Author Topic: 1970 CB750 fouling plugs (Read 8414 times)

kaysystems · « **on:** April 18, 2005, 03:10:46 PM »

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 · « **Reply #1 on:** April 18, 2005, 07:29:38 PM »

Are the plugs sooty black? or oily black?

kaysystems · « **Reply #2 on:** April 19, 2005, 03:42:24 AM »

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 · « **Reply #3 on:** April 19, 2005, 05:42:18 AM »

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 · « **Reply #4 on:** April 19, 2005, 06:10:54 AM »

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 · « **Reply #5 on:** April 19, 2005, 06:16:22 AM »

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 · « **Reply #6 on:** April 19, 2005, 11:09:45 AM »

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 · « **Reply #7 on:** April 19, 2005, 11:14:24 AM »

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 · « **Reply #8 on:** April 19, 2005, 12:58:05 PM »

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 · « **Reply #9 on:** April 19, 2005, 02:32:36 PM »

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 · « **Reply #10 on:** April 19, 2005, 03:01:40 PM »

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 · « **Reply #11 on:** April 19, 2005, 04:44:23 PM »

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 · « **Reply #12 on:** April 19, 2005, 04:55:29 PM »

Good show Chappie!!

kaysystems · « **Reply #13 on:** April 19, 2005, 04:58:17 PM »

Thanks. I'll recheck the numbers later

kaysystems · « **Reply #14 on:** April 19, 2005, 05:26:41 PM »

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 · « **Reply #15 on:** April 19, 2005, 11:19:29 PM »

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

kghost · « **Reply #16 on:** April 19, 2005, 11:27:16 PM »

kaysystems · « **Reply #17 on:** April 20, 2005, 04:43:13 AM »

Quote from: Dennis on April 19, 2005, 11:19:29 PM

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 · « **Reply #18 on:** April 20, 2005, 04:50:32 AM »

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 · « **Reply #19 on:** April 20, 2005, 06:19:52 AM »

Dave,

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

cben750f0 · « **Reply #20 on:** April 20, 2005, 06:24:12 AM »

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 · « **Reply #21 on:** April 20, 2005, 07:40:47 AM »

Quote from: Bob Wessner on April 20, 2005, 06:19:52 AM

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 · « **Reply #22 on:** April 20, 2005, 08:08:20 AM »

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

Bob Wessner · « **Reply #23 on:** April 20, 2005, 08:12:02 AM »

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 · « **Reply #24 on:** April 20, 2005, 08:37:52 AM »

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.

________________
________| _____________|
| |
| |
| |
|___________|

Something like this.

kaysystems · « **Reply #25 on:** April 20, 2005, 08:44:04 AM »

The manual says 26mm float height for the K1 & later. I assume the K0 & Sandcast are the same. They used the same brass floats

David

Bob Wessner · « **Reply #26 on:** April 20, 2005, 08:48:52 AM »

Dave,

Yes, uses the same 26mm. I've been using a small steel rule with a sliding "T" sort of a pain to keep making sure the slide hasn't moved, thought about riveting it in place. To make sure I uderstand, re: your diagram, the square part rests on the carb edge and the arm/rectangular part is rested against the float bottom? And yes, the floats are brass on the K0 also.

kaysystems · « **Reply #27 on:** April 20, 2005, 11:45:39 AM »

Just put all the Honda jets & neesles back. Since it is pouring with rain here today, I had nothing better to do so I took the carbs off & cleaned them with spray carb cleaner to make sure all those small passages are clear. I'll rebalance & go for a ride tomorrow.

David

kghost · « **Reply #28 on:** April 20, 2005, 12:24:04 PM »

Bob,
The notched part acts like your sliding T rule. The square part is just a check to make sure thier not too low. GO-NO-GO. Floats are composite on the K2.

kaysystems · « **Reply #29 on:** April 21, 2005, 11:03:18 AM »

Nice & sunny again today, but only 50 degrees.

Started it last night and ran it for only a few seconds to se if everything was connectoed OK. Wouldn't start this morning - wet plugs.

Changed the plugs and then balanced the carbs (4 cables are so much fun)

Went for a ride. Yes it runs much better, but there is still a little hessitation when trying to accelerate from slow running - let's say 30mph in 3rd or 4th.
I may drop the needles one more time. Currently they are in the second from top, which is still way leaner than those Keysters in the top notch.

Certainly rideable, even if the plugs are still nice & black, but not quite as hairy.

Also trying to figure out how to recut the plug seating faces without removing the head. They are not flat & can give minor leaks, but not enough to cause major grief.

David

kaysystems · « **Reply #30 on:** April 22, 2005, 02:30:55 PM »

Tanks guys for all your info & encouragement. Finally took my wife for a ride today. The stock Honda needles are in the lowest position and it works. Still very minor hesitation after slow running but not bad.

I found some 110 jets (Standard is 120)......maybe I should change them?

Thanks again

Davod

p.s. Maybe now I'll get time to restore the first bike I ever bought. A 1966 Ducati 250 single - overhead cam - 8500 redline - 5 speed transmition. Handles like a dream. Bought it in 1974 & still have it.

News:

Author Topic: 1970 CB750 fouling plugs (Read 8414 times)

kaysystems

kaysystems

MetalHead550

kaysystems

kaysystems

kaysystems

kaysystems

kaysystems

kaysystems

kaysystems

kaysystems

kaysystems

kaysystems

kaysystems

kaysystems

kaysystems