Honda BF5 Mystery starting problem

I am having problems starting my Honda BF5, particularly after it has been run. It has a good spark, the idle and main jets and carb have been cleaned. The fuel is fresh, the filter is new, the fuel pump is pumping fuel. Compression is good. When running she idles smoothly at low revs and develops full power. Cooling water is good. Choke is OK. There is no smoke in the exhaust. She just will not start. The only way I can get her to start is by tipping a teaspoon of petrol into the cylinder via the sparkplug and then after a couple of pulls she starts up and runs as if there was no problem at all. Any ideas ? The engine has been recently acquired second hand and was totally salted up. This has all been cleaned out and new gaskets fitted. So she has probably overheated in the past. Request advice. Yours sincerely Richard BD in the UK.

If the motor runs good at all speeds as you say, then you probably just need to readjust the throttle and/or choke linkage. If someone has had the carburetor off, then may not have reconnected the linkages properly.

When you choke that motor, it should also advance the throttle quite a bit. The idle stop screw should pull away from the stop approximately 3/16" or more. All this work, takes place right at the carburetor.

Take the hood off and pull the choke out as you watch the throttle linkage at the carburetor.
First of all, make sure the the choke is actually closing. That cable could be out of adjustment also.
Second, make sure that the throttle stop screw is away from the stop by at least 3/16" (5mm). You could always manually turn it up higher by twisting the tiller arm.

That motor will not start without a lot of throttle and a good choke.

Mike

Hi Mike, many thanks for your reply. The choke is operating correctly and it does advance the throttle. The engine has a mechanism to stop you opening up the throttle fully when in neutral. So when I was trying to get the engine started one of the things I did was to put the engine in gear and open to full throttle and full choke and try and start. She just would not fire. Other information is that when I did the compression test it read 7 bar I then squirted in a little oil and the compression went to 8 bar. When trying to start the engine if I thought she had flooded it I then disconnected the fuel line opened up the choke and throttle and tried to start a few times. She still did not fire. When she is running at low throttle she ticks over at low revs very smoothly and at full throttle she has full power. The colour of the spark plug electrode, smell and deposits is giving a possible clue as to the reason she is not starting but I am not sure I can interpret the evidence correctly. When I pulled the plug out to look at it during the starting problem the electrode colour was grey/black and clean and not sooty, it was not wet with petrol, it seemed to have a very slight film of oil on it. But it is only very slight. When rested against the side of the cylinder head the spark was good and strong. I was wondering if oil was coming into the cylinder either via the valves or via a worn ring and that this was somehow stopping the engine starting. It was odd how easily the engine started when I tipped petrol directly into the cylinder. There is no smoke in the exhaust. The engine is on the boat on its mooring at the moment and so I cannot get to it easily. Best regards Richard

To Others with the same Honda BF5 starting problem. I went to an outboard repair place and got their advice today. Steve from Bursledon outboards stated that a compression of 7 Bar (100 psi) is too low; it should be about 10 Bar (145 psi). So the cause of non-starting is likely to be a lack of compression causing the fuel not to vapourise. The fault is likely to be either sticky or broken piston rings or leakage past the valves. Re-check the valve clearances which should be 4-6/1000. Carry out a leak test. Bring the piston to top dead centre with both valves just rocking. Put the engine shaft in a water tank and remove the oil dip stick. Apply compressed air to the spark plug via an adapter and leak testing device which has two gauges one to measure pressure and the other flow loss. If air bubbles out of the dipstick the problem is piston rings. If air hisses out of carb - inlet valve. If air bubbles into the water tank - exhaust valve. If it is piston rings fill up cylinder with some oil and leave for a day; this might free a sticky ring. Also go back to the carb and clean all components and blast through chamber holes with compressed air. Really check the jets and make sure the float is level. best regards Richard BD

Further to the above I do not have an air compressor but there must be a way of making a small device which could do the above leak test using readily available equipment. I am thinking of an adapter plug which screws into the spark plug and has a pipe with a T junction and one pipe going to an air reserve such as a wheelbarrow wheel and the other which goes to a car tyre pump. Or maybe just apply air pressure to the cylinder head using a car tyre pump. Other barking mad ideas welcome. Richard BD

The repair place is correct, depending on your serial number. The early 5's had lower compression...64 psi plus/minus 14 @600rpm. For frame numbers 2000001 and later. it is 156 plus/minus 14 psi @ 1000 rpm.

Definitely check the valve clearances and the timing marks.

Mike