Think this one's for Paul...

[ChuckG]

Anyone else chime in, but this us uber high-end FUBAR stuff.

Car seems to have a flat spot, almost a miss at mid throttle. Plugs and wires are new and look okay. I have an adjustable fuel regulator, new AC/Delco pump, good filters, Top Teir gas, etc all new distributor w/electronics and the car just has a bit of a stutter. Injectors are all 16.5 ohms.

Any guidance where to start looking? No Check Engine light yet.

I do get a horrible vacuum let off when I fill the tank and the HVAC doesn't blow as hard as it might. Vacuum problem? Failed canister or valve? Maybe a mis-route?

Anyone have a good vacuum schematic for the 1985 TPI coupe? Better still a very reputable and fair Vette shop in SE Michigan (Westside of Detroit) Canton area?

PS new clutch/pressure plate/new slave is SMOOTHE.... But the old had a jerky element that made the car feel so much faster. It's still quick. Ask that confused kid in the '09 Bullitt!

[c4c5specialist]

HI there,
check your throttle position sensor voltage sweep from idle to WOT as its critical to accurate operation.
The only other thing may be the MAP sensor.
Allthebest, c4c5

[ChuckG]

I do have the TPS tool, but will have to investigate the MAPS. Thanks!

My other thoughts are bad fuel pressure regulator diaphragm, or vacuum canister/valve. Can I skip those? that venting on fill up is very disconcerting and started about the same time as the less-then-full throttle mild stutter I'm feeling. Idle is off a lot too, more than can be attributed to the cam.

I'll dig in when the weather is good and report back. Thanks!

[c4c5specialist]

Hi there,
Going to Carlisle to get more service manuals.
Hoping to get a replacement for my 1984/1985 sets and if so, we can continue with the diagrams when I get back.
Allthebest, c4c5

[ChuckG]

So far, the TPS seems to checkout just fine (.54V), but I will say the problem is most pronounced when the car is cold.

I'm also not 100% happy with the EGR vacuum line. Seems to be a loser fit than I would like.

Also considering the fuel pressure regulator diaphragm as a possible problem area. My idle is a bit erratic, from 740-850, more than can be explained by cam alone. The mass air is new, BTW.

[c4c5specialist]

HI there,
Im not talking just at idle.
You must check the voltage through its entire sweep of the throttle plate.
Allthebest, c4c5

[ChuckG]

Does the spec change from .54 volts with an RPM increase? I tested the voltage, revved some, rechecked.

I'd pull a code, but my DIC has been finicky all summer. On my winter fix list!

I did replace the TPS a few years ago with this one, marked "Wells" (all available locally--local Chevy dealer thinks Corvettes started production in 2008, or whatever year he's trying to sell that day). Think I do have my original, but imagine an AC Delco can still be had. Eckler's lists it at $80, but will call around before I buy.

Mileage has been suffering too, if that's any help. The car did drop power and then conk out once at about 50MPH, but restarted immediately. Drove odd the whole way home, but now just has a "dead hole" kind of feel when it's cold. Wires seem fine.

I've read the MAF system (mine is new) negates the MAP sensor. Then I see Zip lists one for '85. Confused....

[c4c5specialist]

Hi there,
When you are doing a voltage test, the engine should be off, key on.
You set your multimeter up as you normally would, and then open the throttle in a normal fashion all the way to WOT.
You are looking for a STEADY increase in voltage from .54 to around 4.5 volts. Any substantial change in voltage, dip or spike, would probably indicate an issue.
Let us know how you make out, c4c5

[ChuckG]

Sorry for the delay. Much rain & early fall weather.

TPS checked out within specs. Fuel filter is new too. All injectors at 16.5 .

Thoughts on the stock throttle body being too small for the modifications?

If not, time to dig into the vacuum lines. Grrr!!!

[ChuckG]

Okay, TPS voltage reports back fine.

Next thought is MAF burn-off. Is there a way to test it?

Now, the car coughs a bit when cold, but when up to temp, runs good. Mileage mostly around town in 18.8. Best it ever was WAY before the cam was 19.1.

Any thought this stock TBI is choking the cam?