triple carb needles?

[BJ7archaeologist]

Fellow Healeyoids!
I have a 63 BJ7 and I have three 1 /3/4 inch SUs HS6. The manifolds are bored out to accept them. The plugs foul easily and the car is hard to start even when choked. I have standard BC needles and red springs. According to Austin Healey Club Midland Centre, this is the correct combination to have for smooth running. I have stock exhaust, BJ8 cam, mildly ported head. Running 91 octane pump gas. I also have NGK BP5ES plugs. Do I need hotter plugs? Weaker needles? The SQ needle is the weak needle for this application and it is only .001 less taper at idle. I can't believe that would make much difference. Suggestions?

 

[red57]

Don’t know if this will help or not. First, disclaimer – I am not a pro and these are only my own observations/opinions.

I have a similar set-up. I run triple HD6 on bored out manifolds, a much longer duration cam, ported/flowed head, headers, 9.5:1 compression ratio and run 92 pump gas. I’ve been playing with this carb set-up for a couple of years and finally have things near right, but it has been a long uphill battle….

In my experience, each engine factor like compression ratio, flow characteristics, cam timing, etc. will require slightly different needles. Your motor is probably enough different from mine that what I use won’t be right for you – whenever we deviate from stock, we are asking for issues….

First, I have found the red springs are correct, in mine they allow the pistons to reach top between 5500 and 6000 (I red-line mine at 6). You don’t want the pistons to reach top any earlier or the needles won’t be metering fuel as the revs/air flow go up, conversely with heavier springs the pistons won’t make it to the top and that will reduce air flow/power.

2nd, I don’t think you want hotter plugs. I run BP6ES or BP7ES, I have always been told to run the coldest plugs you can without fouling - and BP6ES are the most common ones for our engines.

Needles, this is the heart of the issue. I have made my own because I have not been able to find any off-the-shelf ones that work for me.
I have tried SQs and they can work for idle & low speed/throttle driving but are waaay too lean at full throttle. Have not tried BCs but think they would also be too lean at full throttle/high revs. I have tried RDs and they are quite drivable for easy street driving at part throttle/lower revs but still too lean above station 8 or 9 for full throttle/high revs – you may want to try a set but be aware of the risks you take.

If you are prepared to really get into it, I recommend Des Hamills book “How to Build and Power Tune SU Carburettors” . It has some very useful info on setting the carbs up and modifying needles – some parts of his approach I disagree with but the basics are spot on.

First, I recommend you make sure all jets are the same height using the tail of a vernier caliper, use a flow meter like a UniSyn and make sure you have the same air flow at idle AND off idle. Make a set of the calibration rods as Hammill shows to make sure all three pistons are rising to the same height – it is vitally important that the air flow and piston height is the same for all three carbs. They will also show you what station of the needle is in play at any given throttle/load situation so you can figure out where it is lean or rich (I’ve done a lot of local driving around with the hood off so I can watch them in different load situations – better yet would be a rolling road dyno if one is available in your area). The book has some good advice on this but it is limited – I got so desperate after melting pistons & warping valves on the track that I invested a grand in two 5 wire Air/Fuel meters (one in each collector to read front half & rear half of motor) and an aircraft EGT gage that has 6 probes so I can monitor the exhaust temps. for each cylinder (trying to stay under 1250F or so). The more you can know about what is going on in there, the better chance you have of getting it right – at least that was my theory.

BTW, your question about .001 difference making a difference? It sure does, even .0005 can make a marked difference.

The current needles I am using started life as SQs because I needed leaner in some spots that the BC or RD and much richer in other spots. If you want more, you can PM me and I will give you the needle profile I have come up with for mine but, again they may not work well for your motor. Sorry to be so long winded but it is a complicated thing and I am just an amateur – there may be lots of others who can give you better advice.
Dave

 

[BJ7archaeologist]

Thanks a lot Dave. I ordered some SQ needles and I will try them out once they arrive. If they don't work I will likley go back to stock or try some 2 inch carbs that I have. The triple carb setup just looks so beautiful. I do have an extra 1.5 inch triple carb set as well.
Randy