Topic: 1981 CB900c -- everything fine but no gas into cylinder

[Hondawggie]

Bought a 10,600 miles  1981 CB900c.  Low miles!  A project though.

Here's what I had to do and have done:
- new starter relay
- new battery
- clean carbs


Last night I spent 3 hours disassembling the top dome and float bowl and all jets from the bank of 4 Keihins.   All the jets were clogged.  Carb cleaner to the rescue.  Even the float gas valve needle's little spring-loaded pin was stuck in all 4 carbs. 

So I sprayed the heck out of them.  After all jets were out and soaking in carb cleaner, I sprayed carb cleaner from an aerosol can with one of those red plastic straws, into all the orifices -- full blast. 

I verified the choke butterflys work in all 4 carbs.

I pulled the tops off each carb -- pulled the throttle slide and needle jet out of each carb, cleaned the needles and the carb throats and the throttle slides.   Cleaned the float bowls too.

Put the carbs back on today.  Have a new battery in there.  The bike will not start. 

Here's what I then checked:

"Is it getting spark?"
Yes it is, spark plugs laying grounded on the head, nice healthy sparking.

"Is gas getting into the float bowls?"
Yes it is, I loosened the drain screws at the bottom of the float bowls -- and fresh gas comes out as expected.

The tell tale is this:  I pull the spark plugs and despite lots of choke, lots of WFO throttle -- the damn cylinders *should* be flooded after all that cranking, choking, and full open throttle.

The spark plugs are bone dry, not even a *scent* of gasoline.

Something is stopping gas from getting into the cylinders. 

- I have spark
- I have freshly cleaned carbs and there's fresh fuel in them
- brand new battery

Just to make sure, I hooked up my portable starter/charger to the battery.  The bike cranks and cranks, no gas gets to the cylinders.

These carbs are the "CV" type, Keihin style.  Probably makes no difference.

Tomorrow, despite the fact the bike looks brand new and has only 10k miles on it, I'm considering pulling the valve cover to check if the intake cam is missing, or the cam chain is broken, or something.

That's the only reason I can think of that would dead-stop all that gas in the float bowls from flooding the cylinders.


Is there anything else (besides a broken cam chain/missing intake cam/intake valves never opening) that would stop gas from moving from the carbs to the cylinders?

[MoMo]

certainly check compression first.  That model has an accelerator pump I think, was it cleaned and were you able to pull out the pressed in pilot jets to clean them?  good luck as they were nice cruisers...Larry

[Hondawggie]

certainly check compression first.  That model has an accelerator pump I think, was it cleaned and were you able to pull out the pressed in pilot jets to clean them?  good luck as they were nice cruisers...Larry

Question for you, with a float bowl full of gas, and clean jets, if I was cranking this motor for 5 minutes (not all at once, I took breaks to allow the starter to cool) -- and I ask this because I've never worked on this particular model, nor this type of Keihin carb -- after 5 minutes of cranking the motor, would you expect to see wet plugs or at least smell gas?   

I hadn't thought to check compression -- thanks for that -- maybe the rings are stuck and the intake stroke isn't creating a vacuum on the carbs, but instead sucking air from the crankcase, past the stuck rings.

I saw the accelerator pump but did not disassemble it, but I'm thinking that would not be the cause for no gas entering the cylinder....  the accelerator pump probably needs cleaning, but I need to fix this "no gas getting into cylinder" issue first

[MoMo]

If pilot jets are clogged you'll have trouble, were you able to pull them out to clean??

[Hondawggie]

If pilot jets are clogged you'll have trouble, were you able to pull them out to clean??

I'm used to Mikuni pilot jets I guess;  and I'll assume you're speaking not of the pilot air screw but the pilot jet inside the carbs.

What I found in the float bowls of these Keihin carbs surprised me.  In the same orifice tunnel as the screwed-in pilot jet, at the entrance to that tunnel was what looked like the round-style main jets.   Of course I knew it wasn't the main jet, which is found in the same tunnel as the needle jet.

But sweartagod there is a round main-jet-looking jet and when I took that out I found deep in the tunnel the pilot jet.

In two of the carbs I successfully unscrewed the pilot jets and removed them -- they were clogged.  I cleaned them thoroughly. I used a sewing needle whose diameter was well less than the pilot jet to clean the inner shafts, using carb cleaner liberally.

Two of the pilot jets I could not get out (and damaged the screw head in trying).

So for those two, I took the same sewing needle and using a long, thin needle nose pliers, cleaned out the inner bore of each one and then full blasts of carb cleaner.  Several times.

