Vintagevetter

About a year ago, I had a Holly Sniper installed on my 69 Corvette. The engine is a 350 small block, built to 1970 Lt-1 specifications. 370 HP System was installed by a professional engine builder. When I first got the car
back, the Sniper system was fantastic, with a throttle response, I had never had with the car before, right on up to redline, with no missing, no flat spots, just pure power. Start up was great also no more pumping the carburetor.
After a few weeks, I noticed little problems when shifting to a higher gear, there would a little hesitation when you got back into the throttle. On steady cruising on the freeway, if you fed in a little bit of throttle to speed up, the
hesitation would happen again. Other problems, I notice my idle was "hunting" between 700 and 900 RPM. One morning I went out to start the car, and as soon as it fired up, the idle when to 2200 RPM and stayed there. It was
only when I put it in gear and eased out the clutch and put a load on the engine, did the idle drop back down to normal. I took the car back to my mechanic, and he check the settings with the handheld display, He changed a few
settings, and it seemed to be running OK...(still didn't get rid of the idle hunting thought. I drove it a few days and all the little problems came back. After talking to several knowledgeable people about the problem who had worked
with the sniper, I got several different answers to my problem. One guy said he thought it might be an electrical problem, and that the Sniper doesn't like the voltage to fall below 11.5 However my display monitor showed 12.6 volts
and the alternator was putting out 14 volts. The second guy, he thought it might be a vacuum problem, and the Sniper didn't like low vacuum. As you know the C3's have a LOT of vacuum lines for the headlight doors, and windshield
wiper door., so there could be a leak. A third guy thought it might be a timing problem, as he increased the timing on his Big Block El Camino in 2 degree increments, starting at 15 degrees up to 26 degrees, and his El Camino was running great.

So if anyone out there has some ideas to throw out, what my problems might be...and perhaps the 5 most common problems that cause a Sniper NOT to run right, I would be happy to hear your comments thanks

4-vettes

a "Hunting idle". 3 things come to mind. 1 Target AFR set to lean. 2. sticky IAC or improperly setup IAC. (At how many steps is it idling at?). 3 Vacuum leak.
As per hesitations, what are your target AFR's Set to???
if it ran good on the base map but as it learned it ran worse. sounds like target AFR's might be set to lean, also accel fuel can be adjusted. (basicly an electronic accelerator pump).

Jebbysan

Where is you fuel pressure at? You need 58psi.
Put a vacuum gauge on it and observe the needle......disconnect the vacuum tank (that runs the headlight doors and wipers) and plug the line then run it......
I am ruling out timing because it ran fine at first.......

Jebby

trevorhowe1212

I had a somewhat similar issue with mine after 1 week. Drove the car 2 or 3 times to start the learning process. The next weekend I was having a lot of issues with high idle and going super lean and back firing on a slow acceleration from stop. I found out my IAC was sticking all over the place. Even when the sniper was commanding 0% on the handheld I could feel it sucking a lot of air. Thought I had a bad IAC. But a little foil around the IAC solved my problem proving I RFI issues. Was also easy to see the RFI on a datalog. Points were jumping all of the place.

I thought I had all of the spark plug wires far enough way. But I had routed my #2 wire in front of the distributor to cross over to the other side. And even though it was up against the cap as far away from the IAC as it could be, it was still causing my issues. I routed my #2 wire over top of the cap between the #3 and #4 terminals and all of my problems were away. For safe measure, I also put on ferrite beads on all of the sniper wires.

Rescue Rogers

I just started my car from sitting all winter and the IAC (IDLE AIR CONTROL) was at 30% at idle...If your IAC isnt set to the TPS (THROTTLE POSITION SENSOR} then your EFI will be getting confused. so when you turn the key on , before you start it look at he handheld and see what your IAC and TPS say. THey should both be zero. Everytime you cycle the key the TPD resets its zero. If not you need to re-calibrate them. What I did was looked online and found a bunch of Holley videos on rough idle and watched them all, looked at IAC tuning and TPS tuning. THey are all good info. Eventually I found a write up and that solved the issue. IF yout TPS doesnt read zero you can rezero it.
If your IAC is high, turn off the key, turn your idle set screw one full turn in, it will increase your rpm to about 2000. start your car and look at your IAC. Cover the IAC hole with your finger and you should feel a lot of suction. Leave it there and turn your idle set screw down until you get to the set idle rpm. you should see the iac go to zero. .once it does turn off the key and restart it, both should read about zero at idle or just above.

THe other cause is you are developing a vacuum leak, get a can of starting fluid or carb cleaner and lightly spray the base of the carb, the pcv valve and hose and any other vacuum hose that goes into the carb or manifold. If the RPM increases you found your leak.

Factoid

Another cause is the CTS. Read the temp on the handheld and if it doesn’t seem to be tracking actual, replace it. It is cheap and easy and often fixes the wandering idle issue.