Carb Advice Please

[andy_s]

Hi guys,

Am building an engine for a track mini. I was going to use twin 1.75" SU carbs, but have been speaking to someone who said I'd be better with a single 1.75" SU. The engine spec. is-

1275GT A Series
Stage 3 Cylinder Head (unleaded conversion)
Kent 286 camshaft
Maniflow LCB exhaust manifold (1.75” )
New Oil Pump
Lightened Duplex Timing Gear
Lightened Steel Flywheel

1275GT gearbox
AP Racing Clutch Plate
Central oil pick up
Balanced Crank & Con rods
A Series 1:1 Ratio Straight Cut Drop Gears

So, am I best to keep my twin carbs, sell them to get a single 1.75, or sell them to get something different all together?

Thanks a lot,

Andy

[philster]

Twin 1.75 would be a bit OTT for that spec, a single HIF44 would be more than adequate or maybe a pair of 1 1/2's

[RikWalsh]

Agreed, I would have thought that a set of twin 1.75 carbs would have been a little over kill...




Rik

[andy_s]

When you say over-kill, what does that actually mean, what would happen? Also, I've forgotton my sizes, what size is a HIF44?

Thanks

[NuFab]

Ok, I'm writing this with a little experience and quite a lot of reading around, so I hope it makes sense.. if it's wrong then hopefully someone will correct me.

Basically, using a smaller carb will result in faster airflow, as the air is flowing through a smaller tube. As the intake into each cylinder is so quickly followed by the next, its pretty much a constant flow of air through the carb and intake and into the engine. Now, if the air is flowing quickly, this aids the quick filling of the cylinders on each intake stroke.

Obviously, if the carb is too small, it can't flow enough air and you'll lose power. If it's too large, it'll flow plenty of air but at a low speed, cutting out the effect I've described above, and losing power.

So... use a 1 3/4 carb, or two 1 1/2 carbs, but the 1 3/4 will be too small and probably lose power until you're at the top of the rev range.

[andy_s]

Thanks a lot, that makes sense. Anyone else got any opinions on what carb I should use?

[Mr_Ed]

Rhubarbs Rhubarbs Rhubarbs

[nightrain]

A single 44 will usually smash the float chamber off the side of the carb if your engine moves too much.

The HIF44 is exactly the same as the HIF38, except 44mm instead of 38mm. ie. the float chamber is under the carb, the clues in the name HIF means 'Horizontal Integral Float'.

[Brumski]

Regardless of whichever you go for, you will need to modify a battery box to put into your bulkhead.

Er.. why on earth would you need to do this...?

I've run twin HIF 38's and twin HS4's in the past and with the correct manifold they fit fine using K&N cone filters and stub stacks inside them.

No need for this level of bulkhead modification for just a set of twins.

[NuFab]

I certainly hope you don't need to modify the bodywork, because that's why I'm going with twin HIF4s rather than a weber!

Brumski, which manifold did you use with your twins? I am a bit concerned about clearance, but I'm using the Maniflow steel inlet so hopefully it'll fit well.

[philc]

you will have no problems with that manifold, just don;t go anymore that 1 3/4 exit you will loose torque

[Brumski]

I certainly hope you don't need to modify the bodywork, because that's why I'm going with twin HIF4s rather than a weber!

You won't need to modify it...

Brumski, which manifold did you use with your twins? I am a bit concerned about clearance, but I'm using the Maniflow steel inlet so hopefully it'll fit well.

I used the standard old school one with a bit of porting to clean up the flow.

You'll be fine with the maniflow one. Knoxy has just fitted twin HIF38's on one of those and they fit fine without body modifications.

[jonnyboi]

thought about using a downdraft weber? maybe a 28/36 or something along those lines. No need to modify bulk head?

[nightrain]

28/36 much better carb than the HIF44, people often discard the weber because of vizards graphs in the bible. They don't seem to realize, vizard tested the 28/36 with very small chokes, just make sure you don't get the fixed choke variety of the carb.

[jonnyboi]

ive been madly recommended to use this carb as they are good all rounders being double chocked it offers good economy driving and good performance.

[nightrain]

Honestly it is a excellent carb, just about everyone I know has at some point had one of these fitted to there 1380. Ok most have progressed onto the 45's but you won't be disappointed and set up correctly much better than the SU.
Depending on manifold you may need to raise the rear of the bonnet.

[jonnyboi]

i had a 28/36 but i lots it and its rhubarbed me right off because now im stuck with my dellorto DHLA 40, which is a good carb but i think it will struggle on a 1380 top end wise?

[nightrain]

Now I'm not been funny here, but what have you actually got for your motor ? If your still looking at buying parts, especially a carb how much have you got to spend. Scrimping on parts at the build stage is nearly always why people end up been disappointed. Building a nice FAST and I don't just mean a quick A-series costs money.

With your above spec, if you've got the cash and are looking to spend it on a carb buy a 45. If and you have already got a 40 use that, but maybe reconsider your cam choice.

[jonnyboi]

tbh i still havent decided on a carb yet, but ive fitted a box in the bulkhead so i can try a few too see which works best with my type of driving. I have access to a 45, a 40 and soon a downdraft 28/36 so im going to try them out to see which suites my style of driving. Maybe something you should concider?

[the wing commander]

You need to have an idea of how many bhp your engine will produce. I am guessing that with that level of spec, you will be talking around 90bhp. A single HIF44 will be okay for that level of power output, and maybe just about up to 100bhp, but after that you need to think about alternatives. Some have already been discussed here. Webers are very thirsty indeed for road use, and you have to modify the bulkhead, not a pleasant job, and to be done properly has to be fully seam welded. You don't want fuel joining you in the passenger compartment. I would go with a single HIF44, and then see what the rolling road brings out. Your tuning person will tell you whether the carb is limiting performance or not.

[nightrain]

tbh i still havent decided on a carb yet, but ive fitted a box in the bulkhead so i can try a few too see which works best with my type of driving. I have access to a 45, a 40 and soon a downdraft 28/36 so im going to try them out to see which suites my style of driving. Maybe something you should concider?

Consider what, your driving style ? So the engines assembled built to the above spec ? With that cam there's only one way to drive it properly and thats in the powerband ?

If you drive around like a nancy, you really need a different cam............

[nightrain]

Webers are very thirsty indeed for road us

This is just a common misconception ! If driven with restraint, my 1380 with a 45 will return 42 mpg, ok it's fairly difficult as planting your foot is quite addictive with these engines. The key is to get it set up correctly throughout the rev range and not just for top end performance. Something most rolling roads don't do !