On a roll
Last issue, we left the 2500TC story having visited Colin Radford of the 2000/2500/2.5PI Register and compiled a list of jobs that needed doing. But before wading in with the spanners there was one more assessment we were keen to carry out, and that requi
This issue we visit the Sleaford Mini Centre who, as well as specialising in Issigonis’ Mini marvel as the name suggests, also possess a rolling road and lots of classic tuning expertise. But why take a bog-standard estate car to a rolling road? After all, we are hardly looking for maximum power figures that will give us bragging rights down the pub! However, power runs are only one side of a rolling road session; of far more interest to us is the opportunity to fine tune an engine so that it is working to the best of its abilities. The £80 this costs will be money well spent – not only can it deliver the most bhp per £ by stopping us from spending a small fortune on parts that are simply not necessary, it will also highlight problems with the fuelling, ignition and basic state of engine health. As a result, we will be able to direct the money we do have to spend towards those areas where it is most needed.
Before heading to Sleaford though, I had to do something about the breather hoses linking the rocker cover to the carburettors. The one to the rear carburettor in particular was perished and collapsed, and this could have been enough to unbalance the fuelling and limit the benefits that we could get from the rolling road. So I ordered a new set of three pipes from Chris Witor, and fitted these one snowy morning. It was so cold that I had to warm the ends of the pipes up gently with a hot air gun to make then supple enough to slide onto the plastic T-piece, but they look so much better in the engine bay and are strong enough to support their own weight rather than sagging to rest on the manifold.
I also replaced the hardboard that sits on the transmission tunnel and goes around the gearstick. This had warped, and the gearstick gaiter had come free. Given the long travel on the gearstick, particularly in reverse, this left quite a gap for transmission noise to get into the cabin. I cut a replacement out of 6mm plywood, which is a little thicker than ideal but did at least give me something solid to staple the gaiter to. I also transferred across the old vinyl-and-foam cover.
The new panel wouldn’t clip securely into place using just the original clips, but this was clearly an ongoing problem as the previous cover had been screwed into place. I simply reused the three holes that had already been cut into the vinyl and followed suit by screwing it to the transmission tunnel. Finally I traced a weak screen wash to the non-return valve in the bottle being blocked with gunge. With that cleaned out, I was ready to visit Steve Olive and Mike Greaves at the Sleaford Mini Centre.
First Mike explains a little about what to expect, saying: “When tuning, it is mainly a matter of looking at the fuelling and the ignition timing. Nine times out of ten, measuring the fuel in the exhaust will give you a good indication of the engine’s general state of tune, and also how worn it is.
“Depending on the state of tune, you generally start at factory settings and then modify them to suit the particular engine. We check the fuelling by measuring the air/fuel ratio in the exhaust fumes. Lambda – or 14.7:1 – is your best burn, so that is what you want for cruising and low throttle openings, but maximum power occurs at 12.5:1 because you obviously need more fuel to produce the power. You have to make some decisions as to where your goals lie as you can’t tune it for both maximum power and maximum economy, but if you have it tuned correctly for power, then you will be using less throttle to achieve that performance, so in turn you will be using less fuel.”
And so Mike sets to work on our Triumph, putting a clip around one HT lead so the computer can read the rpm and a probe in the exhaust to measure the air/fuel ratio. Straight away, we can see that the engine is running extremely rich at tickover – 9.5:1 on the air/fuel mix at 830rpm. The first gentle run takes it up to 2000rpm. Nothing falls off and the engine doesn’t sound like it is about to self-destruct, so Mike then pushes it on up to 4000rpm. This is the rev limit we have agreed upon given that there is endfloat on the crank that needs addressing – peak power only comes in at 4700rpm, but it would be foolish to ignore Colin’s advice last issue that we should take it easy until the bottom end was sorted.
Mike examines the readings and says:
“Power measured at the wheels was 68.7bhp. The graphs generally don’t look too bad, although power is dropping off too early at 3650rpm. The fuelling gets better at higher revs, indicating that it is only running rich at idle. This shows you have the correct needles, ones that have been calibrated to match the small inlet pipes feeding the airbox, which act as the main restricting factor for airflow into the engine. We get most problems regarding needles when people change the air filters to let more air in, but don’t also change the needles to match this with more fuel.
“Timing is more of a problem on this car. This is over-advanced at 25° on tick-over, rising to over 40° at full advance. The spread is about right, but it should be starting from about 8° at idle. This could be contributing to the slightly rough sound of the engine because that kind of advance can make a car sound a little rumbly, not to mention suffer from pinking or difficulties in starting of a morning. You probably haven’t been having starting issues though, because the rich mixture means you effectively have choke on all the time!
“The points gap alters the ignition timing, so you always set this first,” he continues. “If that doesn’t correct the timing, then you move the distributor. The gap on these points is 27 thou’ instead of about 14 thou’, so that will be a large chunk of your problem. There is a bit of wear in the distributor, but nothing excessive. Fitting an electronic ignition would be a good idea because play in the shaft alters the gap on points, which then affects the timing. With an optical eye instead of mechanical contact breaker points it is just a matter of breaking a beam, so that is not affected by sideways movement in the shaft. It doesn’t solve the wear issue, but stops it from having an effect on the timing.”
