Topic: 1971 CB750K1 #2 Carb not getting fuel to cylinder

[dharris324]

Hi all,

New to the forum.  Just aquired the bike as a Christmas gift from my Father.  Runs fine but #2 cylinder not functioning well.  Have a 4 into 2 exhaust and the left pipe does not get as hot as the right one.  When I put my hand over all but the #2 intake the idle goes down like it should.  Added seafoam to the tank and used a spray bottle (with seafoam) to clean out the intake while running.  Still no go on #2.

What could be my problem?  Am suspecting trash in the main jet but have not checked it as of yet.  What are your thoughts?

Thanks in advance!
Dan

[Gordon]

Check the easiest things first.  Make sure that spark plug is sparking, and remove the drain plug from that carb to make sure it even has fuel in the bowl.  It's also not a bad idea to check the compression, but go ahead and check all cylinders while you're at it.  If all of those are good, then it's time to pull the carbs and give them a thorough cleaning. 

These carbs have very small holes and passages in them and they get clogged easily if not ridden regularly or maintained well.  Running carb cleaner through the fuel system can't clear out a passage or jet that's completely clogged.   

[dharris324]

Thanks Gordon,

The bike has not been cranked since 1977. It has been sitting in a basement since then. A freind of my Fathers aquired it and had it at his place working on it for 8 months but never started it.  He cleaned it up including the carbs (bowls are immaculate) and ran some cleaner in the crankcase and let it sit for a while.

When I got it I had to get a few choice parts like a petcock seal to keep the gas in the tank (which has light rust and I'm suspecting that's my issue), a tank cap latch (not cheap), battery, and speedo cable.

I have also repaired a pinhole in the #4 float which works fine now (soldered it).   All 4 bowls getting fuel.  Drained and refilled crankcase twice with oil (with seafoam as well) Also changed the oil filter.

All 4 cylinders getting fire as well.

The bike also has a major problem with 2nd gear.  Will not stay in gear and grinds.  Have to skip 2nd when I ride it.  Suspect the gear dogs and forks (probably why it sat in the basement for so long).  Could this problem be in the clutch linkage by any chance?


Thanks,
Dan

[Gordon]

Hopefully your Dad's friend did a good job of preparing the carbs for storage.  Even if he did, though, you should still at least drop the bowls and pull out all the jets to see with your own eyes that they are all completely clear.  I'm betting the tiny pinholes in the top portion of the #2 idle jet are clogged. 

It's possible you have problems with the gears, but it wouldn't hurt to go through the clutch adjustment procedure just to see if that makes any difference. 

[dharris324]

I checked the idle jets (and pinholes) in all four carbs and they were clear a couple days ago.  Will pull the bowl on the #2 carb tomorrow to see whats behind the main jet.

As for the clutch adjustment I will have to read up on how to do.

By any chance, do you know where I can get a new Master cylinder for my bike?

Thanks for the input.

[Johnie]

Did you check the gas tank for rust, etc. You could be getting crap into the carbs from the tank. You can run an in-line fuel filter. As far as the master cylinder, why would you need a new one. You can rebuild it unless there is some major damage there.

Edit: I just saw your other post down the line about the damage to the master. If you can not find one here they are on eBay, but who knows what condition? I think you will find one from a member here. Just give it a bit and post it in the wanted section if you have not already done so.

[ekpent]

Follow Ron's post on that tranny issue. If its popping out you will probabably have to go in.  http://forums.sohc4.net/index.php?topic=59158.0

[dharris324]

Hi Johnie and ekpent,

I read that post earlier this week.  That is where I got the notion it may be a serious "crack the case" affair.  Next time I'm out I will get that inline FF.  The tank is putting out some garbage since I added seafoam.  It's breaking up whats inside the tank and ending up in the carbs.

As for the Master cylinder.  I see whats on ebay too and really would like to steer clear of anything there as of yet.  Will post to the wanted section (thanks!)

Thank you both!

[Johnie]

I had a bunch of junk in the tank of my K4 when I got it last July. Take a gander at the pic below. I dumped the gas out and filtered it as it came out. Then dumped it back in the tank, filtered it out of the tank, etc. I did this for an hour until there was nothing left but clean gas. I then put on those in-line filters and ran it the rest of the summer with no problems.

[Kevin D]

It's breaking up whats inside the tank and ending up in the carbs.

Do you have the screen in the petcock? It should catch the chunks before they get to the carbs.

