Author Topic: CB750 - 16mpg, Reaks of gas, and smoke from breather. help! (Read 7022 times)

Blitzburgh207 · « **on:** June 02, 2011, 04:14:00 pm »

WTF is going on! haha.

I'm losing gas from somewhere - just don't know where. I recently shimmed my needles (k8) and installed 120 mains to compensate for my pods and cycle-x 4-2-1. Got about 40mpg the first time i filled up and now I'm down to 16!!
There is fuel dripping from the air (fuel) screw on my #2 carb when runing at idle and when off, but because it's wet elsewhere, I can't be sure it's coming from the air screw.
Just noticed thick white smoke coming from crankcase breather hose AND Oil tank breather hose.
It leaks gas all over my tranny if I don't turn the gas off when I park it. When I do turn the petcock off when I park it, it still smells like fuel when I return to the bike.

Ok so I figure this is a result from a faulty petcock and shimming the needles, correct? Is there something else I need to adjust when I raise the needles?

It runs great above 20mph but below 20 it kind of runs crappy. I've adjusted the air screws from 1 to 2 turns out (runs better towards 1) Timing and sync are perfect (checked again last night), but idle to 1/4 throttle is shotty to say the least, but above 1/4 throttle is perfect

.

Anyone with a similar setup who has had this problem!?

Thanks guys!

Retro Rocket · « **Reply #1 on:** June 02, 2011, 04:27:31 pm »

You need to check your oil for gas, if it has found its way into your crank case you risk an engine failure..

Steve_K · « **Reply #2 on:** June 02, 2011, 05:03:05 pm »

I would check whether the float valve is sticking or the float level is off. Try tapping the leaking carb with a screwdriver handle to jar the thing shut. Is the float chamber drain screw snug? Dirt can foul the float valve, too.
Steve

Coyote13 · « **Reply #3 on:** June 02, 2011, 05:12:35 pm »

A little smoke is not abnormal, but what you're describing sounds like you're running super rich off idle. Do yourself some plug chops and get back to us. Idle, mid throttle, and 3/4 to WOT.

And definitely check your oil. Nobody likes a blown up motor.

Spanner 1 · « **Reply #4 on:** June 02, 2011, 05:52:02 pm »

1 or more stuck floats allowing 'overfill' all the way into the motor.... I would not run your bike until resolved. Possible cause ; stuck float or floats not closing the fill valve, too high fuel level in bowls, or a 'sunk' float. What year your bike ?

Blitzburgh207 · « **Reply #5 on:** June 02, 2011, 06:43:05 pm »

I just got done changing the oil. The old oil smelled like gas. I have no choice but to ride about 30 miles home tomorrow where I have the tools to work on it, so I figured thats as safe as I can be $:-\$ ?

It's a 78 w/ the PD carbs. I've messed with/cleaned/disassembled the carbs a lot but I guess I don't understand how to adjust the float height?

Quote from: Steve_K on June 02, 2011, 05:03:05 pm

I would check whether the float valve is sticking or the float level is off. Try tapping the leaking carb with a screwdriver handle to jar the thing shut. Is the float chamber drain screw snug? Dirt can foul the float valve, too.
Steve

I don't think I have a drain screw?
Something to point out is that since I bought the bike (February), the no. 2 carb (the one leaking) has had this wierd orange/yellow/brown residue in it. Since cleaning it the first time, the residue is only on the brass and the floats, and shows a fuel level line.

Thank you guys for the help I really appreciate it.

Bankerdanny · « **Reply #6 on:** June 03, 2011, 02:49:09 pm »

The residue is gunk from the tank, I got that in mine even with a fresh screen. I finally added an in-line filter to see if I can catch more of it.

Your bike has the same style carbs as my 550. The drain screws should be immediately obvious, they screw in from the side at the bottom of the bowl, the 1-2 face to the left and the 3-4 to the right. The screw is sealed by a small o-ring.

Blitzburgh207 · « **Reply #7 on:** June 04, 2011, 06:48:03 am »

Ohhhh I always thought those were set screws for the overflow tubes haha. Guess I don't have to dump fuel all over the place when I pull the bowls off huh

The leak is coming from above that though, and I just remembered that last time I took the accelerator pump off to check it out I think I was missing the little bellow rubber that goes on the slide.
You think that could be part of my problem?
I'll be home tomorrow and I plan on working on it all day, so I'm sure I'll have more questions to ask

Thanks for the help guys!

P.S. Is there any way to be sure I've solved the problem? No leaks = no fuel in oil? I really don't want to be without my bike this summer $:-\$

Blitzburgh207 · « **Reply #8 on:** June 07, 2011, 06:30:44 pm »

Sooooooo, this is kind of embarassing, but after spending two days completely disassembling the carbs, cleaning, soaking, re-cleaning, and reassembling I finally realized my issue when re-assembling them: Last time I adjusted the needle position I installed all of the slides backwards lol. DOH! two days work for a 20 minute fix. Whatever, I bet I have the cleanest rack of 750 carbs in Maine!

I also swapped out the throttle (it was tight and didn't snap back before) and it feels like a new bike now.

