Surge at low throttle input just off idle

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rick.tts@gmail.com
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Surge at low throttle input just off idle

Post by rick.tts@gmail.com »

Please read fully before answering , this is an odd one
This is a 140 engine, .040 pistons and barrels, 280 cam and wolf carbs,I am not a fan of his work, and Petronix ign that has been running great for 3 years about 5k miles. Now that that is out of the way. I suddenly have this surge like the Vacuum adv is kicking in and out. went through the dist. and vac is fine, moves on throttle, up, can pull 20 inhg and it holds, mech adv working to spec. Timed at 10 degrees advance at idle, you can step through this issue and goes like a rocket but light acceleration gets the surge above idle, IE pulling up a driveway, or steady state on road. no load there is no issue, only under load. fuel is clean this came out of no where, no vac leaks at carbs.....very strange I will provide phone number that we can chat at no problem if asked...Hopefully someone can help...Thanks
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terribleted
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Re: Surge at low throttle input just off idle

Post by terribleted »

Why do you have your initial timing set so low? Spec is 16-20 degrees. what happens when you time it at spec say around 18BTDC? You can eliminate any questions of Pertronics issue by temporarily installing a standard point plate and points setup to see if it effects the symptoms.
Corvair guy since 1982. I have personally restored at least 20 Vairs, many of them restored ground up.
Currently working full time repairing Corvairs and restoring old cars.
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66vairguy
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Re: Surge at low throttle input just off idle

Post by 66vairguy »

T ted made good points. Your "surging" could be any number of issues, but let's start simple.
Timing can cause this problem. Just 10 degs. BTDC does seem too retarded and makes me wonder about the static timing. I find a lot of incorrect distributors (or parts in them). First you have to determine WHEN the mechanical advance starts. Note on some Corvair distributors it's as low as 600RPM. Make sure there is NO vacuum from the carburetor port to the distributor at idle. Sometimes folks have to set the idle to keep the engine running so the throttle plate allow the vacuum advance to work at idle (this is why the books say to disconnect the vacuum advance hose when setting the timing).

First I would disconnect the vacuum advance hose and plug it so the carburetor isn't affected. If that solves the surge issue, then you are getting too much timing advance at lower RPM. Also check to make sure one of the mechanical advance springs isn't broken.

However only 10BTDC makes me think something else is off. I'd check the harmonic balancer (there should be a cut mark between the inner and outer parts separated by the rubber. Sometimes an old harmonic balancer will slip so the timing mark is off.

You did not mention which Pertronix unit you had or what coil you are using. Faulty Pertronix products have been known to cause surging. Just me, but I won't use the Pertronix Flamethrower coils. So you may have to put in points AND a good coil to determine if it is a Pertronix issue.

Just a few things to check. Good luck

rick.tts@gmail.com
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Re: Surge at low throttle input just off idle

Post by rick.tts@gmail.com »

I no longer have the breaker point plate or I would have reinstalled it for a try, the harmonic balancer is dead on, marks are dead on, 10 degrees with vac hose off and plugged is where it has always run and never had this issue. Petronix flame thrower 1.5 ohm though they recommend a 3 ohm for 6 cyls, why I dont know, they cant tell me. 12v to coil, good grounds everywhere. mech advance pulls back strong so I think the springs are ok.
petronix ignitor D-57-22 lot#18AH-NV5

rick.tts@gmail.com
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Re: Surge at low throttle input just off idle

Post by rick.tts@gmail.com »

BUMPED TIMING TO 16DEG, NO DIFF

rick.tts@gmail.com
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Re: Surge at low throttle input just off idle

Post by rick.tts@gmail.com »

BUMPED TIMING TO 16DEG, NO DIFF

Also forgot to mention does this with Vac hose off dist. and plugged thinking it is Carb, filter was clean

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Re: Surge at low throttle input just off idle

Post by terribleted »

Is the distributor correct for the engine? There are many different Corvair distributors including different ones for a 140 made to go behind an automatic and one to go behind a stick shift. Getting the numbers off the side of the distributor body and referencing them to a list of Corvair distributors by application will let you know this (listed in the CORSA Tech Guide for sure). Vacuum advances also vary some and particular number advances were used based on application. I would get a complete points plate set up with points and condenser and use it for trouble shooting. After that I would put it in the glove box in case the electronic unit fails so I could be back on the road in 5 minutes instead of being towed.
Corvair guy since 1982. I have personally restored at least 20 Vairs, many of them restored ground up.
Currently working full time repairing Corvairs and restoring old cars.
https://www.facebook.com/tedsautorestoration/

