Topic: Spark plugs tuning

[aminemed]

Need your help guys to understand how to properly tune my cb 750 k2. My bike was running only on 3 cylinders, i changed my spark plugs bike ran great for a short time and this morning i removed cylinder 1 and 4 plugs to check them as they are easily accessible and here is what i found: cylinder 1 spark plug is almost clean as new and 4 is black as hell! What is wrong ?

[Deltarider]

The easiest first. Check HTlead and sparkplugcap. The HTlead should be clean, free of grease and contact GROUND as little as possible. Any (intermittent) arcing at idle between cap and/or HT lead and the head? This is best noticed in the dark. Sparkplugcap resistance should be around 5kΩ. Is the cap firmly connected to the HTlead? With plug hold against engine, does it spark OK whilst cranking? BTW, what type plug do you run?

[aminemed]

What is HTlead please ?

[BobR]

What is HTlead please ?

 Spark plug wires. Bob

[aminemed]

I m running D8REA i know there is already a resistance in my spark cap but will this resistance create this difference ?

[BobR]

I m running D8REA i know there is already a resistance in my spark cap but will this resistance create this difference ?

 Only if the resistance is wrong. Basically Deltarider suggested checking easiest, simple stuff first. I might swap caps and see if it follows it and check for arching in the dark as Deltarider suggested. If nothing found then move onto fuel delivery....You said it was running on 3 cylinders before the new plugs- was #4 fouled then? Cant tell from the pic if its sooty or ,worse oily.

[aminemed]

It was cylinder 4 that was not running and it is the cylinder.from which i removed the black spark plug and it is sooty

[TwoTired]

If spark plug resistance were the issue, it would affect all four cylinders in the same way.
Carbon deposits are formed when there is not enough oxygen to combine with available fuel during the combustion cycle.  The heat from combustion then turns the unburnt hydrocarbons to soot which deposits on the internal surface of the combustion chamber.  Soot on the spark plug electrodes is evidence that the air/fuel ratio is wrong for that cylinder.  Once soot is formed on the center electrode insulator, it forms an electrical path to shunt energy away from the spark gap.  This is a secondary problem caused by the errant fuel mixture issue. You can try using a blow torch to clean the plugs you have, or replace them new.

Please state tune up history, as well as the status of the last routine 3k tune up check list.

Does the bike have stock induction, exhaust?

Are the head pipe temperatures even on all four when clean plugs are installed?

Any history of carb work or internal parts replacement?

[aminemed]

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

[Deltarider]

I m running D8REA i know there is already a resistance in my spark cap but will this resistance create this difference ?

Some bikes just don't like resistance in the plugs and in the caps. If resistance in total is close to the limit, the weakest will start malfunctioning.
About the fuel smell. Do you have any indication carbbowl #4 is overfilling, like when the valve needle is not closing completely? If the fuel drain tube is fitted, you would see fuel dripping when bike is on the main stand and petcock is ON, provided forsaid drain tube is open (!).

[evinrude7]

are all float heights correct? 

[BomberMann650]

are all float heights correct?

+1

[aminemed]

I need to take my carburetor and check that. This should take a while as I'm not an expert

[Deltarider]

But before you do the clear tube test, do an even simpler test: see if carbbowl #4 leaks fuel when the petcock is left ON and bike is on the main stand. This is an easy check, provided the drain pipe and/or tube (at the bottom of te carbbowl) is open. When in doubt it is open, simply detach tube, connect another tube and blow a little air, just a little to verify it is open. So if, with a good open drain tube and the petcock open and the bike on the central stand, the carbbowl leaks fuel, it's an indication valve needle is not closing completely. First remedy then would be to tap (gently) that floatchamber with the stub end of a screwdriver and see if dirt frees itself from the fuel valve and needle. Please report back. 

[evinrude7]

remove carbs from bike.  cut an old credit card into a U shape but with 90* corners.  from the inside bottom of the U to the tops should be the factory spec height of your floats.  turn your carbs sideways to measure and make sure you're not depressing the needle valve. 

clear tube after this.  you'll be a lot closer. 

[aminemed]

Bike is also backfiring at deceleration. It was quite funny the first time but not funny any more when i hear that big explosion at every deceleration

[The Machine]

I had problems with sooty plugs and backfiring when decelerating. Have you checked your air mixture screws? My mixture
screws were turned out too much, adjusted them to 1 1/4 turn out and problem went away. Give that a try.

