Carbs leaking, starting problems (still)

[alehman]

I have a very mysterious bike. Really, I have two issues, despite painstaking efforts: the #1 (and now #3 and #4) bowls will not stop leaking through the drains, and the bike simply won't start.

As for the leaking, I have done many things trying to solve it. I have cleaned and rebuilt the carbs. It's not the drain screws, I've checked that. I have checked the floats in gasoline, and they all work. I have cleaned the needle valve seats and replaced the needle valves. I have even adjusted the tangs on the floats slightly. After all those fixes I ran water through the fuel line to test if it still leaked and it didn't at all, but when I attached it to my tank it started leaking right away. The only thing I can think of is that there's junk in my tank that's getting into my carbs and messing with the seating of the needle valves. There's nothing large or visible but the gas is kind of rosy red like rust. Could that be my problem? Other than that, fuel delivery seems to be fine, so I'm ruling that out as something that would prevent the bike from starting.

To the second issue. A little background--- the bike has backfired before through the carbs, which leads me to think that either the valve timing or clearances is off if combustion gasses can escape through the intake valves. I have checked the valve timing marks on the cam sprockets, and they're pretty aligned with the valve cover surface as they should be. They're maybe a few millimeters off, but if I adjusted the chain a tooth, it would be even more off. So no problem there. I have also checked the clearances- they're pretty on spot. So it seems like compression shouldn't be an issue for the bike starting. I'm going to run a proper test soon, but I still can't figure it out why it backfired through the carbs.

I have spark, but it's pretty thin. Shouldn't it still fire with minimal spark though?

What seems to be my issue??

[James]

some things i would try:
run an inline fuel filter
drain the tank
SeaFoam on a fresh tank
inspect bowls with magnifying glass for cracks
wait for answers from internet forums.

[shinyribs]

One thing I ran into on mine was the needles.I went and bought new needles right off the bat. They were the rubber tipped kind. They poured fuel constantly. So I dug my old solid metal tipped needles out of the trash and reused them. If you have any of the metal tipped needles,put some tooth paste on them and ''lap'' them into the seats like you would when lapping valves. Nearly 7,000 miles later and never a fuel leak.

On the backfiring thru the carbs. I cant explain this,but on some older carbureted engines they will tend to spit back thru the carbs when you go to start them. It seems that would only happen if combustion is reversing,like you explained,but sometimes when they lean out while starting that can happen. A good working choke and a correct idle mixture setting can cure this. IF that is even your problem

Don't you just love ''mystery'' bikes? Good luck bud!

[jeremy]

shinyribs has some great ideas.

what year is your bike and ill see if I can find some carb settings for you.

you can email

[e-mail]  put cb750 in the subject line.

[jeremy]

sorry just saw your signature

1979 cb750

float level should be 15.5 mm

if the tank is dirty clean that up

tapping on the float bowls with the handle of a screwdriver sometimes works too.

[seestheday]

Sounds like you have to clean your tank.  I think your floats are either sticking, or the float needles are damaged/incorrect.

When you checked the floats, did you just check to see if they floated, or did you check to see if you got a good seal?

When you reassembled your floats, did the needles slide in and out of the seats easily?

To check to see if they're sealing, try this:
- With the carbs off the bike, and after cleaning the float needles, float needle seats, bowl etc , reassemble the carbs.
- Turn the whole bank upright and level
- Attach overflow tubes and direct into a container to catch fuel in case they leak
- Pour CLEAN fuel into the gas line in nipple

If everything is working properly, you'll be able to fill up the carb bank until gas starts overflowing out of the gas line in nipple.

If you leak out of the overflow tubes, you know you have an issue.  Take off the bowls that you're having trouble with and inspect the float needles and seats again.

[alehman]

Hey all, thanks for getting back to me. I forgot to mention that I am running an inline fuel filter.

Shinyribs, I did actually just replace the needles with new rubber-tipped ones (the old ones were also rubber) so it's funny you should mention your issue with them. I'm wondering if they just don't sit correctly. As for the fuel, I'll try enriching the idle mixture--- there is no airbox on the carbs, so it makes sense if it is running lean. I didn't know that about the carb backfiring.

seestheday--- when I checked the floats, I dipped them in a little container of gasoline, and they seemed to be very buoyant. It's funny because before I checked them, it was consistently just the #1 that was pouring, and then when I reassembled them it was #1, #3, and #4. I'm flummoxed. I'll definitely give that test a shot, because at this point it's either the needles themselves not seating correctly or particulates in the fuel messing with the seal.

