Topic: Desperate with CB750F Keihin Carbs

[Professor]

Hi Group. Please excuse me for posting about a DOHC 4, but I'm desperate.

I have an '82 750F having about 9000 miles which I bought recently from a Florida dealer. The machine has a four into one exhaust system.

I've gone through the carbs four times and cleaned everything as thoroughly as I could. Used a total of at least four cans of carb cleaner spray and lots of compressed air. Installed four carb kits which I got from Cycle ReCycle. Kits contained new float valve, primary main jet ( # 68 ), secondary main jet ( #102 ), main nozzle, pilot needle, and a number of springs & "O" rings.

Compressions are 185, 180, 184, and 185. Battery is new and fully charged. Spark tests to about 20 kv using a spark tester from Auto Zone. Spark timing and advance working as designed. Valve clearances all within specs. Carbs balanced to within 1/2 " vacuum. Gas level in float chambers verified with clear plastic tubing as being all the same and about 1/4" below the top of the float chamber. Air cutoff valves are are working as verified by applying vacuum to the actuation ports in the carb throats. Pilot jets adjusted to give smoothest idle at 1100 rpm.

The problem is that after only driving a couple miles all four plugs carbon foul right out, and after about 15 - 20 miles are fouled enough to cause severe missing. Fouling is soft black carbon which is not wet. Other than the plugs fouling severely, the bike really runs quite well and pulls quite strongly up to 7000 rpm which is as high as I've ever taken it so far.

If you install a clean, or new, set of D7EA plugs, start the engine and let it idlle a few minutes and then pull a plug, the center electrode insulator appears a medium brown. Drive the bike one mile at low throttle settings and 3000 rpm or less and pull the plug again, it is already severe;t carbon fouled.

I'm out of ideas with the plug fouling but it certainly seems that there must be something yet wrong with the carbs. I'd sure appreciate any suggestions that anyone might have.

[eldar]

Well I am not sure what the standard plugs are for the DOHC but the SOHC uses D8EA as the standard. And so for our bikes, the D7 is a colder plug. SO you have a couple of simple options, First is IF the D7 is the standard plug, try the D8. If the D8 is your standard, running the colder plug will foul unless you ride in hot conditions and it could still happen then, put in the D8 or D9ea. If you got a book with the bike, look in it for a your hotter range plug # and try it first. You may also want to adjust the height on the floats, may be too high, I could not say what the height should be.

[Professor]

Wow, thanks for the quick reply. The D8EA is also the standard plug for the DOHC's. According to the NGK website information going to a lower number ( 8 to 7 ) indicated a hotter plug, which is really why I installed the D7EA's. Maybe, as is so often the case today, the manufacturer info is somehow wrong, which wouldn't surprise me a bit.

I do have some D9EA's around here somewhere and I'll stick them in in a few and take a look at what happens.

I believe the floats as are all OK as the are the plastic non-adjustable variety and none appear to be leaking.

Thanks again for the reply: wow it's hot here in Florida!

[TwoTired]

 Aren't those CV carbs on that bike?  Is the bike comprised of all stock components?

[Einyodeler]

 With NGK and ND plugs the higher the number the colder the plug and vice-versa,with Champion and Bosch plugs the higher the number the hotter the plug.So the D7EA is a hotter plug than the D8EA.

[Professor]

Yeah, they are CV carbs, and I believe that the bike is all stock with the possible exception of the four into one exhaust. Pics I've seen show four into four or four into two. I just can't say for sure about the exhaust as I haven't been riding for thirty years and haven't kept up on it. I'm relearning fast though: the hard way!

Thanks.

[TwoTired]

Okay, then you've got too rich of a mixture when the vacuum slides pull the main needles out of their jets.  If you don't have a needle height adjustment, then its a smaller main jet size, a fatter/longer slide needle, or a smaller needle jet.

If you don't want to replace those parts yet you could try lowering the fuel level in the bowls an 1/8 or 1/4 inch and drive for effect with clean plugs.  This should lean up the mixture a bit and tell you if your are on the right track.

This assumes that all holes in the emulsion tubes feeding that needle jet (behind the main) are clear and clean as well as the air jet and passageway in the carb body that feeds the emulsion tubes.

You follow?

[eldar]

Is it lower number = hotter plugs? I thought it was the other way around? Well I am not sure about your floats but from what I have seen, almost all floats should be adjustable. there is a tab that the float needle hooks on that can be bent to control float height.  One thing also, since we dont know what all you have done with the bike or the shape it is in. Check out the seats for the float needles the make sure they are clean.

[MRieck]

Higher # = colder plug.

[luckysox]

If you go to this group they might be able to better help you. They deal with the DOHC  750  900   1100  Super Sports   http://cb1100f.net/index.php

[Zeke]

Keep in mind that 3000 is really way too slow to be running your engine for any length of time.

