Timing or Fuel? "Flat Spot" Has Me Stumped...

[Tommy F.]

My '75 is back from paint , with a fresh rebuild, (using the Comp Cams 354 H grind that I wrote about earlier this summer)and largely reassembled. Now I'm trying to work on driveability issues.

Initial idle problems ( loading up and stalling when placed in gear ) were cleared up with a Summit rebuilt Qjet....engine starts fine, idles great.
Problem occurs when advancing the motor past 1500 RPM ( had the same issue with the original Qjet that I rebuilt).

I have the initial timing set at 8 BTDC
Centrifugal advance is giving me:

0 at 750
22 total (14 advance) @ 1500 rpm
Can't maintain a steady state above 1500...have to "race" the motor, which does run the advance higher, but I can't say how much.

Using the stock VAC. ( gives me 30 total at 750 rpm ..13" vac)

While trying to steadily increase RPM, when it hits 1500, vacuum begins to drop, carb gets that "sucking" sound. If I keep advancing the throttle, engine slowly comes up, but is really sluggish. Waaay to much throttle for the engine speed. No "lean" backfire or spitting. On the street, the motor sounds fine until I try to push it past the flat spot...

New dual exhaust, no heat riser.

Fuel tank was clean when the resto was done, I installed a new screen, all new fuel hoses, new fuel pump. I guess I could have pinched a fuel line somewhere at the tank or by the pump.

This is the problem with a total frame on that included a distributor and carb rebuild....too many places to look.

One minute, I'm convinced that it is running out of gas, the next, I believe that it is ignition...

I'm open to all suggestions ....

Thanks,

[Rich P.]

Re: Timing or Fuel? "Flat Spot" Has Me Stumped...

Tom,

If it runs fine at a higher RPM than the flat spot I don't think its a kinked fuel line or fuel filter, that usually starts at a certin RPM and continers up.

Rich

[Timothy B.]

Re: Timing or Fuel? "Flat Spot" Has Me Stumped...

Tommy,

I don't know anything about the comp cam 354 but I can only suggest that if it's a high performance cam on a stock 1975 corvette rebuild with stock compression you have some tuning issues.

Stick with the timing FIRST and get a fast advance curve and inital timing set properly. Is the vacuum advance hooked to ported or manifold vacuum??

A camshaft with lots of overlap could cause low engine vacuum and require some carburetor tuning as well. I don't know alot about Q-jet carbs but it sounds like you are describing a transision problem from idle circuit to main circuit and low vacuum could have alot to do with the circuit timing when that happens.

[Tommy F.]

Re: Timing or Fuel? "Flat Spot" Has Me Stumped...

Timothy,

I should have edited the Cam number ( Comp 350H ) ....relatively mild cam.

One reason that I am throwing darts at this thing is that during the restoration, I rebuilt the original carb myself. Same story for the HEI, Transmission, Suspension etc

I had never driven the car when I brought it into my garage, so I really didn't have a baseline to judge. Now I'm starting to doubt everything that I touched ( normal reaction, right?).

The VAC is connected to a full time source.

I believe that the curve up to the problem point is on par with the specs ( 15 @1500 ). I have gone as far as removing the "new" springs and weights that were installed during the rebuild, and replacing them with the originals...

I truly believe that there is a simple solution staring me in the face, but I just can't see it.

Happly Labor Day to all,

[Timothy B.]

Re: Timing or Fuel? "Flat Spot" Has Me Stumped...

If I understand you correct, you are saying that if you increase the throttle with the idle screw there is a dead spot at approx 1500 RPM. Does closing the choke manually cutting off air supply help through this spot?

Make sure there is no vacuum leak.. Do you know if this Q-jet is a reverse idle carburetor, the reverse idle carbs have a adjustable air bleed on the idle circuit that helps lean or richen the mixture.

Make sure the vacuum advance control is matched to the engine idle vacuum so you have full vacuum advance at idle.

[Clem Z.]

Re: Timing or Fuel? "Flat Spot" Has Me Stumped...

make sure the pump linkage is in the inner hole in the pump arm

[Arland D.]

