Hi! I've been working on tuning up a '70 GS 350 I bought last year. Last fall I rebuilt the original Q-jet, and I think it's mostly correct inside. Seems to run pretty well under full throttle and at idle, but I have a hesitation when gently accelerating away from a stoplight. When I first ease into the gas it bogs down for a second and then starts to move. Any ideas? The most common suggestions seem to be retarded timing due to a stretched timing chain (I set the initial timing to 6 degrees per my shop manual) or the accelerator pump. Accelerator pump squirts fuel into the primaries when I move the throttle by hand, not sure what else I can check on there. It also starts hard when cold, but I think adjusting the choke will fix that. Thanks a million, Justin
Stock cam? If so, I would check the accelerator pump, the float level and also the secondaries. If secondaries are too loose they will allow too much air flow and create a bog off the line (on a hard acceleration). Bogs are generally a lean condition. Also make sure they throttle blades are not open too much at idle. This will cause a lean condition by not allowing the off idle circuitry to pick up fuel. If you have an aftermarket cam with a lot of duration, you may have to rework off idle circuitry.
Thanks for the quick reply! I've heard about the spring adjustment on the secondary air flaps causing that notorious 'quadra-bog', but I assumed that that was only applicable to hard acceleration. My problem involved gently pulling away from a light as soon as the throttle is cracked. Might be a problem with the throttle plates not sealing, they fit well when I had the carb on the bench but a linkage could be hanging up. Idle RPM is steady at 600rpm in gear (automatic). I'll pull the air horn plate off and double check the float level too. Engine is bone stock with 138k miles, I'm going to do a compression check to make sure I don't have a problem with the rings or valves (even though it doesn't smoke at all). Is there a simple test to see if the timing chain is stretched?
You can test the timming chain by putting a socket and ext with a ratchet/breaker bar on you harmonic-balancer bolt-pull the dist cap and watch the rotor when you move the balancer back and forth. You then can kind of picture the amount of 'slop' in the chain/gears. May not be your problem though.
hookup a vacuum gage. I had a bog condition like you described, and it was caused by a carb base gasket leak. Found the leak fixed it....no more hesitation. Just another thought.
A stretched timing chain will cause a small change in valve timing. That is not the same as ignition timing. They are different things. Have you owned this car since new? If not, how can you be sure you have the original distributor in the engine. Chances are it has been changed sometime in the last 39 years, and if that is so, stock timing specs go right out the window. I would encourage you to read my Power Timing thread in the FAQ forum. It is a sticky. http://www.v8buick.com/showthread.php?t=63475 If you have a timing light, you can power time the engine after installing some lighter advance springs. I think you should also check to see that your vacuum and mechanical advance is functioning properly. A defective vacuum canister(common) will cause a tip in hesitation as you describe.
Larry, I've read your thread from end to end and plan to do that as soon as the engine runs properly. Update from tonight: I don't know how to identify the distributor, there's a patent number but no GM part no. that I can see. Here is a picture if anyone is curious: http://www.personal.psu.edu/jmw474/buick/IMG_3080.jpg Vacuum advance works, per my vacuum pump. It is attached to the ported vacuum source and I verified that that port supplies vacuum when the throttles are cracked open. On a hunch I advanced the timing to about 10 degrees, and BOOM no more hesitation! Of course, the idle speed was also increased about 200 RPM, so that could be doing it. Tomorrow I'll reset idle fueling and high/low idle speeds to see if the problem is solved.
Ok, I see where the stamping should be, I'll check it tonight when I get home. Thanks atlantabuick.com! Production ID casting number location Production IDs broken out by model and year
You think that maybe I'm not getting enough timing from the vacuum advance during tip in? I'll try that this evening, as well as check my total mechanical advance.
In my opinion, the vacuum advance should be connected to manifold vacuum. Everyone has their personal preference though. When it is connected to manifold vacuum, the extra advance is available at idle and low speed. This enhances throttle response and the engine runs cooler in low speed and stop and go driving. Manifold vacuum responds instantaneously to throttle position, and the extra advance goes away faster when you punch it, just what you want. When you power time the engine and get your mechanical advance all in at about 2500 RPM, you will need to limit the vacuum advance canister to 8-10*, but it should still be connected to manifold vacuum.
Checked the distributor number, it matches the year and model. Set static timing to 6-8 degrees and adjusted the idle and idle fueling. Added vacuum advance connected to ported vacuum and saw 22* of timing in drive at 600RPM. This seems to help, but it was pouring down rain here in Pennsylvania this evening and I couldn't take the car for a test drive :rant: Started to play with mechanical advance, but the kit I bought seems to have really weak springs in it (black, silver, gold). The gold springs were the stiffest, and brought all the timing in by 1700rpm. I'll check the part number of the kit when I get to work tomorrow. Off-topic: How do you like your Riviera? A friend of mine has a black Rivi with the series 2 L67, cam, heads, nitrous running in the 12s. I my self have a Grand Prix GTP with the same motor in it and some work done. Good engines, in my opinion!
I also noticed that I had a probable vacuum leak with the air cleaner / snorkel installed :spank: . Previous owner had the vacuum lines for the ram air doors hooked up to the manifold vacuum port on the front of the motor, so I just kept hooking it up there every time. Tested that line with my vacuum pump and it won't hold vacuum at all. Oh well, one more thing to fix. The spring kit I bought was Mr. Gasket #928G or #928 "Advance Curve Kit for 1969 and up Delco Point Dist."
