Visit our Twitter feed for more great content
TRExp

Spitfire & GT6 Forum

Why is fuel spitting from front carb on deceleration

Moss Motors
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor

LGMTR6 Avatar
LGMTR6 Larry Miceli
Mount Dora, FL, USA   USA
1952 MG TD
1954 MG TF 1500
1959 Triumph TR3A
1967 MG MGB GT    & more
My brother is working on putting a newly rebuilt set of SUs onto a 1980 Spitfire.
The carbs worked fine on the car they were removed from.
The issue is that the front carb now burps fuel from the overflow if the RPMs are above 2500 and you then let off the gas quickly.
It doesn't happen if you slowly let off the gas.
So, bad float adjustment?
Bad needle and seat?
or too much fuel pressure?
Or any other ideas?
Thanks.



Larry Miceli
Mount Dora, Florida

52 MG TD (restored)
54 MG TF 1500 (project)
56 Triumph TR3 (project)
67 MGB GT (restored)
68 Triumph TR 250 (nice driver)
71 Triumph Stag (driver)
73 Triumph Stag (project)
74 Triumph TR6 (restored)

I've just never really figured out the "sell one when you get another" part of it all...



Edited 1 time(s). Last edit at 2019-02-08 01:15 PM by LGMTR6.

. Hide banner ads & support this website by becoming a > Gold Supporting Member <
65or66 Jim B
Lake village, IN, USA   USA
1965 Triumph Spitfire MkII
1971 Triumph Stag
1973 Triumph Spitfire 1500 "Jusanudda Munny Pit"
moving from one engine to another may have knocked some crud loose, affecting float needle and seat.

Hopefully it's a stock (correct pressure) fuel pump?

You'll need to check float/needle/seat, and clean out the bowls while you're there. Look for junk in the bottom of the bowl. Tiniest piece of near-nothing can cause malfunction.

If the rubber fuel lines on the car are falling apart, that can get into the carbs too.

65or66 Jim B
Lake village, IN, USA   USA
1965 Triumph Spitfire MkII
1971 Triumph Stag
1973 Triumph Spitfire 1500 "Jusanudda Munny Pit"
forgot to ask, where is the fuel spitting from? And does it spit fuel at idle?

. Hide banner ads & support this website by becoming a > Gold Supporting Member <
TheZster Avatar
TheZster Steven Z
SAINT LOUIS, MO, USA   USA
1978 Triumph 1500 "BLK-BRY"
needle/seat...… If I were to take a shot at it...…..Engine is decelerating - but pump is still pumping through the process as it relies on rpm.... carbs not asking for gas...… but getting overflow...… something in there isn't shutting down flow.... needle/seat.....

Z

65or66 Jim B
Lake village, IN, USA   USA
1965 Triumph Spitfire MkII
1971 Triumph Stag
1973 Triumph Spitfire 1500 "Jusanudda Munny Pit"
also, were the carbs taken apart during the transfer from one engine to another? maybe someone else can remember better, but I think if the top cover/piston/diaphragm is installed incorrectly, it can cause this problem?

Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
Are your plugs sooty?

spitfire50 Avatar
spitfire50 Paul Mugford
Rochester, N.H., USA   USA
In reply to # 1595215 by Doug in Vegas Are your plugs sooty?

Doug,
With fuel coming out the overflow when things go wrong they may well be. That doesn't say anything about float level. Fuel out the overflow shows fuel is above float level. At least the front pair of plugs might be affected.
All the best,
Paul

spitfire50 Avatar
spitfire50 Paul Mugford
Rochester, N.H., USA   USA
In reply to # 1595106 by 65or66 moving from one engine to another may have knocked some crud loose, affecting float needle and seat.

Hopefully it's a stock (correct pressure) fuel pump?

You'll need to check float/needle/seat, and clean out the bowls while you're there. Look for junk in the bottom of the bowl. Tiniest piece of near-nothing can cause malfunction.

If the rubber fuel lines on the car are falling apart, that can get into the carbs too.

