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Subject: "SU Carb tuning" Locked thread - Read only
 
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mikecyc72usa
Member since Oct-17-08
52 posts (Believes that Paul McCartney STILL drives a Spitfire)
Apr-29-10, 04:15 AM (PST)
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"SU Carb tuning"
 
   On my 78 1500 I have twin SU carbs. So far so good. Starts fine, around town is fine, but at about 70 mph just loses the oomph. As in no more top speed. When I floor it from a standstill, the engine goes great to about 4000 rpm, then seems to start missing/sputtering. I've leaned out the carbs a little, but still no luck fixing it. I have more throttle response at 60% than at 100%. Not sure what's going on with it. Any suggestions? In the future I plan to convert to megajolt and to fuel injection, but until then, I hope to have a strong running car. Thanks.


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  Subject     Author     Message Date     ID  
  RE: SU Carb tuning clshoreteam Apr-29-10 1
     RE: SU Carb tuning tonymrfixitteam Apr-29-10 2
         RE: SU Carb tuning tonymrfixitteam Apr-30-10 7
  RE: SU Carb tuning foxtrapper Apr-29-10 3
     RE: SU Carb tuning bdnutsteam Apr-29-10 5
  RE: SU Carb tuning Joe Curry Apr-29-10 4
     RE: SU Carb tuning clshoreteam Apr-30-10 6
         RE: SU Carb tuning tonymrfixitteam Apr-30-10 8
             RE: SU Carb tuning clshoreteam May-01-10 9
                 RE: SU Carb tuning mikecyc72usa May-01-10 10
                     RE: SU Carb tuning ajohnson38 May-01-10 11
                     RE: SU Carb tuning jjanow2000team May-01-10 12
                         RE: SU Carb tuning tonymrfixitteam May-01-10 13
                             RE: SU Carb tuning mikecyc72usa May-02-10 14
                                 RE: SU Carb tuning bdnutsteam May-02-10 15

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clshoreteam
Charter Member
734 posts (TTN Power Member - a well respected man)
Apr-29-10, 06:00 AM (PST)
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1. "RE: SU Carb tuning"
In response to message #0
 
   Sounds like it's leaning out.
Do you know how to read plugs? It's a good way to determine what's going on at speed and under load.
Often, the carbs will behave differently from each other at speed.
Where did the carbs come from, and what have you done so far?
SU are rather simple devices, only a couple of adjustments needed if they are mechanically sound, and the bits (needles, jets, springs)are correct for your car.

Carter


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tonymrfixitteam
Member since Dec-25-02
2633 posts (A TTN governor, governor)
Apr-29-10, 07:36 AM (PST)
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2. "RE: SU Carb tuning"
In response to message #1
 
The mixture adjustment really only effects the mixture at idle speed.
I'ts the needle profile that effects mixture at larger throttle openings. You may well have the wrong needle for your application.
If you can let us know the specs (and modifications) to your engine, we may be able to give you some guidelines as to the needle you require.

Tony M
74 Spitfire 1500
84 Toyota Pickup
03 VW Golf


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tonymrfixitteam
Member since Dec-25-02
2633 posts (A TTN governor, governor)
Apr-30-10, 09:50 PM (PST)
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7. "RE: SU Carb tuning"
In response to message #2
 
LAST EDITED ON Apr-30-10 AT 09:51 PM (PST)
 

Tony M
74 Spitfire 1500
84 Toyota Pickup
03 VW Golf


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foxtrapper
Member since Feb-27-02
1035 posts (TTN Hall of Famer)
Apr-29-10, 08:56 AM (PST)
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3. "RE: SU Carb tuning"
In response to message #0
 
   Check your ignition. As in suspect high(er) rpm breakup.


