Topic: 1970 CB750 fouling plugs

[kaysystems]

Can't figure this one out.

Bought a CB750 2 years ago. Plugs looke reasonable. After finding a stripped plug I took the top end apart, only to find that the valve timing was one tooth off!!.

Rebuilt with new +.50 pistons, rebore & head skim. Replacement carbs to fix a stripped body. The cap wouldn't stay on. Cleanes, soaked, sprayed, blew them out. Rebuilt the carbs with Keyster kits, including the top washer that had been missing.
The needle is in the middle position.

Boyer Bransden ignition with stock coils. Fed up checking the timing &carb synch.

The bike just won't run clean. Hesitation when hot. Traffic lights are a challenge. The idle screws are 2 turns out. I need new or clean plugs every hundred miles. All the cylinders are the same. Even tried DR7ES & Champion 810s.

What gives? Should I drop the needles, or am I missing something obvious.

Someone please help me, I'm losing my sanity.

David   

[kghost]

Are the plugs sooty black? or oily black?

[kaysystems]

I would say 'sooty'. They are not shiney black, just 'hairy'. When I rebuilt the top end I replaced the valve oil seals. The halve stems & guides appeared OK.

Maybe I should find some '110' main jets?

David

[cben750f0]

mate i am haveing the same problem... and i reckon its the keyster kits... i put my old gear back in and it ran better.... i dont think the bleed air screws are the same, and i don think the needle taper is the same... that is just my experience... oh and the fact that i didnt even get one of their bleed air jets in b4 it broke off in the carb, and the fact that two of the needles and seats i got leaked from the day they were installed..... changed these out and put the old ones back in... and no leaking carbs...i am not sure about them, but other ppl who have use the swear by them... just my thoughts....peace

[MetalHead550]

Sorry to go off topic.  Hey cben750f1!  Sweet bike first off.  I'm gonna take a wild guess and say youre a metalhead .  Hey what kind of metal you into and have you heard of the excellent Aussie deathmetal band Abramelin?  If not check em out.  I'm all about the old school and the underground myself.  Everything from Sinister to Manowar to Iced Earth.  Metal!

[cben750f0]

metalhead... mate... as long as its loud...and from the soul... im there... rock on mate.. rock on....oh and appologies for the tangent...

[kaysystems]

So I lowered all the needles two notches, so now the clip is in the top notch. Idle screws are 1 1/4 turns out. Put the Honda jets back in. Wow .... doesn't stall. But 3 plugs are still black. #1 has a brouwn area on the ceramic, but the rest of the plug is black.

I'm ready to give up & get rid of thisthing.

David

[kghost]

Don't get rid of it yet. Did you clean the plugs after you changed needle clip position? And how far did you ride it after the change? Once around the block isn't really enough to clean the plugs. If its running ok try riding it for awhile after cleaning the plugs then check them again.

[kaysystems]

mmm. No I didn't clean them. I went for a 20 mile ride, some slow bits, some traffic lights and a few miles at 60mph. I'll give the plugs a clean. These are new plugs & the soot just wipes off.
It also vibrates a bit above 5000rpm, and would not be pleasent to ride for long at that speed.
The only non-stock item is a Boyer electronic ignition, which includes an advance mecanism back plate from a later model, but the timing marks appear to be in the same place. I don't think the timing changed over the years. Maybe I'll balance the carbs again.
The head was skimmed. I don't know how much was taken off, I'd guess at least 10 thou. Would that do anything?

I may try to measure the Keyster needles against the stock ones to see if there is a difference.

David

[kghost]

Skimming would raise the compression ratio. but that shouldn't affect the mixture.

You might want to compare the needles. I recall an incident where Keyster kits for Honda GL1000 Gold wings had the Wrong taper profile. Unless the stock needles are really worn or otherwise buggered you might try putting them back in.

[kaysystems]

It's supposed to rain tomorrow, so I'll get my micrometer out & check the taper of the Keyster & others that are supposed to be stock Honda.
Think I might be getting there.
I installed the Keyster kit because the bike was running rich & rough. Maybe I made it worse.

Do you think that the Boyer ignition could have an effect. It has more sparks. 2 per revolution on each cylinder.

I'll update tomorrow about the needles.

David

[kaysystems]

Couldn't wait 'till tomorrow.

The Keyster needle that I measured is consistently .05mm (.0197 inches) thinner over the length of the needle.

The Honda one is 2.245mm at the top of the taper & 1.51mm at the bottom. The taper is 28mm long.

So if we divide the diameters by two to get a radius, and calculate the angle, then drop the needle by 2mm (the equivelant of 2 notches) I figure that the diameter has grown by .035mm

That doesn't make up for the .05mm smaller diameter of the Keyster needle, so even dropping the needle two notches, I'm still richer than the Honda needles as the needle diameter is smaller. Guess I know what I'm doing tomorrow

David

[kghost]

Good show Chappie!!

[kaysystems]

Thanks. I'll recheck the numbers later   

[kaysystems]

Just one mistake. The .036mm is on radius, so it's .07 on dia. This means that the Keyster in it's lowest position is a little weaker than the Honda in the centre location. So I'll still be too rich. I'll try the Honda ones lowered.

David

[Dennis]

You may or may not realize this. Those are pretty big differences between the needles you are discussing.

[kghost]

yup.

[kaysystems]

You may or may not realize this. Those are pretty big differences between the needles you are discussing.

Yup, more than 3% at the bottom of the needle. That's about the same as going from a 120 to 115 main jet. No wonder I'm having problems.

David

[kaysystems]

What setting do you guys use for the float level?

Had an email from one guy who said "I just eye ball them and line the float up with the gasket surface". That sounds a bit low to me.I use a Honda float gauge set to 26mm

David

[Bob Wessner]

Dave,

Where/how did you come by the Honda float gauge? I can't find one to save my life.

[cben750f0]

mate that is the problem i had with my kit... put the original needles back in abd the proble, went away..... though i didnt go into the detail you went into... i just replaced one thing at a time with the originals till it ran better... gald you were able to find a prob... i got one of those float level gauges off ebay about two weeks ago... original NOS honda item... makes checking the floats alot easier.... peace

[kaysystems]

Dave,

Where/how did you come by the Honda float gauge? I can't find one to save my life.

It's a later adjustable item, part number 07401-0010000 that I found at a Honda dealer

Service Honda lists them but I don't know if they are in stock
http://www.servicehonda.com/parts.php?partnumber=07401-0010000&getpart=1&getprice=Submit


David

[kghost]

I just make a go-no-go gauge out of a piece of aluminum scrap. Works great to set the floats

[Bob Wessner]

I did try ServiceHonda, they claim they have it, but not so. Kghost, would you happen to have a tracing or something of the one you made?

[kghost]

No, its out at the airport. It really was low tech. There were two sides. Make one side a square and the other a notch. Make the notch slightly bigger than the measurement you want. Make the square the correct size. With this you can use it over again and no need to look at the ruler, caliper, or whatever.

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Something like this.