[Fot] Stutter at 4K rpm

[Bud R]

A vacuum leak is more evident at lower RPM, 4-5k is lower on a 1300 Spitfire engine. If changing the main jet to a leaner jet helped that might be because you were getting less air through the chokes because of a vacuum leak somewhere else and thus less air through the chokes meant a better air/fuel mix given the air leak bypassing the carbs and not going through the chokes. Easing the throttle gives the engine a chance to deal with the too fuel rich mixture while stomping on it shoots an over rich mixture into the engine and makes it stutter.

At higher RPMs this is not as evident a problem because the air leak is less of a factor when pulling lots of air through the carbs.

Did you check for leaks by spraying carb cleaner around the manifold?  

Bud Rolofson

Extreme Parts Racing (more than just a haircut)

71TR6 CC57365 (Good 6)
71 Spitfire MK IV Race Car #3
69 Spitfire MK 3 (back up FE engine/dinghy car)
93 Minnie Winnie Race Support Vehicle 
77 Z-50A Hardly Davidson Honda Mini-Trail Bike (Triumph Pit Bike)












On Oct 16, 2016, at 9:14 PM, D V <explore8 at outlook.com> wrote:

> Thanks again everyone. I checked the header to head and the exhaust to header connection and everything seemed tight. However I went 2 off hard at turn 4 this morning and my strap that was holding the exhaust is now mostly gone. I never figured out that 180 degree off camber turn all weekend.
> 
> To answer Scott's question, yes. I can brake downshift to 3rd in a turn. Full on the gas out of the turn and it stutters between 4-5k. If I do the same and ease into the gas I can most of the time keep it steady.
> 
> Re working the exhaust system as well as checking the float will be added to my winter maintenance. 
> 
> Sent from my iPhone
> 
>> On Oct 16, 2016, at 2:21 PM, Barr, Scott <sbarr at McCarty-Law.com> wrote:
>> 
>> Is it temporary?  Just as you get on the gas out of a corner and then clears up as the revs rise?
>> 
>> If so, do you have no oil in your dashpots or have you removed the dashpot pistons?
>> 
>> Scott (B.)
>> 
>> 
>> 
>> 
>> 
>> -------- Original message --------
>> From: D V <explore8 at outlook.com>
>> Date: 10/16/16 12:53 AM (GMT-06:00)
>> To: Greg Blake <gblake58tr3 at icloud.com>, fot at autox.team.net
>> Subject: Re: [Fot] Fwd: Stutter at 4K rpm
>> 
>> 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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