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mgtommm
USA
506 Posts |
 Posted - 28/01/2017 : 19:21:36
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Are there any big carbs going unused? My J2 engine is coming down the home stretch and I'd like to set it up with larger carbs than the original little pea shooters.
Thanks all!
Tommm on the gusty shores |
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bloodysalmon
Spain
1482 Posts |
Posted - 28/01/2017 : 19:52:01
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Hi Tom, strangely enough I'd just taken a few photos the other in readiness of selling this sole 1 1/8" SU carburetor. I've had this for many a year and it's surplus to my immediate requirements; I'm not sure if it's correct or at the moment what number it is either - £65 + shipping of course. I also have a new pair of N-type windscreen uprights too at £60
Chris Blood (D0407 & F0753) |
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PeterL
United Kingdom
1734 Posts |
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bloodysalmon
Spain
1482 Posts |
Posted - 30/01/2017 : 11:02:29
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my carb is stamped/cast with 3061 ... hopefully someone will know a bit more about it than me
Chris Blood (D0407 & F0753) |
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Simon Johnston
United Kingdom
6214 Posts |
Posted - 30/01/2017 : 11:35:57
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Tom,
Larger carbs will only be of benefit if the engine needs more air (and fuel) than it's currently able to get from the 1" ones, which implies higher revs and a modified camshaft at the very least. Just fitting larger carbs on their won't produce any more power or torque.
Personally I found Vizard's book to be of little help as it doesn't really discuss carbs as small as ours, i.e 1" and 1 1/8"
Simon J
J3437 |
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PeterL
United Kingdom
1734 Posts |
Posted - 30/01/2017 : 12:53:47
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Simon
What do you think of Vizard's ideas? Richard (son) is about to have a go at my spare 1" carbs...
Cheers
P |
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Simon Johnston
United Kingdom
6214 Posts |
Posted - 30/01/2017 : 13:36:16
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Peter,
It's a few years since I looked at Vizard's book and to be honest I can't recall the details. I just recall that I didn't feel it was of much help for deciding whether my 1 1/8" carbs were better than 1" ones.
Simon J
J3437 |
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PeterL
United Kingdom
1734 Posts |
Posted - 30/01/2017 : 15:40:10
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Richard reckons that doing the mods, i.e. flowing the interior of the carb, makes it perform like the next size up because the breathing is so much better. He has even been tuning the carb on his modern fiat!
If/when he presents me with a tuned pair I will report ...
It was something reported by the 18/80 brigade about the fuel efficiency improvement of fitting stubs, is that what they are called? that really raised my interest in tuning carburettors...
Cheers
P |
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mgtommm
USA
506 Posts |
Posted - 30/01/2017 : 18:04:12
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I changed the carbs on my PB and could not believe the difference. That is the sole reason I'd like to give it a try on the J2.
I also went from 1 1/8" to 1 3/8" on my NB (those carbs now on my PB) with similar results.
The zero to 60 times on PB and NB are both now well under two days.
And Chris, many thanks, happy to take you up on your kind offer.
tommm (looking for a neck brace next) |
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Hornet
United Kingdom
382 Posts |
Posted - 30/01/2017 : 20:14:29
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| your hv carb in the photo is slightly different to mmm carbs , the intake end is slightly larger od you can see the end is thicker , butterfly is still 1 1/8 , the mounting holes are slightly different centres , one original fitment was triumph 7 hp . |
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bloodysalmon
Spain
1482 Posts |
Posted - 05/02/2017 : 12:10:40
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Tom, I haven't heard anything from you since the 30th ... do you still want this single SU carb as I have another party wanting it if you don't? I still presume you wanted the N-type windscreen uprights too?
Chris Blood (D0407 & F0753) |
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JMH
United Kingdom
915 Posts |
Posted - 05/02/2017 : 20:53:47
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2 x 1" is all you really need on 850cc. You gain a bit more at the top end with 2 x 1 1/8", but loose lower down. You end up faster on the smaller carbs. This was the conclusion MBH reached after nearly 40 years of "tweaking". For the last 20 years of ownership he stuck with the 2 x 1". 60 years of tweaks saw everything tried - magnetos, 2 x 1 1/8", 2 x 1 1/4", even Amals (briefly). Bigger valves don't really work either (too much shrouding), It's all in the valve seat area & tweaks to the std manifold + exhaust, with a lot of help from cam timing.
JH |
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Richard Hardy
United Kingdom
2166 Posts |
Posted - 06/02/2017 : 19:33:35
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Tommm
Jeremy is correct. My J type is very quick and on original 1 inch carbs. Ok, the head is all gas flowed and balanced. Another significant ramping up on performance then came with fitting the Brooklands cam we had developed last year. This makes a significant difference with 10% more torque coming in all the way up the rev range to around 5,000 rpm. No more revving needed. Can just drops straight in as a direct replacement and no modifications required.
Richard
Vintage MG Parts |
Edited by - Richard Hardy on 06/02/2017 19:37:10 |
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Simon Johnston
United Kingdom
6214 Posts |
Posted - 06/02/2017 : 22:41:10
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When my J2 engine was first assembled it had a Baynton Jones cam, 1" carbs and an apparently absolutely standard head, i.e. no increased CR. It was put on a rolling road and produced a very disappointing 38.5 bhp at 5500 rpm. For reasons that I won't go into here, the engine was stripped and reassembled and the opportunity was taken to take about 60 thou off the head. The carbs were also swapped for 1 1/8" ones. Back on the same rolling road the power was now 47.5 bhp at 6000 rpm. A third visit to the same rolling road two years later showed 46.8 bhp at 6250 rpm.
This would seem to suggest, and I emphasise 'seem', that the larger carbs did indeed enable the engine to breath more efficiently/effectively at higher revs and get the benefit from the modified camshaft. (This is not a debate about respective camshafts - it's simply a comparison of different carbs on the same engine, albeit with increased CR after the first rolling road session ). Using Mike Hawke's data in his article 'What do we get from tuning' an increase in CR from 6.2 to around 7.0 should only increase the bhp from about 38.5 to around 40.5. In other words, the majority of the extra power achieved was from the larger carbs allowing the engine to benefit from the modified camshaft.
So for a standard engine, or nearly so, I doubt if the larger carbs would bring any benefit, but for a modified one they will enable the engine to draw in more air/ fuel mix and thus produce more power if the camshaft is designed with that in mind.
Simon J
J3437 |
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