QuestionI've been working on a Honda 750. It won't start. I've cleaned out just about
everything I can think of. I took all four carburetors off and cleaned the jets
and everything that I could with a small brush. They are quite complex and
have a lot of little holes. I blew everything out with an air-hose but I'm still
not sure that everything is free. The starter will turn the motor over and it
sounds like it wants to crank but generally doesn't. I can get it to crank if i
pull the choke lever all the way up and twist the throttle about half way back.
When it starts you can't put the choke back down or really more the throttle
or it will knock off. It tends to backfire a good bit when I try to crank it. Do
you have any idea what to check next? I've cleaned the points and made sure
all the lever arms aren't stuck but I'm clueless as to what to look at next.
AnswerWesley, You need to start with a foundation check first.
Does it have compression and how much? 175psi is the benchmark figure.
If all four cylinders are near that and fairly even, then you can go onto the next step. If one or more cylinders are low, check the valve clearances first (.002" intake/.003" exhaust checked on compression stroke), then recheck compression to see if they come back up.
In the carburetors, there are "idle jets" which are about .016" in diameter. These MUST be clean and clear. If you can't see light through them, then they won't feed fuel to the engine for starting and idling.
The float valves must be clean and float levels set to 26mm.
Further upstream, the fuel tank gas cap has a small vent hole that needs to be open for proper fuel flow. Then the fuel petcock should be removed, cleaned and a new set of gaskets installed.
The points need to be clean and shiny, gapped to .014-.016", then set the points plates until the points just OPEN at the 1-4F and 2-3F firing marks.
A running engine requires: compression, spark at the right time, fuel-air mixture at correct ratios and air.
LOTS of tech info here:
http://www.sohc4.net/index.php?q=content/general-tuning
Bill Silver
