SUM V Amal

[Ian R11]

An item discussed several times previously is the best alternative carbs to use on the early bikes. There are several options but over the last week I have fitted and tested a new pair of Amals on my R11 and thought I would share the findings.

I have had them on the shelf for a few months but only just had time to try them. I ordered them from Burlen Ltd in the UK, they stated they had the original specification to build for the R11 and would take several months to supply. I was also told the pre monobloc 276 series carb body is now only available in one hand so the mixture and throttle stops are at the back on the left side carb, they are a little tight to get to and adjust. They do not have a cold start device fitted but starting cold so far is no problem. The build spec is in the photo. Also fitted air intakes with 3 gauzes similar to original on my SUM, not exactly a filter but would stop most things. The new mounting flanges supplied needed some elongation of the holes to fit the studs on the cylinders. I had some flange insulators the correct size.

New throttle cable made and twin fuel pipes fitted, a quick tickle and it fired second kick. As expected some adjustment was needed, the throttle stops and after a little testing I moved the needle jets one slot richer and adjusted the air screw a little. I have covered around 100 miles in the last couple of days and have found them generally very good. There is a small increase in power and slightly quicker throttle response. The bike is a little noisier with more intake noise and slightly more bark from the exhaust creating a perception of more performance. It did tackle a top gear hill on my test route slightly better than with the SUM but didn’t quite match top speed. I can’t get the tick over as smooth, but in gear flexibility and generally on the road performance seems much the same as before.

One noticeable difference is the affect on the advance/retard. With the SUM retard was only used to smooth the tick over, the revs only slightly rising when fully advanced. With the Amals the tick over has been set at same retard but when advanced it revs a lot more.

If you have a good working SUM the gain is probably too small to bother swapping, but if you don’t and needing a setup that works it is certainly a good option. While they are not going to satisfy the concourse or purists seekers among us the cost was £607 for the pair, slightly more than single reproduction SUM at £525 and a fraction of originals if they could be found. I will leave them on for now and fiddle a bit more but will fit the SUM back on the bike at a later date, I think the inlet manifold and heater tubes add visually to the bike. If nothing else it proves to me I got the SUM working pretty well and the grass is not that much greener the other side of the fence.

Anybody tried the Bings?

[bmwmyplace]

Ian I for one thank you for the post I found it interesting. I was contemplating doing the same thing on my R12, as I have some vertical flange Amal Bodies NOS that just require jet blocks etc. I know in the case of the R12 it is supposed to give 2 more HP But My Sums work well. So I guess I will leave well enough alone. thanks again Peter