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Mikuni carb for supercharger

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mummert Geoff M
El Cajon, CA, USA   USA
The Mikuni is flowed at 12" of water column. They flow over 350cfm @20.4" of water column,while being essentially 1-7/8" carburetor. Bored to 1-15/16

they flow 365cfm @20.4" With a top mount idle speed adjust they pick up another 6-8cfm. A 2" SU will NOT flow with a 48mm HSR.

The HSR has much better starting, and idle. The mid range acceleration is greatly improved because you dont spend time waiting for the slides to open.

They open went you want them to. A 48mm HSR already has the main jet extender standard, this is why they have the longer float bowl drain nut.



Edited 3 time(s). Last edit at 2018-06-13 05:05 AM by mummert.

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mgbsc72 Avatar
mgbsc72 Silver Member Ross Holt
Melbourne, victoria, Australia   AUS
1973 MG MGB
2004 MG ZT
Ordered my mikuni hsr48 today and the 2.3 needle and seat , have to wait as a shipment is on its way
Denis any chance you could give me your starting tune so I can order jets and needles etc
I know this is going to be a new learning curve , but I love a challenge .


regards Ross

Denis Avatar
Denis Denis Hill
Bearii, Nth Victoria, Australia   AUS
No problem Ross. Denis



68 B roadster, Daffodil yellow, supercharger, Burgess SC head, SC cam, Mikuni HSR 48 carburetor and engine built for supercharging.

73 BGT V8 conversion starting with a bare shell. Built the engine "3.9L Rover" early in 2016 with high comp pistons and a few other nice bits, plus a T5 ford trans. Started on the body late 2016 and complete late 2017, Did all the work myself, mechanical, body. paint etc.
Finished and going well, great to drive and quick. Now has a nice 3.07 LSD.

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gray Avatar
gray Graham Moore
CAMBRIDGE, CAMBRIDGE, UK   GBR
Ross

do please keep this thread going - as a build thread. very interested to see pics of the bits when you get them - to see how they fit, and to hear on the ups & downs of getting them spot-on

G

PJ MGB Paul Eaglen
Auckland, Auckland, New Zealand   NZL
1977 MG MGB
So Denis,

Given that I have the Moss supercharger running reasonably well, what would be
your recommended process for changing over to a Mikuni?

Run a dyno first.

Then just unbolt the HS6 and attach the Mikuni instead
- Tune as best as possible to idle AFR
- Run some AFR checks at different speeds and loads

then run a Dyno

would you expect it to be able to be tuned well enough for general use of the car
before any needles and jets have to be changed?

I guess,it would be easy to swap the HS6 back while testing etc.

Denis Avatar
Denis Denis Hill
Bearii, Nth Victoria, Australia   AUS
Paul No. Dont expect to wack a HSR48 on and drive it without a fair bit of work in the way of tuning. My engine has the works, Burgess SC head, forged SC spec pistons & Pauter rods, SC cam all balanced, HI-flow extractors, 123Tune. and it needed more air to get the best out of it. Stock engines will see little or no gain as the extra breathing only comes into play from about 4400revs. Many will never see that. The Mikuni gave me 148 WHP at a rev limited 6000 revs 21 HP over the SU but only because it needed it , As I said most wont . I figure a HIF 44 will give up to about 127.7 WHP @ 5400 or approx 150 engine HP so only think of a change if your engine can use it, Most wont. Power graph was almost the same up to 4400 I dont want to sell anybody a pup. (dud).

Denis



68 B roadster, Daffodil yellow, supercharger, Burgess SC head, SC cam, Mikuni HSR 48 carburetor and engine built for supercharging.

73 BGT V8 conversion starting with a bare shell. Built the engine "3.9L Rover" early in 2016 with high comp pistons and a few other nice bits, plus a T5 ford trans. Started on the body late 2016 and complete late 2017, Did all the work myself, mechanical, body. paint etc.
Finished and going well, great to drive and quick. Now has a nice 3.07 LSD.