So the first two pilots (the ones I got out) should be enough to fire at least 2 cylinders.  And I believe the sewing needle/high pressure blasts of carb cleaner into the 2 stuck pilot jets cleared them too.  But at a minimum, at least the 2 known-good pilot jets should be firing up two cylinders -- but again no gas on the plugs, bone dry, not even a scent of gas.

[w1sa]

G'day Hondawggie,

These cv carburettors can be an absolute pain to clean properly, if they have been left a long time without use or proper maintenance. They can also be difficult to adjust/tune correctly if they have been 'fiddled with'.

For starting, (assuming relatively correct settings) the forward mixture screw, chamber and orifice must be really clean. There are tiny o rings with these screws and springs that should all be inplace and undamaged. Did you check the correct mixture screw settings after you cleaned/replaced them?

If the orifice from the idle mixture feed chamber to the carb throat is clogged/blocked you will not get fuel to the cylinders for starting/idle. Similarly result, if the passage from this chamber (which delivers fuel and air to it) is blocked.

Similarly, if the fuel level in the bowls is not correct, fuel movement from the bowls to the idle mixture chamber can be seriously compromised. The idle mixture from the idle mixture chamber is comprised of fuel and air. This air is fed/combined with the idle fuel between the jets and the idle mixture chamber. If that air feed passage is blocked/restricted (I think it starts in the air cut-off valve), it will also affect the correct idle mixture delivered to the carb throat.

For these and other reasons it is actually very difficult to properly clean the carbs insitu.
You need to be able to access and visually test and verify these passages and orifices are completely/physically clear and clean.....a combination of forced air, carb cleaner and light as appropriate usually works well to confirm they are clear and clean. For example looking thru the idle mixture chamber to the orifice (very small hole) feeding the carb throat you can witness light shone down the carb throat and clearly see how clean the oricice is......but typically only with the air box separated for full access/inspection. Similarly, physical and visual access to most of the air cut-off valves is limited without carb separation .........and so on.......

For fuel level testing, attach a clear tube to each carb and open the bleed screw. Fuel should settle about 2 to 3mm below the bowl/body joint/gask line.

Don't use pressure can carb clean into the carbs too liberally while insitu/assembled. It will attack and damage the soft orings and diaphrams in the fuel/ air, air cut-off passages.............................cheers

[Hondawggie]

G'day Hondawggie,

These cv carburettors can be an absolute pain to clean properly, if they have been left a long time without use or proper maintenance. They can also be difficult to adjust/tune correctly if they have been 'fiddled with'.

For starting, (assuming relatively correct settings) the forward mixture screw, chamber and orifice must be really clean. There are tiny o rings with these screws and springs that should all be inplace and undamaged. Did you check the correct mixture screw settings after you cleaned/replaced them?

If the orifice from the idle mixture feed chamber to the carb throat is clogged/blocked you will not get fuel to the cylinders for starting/idle. Similarly result, if the passage from this chamber (which delivers fuel and air to it) is blocked.

Similarly, if the fuel level in the bowls is not correct, fuel movement from the bowls to the idle mixture chamber can be seriously compromised. The idle mixture from the idle mixture chamber is comprised of fuel and air. This air is fed/combined with the idle fuel between the jets and the idle mixture chamber. If that air feed passage is blocked/restricted (I think it starts in the air cut-off valve), it will also affect the correct idle mixture delivered to the carb throat.

For these and other reasons it is actually very difficult to properly clean the carbs insitu.
You need to be able to access and visually test and verify these passages and orifices are completely/physically clear and clean.....a combination of forced air, carb cleaner and light as appropriate usually works well to confirm they are clear and clean. For example looking thru the idle mixture chamber to the orifice (very small hole) feeding the carb throat you can witness light shone down the carb throat and clearly see how clean the oricice is......but typically only with the air box separated for full access/inspection. Similarly, physical and visual access to most of the air cut-off valves is limited without carb separation .........and so on.......

For fuel level testing, attach a clear tube to each carb and open the bleed screw. Fuel should settle about 2 to 3mm below the bowl/body joint/gask line.

Don't use pressure can carb clean into the carbs too liberally while insitu/assembled. It will attack and damage the soft orings and diaphrams in the fuel/ air, air cut-off passages.............................cheers

I normally use gallon-cans of Berryman's Carb Cleaner -- each gallon can comes with a metal basket into which you put the carb body and all metal parts.   When I clean Kaw triple carbs it's dang easy.  I like working on older bikes, real easy.    I have four of these 1-gallon cans. 





So I wanted to avoid total disassembly, hence the approach I took last night:  pull the carb bank off the bike, then sit down for 3 hours and clean each carb one at a time.

I pulled one air cutoff apart and it was very clean so I skipped the rest of them.

I still wonder though, this.  At least two carbs have clean pilot jets.  I cannot explain why no gas reaches those cylinders.  With the choke fully engaged, under normal conditions. a clean pilot jet means you're getting gas into that cylinder.

