[Fot] Stutter at 4K rpm

[Jim Gray]

That’s reversion which is exhaust pulses running back up through the exhaust & back through the motor.

Weber carbies do not like cams with less than 110 degree cam lob centers for that reason.

Is there more fuel present at the filter now than before ? Without the tail pipe you are missing out

on some amount of extraction. Greg Solow has done a ton of dyno work with exhaust systems

and he found that an 18”, slight megaphone shape, was superior to just a straight pipe for HP 

and it’s ability to extract so exhaust coming straight out of a muffler cannot be good and will

affect how the entire system performs.

Jim g

 

From: Fot [mailto:fot-bounces at autox.team.net] On Behalf Of Duncan Charlton
Sent: Sunday, October 16, 2016 8:07 AM
To: D V <explore8 at outlook.com>
Cc: fot at autox.team.net
Subject: Re: [Fot] Stutter at 4K rpm

 

Derek,

I’m not sure the fuel in that location tells you much.  With some carbs you can see a fuel/air fog standing in the trumpets and this can condense and be visible as droplets when you have the engine stopped.

 

Duncan Charlton

Elgin, Texas USA

 

 

 

On Oct 15, 2016, at 10:51 PM, D V <explore8 at outlook.com <mailto:explore8 at outlook.com> > wrote:

 

Thanks everyone for the replies. I will inspect the pipe to manifold and manifold to head to see if anything is loose or any leaks apparent in the morning.

 

I actually went down a main jet 145 to 140 and that seemed like it helped some but didn't clear up the issue (which is going leaner; opposite of what Greg said). 

 

It is missing and backfiring when I am full on the gas. I can prevent it by easing into the gas.

 

I did notice some gas at my air cleaner to velocity stack connection. I wonder if it is a float issue?

 


Sent from my iPhone


On Oct 15, 2016, at 7:35 PM, Greg Blake <gblake58tr3 at icloud.com <mailto:gblake58tr3 at icloud.com> > wrote:

Don't know if my other email made it to you. Backfires on deceleration can be a lean condition. Have you checked your plugs?  Maybe something got "adjusted" with the bump. 

Sent from my iPhone


On Oct 15, 2016, at 12:27 PM, D V <explore8 at outlook.com <mailto:explore8 at outlook.com> > wrote:



Sent from my iPhone


Begin forwarded message:

From: <explore8 at outlook.com <mailto:explore8 at outlook.com> >
Date: October 15, 2016 at 10:30:51 AM CDT
To: <fot at autox.team.net <mailto:fot at autox.team.net> >
Subject: Stutter at 4K rpm

Two races ago I went off track and bent my exhaust pipe enough to break the welded joint near the muffler. In the interest of time, I cut the exhaust at the muffler and secured the pipe to the frame.

At the last race, I noticed the car would backfire at deceleration which I am not surprised but I also noticed a stutter around 4-5k rpm. 

At NOLA this weekend it is more pronounced and comes in when shifting hard from 3rd to 4th. After 5k, it clears up and pulls normally.

It doesn't seem to be affected by turns and more related to full throttle vs easing into the gas.

Backpressure from the loss of muffler or unrelated issues?

1300 spitfire with single Weber 40.

Sent from my iPhone

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