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bdnutsteam
Member since Nov-29-07
80 posts (Learning to live with vibrating rear view mirror)
Apr-29-10, 09:36 AM (PST)
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5. "RE: SU Carb tuning"
In response to message #3
 
   I had a similar problem with my S.U.s after installing. They were very strong relative to the Stromberg until I got to about 4000 rpm,s. In my case it was the wrong needle. I ended up calling Joe Curto and he made a reccomendation which seems to be working great. I am sure that the veterans here will also be able to make reccomendations also. I don't know off the top of my head which needle he sent but after describing the mods I had made to my engine he seemed to know what I needed. I'm so happy with the engine with the S.U carbs compared to the stock Z.S. It's like I got another cylinder. Good Luck.

Bruce Nelson 79 1500


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Joe Curry
Member since Oct-20-03
1328 posts (An "official" TTN Senior Wrangler)
Apr-29-10, 09:32 AM (PST)
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4. "RE: SU Carb tuning"
In response to message #0
 
   When I had twin SUs on my 1500 engine, I had a similar problem. Basicly, it would literally shut off in high gear when I reached 4000 rpm.

I finally tracked it down to the profile of the carb needles. The ones in there were suitable for a stock 1296 but didn't work very well in the 1500. I tried going to Joe Curto for recommendations but what he sold me were for an MGA 1500 engine and that didn't work all that well either.

I ended up changing the profile of the original needles using a dremmell tool and emery paper. With a lot of trial and error, I found something that worked.

When I put the 1296 prepared engine from Tiny Tim into Huxley, I tried those SUs once again (from the Twin Webers I previously had on the engine) and the problem reappeared. Rather than go through that laborious trial and error task once again, I went to electronic fuel injection. Although it was very spendy, I feel like it was money well spent.

Cheers,
Joe


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clshoreteam
Charter Member
734 posts (TTN Power Member - a well respected man)
Apr-30-10, 08:03 AM (PST)
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6. "RE: SU Carb tuning"
In response to message #4
 
   LAST EDITED ON Apr-30-10 AT 08:04 AM (PST)
 
Tony, I disagree about your mixture comment.
On my G-Prod racecar, 'adjusting the flats' on the jet was how we controlled lean/rich at high speed. (that car never idled much anyway).
The SU 'mixture adjustment' moves the jet up or down, which changes the section of the needle (and thus the diameter) that engages the jet opening. This is true for the entire range of piston (and thus needle) movement.
So unless you have a needle without any taper, it does affect rich or lean throughout the whole range, even when the piston is fully raised (as it usually was on the racecar).

You can find charts of SU needle diameters, for each 0.10 inch along the length, and ISTR a spreadsheet tool for needle selection on the Web, should you wish to choose.

But perhaps the issue is also with the springs?

The springs control how much the piston will rise for a given amount of airflow through the bridge section where the needle/jet assembly is located. The position of the piston controls which section of the needle taper engages the jet opening, and thus the mixture strength.

I don't think you mentioned which SU's you have, the 1-1/4" HS2, or the 1-1/2" HS4?
Do you know what motor the carbs were originally fitted to?

If they are HS2, then they are perhaps not optimized for your 1500, since at 4000 RPM it will flow about the same as an 1147 at 5200 RPM, or a 1296 at 4600 RPM.

The HS2 can handle the flow (my G-Prod racecar 1147 would turn 8000 RPM), but HS4 are usually preferred.

Carter


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tonymrfixitteam
Member since Dec-25-02
2633 posts (A TTN governor, governor)
Apr-30-10, 09:51 PM (PST)
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8. "RE: SU Carb tuning"
In response to message #6
 
Quote
--------------------------------------------------------------------------------
The mixture adjustment really only effects the mixture at idle speed.

--------------------------------------------------------------------------------

What I mean is; The mixture adjustment nut has a far greater effect in A/F ration at Idle and small throttle openings than it does at larger throttle openings.
Turning the mixture nut 4 flats can drastically lean or richen the mixture of an idling engine (needle at stages2-4), but would not have such a profound effect when the throttle is open and the needle occupies the jet at stages 6-8.
SU have graphs that plot curves to show how the 'wet' area of the jet enlarges at various stages of a needle profile, as the needle is withdrawn from the jet orifice. It is a curve, not lineal!
To compensate the needles typically have a profile that reduces diameter, in a non-lineal fashion, as the stages progress.
But even then, it's not enough. The carburettor piston must have the pressure from a spring to progressively resist its upward movement,
and in doing so, slightly increase the velocity of air across the venturi (therefore richening the mixture)
I hope this make sense? (it's getting late zzzzzzzz)

