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Discussion Starter #1
Hey guys, I've been having a problem with my '68 camaro. Basicly, I have limited power, off idle stumble, and a slight pop out of the exhaust when shifting. It takes alot to get the rpms up and seems to lose power about 4500 rpm. When idling, a distinctive stumble is heard and throttle response is not that great. Also, if I adjust mixture needles for optimum vacuum, the idle shoots up and I can't get it to idle down. :confused: I built the car and engine so I know a little about this, but as for tuning, I know little as compared to many of you, after all, I'm 19. Thank you for the help!

Specs:
1978 block 350 bored .060 over
holley alum heads 2.02/1.60 valves
9-9.5:1 compression
Holley 670 cfm street avenger carb w/ Vac Sec. and elec. choke 65 pri and 68 sec using 'middle' spring
lunati .477-.507 lift 225-235*@.050 duration 112* lobe separtion cam
4* deg advance of the cam w/ relation to crank
m21 4 spd
long tube headers
3.08 gears
28" rear tires
14* deg initial timing 35* total and Vac. avd. attached to ported vacuum
MSD pro billet dist. with mag pickup
Holley Annihilator electronic ignition system/60000 W coil
The Car weights approx. 3200 lbs
Should be a 400 hp motor
 

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Sounds like your aiming for good tune paremeters on your timing and you really need to get that idle circuit dialed in to start cause that also plays a large roll in transitioning to primaries (off idle stuff). Not sure what you mean by "can't get idle down". Are you saying you have the idle screw backed out all the way and still too fast?

I'm not sure if the VS Avenger series carb has a means to adust the secondary throttle plate opening baseline setting or not but if it does and out of wack, this can cause issues with balancing your idle settings.

Your carb - is the elec. choke connected to power source and is the choke working properly?

Is Vac Adv. disconnected and carb port plugged when timing?

Is that the stock jetting for that carb or has it been leaned out some?
 

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Also, since it sounds like this is a fresh build and your just getting it going thought worth asking about the valve/rocker adjustments. Did you perform those adjustments and do you feel confident they were set properly? This could play into some of the other issues you describe.
 

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I had a similar issue with poor transition from idle a while back involving a small block Chevy 406 with a Holley 83670 - 670 cfm Street Avenger aluminum 4-bbl carburetor with vacuum secondaries and electric choke. Take a look at that thread and see if it helps you at all - http://www.camaros.net/forums/showthread.php?t=229442
 

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As like posts above..
It sounds like u have installed the carb and dizzy oput of the box and not dialed in these to9 establish your tuning specs.
The carb.. u need to adjust the sec butterflies so the idle speed and mixture screws are 1 1/2 turns +/- 1/2 turn
How to described in many older posts.
The "pop" also sounds like u have a lean issue, bew it in the accel pump adjustment or jetting.....ahain how to etc in older threads.
Dizzie
14* deg initial timing 35* total and Vac
your total should be ball park starting point around the 34 to 36 deg at around 3000 to 3200 rpms NO VA ATTACHED....
If u are measuring the degs simply by static reving in the driveway with VA attached, there is no load on the engine, therefore the VA is being activated....under reeal roasd test, the engine is loaded and the VA much lower therefore VA is not actuated.
Also not out of the box HEI systems are set up for polluton/ EGR engines which have a much different curve and combination of cent / VA degs, and have VA on ported vaccuum....generally requires putting in much more cent degs and reducing VA.
Notr EGR engines generaqlly have 20 +degs in the cent and 7 to 10 in the VA unless dialed in with data loggers and knock sensors.

How tos etc covered in many older threads
 

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Discussion Starter #6
Sounds like your aiming for good tune paremeters on your timing and you really need to get that idle circuit dialed in to start cause that also plays a large roll in transitioning to primaries (off idle stuff). Not sure what you mean by "can't get idle down". Are you saying you have the idle screw backed out all the way and still too fast?

I'm not sure if the VS Avenger series carb has a means to adust the secondary throttle plate opening baseline setting or not but if it does and out of wack, this can cause issues with balancing your idle settings.

