MacVette86

I'm having an issue where the engine runs rough/stutters when I slowly give it the gas from idle up to about 2000 rpm. My vette is a 77 with L48, quadrajet and TH350. Here's what I've done to date to try to solve this:

1. New cap, rotor, electronic ignition module.
2. New plugs with .035" gap
3. Checked all plug wires (Taylor Blue Wires) for heat damage, cracks, etc. None found.
4. Car idles at 650 rpm, 550 rpm in gear.
5. Checked all vacuum lines to see if there was a leak, but none detected.
6. Checked carb base and intake manifold for vacuum leaks but none detected.
7. Put vacuum gauge on car at idle, reads 19 hg and is very steady.
8. Checked timing with vacuum can disconnected it's at 16 degrees at idle and 34 degrees all in at 2700 rpm.
9. Replaced fuel filter in the carb.

Do you think I have a carb issue or is there something else I can check?

jnb5101

Make sure the accelerator pump it producing a nice squirt of gas.

MacVette86

Yes, I removed the air cleaner and with the car not running I used a flashlight to check that there was a strong squirt of gas on both sides of the carb when I operated the throttle. BTW, the previous owner was a helicopter mechanic and he kept notes of all the things he did to the car. He installed a rebuilt Q-Jet in 2006 which was 20,000 miles ago.

JagNorb

It may be time to rebuild the carb. 20,000 miles on a collector car can take 20 years. Age deteriorates seals.

MelWff

You do realize the plug gap should be .045? Have you checked for a vacuum leak around the base of the carburetor and any vacuum lines?

dochorsepower

As people on here have found, sometimes carb rebuilds are not good - Contact Lars. The other thing to consider is today's crappy gas.

MacVette86

I'm going to get the Q-Jet rebuilt at a local carb shop. They have 30 years experience with Q-Jets and actually told me to stay with my Q-Jet when I asked about switching to a Holley carb. They claim the Q-Jet will give me better fuel economy and is a better car for a cruiser. They only recommend Holley for all out performance cars or drag cars. Not the answer I expected, but it makes me believe these guys are serious about Q-Jets and can rebuild mine and make it better than ever.

Also, might try Seafoam first to see if it helps before spending the $200 to get the carb rebuilt.

OMF

With 19 inches of steady vacuum I assume this is a stock or very close to it engine. It's good that you have a steady accelerator pump shot, so have you tried to make any adjustments to the idle mixture screws? I'm thinking it's running lean and your not getting any (or enough) fuel out of the transfer slot during transition.
Try to richen it one or 11/2 turns on each idle screw to see if that helps.

MacVette86

Yes, I did turn out the mixture screws approx 2 turns. Made it idle a bit higher, but no change on the part throttle stumble/hesitation. I really did a lot of searching on the forum before posting and tried to eliminate every thing I could as most times it seems the issue(s) are not with the carb itself but rather the ignition/vacuum/timing set ups.

procketus

I've got the same problem.

jnb5101

Off hand I'd suspect the "rebuilt carb". As Lars has posted numerous times, commercially rebuilt carbs are not to be trusted.

commander_47

Q junk is a problematic cheaply built carb, as has been discussed many times.

Get a Holley and some sleep.

mr303

pull your timing back to about 10 degrees (idle) and see how it runs. It sounds to me like you have too much advance (either mechanical or vacuum). This will cause a rough idle and terrible off idle miss.

MacVette86

According to Lars paper on how to set up timing for performance, he states that initial timing should not be lower than 12 degrees and 18 degrees is optimal as long as it doesn't result in more than 36 degrees of total timing with the vacuum can disconnected. So 10 degrees seems low?

Shark Racer

If it's too much timing, pulling the vacuum advance and capping it would probably be enough to determine if that was the problem.

If it is running lean, going fat on the idle mixture screws may help a little, but it would be better to adjust APT. This is not for the faint of heart, though and would require the willingness to dive into the carb.

Is this new, or something that has been going on for a while?

MacVette86

It's been going on for a while. I decided to start from scratch and adjust everything again. I adjusted the idle set screw so I was at 750 RPM in park and 600 RPM in drive. (Note that I`m using the tach on my timing light not the one in the car) Then I seated the mixture adjustment screws and turned them out 4 turns as it wouldn't stay running at 3 turns. 6 turns was the magic number to get me to a max of 20 inches of vacuum. I then disconnected my vacuum advance, capped it and set the base timing at 14 degrees using a dial back timing light. Then I adjusted the idle again as well as the mixture screws just to be sure. 6 turns of the mixture screws still gave me the most vacuum at 20 inches. Net result of all this is it idles a bit better, but it still has part throttle hesitation. So I`m going to run a can of Seafoam in my gas for a while to see if it frees up the card. If not I`m off to get it rebuilt. I will be staying with the Q-Jet as it was working very well when I bought the car, so I suspect it just needs some well deserved TLC after 9 years of service.

mr303

I had this issue with my 74 L48 and I had to dial back the timing, there is an ideal timing setting and then there is what the engine can take. There is an overlap period where you still have quite a bit of vacuum advance and mechanical advance is starting to come in, combined they give too much advance at a too low revs. My suggestion was to pull the timing back to a very safe figure and just drive the car to see what happens, I'm not suggesting that this is the ideal setting for your timing but it will certainly identify if timing is the issue. I wrestled with this problem for quite some time untill I worked out what was wrong with my car.

MacVette86

mr303, what did you set your initial timing at?

mr303

My car can run a maximum of 11 degrees initial. The problem we have is that our vacuum canisters pull in too much advance at idle (about 5 degrees too much) so we are either faced with restricting the amount of vacuum advance or dial back the initial advance by the 5 degrees. I plan to limit my vacuum advance in the future but until then I dial back the initial. My car runs very poorly at 16 degrees (rough idle and miss fires under light load) but runs perfectly at 11 degrees.

Silvertone

Hey Mac, have you also done the suck test on the vacuum advance can? In the past I've seen this type of behavior in a car with not enough vacuum advance off idle.