Topic: Spark plugs tuning

[TwoTired]

Current in the coil circuit only flows when all the gaps are filled with spark plasma.
Pulling a lead off one plug, kills two cylinders because now there is a gap too large to form a plasma channel in the huge gap.  Therefore, no spark occurs in two cylinders, making a diagnosis problematic.
If you ground the lead removed, the plug sharing the same coil should still spark.

The Canadian market 750s had both resistor plugs and resistance plug caps, and ran fine.

The plug cap resistors were acceptable with an accuracy of plus or minus 20% of rated value.  The value meaured isn't that critical.  I will say that the most common finding in a 750 is a plug cap of 5k ohms.  It is mostly irrelevant whether the plugs are resistor or non-resistor type, unless you have raised the compression ratio of your engine, or are using exotic fuels.  This applies to stock coils of known design.

Have you found a corelation to the sooty plug locations with the high measured value if your plug caps?

Cheers,

[HondaMan]

Bike is running with a 4 into 1 exhaust rather than 4 into 4. K&N filters instead of stock air box no other change. Unfortunately I do not have carbs tuning history but from what I know, carburetor reference, it should run 105 main jet. One other thing I can mention, bike smells a lot gasoline when running and it smells so much that I smell gasoline too when I take the bike

This is WHY you are getting dark sparkplugs....
Things you might try include:
1. Make SURE the carb hoses are not leaking vacuum at the head, and at the carbs. These hoses are old, and usually leak vacuum, which will make the mixture rich (yes, rich) because these are pulse-feed carbs.
2. Tape off at least 1/2 of the face of each 'pod' air filter with tape. Duct tape works OK here for a while, until the gas fumes make it fall off.
3. Set the air screws to 3/4 turn with pod filters on the CB750K2. This will make it slightly cold-blooded on cool mornings, but will run OK after warmup.
4. Retard the ignition timing slightly, to help burn off the excessively rich mix caused by the pod filters. Try setting the idle timing in between the "T" and"F" marks, for starters. More advance than stock will make it all worse.
5. Make sure the float levels in the carbs are at 26mm, no deeper.

[aminemed]

Should i also try to go for a different main jet ? My carbs are 657B with main jets at 105

[evinrude7]

Should i also try to go for a different main jet ? My carbs are 657B with main jets at 105

that should be stock main size and jet needle slot is 4.  here's a couple of links that might help. 

http://www.hondachopper.com/garage/carb_specs/carb_specs.html

http://www.hondachopper.com/garage/carb_info/bench_sync/bench_sync.html

[HondaMan]

Should i also try to go for a different main jet ? My carbs are 657B with main jets at 105

That jet size is correct for the 657B carbs. Your exhaust pipes will not affect the tuning enough to require a different jet size.

[Deltarider]

Aminemed, as I understood it, you have only one black sparkplug, right? Also you have found one plugcap to be defective and three that show a too high resistance, right? Have you done the few easy tests I suggested above and did you renew the plugcaps? Did the bike run well before?

[aminemed]

Aminemed, as I understood it, you have only one black sparkplug, right? Also you have found one plugcap to be defective and three that show a too high resistance, right? Have you done the few easy tests I suggested above and did you renew the plugcaps? Did the bike run well before?

yes only one is black and 3 plugcaps have too much resistance and one is defective. Just ordered a new set.  Bike never really well Since i had my K&N filter pods and fitted the bike with the Supersport 4 into 1 exhaust, it runs randomly well, smells a lot gasoline (carb issue as i changed the petcock and fitted a new one). I also noticed that the PO was running NGK DPR7EA9 spark plugs that i changed with DR8EA. This only slight change made the bike run 10 times better