Carb Tuning

I've recently had the carbs off my BJ8 and fitted a complete rebuild kit including new spindles. I found before taking the carbs off that when I examined a plug it was always covered in soot now matter what I did to try and stop it. I've checked the timing and it's spot on.

Anyway, I'm now trying to tune the new setup. I understand that BJ8's like to run a little rich and this is my experience from playing around before taking the carbs off. I have balanced the carbs with a sync gadget and I have a Colourtune plug. I've set the carbs with the Slow Running Valves fully closed like it says in my SU manual for multicarb cars. The setting is currently such that turning the mixture screw less than a 1/4 turn will change the Colourtune from yellow to blue. I have the car running so the Colourtune is on the yellow side and the car runs well. However I still have a layer of soot on the plugs. If I back the mixture off just a little it begins to show signs of running lean, it starts to falter when you depress the throttle as though it's running out of fuel. The soot is easily brushed off and the underlaying colour of the electrodes looks a chocolate brown as you'd expect. It seems to me that the soot is deposited during the idle process. I haven't yet done the test of stopping the car before it gets to idle as this is a little awkward where I live.

So, my question is whether I should ignore the SU Manual and try playing with the Slow Running Valve which seems to me will get more air into the engine during idle weakening the mixture at the idle point. Any thoughts would be appreciated.

Thanks
AJ

AJ, I have HD8 carbs on my BJ7. Years ago I would adjust the carbs exactly as the manual discribes. But in doing that I could not get the idle speed up around 800 or 900 rpms unless i enrichened the jets and it sooted up. In the last couple of tune ups, which has been over about a 5 year period, I have started to crack open the slow run screws just a wee bit. I can achieve a better idle speed this way and i do not have any sooting. I also have removed the hose from the valve cover "T" fitting to the rear carb. I have used an open 2" K & N filter on the "T" fittling. I have not had to touch my carbs between tune ups at all.

AJ

I do not know much about the HD8 carbs but I can tell you that my BJ7 - with HS6 carbs - was playing up and soot was being deposited on the plugs such that the car would misfire and stutter. I spent a week trying to solve it, my Healey guru spent almost two weeks on it and things were looking a bit better until I covered 20 plus miles and then it all started again. After discussions with the guru it was felt that the carbs were over fuelling.

All this was after I had the car for some 13 years with out any problems. My guru had changed all the electrics on the ignition side and out of desperation I changed the jets without much effect and then the needles and that did it. Problem solved. I checked the old and the new needles and there was a world of difference although they were both referenced the same. I am not sure that your rebuild kit contains needles.

I guess that some PO had adjusted the tapers on the needles for some reason - maybe to do with the Racer Brown cam that is in there, but why it should suddenly start to play up still remains a mystery.

It sounds as if you could be suffering the same.

As another thought have you changed the spark plugs at all? I once bought a set of Boch plugs through a 'self fit' firm and the guy told me that they were a good match for the old champions and it turned out that they were quite the opposite, they were long reach alright but were cold plugs not hot plugs, but then the engine would misfire after a short time, you do not have that problem.



Bob

AJ, My principle was correct but I worded it wrong. On the HD8 carbs the book tells you to keep the throttle plates closed and to adjust idle speed with the slow run screws. I finally decided to crack the throttle plates a wee bit and adjust the idle speed with the slow run screws.

Hi AJ,

Having done and followed almost all suggestions over the years I have owned my BJ8, and also having rebuilt the HD8s at least twice, I must agree with Vette and not been able to secure sufficient richness adjusting the Slow Run screw and have opened the throttle plate slightly. This change in procedure gives me high idle but no hesitation on set-off at a traffic light and no misfires or hesitation all the way past 5K.

However, that being said, I have never been able to get an even idle at 750-800 RPMs as I would like and the Slow Run screw has never seemed to be used as the book indicates. I do use a Unisync to balance the 2 carbs and have also installed 2 Color Tunes (1 for front 3 cylinders and 1 for back 3) with little success (have difficulty seeing any color).

Every spring I take another shot at adjusting the carbs and, if the weather ever stays dry, this year will be no different. I have made it much easier to remove the filters by retaining the outside-side nuts and replacing the inside-side nuts with pull-pins. Additionally, I have eliminated any oil contamination from causing rear carb difficulties by installing a home-designed oil catch can and connected it to the valve cover "T" hose prior to the hose's connection to the rear air cleaner (originally planned on a PCV installation). I use a minimum of 93 Octane fuel to diminish/eliminate pre-ignition with an occasional measure of Real Led additive.

All in all, I would appreciate any suggestions on the use of Color Tune as I have found, by following their instructions to the letter has, they do no help.

