Carb Problems
Hi all
I hope you and your families have a great Thanksgiving
Anthony
I hope you and your families have a great Thanksgiving
Anthony
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Re: Carb Problems
Anthony;
I am not too familiar with your car or it's carburetion, but do you have a Holley carb or Rochester Quadrajet? I see you said your car has a 4-speed manual transmission; do you perhaps have a situation where you have a carb from an automatic transmission car that has a dashpot to dampen the throttle return? I have no idea if one was used on a corresponding Auto trans 69, so this is a shot in the dark. But, that would do it. The only other thing I can think of is you might be using a weak throttle return spring. I know on my 63, I have to run a booster spring to get any sort of stable idle. Also, there were a lot of 70's Vettes that used an inner and outer throttle return spring arrangement, specially with Quadrajets. For what it's worth.
Stu Fox -
Re: Carb Problems
Stue,
I have a Quadrajet (7029207)and its correct for my car. The date is a service replacement. I am going to try the spring tomorrow but what I don't understand is why it only does it when I am driving. It doesn't do it when I am in the driveway trying to make it do it.Comment
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Re: Carb Problems
Anthony;
Is your choke working properly?
My 63 has a Carter AFB and, like you say, it works fine in the driveway. I adjust the idle, set the idle mixture screws using a vacuum gauge, and get it just perfect at a standstill. Then, I go out and the first stop sign I come to it has jumped up a couple hundred RPM. That's with the stock spring and, oh yes, I've tried several new ones of those. Adding a light booster spring just makes the difference for me, and it doesn't add any to the pedal feel - just more positive. So, if all your adjustments are correct and you have no suspected vacuum leaks or other maladies, that just might be the ticket.
As mentioned before, QJ's of that era seem to need a little more return spring on some applications. Perhaps someone else can jump in with a little more first hand experience with them.
Stu FoxComment
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Re: Carb Problems
Stu,
I'm not to familiar with carbs but if I had a vacuum leak wouldn't it do it in the damn driveway??? I'll try the spring tomorrow and see if that works.
Thanks again
AnthonyComment
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Re: Carb Problems
Stu,
Happy Thanksgiving!! I went out this morning and looked over the carb. I started the car and started looking for any sort of vacuum leaks.(as you suggested) I found that the PCV valve was bad so I replaced it and the car runs much better. I also put more tension on the spring for now until I can purchase a heavier spring. The next thing I noticed was that as soon as you hit the gas (just off idle) you hear a sucking noise from the carb and it tries to stall. If you continue to press the gas pedal it runs fine. I guess I'll go back out tomorrow and see what thats all about. Once again have a Happy Thanksgiving.
AnthonyComment
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Re: Carb Problems
That symptom (Jeckyl & Hyde idle RPM) is reasonably common with later Shark cars featuring TCS or CEC emissions controls (vac advance that was 'switched' as required to provide/deny spark advance)....
The 'acid' test in diagnosis is to recreate the artificially high idle RPM condition, put your foot on the brake and let out the clutch to point of 'almost' stalling the engine then disengage the clutch. Did curb idle fall back to the lower RPM point and stay there after you FORCED a load on the engine?
If so, that's telling you the distributor's vac advance failed to expell stored vac developed during highway cruising. The root cause is typically one of two things:
(1) The TCS/CEC vacuum switching solenoid didn't release cleanly to vent vac from the distributor and/or its vac vent path is plugged with dirt/grease.
(2) The distributor's vac advance is 'hanging up' for some reason preventing the base plate from returning to its zero advance position OR the advance unit has been replaced with an incorrect substitute unit with the wrong advance profile and it's not 'letting go' as it should when you throttle down from high crusing RPMs...Comment
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Re: Carb Problems
Stu,
I have 62 340HP that does the same thing. Where did you add the booster spring?
JeffComment
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Re: Carb Problems
Stu,
I have the same problem with a 62 340HP. Where did you add a booster spring?
JeffComment
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Re: Carb Problems
Jeff;
I was blessed with having my original model #3461S carburetor stolen from me by a Corvette shop in Downers Grove, Illinois. They replaced it with a 3720SA, from a 64 automatic trans application. This model has the throttle linkage on the carb with a lower arm connection hole for the transmissioin linkage. I call this my "A" carb and, with it, I use a small bracket off the upper left rear valve cover screw. So the booster spring attaches between the lower hole for the trans link to the bracket on the screw. The spring is a small 1/4" diameter piece about 6" long with a loop on each end. It's very unobtrusive, but does the job - and can be removed in an instant for show.