[Yamahawk]

Bike is also backfiring at deceleration. It was quite funny the first time but not funny any more when i hear that big explosion at every deceleration

One of two things is happening here.
1. Your #4 float needle is leaking by, giving you a rich condition, and the smell of gas. (+1 - This could be either the needle has wear or dirt, or the float level is not adjusted correctly.)
2. Your plug wire and cap is arcing, or is not giving good spark to your #4 cylinder. (Also could be both)
I would tap the float bowl with the handle of a screwdriver, and see if it stops. If you still smell gas, then you will have to pull the float bowl and see if the needle and seat can be cleaned. If not, replace the needle and seat.... as a matter of course, check your float height first, and see if it is set to specs. Then check your needle and seat if the float adjustment is in spec.
A complete tune up with setting static timing/points gap and cleaning/replacing your points wouldn't hurt either. Your capacitors(condensers) on the points plate could be the problem with the deceleration backfire too. These are things to check and adjust so you can eliminate them from the problem.
Charlie

[aminemed]

measures my spark plug caps resistance, one is defective other 3 are between 13 and 17 kohms

[flatlander]

i'm not that familiar with the spec for a 750. but should they not be 5k ohm?
if so, then it's time to buy a whole set of new ones.

[Deltarider]

measures my spark plug caps resistance, one is defective other 3 are between 13 and 17 kohms

You may have found the problem. If your measuring is correct, that resistance is a bit high and in combination with "R" sparkplugs, I can imagine it could be too much. Luckily a new set of plugcaps is not a big investment and neither is a set of nonresistor sparkplugs. Notwithstanding this - and also for some peace of mind - the tests I suggested above are all easily carried out. I myself do them as a routine maintenance. If you are curious enough, you could perform a post mortem on the defective cap, by putting a screwdriver in where it connects to the plug, unscrew the copper thingie in there, bring the resistor behind it out in daylight, make a (close-up) pic of it (both ends please) and share that with us. If you still have the original coils and leads, this is a good opportunity to clean the HTleads with a drop of alcohol on a tissue. Before fitting the new caps, I'd clip a mm or so (not too much!) from the lead ends to ensure the new caps can be screwed on firmly.

[aminemed]

is there any good test to assess if I'm running on 3 or 4 cylinders expect bike feeling and exhaust head temperature. I'm under the impression that my 4 head exhausts are not at the same temperature, some are really hot while others are less

[Deltarider]

is there any good test to assess if I'm running on 3 or 4 cylinders expect bike feeling and exhaust head temperature. I'm under the impression that my 4 head exhausts are not at the same temperature, some are really hot while others are less

I can't think of an easier test than flash touching the headers to check all 4 are working more or less. An eventual problem with unequal firing due to worn plugcaps and/or plugs with carbon build up must be adressed before anything else. Plugcaps happen to have a limited lifetime and I remember 10.000 km service packs that besides oil, filters, plugs also contained 4 new plugcaps.

[Yamahawk]

is there any good test to assess if I'm running on 3 or 4 cylinders expect bike feeling and exhaust head temperature. I'm under the impression that my 4 head exhausts are not at the same temperature, some are really hot while others are less

Sure! Pull the spark plug lead off of the suspected cylinder's plug and see if it changes the idle speed. If it does, it wasn't that one lol... keep doing this, until you find the one that isn't functioning... and BE CAREFUL to us an insulated set of pliers to pull the lead, otherwise you may be in for a shock. Or do it prior to starting, remembering what it sounds like, and idle speed.
The really hot exhaust is the cylinder that is running, the cooler one(s) are not functioning properly.

[Deltarider]

is there any good test to assess if I'm running on 3 or 4 cylinders expect bike feeling and exhaust head temperature. I'm under the impression that my 4 head exhausts are not at the same temperature, some are really hot while others are less

Sure! Pull the spark plug lead off of the suspected cylinder's plug and see if it changes the idle speed. If it does, it wasn't that one lol... keep doing this, until you find the one that isn't functioning... and BE CAREFUL to us an insulated set of pliers to pull the lead, otherwise you may be in for a shock. Or do it prior to starting, remembering what it sounds like, and idle speed.
The really hot exhaust is the cylinder that is running, the cooler one(s) are not functioning properly.

This practice is not very healthy for the coils and I wouldn't risk it. The idea behind it is OK, but instead of pulling the cap of the plug, you'd better ground the HT lead which will have the same effect without endangering your coil.

[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