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Stutter after releasing accelerator

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lwgreig Lachlan G
Edinburgh, UK   GBR
Hi guys, new member and very new Spitfire owner!

I recently bought a 1972 mkIV, and am still getting used to its quirks. Recently however, it has started to drag or jump after I release the accelerator - it is worst in 2nd but does it slightly in 3rd as well.

Sorry for the vague description, any ideas what could be the issue?

Cheers guys

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Yellowhawk Valley Avatar
walla walla, WA, USA   USA
1969 Triumph Spitfire "Walla Walla"
1969 Triumph Spitfire "Portland"
1972 Triumph Spitfire MkIV "Spokane"
1975 Triumph Spitfire 1500 "Dayton"    & more
You are right, the description is confusing. It should naturally react like "dragging" (like decelerating?) when you let off the gas. Now the "jumping" part is something new. The lower the gear you are in, the more deceleration that will occur - faster decal.

Do you mean the engine rpm seems to jump around, up and down a few seconds when you first let off? Or that it sounds like it is sputtering?
Dan

lwgreig Lachlan G
Edinburgh, UK   GBR
Hey Dan, cheers for the quick response.

I'll try to be more clear - when I take my foot off the accelerator, the car will jerk in a motion similar to if I was to pump the brake slightly, twice in a row. Similar maybe to when starting off cold with some choke, and the engine misfires a bit.

Again, apologies that I'm coming from a pretty amateur background, this is my first classic!

Lachlan



Edited 1 time(s). Last edit at 2017-12-29 01:55 PM by lwgreig.

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Tonyfixit Avatar
Tonyfixit Tony M
Duncan, BC, Canada   CAN
I'm thinking an ignition problem, perhaps a misfire.

Hook up a timing light and see if the light momentarily stops flashing after you rev the engine.

lef2wander Avatar
lef2wander Gold Member James Thomas
Hatfield, MA, USA   USA
Dash pot oil

Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
Check the air cleaner.

lwgreig Lachlan G
Edinburgh, UK   GBR
I took the air cleaner off the caburettors today and poked around a bit - the paper part of the air filters seemed pretty bent out of shape in lots of places - would this be a large issue?

Also couldn't remember how the hoses fitted onto the air cleaner - but I think I've clamped them back in a reasonable position!

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SpiTazz72 Avatar
SpiTazz72 Bryan H
Magnolia, TX, USA   USA
Strange because I was just in a newer Jeep a few days ago and it did something similar. We found the rear wheels had lose lug nuts. I'd jack up the back of the car and check the wheels and drivetrain for any loose bits or work u joints.
But just a guess. Better safe than sorry.

Yellowhawk Valley Avatar
walla walla, WA, USA   USA
1969 Triumph Spitfire "Walla Walla"
1969 Triumph Spitfire "Portland"
1972 Triumph Spitfire MkIV "Spokane"
1975 Triumph Spitfire 1500 "Dayton"    & more
In reply to # 1504910 by lef2wander Dash pot oil

Yeah, I will go with this as a good possibility too. No oil in there and they needles will drop suddenly cutting off all the fuel, more or less. The ignition check is not a bad idea either.. It sounds like something is shutting the engine clear off for just a second when you do that, so either of these could be the culprit./
Dan

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spitfire50 Avatar
spitfire50 Paul Mugford
Rochester, N.H., USA   USA
In reply to # 1505180 by Yellowhawk Valley
In reply to # 1504910 by lef2wander Dash pot oil

Yeah, I will go with this as a good possibility too. No oil in there and they needles will drop suddenly cutting off all the fuel, more or less. The ignition check is not a bad idea either.. It sounds like something is shutting the engine clear off for just a second when you do that, so either of these could be the culprit./
Dan

Dan,
The dash pot dampers only slow the motion of the piston on rising. They are designed to let the piston drop without extra resistance. Remember you check the centering of the jet by letting the piston drop listening for the click of the piston hitting the bridge.
All the best,
Paul

lwgreig Lachlan G
Edinburgh, UK   GBR
Hey guys, this morning I checked the oil level in the dashpots and they seemed fine, hard to judge how much should be in there?

I took it for a quick run up and down the road, and found that the feeling of the throttle snapping shut doesn't happen when I keep the choke maybe an inch out. I should mention that I'm currently in the south of England which is pretty cold right now. Could it just be that I'm not letting it warm up enough? What are the pros and cons of running with the choke open a small way?

lef2wander Avatar
lef2wander Gold Member James Thomas
Hatfield, MA, USA   USA
With choke out an inch, your running rich. The air/fuel mix has more petrol in it than normal. It's okay for a short run, but say after just a few miles you should be able to come off.(and should)!. With the choke out an inch what is your idle speed. What kind of oil is in the dash pot. SAE 50/40/30, MMO?

Is your thermostat working? Is it stuck open?
HOW cold is the ambient air. 0°C??? Colder?
Do you have an aluminum radiator?
Is the plumbing to your intake manifold correct or defeated.

lef2wander Avatar
lef2wander Gold Member James Thomas
Hatfield, MA, USA   USA
I should mention that I'm currently in the south of England which is pretty cold right now.


I'll trade your weather for mine. It's 6°F here. Yours looked like a low of 42°F today. If that's true it should run off choke after a short period. Mine on the other hand is not going anywhere, I refuse to use the top, I refuse to go out at all.

spitfire50 Avatar
spitfire50 Paul Mugford
Rochester, N.H., USA   USA
Lachlan,
Having the choke open, or rather closed, a bit, only puts some fast idle on. Pulling the choke more lowers the jet which makes the mixture richer. Having the jet lower can also allow the piston to drop to the bridge though it would stick if the jet were fully up. This is a sign that the jet isn't centered and the needle is brushing against it before the piston falls to the fully down position on the bridge.
With the control knob pulled out an inch you are at the point where the jet might be moving. It is a good idea to check if this is the case. With the choke control pushed fully home lift the pistons and see if either stays up a few mm. if that happens, pull the control and see if the piston has dropped to the bridge.
All the best,
Paul

lwgreig Lachlan G
Edinburgh, UK   GBR
Hey guys - thanks for all the response, overwhelmed with possible solutions.

I'm starting to wonder if my relatively poor driving of this Spit has roughed it up a wee bit or there were some underlying issues that I didn't notice on the test drive.

Drove from near London to near the welsh border today which revealed a few worrying details. Even at a very low speed, I was unable to change down to 2nd, just ground heavily until I skipped it and started again in first. Changing up into it is fine! 3rd is also now worrying me - it gives a small protest noise when changing up and now moves quite violently when pressing the throttle, sometimes enough to throw it out of gear.

Really thankful for all the help so far, could all these issues be related?

Lachlan

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