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Low flow on #4 / Excessive DCOE 'air bleed screw' input


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All, I have a situation around cylinder flow / air bleed screws that I cannot visualize after reviewing all my books / posts on this site etc. I set up my twin 40’s and they are running well. A quick summary is below for background from another post:

 

I have a stock (and old) motor and until recently thought a stock cam. I just noticed that it has a yellow and red paint stripe / ring on it so it now appears it is at least a regrind but I have no idea what! I have the IE mechanical Dist, step header and stainless exhaust. I gave IE my set up and then to start just bolted on the carbs as supplied. The markings made by IE on the box state:

  • Main: 120

  • Idle: 50F9

  • Air: No change

There were also 115 Mains and 45 F9( (Idle jet / bleed?) in a baggie that I assume were what the carb came with to IE. As of now I do not know the rest of the set up and wish I had pulled everything out and made notes before installing! I am also not sure if IE verified float levels prior to shipping and as such will need to check this.

 

The good news is it starts well and runs well. No stumbles and seems to stay in the high 13:1, low 14:1 range throughout the range only leaning out slightly further at high RPM's (high 14's). As it is  fun car and not daily driver I am going to go a little richer on the mains to try and get in to the 12:5:1 range and am thinking 125 or 130? (likely 130)

 

My question here is around a flow imbalance and the need to make a large adjustment to the air bypass screw that I encountered in initial set up:

 

After initially balancing the carbs between #2 and #3 I then compared flow from #1 to #2 and #3 to #4. With the latter #4 was flowing less (20% ish) than #3. I was (just) able to level them out by opening the air bleed screw on #4 to bring that flow up (measured with squirrel cage) but the screw is 2 ½ turns out (essentially all the way) which just doesn’t feel right.

 

My questions are what would cause a flow difference between two cylinders on the same carb, I just can’t visualize the ‘flow bypass’?

 

  • Valves? (Set 500 miles ago but will re-do)
  • Could a vacuum leak on #3 or #4 reduce the relative flow on #4? (I cant find one but am always wary of the brake booster take off)?
  • Something else?

 

Many thanks

Sam

Edited by sam1904
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  • sam1904 changed the title to Low flow on #4 / Excessive DCOE 'air bleed screw' input

There is a very good chance that at some point the main throttle shaft has been twisted.  It is very common for someone to tighten the nuts (or loosen a tight nut) on the end of the shaft while holding a wrench on the nut on the other end of the shaft.  This usually causes a twist in the shaft and as such the 2 throttle plates are no longer inline with each other.  If you know for a FACT that you do not have a leak in the manifold and that the engine is healthy (even compression and leak down tests between the cylinders) Close off the idle bypass screws if you have them in the carbs and check the airflow between the 2 throats in each carb,  if they are off you need to "twist" the shafts back to where they are even.  

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at some point the main throttle shaft has been twisted

This has been my experience.

 

An easy test is to pull the covers off the transition ports 

(the big flat screw on the top of the car, right over the 'closed'

position of the throttle plate)

and look down with a bright light.  When you move the throttle

slightly, you'll see the edge of the throttle plate.

Ideally, all 4 barrels are the same- but both barrels of each carb MUST

be in the same place.

 

t

https://www.lainefamily.com/images/WeberTuningManual.pdf

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Thanks all. Next steps

  1. Have pinched off the booster and that appears good.
  2. On a separate post it was suggested that my choke body may be hitting the manifold and creating a gap
  3. I am going to check for a twisted shaft and report back (these are new but I did move the arm from one side to the other..)

 

Thanks all, such a great resource

Sam

 

image.png

 

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Posted (edited)

Quick update. 
Checked the shaft and it appears straight with no issues.
I did however file off the edges of the coldstart housing and it seems to have closed up a gap between the carb and number four runner on the manifold. Will start up tomorrow but appears to be the culprit!

 

9D12FB1E-5FE9-4780-90FC-7AC104E4277A.jpeg

Edited by sam1904
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4 hours ago, Hans said:

What about the gaskets?

Is that your manifold in the picture? Did you have trouble getting fitting seated? In #4?

I used paper gaskets between all surfaces and believe any slight gap that was caused by the casting issue noted above has now been corrected.
 

That fitting shown is a pipe thread and hence does not fully seat, I did actually have issues in that I removed it and re-installed too many times and potentially stretched the thread slightly but believe I have overcome that with liquid PTFE.

 

 Thanks

 Sam

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No they do not, when I removed the coldstart castings there was deep interference on number 4 very mild interference on number 3 and none on 1 or 2. I will be starting the car again in the next day or so but looking at the images it does look like this is the reason why the gasket on number four was not being fully compressed and it does sites there could have been a gap. 

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