Jim,
Though it is good to clean the bowl, anything found there has already passed the needle and seat. Stuff there might get to the jet and cause trouble, but it can't cause the float valve to stick open any more.
If after cleaning the trouble persists, finding new debris in there can indicate there is an ongoing problem of junk in the fuel.
All the best,
Paul

Mgro1959 Rob M
Surrey, BC, Canada   CAN
The piston could be hanging up allowing fuel to keep flowing but unlikely since unlike SU carb these have flexible diaphram and self centering needle. . The flow shut off needle seat valve in the bowl which shuts off fuel is sticking open or float either hanging up or possibly has fuel in it causing it not to shut off. Fuel can only leak out if it is not shutting off in float bowl. Goodluck. Thanks Rob.

Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
In reply to # 1595351 by spitfire50
In reply to # 1595215 by Doug in Vegas Are your plugs sooty?

Doug,
With fuel coming out the overflow when things go wrong they may well be. That doesn't say anything about float level. Fuel out the overflow shows fuel is above float level. At least the front pair of plugs might be affected.
All the best,
Paul

As I've mentioned before: The gasket at the top of a float bowl should never be immersed.

The ZS could use a sight glass.

spitfire50 Avatar
spitfire50 Paul Mugford
Rochester, N.H., USA   USA
Hey guys,
To quote the first line of this thread: My brother is working on putting a newly rebuilt set of SUs onto a 1980 Spitfire. Note: SUs, not ZSs.
All the best,
Paul

brucejon Avatar
brucejon Gold Member Bruce Jones
Santa Cruz, CA, USA   USA
1960 Triumph TR3A
1963 Triumph TR3B "Tupperware TR3"
1969 Triumph Spitfire MkIII
1972 Triumph TR6
While it likely is related to the last thing you did - work on the carbs - you might check the valve lash on the intake valves for the carb that is spitting. If the intake valve isnt closing all the way it will blow back.



60 TR3A (red), 62 TR3B project, 72 TR6, 69 Mk3 Spitfire EU setup
https://spitfiremk3.wordpress.com

LGMTR6 Avatar
LGMTR6 Larry Miceli
Mount Dora, FL, USA   USA
1952 MG TD
1954 MG TF 1500
1959 Triumph TR3A
1967 MG MGB GT    & more
Wow! Great input! Thanks!
Yes, sooty plugs so running too rich. Bowls are clean and all new hoses. Adjusted floats to correct factory settings. And factory mechanical fuel pump. So, I suspect bad needle and seat as well.



Larry Miceli
Mount Dora, Florida

52 MG TD (restored)
54 MG TF 1500 (project)
56 Triumph TR3 (project)
67 MGB GT (restored)
68 Triumph TR 250 (nice driver)
71 Triumph Stag (driver)
73 Triumph Stag (project)
74 Triumph TR6 (restored)

I've just never really figured out the "sell one when you get another" part of it all...

Mgro1959 Rob M
Surrey, BC, Canada   CAN
If is is SU carbs then the needle is probably not centered properly and piston hanging up. Undo the jet nut on bottom with the dashpot piston needle assembly in place raise piston up and down with middle finger a few times letting it seat. Tighten the jet nut with piston seated. This should center the needle and allow piston to seat and shut off fuel.

65or66 Jim B
Lake village, IN, USA   USA
1965 Triumph Spitfire MkII
1971 Triumph Stag
1973 Triumph Spitfire 1500 "Jusanudda Munny Pit"
Paul, where did I say that junk in the bowl will cause a malfunction of the needle/seat? Pretty sure I did not. My point was that if there is junk in the bowl, it surely came past the needle/seat, and is entirely possible that there is more junk to be found in the fuel system. Sorry if that was too vague for ya

. Hide banner ads & support this website by becoming a > Gold Supporting Member <

To add your reply, or post your own questions




Registration is FREE and takes less than a minute!


Having trouble posting or changing forum settings?
Read the Forum Help (FAQ) or contact the webmaster