Re: Timing or Fuel? "Flat Spot" Has Me Stumped...

Tommy,

Recently addressed your exact symtoms in a couple of GM 'other brand' cars that had new cams installed. It required matching the metering rod springs to the new amount of vacuum to ease the transition between circuits. It takes some experimenting and isn't really an exact science so it's difficult to be more specfic. Regards,

Arland

[Tommy F.]

Re: Timing or Fuel? "Flat Spot" Has Me Stumped...

Thanks to Timothy, Clem, and Arland....

I have checked the accel pump, and it is indeed in the inside hole, and it appears to be getting a full and robust shot of fuel when cycled.

Everything that I have read and experienced with the q-jet tells me that low vacuum at idle would tend to keep the power enrichment circuit (power piston) open, which would have it running fat, which it is not.

The motor doesn't run past the "flat spot" and then run fine, it really hits a wall, and won't get past 2000 RPM unless I "flash it" up by blipping the throttle.

As a process of elimination, I'm going to hook a fuel container directly to the inlet side of the fuel pump to rule out any leaks or restrictions between the tank and fuel pump.

If that doesn't show a problem, I'll borrow an HEI from someone, and see if it makes a difference. I rebuilt the current one, with a new module and pickup coil during the restoration....maybe I screwed somthing up during that process...

As much as it sounds like it's running our of gas, there could be a spark problem....

If this doesn't reveal the problem, I'll have to get some instrumentation on this rascal .

Thanks for the help, I'll keep y'all posted....( I'll get some pictures posted too....who knew that fresh Medium Saddle Metallic could look so good ?!)

[Arland D.]

Re: Timing or Fuel? "Flat Spot" Has Me Stumped...

Tommy,

You are exactly right about running rich if you have a low vacuum situation. The scenario these cars faced was 1) higher vacuum with a new cam or 2) weak springs that could not overcome the vacuum. Either one results in the 'flat spot' symptom. Let us know what you come up with. Regards,

Arland

[Clem Z.]

Re: Timing or Fuel? "Flat Spot" Has Me Stumped...

Thanks to Timothy, Clem, and Arland....

I have checked the accel pump, and it is indeed in the inside hole, and it appears to be getting a full and robust shot of fuel when cycled.

Everything that I have read and experienced with the q-jet tells me that low vacuum at idle would tend to keep the power enrichment circuit (power piston) open, which would have it running fat, which it is not.

The motor doesn't run past the "flat spot" and then run fine, it really hits a wall, and won't get past 2000 RPM unless I "flash it" up by blipping the throttle.

As a process of elimination, I'm going to hook a fuel container directly to the inlet side of the fuel pump to rule out any leaks or restrictions between the tank and fuel pump.

If that doesn't show a problem, I'll borrow an HEI from someone, and see if it makes a difference. I rebuilt the current one, with a new module and pickup coil during the restoration....maybe I screwed somthing up during that process...

As much as it sounds like it's running our of gas, there could be a spark problem....

If this doesn't reveal the problem, I'll have to get some instrumentation on this rascal .

Thanks for the help, I'll keep y'all posted....( I'll get some pictures posted too....who knew that fresh Medium Saddle Metallic could look so good ?!)

if it is a lean condition the plugs will be clean and if it is a weak ignition the plugs will be dark.

[Tommy F.]

Re: Timing or Fuel? "Flat Spot" Has Me Stumped...SUCCESS !

Sometimes, you just have to walk away for a while.....After all of the things that I have checked and re-checked in the past 3 weeks, everything led me back to the q-jet.

Summit Racing took the carb (Reman by Clamatic) back, and sent me a replacement right away....That did it !
15 inches Vac @ 750 RPM....Idles like a charm cold or hot....pulls cleanly through 1st gear....no stumbles, hesitation, surging, and it was hot outside tonight ! ( 88 degrees - motor ran between 195 and 205 ). Three weeks ago I was ready to put a bullet in it! Thanks to everyone for the input...I'll post pix next week, now that I can get back to work on the trim!