Then, I have a "B" carb which I bought new as a replacement in the mid 80's which by then was a generic model #3721SB - the latest iteration OEM for L-75's thru 1965 or 66 w/manual trans, as well as the replacement for the 63's - both L-75 and L-76, and 64 L-75's. Each of these carbs have slightly leaner jetting than the previous model. This "B" carb also has the same linkage as the one on the "A" carb for either automatic or manual trans with one exception; it has a hole immediately below the throttle rod attachment hole which can be used to connect the booster spring to. The bracket for the front spring attachment on the 63 straddles and mounts to the two manifold bolts. It has two spring holes in it and I use the outer one for the standard spring and the inner hole to connect my booster spring. Again, it is easily removable for show.
There, you got all that. Whew! I am not real familiar with the 62 application, but I know the carb is a different model yet with some linkage and fuel connection differences and no PCV, etc. However, it is the same basic design and either could probably be made to work on a 62 or 63 w/o too much difficulty.
Stu FoxComment
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Re: Carb Problems
Jeff,
Before adding a spring to your carb, check this. At idle, can you push against the throttle (against the idle screw) and lower the idle speed?
If you disconnect the vacuum advance at idle, and reset the idle to spec, does the idle speed still wander? One item not discussed yet is throttle blade angle. In your 62 where the timing advance system is very simple compared to a C3, a person can make a simple adjustment to check and see if the throttle blade angle is the problem. Advance your base timing 2-3 degrees, and reset your idle speed to spec. Does the wandering improve? This may provide some more direction.
TimComment
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Re: Carb Problems
Jeff;
As Tim has so well advised; there are a number of other items that may be causing you a problem with your idle speed. As I was getting so wordy with my answer to your question, I didn't go into them. I have been experimenting with the two carburetors (A and B, as mentioned) as well as the use of the a 64 Vacuum Advance (not a concern for your 62) and various changes to limit a bad percolation problem I had been having here in Florida. I now have a Phenolic spacer from an L-75 which, with two gaskets, effectively jacks up my carb another 1/2". This in turn affects both my idle speed (blade angle) setting and my idle mixture screw settings. They are all interrelated. When I get my idle speed correct and my best idle vacuum, I remove the carb to check and ensure that the fuel transfer slots are not overly exposed, i.e. they should ideally look like a 0.060" square hole below the throttle blade. If they have too much exposure (more rectangle than square), then I know they are negating the effectiveness of my idle mixture screws. This often is the case when a long duration camshaft is installed in an engine, and even with our Duntov 097 cams we are walking a thin line. The changes I have had to do (some noted above) have put me into that territory.
By the way, I have solved the percolation problem, but now I am experiencing some low speed lean surge for which I will take and swap out my primary Venturi Clusters (from my "A"" carb to my "B" carb) which will give me a 0.037" fixed idle jet in place of the 0.035"'s. I could not adjust out the surge with the mixture screws because, as I eluded to, the screws are on that line of being ineffective due to a greater throttle blade opening needed to maintain my 900 rpm idle speed using the spacer.
So, as you can see, it can get kind of involved.
Stu FoxComment
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Re: Carb Problems
if the throttle arm is against the idle speed adjustment screw and the idle does not come down check for sticking weights in the distributor.Comment
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Re: Carb Problems
Please 'splain how the PCV valve was "bad"?
You didn't mention the "flat spot" in the carb, off-idle before. Was it there before you replaced the "bad" PCV valve?
If you replaced the valve with a different one than specified, then this can very possibly affect your idle/off idle A/F ratio! The amount of spring tension in the valve, and/or its orifice size and rate-of-change will effect fuel metering.
The original problem can be many things in addition to a vac leak. Is there excess play in the primary throttle shaft? Is your carb linkage free, without binding? Lubed? Accelerator pedal interference?
Same problem is fairly common. I also put a stiffer return spring on my Holley, after all else failed to cure the "problem". It is somewhat better now, but idle speed consistency still does not match my '85 with TPI.Comment
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Re: Carb Problems
Jeff;
I think we better take our discussion to another thread, if there are any more questions you have on your 62. We're stepping all over Anthony's thread and his original 69 Quadrajet idle problem. Sorry Anthony, but when you get into that emissions related equipment and it's possible effects on your unstable idle, I have to leave it to some of my learned colleagues. They seem to be offering you some very good insights. I literally left QJ's back in the 70's where they belong, and only recently had to recall from my memory some things to help my son with his 78 P.C..
Stu FoxComment
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