Just my thoughts,
Ray(64BJ8P1)

Ray,
I'm also about to attempt carb adjustment/fine tuning. My tricarb runs well, but warbles and pops occasionally when coasting.
Researching this forum, it seems that using Color Tune has become less dependable due to the ethanol present in most gasoline.
But I have one and want to learn how to use it.
Maybe the experts will chime in...
Good Luck.
Douglas

Thanks for the quick responses. Some more info. I have a Rawlesmotorsport 'fast road' engine fitted. Alloy head, faster cam, petronix ign. I bought it like that so don't have much more info on it.

The plugs are NGK BP6ES. I wondered whether they are hot enough so I checked. They're the right one's according to the NGK catalogue although they are on the cold side of the range according to the chart in their catalogue.

I just did a 'stop the engine at 2000rpm' test. There was still some soot but not as much. Brushed off easily. I have no trouble getting it to idle at 800 rpm, I can get it down to 600 and it's quite smooth. I think Vette has put into practice what I've been thinking about. I'll try tweaking the slow running valves.

I've been using 98 RON fuel. Principally because I was told it has less ethanol in it. I doesn't seem to be any different to the 95 RON fuel. Perhaps I'm wasting money.

I also use a Unisync. I find the colourtune very useful. I fit the plug without the long black tube. I do it with the car in the garage and get the garage as dark as I can by shutting the roller door half way down, I have no trouble seeing the change from blue to yellow in the darker environment inside but I find it difficult to see the transition Blue/Orange/Yellow that I've read about, a small turn of the mixture screw (1/4 turn) changes if from Blue to Yellow quite quickly making it a bit difficult to get it just slightly rich. It was quite nerve racking leaning out the mixture to get it off Yellow to Blue the first time I used it, it must have been running awfully rich.

The only reason I haven't tried tweaking the Slow Running Valve yet is because I'm loath to remove the air filters to balance the carbs again. They're a pain to fit, frustrating, time consuming and they make my back ache. I HATE THEM they're going to drive me nuts.... sorry! Ray, I'm not sure what you mean by 'pull-pins', how do you tighten up against the gasket? I would appreciate a little more info on your idea for fitting the filters please.

The SU rebuild kit included new jets but not needles. The old Standard needles are still fitted. I did think to check them to see if they've been modified but I could find no sign of abrasion on them. Perhaps I should take them out and measure them. Before I set out on this mission I found my car was difficult to start from cold. I would turn it over for ages until it caught and then it would backfire through the exhaust and splutter for a while before it settled down. I eventually traced the problem to the choke. It wasn't coming out far enough. I didn't realise that when you pull the choke out it lowers the jet and squirts a shot of petrol into the intake. It wasn't doing that. I reset the choke at the carbs and it started much easier but I didn't completely get rid of the backfire until I rebuilt the carbs. I think it was running much too rich, hence the nervousness with the Colourtune. It now starts on the button although it does chuck out a lot of smoke when it fires for the first time. It settles down when the choke goes back in. No backfiring anymore.

Thanks again
AJ

Over the years, I have used 2 Colortunes--bought them when they were a lot less expensive--with varying degrees of success on my BJ8. The #2 cylinder would never show any color, except for an occasional white flash, but the others behaved more-or-less as expected. I finally did a compression test and found all cylinders were weak, but #2 was only 60PSI. Out came the engine for an overhaul, and now all 6 behave. When we rebuilt my BN2's engine I used the Colortunes, and they worked exactly as advertised. My plugs always show a little soot around the edges, but that's likely because I just pull them when convenient, and don't follow the recommended procedure (run on the road till hot, then shut down the engine without idling).



I too could not get enough idle speed using only the slow run valves, but I found cracking the throttle plates just a hair cured that, and I could futz with the slow runs to tweak the carbs to conditions when necessary. Plus, I never liked the idea of the throttle plates being slammed--OK, that's an overstatement--against the carb throats. I think there would be some wear on the bores eventually.

Has a one used Gunsons AF instrument to check their mixture strength?

Gunson AF? Do you mean the Digital Gastester G4125. If so the answer is yes. I have one but I'm not sure it's very reliable. I've been trying to find out what the CO level should be for the BJ8 in good tune. My car is runs quite well at the moment, just on the rich/Yellow side of the Blue flame but it starts OK, doesn't hesitate and no backfiring as described above. If I back it off into the Blue region it starts to show signs of hesitation on throttle take up and mutters a bit. I tried the gas tester on it and I'm getting a reading of about 12% for each bank, it jumps about a bit. (twin exhaust). Don't know if that's good or bad. On reflection I think I need to test it again and check the calibration was right as I think it should probably be a lot lower. My research tells me it should be down to about 4.5%. So unless I got the initial calibration wrong, to get to 4.5% I'm going to have to go well into the Blue flame region on the Colourtune which I'm reluctant to do as I think that's going to be much too lean.

