Weber 40 IDA 3C Need help

[Amstaff]

I recently rebuilt my 1981 3.0 and rebuilt the Webers. Now at full throttle the motor kind of bogs down. It also "pops" but only at full throttle. It runs very smooth at anything but full throttle.

And, most of the idle mixture screws make no difference in idle speed as the are screwed all the way in.

Timing is right on the money but I tried advancing and retarding it a bit but it makes no difference.

I am running:
170 Air Correction
F3 Emulsion Tubes
34 Venturis
60 Idle jets

It is basically a stock engine but has 964 cams in it.

Suggestions?

[john walker's workshop]

How about the main jet size? What fuel pressure? 180 air is more common, unless you have small mains and need to pull more fuel from them at higher rpm. Sounds like you're running out of fuel.

[Amstaff]

Mains are 160

Fuel pressure is good holding about 3 lbs

[1QuickS]

145 mains would help, they are more appropriate for 34mm main venturis...rich bog is what I think it is.

Throttle plates too far open at idle and exposing first progression holes, perform "Lean Best" idle mixture adjustment. 55 idles might actually be good for you. If you have a STE Synchrometer (highly recommended) you will see a reading of about "5" when Lean Best is achieved. Then, the throttle plates will be closer to their proper position at idle and then the mixture screws should become effective.

If you cannot get throttle plates closed enough then try opening air correction screws as much as possible to allow plates to more fully close. If that does not help then you can drill a small hole in the throttle plates to provide more air. This is a bit more advanced and requires more discussion.

Get some 145s and try again.

[Amstaff]

145 jets are ordered.

I do not have a STE Syncrometer but I do have a more basic unit that I synced the carbs with. I assume they are two different animals?

Tom

[1QuickS]

Hi Tom,

Yes, synchrometers are not all the same. Air synchrometers I have used and have abandoned for the STE:

• Unisyn
• Carb Tune (a British, multi-tube device)
• mercury columns (four columns of mercury for balancing four cylinders simultaneously)

The STE is the best in my opinion and when the reading is around 5 then you can be pretty sure you are close to a Lean Best idle mixture adjustment. Larger STE readings show throttles are rather open from their design position at idle (edge of throttle plate should cover the first progression hole leaving fuel delivery to the engine at idle solely that from the mixture screw). Large throttle openings require more fuel to keep the engine running, typically the idle speed stop screw (at the end of the throttle body) is manually adjusted to decrease idle speed to 900 but air flow is high since mixture is "fat" and and engine speed will be suppressed due to fat mixture. Achieving Lean Best will optimize idle mixture which results in increased idle speed which requires adjusting throttle stop screws to close the throttles which requires less fuel to idle...all things in the right direction. The STE provides a litmus for achieving this end result and the other synchrometers (not the mercury sticks) only compare air flow between cylinders but do not quantify air flow to a base-line of zero flow.

[Amstaff]

Just bought one so I will let you know how the new jets and syncrometer work.

Thanks again,
Tom

[Amstaff]

I installed the 145 mains and it cured the bogging at full throttle.

Now onto the STE. The directions in the box are terrible!

So number 1 cylinder reads 6, 2 reads 5, 3 reads 5, 4 reads 5, 5 reads 6 and 6 reads 7.

Is the adjustment the air correction screws?

[1QuickS]

Yes, air correction screws adjust idle air flow, typically a 5 reading indicates Lean Best adjustment. Please have a review of info "Tune-up Interval Carburetor Adjustments" on my site: Performance Oriented

There is more info on the site regarding tuning and optimization but this is where to start.

Glad to hear the 145s helped!

[Amstaff]

So I finally got her out for a real ride. For the first 5-6 miles she ran great but once she warmed up it began to sputter. To the point that it stalled on me when I stopped. Luckily I got home.

Figured I would retune the carbs so I adjusted the mixture screws and 2 and 5 did not respond to any adjustments that I made. I backed those two out 2 turns and then reset the air correction screws to exactly 5.