Your carb - is the elec. choke connected to power source and is the choke working properly?

Is Vac Adv. disconnected and carb port plugged when timing?

Is that the stock jetting for that carb or has it been leaned out some?
As for the idle, yes. I have the idle speed screw all of the way out and when the idle mixture screws are adjusted for optimum rpm/vacuum the idle is approx 1600-1700.
Electric choke is connected and working properly (choke blade closes when cold and opens when warm).
Vac. Adv. is disconnected when checking timing and port plugged.
Stock jetting from factory.
To get idle @ 900 rpm, the idle mixture screws are (1/2)-(3/4) from closed.
 

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Discussion Starter #7
Also, since it sounds like this is a fresh build and your just getting it going thought worth asking about the valve/rocker adjustments. Did you perform those adjustments and do you feel confident they were set properly? This could play into some of the other issues you describe.
Yes, I believe they are set properly, but there is always a chance they aren't, although I doubt it.
 

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Discussion Starter #8
I had a similar issue with poor transition from idle a while back involving a small block Chevy 406 with a Holley 83670 - 670 cfm Street Avenger aluminum 4-bbl carburetor with vacuum secondaries and electric choke. Take a look at that thread and see if it helps you at all - http://www.camaros.net/forums/showthread.php?t=229442
Thank you Scott! :thumbsup: This thread has given me some good insight and has confirmed my suspicions that the engine is 'starved' for fuel, so I believe I will change my jet size to approx a 68-69 pri and 70-72 sec. :yes:

Did changing the primary jets fix your problem completely, or did you do anything else subsequently to further tune your carb(change the secondaries as well)? You mentioned the possibility of doing it, but I'm sure if you said you had. Also, given your cam is about the same lift/duration as mine, what timing set-up do have and where do you connect your vac adv as of now?

I apologize for asking so many questions, but I've been tuning this thing for darn near a year and a half :mad:, and I'm ready to nip this problem in the bud. Well, mostly at least.

An old man once told me: "They're never done."
 

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The most important change was getting the idle transfer slot partially covered when the throttle was at rest on the idle screw. Tough to do without removing the carburetor and flipping it upside down on the bench. I also machined on my bench grinder a custom screw bit from a broken #2 screw gun bit that fits in a 1/4" box end wrench and allows me to adjust the secondary idle speed screw (the pain in the A$$ screw on the underside of the throttle plate near the vacuum canister for the secondaries) while the carburetor is bolted to the intake manifold. I tape the bit to the wrench so it stays together as one tool.

Once that was done, I installed the carburetor on the car and used my new custom tool to adjust the secondary idle speed adjustment screw to set idle speed (don't ever change the primary idle speed screw once you set the primary butterflies properly in relation to the idle transfer slot, resist any and all temptation!). Getting the idle transfer slot relationship to the primary butterflies at idle had the the biggest effect on cleaning up the poor transition from idle problem. If you don't get that right, you'll chase your tail trying to solve the off idle hesitation so start there.

As far as jets go I changed them primarily because my plugs were reading too lean. I've ended up (for now at least) with #69's in the primary side & #72's in the secondary side. I'm running a 4.5 power valve as well instead of the 6.5 that the carburetor shipped with. Still have the original accelerator pump shooter in place as changes both larger and/or smaller to that made no positive difference. Still have the vacuum secondary spring that shipped with the carburetor in place as well seeing as the transition issue isn't effected by the vacuum secondaries.

As for ignition timing I can't say for sure what I have, I need to do some more work as the numbers I get from the timing light don't make sense (timing light indicates way more advance than it would run at in my opinion). I need to do some investigating and see if the outer ring of the vibration damper has moved on the hub and/or set the crankshaft at TDC and then re-mark the damper. It's an aftermarket damper with degrees marked off around it but again, the readings from the timing light just don't make sense (way too far advanced).

If I remember correctly, the timing light indicated base timing at idle (900 rpm) of around 26 with no centrifugal and no vacuum advance. With centrifugal and vacuum advance I think it was indicating up around 56 or 58 degrees! And I can advance it further without any apparent issues, but it runs pretty good where it is so until I figure out exactly what is going on, I'm just cruising with it where it is at for now. I currently have the vacuum advance running off of ported vacuum as the poor transition issue seemed less apparent this way. I get between 9-11" of manifold vacuum at idle.

