Tune Up Time

ALAN COCHRANE · Post by **ALAN COCHRANE** » Sat Feb 27, 2016 6:38 pm

Hi Everyone

I've just completed the long promised tune up in readiness for the driving season. I had a splashy misfire originating from the rear carb which only disappeared when I turned the mixture to slightly rich. The other two carbs were perfect with a lovely "Bunsen Blue" seen on their respective colourtune plugs. For the first time I've managed to get the carbs perfectly balanced as well. But I think the rear carb and probably the other two as well are looking at a rebuild in the near future.
Anyway does anyone know what CO level I'd expect? I was going to check it with my Gunson Gas Analyser tomorrow.

Cheers

Alan

abowie · Post by **abowie** » Sat Feb 27, 2016 9:38 pm

5%

A trick with the mixtures is to use a vernier caliper to measure how far below the bridge the jet is set. You can then reset all 3 to the average. Then you can fine tune the mixture knowing that you will end up with all 3 carbs set the same.

ALAN COCHRANE · Post by **ALAN COCHRANE** » Sat Feb 27, 2016 11:25 pm

Andrew

Yes that's exactly what I did. Although the amount of adjustment after that was anybodies guess! Despite that the car has never sounded sweeter. It has a very steady idle at 800 rpm. If only it wasn't for the damn salt I'd be off on a road trip. Emigrate to Australia maybe?

Alan

abowie · Post by **abowie** » Sun Feb 28, 2016 12:08 am

ALAN COCHRANE wrote:Andrew

Emigrate to Australia maybe?

Alan

The sensible members of many countries have done this since WW2.

We have the smartest British, Italians, Greeks, Vietnamese, Lebanese and Sudanese people on the planet. And a fantastic cuisine to match it.

ALAN COCHRANE · Post by **ALAN COCHRANE** » Sun Feb 28, 2016 9:12 am

[We have the smartest British, Italians, Greeks, Vietnamese, Lebanese and Sudanese people on the planet. And a fantastic cuisine to match it.]

Well that would rule me out then!

Alan

Durango2k · Post by **Durango2k** » Sun Feb 28, 2016 10:08 am

... Me too....

Carsten

JagWaugh · Post by **JagWaugh** » Sun Feb 28, 2016 11:12 am

abowie wrote:5%

A trick with the mixtures is to use a vernier caliper to measure how far below the bridge the jet is set. You can then reset all 3 to the average. Then you can fine tune the mixture knowing that you will end up with all 3 carbs set the same.

If everything is in good original condition (correct needles, no false air etc) then 4% would be ideal (and achievable). If in doubt, 5% is a safer target. Above 10% and you're starting to wash the lubricant off the cyl walls. You want the rearmost carb just slightly richer than the others, as the rear of the engine can't shed heat as readily as the front.

The vernier method is good, but the carbs still need to be in good nick. If the carbs and ignition are not right then you can sometimes fiddle with them to achieve a compromise, but not dial in to the optimum.

I usually put new needles in when I do a rebuild: you would be surprised how often it turns out that the needles you remove are not the same profile as the new ones. New correct needles are a lot less expensive than putting a exhaust sensors in.

ALAN COCHRANE · Post by **ALAN COCHRANE** » Mon Feb 29, 2016 9:52 am

Well I unpacked my Gunson gas tester for the first time and actually read the instructions, which is something of a first for me.
It was well worth the effort since my initial reading were 5.8% for the front exhaust and 6.5% for the rear(left and right). This differential didn't surprise me since the rear carb was running slightly rich.
I was going to leave it at that but my tinkering habit got the better of me and I adjusted the mixtures again. Surprise, surprise I got them both down to around 5% and the idle sounded even better than before with no intermittent misfire from the right exhaust this time.
I did however have a problem with the engine stalling after revving and returning to idle. The culprit turned out to be a sticky piston on the middle carb. The inside of the bell showed some quite deep scores which I carefully filed down with emery paper and seemed to move freely once refitted. Any ideas what would cause the scoring-to my mind it can only be the piston but how?
The gas tester is a real multifunction tool and so I decided to measure the rpm at idle to see how accurate the rev counter actually was. I was pleasantly surprised to find it was very accurate. The tester measured 800-820rpm and the rev counter was indicating slightly above the 800 mark. Not bad considering it's still fed off the cam powered generator.

Post by **mgcjag** » Mon Feb 29, 2016 10:23 am

Hi Alan.....before i rebuilt my carbs i went to a seminar at CMC....David Lonsdale the carb restorer was showing us over carb resto.....he said it was important to make sure that you always put the pistons back into the carb they came out of... same with the dashpot covers but most important to keep them the same way around that you took them off

ALAN COCHRANE · Post by **ALAN COCHRANE** » Mon Feb 29, 2016 10:45 am

Steve

I always keep the piston and bell housing together when they are off the car. In this case I only removed them one at a time anyway so no chance of mixing them up.
The pistons have to go back the same way anyway because there's a slot cut down the side to match the brass protrusion on the front of the carb body. Similarly the bell can only fit back on one way due to the screw mount positions.
Thinking back I did find quite a few grit granules inside the bell. I washed these out with detergent and hot water. At the time I thought it may have been caused by the emery paper but now I'm not so sure. Could grit have come from fuel coming out of the jet and then being sucked up onto the inner face of the bell. I have to confess I ran the engine without a filter for a short while when I was having my earlier fueling problems which I can now confirm was caused by the Lucas pump which I've now bench tested.

Alan

Pim · Post by **Pim** » Mon Feb 29, 2016 5:37 pm

How much of a problem do I have then? I bought my car totally disassembled to the last nut and bolt, carbs disassembled as well.

JagWaugh · Post by **JagWaugh** » Mon Feb 29, 2016 7:28 pm

How much of a problem? Let me count the ways....

Actually, it isn't a big deal. Clean the bells and pistons, using a felt pen mark the bells A, B and C, and mark the pistons 1, 2, 3. now do all 9 possible combinations as a drop test and record the values. Repeat the test series a couple of times and average the values. (they should be 7-10 seconds). Now look at your chart of values and select the combination that gives the most consistent value.

I always do a drop test, even if the carbs are marked, you never know if the bells were swapped before someone marked them.

Pim · Post by **Pim** » Sun Mar 06, 2016 10:34 pm

Thanks Andrew, sounds easy enough. Will have a go at that.

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