbl. The Edelbrock is not totally a manual or vacuum secondary and can be somewhat tricky to tune. Mine was opening up to soon before the mixture was rich enough to handle the sudden drop in vac.
Not sure what the timing is. It was set the last time it was worked on to correct the issue, which they failed to correct. They kept the car for a week and ended up installing an electric fuel pump and bypassing the manual pump. The fuel pressure is around 5 1/2- 6 lbs. Its a C4 automatic.
Unless timing was off so badly it was effecting operation on the front barrels would that even be a consideration... To be sure you could disconnect the vacuum timing advance and set base timing to around 20*, if it still stumbles on opening of secondaries timing or ignition isn't your problem... Stumbling on opening of secondaires points to lean mixture... While I know you've tried three good carbs, if above has no effect I'd be raising rear float and/or increasing jet size till there was some improvement...
You haven't given us enough information. How long have you owned the car? Has the engine always had a stumble; if not when did this stumble start? Did the stumble start after a modification to the engine was performed i.e. electronic ignition installation, etc.?
Sounds like fuel delivery issue...As stated: Carb too lean (Not enough fuel) Mis adjusted secondary opening...Blockage in metering circuit not allowing secondaries to supply fuel on demand...
