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Jun 16, 2008 11:20:32 GMT
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when i get my mk2 spitfire (1147cc) on the road, it'll run its standard factory set up - mech dizzy, twin su's etc.
my first mod will be to fit an omex 3d mappable ignition kit that i've got spare in the garage, to liock down the timing etc. i reckon i'll see a huge improvement in the drivability of the engine with this mod.
then i want to move on to the carbs. i've no doubt that the twin 1 1/4" SU's will need a rebuild. also knowing that they go off fairly easily once set up, if i've got to spend money on getting them rebuilt, i could spend money on other fueling options. was wondering what are the benefits of modern bike carbs.
there are plenty of 32mm mikunis on ebay etc for pennies, can be easily split down into a pair and i can easily knock up a manifold/linkages etc.
loads of articles about whacking on bike carbs for power etc, but what about fuel economy. would a pair of mid 90's bike carbs be more reliable,efficient, economic than a fresh set of twin sus for example.
other option wiould be to get a triumph herald single carb manifold and mod a metro hif38/44 on it.
not necessarily bothered about ultimate power but more about economy as i want to build the spit as a lightweight commuter car
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slater
Club Retro Rides Member
Posts: 6,390
Club RR Member Number: 78
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Jun 16, 2008 17:05:34 GMT
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I'm in a simlar boat.. what worries me about the bike carbs is that you don't seem to be able to 'fine tune' them very well. People talk about drilling out jets and what not but surely that just increses fueling fairly unifromly across the whole range.. what happens if you need more fuel at a cirtan point? you have to put it in over the whole range? to me thats goin to waste alot of fuel!!
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Jun 16, 2008 17:07:57 GMT
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just fit injection if you want economy, much better for an everyday car anyway.
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Volvo back as my main squeeze, more boost and some interior goodies on the way.
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B-8-D
Posted a lot
down to one car!!
Posts: 4,038
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Jun 16, 2008 17:11:44 GMT
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I'm in a simlar boat.. what worries me about the bike carbs is that you don't seem to be able to 'fine tune' them very well. People talk about drilling out jets and what not but surely that just increses fueling fairly unifromly across the whole range.. what happens if you need more fuel at a cirtan point? you have to put it in over the whole range? to me thats goin to waste alot of fuel!! they are VERY "finetunable" not a problem different needles and needle hight. drilling the slide. diff slide springs. diff air bleed jets. float hights. air corector jets. and emulsion tubes its all adjustable..... i used to set up many bikes on the dyno and bike cards are the pinacle of carb design... do it...  si |
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slater
Club Retro Rides Member
Posts: 6,390
Club RR Member Number: 78
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Jun 16, 2008 17:47:05 GMT
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Yeh i suppose the drilling method is just a bodge to get them to 'work' afterall. Id like to see someones results... In theory i cant see then hurting the economy too much but the carb(s) are a big part of it and if you get it wrong your pretty much guarenteed to loose.
As for injection its no where near as cheap/easy. You got to do alot of miles or keep the car a long time to justify it relly.
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Jun 16, 2008 18:23:47 GMT
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injection is super easy, plenty of cars you could nick an MFI set up from, cost a few quid from a scrappy and give you better economy, better starting, better cold running, just the convenience factor alone in a daily would be worth it even without the mpg benefit.
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Volvo back as my main squeeze, more boost and some interior goodies on the way.
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Icey
Part of things
Posts: 388
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Jun 16, 2008 18:28:24 GMT
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And with the exchange rate the way it is, you can get an MS kit for ~£130.
IMO if you are after day-to-day driveability, economy and simplicity then injection is the way to go, but thats a whole other debate.
(ChrisW)
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Jun 16, 2008 19:21:10 GMT
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I'm skeptical. Dyno testing won't tell you anything about how the carbs react to motion. My concern about bike carbs is that they're not designed to take lateral g's per se... Bikes obviously tilt abruptly from side to side, but they do most of their acceleration with fuel slosh along a different axis than you'll be mounting them, right?
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Jun 16, 2008 19:31:40 GMT
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Not sure how relevant that is. Carb float chambers are a/ full, and b/ vented via a snorkel... fuel slosh is a tank and injector pump probelem (swirl pot anyone?) not really a carb issue. besides, bikes wheelie too. Drilling jets is a bodge. get a dynojet kit and do it properly.... Dyno testing will tell you if your fuelling is correct across the rev range, which is the whole point of getting it dyno'd. Like the OP, I'm using two bike carbs on a car that has siamiesed inlet ports. I don't see why a set of bike CV carbs should ever be worse than twin SU's can be to set up, its a refinement of the same design. I'm just trying it on the basis that I might as well. See what happens and report on it. Incidentally, I'd use cartbs off a twin cyl bike than try to cut down a four pot set. In fact, I have. Generally though, I'm in the "win" camp. I've done this before (although not quite like how I'm doing this one) |
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To get a standard A40 this low, you'd have to dig a hole to put it in |
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Mr S
Posted a lot
10-4 Good buddy.
Posts: 2,654
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Jun 16, 2008 19:40:06 GMT
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I've got plenty of experience with bike carbs (bikes are my background) and I'm going to be using 8 carbs (2 sets of V4 bike carbs, probably from a VFR750) on my Merc V8.
They are soooo easy to tune, you dno't need a dynojet kit as such, just a selection of main jets (couple of quid each normally) and most of them have adjustable needle heights and you're away! Not to mention the pilot circuit which is also adjustable...
Basically, you set the main jets first so your what is good, then you set the needle height for the mid-range throttle position, then once that's all sorted you set the pilot circuit for the first 1/8 throttle.
