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Tips

carburation

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John Morralee

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I have long suspected that my twin SU carb. /ubbthreads/images/graemlins/england.gif /ubbthreads/images/graemlins/canpatriot.GIF63BJ7 runs too rich (fuel consumption,neat gas,some black soot from exhaust,that 'bubbly' sound when decelerating). When I weaken the mixture via the jet nuts ,only one or two flats, I get stalling on intersections and poor acceleration.Is this the sign to change jets? With current price of gas something has to give!
 
Hi John, First thing I would do would be to take a look at the Plugs and see what they say.I might even try a hotter plug initially, retune the engine and then recheck the cars performance.However, if the jets and needles are old they probably could do with a bit of freshening up.--Cheers Keoke
 
Healey engines like to run rich. In past surveys 12 to 16 MPG around town seemed to be the norm.
 
"When I weaken the mixture via the jet nuts ,only one or two flats, I get stalling on intersections and poor acceleration."

Maybe poor vacuum?
 
Use the pin to elevate the piston about 1/8 inch and see if the engine speeds up or faulters. When right it should stay about the same. There are some good books on adjusting the SUs. As stated above, check the plugs for that brownish powderie appearence. All the plugs should be about the same color too. Sometimes one carb runs richer and this will help you tell. (Then you get to balance them).
The guys here can give you a real education on this stuff.
 
[ QUOTE ]
Use the pin to elevate the piston about 1/8 inch and see if the engine speeds up or falters.

[/ QUOTE ]
TH,
Very good advice. Obviously the first step. Second step would be to get a good shop manual & read it.
D
 
Hello John,

jets and needles on S.U.'s rarely need replacing due to wear, unless the jet is not correctly centered and the needle rubs the jet. Properly set, there is no real mechanical wear on either.
As recommended, look at a manual to understand the workings of these very simple and effective carburettors.
Re stalling and hesitation, check that there is oil in the damper tubes and that your idle speed is not too slow now you have weakened the mixture. Optimum ignition timing is also affected by changes of mixture, as a weaker mixture needs a little more advance.

Alec
 
Back some 30 years ago when I was in a carburetion and ignition theory class at college, the instructor made a statement that has always stuck in my mind. That statement was “90% of carburetor problems is ignition related.” Having said that, check the condition of the plugs, wires, coil, distributor cap and rotor, points, advances, and general distributor wear. Be sure that the ignition timing is correctly set. Check for excessive wear at the chain timing set, etcetera.
 
Hi Alec,
John's BJ7 should have the SU type HS6 carbs. These carbs have the spring biased (to one side of the jet) needles, are made to rub on the jet, & DO wear the needle & jet more quickly than the older carb models. It's possible that the needles & or jets have worn enough to screw up the calibration, especially at idle. However, his statement that the engine responds to a flat or two of adjustment would indicate as you say, not enough wear of these parts to cause trouble.

Big6,
The statement that 90% of carburation problems are ignition related, may be true for people who are not experienced with engine tuning. IF more experience is gained, It becomes easier to sort out the differences, & mistakes, although still sometimes made, become much less frequent. The biggest problem when diagnosing problems by mail is getting a COMPLETE & accurate statement of the problem.

John,
Your statements seem to be more of a general feeling than quantifiable problems.
"63BJ7 runs too rich (fuel consumption,neat gas,some black soot from exhaust,that 'bubbly' sound when decelerating)."

Some black soot is to be expected under certain conditions. Fuel consumption varies greatly with driving conditions, & temperature. Bubbly sound is very subjective.

If you wish to try for improvements:
1- Check plug condition & type (UN12Y or equivalent), I think NGK type BP5ES might work better. Verify gaps at .025" or possibly .030" if there is no missfire.

2- Obviously check condition of points (.014" gap) & plug wires.

3- Set timing to 10 degrees static or 15 degrees advanced at exactly 600 rpm (no more, no less) dynamic. Vacuum advance disconnected. If you have very good fuel or drive at elevations considerably above sea level (3000 to 8000 feet), two to five degrees more advance may be helpful.

