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

Classics Monthly - - Triumph 2500 - WORDS AND PHO­TOG­RA­PHY SI­MON GOLDSWOR­THY

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.

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