Carb Jet experience needed
Thread Starter
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Joined: May 2011
Posts: 722
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From: Karlsruhe (Germany)
Carb Jet experience needed
Hello there,
since our trip to france i know my LS5 is running to rich on the primaries. At high temperatures and in the montains it had heavy missfire and was not good driveable.
But since that i can hear when the mixture is close to the border of missfiring. And even where i live it is at the borderline at hot days. But it was always driveable, had just to be carefull with the pedal in that cases.
I will replace now the 68-jets of my sixshooter by a smaller size.
The problem is, there is no easy access to the primary jets, i have to disassemble the throttle linkage, fuel lines of all 3 carbs etc etc.
So, here is the question, knowing the mixture is borderline rich, what jet size decrease would you recomend?
I dont want to dismount the carb system too often
since our trip to france i know my LS5 is running to rich on the primaries. At high temperatures and in the montains it had heavy missfire and was not good driveable.
But since that i can hear when the mixture is close to the border of missfiring. And even where i live it is at the borderline at hot days. But it was always driveable, had just to be carefull with the pedal in that cases.
I will replace now the 68-jets of my sixshooter by a smaller size.
The problem is, there is no easy access to the primary jets, i have to disassemble the throttle linkage, fuel lines of all 3 carbs etc etc.
So, here is the question, knowing the mixture is borderline rich, what jet size decrease would you recomend?
I dont want to dismount the carb system too often
Melting Slicks
Joined: Mar 2001
Posts: 2,175
Likes: 56
From: Clarence N.Y.
zuendler, I understand that a jet change with your carb setup
would be a pain and time consuming. I would drop down two jet
sizes and see how the engine responds. when you where in the
mountins the air was "too thin" very high air density.
You may have to adjust timing when you do the jet change,only if
it doesn't fell right to you.
I sure hope this helps.
Any other questions you may have just yell. Good luck
Schmidt
would be a pain and time consuming. I would drop down two jet
sizes and see how the engine responds. when you where in the
mountins the air was "too thin" very high air density.
You may have to adjust timing when you do the jet change,only if
it doesn't fell right to you.
I sure hope this helps.
Any other questions you may have just yell. Good luck
Schmidt
Thread Starter
Pro
Joined: May 2011
Posts: 722
Likes: 58
From: Karlsruhe (Germany)
Thx, i readed this from a holley manual too. They recomend always to drop 2 steps, 1 step would make no big difference.
It would be so easy if the primary would be on the first place
Sometimes bigger is not better...
It would be so easy if the primary would be on the first place
Sometimes bigger is not better...
Thread Starter
Pro
Joined: May 2011
Posts: 722
Likes: 58
From: Karlsruhe (Germany)
I hoped someone could tell if the mixture is borderline rich, than i am at least 4 sizes away from the correct value or something like that.
I have really no experience in replacing the jets and the results.
I have really no experience in replacing the jets and the results.
Burning Brakes
Joined: May 2008
Posts: 1,116
Likes: 5
From: Middleburg Florida
Zuendler, I think you have two issues. Timing and mixture. Is the engine at stock specs, or has it been modified? Do you know what jets are currently in the carbs? I'd pull a spark plug or two and "read" the plug. If it's black and sooty (like a chimney) your are indeed rich, and should drop at least two sizes. If they aren't (tan is a better color) you likely have timing issues. Regardless, I'd suggest at least checking (probably changing) the timing after the jet change.
Thread Starter
Pro
Joined: May 2011
Posts: 722
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From: Karlsruhe (Germany)
Do you mean ignition timing or valve timing?
(i have the suspicion that the chain is worn since i have the car)
Anyway, i have exchanged the #68 jets with #66 jets.
Here is the video, it took only 3 minutes
http://www.youtube.com/watch?v=Mj6EtkpH37Q
The result: Response has much inproved, more torque at lower RPM.
With the 68er Jets i could not press the pedal that far in low RPM because it startet to missfire.
Well i think it is still not at its optimum. I would try #64 jets but im not sure if it will get to lean then.
How would i reocgonize a too lean mixture?
My plugs were tan on one side and black on the other side with the 68 jets.
Whats confusing me is that, 66 jets are a little small for a BB dont you think so? With the 68ers i had 12,5mpg in averrange. Thats a common value i think.
(i have the suspicion that the chain is worn since i have the car)
Anyway, i have exchanged the #68 jets with #66 jets.
Here is the video, it took only 3 minutes
http://www.youtube.com/watch?v=Mj6EtkpH37Q
The result: Response has much inproved, more torque at lower RPM.
With the 68er Jets i could not press the pedal that far in low RPM because it startet to missfire.
Well i think it is still not at its optimum. I would try #64 jets but im not sure if it will get to lean then.
How would i reocgonize a too lean mixture?
My plugs were tan on one side and black on the other side with the 68 jets.
Whats confusing me is that, 66 jets are a little small for a BB dont you think so? With the 68ers i had 12,5mpg in averrange. Thats a common value i think.
Last edited by zuendler; May 24, 2012 at 06:10 PM.
Melting Slicks
Joined: Mar 2001
Posts: 2,175
Likes: 56
From: Clarence N.Y.
zuendler as far as jet sizes being small,give the engine the jets it likes. Remember you have 3 carbs,thats a bunch of fuel and air.
Is the engine equipped with headers or stock manifolds?
