Fuel Preference for 1969 with l36
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Pro
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From: Southeast WI
Fuel Preference for 1969 with l36
I recently purchased a '69 with the L36 427/390 engine. I've had the car tuned up by a shop that works exclusively on Corvettes and the car is still running a little rough. For example, it feels like it bucks or lurches slightly on the highway. I suspect that it might be the type of gas I put in it. I've been using the premium grade of gas (93 octane). Do I need to add any fuel additives to the tank for optimum performance? I've heard about an octane booster, but I don't know if it's a gimmick. Also, could the poor performance be due to the unusually cold weather in my area? I live in SE Wisconsin. Could there have been a build up of water vapor in my fuel line? Any advice you could suggest would be very much appreciated. Thanks!!! :steering:
Melting Slicks
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From: TEXASTEXASTEXASTEXAS TEXASTEXASTEXASTEXAS
St. Jude Contributor
Re: Fuel Preference for 1969 with l36 (Tele_Man)
The lurching could be a whole lot of things, but I doubt that it's the gas. The L36 is picky about fuel, but what you'll get is pinging, not lurching, from insufficient octane. Octane boosters are pure crap, IMHO--they don't boost enough to do you any good. You've got two choices if you've got pinging, and that's retard the timing or get better gas. I'm currently running 1/4 tank blue aviation gas and half a bottle of Marvel Mystery oil per tank, which give me excellent performance & no pinging. Now, truly crappy gas that's old or has water in it could possibly cause your problems, though.
Regarding the lurching--I'd look for vacuum leaks & I'd tune and/or rebuild the carb. Not a bad idea to check and make sure your distributor is functioning right (the weights swing freely, the vacuum advance can works right). Also check your plug wires & make sure you aren't getting any arcing.
JB
Regarding the lurching--I'd look for vacuum leaks & I'd tune and/or rebuild the carb. Not a bad idea to check and make sure your distributor is functioning right (the weights swing freely, the vacuum advance can works right). Also check your plug wires & make sure you aren't getting any arcing.
JB
Melting Slicks
Joined: Oct 2001
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From: Halifax, Nova Scotia, Canada
Re: Fuel Preference for 1969 with l36 (Tele_Man)
Does it happen at cruise speed, at free point (while neither accelerating or de-accelerating) ?
If so, it may be a "lean surge". I have no experience with a BB engine, however a small block with a Qjet would exhibit this symptom.
A Qjet runs very lean under this condition.
If so, it may be a "lean surge". I have no experience with a BB engine, however a small block with a Qjet would exhibit this symptom.
A Qjet runs very lean under this condition.
Le Mans Master
Joined: Sep 2000
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From: Orlando Floriduh
Re: Fuel Preference for 1969 with l36 (Tele_Man)
I went through all of this. Amoco 93 is much better than anything else I tried. Not all 93 octane fuels are the same. BB's are very finiky as to the fuel.. Recurve the distributor as follows 4-6 deg initial, limit the total to no more than 28 (in by 3000) and do not use the vacuum advance. The surging at cruise may be from the aditional advance from the vacuum advance.This is what worked for me. There are more things to check.. A replacement Q-Jet may not be calibrated correctly. Check the carb number to see if it is an acceptable BB unit.
[Modified by silvervetteman, 5:49 AM 2/11/2003]
[Modified by silvervetteman, 5:49 AM 2/11/2003]
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From: Southeast WI
Re: Fuel Preference for 1969 with l36 (JB)
Thanks for your reply. My car is lurching or surging, not pinging. The car just had a MAJOR tune-up. The carb is just a few years old, the distributor is brand new. The plugs and wires are also brand new. The problem was noticed at constant highway speed. Does that help pinpoint the issue?
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From: Southeast WI
Re: Fuel Preference for 1969 with l36 (Barry's70LT1)
Thanks for your reply. Yes, this problem was noticed at constant highway speed. I was cruising at ~65 mph. If it's a "lean" problem, what is the solution?
