• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

lousy fouling plugs

untill62

CB750 Enthusiast
Messages
48
Reaction score
0
Points
6
Location
Middleton, Wi
Need help with my 1974. Within 10 miles after putting in new plugs, they start to carbon up. I use NDG D-7ES, new gas, new air filter. I have to run a wire thru the slow jet at least twice a year to open it up. Slow jet is marked 40. Not sure if that has an impact or not because it runs pretty well over 3000rpm, but sputters under and then I look at plugs and they are carboned up. I did awhile ago lightly clean the cylinders and put on new rings, not that it should matter? Boy, I hope someone has a suggestion, I'm lost.
 
Last edited:
You probably have the idle jets worn to a bigger size by running a wire through it. It takes a very minute change is idle jet size to make a big difference. Will the idle mixture screws make a change in how it runs? 40 is the standard jet size and I have never seen a time to go with a smaller jet so I wouldn't suggest that. Only other thing would be the needle jet position as it does affect off idle throttle opening.
 
Ya, I had heard online someplace else about running a wire thru the slow jet, but what else could I use to open them up from time to time?
I don't head idle mixture screws. I do use carb synchronizing tool when needed, but I'm not sure how to make a change that would effect the plugs fouling. I think it's too rich, right?
 
Do you think I should buy new slow jets in case I did damage mine? Also how would I adjust the carb to be less rich? I need to start trying different things.
 
You should replace all the jets and never stick wires in them. Use compressed air if you need to clean one out. There's no reason to "open them up from time to time" unless your tank is full of dirt or you let it sit for a few years with out draining the carbs. You can add tiny inline fuel filters to both lines to help catch anything that gets through the crude wire screen Honda called a filter in 74.

You should check your spark too. A weak spark can cause fouled plugs.

Anyone here who has changed over to electronic ignition with some new coils and plug caps will tell you the difference is amazing. I live in CA and never even use the choke anymore. The bike starts instantly even after sitting a month or two, runs better then ever, gets better mileage, and hasn't fouled a single plug since I swapped over.
 
I would start with new idle jets in the stock 40 size. Jetsrus.com has jets...stock keihin and cheaper aftermarket that work perfectly fine.
 
Hi again. Well I did buy new slow jets(stock 40) and obviously new plugs(D7EA). Do you think I would need new Main jets as well? I didn't think so, since it seemed to run smooth over about 3000rpm?I put inline fuel filters in because of a comment someone had about the stock screen isn't very good. I also turned the idle air screws out one turn and will not touch them again, as per the Clymer manual I have.I haven't started yet. But thru all of these emails, no one has said if there are any carb adjustments I can make to carbs so it doesn't run so rich, or is it just what I have done so far good enough.
I was wondering about the coils, since they are original(1974) I think i'll wait for a response before I fired it up again. Thanks
 
Like said the ignition will affect plug readings just like too rich. AND, compression will as well, you mention ring work there and indicates older engine. Compression test will tell the tale. If you got any weak cylinder you can forget ever getting plugs to run clean, compression increases heat to keep the plugs burned clean so they don't foul. Early fouling and dark plugs are the hallmark of a worn out engine.

Point ignitions flat suck and electronic is the way to go here as well. Plugs burn much cleaner with them.

FYI, I have used pieces of wire to clean jets out for 45+ years and never screwed up a jet yet, you just have to have some sense about it. I prefer to use high pressure air but ethanol in fuel now can dry carbs and jets up to clog the crap out of them now and you have to have a backup plan if the jet does not blow back open with air.
 
plugs

Like said the ignition will affect plug readings just like too rich. AND, compression will as well, you mention ring work there and indicates older engine. Compression test will tell the tale. If you got any weak cylinder you can forget ever getting plugs to run clean, compression increases heat to keep the plugs burned clean so they don't foul. Early fouling and dark plugs are the hallmark of a worn out engine.

Point ignitions flat suck and electronic is the way to go here as well. Plugs burn much cleaner with them.

FYI, I have used pieces of wire to clean jets out for 45+ years and never screwed up a jet yet, you just have to have some sense about it. I prefer to use high pressure air but ethanol in fuel now can dry carbs and jets up to clog the crap out of them now and you have to have a backup plan if the jet does not blow back open with air.

thanks for the reply. I put new rings in barely 100 miles ago. I have not used the bike more than that in 2 yrs. It just has not run well. I am going to buy electronic ign. and now that winter is almost here, I have a while to work on it. I noticed also that the plug wire to boot on a couple cylinders is loose and just touches, so with the new coils will come wires and boots.
Will try again in spring
 
If they have ethanol laced fuel where you are at then letting them sit long periods is helping making your issue much worse. The carbs on old school stuff REALLY hate that as the ethanol allows the fuel to evap much faster out of fuel bowls in carbs to stick things up like your idle jets. VERY common if it sits. I have the problem with a CB550 if it sits longer than 2 weeks but I can always get it back to running perfectly. Ethanol is pretty transparent if the vehicle is used a lot but let them sit and brother are you asking for troubles. These will stop up the pilot jets lickety split letting them sit, along with sticking the needle valves in place in the carbs to not work.

Your ring thing could be part of a problem, you mention 'lightly cleaning' the cylinders, that is screaming danger all over it to me depending on what you did there. The quick plug fouling could be from that easily too. Compression test numbers would tell the tale there.

How long since valve seals changed? Those can do that too.
 
I would be checking the plug caps. Standard ohm reading is 5k ohms but over time they break down and cause a weak spark.
It's the cheapest and quite possibly the most effective result.
 
Not there.........he's carboning them too quick as it is, an 8 plug is colder than 7 and will do that even quicker. Can't just throw wrong plug in there willy nilly. Gotta have some thought behind it.
 
Back
Top