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motorcycle carbs on 79 Spitfire

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brucejon Avatar
brucejon Gold Member Bruce Jones
Santa Cruz, CA, USA   USA
1960 Triumph TR3A
1963 Triumph TR3B "Tupperware TR3"
1969 Triumph Spitfire MkIII
1972 Triumph TR6
In reply to # 1594427 by tlcgt6 the 123 distributor allows you to set the ignition curve electronically with a computer of even with a phone as you drive down the road.

I can imagine it now.... "but officer, really I wasnt texting. I was optimizing my advance curve... really...."



60 TR3A (red), 62 TR3B project, 72 TR6, 69 Mk3 Spitfire EU setup
https://spitfiremk3.wordpress.com

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OFRacer Avatar
OFRacer Mike H
Poughkeepsie, NY, USA   USA
There's a set of Keihens on eBay for a 1300/1500 spit. They've been there for a while so the price may be negotiable. I considered them but decided to stay with my HS4s.
NFI, not my stuff, just passing it on.

https://www.ebay.com/itm/Triumph-Spitfire-Midget-1500cc-1300cc-Keihin-Cr-Special-Custom-Carburetors-CR33/283275702913?hash=item41f48c3281:g:ppgAAOSwKtlb-Yk~:rk:16:pf:0

mike h

65or66 Jim B
Lake village, IN, USA   USA
1965 Triumph Spitfire MkII
1971 Triumph Stag
1973 Triumph Spitfire 1500 "Jusanudda Munny Pit"
I was just thinking of a set of Keihin from a 750cc bike would be about right for a Spit. That ebay offering has the intake manifold already done!

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mtnwings Avatar
mtnwings Greg B
Ellenville, NY, USA   USA
Thank you, I will look for that ebay add. I believe the 750 only had 32 or 34 mm carbs, but offered in either CV or not, both Keihin carbs

mtnwings Avatar
mtnwings Greg B
Ellenville, NY, USA   USA
Thanks, Too much money for me, besides, I want CV carbs, I am going to try dual carbs first, if I am not happy or I am bored I will try the cooler looking quad set up. These carbs can not be pressurized, not meant to be pumped with pressure . Read someone tried them thru as fuel pump. I am thinking the tank is high enough for gravity feed.

IanF Ian Furqueron
Croydon, PA, USA   USA
In reply to # 1594616 by 65or66 I was just thinking of a set of Keihin from a 750cc bike would be about right for a Spit. That ebay offering has the intake manifold already done!

That is what I've read as well - since most motorcycle engines generally have a redline about 2x that of a Spitfire (that isn't fully race-prepped), then a 750 cc bike carb set-up is a good starting point for a 1500cc car engine. My understanding is Chris C (the guy who is most known for offering MC carb kits for Spitfires) spent a lot of time working on the fine-tuning to get them to idle correctly on the car engine with its generally lower idle speed.



"Lisle" - '72 GT6 basically stock and original. For now... T-9 conversion pending.
"Winnie the Poo" - '79 Spitfire 1500. Rubber to chrome bumper conversion, otherwise stock at the moment.

clshore Carter Shore
Beverly Hills, FL, USA   USA
Carbs are sized for airflow volume, which is the product of RPM, displacement (divided by 2 for a 4 stroke), and motor Volumetric Efficiency.
Carbs meter fuel based on the vacuum created by airflow velocity, so carbs that are too big will usually yield poorer fuel metering accuracy
than properly sized ones (hint, that's why classic Webers are available with different sized venture).
The rule of thumb is to size the carbs about the same as the intake valve diameter, but if the inlet manifold or head ports are poorly designed,
that has to be taken into account too.

It's been asserted in other threads that carbs should be placed as close to the head as possible, which is absolute rubbish.
The 'ram' or tuning effect of inlet tract length is well known, and inlet event order, when one carb feeds multiple cylinders (as in dual carbs on Spitfire)
can result in unequal mixture strength, that's why there is a balance tube.