[Duke McDukiedook]

Clean your carbs again, zero in on #2 and blow out all orifices with compressed air (around 80psi), you'll need an air compressor for this task. Check your float levels before reassembling and get some rebuild kits if you have to (Keyster is OK, just don't use their needles).

[dharris324]

Thanks all,

I do have the screen in the petcock.  Just got finished prying the carbs off the engine.  Not a pretty sight.  Will have to purchase the carb boots that connect to the top end.  They were dry rotted and hard.  The #3 carb leaked gas all night and had a puddle under the bike even though the petcock was off.  Must have been what was in the bowl.  The petcock does not leak.

Will go through the entire carb system and get some boots and 4 carb kits.  The fun begins!

[dharris324]

I think Im gonna have a heart attack!  The carb intake boots on ebay are $89.00.  Does anyone have a good used set on hand?

[Gordon]

I think Im gonna have a heart attack!  The carb intake boots on ebay are $89.00.  Does anyone have a good used set on hand?

$60 for a set from David Silver.  Probably about the same direct from Honda. 

[dharris324]

Thanks Gordon,  I'll check on that.

[dharris324]

Some nice fellow in this forum was giving away a complete transmission assy.  Just sent him a Paypal payment for the shipping.  Comes with all gears, forks, shift cylinder and linkage.  Makes me wanna cry with all the love.  Group hug! (not).

[dharris324]

Just finished pulling all 4 bowls and removed all brass parts to inspect them.  Was hoping to find crud somewhere but no such luck.  They look clean but I could tell when I poured out the bowl gas there was dark brown silt in the bottom.

Also I found the float I soldered the other day (#4 carb) was still letting gas inside.  Will have to fork over more cash for a new float now.  Will disassemble carbs and give them a good soaking and then break out the air compressor for the passages.  What kind of cleaner do you guys recommend for soaking carbs?

[ekpent]

Congrats on the tranny parts. When the engine is out you can do some nice cleaning  etc.Thread on right now about GUNK carb dip,I like it for carbs and other degreasing. On my 71 I swapped out the sometimes troublesome old brass floats (2 were bad) for some newer style plastic ones I had around,works great.You will have to deal with that rust issue in tank,its easily gets past the screens. I have had very good luck useing the Works toilet bowl cleaner many times.Make sure you have check your tapetts and those coil wires and boots concentrating on your #2 if that on is a problem. Good luck and post us a pic of your bike please.      Eric

[CB750Winger]

   Dan, on your master cylinder, try partsnmore.net. They sell a new one including the lever and brake pressure switch for I think around $70. It's not OEM but it's new.

[dharris324]

Thanks for the great info guys.  Really do want a set of those plastic floats.  Will check on that master cylinder as well.  I always heard the Gunk products were good to use.  Will get some and try it out.

Here is a pic of the bike.  She's in great shape and no rust on any of the chrome!

[dharris324]

Here is another angle.  Having trouble with photo restrictions:

[dharris324]

Hi All,

After cleaning my carbs after removal, I reinstalled them and now the #2 carb backfires out of the intake port.  The bike also backfires out of the tailpipe on the same side (sometimes with flames).  At least now the #2 carb is doing something besides being a paperweight.

Turning the air screw and or putting my hand over the intake on #2 has no effect on the idle.  The bike revs up like it is running on all 4 carbs but  pops a lot. when reved and ridden. Getting spark on all 4 plug wires.

What gives guys?

[Johnie]

Nice looking gold there. I skimmed back through the threads and did not see if you mentioned anything about a bench sync before you put those carbs back on. Did you do that?

[ekpent]

Just for kicks the next time you start it up and get it fairly warmed up shut it down and take a wet rag and dab it on the header pipes near the engine and compare the "sizzle" sound. See if that number 2 is running cooler,maybe still not fireing correctly.

[dharris324]

Thanks guys,

From what I understand,  the carbs were synch'ed by my fathers freind who had it previously.  I really do not know how to do this (haven't read up on it).  The carb cylinders look in line with each other when looking through the intakes though.

The #2 exhaust does not sizzle like the rest of the pipes when using the spit on finger method.  Sometimes I can do this with no spit at all because it is just warm.  I know it is getting fire, I just checked all of the spark plugs yesterday.

I think something is still stopped up in the #2 carb because sometimes it runs fine on all cylinders and all exhaust pipes get hot.  Seems to act up when cold especially on a cold day.  Really hard to crank up and when I use the choke It fouls the plugs real easy.  When it warmed up today it ran like a champ.  When the backfiring (intake)starts it seems like the exhaust gets hotter.  When it does not backfire thats when the carb is not working and the exhaust gets cold.