It sounded better sitting in the driveway so I shut her down and decided to pull the plugs to see what they looked like. As I am pulling them I hear gurgling from cylinder 3 and 6. If I pop my hand on top of the velocity stack the gurgling stops so I know it is those two cylinders. Fuel pressure dropped to zero and 6 stopped but 3 continued to gurgle. Any idea why?

Here are the plugs. Left to right 1,2,3



Bottom to top 1,2,3

[Amstaff]

Left to right 4,5,6

[Amstaff]

Originally Posted by Amstaff

So I finally got her out for a real ride. For the first 5-6 miles she ran great but once she warmed up it began to sputter. To the point that it stalled on me when I stopped. Luckily I got home.

Figured I would retune the carbs so I adjusted the mixture screws and 2 and 5 did not respond to any adjustments that I made. I backed those two out 2 turns and then reset the air correction screws to exactly 5.

It sounded better sitting in the driveway so I shut her down and decided to pull the plugs to see what they looked like. As I am pulling them I hear gurgling from cylinder 3 and 6. If I pop my hand on top of the velocity stack the gurgling stops so I know it is those two cylinders. Fuel pressure dropped to zero and 6 stopped but 3 continued to gurgle. Any idea why?

Here are the plugs. Left to right 1,2,3

Bottom to top 1,2,3

[Amstaff]

Originally Posted by Amstaff

Left to right 4,5,6

Bottom to top 4,5,6

[911pcars]

Some random thoughts. Perhaps Paul can comment:

Check float bowl level?
Blocked idle passages?
Shape of idle mixture screws (3CI type or previous tapered shape?)
Insulation spacers under the carbs?

Sherwood

[Amstaff]

I set all of the floats with the float gauge. They are all set to the top line on the gauge.

I just rebuilt them and cleaned all of the passages. When assembled they were all clear. How do I check to see if the idle passage is blocked?

[911pcars]

Originally Posted by Amstaff

I set all of the floats with the float gauge. They are all set to the top line on the gauge.

I just rebuilt them and cleaned all of the passages. When assembled they were all clear. How do I check to see if the idle passage is blocked?

Remove each idle mixture screw and squirt carb cleaner into each using the straw nozzle. Observe carb cleaner exiting the idle passages in each carb bore.

Have you tested the fuel pump for pressure and volume? Malfunctioning FPs have been known to produce less volume as fuel requirements increase (at higher engine speeds).

Are you using a FP regulator? Does it include a FP gauge?

Sherwood

[Amstaff]

Originally Posted by 911pcars

Remove each idle mixture screw and squirt carb cleaner into each using the straw nozzle. Observe carb cleaner exiting the idle passages in each carb bore.

Have you tested the fuel pump for pressure and volume? Malfunctioning FPs have been known to produce less volume as fuel requirements increase (at higher engine speeds).

Are you using a FP regulator? Does it include a FP gauge?

Sherwood

I will pull them and shoot some carb cleaner through them.

It has a regulator with a gauge and holds the pressure pretty steady between 3 and 3.5 lbs.

[911pcars]

Originally Posted by Amstaff

I will pull them and shoot some carb cleaner through them.

It has a regulator with a gauge and holds the pressure pretty steady between 3 and 3.5 lbs.

Not suggesting the pump is the culprit, but fuel volume requirements change as engine speed/load increases. The pump could be fine under normal conditions, but deficient under full throttle. A basic pump test should produce about a quart of fuel per minute.

Some basic tests here to review:
All you need is a fuel pressure gauge to get you started

Sherwood

[1QuickS]

Check "Hidden Galleries" for blockage, these deliver fuel to the idle circuit and cannot be blown through unless you block certain passageways. Review this article "Clearing "Hidden" Fuel Gallery" on this web page: Performance Oriented

Then perform Lean Best idle mixture adjustment.

[jass]

911pcars mentioned in an earlier post the possibility of needing insulation spacers under the carbs. I had a similar issue only after warming up and after putting the insulation spacers under the carbs the problem disappeared. Perhaps Paul has observed that in the past as well?