1147cc Avatar
1147cc Silver Member Douglas Hansen
Westminster, SC, USA   USA
a single Mikuni HSR43 would be perfect for a stock 1500



Douglas Hansen
New Parts; Engine Rebuilds; Sheet Metal work and Advice.
http://www.1147cc.com

mtnwings Avatar
mtnwings Greg B
Ellenville, NY, USA   USA
Thank you, great info.
If you don't mind I have a few more questions:
I just picked up a 76 parts Spit yesterday, engine # FM-49562 UE, head #RRC 0589 11 M 5, body# 44094 FM Do these numbers tell me anything?

It is also equipped with a Piranha ignition, is this any good? The tach cable goes into a white box with wires then out to the dash, there is a set of rotating numbers in this white box, what is this?
Thank you for any help with these questions
Greg

mtnwings Avatar
mtnwings Greg B
Ellenville, NY, USA   USA
Engine will not be stock, better cam, milled head, 4-2-1 header, electric fan, sans all environmental crap, better ignition, all this cries for more fuel. Thus the dual carb/size questions. I love it that so many have tried the different options. So many different pinons, different setups, it's hard to know which way,(size) to go. Yes, I have heard it from the guys in the red Maserati's that "size isn't everything" . LOL

Tonyfixit Avatar
Tonyfixit Tony M
Duncan, BC, Canada   CAN
In reply to # 1594695 by mtnwings

It is also equipped with a Piranha ignition, is this any good? The tach cable goes into a white box with wires then out to the dash, there is a set of rotating numbers in this white box, what is this?
Thank you for any help with these questions
Greg

The Piranha like all EI's is good until it isn't. Mine, @ 1974, lasted 6 months.
But NO OE manufacturer used an optical ignition system (at least, not for long!)

That box is a sevice interval timer, I belive it lights an idiot light.

mtnwings Avatar
mtnwings Greg B
Ellenville, NY, USA   USA
Hi, The 4-2-1 headers I found on ebay just came in, great price but no collector from 2 to 1. My question is this: The ports are round not oblong like most all headers I have seen, 1 3/8 inch diameter. Also they are shorter than ones I see on line, only 13 inches from head to collector. Any idea what these are? I can match the ports, no problem but should they be longer for better performance? Also, the collector from 4 to 2 ends up just 1 1/4 diameter , shouldn't that be larger? I believe the four pipes from the head are 1/1/4 wont that cause a lot of back pressure? I can cut the collector up 1/2 an inch and weld on larger diameter 2 into 1 pipes to the exhaust system, any thoughts?
Thanks
Greg

mtnwings Avatar
mtnwings Greg B
Ellenville, NY, USA   USA
Received my dual carb manifold today, now I need those dam 38 to 42 mm CV Keihin carbs, headers came in yesterday, yee haa.

GeorgeOhr Nonya Business
Yes, confused, USA   USA
IIRC at least one of the CX carbs had a pump on it.

clshore Carter Shore
Beverly Hills, FL, USA   USA
In reply to # 1594743 by mtnwings Hi, The 4-2-1 headers I found on ebay just came in, great price but no collector from 2 to 1. My question is this: The ports are round not oblong like most all headers I have seen, 1 3/8 inch diameter. Also they are shorter than ones I see on line, only 13 inches from head to collector. Any idea what these are? I can match the ports, no problem but should they be longer for better performance? Also, the collector from 4 to 2 ends up just 1 1/4 diameter , shouldn't that be larger? I believe the four pipes from the head are 1/1/4 wont that cause a lot of back pressure? I can cut the collector up 1/2 an inch and weld on larger diameter 2 into 1 pipes to the exhaust system, any thoughts?
Thanks
Greg

Unless you are a racer seeking to squeeze the very last HP, none of those issues will make any practical difference on your street car.
Notice the size/shape of the exhaust vs intake ports on the head.
Lacking a dyno, any mods you make are pure guesswork.
Spitfires are not high RPM motors, if you make them run high, you will have reliability issues (conrod bearings).
Better then on a street car to optimize for better low and midrange torque …
longer secondary 2-1 pipes
cam grind and timing
ignition advance curve

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