I'm having an issue with my Q-Jet surging. This only happens after the car is at operating temp and only at around 900-1500rpm or so. I played around with the float level to no avail then decided to change the needle and seat and reset the float level. This helped the situation but its still there, it just took longer for the surging to start happening. I put a fuel pressure gauge on and drove the thing around and the pressure seemed fine and I didn't notice anything different when the problem started to occur. Put a screwdriver on the power piston and cracked the throttle and it appears to be working fine. I also checked the plug wires and stuff which all looked fine as well and sprayed some carb cleaner around the base of the carb but didnt notice any change in RPM. At an idle in park I get around 17" of vacuum. Here are some specs. 72 Riviera GS stock except for stage 1 valves and mild port job on the heads, TA shorty headers and 2.5" exhaust The carb is an 800 cfm unit with .075 jets, 44B primary rods and CV secondary rods. At this point I'm not really quite sure what to check next. Could the primary rods be too small causing this surging? Could the float level still be a little off? Any suggestions would be greatly appreciated
The surge is still there but it took longer to start happening, but once it did it eventually got a lot worse than it did with the advance hooked up.
Have you done anything to the distributor? What's your total timing? With the engine running at idle, do you see any fuel leaking inside near the boosters?
I didn't see anything leaking by the boosters at idle but did notice that the gasket between the body and throttle plate was wet, it was also a pain to start. I got a recurve kit from Crane and went through your timing thread and the car is at 32* of total timing with an additional 10* of vacuum advance, all in by 2300 RPM. Other than that it's a stock distributor with points.
I would also take a look and see if there is a bushing on your advance pin. If there was one, and it fell off, you will have too much advance. Usually, surging is too much advance.
. I got a recurve kit from Crane and went through your timing thread and the car is at 32* of total timing with an additional 10* of vacuum advance, all in by 2300 RPM. Other than that it's a stock distributor with points.[/quote] Are you saying that your initial advance is 10 and you have 32 total, all in at 2300 RPM ?
Are you saying that your initial advance is 10 and you have 32 total, all in at 2300 RPM ?[/QUOTE] No, that is not what he said. The total timing is 32* all in at 2300 RPM. He has an additional 10* of vacuum advance. At light cruise, he would be at 42*. Wide open, 32* He did not mention his initial timing.
17" of vacuum at idle sounds a bit low to me. Is the pcv valve new and properly hooked up? Another long shot or two, does the brake booster have a vacuum leak, or the climate controls? At least eliminate them first before going to far.
The points and condenser are fairly new, less than 3 months I'd say and I did try a new coil as the existing one is fairly old but that didn't help at all. There wasn't a bushing on the advance pin Vacuum is a tick low but has always been around 17". I'm the 4th owner that I can figure out, maybe somewhere along the line a cam was put in it? I'm not sure how large of a cam would drop about 3" of vacuum but it's just a theory. How would I go about checking if the brake booster or climate controls have a vacuum leak?
You said stock engine. 17" is low for a stocker. For a small cam, it may be normal. Pinch off the vacuum hose to the booster, and the supply hose to the vacuum canister on the firewall. Watch for an increase of vacuum.
There is a difference between ''Surging'' and ''Bucking''.... bucking usually occurs when the timing is high and the engine is ''lugged'' at low speeds.... ''Surging'' is when driving at a steady speed and the engine rpms go up and down with a steady amount of pressure on the accelerator pedal....the usual causes of surge is a lean condition...or fuel percolation from too much heat to the carb....causing the fuel to boil and the bubbles get into the emulsion tubes and passage ways and making the mixture lean.... bringing up the question,,,,is there a heat shield under the carb???? or is the heat valve on the exhaust manifold stuck , putting too much heat to the intake manifold????... or if there is a flapper on the air cleaner snorkel not working making it pick up too much heat from the exhaust manifolds..... I take it that you have not changed the jets or the needles any at all....
Doc, by your definitions I would have say it's doing both. If I leave from a stop, increasing pressure on the pedal the car gets to a point where it starts buck and at certain RPMs with constant pressure on the throttle it starts to do the same thing, which I guess would be surging then. To answer a few of your questions there is no heat shield under the carb, just a gasket to the manifold. The passages from the heads that allow exhaust gas into the manifold were plugged when I had the heads off about 2 years ago and the air cleaner is an aftermarket K&N 3" circular deal. The jets are .075" with 44B primary rods and CK secondary rods (the secondary rods are from edelbrock and measure the same as CV rods, so not to confuse anyone from my previous post) I went to check a few things this morning and before starting the car I noticed the gaskets (I have them doubled up) between the air horn and the body were wet with fuel, this after running for maybe 5 minutes last night to make sure no fuel was dripping from the nozzels at idle. Ran the car and checked for vacuum leaks with the gauge hooked up and did not find any. The vacuum read a fairly constant 19-20" at idle in park so my previous reading of 17" must have been before the engine was completly warmou: I checked the plugs and found 1, 3, 2 and 4 to be carboned up while 5, 7, 6 and 8 look fine. None of the plugs were fuel soaked nor did they smell of gas. Now I'm really confused
could you possibly have cracked rubber connections in the fuel line between the fuel pump and the gas tank letting air in?
I too have a Crane adjustable vacuum advance. If I run too much advance out of this cannister at speed I get surge from the engine.
I looked back by the fuel tank and saw two short rubber lines on the outer sides of the tank that were split pretty bad and replaced them but the issue was still there.
