26VA carb adjustment

[Peter Naulls]

I've just replaced the block and pistons on the Ruby engine. Over the winter I also rebuilt another 26VA carb. The old one performed very well but the spindle was badly worn and the idle was a little high.

All back together and initial runs around the block have been successful /uneventful. It idles smoothly and pulls well but on the flat it hunts as if the mixture is too rich. The jets are 50 and 57. I've swapped the jets for the old ones (same sizes) but it makes no difference.

Are there any other factors that would cause it to run rich? or am I barking up the wrong tree and it's just tight after the rebuild?

[David Stepney]

I have had a similar problem with my car after it’s recent rebore. The engine was initially quite tight and needed a richer mixture to idle properly. After about 120 miles,the engine is now a lot freer and I have reset the mixture. The car now runs as it should. I expect your car will be the same once it has a few more miles under its belt.

[Peter Naulls]

Ok thank you David. I was tempted to play around with the float height but perhaps I should leave well alone until it has some miles on it. I shall keep driving and see what happens.

[David Stepney]

As I recall, in the days when your new car needed running in, the carbs were set rich at the factory and the idle was rather on the fast side to compensate for the tightness of a brand new engine, One of the operations at the first service (usually at either 500 or 1000 miles depending on the manufacturer) was to check the timing and reset the carb.

In the instant case, I had to quicken the idle setting and richen the mixture by about an eighth of a turn. The adjustments are now back to where they were previously.

[Peter Naulls]

That makes sense, and I can understand why it would be more evident on idle. What puzzles me is why mine appears to be running rich, and why under very light load, especially as the jets are the ones that were in it before. Time will tell if it is tightness but I can't say I've experienced anything similar with previous re-bores.

I might run it along a long flat road and check the plug colour to see if that gives me any clues.

[David Stepney]

The only other thing I can think of is that the float may be sticking.Mine did on initial assembly, noticed when I primed it and fuel came out of the vent hole at the top. A quick smack with a spanner on the side of the fuel chamber cured it, but there are other more subtle ways it can play up.

[Peter Naulls]

I had a bright idea today and swapped the bowl for the one off the old carb. It runs like a dream now. 

I don't know if there is some fault with the new bowl or if it is subtly different in some way. Either way it will do as a spare to stash under the driver's seat.

Thanks again for your help David.

[John Cornforth]

Hi

Fixed jet carbs like the Zenith are quite sensitive to float bowl fuel level. Too high and it will tend to be be over-rich, and even "siphon" out with the engine stopped, especially when hot. Too low and it will be weak at low airflow, giving a flat spot between idle and full power. At one time, replacement needle valves were supplied with a selection of different thickness washers, to allow the level to be tweaked.

I suspect your float bowl change has altered the fuel level for the better.

[GM6VXB]

Another thing to check is the Emulsion block, No 33 in the Zenith carburetor bulletin.
Had a few 'strange' problems with both tickover and accelerating, and both times found
it was due to obstructions within the block.
Removal and an ultrasonic clean cured the problems.

Martin.

[Hugh Barnes]

There was a very helpful thread a while ago where the detailed differences of various carbs and their matching float chambers were shown (details from the ever helpful Ruairidh) I suppose it's possible you have had a bowl mismatch and you have resolved that by changing the bowl over?

https://www.austinsevenfriends.co.uk/for...4#pid27324