Running super lean off idle

[Zed Head]

On 5/12/2020 at 8:03 PM, RS02 said:

When I tried to drive it on the street and give it more than 1/4 throttle, the car stumbles badly and wants to die. 

I would start from scratch with this.  What are any and all possible causes for an engine to lose power when the throttle is opened?

[Zed Head]

One possibility is that the pistons aren't rising when the throttle is opened.  This keeps the needles low, reduces fuel input to the air passing by, and makes things lean.  The pistons' rate of rise is controlled by the damper oil.

So, it seems like you have sticky pistons.  What kind of oil are you using?

[Mark Maras]

 Post #1 you mentioned "Rebello modified" carbs and you thought they had been bored out. Can you give us more information about the modification?

[RS02]

1 hour ago, Mark Maras said:

 Post #1 you mentioned "Rebello modified" carbs and you thought they had been bored out. Can you give us more information about the modification?

I believe that they have been bored out to 50mm and it came with a custom ground needle... The needle looks like quite a bit of material has been taken off when compared to the stock SUs I had lying around. 

[RS02]

1 hour ago, Zed Head said:

One possibility is that the pistons aren't rising when the throttle is opened.  This keeps the needles low, reduces fuel input to the air passing by, and makes things lean.  The pistons' rate of rise is controlled by the damper oil.

So, it seems like you have sticky pistons.  What kind of oil are you using?

This is where it causes a bit of confusion. I searched this form and found in another post that slow the rise of the piston actually richens up the mixture. The "more piston raised more fuel" method is only true when the engine is at WOT. Running no oil and light spring will actually cause the car to run bad at any other throttle position besides WOT. 

I am running straight SAE 20wt oil (3-in-1 small engine oil), I can lift both pistons all the way up and both dampers are working to make them rise slow and drop fast. 

[Zed Head]

23 minutes ago, RS02 said:

This is where it causes a bit of confusion. I searched this form and found in another post that slow the rise of the piston actually richens up the mixture. The "more piston raised more fuel" method is only true when the engine is at WOT. Running no oil and light spring will actually cause the car to run bad at any other throttle position besides WOT. 

I am running straight SAE 20wt oil (3-in-1 small engine oil), I can lift both pistons all the way up and both dampers are working to make them rise slow and drop fast. 

My comment was just from the basics of Venturi and Bernoulli.  Where did your pasted picture come from?  Actually, there's a balance between air velocity and pressure and needle position (effective orifice size).  That's where things get messy.

As I understand things the oil controls the rate of piston movement, the speed, but not the steady state.  In other words a thick oil and a thin oil should put the piston in the same place at steady RPM.

I'm no carb expert but a set of stock carbs might be a good starting point just to see how the combination of engine parts work.  Then go to the "upgraded" carbs.  You don't really know what has been done to the ones you have.

Edited May 15, 2020 by Zed Head

[RS02]

57 minutes ago, Zed Head said:

My comment was just from the basics of Venturi and Bernoulli.  Where did your pasted picture come from?  Actually, there's a balance between air velocity and pressure and needle position (effective orifice size).  That's where things get messy.

As I understand things the oil controls the rate of piston movement, the speed, but not the steady state.  In other words a thick oil and a thin oil should put the piston in the same place at steady RPM.

I'm no carb expert but a set of stock carbs might be a good starting point just to see how the combination of engine parts work.  Then go to the "upgraded" carbs.  You don't really know what has been done to the ones you have.

I was looking at different posts on the forum and from what I can gather you are right, the spring controls the piston position, the damper controls the rate which the piston moves at. The picture was from an old article for tuning those British H2 or HS2 carbs.

The stockers are not in working order.  I think I might try to run without damper or even change the needles to the stock ones that I have. At this point I think I'll try with the resources I have here and see how they changes things.

Edited May 15, 2020 by RS02

[Captain Obvious]

The damper is a "transitional" device to temporarily slow the rise of the piston when there is a rapid increase in the air speed through the venturi. This has the effect of richening up the mixture for a short period of time. The damper does nothing to affect the piston position during steady state conditions. If you're already running lean on transition, then running without a damper will only make that worse.

@RS02 , You've got so much stuff going on that I find it hard to get a "baseline" from which to operate.

I don't know anything about "Rebello modified" carbs, but my (non-expert) suspicion is if they bored out the throat and changed the venturi area, it will have the effect of leaning out the mixture at the same airflow. So unless you're pulling in more air to make use of the new larger venturi area, then things are not going to work out well. A thinner needle may help mitigate that situation, but I wonder if those carbs were more intended for flat out WOT (track) performance more than street driving.

I agree with the sentiment above that it would be a good idea to drop back to a set of "normal" SU's and see what happens. Just so many variables...

[RS02]

10 hours ago, Captain Obvious said:

The damper is a "transitional" device to temporarily slow the rise of the piston when there is a rapid increase in the air speed through the venturi. This has the effect of richening up the mixture for a short period of time. The damper does nothing to affect the piston position during steady state conditions. If you're already running lean on transition, then running without a damper will only make that worse.

@RS02 , You've got so much stuff going on that I find it hard to get a "baseline" from which to operate.

I don't know anything about "Rebello modified" carbs, but my (non-expert) suspicion is if they bored out the throat and changed the venturi area, it will have the effect of leaning out the mixture at the same airflow. So unless you're pulling in more air to make use of the new larger venturi area, then things are not going to work out well. A thinner needle may help mitigate that situation, but I wonder if those carbs were more intended for flat out WOT (track) performance more than street driving.

I agree with the sentiment above that it would be a good idea to drop back to a set of "normal" SU's and see what happens. Just so many variables...

I wish I had a set of normal carbs... The set I have is not in working order and that's why I bought the rebello units in the first place.

[Captain Obvious]

Yeah, that's not an option then.

So how about pics... Can you take some pics of the carbs you do have? Do you have any documentation that came along with them?

Also, earlier you said "both dampers are working to make them rise slow and drop fast." - How did you test the dampers?

[Mark Maras]

 

3 hours ago, RS02 said:

I wish I had a set of normal carbs... The set I have is not in working order and that's why I bought the rebello units in the first place.

I want to know about the normal set of carbs you have. What's wrong with them?

[7tooZ]

On 5/15/2020 at 5:06 PM, RS02 said:

 

Sent you a PM on a different subject.

Don