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Idles at 2800 then quickly dies. HELP!

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Brad.Cogan Avatar
Brad.Cogan Bradley Cogan
RAF Cosford, Shropshire, UK   GBR
1977 Triumph Spitfire 1500 "Wray"
1989 Toyota MR2 "Coral"
2007 Fiat Grande Punto "Pepper"
I decided to tune my carbs and yeah I've made things far worse. I've reset the mixture screws to 12 flats below flush and have moved the throttle screws/arms (screws on top) so the throttle linkage hits the arms at the same time.

Now it starts but idles at about 2800RPM then dies after a few seconds if I don't put my foot on the throttle. It seems to be 2800RPM or 0.

How should I go about getting a steady low idle?

Thanks!



Brad Cogan

1977 Triumph Spitfire 1500 'Wray'
1989 Toyota MR2 Mk1b AW11 1600 twin cam
2007 Fiat Grande Punto Active 1.2 'Pepper'

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Lizzard d id
san jose, CA, USA   USA
Sounds way rich , try going leaner . "12 flats below flush" , try putting them at flush , does it run better ?

Wolfcreek Steve Steve P
Central, WI, USA   USA
Have you messed with the PCV valve, is it working properly?

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Brad.Cogan Avatar
Brad.Cogan Bradley Cogan
RAF Cosford, Shropshire, UK   GBR
1977 Triumph Spitfire 1500 "Wray"
1989 Toyota MR2 "Coral"
2007 Fiat Grande Punto "Pepper"
I've managed to adjust the throttle screws to get an idle of 1400. It's up to temp now.

Lifting up the pistons a mm should increase the RPM then drop back down again yeah? Right now the RPM is dropping when I lift the pistons. I've adjusted the nuts clockwise (down I think) but it still drops when I lift the piston up. I've turned them about another 8 flats so I'm worried I'm doing something wrong here.

P. S I dunno what a PCV is. I should've pointed out I have twin HS4 SUs.



Brad Cogan

1977 Triumph Spitfire 1500 'Wray'
1989 Toyota MR2 Mk1b AW11 1600 twin cam
2007 Fiat Grande Punto Active 1.2 'Pepper'

Born Loser Avatar
Born Loser Silver Member Matthew Taylor
Land O Lake, FL, USA   USA
Balance the airflow between them FIRST. If one is sucking harder than the other, the mixture adjustments won't have any effect at idle for one of the carbs - it will seem dead. The 12 flats is 2 turns, and that is actually a good starting point. But be SURE the airflow is balanced FIRST. High RPM is because of too much air - either your idle, choke fast idle, or the vacuum line from the valve cover, or an actual vacuum leak.



Matthew
1960 Triumph TR3a
1970 Triumph Spitfire MK 3
2012 Mini Cooper SS Convertible
2018 Jaguar F-Pace

Lizzard d id
san jose, CA, USA   USA
Well if its running and warm you can turn the adjuster up or down and see what the effect is on a running car .

Lifting the piston and not getting an increase in rpm still sounds rich to me .

Lizzard d id
san jose, CA, USA   USA
In reply to # 1510812 by Born Loser The 12 flats is 2 turns, and that is actually a good starting point.
For a stock set up with stock needles . I'm thinking he doesn't have a stock set up . "I should've pointed out I have twin HS4 SUs."

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Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
Start with both of the throttle butterfly valves closing fully and slight slack on the throttle cable and work up from there.

Brad.Cogan Avatar
Brad.Cogan Bradley Cogan
RAF Cosford, Shropshire, UK   GBR
1977 Triumph Spitfire 1500 "Wray"
1989 Toyota MR2 "Coral"
2007 Fiat Grande Punto "Pepper"
In reply to # 1510814 by Lizzard
In reply to # 1510812 by Born Loser The 12 flats is 2 turns, and that is actually a good starting point.
For a stock set up with stock needles . I'm thinking he doesn't have a stock set up . "I should've pointed out I have twin HS4 SUs."

That's stock over here. I have a high compression head and electronic ignition but apart from that standard.



Brad Cogan

1977 Triumph Spitfire 1500 'Wray'
1989 Toyota MR2 Mk1b AW11 1600 twin cam
2007 Fiat Grande Punto Active 1.2 'Pepper'

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Brad.Cogan Avatar
Brad.Cogan Bradley Cogan
RAF Cosford, Shropshire, UK   GBR
1977 Triumph Spitfire 1500 "Wray"
1989 Toyota MR2 "Coral"
2007 Fiat Grande Punto "Pepper"
I've balanced the carbs with a syncrometer and found that the throttle linkage arms hit the carb throttles at different times. I've unclamped and adjusted those now so they match.

I thought if you lifted the piston and the RPM dropped it was lean not rich. I might be wrong though.



Brad Cogan

1977 Triumph Spitfire 1500 'Wray'
1989 Toyota MR2 Mk1b AW11 1600 twin cam
2007 Fiat Grande Punto Active 1.2 'Pepper'



Edited 1 time(s). Last edit at 2018-02-01 12:02 PM by Brad.Cogan.