Have you tried running it on the highway around 5Krpm?  If it doesn't foul out then it seems that your pilot screws are simply too rich -- because at 3000 you aren't really even into the mains/needle very much.

Zeke

[Professor]

Again, thanks to all.

I'd like to lower the floats to see what happens, but unfortunately the floats aren't adjustable as they are 100% plastic and have no metal tang to bend where it rides on the float needle. I dunno, maybe somebody put the wrong floats in?

On the highway at 5000 - 6000 rpm the thing pretty much holds it's own in respect to plug fouling: doesn't build up, doesn't go away either. Drove a 180 mile trip @ 65 - 70 mph with no real problems as far as losing cylinders due to plug fouling. It seems to be mainly the low midrange where the worst rich condition seems to be, hence the 3000 rpm test drive.

Took the air cutoff valve off # 1 carb last night and blew carb cleaner through the passages while carb still on the bike. That possibly may have accomplished something as during a short test drive afterward the engine was running pretty bad. I'm hoping this means that some gunk got blown out of the passages. We'll see today as the day wears on.

Thanks for the tip on the cb1100F group. Have made a couple of posts there as well.

Many thanks to all and would it be good if I let everyone know what the final solution is? Assuming I ever get it fixed?

[MRieck]

Are you sure the needles are correct (OEM and not Dynojet). Your needles and/or needle jets can be worn out too which results in a rich condition. High float level will foul plugs quickly at idle and cause a rough idle too. Good luck getting to #2and #3 air cut off valves.

[Professor]

Unfortunately I don't know whether the needles are OEM or not; is there possibly a way to tell by looking at them? Have checked the float levels by hooking a piece of clear tubing to the bowl drain and then opening the drain screw. Fuel rises in the tube to the same level as in the bowl. Appears to be something like 1/4" below the top of the carb bowl. Unfortunately there's no way to adjust the float level as the floats are plastic and have no metal "tang" to bend; oh well...

Defeated the carb #1 air cutoff valve this AM by plugging the actuation port with a gasket: unfortunately no change in that cylinder fouling. Next, am going to remove the air cutoff diaphram in carb #1 and see what happens. I'm wondering if for some reason it may be actuating to the "rich" mode unnecessarily.

Wow what a pain!

[pctech]

I am new to the group and hope you will put up with me, Have you tried the  different plug sets, 7's 8's 9's, I was running 8's  and was having the same problem, I was told NGK 9's where hotter, I installed them and they work great. I have not fouled a plug in 2 months, the bike starts up better and warms up better, since you have gone the hard way try the plugs.
All I know it works great now.
Also you did not say what size jets main and secondary you are running, and also do you have pod filters or stock air box, that makes a HUGE difference with this bike. Let me know how you are doing. Mark

[Professor]

Very interesting about the plugs. I have some D9's which I'll stick in this morning. It wouldn't surprise me a bit to find the NGK website info is wrong.

The bike presently has #68 jets on the primary mains and #102's in the secondary mains. Before going through the carbs ( the fouling was happening beforehand ) it had #68's in the primary mains and # 110's in the secondary mains.

The air filter system is completely stock.

Thanks for the info.

[cb650]

Did you also check the airbox and make sure it isnt blocked.   Also make sure the choke is opening up all the way.   Simple but sometimes over looked.   




                     Terry

[Dennis]

D9's are COLDER than D8's.
NGK's get hotter as the numbers get lower!

Mike knows what he is talking about!
It's been a while since I've been to the NGK site but I don't recall seeing any errors there.

[Professor]

Thanks for the updates.

The airbox isn't plugged, unfortunately.

That's what it seemed to me re: plug heat ranges, but I have seen such company references ( including the outfit I used to work for ) which were wrong.

Even though I've been all through all four carbs four times, using lots of carb cleaner spray and compressed air, and done all manner of checks ( including a few minutes ago tearing into the vacuum operated fuel cutoff: no joy ), I guess i'm going to take a "kill it or cure it" approach.

Next step is to remove all the jets from #1 carb ( on the bike ) and take the airhose to all the passages using about 50 psi. Got nothing to lose at this point.

Thanks again to all.

[eldar]

This is a dohc but I would still say to move to a smaller low speed jet. I did find that the plastic floats in those years are non-adjustable, supposedly. There may still be a way but I could not tell you. So try different plugs and make sure they are not a resistor plug since I think this bike used resistor caps. switch to a smaller low speed jet. Also try running the engine with no filter but dont ride it. See if this helps a little.

[luckysox]

Just for refernce here is a chart for Honda carbs    http://www.hondachopper.com/carb.specs.html