Flat Spot/Stumbling at 1800RPM

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LilRedDevin
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Flat Spot/Stumbling at 1800RPM

Unread post by LilRedDevin » Fri May 19, 2017 4:06 pm

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!

skipvair
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Re: Flat Spot/Stumbling at 1800RPM

Unread post by skipvair » Fri May 19, 2017 6:57 pm

I will try find the website inglesinduction who specialize in webers. I am sorting through minor issues with 2 3 barrel 40 IDA's and they have great tech on the 44's


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bbodie52
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Re: Flat Spot/Stumbling at 1800RPM

Unread post by bbodie52 » Fri May 19, 2017 7:52 pm

From someone who knows nothing about tuning Weber carburetors...

If the Weber two-barrel was configured with a progressive throttle linkage -- opening one carburetor throat as a "primary" and then bringing on he other as a high rpm "secondary", you might not experience that "flat spot" when slow air flow stalls fuel flow in the venturis as the carburetor transitions from low speed to high rpm airflow through the venturis. The engine may be suffering somewhat from being fed too much carburetor CFM that leans the fuel-air mixture at the engine RPM you mentioned, which gives it a performance "flat spot". Higher RPM produces greater airflow through the venturis, which produces better atomization and an improved fuel/air mixture.

The accelerator pump characteristic described below may point to a cure. If you were able to tune the accelerator pumps to generate a long-duration output with greater volume, the mechanically injected boost of gasoline might help the engine to push past the lean period you are apparently experiencing

IDF Carburetors
Low Speed Circuit Tuning

:link: http://www.redlineweber.com/html/Tech/i ... ntrols.htm
Pump By-pass Valve:

The pump by-pass valve is designed to allow fuel into the pump circuit and when depressed, by-pass a percentage (hole on the side of the valve) of the fuel delivered to the accelerator pump nozzle/jets. We have included the zero by-pass valve in our jet kit. This will deliver all of the available fuel to the engine and not by-pass fuel back to the fuel bowl. This will increase the duration and volume of the pump shot with the original pump jets. To decrease the duration and increase the fuel volume we have included in our jet kit the larger 55 pump nozzle/jets. This is for maximum fuel delivery from the accelerator pump circuit. The accelerator pump by-pass valve is located in the bottom of the fuel bowl, one is required per carburetor.
Also, think about this...
Arriving at the correct carb/venturi size

When selecting Webers, the most commonly asked question is "Should I have 40s or 45s" coupled with "Surely the 45s will give more power". This shows a basic misunderstanding of the construction and principles of operation of the DCO series. It is not the barrel size (40 or 45) which determines the airflow and therefore potential horsepower; it is the size of the main venturi or choke. Selection of the correct main venturi size is the first step in selecting the carburettor.

It is easy to make the assumption that biggest is best when selecting a main venturi size, but the purpose of the main venturi is to increase the vacuum acting on the main jet in order to draw in and effectively atomise the fuel mixture. The smaller the main venturi, the more effective this action is, but a smaller venturi will inhibit flow. A large venturi may give more power right at the top end of the power band, but will give this at the expense of lower RPM tractability. Only a circuit racer will benefit from this sort of compromise, on a road car, driveability is much more important. 95 percent of the time, a road engine is nowhere near its peak power, but is near its peak torque for 75 percent of the time. It is much more important therefore to select the main venturi for best driveability, once the venturi size has been selected, then the appropriate carburettor size can be arrived at.
:link: http://www.s262612653.websitehome.co.uk ... webers.htm

Perhaps reducing the venturi size would produce a better match for your street-tuned Corvair engine. The above article may give you some idea of just how tuneable Webers are... that is what you pay for when choosing Weber carburetors. The section on Idle Jet selection may give some useful insight to help get your carburetors right for your engine and your intended usage. There is a lot of trial and error and experimentation involved.
Brad Bodie
Lake Chatuge, North Carolina
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LilRedDevin
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Re: Flat Spot/Stumbling at 1800RPM

Unread post by LilRedDevin » Fri May 19, 2017 7:58 pm

Thanks bbodie52. The carbs are NOT progressive. All 4 venturi are sync'd the same.

I don't disagree on the accellorator pump but most of the webber tuners seem to think that would be a band-aid fix or mask the underlying issue. I'd prefer to get it to run well on the idle circuit 650-2500RPM and have the accelerator pump work well on top of that. Something I will look into further though!

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bbodie52
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Re: Flat Spot/Stumbling at 1800RPM

Unread post by bbodie52 » Fri May 19, 2017 8:24 pm

I know the Weber linkage is not progressive -- I believe that is part of the problem. Too much air overwhelms the small displacement engine and log-style common intake manifold.

I added a second part above, with regard to reducing the venturi size to match the carburetors for street use, instead of tuning the carbs for high speed racing.
Brad Bodie
Lake Chatuge, North Carolina
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