CB750 1980 unburnt fuel

[Motorhead]

Hey all, maybe someone had this problem before.
My CB750 is running OK, but the exhaust smell is unbearable, people closing windows when I stand next to them at lights . Makes me think it doesn't burn all the fuel, or very rich.
No smoke.
The bike has Dynojet stage 1 with a free flowing 4 into 1 (Laser/Jama) and a K&N filter in a stock airbox, Dyna 3ohm green coils with stock ignition pickups.
I did try all possible idle settings, but none really made a difference.
Used to ride with idle screws 2.5 turns out, now 1.5 - doesn't really change anything.
Any ideas?

 

[Dave]

Could be many things, a sticky float, sticky slide, plugged passage in the carb, bad gas, dirty air filter. Pull the plugs and check the color to confirm it's running rich first.

 

[Motorhead]

Carbs sync fine, if that stands for anything, and the engine works smoothly at any RPM. No difference in color between plugs, but funnily I can't tell if they're slightly rich/cold or lean/hot. But within safe range.

 

[Motorhead]

Increased the gap as per Randakk's recommendations, will see if it helps.

 

[amc49]

'...funnily I can't tell if they're slightly rich/cold or lean/hot. But within safe range.'

The most ambivalent, impossible statement I've seen in a while. You can't tell but then pronounce them as 'safe'; there's enough incompetence there to choke a horse.

By locking yourself out like with that and the idle screw setting issue you have now said you can't tell what any adjustments are going to do anyway. In short, impossible for you to do anything there as you cannot tell what difference any change makes. May as well park the bike or pay others to fix it.

You have self limited yourself to be unable to fix the problem.

 

[Motorhead]

I was trying to say that it appears to me that the plugs are within normal range, definitely not very rich or very lean.

BTW, as much as it matters, this bike is my daily ride for the last 18 years (with a 4 year brake in favor of a VFR750), I ride it to work every day. It does about 14,000 km a year. Open roads /city traffic is about 50/50.

 

[Motorhead]

Could be many things, a sticky float, sticky slide, plugged passage in the carb, bad gas, dirty air filter. Pull the plugs and check the color to confirm it's running rich first.

Thanks Dave, I use RON98, air filter was cleaned and re-oiled just about 5,000 kms ago.
About the carbs - is there anything specific I should look for? You meant air passages?

I checked the plugs again comparing to a chart I found at precisionmoto.com and they are good, very slightly rich. My idle screws are just 1.5 turn out right now.

BTW, floats were replaced with OEM new floats and float needles about 15K kms ago.

 

[Motorhead]

Increased the spark plug gap to 0.9mm. The bike runs much better now, revving up to 9000rpms easily in any gear (didn't try to push it any harder). There's still strong exhaust smell on idle.

 

[Motorhead]

Since I heavily depend on this bike as the daily transport, I service it regularly, replacing oil every 5K kms, checking valve clearances and syncing carbs every 10K kms. I have installed the OEM fuel petcock with screen about 2 years ago, last time I removed and cleaned the carbs was about that time as well.
Carbs are VB52 - pressed in slow jets, no accelerator pump, no fast idle adjustment.

 

[Dave]

Most of us have bikes that spend most of their lives parked inside a garage. According you the info you've given, you've put 196 thousand miles on this bike?

That's totally fucking outstandingly amazing dude!

First, here's a clip from "Common mistakes in carb rebuilding".

2. Not doing a valve adjust and compression test. These MUST be addressed before doing ANY carb tuning. There is no point chasing carb gremlins unless you have good compression.

3. Failure to perfect all ignition issues before tackling carb work. There is no point chasing carb gremlins unless you have perfect ignition.

Without a rebuild of the top end in there somewhere one would think you might be burning oil. Let it idle for several minutes then rev it up while watching the pipes, do you see a puff of smoke? Oil is white smoke, gas is black smoke. Open your oil tank and smell it, take a big whiff. Open the gas cap and smell it, NOT a big whiff, a small one LOL. Now compare that to the smell from the exhaust.

The plugs should be clean and brown. If they are black, that's too rich. If they are black and dirty, that's from oil. You may be able to open the plug gap even farther since you have modified the ignition. Try 1.1mm and see if it's better or worse. Do you ever foul plugs?

Do you always use the same gas station? I don't know how it is in Israel but here in America some of the gas stations will illegally add crap to increase their profits.

VB52 Carbs?
o Hidden Emulsion Type Idle Mixture Screw
o Non-Replaceable Slow Speed Jet, Some Models
o Non-Adjustable Jet Needle, All Models
o Air Cut Valves, All Models
o Twin Sequentially Functioning Main Jets
o Accelerator Pump, All Models
(Click for source)

 

[Motorhead]

Dave, I don't even have a garage ). Lucky I own sufficient storage space on the ground floor of my building, for tools and spares for this bike and an old car I have.

I didn't always use the bike so heavily (there was a period when I worked a walking distance away from home), but indeed it's done about 100,000 miles since the big overhaul.
Since they started digging the subway in Tel-Aviv, the congestion has gone so bad that driving a car to work is almost out of the question.