Le Mans Master
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From: End of the Ike IL
Cruise-In V Veteran
Cruise-In VI Veteran
Re: Fuel Preference for 1969 with l36 (Tele_Man)
I've got an L36 in my '68, run Amoco/BP premium with timing set so as not to ping, and have no lurching at any speed. You said you had a major tune-up, but did this include new gas line filters and fuel pump pressure check? Might be worth a check to make sure it isn't a fuel delivery problem. Maybe Lars can speak on this.
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Melting Slicks
Joined: Oct 2000
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From: TEXASTEXASTEXASTEXAS TEXASTEXASTEXASTEXAS
St. Jude Contributor
Re: Fuel Preference for 1969 with l36 (BlueL36)
Tele-Man, as you're seeing from the replies, this one's hard as hell to pin down. I had a surging sort of feeling on light acceleration from cruise when I first bought my car & spent many, many hours trying to figure it out & made many posts here about it (you may want to do an archive check on the subject). What finally worked for me was a combination of things.
1. disconnected the vacuum advance, as Silvervetteman suggests.
2. changed out the jets and rods in the carb for a more optimal fuel flow (see the carb tuning paper at http://www.corvettefaq.com),
3. poured transmission fluid slowly through the carb while it was running to clean out carbon,
4. set it at 2 degrees initial advance and about 28 degrees total advance, and
5. fixed the power piston in my carb. The power piston had been mutilated by Holley when they rebuilt the carb (they cut off the pin on the bottom, something they do to Qjets which no one seems to be able to explain, but which definitely seems to contribute to driveability problems) Again, Silvervetteman pointed that one out to me--thanks, Larry!
Runs smooth as glass now, even with cheap gas, though it pings like crazy without the 93 octane + av gas.
JB
[Modified by JB, 8:59 AM 2/11/2003]
1. disconnected the vacuum advance, as Silvervetteman suggests.
2. changed out the jets and rods in the carb for a more optimal fuel flow (see the carb tuning paper at http://www.corvettefaq.com),
3. poured transmission fluid slowly through the carb while it was running to clean out carbon,
4. set it at 2 degrees initial advance and about 28 degrees total advance, and
5. fixed the power piston in my carb. The power piston had been mutilated by Holley when they rebuilt the carb (they cut off the pin on the bottom, something they do to Qjets which no one seems to be able to explain, but which definitely seems to contribute to driveability problems) Again, Silvervetteman pointed that one out to me--thanks, Larry!
Runs smooth as glass now, even with cheap gas, though it pings like crazy without the 93 octane + av gas.
JB
[Modified by JB, 8:59 AM 2/11/2003]
Melting Slicks
Joined: Oct 2001
Posts: 2,065
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From: Halifax, Nova Scotia, Canada
Re: Fuel Preference for 1969 with l36 (Tele_Man)
Thanks for your reply. Yes, this problem was noticed at constant highway speed. I was cruising at ~65 mph. If it's a "lean" problem, what is the solution?
To keep it simple, try disconnecting the vacuum advance to the dist.
Don't forget to plug the hose.
If this improves the condition, then check to see if you are using ported vacuum or not.
Ported vacuum = No vacuum to the dist. at idle.
If it improves and you have vacuum at idle, switch to a ported vacuum connection.
Also, Qjets have been know to have the power piston (power valve) stick in the closed position. This will cause a lean condition however not normally at cruise speed.
You can check this by pushing a pencil down the tube on the top of the carb.
(By the choke)
With the engine off, eraser end of the pencil, drop it in the tube. Then gently push down. There should be some spring resistance. Travel is about a half inch. . If you cannot depress anything, then the piston is likely stuck on the bottom. (Normally where they stick - in the closed position)
Keep us updated on your progress.
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From: Southeast WI
Re: Fuel Preference for 1969 with l36 (Barry's70LT1)
Many thanks to you and JB for the very helpful suggestions. As soon as the weather warms up, I will investigate the problem armed with this new info. You guys are great! :steering:
