Topic: The only thing SCREAMING now is the bike on the highway!

[medic09]

Just a thought, did you drain them right after running it when all four were hot? It would seem, because they were all hot, they were getting fuel. On the other hand, if you drained them, say the next day after running it, did you find any fuel residue on the engine case or starter cover beneath the carbs. Maybe #2 had more of a leak at the drain screw and it all drained out whereas #3 wasn't as bad so some was still left.

Except for an aborted attempt to start this morning, the bike has sat cold for over a day.  I didn't notice any leakage/residue on the engine case when I went to drain them.  The bike is hooked up to an auxilliary shop tank, and I expected there to be fuel since I had tried starting this morning.

 

[eldar]

What parts did you replace with the keyster kits? Was it just seals and float valves? There has been a number of people here who have had issues with those kits. But that seems to be mainly with the needles and float valves.

After draining, did you reset everything and turn your fuel on and drain again? The result you got is a little strange unless you are not getting proper fuel flow.

[medic09]

What parts did you replace with the keyster kits? Was it just seals and float valves? There has been a number of people here who have had issues with those kits. But that seems to be mainly with the needles and float valves.

After draining, did you reset everything and turn your fuel on and drain again? The result you got is a little strange unless you are not getting proper fuel flow.

I replaced the whole works: jets, float valves, gaskets, etc.  Just not the needles.  Fuel flow from the tank and hose is fine.  Whatever the problem is, it starts at the carb rack.  I haven't tried running the fuel again, but I might as well.

[medic09]

What parts did you replace with the keyster kits? Was it just seals and float valves? There has been a number of people here who have had issues with those kits. But that seems to be mainly with the needles and float valves.

After draining, did you reset everything and turn your fuel on and drain again? The result you got is a little strange unless you are not getting proper fuel flow.

I just tried it again, with almost identical results.  #1 yeilded 38.5cc and drained easily.  #2 zero.  #3 zero with seepage around the screw.  #4  ~35cc with a little seepage around the screw.

 

[mrbreeze]

Why do you have to pull carbs to get at your floats? Don't tell me the bowls are screwed on!!!! On the older carbs, there is a big clip to hold the bowls on. Sounds to me like #'s 2&3 floats are not dropping. Try this....turn the gas on and remove the drain screw on #2. If nothing comes out....try tapping the bowl to shake the float loose.Can you see what I'm getting at? If the floats are stuck at the top,the needles aren't letting any fuel in. They may not stick every time but it only takes once for it to run out of fuel to that cylinder. Once you get that straightened out,make sure all bowls are holding the same amount of gas. Your #'s 1&4 are close but not dead on.

[medic09]

Why do you have to pull carbs to get at your floats? Don't tell me the bowls are screwed on!!!! On the older carbs, there is a big clip to hold the bowls on. Sounds to me like #'s 2&3 floats are not dropping. Try this....turn the gas on and remove the drain screw on #2. If nothing comes out....try tapping the bowl to shake the float loose.Can you see what I'm getting at? If the floats are stuck at the top,the needles aren't letting any fuel in. They may not stick every time but it only takes once for it to run out of fuel to that cylinder. Once you get that straightened out,make sure all bowls are holding the same amount of gas. Your #'s 1&4 are close but not dead on.

Ya know, you're really getting on my nerves...   First you rub it in about how simple the old chokes were; now the float bowls!  Yes, on the 77-78 the bowls SCREW ON.  The only way I can see to open them is take the whole rack off.  Go ahead, rub it in... "On the older carbs..."   

[mrbreeze]

Sorry about that Medic.....but do you see what I'm saying about the float hanging up? Try tapping on the bowl with the drain screw out (petcock on) and see if you get fuel delivery. Thats the only thing I can see that would stop flow to the 2 middle carbs. Good luck and again....sorry about the jab on the 78's.

[medic09]

Sorry about that Medic.....but do you see what I'm saying about the float hanging up? Try tapping on the bowl with the drain screw out (petcock on) and see if you get fuel delivery. Thats the only thing I can see that would stop flow to the 2 middle carbs. Good luck and again....sorry about the jab on the 78's.

Jab away!  The whole thing's pretty funny/absurd really; just if it were someone else! 

I had the same thought about the floats; but I suspect that even if I jar them loose the problem won't be solved without getting a look inside.  Otherwise, they may just stick again.  I'll try tapping on them first, but will probably take the rack off to open them and see what's going on, unless someone like TT comes up with a better insight/idea.

Just remember, those screw-on bowls are part of the evolution that led to the 78 being the pinnacle of SOHC4 development!   

[wardmoto]

Your starting problem is perhaps linked to the fact that you are trying to run it on 2 cyclinders!  Obviously you have a fuel delivery problem, on my 350F, number 2 and 3 fill first and then the fuel travels through interconnects(that have no part #!!!!, it is a long story) to fill 1 and 4.  Is it similar on your ride?? I bought a keyster kit for my 350F and from the time I bought it and installed it, to the time I put the carbs back on the bike at the conclusion of the build, I put all the Keihin parts back in.  I heard too many stories and with my mods and the fussy nature of the 350, I didn't need to complicate anything more than I already had.  I know people have used these kits with no problem and perhaps I would have had no problem as well, but the oem stuff was in good shape.  I did "lap" the float valve to its seat with some Mothers.  Unfortunatly, you are gonna have to pull the bowls at some point and watch for flow out of each carb...

[eldar]

well as a bit of a messy test, leave the bowl off #2 and turn on the fuel. See if any comes out. if not, see if the float is dropping. if not, pull on it and see what happens. If it frees up then pees fuel, pull the rack and check to see why the valve is sticking so bad. If still no gas, you will need to pul the rack and find the blockage inside the carbs.

[TwoTired]

By now you should be pretty confident that the float valves aren't opening up on a couple of carbs.  This is a bad thing, btw     

I've never used Keyster kits.  But, so far I've head more strange things about them than positive.

Were the float valve needles rubber tipped?
Is that the same rubber that you've noted the drain screw seals have?  Hint.

If the screwdriver handle tapping near where the float valves are doesn't yield you a gusher, do you have a bicycle pump, or other means by which you can apply some extra pressure to the fuel inlet to get the float valve to open?

If the float pivots are stuck this might not help much.  But, if the valve needles are stuck in their seats, a little extra pressure besides fuel under gravity might just free the valve to get fuel in there to dissolve dried fuel deposits and lube things up again.  Of, course the other valves that aren't stuck may just relieve enough pressure to make the attempt futile.

Dunno.  Depends on how opposed you are to taking the carbs off to look inside and find out WTF?  However, there is a possibility that carb removal/install practice is in your future anyway.

Good luck!

[eurban]

An evolution of the 77/78 style accell pump carbs is that you don't need to remove the drain screw to empty the bowls.  The "drain screw" is actually a sort of valve that when opened slightly alows fuel to flow out of the brass nubs of the "overflow" tubes.  What this means is that you can attach tubing to the nubs (like the bike came with when new) run all four tubes down right between the swingarm and frame cross member.  Put your fuel catching device down there and its super easy to drain the bowls and or see if fuel is a flowin.  I have clear tubing attached to the nubs and its even easier to see what is and what is not draining out of the carbs.  Even better, when you get your float needles actually allowing fuel to flow you can hold the clear tubing up along the carb bodies, open up the drain screws slightly and see at what level the fuel actually is in the bowl.  You can compare all the levels to see that the are very close to each other and just a bit below the bowl to carb body seam.  You would need to fabricate some sort of drain screw adapter to accomplish this on the older style carbs so don't get too much carb envy!  Hope you figure out whats keeping the fuel from flowing . . .Sounds like you have finally hit the nail on the head!

[keiths]

When I first rebuilt my carbs I didn't realize that one of my carbs wasn't getting any gas. It may have worked for a while but when I went to drain the gas for the winter none came out of number 2.
I added Yamaha carb cleaner to the fuel line just below the petcock. Before I had added 30 cc's (mixed with gas as dirrected) the gas was gushing out the float bowl drain.
It worked for me. It might be worth a try.

[medic09]

By now you should be pretty confident that the float valves aren't opening up on a couple of carbs.  This is a bad thing, btw     

I've never used Keyster kits.  But, so far I've head more strange things about them than positive.

Were the float valve needles rubber tipped?
Is that the same rubber that you've noted the drain screw seals have?  Hint.

If the screwdriver handle tapping near where the float valves are doesn't yield you a gusher, do you have a bicycle pump, or other means by which you can apply some extra pressure to the fuel inlet to get the float valve to open?

If the float pivots are stuck this might not help much.  But, if the valve needles are stuck in their seats, a little extra pressure besides fuel under gravity might just free the valve to get fuel in there to dissolve dried fuel deposits and lube things up again.  Of, course the other valves that aren't stuck may just relieve enough pressure to make the attempt futile.

Dunno.  Depends on how opposed you are to taking the carbs off to look inside and find out WTF?  However, there is a possibility that carb removal/install practice is in your future anyway.

Good luck!

The float valve needles are rubber tipped; but the rubber at the drain screws appears to be o-rings chewed up by the drain screws. 

I suspect your last suspicion is the correct one:  I have carb rack removal/installation practice in my future.

[techy5025]

Bob...are you ready for this? 

Jim

[mrbreeze]

Bob...are you ready for this? 

Jim

Just another day at the office

[medic09]

Bob...are you ready for this? 

Jim

Just another day at the office

I could get worried (paranoid?) watching an exchange like this...

[medic09]

Okay, so what I've found is that the floats in #2 were mounted upside down.  Now, who the hell did that!   

(Question:  since I *measured* float heights on this job, how did I miss that?!  )

I didn't find anything obviously wrong in #3, but I made sure that the float and the attached valve move freely.

Any suggestions before I put these back on and see what happens?

Thanks all!

[Loas 78 CB750K]

Could you  post some pics? I have a 78 CB750K as well and the choke cable was not hooked up when I bought it, which was about 2 weeks ago.

[medic09]

Could you  post some pics? I have a 78 CB750K as well and the choke cable was not hooked up when I bought it, which was about 2 weeks ago.

I'd be happy to, but my digital camera died last week.  One more thing to replace... 

Congratulations on your new aquisition.  Is your problem where/how to hook up the choke cable?

[750goes]

Hey doc, been a while since I looked at this thread - you have come a long way grasshopper....

to test your carbs before putting back on the bike - put them correct side up on the workbench and hook up a test tank - (one from the lawnmower sounds good...turn on the fuel tap and see if they leak...then do your measuring trick with a syringe for all 4 carbs... try and get your float heights the same across all four, then measure the amount of fuel in each bowl to be roughly the same.....

I had an issue with the idle rising as the bike got warmer - I bought 8 new hose clamps for the intake rubbers and now 99% of idle issues are resolved.. I kept checking and gradually tightened all the hose clamps as they warmed up...

 

[medic09]

A note about using syringes for measuring gasoline (or other solvents).  It destroys the rubber on the plunger VERY quickly.  Only good for a few moments, but of course great for measuring small amounts.

Carbs reinstalled.  Doesn't quite want to fire up.     I'll get back to it when I return from training this afternoon...   

I used a steaming hot towel to warm the boots ("insulators") thsi time 'round.  Works great!  Looks like I'll yet be getting a bit of practice on this topic...     

[medic09]

Okay, got the beast started.  Air cleaner still off for the moment, hooked up to auxillary tank.

Very hard to start.  Full choke, several attempts.  Clear the cylinders by 'kickstarting' with key off.  Try again.  More of the same.  Finally starts enough that by adding some throttle to the full choke, the bike will run.  Has to run above 2000 rpm for a few minutes before it will idle on full choke.  Won't idle stably below about 1200 rpm.  After a few more minutes it will idle without choke.

Still haven't compared the amounts in the bowls.  And do I need to resynch?

Stock jets, but I am at 7000 feet.  Stock air box with imitation K&N oiled filter.  2 into 2 exhausts (don't know what kind; they came on the bike and seemed to work well).

What now?  How do I achieve easy starting; and easy, lower rpm, stable idling?

At least we're getting somewhere.   

[medic09]

Okay, drained the bowls again for comparison.

1,2,4 all drained close to 40cc.

3 didn't drain anything, but again was seeping around the drain screw.  The bike starts, and I ran it around the block (backfiring) with the airbox off.  Not much of a test, but it runs okay in 1st and 2nd gears.

I suspect that just the drain tube on 3 is somehow occluded, and that the carb is otherwise okay.  Is that plausible, or am I just in denial?  I didn't see anything obviously wrong when I took the rack off last night (well, except for those reverses floats in #2  ).

 

[750goes]

open the drain screw on #3 - turn on the fuel - if nothing coming out...give the bowl a tap with end of screwdriver handle - gently..if still nothing coming out - you ain't getting nothin IN...

check your fuel delivery for blockage or OCCLUSION - (no medi speak here).....pull the fuel line delivery off between 3&4 - blow some compressed air in the T section - can you hear it come out the #3 drain screw hole ?? if not you got a blocked artery - accounts for some of the backfiring as well - running lean or not at all...more examination required medico...