Carbs Vs Injection   By Dave Walker

I got sucked into this one via an internet interest group. James Mortimer had bought himself a nice Westfield with an 1800 Zetec engine on 40DCOEs. It was a standard engine but with the 130 bhp cams, not the lesser spec 105bhp jobs. The idea was to have a car that he and his wife could enjoy that summer without the hassle of building it. However there was one small problem; a flat spot on light throttle.

If you saw the car you would know that James isn’t a man to muck about, you could eat your proverbial dinner off any part of the car. He asked around and was told that he needed new carburettors. Not having money to burn he then shopped around for the best deal and duly fitted the new carbs. That’s when the real trouble started. The new carbs had a bigger flat spot than the old ones.

He got on the phone and started asking various experts what he needed to do to get the car to stop kangarooing down the road on light throttle. One renowned expert told him that the Emerald ECU that was fitted was causing the problem and that James needed to buy his in-house ECU. Having been caught with the new carb story James was reluctant to spend more money without further checking around. That’s where I got involved as it was my Emerald ECU that was being blamed for the problem.

Obvious

It’s bloody obvious that it wasn’t an ignition issue but it’s hard to convince people who are new to engines. James is a Jeweller by trade and he knows a good watch when he sees one but his automotive experience had so far been limited to being sold two new carbs which he really did not need.
After a chat on the phone I suggested he being the car up to Norfolk and I would give him a free day on the rollers and look at the problem. The car duly arrived and straight away the problem was obvious – to me at any rate. Snap throttle opening stalled the engine but we found some bigger pump jets and that cured that one.

The part throttle stutter was a progression problem. Unfortunately we had only a limited number of jets available; basically my leftovers from ten years back. I did the ignition mapping and checked the full throttle fuelling but maximum power was only 120 bhp or so. The day was not an unqualified success because the stutter was still there and the power wasn’t what it should have been. I explained to James that what it probably needed was the progression holes re-drilling but this is a one-way street and the carbs are not much use again if you get it wrong.

Then the internet experts got involved. “Maybe it’s this, maybe it’s that” it went off at all sorts of tangents. Some experts guaranteed they could sort the problem in a matter of minutes but none of them actually did. I think the expression is: “well done is better than well said”.
I have never actually banged my head off a brick wall but I am told that it hurts. For pity’s sake, it’s a simple mixture progression problem. Now I really felt the need to prove a point. I talked to Jenvey Dynamics who make the throttle bodies and they agreed to supply some kit at cost price for a back-to-back test. I threw in an ECU upgrade and another free day on the rolling road. Jenvey knew what the result would be just as well as I did but by now I was on a crusade.

James made a superb job of fitting an SBD swirl pot and he installed some high pressure fuel lines and an injection pump. The cams were checked and found to be 105 bhp items! These were replaced with the right profile. He came up to Emerald still on the carbs and we fitted the bodies. I had to make up a simple (temporary) throttle cable bracket and I had to test some second-hand injectors we had laying around and then install everything ready for mapping. James was a bag of nerves because he didn’t know what the outcome would be and his wife had recently given him some advice: “sell the car, it’s a lemon”. If this didn’t work he was going to have to take her advice.

Would it start? Of course it would… I’ve done hundreds of these bloody things. It took a few turns to clear the injectors of the test machine fluid and then it burbled into life. I had a good base map to work from and a few taps of the keys sorted the idle mixture and it sat ticking over as smooth as you like. James leaned over and started snapping the throttle open. “It works, it works!” and a big grin came over him.

I started mapping but we ran into a minor glitch after about ten minutes – the fuel rail sprang a leak. This was due to a cut in an o-ring (assembly glitch) which we replaced and were under way again about half an hour later. I had specified the longer 90mm intake trumpets as they give the best result on this engine but James’s air filter would no longer fit. James managed to force the filter on but the trumpets were hard up against the foam – reducing the air filtering area by about 70 per cent. On the first power runs we only had 130 bhp. Karl saw the filter and had a bit of a giggle. We pulled it off and remapped to suit the improved breathing. The end result was 152bhp.

More to the point it was smooth everywhere in the load and rpm range and no more kangaroo impersonations. The whole point of injection is that you can get the fuelling spot-on everywhere regardless of what the engine breathing is doing. A carburettor is a very poor compromise by comparison and it is only since we got accurate mixture reading/logging kit that we started to understand how bad carbs really were.

James’s report

“The journey home was fabulous, 200 miles of bliss, the car is completely changed and a joy to drive. Fuel consumption has improved; I’m getting about 40 miles more to a tank full and the performance has gone up, it’s a win, win, win situation. On the way home there were road works on the M25 for about 12 miles, the car was happy in fourth gear at 40mph and just went when you put your foot down. On carbs it would have been third gear and kangarooing down the road.

The only problem I’ve got is this stupid grin on my face all the time and apparently it’s getting annoying. Work is also becoming an issue because all I want to do is get out there and drive the car. I think it’s safe to say that I am a total throttle body convert!”