A little lean, but hopefully a little jetting and I'll have my acceleration back!

wrenchmuch · « **Reply #9 on:** June 08, 2011, 06:20:56 pm »

Make sure your accelerator pump diaphram is in good shape . Bike bandit shows it at $69 for the assembly .
http://www.bikebandit.com/houseofmotorcycles/1978-honda-cb750k/o/m9458#sch401941

gnarlycharlie4u · « **Reply #10 on:** June 09, 2011, 01:41:18 pm »

Quote from: wrenchmuch on June 08, 2011, 06:20:56 pm

Make sure your accelerator pump diaphram is in good shape . Bike bandit shows it at $69 for the assembly .
http://www.bikebandit.com/houseofmotorcycles/1978-honda-cb750k/o/m9458#sch401941

sirius has it cheaper. $15 http://www.siriusconinc.com/pro-detail.php?pid=&product_id=1094

also... check out the hondachopper article on cleaning the pumps:
http://www.hondachopper.com/garage/carb_info/accelpump_valve_repair/page1.html

But yea that's really unfortunate about the slides being in backwards. Thanks for manning up and admitting what happened. Not only does it solve the mystery but it shows other members that we are in fact human, and do occasionally make stupid mistakes.

Duke McDukiedook · « **Reply #11 on:** June 09, 2011, 02:05:58 pm »

You might want to check to make sure you installed your floats correctly.

gnarlycharlie4u · « **Reply #12 on:** June 09, 2011, 02:21:43 pm »

Quote from: Industrial-sized Dukiedook on June 09, 2011, 02:05:58 pm

You might want to check to make sure you installed your floats correctly.

haha yes this is always a good point. I put mine in upside down the first time, but luckily I caught it when I went to set the float height.

Blitzburgh207 · « **Reply #13 on:** June 10, 2011, 03:52:25 pm »

Well unfortunately, I've got bigger problems now. It wont start now. I accidentally left it out in the rain for a couple hours two nights ago.
When I turn the key to the on position, it tries to start without me hitting the start button... It tries to start but wont tun over - even with the kick start or when i tried to jump it. Not even a cough. Am I in serious trouble here??

gnarlycharlie4u · « **Reply #14 on:** June 10, 2011, 06:23:01 pm »

Quote from: Blitzburgh207 on June 10, 2011, 03:52:25 pm

Well unfortunately, I've got bigger problems now. It wont start now. I accidentally left it out in the rain for a couple hours two nights ago.
When I turn the key to the on position, it tries to start without me hitting the start button... It tries to start but wont tun over - even with the kick start or when i tried to jump it. Not even a cough. Am I in serious trouble here??

sounds like you might have fried some electrics. time to bust out the multimeter

Spanner 1 · « **Reply #15 on:** June 10, 2011, 07:40:57 pm »

Stuck start button or short in start button caused by the 'carbon' ( conductive ) on the contacts getting wet and leeching across the switch causing the solenoid to engage the starter..... open up the R hand control and spray the start and kill with a good quality contact cleaner.......... good luck.
Go ahead and open the L hand too and do it all.

JDWells · « **Reply #16 on:** June 10, 2011, 08:55:07 pm »

Bump spanner. I work at a shop in SC and this is kind of common. They get wet and you get a short through the switch so the button is always pushed kind of. Also check your solenoid. They get stuck all the time. Usually you can just hit it and it'll let go but not always. Is the motor seized? You said it won't turn over. If it just won't start then it's most definitely electrical.

Blitzburgh207 · « **Reply #17 on:** June 11, 2011, 11:03:01 am »

No, the motor isn't siezed. I can rotate the crankshaft w/ the kickstart - it just won't fire. I'm out of town for the weekend but I'm going to go over it real good when I get home. Hopefully it's dried out by then.

So here's the deal: I don't think I've helped myself out w/ this situation much haha.

Stupid move #1: Like a week ago when I was adjusting the timing, I ditched the gasket for the cover. The p.o. had siliconed it on and I couldn't get it to seat properly so I just ditched it figuring it will never see rain anyways and I'll just get a new one... oops.

Stupid move #2: I was trying so hard to get it to start so I could get to work that the battery is near dead now $:-\$ Will the kickstart still start it with no real charge in the battery?

Quote from: Industrial-sized Dukiedook on June 09, 2011, 02:05:58 pm

You might want to check to make sure you installed your floats correctly.

What. Just because I installed the slides backwards doesn't mean I install everything backwards. Lol - jk jk.

But anyways, that's where I'm at with it now. I really appreciate everyone's help. I'm pretty good with a wrench, but electrical

forget it.

Oh and thanks for the link gnarlycharlie! I had no idea about the inner workings of the accel. pump like that. I'll do that when I get home too. The diaphragm is fine - clean and flexible, but I didn't know I could give it further attention.

JDWells · « **Reply #18 on:** June 11, 2011, 01:45:33 pm »

Quote from: Blitzburgh207 on June 11, 2011, 11:03:01 am

Stupid move #2: I was trying so hard to get it to start so I could get to work that the battery is near dead now $:-\$ Will the kickstart still start it with no real charge in the battery?

It probably will but maybe not. Either way it would be dumb to try to diagnose an electrical problem with a dead battery because it could cause electrical problems. Charge it and go from there. 12.7 volts is full and 12.4 volts is dead technically. Batterys are pretty sensitive.

And get a new gasket...

Blitzburgh207 · « **Reply #19 on:** June 15, 2011, 06:24:05 am »

Well, found my short haha. The cable for the start button and killswitch was getting caught in the steering stop; effectively cutting it in half. That explains why it would try to start with the key on at full lock! lol. So for all you noobs like me: double check your routing.
Luckily i have an extra one on my parts bike. So with any luck, I'll be back on the road tonight :-)

Blitzburgh207 · « **Reply #20 on:** June 15, 2011, 06:10:39 pm »

So I replaced the rh controls and she started right up

I also put the airbox back on, threw some 130 mains on and it runs like a dream! Full, smooth power though all the gears. I couldnt be happier

Guess I can start on some aesthetics now!

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Author Topic: CB750 - 16mpg, Reaks of gas, and smoke from breather. help! (Read 7022 times)