With the points gap set up correctly, the timing advance had reduced to 16 degrees. Mike’s next step is to give it another run on the rollers, just to see what effect this has had and make sure we are moving in the right direction. The change is
impressive. It hasn’t made a massive difference to low-down power, but the top end has moved up from 68bhp to 73bhp, plus the new curve carries on rising past our 4000rpm rev limit. The air/fuel mixture is also leaner even though we have not adjusted this yet, simply because the engine is burning the fuel more efficiently.
With this conformation that we are definitely moving in the right direction, Mike then adjusts the distributor to set the timing at 10° BTDC. “The manufacturer data is based on old four or five star fuel and a new engine, so you have to be a little bit flexible and get what’s right for your car,” he explains. “Idle speed is now down to 750rpm too as a result of changing the points gap. If I now adjust the timing at the distributor, see how it drops further to 700rpm.”
Another power run shows how tuning is always something of a compromise. This time there is a slight reduction in top end power, but it has gained torque all the way from 2000 to 3100rpm, and that is what you are going to use on the road. If we were still trying to chase that last one or two horsepower, then we’d need a custom distributor whose bob weights and advance curve could be tailored exactly to our engine. As it is, for real world driveability our timing is exactly where it wants to be.
Because the engine is running better, the air/fuel mixture has improved slightly at idle, but the next things to check are the carburettors. Taking each damper out and putting it back in while the engine is running will affect the idle speed as it sucks the dashpot piston up and pushes it down. The important thing is that you want both carbs to do the same thing – balance is key in a twin carb set-up.
Mike checks the throttle spindles for wear, but can’t detect enough movement to cause him any concern. Taking off the dashpots and putting the pistons and needles to one side, he then looks down the carburettor bodies at the jets.
“These jets are nice and round and not worn oval,” he says. “But one jet is sitting slightly lower than the other. We want to lean the
engine off, so we will lift the lower jet on the back carb to match the jet height on the front one. Then, once they are level, we’ll know that if any subsequent adjustments are made equally to each carburettor, they will stay in balance.”
And so we run it up on the rollers one more time, and it sounds great. Back at idle, the ‘fluffiness’ from the exhaust when you rock the car is gone too. “It’s still ever so slightly on the rich side,” concludes Mike, “so you could change the needles and/or the jets, but if it were my car I wouldn’t worry about that. It certainly sounds much smoother as the revs pile on. To be honest it didn’t need a lot of work. The points gap was probably the biggest thing, as that was throwing out the ignition timing.”
So what figures did we get? For power we ended up with 78.3bhp at 4050rpm, and for torque we got 115 lbf.ft at 2900rpm. Those were at the wheels, of course. It is a rough science, but you would generally reckon on adding up to 20 per cent to get flywheel figures. That in turn would give us 93.6bhp and 138 lbf.ft. They compare well to factory figures of 99 bhp and 133 lbf.ft, and don’t forget that the factory bhp was achieved at 4700rpm, whereas ours was still rising at our self-imposed rev limit of 4000rpm.
All in all we have a healthy engine that seems perfectly in keeping with a recorded mileage of just under 60,000. And as for a rolling road tune-up, I think this has to be one of the best investments you can make in your car. Pound-per-bhp it is superb value, and while you may not get something physical like a K&N air filter or some fancy plug leads to show for your money, getting what you already have to work more efficiently may mean you no longer need to spend money on other parts, and will certainly save you money on fuel. Plus, if you have been making other changes, it will bring them all together so they work in unison to achieve their maximum potential.
Our thanks to Steve and Mike at the Sleaford Mini Centre. They can perform power runs on any two-wheel-drive car up to 500bhp, and tune anything on carburettors. Call them on 01529 460049 or visit www. sleafordminicentre.co.uk
3 The vinyl and foam had also come unstuck from the parcel shelf – contact adhesive and overnight clamping fixed that.
4 The carburettor breather hoses were perished and one was collapsing entirely, which would upset the balance.
5 A set of three new hoses from Chris Witor cost a total of £21.84 including delivery, and arrived promptly.
6 Non-return valve in the screenwash pick-up pipe needed cleaning to restore proper cleaning action.
1 The hardboard surround to the gearstick had become warped, and would no longer hold the gearstick gaiter in place.
2 A replacement cut out of 6mm plywood gave a flatter surface for the vinyl, and something to staple the gaiter to.
7 At the Sleaford Mini Centre, the 2500TC was strapped down to hold it in the rollers.
8 A clip was put around one HT lead so the computer could pick up engine RPM...
10 A fan was readied to give cooling airflow into the radiator, but in sub-zero temperatures it was not needed.
9 ...and a probe was clamped to the exhaust tailpipe to measure the air/fuel ratio.
11 Mike took the car up to speed on the rollers, but kept it to our self-imposed red line of 4000rpm.
12 The computer collated the figures and gave Mike his first set of data from which to work.
19 Final printout overlaid the last run with the first, and showed a big gain in performance and fuel efficiency.
14 This timing problem was caused to a large extent by the points gap being too wide at 27 thou’.
13 A timing light showed that the timing was too far advanced, sparking at 25° before TDC even at idle.
18 The jet on the rear carburettor was adjusted until it matched the height of the one at the front.
16 Pulling the dashpot plungers up and down caused the engine revs to change, but crucially by the same amount for each carb.
17 Mike’s home-made carburettor piston and dashpot storage unit is actually a triple carb model!
15 Setting the points gap to 14 thou’ showed an immediate improvement to timing, power and fuel economy.