Corvair DiagramCorvair Photo
Corvair Center
home forum corvairs calendar links Corvair Podcast
California Corvairs
Clarks Corvair
Clarks Corvair
“Performance
“CORSA"



Chevy Corvair License Plate
Chevy Corvair Chrome Wheel
Corvair Center Forum :  Corvair Center Phorum The fastest message board... ever.
Corvair Center 
Pages: 12Next
Current Page: 1 of 2
Flat Spot/Stumbling at 1800RPM
Posted by: LilRedDevin ()
Date: May 19, 2017 03:57PM

I've owned Tom Schrum's 1962 Devin C for a few years and I've always had a bit of trouble getting it to run properly. I've made a lot of progress but could use some input regarding a flat spot around 1800RPM. I should note that under my care the car is only fed 100 Octane race fuel. In ran even worse when I got it on 91 Octane pup gas w/ 10%-15% Ethanol.

The Engine:
- Engine #: TI208RB
- Dual Webber 44's
- Electric fuel pump + pressure regulator
- Pertronix ignition & Flamethrower II coil
- Tom's own custom cam grind

The symptoms:
- Car starts and runs well
- Car drives very nicely at freeway speeds
- On city streets with stoplights/stop signs the car feels like it wants to die when accelerating away from a stop sign. You can feather the throttle and "push through it" and once the car hits 2300-2400RPM is takes off like a rocket and revs for the moon.
- With car idling at a stoplight or at home in the driveway you can feel the flat spot when pushing the accelerator pedal. You can hear a hissing sound from the carbs and the engine wants to stumble. Seems like the sound is coming from the carb on the passenger side bank of cylinders.
- The flat spot/stumble is worse when the car is warmed up and having been driven a while. Very bad if the car is shut off to get gas, lunch, etc. and then driven again within 10-30minutes.

What I have done so far:
- Replaced fuel filter
- Checked timing: 14º at 650RPM. Advances when revved.
- Replaced Plugs, plug wires, rotor, cap, and coil. Replaced with the exact components Tom had on the engine. Ignition is Pertronix. Plugs looked really good in color too.
- Checked to ensure carb bolts to intake manifolds were tight.
- Replaced jets in the Webber 44's with the same jets Tom had written down when he built the engine (this alone sorted 90% of the drivability issues I had previously experienced).
- Adjusted all carb linkage to ensure both carbs were fully "off" with no pressure applied to pedal (they had been rigged to slightly open the throttle at idle).
- Carb accelerator pumps do squirt
- Adjusted and balanced the twin Webber 44's. Not saying they could not be better tuned by a more experienced mechanic but they are balanced and at a much better place than they were.

What I have not done:
- Inspected valve train
- Compression test
- Rebuilt carbs

Would love to hear from the experienced Corvair owners here. Suggestions for other things to inspect & fix. Suggestions for better carb tuning. Suggestion to make it run correctly. I'm all ears. Very difficult to find mechanics knowledgable about these engines so I'm trying to learn all I can to ensure I can get this engine running properly and keep it running for a very long time.

Thanks in advance for any and all input!

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: Lane66m ()
Date: May 19, 2017 04:08PM

Not being experienced enough to suggest any tips, I think additional info would be needed.

PG or manual?

Petronix I or II

Petronix II is known to not play well with PG cars.

Al Lane
Ellabell GA

1966 Monza Coupe, 110 hp, 4 Spd
1966 Corsa Turbo Coupe, 180 hp 4 Spd
1964 Greenbrier Deluxe, 6 dr, 80 hp car engine, PG
2015 Chevrolet Malibu 2LT
2015 GMC Sierra SLT Crew Cab
1968 Camaro SS Coupe 350 CI 295+ HP PG
1947 Farmall A tractor 15 hp


Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: LilRedDevin ()
Date: May 19, 2017 04:34PM

Manual Transmission w/ Posi diff.

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: MattNall ()
Date: May 19, 2017 04:39PM

It's a PG!!! Devin C's are all that way!! GGGGGG


I suggest Adjusting fuel pressure!! WEbers are super sensitive!

Here's a Ladies version of a Devin.....I sold it to a50 year old woman!

PG 145 ci 2nd place specialty class [ 3rd fastest ] at Flagstaff 2002.

Attachments:

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: LilRedDevin ()
Date: May 19, 2017 04:50PM

- Pertonix II
- Manual Transmission w/ Posi diff.

