Tuning Advice sought on carbs

[tonyX]

hi, newbie on this group here, should have joined earlier. Anyway I have a '74 X 1/9, the ones here in NZ are UK spec vehicles. Obviously it is (was ) a 1300/4spd, which was embarassingly slow on hills, so Ive transplanted a 1500/5spd unit to which Ive done a little work. Firstly for anyone interested, yes the new unit does more or less drop straight in without any metal needing to be cut, but (I found) you also need the 5spd drive shafts, as the 4 spd uses those tripode type things. Ive had some head work done, and a 'fast road' cam regrind, by ppl here who are generally held to be reliable (Harris Developments and Franklin Cams, respectively, but obviously that wont mean anything to anyone outside NZ). I also fitted a single 40 DCNF, mounted on an enlarged standard manifold, so yes it does have a lean bog on a RH corner. Work in progress. I had a tubular exhasut manifold made up mainly to avoid hot-start issues, having a huge heat sink right under the carb isnt a good idea. A few pics attached (or should be, this is the first post Ive made).
But my specific question is this. It seems to run out of puff at about 5500 rp, just no point in going any further - midrange torque is great, a pleasure to drive, but just wont really wind out. Would there be benefit in a double DCNF setup? And if so, why is there benefit. Surely with the single setup, at any one time, each cylinder is sucking on both barrels: with a double setup, the same is true as the carbs dont 'talk' to each other (ie no balance tube or anything, as far as I can see). Is the benefit from intake geometry? Or am I wrong with the balance tube observation. Perhaps there is something fundamental I dont understand. many thanks for any advice!

 

[bugsymike]

hi, newbie on this group here, should have joined earlier. Anyway I have a '74 X 1/9, the ones here in NZ are UK spec vehicles. Obviously it is (was ) a 1300/4spd, which was embarassingly slow on hills, so Ive transplanted a 1500/5spd unit to which Ive done a little work. Firstly for anyone interested, yes the new unit does more or less drop straight in without any metal needing to be cut, but (I found) you also need the 5spd drive shafts, as the 4 spd uses those tripode type things. Ive had some head work done, and a 'fast road' cam regrind, by ppl here who are generally held to be reliable (Harris Developments and Franklin Cams, respectively, but obviously that wont mean anything to anyone outside NZ). I also fitted a single 40 DCNF, mounted on an enlarged standard manifold, so yes it does have a lean bog on a RH corner. Work in progress. I had a tubular exhasut manifold made up mainly to avoid hot-start issues, having a huge heat sink right under the carb isnt a good idea. A few pics attached (or should be, this is the first post Ive made).
But my specific question is this. It seems to run out of puff at about 5500 rp, just no point in going any further - midrange torque is great, a pleasure to drive, but just wont really wind out. Would there be benefit in a double DCNF setup? And if so, why is there benefit. Surely with the single setup, at any one time, each cylinder is sucking on both barrels: with a double setup, the same is true as the carbs dont 'talk' to each other (ie no balance tube or anything, as far as I can see). Is the benefit from intake geometry? Or am I wrong with the balance tube observation. Perhaps there is something fundamental I dont understand. many thanks for any advice!

Would lagging the exhaust manifold help re heat, I had to do that for a turbo on a diesel in a marine project.
Would larger main jets help on top end, has it been run on rolling road to see if leaning out at 5500rpm or maybe a hard drive at that point followed by clutch in and ignition off then pull spark plugs to see if weak mixture as a simple test.
Many years ago I shared a seat racing karts at Thruxton, the other driver had problems with oiling up plugs so a smaller main jet was used but not altered for my "harder driving" the result was an engine seize at high speed due to a very weak mixture, luckily my reactions then were quicker than now.
Re benefits of quad carbs against twin choke could be with the basic twin choke set up the fuel/air flow has to change direction to feed different cylinders unlike multi carbs/carb per cylinder?
Re the fast road cam, is it designed more for mid range than top end like a race cam which may be undrivable around town, but allow better breathing at top end revs?
Is there any chance fuel pump is not giving enough flow at high revs?
On a different line, a lightened flywheel helps with rapid pick up. As a poor apprentice I fitted a 1500 engine in a Ford Anglia, but then found an improvement by fitting the original 997cc flywheel whilst still using the 1500 clutch, it would rev to 7000rpm. Doubt if the 1300 Fiat one would fit the 1500 as Fords in the old days were very interchangeable. It could still be skimmed if local tuners feel it a good idea.

 

[tonyX]

Would lagging the exhaust manifold help re heat, I had to do that for a turbo on a diesel in a marine project.
Would larger main jets help on top end, has it been run on rolling road to see if leaning out at 5500rpm or maybe a hard drive at that point followed by clutch in and ignition off then pull spark plugs to see if weak mixture as a simple test.
Many years ago I shared a seat racing karts at Thruxton, the other driver had problems with oiling up plugs so a smaller main jet was used but not altered for my "harder driving" the result was an engine seize at high speed due to a very weak mixture, luckily my reactions then were quicker than now.
Re benefits of quad carbs against twin choke could be with the basic twin choke set up the fuel/air flow has to change direction to feed different cylinders unlike multi carbs/carb per cylinder?
Re the fast road cam, is it designed more for mid range than top end like a race cam which may be undrivable around town, but allow better breathing at top end revs?
Is there any chance fuel pump is not giving enough flow at high revs?
On a different line, a lightened flywheel helps with rapid pick up. As a poor apprentice I fitted a 1500 engine in a Ford Anglia, but then found an improvement by fitting the original 997cc flywheel whilst still using the 1500 clutch, it would rev to 7000rpm. Doubt if the 1300 Fiat one would fit the 1500 as Fords in the old days were very interchangeable. It could still be skimmed if local tuners feel it a good idea.

thanks for the response. All food for thought. With unleaded fuel its a bit hard to take a diagnostic plug cut alas, but I'll try the richer mains to see if that makes a difference. Flywheel has already been lightened. I dont think revving freely to 7000 is asking too much for a fast road cam, esp for these Fiat engines. Im still going to try locally for a twin carb setup but pretty scarce arouind here...

 

[porta]

I had a 128 coupe 1300 where the twin choke carb failed, it was worn, jets seized and basically not worth repairing. A 1500 Lancia Delta was traded in that had been modded, it had twin twin chokes on a two to four manifold.
As the car was headed straight to the auction (it had lived a hard life) I asked if I could have the carbs and manifold…the swap was a revelation, quicker pickup and much faster between gears, with very negligible effect on fuel economy