Rich mixture
QI recently bought a 1979 Honda Accord MKI with 30,000 miles from new. It’s a Honda-matic and, when you are driving, it has a hesitation at about 2000rpm, as if it is taking a gulp of air. The carburettor – a Hitachi twin-barrel type – has been stripped and cleaned, but the hesitation is still there. There is also a strong smell of petrol after a run and the tailpipe is sooty. I’m getting 24-25mpg and there are no visible signs of a leak. I have tried weakening the mixture, to no avail, and the spark plugs look fine.
I wonder if it could be the float? Alternatively, can you suggest where I might find a carburettor specialist that is capable of sorting it? Most garages are used to modern cars or fuel injection and haven’t a clue about these. It’s no good taking it to a Honda main dealer as they won't have any idea, given that its nearly 40 years old. Karl Sheridan
AI am making the assumption that the other mechanical adjustments that could cause this problem, such as valve clearance and ignition timing, have already been addressed.
The strong smell of petrol and the sooty exhaust suggest a rich mixture, but I would also expect this to leave a trace of soot on the spark plugs. The float and needle valve would be the first point to check for a possible over-fuelling problem. This is easily checked on the Hitachi as you should have a sight glass which has a white dot, indicating the correct float level. The data for the carburettor advises that the float should be level to the body when the needle valve is correctly adjusted and closed. Clearance between the float and needle valve when the float is at its lowest point should be 6.1-7.6mm.
Other possible problems would be the accelerator pump system or the setting of the secondary throttle adjustment and operating diaphragm. There is also the possibility that the jets are worn.
As you know, carburettor specialists are not as common as they once were. I did find one – at http://bit.ly/29ptjbs – who should be about 40 minutes from you. Not having used this company this is only a suggestion, not a recommendation.