I got the bike in the late 90's, it burnt more oil than fuel (well, not, but at least two quarts for 600mi). Then I had it fully overhauled top and bottom with all OEM parts.
So, yes I do know what burning oil smells like LOL.

Since then, I do all the service on the bike myself.

Today the bike uses about half a quart for every 600 mi. in average. I'm riding it pretty hard (for its age), around 7000-7500 rpm on the highway. It also has a very small oil leak at where the pickup wiring exits from under the left cover.
In Israel, it's pretty warm all year round and gets very hot in the summer. That's why I'm considering adding an oil cooler.
I use Castrol Power1 15w50 semi-synth motorcycle oil. Used to use Motul, but then someone told me that Ester additive was bad for air-cooled bikes. So Castrol since two oil changes ago.

I'll take a picture of a plug tomorrow morning (it's 10:30pm here now) and post it here. Seems very dark brown to me, not dirty. Has some grey spots on the outer circumference. I'd be glad if someone took a second look at it.

I'll do the sniff test again, LOL, but it definitely smells like gas. No real smoke, just bad, obnoxious smell of exhaust fumes.
Speaking of fuel - in Israel they add shit mostly to diesel fuel. Unleaded is just generally of lower octane than stated, I heard our 98 is more like European 95... That's why I use 98, and also for the reason not many people use it, so there's less point for the stations to mess with it. I used to run 96 leaded, and it was perfect, until it was banned.

Will get back with plug pic tomorrow.

Sorry for the long preface, but me and that bike have a very long history together too)))

Thanks!

 

[amc49]

So what Dynojet parts were used? May well impact the issue.

If you open the plug gap wide enough you may spit out the ignitors, the power transistor rebels to fail past a certain amount of gap, just like going to lower amp coils. I'm surprised the 1 mm. gap fires reliably. The ignitors fail if pushed hard. Check the pulsers and see if you can get them closer than the min. of .016", I ran mine closer to like .013" better sharper signal to ignitors.

Here in Texas you need an oil cooler on 750 as soon as you go over say 5000 rpm constant, the oil pressure drops at least 20 psi if you have a gauge on bike. BTDT.

The valve guide seals toast pretty quick even using Honda ones in solid heat like climate. Here in Texas the cam chain tensioners are beginning to crack up rubber at 35K miles. Seals will be hard as rock then too.

Where can you even get up to 7000 rpm for long periods, the desert? That's pretty freaking hard use, the engines are made to do it but longterm that eats cam chain tensioner parts, the front guide not touching (faulty OEM design) lets the chain flap at rpm like crazy and kills the chain if you go longterm like that.

Stock pressed in pilot is 35, being you can't remove them the cross passages at the top that bleed air can stop up to make your issue. You should be able to at some point close the idle mixture screws enough to kill engine, if not something is wrong there.

The engines generally like paper airfilters like OEM instead of K&N, even in the stock airbox. The added restriction opens the slides more.

Even idle set correctly you will get some smell of HC out the pipe if not using the emission lean idle settings and you get within 12 inches of pipe end. Slight burned oil will mix with fuel smell to be stronger smell with no smoke if the ignition firing right like you say. It doesn't have to smoke at idle to burn oil, the smoke and pure oil smell only show when it gets worse. People seeming to notice it says some oil mixed with fuel there to me.

What plug used? It determines the color too.

 

[Motorhead]

The plug. NGK DR8ES
The plugs are gapped 0.9 mm.

The plug says 'lean', right (the grey electrode and grey spots on the perimeter)? It wasn't like that the last time I checked...

Yes, when I screw the idle screws all the way in, the engine dies.

I increased the gap as per Randakk's advice because of 3ohm coils. Would closing the gap at the pickups do the same as gaping the plugs back to stock?

I don't live in the desert, but in the grater Tel-Aviv area, at the Mediterranian Sea. It's more humid here. In the summer temps are around 86F to 92F with 70% RH. It's like NY in the summer, just not as bad.
Now, when you say "Long periods" you mean Texas long LOL. My typical daily use of the bike looks like this - about 15 minutes on the highway, and I do 7000rpm all that time, if the road is not congested. Then it's another 15 minutes filtering through city traffic. Then back home in the evening in the same manner.

I don't have an oil pressure gauge on the bike and I understand it's not as straightforward a job to install one on DOHC as it is on SOHC.
I want to install a cooler, I'll post a picture of it.

About the cam chain flap - I heard that it was rather a problem with CB900's due to taller cylinder block. Mine makes no excessive noise and feels nice and tight when I inspect it while having the cam cover removed for valve adjustment.

 

[Motorhead]

This is the Dynojet kit I have installed (some 14 or 15 years ago I think).
I use #100 main jet with stock #69 primary jet. Note that Stage 1 says K&N air filter #HA-1097 which is what I have, or stock. You say stock is better?

 

[Motorhead]

This is the Derale oil cooler I have. It's new, but I got it without the rest of the kit, so bought a Cycle X oil cooler adapter.
Would that be a good solution, at least until I come up with the CB900 pump+pan+path combo?

