Twin Carb balance

M

Malcolm

New Member
Does anyone have experience of balancing twin SU's (HS8)using vacuum gauges on the manifold side of the carbs ?
I have used a crypton balancer which goes over the inlets but with this I could only check one carb at a time and I am convinced that it had a choking effect on the carb being measured, and so creates its own imbalance .
I am considering purchasing a pair of Davida vacuum gauges and proposing to use the engine breather tubes at the back of the carbs to link them up. (with breather pipes disconnected) thus avoiding drilling the manifold.
Are there any pitfalls with this method before I invest in the gauges ?
Any tips appreciated - runs OK, pulls from 1500 rpm until rev limiter cuts out at 6,500rpm , but tickover is erratic and fuel consumption is at best 20mpg driven carefully. Distributor, points, carb floats, needle valve, fuel pump, etc all fully overhauled , compression is even across all four pots and valve clearances fine, timing is OK, static & dynamic.
Could be a sticking valve but head was recently decoked, seats and guides were fine, with no noticable play back then.
 
I would be very interested in any solutions you find to come up with your problem. I have also used the Gunson Carbalancer, which I thought worked OK for me (but I did have worn carbs). However, since having some engine work done recently - timing chain, full overhall of the carbs etc, I am still suffering from poor (lumpy) idling. Also when I rev the engine and let her return to idle, she always dips down to 500rpm or a bit less before recovering - can be unnerving on the road as she gets very close to stalling.
I am going to retune her this weekend, in case the garage only did half a job, and see what happens.

Regards,
Brian.
 
Your lumpy idle could be down to a slightly rich mixture, or an imbalance of mixture.
I use both the Carbalancer, and also a "colourtune" which allows you to see the colour of the combustion, which helps to get the mixture correct on both carbs.

I personally find twin carbs a nightmare to tune, not only do you have to get the airflow equal, but you also need to get the mixture the same on both. But then I think mine are VERY badly worn, even after full rebuild with new jets and needles. They require constant choke adjustment until fully warmed, and give a terrible flat spot and even pop-back at about 2000rpm.

When my first 2.2tc engine failed not long after buying the car, I part stripped it to inspect, and noticed that the front (1&2) pair of inlet ports had a good 1/4 inch of carbon baked over the entire port surface (not the back of the valve), suggesting very bad carb adjustment.

I never have problems passing the emissions test though, usually below 2% co

Richard
 
I would be interested to hear how you managed to tune using the carbalancer. I have great difficulty using the device due to the position of the air cleaner studs. The haynes book suggests using rubber pipe to extend the air intake, but I've never found any rubber tubing that fits. Anything that I've managed to stuff in the hole, I've been unsure of getting a true air tight seal.
 
A lumpy idle can be caused by a small air leak on the inlet manifold. Check for leaks in the advance/retard tube and make sure the capsule diaphragm is ok by sucking or blowing down the tube. Test for manifold leaks by trickling some wd40 or light oil on the joins between the carb and the manifold and the manifold and the engine. Do this at 1-2000 rpm and look for any smoke in the exhaust due to ingested oil being burned.
 
I have used a carbalancer before to good effect.

I also rely heavily on and old length of rubber tube ( normally the heater overflow pipe ) which I place just at the mouth of the carb to simply listen to the air flow. By moving the tube from carb to carb you can hear any imbalance in the sucking sound. You then simply adjust the throttle stops and linkages until the sucking sound is identical between the two.


Works for me !!!!!!!!!!!!
 
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