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: steve c goodman ()
Date: May 19, 2017 04:54PM

Is your vac adv canister working or are you using centrifugal adv only?

best wishes, Steve
Rear Engine Spec. Inc. Golden, Colo.

1962 spyder 3.0L turbo 1965 Crown V8
1967 monza 110/4 1968 monza 110/4
1971 amante gt 110/4
CORSA/RMC/PPCC

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: ROD ()
Date: May 19, 2017 05:20PM

timing , wrong distributor probably not the carbs.... do your webers even have a vacuum port for the dizzy ?

Rod Tetrault
El Cajon , CA
65 Corsa Yenko Clone / 65 Corsa EO Creampuff Vert
66 Corsa "JIMISH" Mid engine Turbo LS1 currently 2nd fastest Corvair in the USA
Class 5 Corvair powered Baja
61 Vintage Rampy
Corvair powered Buggy x 3
Enough hidden parts to build a space ship

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: LilRedDevin ()
Date: May 19, 2017 06:18PM

Rod,

There is NO vacuum advance installed on the distributor and no vacuum ports on the Webber's. The car is running the Pertonix II electronic ignition in the distributor. Timing is set at 14º at 650 RPM.

I do not question your logic BUT the car was very competitive when Tom Schrum owned it racking up over 50 TToD auto-x wins. Tom Passed a few years back so all I have to go from are his notes and saved receipts.

So yes, it very well COULD be ignition related. I would not rule it out. But the car ran well for Tom in this configuration so I'd hate to start changing parts without a way to test the part is not working optimally first.

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: MattNall ()
Date: May 19, 2017 06:23PM

Pet II's are hard to test..... see if you can borrow a 1 or 2 from someone local...

They are know to fail in weird ways...


What Fuelpump are you using? a regulator? if so, what pressure on a Gauge?





MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
..............................110-PG.................................................Webered-Turbo

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: ROD ()
Date: May 19, 2017 06:39PM

Its the timing. the pegged distributor is only using the centrifugal advance so you get easy to start and good pull above 2000 -2500 rpm. you can adjust timing to make it run right but it wont want to start..... some webers you can add the vacuum port then go back to a vacuum control on the distributor that will smooth out the drivability. I'm sure he did fine racing around but the normal drivability is the trade off. the pertronix or points wont effect this.

you can try a better curve distributor like a Mallory if you want centrifugal alone.

Rod Tetrault
El Cajon , CA
65 Corsa Yenko Clone / 65 Corsa EO Creampuff Vert
66 Corsa "JIMISH" Mid engine Turbo LS1 currently 2nd fastest Corvair in the USA
Class 5 Corvair powered Baja
61 Vintage Rampy
Corvair powered Buggy x 3
Enough hidden parts to build a space ship

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: ROD ()
Date: May 19, 2017 06:54PM

you mentioned re adjusting the air volume at idle by resetting the butterflies to be completely closed .... what number are you now getting on the sync tool ?

Rod Tetrault
El Cajon , CA
65 Corsa Yenko Clone / 65 Corsa EO Creampuff Vert
66 Corsa "JIMISH" Mid engine Turbo LS1 currently 2nd fastest Corvair in the USA
Class 5 Corvair powered Baja
61 Vintage Rampy
Corvair powered Buggy x 3
Enough hidden parts to build a space ship

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: LilRedDevin ()
Date: May 19, 2017 07:28PM

Rod,

I'm using the GERMAN SK Snail Type Syncrometer as suggested by one of the Webber vendors. Reading 8 on all four venturis at idle (just below 700RPM).

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: ROD ()
Date: May 19, 2017 07:45PM

IDA or IDF webers ?

yes this style like here on my bench ...... I set webers at 7 but if you are already closed butterfly at 8 I don't see how you are going to lower the air volume. were there holes drilled in your throttle plates ?

Rod Tetrault
El Cajon , CA
65 Corsa Yenko Clone / 65 Corsa EO Creampuff Vert
66 Corsa "JIMISH" Mid engine Turbo LS1 currently 2nd fastest Corvair in the USA
Class 5 Corvair powered Baja
61 Vintage Rampy
Corvair powered Buggy x 3
Enough hidden parts to build a space ship

Attachments:

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: LilRedDevin ()
Date: May 19, 2017 07:54PM

Webber 44 IDF's. Throttle plates are not drilled.