Tony M
74 Spitfire 1500
84 Toyota Pickup
03 VW Golf


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clshoreteam
Charter Member
734 posts (TTN Power Member - a well respected man)
May-01-10, 05:21 AM (PST)
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9. "RE: SU Carb tuning"
In response to message #8
 
   Good points, apologies if I misinterpreted your earlier post.
I'd still like to know which model carbs we are talking about ...


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mikecyc72usa
Member since Oct-17-08
52 posts (Believes that Paul McCartney STILL drives a Spitfire)
May-01-10, 10:37 AM (PST)
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10. "RE: SU Carb tuning"
In response to message #9
 
   Ok, sorry for the delay. Here is what we're dealing with.
Pertronix electronic ignition. SU HS4 dual carbs. Pacesetter header hooked up to stock exhaust system.

I don't know what car the carbs were on originally. Not sure what needles are in, also. I'm going to really clean the carbs in a few minutes, and see if that helps any. They aren't that dirty, but you never know. I'm going to recheck the adjustment, also. I'm still suspecting an improper jet setting. Details later.

Thanks


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ajohnson38
Member since Aug-20-03
744 posts (TTN Power Member - a well respected man)
May-01-10, 11:41 AM (PST)
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11. "RE: SU Carb tuning"
In response to message #10
 
   While the Jets can adjust the mixture throughout the whole range I suspect that the needles are wrong.

Did you buy these carbs new or did they come from a different application?

The needles will have the type stamped on them at the top.

aaron

Aaron Johnson
#87 H-Prod Spitfire MkIV Southern Illinois Region SCCA
http://hpspitfire.servebeer.com


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jjanow2000team
Member since Mar-27-03
85 posts (Wonders how many posts are required to stop the torment)
May-01-10, 04:55 PM (PST)
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12. "RE: SU Carb tuning"
In response to message #10
 
Here is a link that lets you compare different needles.

http://www.mintylamb.co.uk/suneedle/


There are other programs that do the same but for a reasonable price, put in the car information and it gives a recommendation. But to use it fully it needs to be registered for a fee. here is the link..

http://www.mintylamb.co.uk/suneedle/

Hope this helps,

John
1979 Spitfire


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tonymrfixitteam
Member since Dec-25-02
2633 posts (A TTN governor, governor)
May-01-10, 05:06 PM (PST)
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13. "RE: SU Carb tuning"
In response to message #12
 
I'm thinking that the the standard for the 1500 ABT needles should proberbly work, but AAQ needles might be better.

One other thing, check you fuel filter!

Tony M
74 Spitfire 1500
84 Toyota Pickup
03 VW Golf


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mikecyc72usa
Member since Oct-17-08
52 posts (Believes that Paul McCartney STILL drives a Spitfire)
May-02-10, 08:37 AM (PST)
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14. "RE: SU Carb tuning"
In response to message #13
 
   I'll check the needles tonight. The carbs were bought used, but rebuilt and are in great condition. I'll check the fuel filter again, as I just replaced the entire fuel system, gas tank, lines, etc. No arcing or spark issues, checked that last night. It continues...


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bdnutsteam
Member since Nov-29-07
80 posts (Learning to live with vibrating rear view mirror)
May-02-10, 05:00 PM (PST)
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15. "RE: SU Carb tuning"
In response to message #14
 
   For what it's worth I ended up with ABD needles from JOe Curto. The most reccomended needles I found while researching last year were AAT or AAQ I really don't know enough about the subtle differences between all the needles so I needed to rely on others. Like I said in my earlier post, without trying different needles to see how they might compare I have been pleased with the ABD. By the way I had ADN in the carbs when I got them. And like I said before they were fine up until about 4000 rpm's and then they died. I know most of the Hs4 setups, if not all come from Europe, and quite possibly not even from a Spitfire.

Bruce


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