Reason I ask is that it is usually easy to remove a header to check the
exhaust port in the head for coloration. Black and dirty is way rich,a lean condition would be real a real light color.
Is the engine equipped with headers or stock manifolds?
Reason I ask is that it is usually easy to remove a header to check the
exhaust port in the head for coloration. Black and dirty is way rich,a lean condition would be real a real light color.
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Racer
Joined: Nov 2008
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I thing a slight timing adjustment might solve your problem. I used to travel to mountainous area's to hunt and my mechanic (from Colorado) would make the adjustment for the thinner air. I would also have it adjusted for Mexican fishing trips because of the lower octane gas. Your cars different carb set up is an unusual and you might have to try A lil of both the re-jetting and slight timing adjustments.
Thread Starter
Pro
Joined: May 2011
Posts: 722
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From: Karlsruhe (Germany)
Hm, you think i could have to much vaccuum advance?
This could be true.
Total vaccuum is adjusted some degrees before pinging under maximum load (with no vaccuum then of course)
Well i can remove the the vaccuum advance for a test and see what will happen.
@Jimmy: My car has sidepipes, removing the headers is a ugly job :/
This could be true.
Total vaccuum is adjusted some degrees before pinging under maximum load (with no vaccuum then of course)
Well i can remove the the vaccuum advance for a test and see what will happen.
@Jimmy: My car has sidepipes, removing the headers is a ugly job :/
Last edited by zuendler; May 25, 2012 at 09:18 AM.
Drifting
Joined: Jan 2012
Posts: 1,734
Likes: 276
From: Port Huron MI
I have one word for you, Zuendler: LARS.
Okay, technically, now I have more than one word. But the man is THE expert on fine tuning these babies. See his site or email him for papers that'll tell you everything you need to know.
Great trip, by the way, I followed along closely. Thanks for sharing.
Okay, technically, now I have more than one word. But the man is THE expert on fine tuning these babies. See his site or email him for papers that'll tell you everything you need to know.
Great trip, by the way, I followed along closely. Thanks for sharing.
Thread Starter
Pro
Joined: May 2011
Posts: 722
Likes: 58
From: Karlsruhe (Germany)
Hello Stroh, thx,
i did the timing adjustment with his method 1 year ago. (Setting 36° advance at 3000rpm or something like that, then driving and here for pinging and so on)
But i could imagine, that my vaccuum advance actuator does not really match to my setup and it gives to much advance to the timing at part load.
This problem only occurs at part load, the engine runs very good at full load.
And i have an idea why i hadnt this problem in the past so much: I put Engine Restorer to my motor oil. They promissed a better compression etc etc. (My engine was never restored, its over 40 years in the car) I could imagine, that it now produces more vaccuum too.
Under full load there is no vaccuum anymore - like my problem
It would all match together...
Well, i will first try to go a little bit back with the timing and see what happens.
Thanks for all your help and ideas.
i did the timing adjustment with his method 1 year ago. (Setting 36° advance at 3000rpm or something like that, then driving and here for pinging and so on)
But i could imagine, that my vaccuum advance actuator does not really match to my setup and it gives to much advance to the timing at part load.
This problem only occurs at part load, the engine runs very good at full load.
And i have an idea why i hadnt this problem in the past so much: I put Engine Restorer to my motor oil. They promissed a better compression etc etc. (My engine was never restored, its over 40 years in the car) I could imagine, that it now produces more vaccuum too.
Under full load there is no vaccuum anymore - like my problem
It would all match together...
Well, i will first try to go a little bit back with the timing and see what happens.
Thanks for all your help and ideas.
Thread Starter
Pro
Joined: May 2011
Posts: 722
Likes: 58
From: Karlsruhe (Germany)
ok, here are some results:
Before i drove home from my office i turned the distributor some degrees to "late".
Thought the problem was better.
I stopped at half way and turned it some more degrees to late position.
Felt not really an improvement.
At home i turend it again some degrees, thought it would improve a little, but still far away from "good". Additional the engine was lazy now, missing much power due to the late timing.
Then i disconnected the vaccuum advance.
The engine run very nervous at part load, had lean popping while cruising...
The "problem" was more or less still there, addition to others
So i think it is really no problem of to much advance timing.
Then i set timing back to the early position (maybe a little bit more) and connected the vaccuum advance.
And now i think the "problem" is smaller than it was before. Still not perfect, but the car is good driveable.
Well i think it could also be, that the sparks are to weak, or the valve timing is not perfekt. The engine just doesn´t have the torque below 2000rpm as it could have. Over 3000 everything is fine
Before i drove home from my office i turned the distributor some degrees to "late".
Thought the problem was better.
I stopped at half way and turned it some more degrees to late position.
Felt not really an improvement.
At home i turend it again some degrees, thought it would improve a little, but still far away from "good". Additional the engine was lazy now, missing much power due to the late timing.
Then i disconnected the vaccuum advance.
The engine run very nervous at part load, had lean popping while cruising...
The "problem" was more or less still there, addition to others
So i think it is really no problem of to much advance timing.
Then i set timing back to the early position (maybe a little bit more) and connected the vaccuum advance.
And now i think the "problem" is smaller than it was before. Still not perfect, but the car is good driveable.
Well i think it could also be, that the sparks are to weak, or the valve timing is not perfekt. The engine just doesn´t have the torque below 2000rpm as it could have. Over 3000 everything is fine