PJ MGB Paul Eaglen
Auckland, Auckland, New Zealand   NZL
1977 MG MGB
Hi Denis,

My engine was built specifically for a supercharger so
8.1 compression,
large valves
supercharger cam (low overlap)
roller rockers
ported but not flowed head
big exhaust
lightened flywheel
etc.

I get about 100HP at the rear wheels with low boost pulley and 5lb boost
and have not dyno'd the high boost pulley yet and don't know what boost for that but with such low
boost it would appear the engine is breathing well.

I have had some problems with the HS6 especially on long hills so changed
the needle valve in the float chamber but things are not perfect. I have had flooding with the largest
needle valve (1.25) and now using the .96

It certainly takes a long time to warm up and the cost of the mikuni
is not impossible in comparison to what else has been spent, so I am thinking about it.

I really want to spend a bit more time improving the HS6 before I change it out
but would appreciate knowing more what's involved in the change.

You have the larger harmonic pulley and a 123tune but otherwise our engines are not too different.

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Denis Avatar
Denis Denis Hill
Bearii, Nth Victoria, Australia   AUS
Paul you would not benefit from a Mikuni at that boost. I went to mine when only when the engine hit the wall before the cam, and flow was the most likely. If you get into mid 120s whp and still not a lot of revs think about it then. The highest boost I saw with the HIF was 8.5 lb but the Mikuni gives 10lb with no other mods and max HP went from @5400 to @6000 and still heading in the right direction. I have the Moss high boost pulley and Hi-flow harmonic balance which is worth about 1 lb over the Moss crank pulley.
I have not tried HS6 myself but opinion seems to be that that the HIF44 flows a little more.
Denis



68 B roadster, Daffodil yellow, supercharger, Burgess SC head, SC cam, Mikuni HSR 48 carburetor and engine built for supercharging.

73 BGT V8 conversion starting with a bare shell. Built the engine "3.9L Rover" early in 2016 with high comp pistons and a few other nice bits, plus a T5 ford trans. Started on the body late 2016 and complete late 2017, Did all the work myself, mechanical, body. paint etc.
Finished and going well, great to drive and quick. Now has a nice 3.07 LSD.

mgbsc72 Avatar
mgbsc72 Silver Member Ross Holt
Melbourne, victoria, Australia   AUS
1973 MG MGB
2004 MG ZT
Hi Paul

My engine was also built for the supercharger after a head gasket blew at sandown race track it blew a piece of head away and the blow off valve
across the engine bay . The engine restarted on two cylinders after I stuck a coke can over the hole in the manifold
The normal rebuild plus head work balance gapless rings 18 v rods Hans cam and balancer etc

I should have used forged pistons but old stock ACL made in oz low comp pistons at $180 a set were just to cheap !

The new engine had 100hp at 4900-5000 when very tight on the hs6 carb , The hif that is on now is a lot better
The only problems left are that a thick damper oil makes throttle responce slow and smooth, a thinner oil eg 30-60
great responce but to big a lean spike come flat spot .

I have now drilled the su piston lift holes back to 3.5mm as my boost in 1st and 2nd was going to 6psi
then coming back up again . My max boost is around 9psi but lower with thicker damper oil
I have also had some icing up in the cold weather causing a stuck piston then a back fire

Having done lots of tuning with the su needles and damper mods etc I want to give the mikuni a try
My car is running a taller diff which makes the lean spike a bit worse
If you
want to stay with the su I would go over to a hif44 they are better
Also check your hs6 my one with the kit had very had machining inside the damper shaft causing sticking
The best needle is a modded bcf and a 8 oz spring
regards Ross

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Denis Avatar
Denis Denis Hill
Bearii, Nth Victoria, Australia   AUS
I recon my old HIF44 was as good as FI Once the early air filter problem was sorted and back to the the Moss BCA needle, 90 ( I actually used heaver) damper oil it was almost perfect. Just not big enough for my application. I do wonder what other changes Moss may have had done to the SC version of HIF as off the shelf HIF seem to require a bit more work than just needle, spring and piston lift to be as good as the OE. Denis



68 B roadster, Daffodil yellow, supercharger, Burgess SC head, SC cam, Mikuni HSR 48 carburetor and engine built for supercharging.