Born Loser Avatar
Born Loser Silver Member Matthew Taylor
Land O Lake, FL, USA   USA
In reply to # 1510819 by Brad.Cogan
In reply to # 1510814 by Lizzard
In reply to # 1510812 by Born Loser The 12 flats is 2 turns, and that is actually a good starting point.
For a stock set up with stock needles . I'm thinking he doesn't have a stock set up . "I should've pointed out I have twin HS4 SUs."

That's stock over here. I have a high compression head and electronic ignition but apart from that standard.

Thats how I read it - stock over yonder. Its in your info.


In reply to # 1510821 by Brad.Cogan I've balanced the carbs with a and found that the throttle linkage arms hit the carb throttles at different times. I've unclamped and adjusted those now so they match.

I thought if you lifted the piston and the RPM dropped it was lean not rich. I might be wrong though.

Exactly right. Dropping RPM is lean. Be sure you are BARELY lifting though. A big lift will cause a drop regardless.

So now that the airflow matches, try adjusting the mixture again. One at a time. I have seen lots of advice about them being the same, but in my experience they are "close", but never exactly the same. Maybe the were 40+ years ago. But dont worry if they each require a little different setting.



Matthew
1960 Triumph TR3a
1970 Triumph Spitfire MK 3
2012 Mini Cooper SS Convertible
2018 Jaguar F-Pace

Brad.Cogan Avatar
Brad.Cogan Bradley Cogan
RAF Cosford, Shropshire, UK   GBR
1977 Triumph Spitfire 1500 "Wray"
1989 Toyota MR2 "Coral"
2007 Fiat Grande Punto "Pepper"
I think my main concern is how fair I'm turning it and it's still dropping. I'm lifting each piston by about a mm with a screwdriver.



Brad Cogan

1977 Triumph Spitfire 1500 'Wray'
1989 Toyota MR2 Mk1b AW11 1600 twin cam
2007 Fiat Grande Punto Active 1.2 'Pepper'

14GPDJENGINEERING Avatar
Silver Spring, MD, USA   USA
Use the piston lift pin. It is more consistent than a screwdriver and allows lifting with air cleaners in place..



Dennis smiling smiley

Born Loser Avatar
Born Loser Silver Member Matthew Taylor
Land O Lake, FL, USA   USA
In reply to # 1510840 by 14GPDJENGINEERING Use the piston lift pin. It is more consistent than a screwdriver and allows lifting with air cleaners in place..

Sure wish they all had them, those are nice.



Matthew
1960 Triumph TR3a
1970 Triumph Spitfire MK 3
2012 Mini Cooper SS Convertible
2018 Jaguar F-Pace

Darth V8R Avatar
Darth V8R Vance Navarrette
Beaverton, OR, USA   USA
1980 Triumph TR8 "Wedgie"
In reply to # 1510803 by Brad.Cogan I decided to tune my carbs and yeah I've made things far worse. I've reset the mixture screws to 12 flats below flush and have moved the throttle screws/arms (screws on top) so the throttle linkage hits the arms at the same time.

Now it starts but idles at about 2800RPM then dies after a few seconds if I don't put my foot on the throttle. It seems to be 2800RPM or 0.

How should I go about getting a steady low idle?

Thanks!

Brad,

Take it from someone who has been there, avoid the "lift the slide" test. It is exceedingly difficult to do properly, and if the engine has been modified it probably will not work at all. There will be some, no doubt, that say the slide method works well for them. I am very happy for those individuals, but you do not know the configuration of their engine (Cam, ignition, carb, emissions equipment, compression, blah, blah, blah) and therefore cannot assume that their success will translate to your situation.

I will assume your valve lash and ignition timing are correct (10 BTDC with distributor vacuum disconnected).

Set your mixture to the middle of its range. For Zeniths the needle carrier is flush with the bottom of the air valve (slide). For SUs, whatever the manual tells you. But make sure it is in the middle of its range. Start the car, and set the idle speed (as best you are able).

Allow the motor to fully warm, and take the car for a brief drive. Return home, shut down the engine, and pull a couple spark plugs. They should be a light tan color on the tip of the plug (on the ceramic insulator that wraps around the center electrode). If it is dark or black, lean the carb - for a Zenith that is rotating the needle adjustment 1/4 turn counter clockwise. If it is white or nearly white, enrich the mixture (1/4 turn clockwise on a Zenith). Once again, follow the manual's guidance for SUs - I have not used them myself.

Reinstall the plugs, start the car, take it for another driver. Repeat the adjustment process until your plugs are the color of corrugated cardboard.

Job done.

BTW, a Color Tune or an O2 sensor works well too, but the lift the slide test is notoriously unreliable as our cars have passed through many hands, and in general are no longer stock. Your use of an SU carburetor being a case in point. So use the above method which is insensitive to engine modifications.

BTW, if your engine is extensively modified, you may not be able to get the plug color correct, and you will need to go to a richer or leaner needle to get the correct mixture. Report back here if that is the case.

Vance



1980 Platinum Metallic TR8, navy blue interior
Bare metal respray, Crower cam, raised compression
ported heads, modified Zenith carbs, 0.060" overbore

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