 

[Motorhead]

The plug says 'lean', right? It wasn't like that the last time I checked...

 

[amc49]

Plug nowhere near lean, if ignition really firing hot could even be a bit of oil in there. Lean will be pretty much white or very light, you're dark there. Dynojet always hooked up with K&N, means nothing. There was a guy long ago (passed away now) that was in Montana US IIRC and he went after Dynojet about why the kit didn't seem to run so well, and after several special one-off efforts by the owner of Dynojet himself, they gave up trying to truly get a real good running setup on these first gen DOHC bikes. The kit at least for these presents issues that nobody I know of has ever cured. As well, you can replicate the exact same performance without using any of the parts at all. The 'typical' power gain there is often clouded in smoke and does not exist. The dyno itself gets tweaked to make better power readings, pretty much all dyno owners know how to screw with the numbers and why dyno numbers don't mean a thing, at least to me. These CV carbs work somewhat different from most others in that they have a pretty big leak even when new, the top is not sealed off with rubber like almost every other one out there. As a result very easy to mess with the carb to have slides not lift as much as before and then the engine runs rich even with a jet that before ran lean. Engine condition itself as far as compression affects greatly how far and fast the slides open. Dynojet screws with your mind as well with their jets, the same number jet is NOT the same hole size as OEM Honda and that can confuse things too. Dyno jets being slightly bigger than the number dictates. So, your 100 secondary main may be flowing more than a Honda 100 would. 100 being pretty much what comes in VB52s, some may have had 98s.

yes that's the big tube Derale that would work with OEM 900 oil pan setup. I went to look at the oil adapter, it is the normal type that has the cooler at full pressure. You need quality oil lines and fixture there.

You can have cam chain 'nice and tight' and still encounter chain whip, the front guide OEM mounts at least 3/8" away from ever touching the chain, the design mistake. Should have been lightly kissing it to stop flop, there is a certain amount of whip that occurs there that will be there regardless of chain tight and why the guides need to kiss. Honda stayed with their old thinking, most of their smaller engines space the front guide similarly and I've seen the front of engine almost sawed in two from whip from normal chains if run at high engine speeds. 900 does it worse but the issue is present on 750 too, the unsupported chain distance there is too great. Chordal vibrations are what set chain to whipping, the tighter it is the worse the whip.

Closing pickup gap has nothing to do with spark gap, two different things. Pickup gap closed improves the signal which is read by ignitor to fire coil. Simply a more consistent firing point there. The magnetic force gets weaker with more airspace and the triggering then gets a little sloppier. The stock coils are 3 ohm FYI. Whenever you open plug gap up though you are loading the coil and ignitor harder due to having to jump across more airspace. Coils like all electrical take the easy way out and develop less spark with less plug gap. Open gap up and ignitor then tries to fire the wider gap up until it no longer can, how you can pull plug wire checking for spark and if you hold plug too far away from the ground you can actually pop the ignitor or whatever else electronic box is controlling the spark. A no-no on many cars, the best way to blow out the 'black box' there is. Simply having no plug in wire and turning engine over with ignition on can damage them when the box tries to fire a plug through infinity.

Run a compression test, new engine is around 170 psi and an engine down on squeeze can look rich just like one that has ignition issues to not fire hot. More than one way to be rich, even with lean settings. Less depression at slides from engine low on compression will make carbs run rich too when the slide action falls off.

 

[Motorhead]

I did run a compression test, but could only use a press-in fitting (the srew-in adapter was obviously leaking). Came up with all cylinders around 160-165 PSI. I didn't write the figures down, I saw that compression was within spec (manual says 170 +/-14psi) and forgot about it.
Now, the gauge could be not of the highest quality - http://www.walmart.com/ip/Equus-3614-Innova-Compression-Tester/14644668, but that's what I have.
Theere also could be carbon buildup compensating for engine's loss of comp?

I have ordered iridium DR8EIX plugs. When I have them, I'll measure compression again when replacing plugs.

Are you strongly against aftermarket cooler adapters which mount at the oil filter location? Even as a temporary measure?

 

[Motorhead]

BTW, how much oil consumption is OK? Or a healthy engine will not use any at all?

 

[amc49]

I personally do not like to run full oil pressure through the cooler, if a line blows (much more likely since line loaded at pressure too) you have lost pressure to the engine. The 900 oil system can leak and not blow up engine before you discover the issue if you are sharp witted. The extra oil pump section simply pumps oil through the cooler and an open end right back in pan, the lines and cooler are MUCH more reliable that way. You have a leak but the other section still supplying full pressure to motor. If you catch before all oil pumped out you have hurt nothing.

Compression fine at whatever time you took that reading.

The front cam chain guide thing...........

http://s728.photobucket.com/user/WBHYDE/media/DSC02863Large.jpg.html

Dunno if you are familiar with that guy but possibly the top early Honda DOHC racer in the world (New Zealand) right now. Lots to be learned looking through those pics and plenty of one off race pieces available nowhere else.