I want to see if tweaking the Slow Running Valve, which should weaken the mixture at idle, fixes the problem first. The thought of taking the air filters off again to re-balance the carbs is delaying things, it took me ages to put them back on last time. If that doen't work I'm thinking I might try a set of hotter plugs, maybe the NGK BP 7 or 8ES to see if that fixes the problem as my car seems to run quite cool at about 80 degs most of the time.
AJ

I have bought 3 colour tunes - 1 new and the rest from Ebay My carbs are HS6s and adjusting the jets can be a PITA from the get atable point of view.

My set up consists of two trumpets on the carbs so life is a little easier without the pancake filters. The plugs are NGK BP5ES

The timing is adjusted using the Gunsen Gun

I use 2 at a time, one for each carb and install them on 2 and 5. I can honestly say that I do not have much trouble in tuning them in, I do check the plugs after a test run and find that I may have to just tweak the jets a tad, my engine ticks over at around 750 rpm once warmed up and the plugs are that beautiful shade of nut brown.



Bob

Hi AJ,

As previously mentioned, I modified my original air cleaners by retaining the outside nuts to secure the cleaners to the carb bodies and drilled a hole through the troublesome inside bolt extensions and secure with a washer and push pin. Adjustment the depth of the bolt in which the push pin is installed is how tightness of the inside filter connections is made. This easy modification makes filter removal and reinstallation quick and easy and you can always go back to using a nut for originality purposes.


Ray(64BJ8P1)

The specifications of the engine you describe will need a lot more air flow than a standard engine and I do not believe you would be able to get a proper idle mixture by tuning it as the manual describes. You would be trying to idle the engine on just what air/fuel mixture is passing thru the slow run valves.
I use the highest octane I can buy as a precaution.
When it comes to using a Unisync, it's not needed on HD8 carbs. (leave the air filters on). If tuning as the manual says, then the throttle plates would both be closed and there would be no difference in air flow thru the carbs. Hence nothing to balance.
If you are going to crack the throttle plates a wee bit, you can set them both equally by just the equal amount of the turn of the throttle stop screws or by watching the throttle shafts as you turn the screws. You might argue that this is not ensuring that there is the same volume of air passing over the throttle plates at idle but I would then argue that any air flow measurement via a unisync type instrument or a hose to the ear is equally inexact. The characteristics of the intake air track are too varied to be that precise. So close the throttle plates and do it by the book or open the throttle plates a wee bit by an equal amount of the turn of the idle stop screws and watch the rotation of the shafts.

I use a Unisyn and crack the throttle plates. To get them balanced, I close the slow run valves and balance on throttles only, then open the slow run valves 2-3 turns and balance again, then close the down the throttles to get the desired idle and balance again (you can do in reverse). Also, you need to balance them off-idle to make sure the throttles are opening equally. I put a washer or a quarter (coin) under the stop on the manifold to hold a constant RPM. I use a couple Colortunes, but usually have to tweak the mixtures and idle on the road as no-load testing is basically a SWAG. It's also a concern that the entire linkage--from pedal to carbs--is in good nick and adjusted (the manual has a fairly elaborate procedure).

Not to hijack the thread, as I believe this may be a tuning issue, but my BJ8 has started dieseling--aka 'running on'--on shutdown more than I'd like (an extra crank or two ain't bad; my car will go for 10 seconds or more). The car has good compression--180PSI give or take on all 6--but the best gas I can typically get get is 91-octane (91 AKI, equivalent to European 95). It idles at about 700-750RPM, and timing is set per the manual. Any suggestions?

mgtf328 said

I want to see if tweaking the Slow Running Valve, which should weaken the mixture at idle, fixes the problem first. ​

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Just to be clear, the slow run adjustment screws do NOT change mixture. SU carbs have only one place that mixes air & fuel and that is the main jet. The 'slow run' is just a port that bypasses the butterflies which is why you tune HD8s by closing the butterflies completely and set the idle speed by changing the amount of premixed air/fuel you let into the engine. Other SU (H, HS) carbs do not have this slow run port and you must use the butterflies to set the idle. However, the effect is the same whether you hold the butterflies open a bit or use the slow run port - more or less air/fuel but no mixture/ratio change.