Right now I have her timed via the good old ear and manifold vacuum reading (yes, I know, not the best way but it's just until I get time to do the investigation on why the timing light indication is so far off) although I did check it with the light at the same time. I get no octane knock at all, even in 4th gear trying to lug it up a hill, and that's running regular 87 octane. I don't know the compression ratio of this engine either, perhaps over the upcoming winter season I'll get the time to sort it out but for now, she runs very strong with the transition from idle issue almost completely eliminated. I'm currently getting 13-14 mpg depending on how much "fun" I have with the throttle and that's with 3.73 gears and an M21. When I purchased the car, she was getting just barely over 6 mpg and ran poorly compared to how she's running now.

Hope this information is helpful and good luck,
Scott


Thank you Scott! :thumbsup: This thread has given me some good insight and has confirmed my suspicions that the engine is 'starved' for fuel, so I believe I will change my jet size to approx a 68-69 pri and 70-72 sec. :yes:

Did changing the primary jets fix your problem completely, or did you do anything else subsequently to further tune your carb(change the secondaries as well)? You mentioned the possibility of doing it, but I'm sure if you said you had. Also, given your cam is about the same lift/duration as mine, what timing set-up do have and where do you connect your vac adv as of now?

I apologize for asking so many questions, but I've been tuning this thing for darn near a year and a half :mad:, and I'm ready to nip this problem in the bud. Well, mostly at least.

An old man once told me: "They're never done."
 

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Had the same problem on my 67. Make sure your timing is where you want it then take apart your carburetor and clean her up mine had carbon build up and was dirty I cleaned it all up put it back together then made sure all the spark plugs were good. Turned on the car reset the air fuel mixture screws then the idle screw. And bam! All better. Hope this works for you buddy. When I have a problem i can't solve myself I come on here and I like to hear all the different problems and solutions there might be. Good luck! :)
 

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Hey guys, I've been having a problem with my '68 camaro. Basicly, I have limited power, off idle stumble, and a slight pop out of the exhaust when shifting. It takes alot to get the rpms up and seems to lose power about 4500 rpm. When idling, a distinctive stumble is heard and throttle response is not that great. Also, if I adjust mixture needles for optimum vacuum, the idle shoots up and I can't get it to idle down. :confused: I built the car and engine so I know a little about this, but as for tuning, I know little as compared to many of you, after all, I'm 19. Thank you for the help!

Specs:
m21 4 spd
3.08 gears
28" rear tires
This sounds like you have a vacuum leak somewhere. You have to back out the idle screws all the way to get enough fuel to stabilize the idle. Plus the engine won't idle at a slow speed. This sounds like it's too lean because you have an air leak somewhere ; possibly a vacuum hose, carb gasket or intake gasket.

Your combo has a very "tall" gear for acceleration. 3.08 gears with a 28" tall rear tires and a 2.2 first gear = slow acceleration.

For good acceleration you want a 1st gear X rear axle ratio of close to 10.

You have 2.2 X 3.08 = 6.776.

A 2.52 M22 trans with a 3.73 gear would accelerate much faster
2.52 X 3.73 = 9.4
 

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Tough to do without removing the carburetor and flipping it upside down on the bench. I also machined on my bench grinder a custom screw bit from a broken #2 screw gun bit that fits in a 1/4" box end wrench and allows me to adjust the secondary idle speed screw (the pain in the A$$ screw on the underside of the throttle plate near the vacuum canister for the secondaries)
As mentioned in many other threads...the soln is simple
take a rivet, flatten the end with a hammer into a small screw driver blade... then bend the shaft to near on 90 degs...u now have a tool to get to the adjustment stop screw under the bottom edge of the carb...while the engine is running...
DONOT close them up so the butterflies sit in the bores... this causes binding and damage to the butterfly alignments and huge tuning headaches

This sounds like you have a vacuum leak somewhere. You have to back out the idle screws all the way to get enough fuel to stabilize the idle. Plus the engine won't idle at a slow speed. This sounds like it's too lean because you have an air leak somewhere ; possibly a vacuum hose, carb gasket or intake gasket.
yes...the "pop" also look to a faulty/ dirty/ or PCV valve incorrect calibration.
 