One more question. When I was retightening the valve cover bolts yesterday after checking clearances, I actually snapped not one, but two bolts (bad, I know). I didn't think it would really affect whether or not the bike would start, so I continued putting everything back together and tried to fire it. Of course, nothing happened, but then I heard a hiss of gas escaping. I tried it again and listened for the source and it seemed to be coming from the area of the valve cover with the broken bolts. Granted, I might be completely wrong about where it's coming from, and I'm going to double check. But would it make sense for there to be a pressure buildup under the valve cover? It wasn't there before the bolts broke, and I don't understand why there would be positive pressure under there. Whatever it is, it makes me nervous....

[seestheday]

You snapped the steel valve cover bolts on the aluminum threads?

If you mean that you stripped the threads, then I wouldn't start it until I fixed it. There are posts that can break and you don't want a large amount of aluminum floating around your cams.

Also, go out and buy a torque wrench :-).

Sent from my BlackBerry device on the Rogers Wireless Network

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From: "alehman [via Honda CB750'S]" <[hidden email]>

Date: Mon, 11 Jun 2012 14:21:24 -0700 (PDT)

To: seestheday<[hidden email]>

Subject: Re: Carbs leaking, starting problems (still)

Hey all, thanks for getting back to me. I forgot to mention that I am running an inline fuel filter.

Shinyribs, I did actually just replace the needles with new rubber-tipped ones (the old ones were also rubber) so it's funny you should mention your issue with them. I'm wondering if they just don't sit correctly. As for the fuel, I'll try enriching the idle mixture--- there is no airbox on the carbs, so it makes sense if it is running lean. I didn't know that about the carb backfiring.

seestheday--- when I checked the floats, I dipped them in a little container of gasoline, and they seemed to be very buoyant. It's funny because before I checked them, it was consistently just the #1 that was pouring, and then when I reassembled them it was #1, #3, and #4. I'm flummoxed. I'll definitely give that test a shot, because at this point it's either the needles themselves not seating correctly or particulates in the fuel messing with the seal.

One more question. When I was retightening the valve cover bolts yesterday after checking clearances, I actually snapped not one, but two bolts (bad, I know). I didn't think it would really affect whether or not the bike would start, so I continued putting everything back together and tried to fire it. Of course, nothing happened, but then I heard a hiss of gas escaping. I tried it again and listened for the source and it seemed to be coming from the area of the valve cover with the broken bolts. Granted, I might be completely wrong about where it's coming from, and I'm going to double check. But would it make sense for there to be a pressure buildup under the valve cover? It wasn't there before the bolts broke, and I don't understand why there would be positive pressure under there. Whatever it is, it makes me nervous....

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[alehman]

I'm not sure exactly what the damage is yet, but I think one of them is stripped and the other one definitely broke. if one's stripped, it shouldn't be a problem to get out, and I'm sure I can figure out the other one. I really hope that it was the threading on the bolt that gave and not the threading that the bolt screws into. What do you think about the escaping gas though?

[robs750]

The pressure you hear is coming from the cranckcase.The oil flows from the head down oil passages to the crankcase.The pressure goes up those oil passages.
You probly did not hear it before because the valve cover was sealed.

[Lucky 1]

This whole conversation is scattered all over the place WOW!!

This is a 1979 CB750 .

The carbs should come last in the tuning up process.
Of coarse the gas leaking is an emergency.
QUOTE: "the gas is kind of rosy red like rust"

Take the tank off and CLEAN it. IF it needs to be sealed do it.
I recommend Caswell's tank sealer with one can of white colorant from tap plastics.

As far as the float valves... if it was me I would remove all of them and throw them in the garbage can.
End of that story.
Then get 4 brand new valves WITH seats and install them. Then set the float with a float gauge like the manual tells you. 15.5mm or .610 thousandths.
QUOTE: "I have even adjusted the tangs on the floats slightly."   Just set them to the specifications in the book.

WHEN YOU PUT water in the fuel line that means ANY WATER left in the carbs anywhere means all of the water would have to be removed. Even ONE drop of water can keep the carbs from working right.
GAS and WATER do not mix.


Now ALL the water MUST BE REMOVED from the system. That means the carbs will have to come off for cleaning.

You know i do not even think you should be working on this bike. For your own safety .
STATEMENTS like: "and they're pretty aligned with the valve cover surface as they should be. They're maybe a few millimeters off"  I want to tear my hair out.

LIKE I said all issues with the ENGINE must be done correctly before the carbs !!!