Most bikes that I've run across, if they have clogged jets, will start with the choke on then die when you take the choke off.

Not this beast.  Bone dry cylinders.  All those orifices I cleaned, and associated jets -- should get *some* gas in those bores!

Unlike older style floats, with a little metal tab you can bend to adjust fuel level -- these carbs have all-plastic floats.  There's no adjustment tab, it's all hard plastic -- I suspect Keihin engineered these  "fixed" plastic floats to provide the proper fuel level.  It's ease enough to check, I've done it many times, but in this case I suspect the fuel level is correct, due to the un-settable floats.

I cannot get around zero gas in all bores; should have fuel or scent of fuel in at least two of the bores.

Before I pull these carbs for a 2nd time, disassemble them completely and put them in Berryman's for overnight, I want to exhaust all other possible causes.

I have an intake vacuum test device -- an expensive electronic one from England -- I should bust out and see if I'm completely missing vacuum on the intake side of this engine.

I spent 45 minutes on each carb.  Meticulous and thorough.   Should be *some* fuel in the bores.   After all the choke mechanism is working fine.

[w1sa]

Yes, by all means test for intake vacuum/compression  etc......................but regardless of what we often think should be the case, it isn't always so........you need to tick off/eliminate whatever might cause the problem you have.

IIWM, (Given the situation you describe)..........The picture I would want to see is (after normal 3000ml maint. check up)
- test for normal fuel flow/delivery by clear tube method to a level close to 1/8 in (3mm)
- after cranking for a start-up (ass. no start) pull plug(s) and check for fuel...
   *if no fuel on plugs
- unscrew mixture screw(s) and check for fuel in mixture chamber(s)
 * if no fuel in mixture chamber......the idle circuit is NOT clean/clear
 * if fuel is in mixture chamber...The orifice to the carb throat is (prob) NOT clean/clear (and possibly some of idle circuit).

.....cheers 

[Hondawggie]

Previously I noted this behavior, as I cranked over the engine and stared at the see-through "flying saucer" shaped inline fuel filter:   

- with the petcock turned 'On', the see-through inline fuel filter filled up with gas about 3/4 of the way
- when I then cranked the engine, *all* the gas was instantly sucked out of that inline fuel filter.

And yet the bike wouldn't start.   

And I previously checked, yep the float bowls all had fresh gas in them.

So why wouldn't the damn thing start?  The carbs had gas in them after all.


Today I *bypassed* that wretched freaking remote fuel diaphragm and put the outlet hose from the gas tank petcock right into the fuel inlet for the carb bank.
** EDIT: **  And today, I did *not* see all the gas instantly sucked out of the inline fuel filter when I started cranking the engine.

The dang engine started up.   It's only running on 3 cylinders, but that's my schitty job on the first carb clean I did last week coming through, easily fixed.

So I pulled the carbs -- this time I'm disassembling them *completely*, taking them off the mounting rail, and putting each carb and its parts into a separate gallon can of Berrymans carb cleaner that I've been using for a few years now, I've got 4 cans of it to keep each carb's bits together and never mixed up:


 

The other thing I found today was this:  the idle adjustment was *way* too low on the carbs so I cranked it in, the idle adjustment black knob just below the carb bank when mounted on the engine.   It was too low to keep the engine going.  I don't know if that played a role in why the bike wouldn't even sputter/cough/start last week.   Today the bike would only sputter, it sounded like it *wanted* to run, but wouldn't stay running.  Then I thought "hey, maybe the idle adjustment is way off, not a high enough idle to keep the motor going" sure enough that did the trick to keep the engine going.


So a few days back when I couldn't get it to even *fire*, cough or sputter -- I still do NOT understand that at all.

Because the float bowls had gas in them.  And the carbs have gas in them today too,  but the bike started up today.

How could that remote fuel diagram deal (1) fill the carbs with gas, but then (2) stop all gas from getting to the engine?  I just don't see it.

** EDIT: **    Forgot to mention one other change I made today, which may be important.  Recall that the gas cap on a motorcycle gas tank has a small hole to allow air into the tank -- that hole allows gas to flow freely out of the tank and down into the petcock.  If that hole wasn't there, a vacuum would get created as the gas flowed down into the petcock and eventually that vacuum would very much slow the flow of gas from the gas tank into the petcock.  That hole in the gas cap allows air into the tank as the gas flows out of it, thus no vacuum gets created so the gas flows freely. 

So today to eliminate that possibility I took the gas cap off the tank to let air in there while I tried to start it.

[Tugboat]

Ditch that stupid vacuum fuel thing and just run a single line to a brass "T" and down to the carbs. Run an inline filter too if you want.