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Discussion Starter #13
I had a similar issue with poor transition from idle a while back involving a small block Chevy 406 with a Holley 83670 - 670 cfm Street Avenger aluminum 4-bbl carburetor with vacuum secondaries and electric choke. Take a look at that thread and see if it helps you at all - http://www.camaros.net/forums/showthread.php?t=229442
Well I adjusted the butterflies like in your picture, fixed my float levels, and discovered the throttle linkage bound at idle, not allowing the blades to close all of the way so it idles much better now. As for the 'lean condition', it still exists. Still pops a little when shifting, but not as much. I would post pics, but they exceed the quota.

I think I am going to re-jet this weekend to a 68 pri and a 70 sec. and see what that does. I have a friend with all of the jets, pv's, etc. :yes:

As far as a vacuum leak, I doubt it. Carb seal is good, no whistling, etc. I am going mess around with timing curves, put my vac. adv. back on and experiment. Thanks for the insight on the transfer slot for both sides, I would have never known to do that!
 

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A step up of 2 jet sizes on the primaries only will fix a lean condition as u describe.
If no ther leaks secondary issue and adjustment now correct...ie idle speed and mixture screws are 1 1/2 turns out +/- 1/2 turn.
1st check the PCV valve is correct rating and clean.
It only be open under light loads to WOT and closed under higher vaccuums like idle and deceleration, lift off throttle.

Basic rule: NEVER make more than 1 change at a time.
 

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Before you re-jet, start the engine and spray some starting fluid or carburetor cleaner around all the potential vacuum leak prone areas (hoses, manifold to head mating surfaces, carburetor base gasket, throttle shafts, brake booster vacuum connection) and listen for an rpm increase each time you spray a quick shot. If the rmp's increase for a short period and then drop back down to previous idle rpm, you have a vacuum leak. You won't always over even usually be able to hear a whistling/vacuum leak but the spray method works very well.

It's much easier to find and fix any vacuum issues before re-jetting. Also make sure your PCV valve is one for a lower vacuum engine, if you purchase one for a 67-69 302, it should be a good match. If these two steps don't better the situation, then go ahead and try the re-jetting and go two jet sizes each time until you get close then you can fine tune one jet size at a time from there.

Good luck,
Scott


Well I adjusted the butterflies like in your picture, fixed my float levels, and discovered the throttle linkage bound at idle, not allowing the blades to close all of the way so it idles much better now. As for the 'lean condition', it still exists. Still pops a little when shifting, but not as much. I would post pics, but they exceed the quota.

I think I am going to re-jet this weekend to a 68 pri and a 70 sec. and see what that does. I have a friend with all of the jets, pv's, etc. :yes:

As far as a vacuum leak, I doubt it. Carb seal is good, no whistling, etc. I am going mess around with timing curves, put my vac. adv. back on and experiment. Thanks for the insight on the transfer slot for both sides, I would have never known to do that!
 

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Discussion Starter #16
Before you re-jet, start the engine and spray some starting fluid or carburetor cleaner around all the potential vacuum leak prone areas (hoses, manifold to head mating surfaces, carburetor base gasket, throttle shafts, brake booster vacuum connection) and listen for an rpm increase each time you spray a quick shot. If the rmp's increase for a short period and then drop back down to previous idle rpm, you have a vacuum leak. You won't always over even usually be able to hear a whistling/vacuum leak but the spray method works very well.

It's much easier to find and fix any vacuum issues before re-jetting. Also make sure your PCV valve is one for a lower vacuum engine, if you purchase one for a 67-69 302, it should be a good match. If these two steps don't better the situation, then go ahead and try the re-jetting and go two jet sizes each time until you get close then you can fine tune one jet size at a time from there.

