It's pretty basic on the springs; just like the SU's (CV carbs as well) if the spring is to soft then the needle will rise to quickly and go rich because the needles rise more fuel flows. Think of lighter or heavier oil in an SU. When you get up and running make sure you have an air/fuel meter or head to the dyno. It's not uncommon to have the full throttle jetting perfect but then find out when you roll on the throttle the carbs temporarily go rich. The SU tuning link is a good guide even though your CV carbs use different method of changing the jetting. My buddy did a rack of 6 Yamaha Carbs on his Ford straight 6 engine and beyond the normal jetting he played with various spring rate to get it really dialed in.

Tom

Thanks again Tom.

I fired it up today. Revs quite nicely and I'm pretty impressed with the throttle response. The carbs impress me, they even have vacuum reference lines for tuning and the synchronizing adjusters are well designed. Only issue I have is the idle adjuster is way under and tricky to get to when things are hot.

I have a fuel issue with the first two carbs where they don't seem to be getting as much fuel as the two closer to fire wall, sometimes I wonder if they're getting any at all.

The two closer to the firewall are lower than the first two so I'm hoping that it isn't gravity related, but rather a blockage.

I'll get to pulling the carbs apart. Fingers crossed I find some crap....but not too much.

I have a short video but I'll hold off until it's running nicely.

The lack of fuel to the front carbs isn't to do with the restrictor you inserted in the line to limit pressure (and flow) is it?

You should not have a problem with flow overcoming the needle and seats, it is purely pressure that will affect them.

Where is the fuel line connected to? The rearmost carb? How does it then get fuel to the remaining carbs, via an internal rail? If it is a rail I would think that the first two carbs are taking the bulk of the fuel and there isn't enough fuel left to keep the front carbs supplied?

Hey Dean,

Nah the restrictor is a off the shelf motorbike fuel tap located closer to the fuel pump, that seems to be working well, and I'm confident it won't strain the facet pump.

I found the carb closest to the radiator was completely clogged in it's fuel supply into the carb and bowl, so that has been cleaned up and put back together, all galleys cleaned and I blew air through all to check flow and seal. The second carb seemed fine so I'm not sure what is happening, I should have had at least had 3 cylinders earlier on....

The fuel rail built into the carb rack begins halfway along the rack, so between carb 2 & 3 and dead centre along the rack, so the feed is fairly even rather than staggered.

I have spark on all four cylinders, the two back cylinders are doing all the work and the two front ones aren't doing anything, no firing.

I have checked all spark plugs and swapped leads, still no proper firing on first two cylinders closest to radiator. Upon inspection of the plugs there is no evidence of combustion, no soot, no carbon. Just a clean plug, no sign of fuel. Hopefully I have cleared the passages and it'll all work, unfortunately I've run out of battery, so I'll have to wait till the second car gets back.

Spewin.

Had to give the motorbike carbs the flick! I couldn't get all four cylinders to fire.....

I'll pull them apart again and have another look.

Has anyone got a smart way to see if they are supplying fuel? I manually choked them and felt/saw fuel that had spat back so I presumed they were fueling.

I then threw the sideraught Mikuni 40's on and it runs sweet...... weirdest thing ever?!

EXACT SAME PROBLEM AS ME!! I HAVE THE MIKUNI VERSION OF THESE AND HAD THE SAME PROBLEM OF TWO CARBS FLOODING AND TWO BONE DRY AND THE ENGINE RUNNING ON LIKE 2 OR 3 CYLINDERS. WHAT THE?? MINE WERE ON MY A15.

Just thinking about this boys, it's not the front to rear lean the carbs are on is it? In a bike they would sit level left to right i.e. along the length of the carbs. But in the datto they will be tilted down from front to rear (or left to right idf they were still on the bike?).

Yo Dale, when you gonna come on a cruise?!


Deano, that's what I was wondering before.....but;

I also stuffed up a little and fiddled with the ignition between the induction changeover. So when I took the bike carbs off I also muddled with the ignition timing, which is a variable that I didn't want involved!

Strange things though as I am absolutely convinced they were fueling......bugger.

The throttle response is pretty amazing though with the Mikuni 40's - neighbors weren't too happy with the noise...

Chris,

how does the fuel flow work on those carbs? Does each carb have it's own float/needle and seat? Or is there only the one needle and seat at the inlet and then each carb bowl is filled from the overflow of the previous (or by an internal passage?)? My bike is injected (and there are only three TB's) so no help there!

You should be able to check if the front/rear level of the carbs is the issue by simply jacking the rear of the car until the carbs are level.

The carbs share a built in rail that spans the entire rack and the feed to the rail begins between carb 2 & 3, you may be right about the angle, there is a distinct yet minimal angle across the carbs but I'm still not convinced that that's the reason (althought they certainly do sit east-west on a bike).

Each carb is fed from the rail into it's own bowl, there are no restrictions, junctions, bottlenecks to warrant any carb getting more than the other.

The more I think about it the more I reckon I got the timing wrong, but I checked it at least ten times. So I have no clear idea what's going on.

I'm stripping the carbs, completely rebuilding them and trying again. In the meantime I have ordered another set (Mikuni equivalents) to try and get it going again.

But there only three things;

Spark- Timing of spark

Carbs- Faulty?

Manifold/Rack- Angle

As for timing I'm running sidedraughts now with firing on all four.