BTW, Bob is absolutely right about checking balance at some off-idle setting to insure the butterflies are opening equally. I think this is even more important than idle balance.
Dave

Having issues with dieseling and a BN2 that is really sooting-up its plugs, I think spark plugs should be considered in 'tuning' discussions. I had Champion N3Cs in the BN2, probably because that was what the car had when we bought it, but the Champion website calls for 5s. I have a set on order to see if that addresses the soot issue. I'm running Champion RN12YCs in the BJ8, and wondering if colder plugs might address the dieseling issue?

Sorry I'm getting confused, I think I may have been doing this wrong.
I'm using the SU Workshop Manual I got from Burlen and the Service and Tuning Guide I got with the HD8 rebuild Kit CRK 216.
There is a note at the beginning of Section 1 on tuning HD mulitcarb cars that says that 'whenever the throttle adjusting screws are fitted, they and not the Slow Running Valves should be used to adjust idling speed. Screw down the Slow Running Valves (which must remain closed) and adjust the throttle adjusting screws'. That's what I've been doing as I thought Burlen would know!.
Dave is saying something different and checking the two workshop manuals I have (Haynes and Bentley) they agree with Dave.
I realise the SRV's bypass the jet/needles but they're supposed to be closed aren't they? I've managed to get an idle speed of 750 using the throttle adjusting screws only which resulted in a little soot fouling of the plugs at idle. Hence my question as to whether opening up the SRV's a little would lean out the mixture at idle reducing the sooting up.
On another point, I've been adjusting the mixture using the Colourtune with the car running at idle. Obviously the Throttles are slightly open. Is that the correct way?
I'll start again!

The HD8 carbs were originally designed to be idled on the slow run valves. As written in the Bentley Manual and the original BMC Manual. They never really worked well that way and it depends alot on the engine they are installed on. From reading other sources I believe I have surmised that upon finding that the slow run valves were not sufficient to do the job, certain authorities have written directions to close the SRVs completely and adjust the HD8 the same as you would other SUs and adjust them using the jets adjusting for fuel and the throttle plates for air. I have had good luck with cracking the throttle plates just a touch and then using the SRVs to do the final adjusting of idle. Realize that by the way I do it, the engine is not intended to idle on the minute opennng of the throttle plates. That is just an assist to get more air. I still need to adjust the SRVs. This is very much what is done on and American 2 or 4 barrel carb where the throttle is openned but the idle quality is achieved with the idle adjustment screws. As said by red57 the richness or leaness of the mixture is controlled by adjusting the jet. The air can be done either by closing the SRVs and adjusting the throttle plates or by just opening the throttle plates a wee bit and then using the SRVs to achieve the idle you want. I can tell you that I have never bothered to try to achieve a balance between these two HD8s in the 20 years I have been running this car other than to set the throttles visually before I reinstalled carbs or by just watching the shafts as I turned the throttle stops. This car runs excellently.

mgtf328 said

There is a note at the beginning of Section 1 on tuning HD mulitcarb cars that says that 'whenever the throttle adjusting screws are fitted, they and not the Slow Running Valves should be used to adjust idling speed. ​

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I think this is what has caused confusion - not all HD8s are the same, we are talking about the HD8s as fitted to Healeys but the HD8s as fitted to Rover 2000 do not have a slow run ports (and have several other unique features) and you must use the throttle plate screws. So, for whatever reason, SU decided to do it one way on our cars and a different way on Rover 2000 - why?? Beyond that, there may be other configurations of HD carbs I am unaware of, I'm no SU expert.

vette said

Realize that by the way I do it, the engine is not intended to idle on the minute opennng of the throttle plates. That is just an assist to get more air. I still need to adjust the SRVs. This is very much what is done on and American 2 or 4 barrel carb where the throttle is openned but the idle quality is achieved with the idle adjustment screws.​

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Sorry but this is not the same. On American carbs the idle screws richen or lean the mixture but in our case all mixture ratio is done with the main jet and needle. So, the air/fuel mixture entering the engine is set by the jet & needle and the volume of that mixture entering the engine is controlled by either the throttle plates or the SRV - I'm not sure why getting some of the charge from the throttle plates and some from the SRV is any better than all from one or the other since it is the same pre-mix.

Sorry if it seems like I am arguing, I just know I tend to overthink things sometimes and need to get back to basics.

Dave

Red57, I do over think things. Also at my age I operate from memory which fails at times. I believe you are right about the idle control on the American carbs. As to your query about either/or, SRV or throttle, i do not believe you can get enough volume thru the SRV to do the idle justice. Therefore you either do it on the throttle plates or a combination of both. Now that I’m thinking of this a bit, I believe I have a pair of SRV screws that looked like the tips had been filed down a bit. I took them out of my carbs the first time I tuned it up after I bought it, 20 yrs ago.