73 BGT V8 conversion starting with a bare shell. Built the engine "3.9L Rover" early in 2016 with high comp pistons and a few other nice bits, plus a T5 ford trans. Started on the body late 2016 and complete late 2017, Did all the work myself, mechanical, body. paint etc.
Finished and going well, great to drive and quick. Now has a nice 3.07 LSD.

galotis Avatar
galotis George Van Horn
Collegeville,, PA, USA   USA
1980 MG MGB "Ms. Blue"
Denis, I went to FI, when I keep running out of fuel at the top of 3ed, no matter what inlet jet I added. the pump was a Holley 560lph so it was the carb just could not hold enough fuel in the bowl and the inflow was not enough to keep up with the CFM that the SC was pulling. now the motor at that time was a .060 over with 1.7in intakes. now were .020 over and 1.65 intakes. I just got the Water/meth dialed in so now with full timing (30 deg) at 6500 and 12lbs of boost. The shop with the dyno had a air flow meter we put on the intake, at 6500 rpm the SC was pulling 326cfm, SC spinning at 12600rpm

Denis Avatar
Denis Denis Hill
Bearii, Nth Victoria, Australia   AUS
George I am a great fan of water/meth when your getting into higher compression or/ and higher boost area. I found fitting it made very little difference in itself , but the few degrees gained in advance made all the difference. Mine only comes in over 5lb and I could set it higher but at that its economical and great insurance. I have never run my engine over 6000 but the power is still climbing at that. The engine has all good stuff inside so maybe one day.
Interesting what Geoff said about the Mikuni flow. V other carbs I have heard similar before. Its just a big smooth hole.

Denis



68 B roadster, Daffodil yellow, supercharger, Burgess SC head, SC cam, Mikuni HSR 48 carburetor and engine built for supercharging.

73 BGT V8 conversion starting with a bare shell. Built the engine "3.9L Rover" early in 2016 with high comp pistons and a few other nice bits, plus a T5 ford trans. Started on the body late 2016 and complete late 2017, Did all the work myself, mechanical, body. paint etc.
Finished and going well, great to drive and quick. Now has a nice 3.07 LSD.

Denis Avatar
Denis Denis Hill
Bearii, Nth Victoria, Australia   AUS
For those interested in the Mikuni HSR 48 Here is a pic of the needle mods from where I did mine four years ago. All the mods are in the taper and start 4mm AFTER the start of the original taper. The OE needle is the one that came with the carb . The one I bought to modify was shorter than the OE but that does not seem to matter. It is important not to alter the rest of the needle otherwise "cruise" mixture will be too rich.

Denis



68 B roadster, Daffodil yellow, supercharger, Burgess SC head, SC cam, Mikuni HSR 48 carburetor and engine built for supercharging.

73 BGT V8 conversion starting with a bare shell. Built the engine "3.9L Rover" early in 2016 with high comp pistons and a few other nice bits, plus a T5 ford trans. Started on the body late 2016 and complete late 2017, Did all the work myself, mechanical, body. paint etc.
Finished and going well, great to drive and quick. Now has a nice 3.07 LSD.


Attachments:
needles.pdf    238 KB

mgbsc72 Avatar
mgbsc72 Silver Member Ross Holt
Melbourne, victoria, Australia   AUS
1973 MG MGB
2004 MG ZT
Thanks for the pic Denis
I have found a great carb test online it tests all the big harley carbs including the hsr 45 and 48, they did the results at 10 inches
The mikuni hsr 48 was 265 cfm at 10 inches not the 20-25 inches that su carbs have been tested at .
When you convert to 20 inches = 375 cfm The mikuni hsr 48 flows more than a su hs8
I will attach file

regards Ross


Attachments:
CarbShoot.pdf    881.1 KB

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