1965 Fuelie stumble 2500 rpm and up.

[Michael T.]

Fuelie stumble 2500-3000 rpm

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1965 Fuel injection. Starts beautiful and runs nice until you reach 2500 rpm and up. The stumble is most noticeable when your into the throttle and accelerating. Surging if your slowly accelerating. Feels like it's starving for fuel. Only does it under a load, in neutral it seems to rev up just fine. High pressure fuel pump rebuilt. New points. Any ideas? Thanks in advance.

[Don H.]

Re: 1965 Fuelie stumble 2500 rpm and up.

Did you replace the condensor with the points? Also the coil and all other ignition parts. Don't assume FI, eliminate everything. Good luck, Don H.

[Loren L.]

Re: 1965 Fuelie stumble 2500 rpm and up.

If it is as originally built, you have probably fouled the spark plugs by putzing around in too high a gear at too low revs; silicone spark plug wires that are more than one hour old are also suspect; clean/gap/replace the plugs and if it goes away, make it a point to find a main artery with a 50 MPH limit where you can "clean" them out with some dashes to 60 MPH in second gear, like weekly.

[Gene M.]

Re: 1965 Fuelie stumble 2500 rpm and up.

Fuelie stumble 2500-3000 rpm

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1965 Fuel injection. Starts beautiful and runs nice until you reach 2500 rpm and up. The stumble is most noticeable when your into the throttle and accelerating. Surging if your slowly accelerating. Feels like it's starving for fuel. Only does it under a load, in neutral it seems to rev up just fine. High pressure fuel pump rebuilt. New points. Any ideas? Thanks in advance.

Mike, you need to give a bit of actions prior to the problem. Has it just started, was something changed? What plugs, wires, points, condenser? What's condition of suspected parts? Timing setting? Dwell?

[Richard G.]

Re: 1965 Fuelie stumble 2500 rpm and up.

In the 80's my wife used to drive our 63 340hp to work. It was only a couple miles away. After a couple weeks of the short commute the Corvette would hardly do freeway speeds till it was blown out, like Loren mentions. It did have 90 to 100K on the clock, and may have been better when new. However with modern fuel injection we just forget how it used to be crappy fast idle and all. I remember going out after warming up the car to a 2000 rpm driveway idle after the Chevy warmed up and no one was there to kick off the choke.

[John D.]

Re: 1965 Fuelie stumble 2500 rpm and up.

Mike, uy a NAPA Condenser. RR175. Do Not use a DR condenser. Hopefully your coil is a known good original Delco coil. Or maybe an auto store one.
Assuming that it's not electrical makes it difficult as your FI unit could have a number of issues.
Starting from the Passenger side corner.
Drive cable slipping in distributor.
Hi-pressure pump has issues although you said it was rebuilt. Did you sent it out for rebuild or do it yourself?
Float settings although not as critical as a carb they should be close. 2-9/32 for float level. 2-27/32 for drop although for 50 years I have set them at 2-25/32.
Issues with the accelerator pump which is called spill valve. Problems with the internal linkage called axle link.
Bad main diaphragm. Warped main diaphragm cover. Loose fittings causing vacuum leaks.
Main signal line from air meter to fuel meter missing the rubber seal or o'ring where the line screws into the air meter..
Clogged nozzles. Clogged nozzle lines and center fitting with the check valve.
Choke not open.
And the list goes on.
I like all the replies before I chirped in.

Look in the 63 shop manual trouble shooting section although it's not the best. Good luck, John

[Michael T.]

Re: 1965 Fuelie stumble 2500 rpm and up.

Thanks John
I wanted to provide an update and thank everyone who has provided advice. Jerry Bramlett has been wonderful walking me through the steps over the phone and via email. He had me check the distributor mechanical advance and vacuum advance for proper bushings. It's timed at 12 degrees initial and 36 degrees by 3000 rpm. We checked to make sure it had the right timing degree plate and finned harmonic balancer. Jerry had me check the coil ohms(1.2) and ballast resistor (1.9) I've ordered a new coil, plug wires, cap from K&B Products and a Black dot ballast resistor from Paragon. The economy stop and power stop were way out of adjustment. 1/4" threads showing from the locknut on the economy stud. 7/8" between both studs. My idle fuel screw was at 3 1/2 turns out to compensate for the lean condition. Jerry had me set the economy stud so just one thread protruded from the locknut and the distance between the two studs to just under a 1/2". Next I hooked up a vacuum gauge to see when the ratio lever moves off from the economy stud. At 3 inches it would leave the stop. I removed the two screws holding the enrichment diaphragm and rotated the whole unit 3 turns which changed the setting to 7 inches before the lever leaves the economy stop. I was then able to start the engine and adjust the idle fuel screw to 1 1/2 turns out(1/2 turn back from killing the engine with a lean mixture) and set the idle to 800-850 rpm with 12 1/2" of vacuum at idle. I rode tested the car and it's amazing the transisition that's taken place. With 3.55 gears you had to slip the clutch to get it going, now it just takes right off. I still had the stumble at around 2700 rpm and only when the pedals to the floor. I brought the car back and installed an RR175 condenser. Took the car out again and got a slight miss at 3300 rpm and once again, only when your foots on the floor. If you back off a little, it's winds up perfectly. Tomorrow I'll install a new set of spark plugs and remove all the shielding and try it again. As soon as the new coil, ballast, plug wires and cap arrive, I'll try them. Lastly Im sending my HP pump to Rollin Jones in OH.

[Michael T.]

Re: 1965 Fuelie stumble 2500 rpm and up.

Final update, the stumble is gone. The car will wind up to 5000 rpm perfectly now. I can't say enough good things about Jerry Bramlett. He's a first class act and incredibly knowledgeable when it comes to these engines and fuel injection units. Today I removed all the shielding and the AC R43 plugs an installed Autolite 295 plugs gapped .035. The car idles at 800 rpm, has 12" of vacuum at that idle and pulls like an Ox into the upper rpm band. No surging and even pulls away from a dead stop with 3.55 gears with ease. I still have to change out the plug wires, coil, ballast resistor when they arrive, along with sending out the HP fuel pump, but for now, I'm really satisfied with how it's running.

[John D.]

Re: 1965 Fuelie stumble 2500 rpm and up.

Michael, Glad your got the problem resolved with Jerry's help. You certainly had a really cold spark plug. Glad the RR175 helped some also.
I wonder what cam is in your engine. TO be able to idle a 30-3- cam down to 800RPM is saying something. Also you have a lot of vacuum for a 30-30. Nice.
I never ran a distributor shield as they caused shorts. Happy Motoring. John

[Michael T.]

Re: 1965 Fuelie stumble 2500 rpm and up.

Thanks John, I ordered an Innovate LM2. I'll make up some extensions for the tail pipe probes. I'm curious how close I am to optimum.
The condenser did help. I just received the other parts I was waiting for and will install them later this week. I decided to raise the idle. 800 might be too low for every day driving. I'm now at 850 rpm with it bouncing up to 900. Vacuum is at 12"-12 1/2". Valves adjusted to 30-30. Engine has very little miles since its rebuild.

[John D.]

Re: 1965 Fuelie stumble 2500 rpm and up.

Michael, Nice that you adjust the valve to at least 30-30. Some like them tighter but the more valve lash you have the greater the vacuum and as you all know the FI's need vacuum.
Thanks, John