Question about Honda CB 750 Nighthawk Motorcycles

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Carburetor problem or ? 1975 Honda CB750K, #4 Cylinder about 160 degrees F cooler than other cylinders. Compression all about 135 psig, overhauled the carbs fuel in bowl is clear and levels are #1 & #3 is 45ml (1.5 oz), #2& 4 is 40 ml (1.4 oz) Used a Fluke non contact thermometer and got the following temperature on fin at 12:00 position above exhaust pipe, outside temperature was 25 degrees C ( 77 degrees F), bike brought up to temperature and at idle not moving #1& 2 read 134 degrees C (273 degrees F) #3 read 132 degrees C (270 degrees F) #4 read 44 degrees C (111 degrees F) Checked spark and was strong, has an electronic ignition system HELP very little hair left?

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Be certain that fuel is getting into the float bowl on the bad carb. If so, and the fuel level is at the correct height, the problem would be either the slide is not raising as it should, the main jet is plugged up or there is a blockage in the air clearer/manifold.
The compression is good so a valve can't be stuck. You said you have spark. The only thing left is a fuel problem or air problem. Please rate my answer. Thanks.

Posted on Aug 19, 2010

Testimonial: "Thanks I will check the slide and air manifold"

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Just fixed a similar problem on my '77 GS750. No temp gauge, but one cylinder was cooler than the rest (spit test!). Turned out to be simply a duff HT Cap. I had also checked the spark, but it looked as 'blue' as the others.
I would also check the temp on the downpipe for each cylinder as this would eliminate any issues with the thickness of the fins/heat path etc.
Does the bike run well?
Hope yours is as easy to fix as mine!

Posted on Aug 19, 2010

Testimonial: "I will check high tension leads, bike does run fine"

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1 Answer

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It will be very helpful if you post the type of car you are having problems with so we can try to help. But rule of thumb is that if you are getting oil into cylinders, your piston ring or rings are failing... Depending on the mileage of your car that means that the engine needs an overhaul. A good mechanic can do a compression test on your engine for a very reasonable price so you can get an estimate of the parts and labor needed. Good luck!

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Popping noise could occur due to the malfunction in the ignition system, leaky valves, weak cylinder compression, carbon accumulation in the combustion chamber or lean fuel.
1. Ignition system malfunction (Misfire).
Check:
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My boat motor has low compression @ number 1 cylinder how can I fix it


2 possibilities:
-1 Your head gasket is burned. Caution, some Mercury engines have no head gaskets. I could tell if I had your serial number.
-2 Your cylinder has seized. If it's the upper cylinder, you'll have also to check your cooling system! Look at your spark plug if you can't see some 'aluminum' deposit on it. Or, inspect the inside of that cylinder with an endoscope.
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It's in any case better not to use that engine in this state.

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1 Answer

Low overall compression in 1996 honda accord 4cyl 129.75 psi avg.


the compression pressures are on the low side. The indications are worn rings as the pressure readings come up when the ring seal is improved with the addition of oil. The bores will be worn in a taper and if the head was to come of there would be a noticeable lip where the top ring stops in the cylinder
rings and cylinders wear equally but rings suffer from a loss of tension, and an increased ring gap that allows more compression to escape to the sump
Do you need to do something about it--
if there is an oil consumption problem( smokey) then yes
If the economy is acceptable to you then --no --not immediately
What should you consider in making a decision----
1. good engines are available from japanese spares ( imported engines with less than 30.000klms on the engine for a fair price
2. consider what is termed a "short motor" which is the block,crank, pistons ,rings,oil pump and sump all new
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1 Answer

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If cylinder compression is below 120 psi as an average, you should be looking into a rebuild event-rings and valves need attention. If any cylinder is below 100 psi, it may not even fire, certainly will produce no power, and could be a head gasket failure, very worn rings, or valve problems.
Remember the important thing is to find compression readings that are very close to each other. I have an '87 Sentra with 75,000 miles(true!)-lowest comp. is 160, highest is 165 psi-runs great, good power. Also a '79 Datsun pickup (with god only knows how many miles-about 200,000 maybe). Compression is about 125-135 psi, fairly well balanced-noticeable loss of power, very little oil burning, still runs fair to good-dependable, at least.
That should give you an idea of what to look for. Good luck.

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