[alehman]

The water in the carbs was a quick, un-messy way to check the floats. I know it doesn't mix with gas, and it's actually really dangerous to get into the combustion chamber in part because water expands something on the order of 1700 times in volume when it turns to steam. That's bad news. The carbs were off when I tested them that way, and went nowhere near the engine while there was water in them, and I dried them with a compressed air hose.

Do you have any pointers on how to get the valve timing mark on the cam sprocket perfectly level with the valve cover surface? It's about a millimeter or two off. Neither the FSM or my clymers is specific on how to do that. I'll check out that tank cleaner. Thanks for the pointers.

[shinyribs]

Alot of guys have had good success cleaning their tanks with muriatic acid or CLR. There are a few posts on here talking about using the CLR with really good results just recently. The CLR is much less violent and I would imagine a lot friendly to use.

  I know Lucky just lined his tank and has had no problems with it,but there seems to be alot of guys with lined tanks having serious issues with them. Both DIY lined tanks and professionally done.  A clean tank full of gas won't rust,even without a liner. Once you get it clean(if needed) just try to keep it full for the most part and you will be okay. Ever hear of people lining tanks in classic cars or muscle cars?Nope. Tank liners seem rampant in the motorcycle world.I dont know why. Just putting it out there. You decide for yourself on that one

 Measuring your floats with a tool would be ideal,but just getting them level works for most of us here,including me. These bikes go up and down hills and lean both way at several different degrees of angle on any given ride. The float level really doesn't have to be rocket science. As long as it doesn't starve for fuel while riding or slosh out you'll be just fine. Level works.

Good luck on your project and don't let anyone discourage you.You CAN do it.Sometimes you just have to pull a little hair out in the process

[alehman]

Thanks shinyribs, I really appreciate the help.

[seestheday]

Lucky, where can you get a new float needle and seat for a DOHC?  Will the ones that they sell for the SOHC's work?  Are they the same?

Sirius, parts and more, and old bike barn don't have any listed.

[seestheday]

I think I'd rather have the threading on the head go than the bolts.  The DOHC valve cover bolts are unique and I'm not sure where to buy them.  If you're not sure, it was probably the threads in the head that gave out and not the bolts.  The head is aluminum, and the bolts are steel.  Steel is much stronger than aluminum.

Which ones broke?  Was it the ones on the outer edges, or in the centre?
There are posts that the bolts go into that can break.  Happened to me (http://honda-cb750-s.456789.n3.nabble.com/Heli-coil-Timesert-question-td3931333.html).

If it's on the edge you can fix it with a heli-coil while it's mounted in the frame.  If its one of the centre ones you're probably going to have to pull the engine to get access to drill it out straight.

Also, seriously, go out and buy a torque wrench.  It won't always help (mine broke under the torque specified in the FSM and I suspect it was already mostly broken) but it will help a lot of the time.

[Lucky 1]

Yes many of them are the same part!!! Used on many bikes and on different brands.

[shinyribs]

Lucky is right. There were several bikes that used these carbs.Not just CB's. My keihin roundtops for example,I dont know what other bikes came with them,but if you buy jets for them from 'vintage' parts places you will pay vintage parts prices. I got my main and pilot jets from my local bike shop and paid about 1/3 of what the online companies want for them. I got two sets of each for about $40. Thats 16 jets total! They really hold you up on the prices on these CB parts if they know they can get away with it.

[alehman]

I may be mistaken, but I'm not sure if the seats in DOHC carbs come out. Seestheday, I believe it was two of the outer ones. Strangely enough, I was using a torque wrench and the breaking/stripping occurred when I was within the recommended torque range. Go figure. Don't the valve cover bolts tighten into the cam bearing brackets, which are also steel?

[seestheday]

I know that some definitely thread into those posts that you can see in the thread I linked to and that they are aluminum. Now I'm 2nd guessing if they all do.

Sorry to harass you about the torque wrench :-).

Sent from my BlackBerry device on the Rogers Wireless Network

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From: "alehman [via Honda CB750'S]" <[hidden email]>

Date: Tue, 12 Jun 2012 15:33:42 -0700 (PDT)

To: seestheday<[hidden email]>

Subject: Re: Carbs leaking, starting problems (still)

I may be mistaken, but I'm not sure if the seats in DOHC carbs come out. Seestheday, I believe it was two of the outer ones. Strangely enough, I was using a torque wrench and the breaking/stripping occurred when I was within the recommended torque range. Go figure. Don't the valve cover bolts tighten into the cam bearing brackets, which are also steel?

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