Located in Snellville, Georgia

66vairguy
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Re: Surge at low throttle input just off idle

Post by 66vairguy »

rick.tts@gmail.com wrote:
Sun Sep 13, 2020 2:12 pm
I no longer have the breaker point plate or I would have reinstalled it for a try, the harmonic balancer is dead on, marks are dead on, 10 degrees with vac hose off and plugged is where it has always run and never had this issue. Petronix flame thrower 1.5 ohm though they recommend a 3 ohm for 6 cyls, why I dont know, they cant tell me. 12v to coil, good grounds everywhere. mech advance pulls back strong so I think the springs are ok.
petronix ignitor D-57-22 lot#18AH-NV5
I have no issue with a proper Pertronix setup, but good grief their directions and catalog are AWFUL!! The Pertronix catalog saying a 3 ohm coil for six cylinders is B.S. and has been wrong for many years, yet they don't fix the issue (and apparently don't know why they did it). Back in the 40's, 50's, and 60's there were different coil impedance values for six cylinder engines.

The Corvair ignition is very similar to the 60's small block V8 engines. They use a 1.8 ohm wire ballast and a 1.3 to 1.4 ohm coil spec. for a total of about 3.2 ohms - (total impedance for durability of points in any old car is at least 3.0 ohms) The higher output Corvair Delco coil (110,140, 180HP) was very similar to the the one used for 283 and 327 c.i. engines (not the racing versions) in you basic 1965 Chevrolet. In fact that's what I ask for when installing a new coil in a stock Corvair. IF the ballast is missing (it's in the wiring harness), then that needs to be fixed, or use the 3.0 ohm coil.

I use the Pertronix basic Ignitor with the stock ballast and coil. Works fine. I do NOT recommend the Ignitor II.

Since this surging issue happened WITHOUT changing anything the distributor is probably correct -- HOWEVER you need to check to make sure you notched the distributor cap for the thicker wire bundle of the Pertronix Ignitor. If the cap is full of metal filings then the cap didn't sit square. Not saying this is your issue, but I run across it in on Pertronix installations.

At this point the basics suggest installing points and a good coil to test the ignition. Next you can install a good spare carburetor. BTW did you check you fuel pressure. S/B 4-5PSI. If higher or lower the carburetors will not work well.

66vairguy
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Re: Surge at low throttle input just off idle

Post by 66vairguy »

BTW - you said you have a surge condition. If it is actually a hesitation when going off idle, then that is bad accelerator pump cups. The old black material cups seem to go out after 3-5 years. Now folks are using the blue Viton cups - time will tell if they are better.

rick.tts@gmail.com
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Re: Surge at low throttle input just off idle

Post by rick.tts@gmail.com »

CAP HAS BEEN SITTING CORRECTLY, NO FILINGS, LIKE I SAID, THIS JUST OCCURRED AFTER YEARS OF DRIVING, BLACK ACCELL CUPS ON THESE, BUT PLENTY OF SQUIRT. I DONT HAVE EXTRA CARBS TO TRY BUT GETTING KITS TO TEAR THEM DOWN AND CHECK FOR ANY DIRT EVEN THOUGH FILTER WAS FINE. JUST FOR DRILL CHECKED ENGINE GROUND CABLE ALL IS GOOD WITH THAT

66vairguy
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Re: Surge at low throttle input just off idle

Post by 66vairguy »

You said you checked the vacuum advance for leaks, but did you try driving it with the vacuum advance vacuum hose disconnected? If that corrects the issue the vacuum advance spring in the canister may be broken or weak and going to maximum vacuum advance just off idle when the carburetor port sends vacuum to the vacuum advance canister.

May not be the issue, but easy to check before you replace ignition parts or carburetors.

66vairguy
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Re: Surge at low throttle input just off idle

Post by 66vairguy »

Rick ---- Check you PM's.

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Gregory_Miller
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Re: Surge at low throttle input just off idle

Post by Gregory_Miller »

Don't ask me how I know this, but if one of your idle siphon tubes is clogged it will idle on 3 cylinders but when you try to make power off one idle transition slot only it will surge and buck. I'd check to see if that might be your issue. One way to fond out is to cover the carb inlet with your hand and if you have no difference in idles when covering one but it dies when covering the other, you know which one is working and which one is not.

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