QuestionQUESTION: Hi Bill, you have helped me alot in the past, from posted answers and directly from you.
I have hard starting Honda CB750K. I have replaced plug wires, drained tank and carbs and added fresh gas w/stabil. Battery new. Plugs are all getting fire. Carbs are vacuum assist and all are working. This bikes occasionally runs and when warmed up runs great. Today it ran barely 1 minute. I checked header pipes and left 2 were warm but right 2 were cold. I tried to crank again and got a horrendous back fire. The 2 right cylinders hook to different coils,so I don't think its a coil. I don't know what the problem is.
EdT
ANSWER: Ed, reading between the lines, I am guessing that this is a DOHC Honda 750K model, but I don't know the year, which is important due to whether or not it was equipped with a vacuum operated fuel control system or not. That part was used in the 1981-82 models.
http://www.cmsnl.com/honda-cb750k-750-four-k-80-us_model7245/partslist/E++19.htm
The vacuum line and the fuel lines enter the carb rack between the #1 and #2 carburetors. If there is a restriction to fuel flow, then those carbs are going to get fuel first, as they are closest to the source. Even without the automatic fuel control, the fuel inlet is in the same location. If the fuel tank petcock filter screen is plugged up, fuel will trickle down the line to the 1-2 carbs first and perhaps will fail to feed the 3-4 carbs.
http://www.cmsnl.com/honda-cb750k-750-four-k-80-us_model7245/partslist/F++13.htm #8
As you have noticed the 1-4 and 2-3 cylinders are fed from separate coils, so should not contribute to the 3-4 cold pipe conditions. These bikes have been known to have failed spark units, but again there are two, one for each coil set. If you have spark at all 4 plugs, then the issue is fuel.
It wouldn't hurt to do a compression check and have the valve clearances adjusted, if you have low cylinders. You are looking for readings around 160-175 psi.
Do check your spark plug caps for 5k ohm resistance, as these can cause backfiring when out of range, but I think your problems are fuel related, at this point. The fuel tank must be totally clean inside, with a fresh fuel screen to ensure that fuel supply to the carbs is maximized.
Bill Silver
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QUESTION: Bill, thank you for your time and effort you have given me and so many other cycle fanatics.
The year of bike is 1981 Honda CB750K. It is DOHC and has vacuum operated fuel control system.
I did a compression check and get at least 110-125psi on 4 cylinders; rubber boot on compression tester was on the big side and not a good fit. I also checked spark plug caps and got 5-6k ohm resistance. I have noticed plugs are wet w/fuel and bead blasted and cleaned and regapped to spec. I replaced main fuel line from tank and from vacuum to carb 1-2. I also noticed a loose hose from vacuum, another slightly larger hose that connects in vicinity of carbs 2&3 and a still larger hose from lower end of engine/transmission; all 3 exited the rear of engine. The first 2 hoses appear to have never been attached to anything but the third and largest hose had clamp marks and may have been emissions related. After I did all this, the bike fired right up and ran perfect. I left it alone for a few hours and cranked again with some trouble. I ran it around the block and it ran perfect.
Other than cleaning plugs and replacing a gas line I am not sure the problem is fixed. The low compression is a real concern but not one I really want to tackle. I don't have a good book for this bike and really don't know what to do about these loose hoses. Your thoughts are appreciated
EdT
AnswerEd... Looking at the other illustrations for the carburetors:
http://www.cmsnl.com/honda-cb750kb-1981-four-general-export-kph_model14706/parts
You will see that there are vent hoses for the float bowls, and a pair of vent hoses that help maintain the atmospheric balance in the float bowls via the interconnected vacuum T fittings and other interlinking fittings.
The main focus is the automatic fuel valve... one vacuum line from the intake manifold connection to the fuel valve diaphragm, a second small line that vents the diaphragm side, two fuel lines going to the carburetors. If you have the right size fuel line, you can just bypass the vacuum valve altogether, using the pre-80 fuel line patterns. There are other emission hoses in the system that are part of the evaporative and crankcase venting systems.
If your four cylinders were all reading 110-125, there could be several reasons for the low readings.
1. The throttle needs to be held wide open when checking compression
2. The vacuum throttle slides block a lot of air from going through the carburetors anyway.
3. The cylinders actually have low compression either due to wear or needing a valve adjust.
4. The cam timing is incorrect for either intake or exhaust cams.
Most aftermarket repair books should have vacuum line diagrams for the emission control hoses. There may be a diagram in the owner's manuals, as well.
Use of non-metric hoses causes many problems with fuel leaks and vacuum fitting leaks.
Carburetors should be synchronized and the idle screw limit caps removed to aid in a more even engine idle and part throttle response.
Bill Silver