Setting it up on a dyno is best, failing that, seat of the pants dynos work for me!
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Suzuki GSXR1000 K2
BMW R1150GS
BMW K1200RS
Chevy K5 Blazer
Chevy Suburban LT
Jaguar XKR
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Jun 16, 2008 22:29:10 GMT
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Float chambers "full" when the engine is running? Fuel slosh isn't normally a carb issue? I'm blown away, because these two statements are 180 degrees from my experience. I'm sure I'm not the only one here who has killed Saturdays messing with float bowl baffles, or fitted a center-hung-float carb (versus a side-hung-float carb) trying to overcome surges or flat spots under cornering... Two articles that seem to echo my experience: en.wikipedia.org/wiki/Carburetor#Float_chamberwww.britishv8.org/Articles/MGBV8-Carburetor.htmHowever, your point about bike wheelies is quite good. Some cars wheelie too...  |
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Jun 16, 2008 22:35:59 GMT
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Placement of these vent tubes can be somewhat critical to prevent fuel from sloshing out of them into the carburetor, and sometimes they are modified with longer tubing. presume you mean this bit (god, thats a long winded wiki) Thats the bit I called a snorkel. Not wanting an argument, your mileage may vary as they say. I've never had to do more to a float chamber than clean out the needle valve and empty the silt. Tanks, however, I've welled, baffled and swirlpotted cos of the tendancy of injector pumps to suck air on corners. edit typo correction  |
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To get a standard A40 this low, you'd have to dig a hole to put it in |
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I have heard of people with motorbikes fitting SU's to increase fuel economy. To be honest i would say a well set up SU would be among the best carb from an economy point of view.
SU's meter on load, not on throttle position. Correct me if i am wrong but bike carbs needle goes up and down with the throttle, not the load?
Not really sure where all the SU hate comes from, i think they are a good carb.
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I did mention that I already had an omex 3D mappable ignition kit, so going the whole EFI setup isn't an option (extra cost than necessary). my old 72 mini running standalone engine management via hayabusa TB's pn c20xe lump, so the concept of retro fitting EFI isn't a problem. cost is though. having read about su tuning for economy from the vizard book, I know that su's can be tuned, and are a great carb for such an old design (no SU bashing here). However, as the setup on the spitfire are twin su's, need rebuilding and have the tendancy to "go off" now and again, I wanted to consider other options. if i'm gonna spend money etc etc edit:- also, i've been looking around for a few weeks now trying to read up on bike carbs but internet searches just throw up bogg bros and big power set ups. where as i'm interested in economy. for best economy, i'd be better off buying a EFI 1.0L corsa or summat but i think a mk2 spitfire (tax free too) is a a tad more interesting to drive. if setup right it'll sip dinojuice, but there no harm in tinkering. |
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Mr S
Posted a lot
10-4 Good buddy.
Posts: 2,654
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*Most* bike carbs are CVs, and work off vacuum pressure from the inlet manifold. They're not like flatslide bike carbs (where the cable is connected directly to the throttle slides). So although they do react to the throttle being whacked open, they aren't directly linked IYSWIM
I've never really fiddled with SUs (or any car carbs for that matter!), so can't really comment on any comparisons!
Cheers, Martin
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Suzuki GSXR1000 K2
BMW R1150GS
BMW K1200RS
Chevy K5 Blazer
Chevy Suburban LT
Jaguar XKR
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Jun 17, 2008 11:30:29 GMT
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Could a late rover mini injection setup not be adapted to fit? similar siamese design isn't it?  *n |
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Top grammar tips!
Bought = purchased. Brought = relocated
Lose = misplace/opposite of win. Loose = your mum
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Jun 17, 2008 15:09:23 GMT
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Could a late rover mini injection setup not be adapted to fit? similar siamese design isn't it? *n I had a brand new mini (in '95, motability finance ...) with the bosch monopoint on. half an hour after its first service it had an HS4 and a pico pipe. 1275 should never be slower than a hammered to death 998 (which is what I had before). |
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To get a standard A40 this low, you'd have to dig a hole to put it in |
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Jun 18, 2008 14:32:36 GMT
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mini mpi injection hardware is OK, but the mems/mini software is pap. designed to meet emmissions and thats all really
aftermarket mini based (ie siamised ports issue) ecu made by Canems is very good. uses 2 lambda sensors and trick programming to ensure the engines 2 outer cylinders don't suffer from charge robbing/running lean. standard 998 engines on canems produce smae poer as 1275 mpi factory engines.
back to thread - what about bike carbs for economy?
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bazzateer
Posted a lot
Imping along sans Vogue
Posts: 3,653
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Jun 18, 2008 14:48:55 GMT
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I've got some bike carbs I intend to experiment with on either the Imp or the Vogue one day.
I've recently fitted twin 1 1/4 SUs on the Imp and the first tank-full, about 200 miles motorway + 100 or so on A roads (last hundred running on 3 with a failing h/g) returned an average of 38mpg. This should be well into the 40s when the h/g is sorted.
Not sure where the idea that SUs 'go off' is from? If properly set up, then left alone, they go on for ages with no problems (my Frogeye did 12,000 miles as a daily commuter in all weathers with no need to adjust the SUs).
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1968 Singer Chamois Sport
1972 Sunbeam Imp Sport
1976 Datsun 260Z 2+2
1998 Peugeot Boxer Pilote motorhome
2003 Rover 75 1.8 Club SE (daily)
2006 MG ZT 190+ (another daily)
2007 BMW 530d Touring M Sport (tow car)
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