I could probably get the air down to 7 on the sync tool and still have it idle BUT with the lumpy cam it does seem happier idling a bit higher. I believe that's one reason the throttle linkage had been rigged to slightly open the throttle even when your foot was off the pedal. They were sort of cheating the idle circuit a bit that way and running it slightly faster at idle. I understand I could probably achieve the same or better results with more ignition advance (currently 14º at 650RPM) and less intake air but I've been scared to death of too much advance on these engines based on other owner's horror stories.

I do like your suggestion that perhaps I should look at a different distributor/ignition set-up. So much of this engine is "hot rodded" but other than the Pertonix the ignition seems to be stock (minus vacuum advance of course).

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: MattNall ()
Date: May 19, 2017 08:29PM

LilRedDevin Wrote:
-------------------------------------------------------

> So much of this engine is "hot rodded" but other
> than the Pertronix the ignition seems to be stock
> (minus vacuum advance of course).

============================================================

What little I knew Jim.... I'd put the distributor on a Machine or use a "dialback " timing light with tach" to see what you have...





MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
..............................110-PG.................................................Webered-Turbo

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: ROD ()
Date: May 19, 2017 08:34PM

yeah they like a bit of a high idle. with the plates closed I thought you were going to say 5 or 6 and I would say open them up to 7 . once the air is up then you tweak the lean fat condition to rid the thing from popping. at least you have IDF's they are supposed to be more street friendly than IDA . I would try a better centrifugal advance distributor.

you can play with the corvair one but you need someone that can tell you what its doing with a sun machine. A while back I mentioned to Vairchet I had a few pegged vair Dizzy's but haven't gone to see him yet. I just want to know where they are at so I can duplicate them if needed. I guess a really good tach and digital timing light with degree counter would do it if you had enough hands to help.

those jets are pretty big I rarely need to replace them. just clean and check them with jet feeler gauge.

Rod Tetrault
El Cajon , CA
65 Corsa Yenko Clone / 65 Corsa EO Creampuff Vert
66 Corsa "JIMISH" Mid engine Turbo LS1 currently 2nd fastest Corvair in the USA
Class 5 Corvair powered Baja
61 Vintage Rampy
Corvair powered Buggy x 3
Enough hidden parts to build a space ship

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: ROD ()
Date: May 19, 2017 09:23PM

ahh Matt beat me to it

Rod Tetrault
El Cajon , CA
65 Corsa Yenko Clone / 65 Corsa EO Creampuff Vert
66 Corsa "JIMISH" Mid engine Turbo LS1 currently 2nd fastest Corvair in the USA
Class 5 Corvair powered Baja
61 Vintage Rampy
Corvair powered Buggy x 3
Enough hidden parts to build a space ship

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: MattNall ()
Date: May 19, 2017 10:00PM

Tom's abilities.....

Phil Dally wrote:

My 148.253 mph pass had me at 7200 rpm through the trap.

It was the Schrum II motor with hydraulic lifters...no floating.





MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
..............................110-PG.................................................Webered-Turbo

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: RobertC ()
Date: May 20, 2017 12:14AM

Have no clue on how Webers work - Is 1800 rpm where the carb switches from the idle circuit to the main circuit?

Have any idea on what is in the distributor? Shaft? Cam? Weights? Springs?

Vaguely recall somebody - Dayton Wheel Products? (could be IECO, Crown, Otto???) sold a kit (cam, springs, weights) that "changed" a Corvair distributor to all mechanical advance. This was late 70's / early 80's per my feeble memory.

Autocrossing - the only time the engine would be at 1800 rpm is when accelerating onto the course and when stopping after completing the course... I am sure he "managed" the flat spot if it was there at that time.

Options: ReplyQuote
Re: Flat Spot/Stumbling at 1800RPM
Posted by: LilRedDevin ()
Date: May 20, 2017 07:48AM

Thanks for sharing MattNall and congrats on your timed run, that is impressive!

Tom but great motors. That is one of the reasons I am putting forth the effort to get this one running right... hopefully.

The motor pulls strong and revs fast. I just need to work on ensuring it is receiving right fuel, air, and spark at all revs.

Options: ReplyQuote
Pages: 12Next
Current Page: 1 of 2


Sorry, only registered users may post in this forum.
This forum powered by Phorum.