4- Verify that the vacuum advance is working as it should. This advance is intended specifically to improve part throttle gas mileage. You can verify this by applying vacuum to the advance unit & observing the point mounting plate rotating & returning.

5- A 180 degree F thermostat vs a 160 degree, will help fuel mileage.

6- Last & most costly, replace main jets .100" diameter, & metering needles type BC for your HS6 carbs. It is nearly impossible to get accurate wear measurments on the jets & needles. As I said above, the fact that the carbs respond to small jet height adjustments makes this the least likely problem.

These items may or may not make any improvement in fuel mileage. You will just have to try it. Let us know how it goes.

Regards,
D
 
Thank you for advice.I have tuned to the manual and 'SU carbuetters tuning tips and techniques'by G.R Wade and others ; isbn 1 85520 2557 (very good book as it includes SU pumps).Maybe the answer lies in that I am expecting too much re. fuel consumption.Do others agree that 12/15 mpg is usual? If occasional soot and neat gas from the exhaust is the norm (on starting only) I have no complaints because overall performance is fine.Will play around and report back.Thank you all again.John M.
 
Now Piman, as an old Volvo owner I can tell you that those HS6 carbs will wear the jet orfice to an oval shape and degrade the needles accordingly. This can be just the result of old age or slightly misalinged Jet bearings. /ubbthreads/images/graemlins/yesnod.gif---Keoke
 
Hello John,
performance cars tend to be tuned 'rich' (relatively) and dark exhausts are common. Black smoke should not be present except possibly on full choke, and the liquid you see out of the exhaust is likely to be water not fuel.

Dave,
I must admit to thinking that the 'Big Healey' was too old for the biased needles or perhaps they were introduced earlier than I imagined. I know S.U. had a few modifications as emmission limits dropped, none of which helped reliability.

Alec
 
Hello Keoke,
I guess that those 'old Volvos' must have had biased needles as well. But that is not true of a lot of HS carburettors. Note my earlier post with regard to 'modern improvements' and reliability. Badly aligned jets will do the running of the car no good whatsoever and should be quickly detected (and corrected)

Alec
 
Nope Pieman, Fixed KN Needles I have never seen a biased needle in an HS6 Carb.---Keoke
 
Re: carburetion

Hi Alec,

The HS carb came in several versions. Some of the later ones had a biased needle, others such as the Healey had fixed needles. I apologize, you are correct.
---------------------
Keoke,
"Nope Pieman, Fixed KN Needles I have never seen a biased needle in an HS6 Carb.---Keoke"
---------------------
I think the "KN" needle is a biased needle used in some Volvos with HS6. I guess it could be converted to fixed. It is about 20% richer than the suggested "BC" fixed needle for the Healey. All of my references show a "BC" as standard for a BJ7 with HS6 carbs.

There were a number of HS6 carbs used with biased needles in the early seventies. A quick check shows a number of Austins, a couple Mini's, Leyland International, MGC, some Rovers, the B188 Volvo, & Wolsely using them. Maybe the ones you looked at had all been converted. The biased needle wasn't one of the better ideas.
D
 
Re: carburetion

I still have not seen a Biased needle in the early Volvo cars Dave of course I have never seen a ball bearing suction chamber either but they are out there.---Keoke
 
Re: carburetion

Hi all,

From what I understand (which, admittedly, is very little) the low compression and long stroke on a Healey means it needs a rich mixture to idle. I've been getting a little black soot, and I love that "bubbly" sound on deceleration. I get about 23 mpg on the road in my four cylinder, before and after the engine work with the carbs being totally rebuilt about six yeras ago, only freshened up recently. Is that reasonable mileage?

Sharon
BN1
 
Re: carburetion

That sounds like pretty good mileage to me Sharon are you going to CHW this year ? I think its going to be a bit warm.---Keoke /ubbthreads/images/graemlins/yesnod.gif
 
Re: carburetion

Hi Sharon,
I think you are correct about the idle mixture. It may also be caused by the combustion chamber design.

I get about 19 to 23 on the road. 23 when taking it easy which is seldom, 19 when pushing hard, most of the time.
D
 
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