Ok, took the GS out for a cruise last night Hesitation is still there when the engine is cold, leaning towards a fast idle problem or choke closing too far. I have another spring kit on order so I can fix my mechanical advance, hopefully for the last time! Ha! I also ordered a new vacuum advance in case my old one is responding too slowly.
Since you mentioned the difficult start cold, I suspect that the choke is involved here. If the engine is fully warmed up, does it run well? If not, the accelerator pump or idle mixtures may require adjustment. Is the air cleaner pre heater operative? These carbs were calibrated for 120 deg air. Much under 80 degrees will directly fuel metering due to air density issues. As to the cold issues, they include the choke's completely closing when cold, assuring sufficient start up fuel. The choke pull-off affects the engine's ability to run properly as the choke coil heats up. Lastly, be sure that the choke does not open too quickly, causing lean outs. PM me with questions. Ray
Ray, Thank you for your reply! I need to check the choke to ensure that the rod length is correct, I understand that to do that I must disconnect the linkage from the coil and check that it fits in the gauge hole. What I do know is that the choke is fully closed on cold start, and fully open when the engine warms up. I also replaced the choke pull-offs with brand new units, and they are not binding up in any way. Now that you mention it, no the pre heater is not connected. The flexible tubing was falling apart when I bought the car, and since I was planning on installing headers anyway I just removed it entirely. I have to check this problem again on the next cold start, I've been playing with timing quite a bit and as you know that can affect all kinds of things!
Lysander, Actually, now is a good time to learn choke operation. First, let me say that GM, in its efforts to meet emission standards, set their chokes a little lean, and with time, this situation became worse. The result is that, in most cases, the choke no longer closes above 50 degrees, making starting and cold driving is test at best. Regardless of how it is measured, the choke rod's job is to push the linkage, closing the choke blade. Pushing the throttle open about half way, and slowly allowing it to close, on an overnight cold engine, REGARDLESS OF THE TEMP, should completely close the choke, with some tension behind it. That means that the choke should not merely close, but there should be some tension behind it. This will allow the choke to overcome the opening force behind the incoming air, which, by design, pushes it open. Once the engine starts, engine vacuum pulls the choke pull-offs back, allowing about a 1/4 " of air to enter the engine. If this does not occur, the engine will load up, making it necessary to hold the throttle on the floor to clear the flood. Once the engine begins to warm, the choke coil, as it warms, slowly opens the choke, allowing an increasing amount of air, leaning the mixture. If, for example, the engine fails to return to warm idle, either the choke rod is too long, or, as is typical, the coil no longer extends fully, requiring its replacement. On the other extreme, if the choke opens too fast, there will be hesitations on mild throttle applications, as would be used during warm up. If you experience these hesitations, try enriching the rod slightly, evaluating over a week's time before making another adjustment. Make sure, of course, that the exhaust crossover is still open, as this is where the coil gets its heat. If it is not open, you will need either a mechanical choke cable, or the installation of an electric choke system. One thing at a time. Forget measurements. Carefully observe the operation of the various linkages, determining the operation of each, and either enrich for hesitations, or lean for flooding. Measurements became a starting point for me in the engine performance section of the GM dealerships in which I worked. I never had a disappointed customer, and my customer satisfaction percentage never dropped below 99%. With respect to the heated air cleaner, these carbs typically were rich enough to deal with the slightly denser air resulting with cooler air. Let's start with the choke operation, and before doing anything, make sure that the engine performance is perfect when warm before making the choke mods. Questions? I'll be happy to answer. Ray
Larry, since you mentioned manifold vacuum for the advance. Where on a 71 350 would I hook it to? thanx Tim
If you have a Q-jet, the nipple on the passenger side is manifold vacuum. If the is a vacuum hose there already, you can tee into it.
One you have determined that the distributor is working correctly, and there are no other contributing factors, go back to the carburetor. FWIW, it makes no difference what source you choose to use to operate the vacuum advance for a right off idle stumble/hesitation/miss etc for light throttle appliction. The q-jet has a well located ported source, and ALL the timing comes in with the slightest movement of the throttle, same as it would if MVA was already being used. If an EGR source were being used for the advance, their source location is high in the baseplate, and timing will come in late and usually not enough to fully employ the advance. Bottom line here, check your source location with a vacuum gauge and verify spark advance with a timing light. Back to the carb. The accellerator pump is NOT a play in very light off idle throttle movements. The relationship between the primary jets and metering rods are the main players, with some off idle fuel delivered from the transistion slots (idle fuel) and at higher throttle openings from the lower idle airbleed. The very first thing I would do would be to remove the airhorn from the carburetor and check the power piston, spring under it, and the jets and metering rods. We see a high percentage of carburetors with bent power piston hangers, having the rods all over the place in the main jets. If the carb is not "virgin" fully expect the hanger and metering rods to be out of their correct location. The APT system is a big player with the early Buick carburetors. Most builders don't even know it exists, or even how it works, as we continue to see folks chasing the metering all over the place with jets and rods, when they could simply free up the APT adjustment screw, raise the metering rods slightly, and have plenty of fuel for part/light throttle operation......Cliff
