calling engine/carb experts - 1776 won't run at speed?

Hi all,

Just got my 1776 in my bus, fitted with twin dellorto DRLA 40s.

Runs fine round town up to about 3k revs, although when it's cold it does hesitate a little and pops under load (e.g. pulling up a hill), although this gets better when you've done a couple of miles.

Took it for a spin on the motorway and got up to 60ish fine, but then started to die - with my foot down speed just kept dropping - luckily it's only a small section of motorway so by the time it got down to 45mph I was able to turn off the motorway, then as soon as I came off the slip road, it pulled ok up to about 60 again.

Spoke to the guy I bought the carbs off who suggested that it sounds like fuel starvation.

I have connected the fuel feed into the smaller barb on the fuel union on the left carb then a larger pipe from the larger barb on the same fuel union to the larger barb on the right hand carb.

He suggested that I may have the fuel pipe connections on the left carb the wrong way round, and when on the main circuit (i.e. not the idle circuit) not enough fuel is being delivered to the carbs so the float chamber is being emptied and not enough fuel is getting through to the carbs as the flow of fuel is restricted by a small hole in the smaller barb in the fuel union.

Did notice however a previous post before I fitted the carbs where Zed stated that it doesn't matter which way round the fuel lines are connected on the left hand carb.

can anyone shed any light on this or suggest what else could be causing the problem?

Got an SVDA dizzy with the vacuum pipe connected from the dizzy to the brass tube on the left carb that sit's on it's own slightly above the other 2 brass tubes (which I think are to connect a gauge?) so think that's connected up right.

Any suggestions?

 

Those symptoms could be electrical, the HT system heats up at the constant higher motorway speeds and starts to break down, when you ease down and start motoring at a less constant speed it kicks back in again.

 

I'd try that. Both connections are exactly the same size on my double banjo. In the big one and chain off the small to the other carb makes sense. I had an over fuelling problem myself, mainly in the carb on the end of the chain. That led me to believe that chaining them up does lead to different pressure at each carb, so I blocked one connection on the double and T'd off to each carb seperately.

 

does it sound like fuel starvation to you Zed?

 

Are you sure your throttle linkage is actually opening the throttles more than a small amount.
You might just be running out of power on a slight uphill section.

 

The small feed pipe on the carb, isn't a feed pipe... It's actually a return for sending excess fuel back to the tank...

So the carbs would've originally been set up in a loop... One carb would've had two pipes the same size, and the other one large and one small pipe

 

it could be several things tbh...^^^ I'd check what Mike said above first to rule it out, then i'd double check the total timing with the vac disconnected....book timing is good for stock set-ups, and not necessarily good for a modified 1776...
you can eliminate the fuel feed issue by running a line from the pump/regulator to a T then to each carb...if you want to be exacting, you can cut each line to the carb the same length (although in practice it doesn't make any difference as pressure is pressure and flow is flow)
If the fuel pressure is adequate (btw 2.5 and 3.5 PSI at idle) it should be more than up to the job of supplying both carbs at higher RPM
if the engine is physically slowing down, rather than just running like a crock above 60, then it does sound fuel delivery related, but it could just as easily be too much fuel from too large mains or the chokes are too big..it's a moveable feast unfortunately

did you fit a new fuel filter?

 

^^^this

 

My dell set up is
Mechanical Fuel pump to Filter King fuel pressure regulator to t piece to one fuel pipe direct into each carb (each carb is now single banjo version)

I hope that is helpful

 

thanks guys - all very helpful and very much appreciated.

Timing is spot on, although I must admit thinking about it I did check the timing with the vac pipe still attached - not sure if that will make a massive amount of difference.

Think the carbs were jetted about right for a 1776.

When I had the carbs on the engine out of the car, the butterflies were opening fully, but I will check that again now it's all in the us & connected up (although it pulls like a train up to about 3k revs).

Hopefully it's as simple as this fuel supply connection, so I will run a line from the fuel pump to a T (fuel line from pump is 6mm so will need a 6mm-8mm-8mm T) then run 8mm pipes to each of the larger tubes on the banjo's then blank off the smaller tube on the left banjo (although what happens to the excess fuel?).

fuel filter is new, but I must admit I guessed at which way round the fuel lines go on the pump as there is no 'in' or 'out' arrows - I assume that if I had got the pipes the wrong way round it would not run at all??

Thanks again - will let you all know how I get on when I locate a suitable T piece.

 

it will give you a false reading if the vac's still attached...your total timing will be retarded if you've set it up to 32 degs @3500 with vac...in some circumstances you can be running at 45+ degs of advance safely

do this before messing with fuel lines and go for a test drive

 

by the same token, if you've set up the idle mixtures with the vac connected, this will also be incorrect...which could account for the lean running under load when cold

 

Or you could buy a single banjo and then you don't need to worry about fuel leaking out of the blocked off pipe.

Or if you block the pipe off make sure you use a proper fuel clamp at BOTH ENDS.

Here is athread about it!

http://thelatebay.com/index.php?thr...serious-fuel-leak-learn-from-my-errors.50347/

 

Just remember at up to 3000 rpm you are on idle jets.

It could be that the main jets are oversized and flooding the engine under load.

Or the fuel level rises after you floor it and dribbles down the carb throat and floods it. In that case it will go extra burbly and wobbly around 30mph on a flat road in 4th.
For myself this would be easy to diagnose as the little PLX devices meter on the dash would be saying 10:1 air fuel ratio instead of 12.7:1 as I floored it. So I would fit either a larger air correctors or fit a smaller main jets .Meter is £170 plus fitting. Almost as cheap as a couple of rolling road sessions.

 

One day someone else will buy one Mike. I'd like one, it's on the list.

 

I will have to knock up a tube with a lambda sensor bung in it for poking up other peoples exhausts and bring it to techenders.

 

I looked into it when I got my exhaust which has a place to fit them either side, then when it came to which exact sensor to get to work with a cheapy Chinese guage I gave up. lol And it was adding up to more ££'s than I could spare at the time.

 

same here....looking at the innovate stuff...LC2 coming down in price atm
for now, it's the trusty old colourtune....

 

re-connected the fuel supply from pump to a Tee than branched off at 8mm to both carbs (larger barb) then blocked off the smaller barb on the left carb this morning.

Then took it for a spin - woohoo, 75 on the motorway & still pulling - goes ok for a 1776 !! (splitty, don't forget!).

sorted - thanks all.