Good luck,
Scott
Well I checked for vacuum leaks, none to be found. After this weekend, I will post findings of going to #35 shooter and 67 pri from #31 shooter and 65 pri jets. Hopefully it will fix the lean issue. I will also play with othe advance curve/stop bushing/initial timing set-ups to see what's best.
 

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I will also play with othe advance curve/stop bushing/initial timing set-ups to see what's best.
If u play you will end up going in cicles, with the end result odf a dizzie with a patched / compromised tunining rather than correct.
THE ONLY way to dial the timing is 1st set the dizzie to ball park specs....
THEN dial the carb circuits (idle, power and cruise) individually to correct AFRs
THEN establish independently your dizzie tuning specs (idle advance, total advance, VA degs, VA vaccuum start and finish pionts) locking the the cent weights.
THEN once those dialed in specs are known, then make the internal adjustments to the dizzie....
Then re check your AFRs which if done right will be as close as u can get without getting into laboratory type dial in equipment.
I have been doing this sort of stuff for over 30yrs, on CNG, LPG, non egr, and vintage pre war engines to get them to run as well if not better than they originally did from factory on modern fuels.
 

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Discussion Starter #18
Well I got the 67's in the primary side, and the #35 squirter. Runs way better. Off idle stumble is gone, idle is good, at 950 rpm, and sounds great. Two things i found as well: first I had a burnt plug wire on the #2 cylinder and an exhaust leak. Both fixed now.

The only thing now is: If I accelerate anywhere above 2500 rpm and let off the throttle to maintain speed(no more acceleration), I get the same "backfire" popping from the exhaust. It also occurs if I accelerate at a slower rate and exceed 2500 rpm.

Is this a lean condition issue? It's the same popping as described in previous posts.
If it is a lean condition, do I need to move up jet sizes in the primary again to say, 69.
And if so, do I need to change the secondaries to two sizes greater? (71)
Current jetting: 67 pri. 68 sec. #35 shooter #65 power valve silver spring in the vac. sec.
 

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So it appears going richer is heading in the right direction. Now the task is to find when the primary and secondary circuits are each rich enough. When you are getting the popping out the exhaust, are you still in just the primary circuit or are the secondaries opening up?

To determine this you can temporarily wire the secondaries closed and see if anything changes. What you describe sounds like a lean condition to me. Have you pulled the plugs to see what they show as fas as mixture goes (both before and after re-jetting)?



Well I got the 67's in the primary side, and the #35 squirter. Runs way better. Off idle stumble is gone, idle is good, at 950 rpm, and sounds great. Two things i found as well: first I had a burnt plug wire on the #2 cylinder and an exhaust leak. Both fixed now.

The only thing now is: If I accelerate anywhere above 2500 rpm and let off the throttle to maintain speed(no more acceleration), I get the same "backfire" popping from the exhaust. It also occurs if I accelerate at a slower rate and exceed 2500 rpm.

Is this a lean condition issue? It's the same popping as described in previous posts.
If it is a lean condition, do I need to move up jet sizes in the primary again to say, 69.
And if so, do I need to change the secondaries to two sizes greater? (71)
Current jetting: 67 pri. 68 sec. #35 shooter #65 power valve silver spring in the vac. sec.
 

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Discussion Starter #20
So it appears going richer is heading in the right direction. Now the task is to find when the primary and secondary circuits are each rich enough. When you are getting the popping out the exhaust, are you still in just the primary circuit or are the secondaries opening up?

To determine this you can temporarily wire the secondaries closed and see if anything changes. What you describe sounds like a lean condition to me. Have you pulled the plugs to see what they show as fas as mixture goes (both before and after re-jetting)?
I believe just in the primary circuit. I drove the car easy and put a paper clip on the vac. sec. pushrod. If I accelerated slowly and reached ~2500 rpm, instant popping. When I got back the paper clip had not moved. Sounds like I need bigger primary jets. Response is good, so I'll keep the #35 shooter nozzle.

As for plugs, no I have not pulled them. I can do so though, to check bigger jets vs. current setup.

Even if I get bigger primary jets, do I need to change the secondaries? I've heard yes and no.
 
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