Topic: Trixie - 1977 CB750K - Project

[lucky]

Thanks!  Yeah I actually had that exact page open.  I was thinking of taking a bike to a local shop to get their opinion but I feel like I would regret doing that.

Yes you would regret that.
You would regret it even more if you let them work on it.
I am not saying they are ALL bad or incompetent but most of the bike mechanics now do not know much. I read the stories.

[BrockSamson]

I am having a problem with the bike.  It was idling fine and now all of a sudden the RPMs are running out of control.  As soon as I kick it shoots straight to 4k RPM and I have to cut it off.

The idle is all the way out, so that is not the problem.  I am very confused how it could go from a stable idle at about 1400 RPM and now its acting all goofy.  Anyone know whats going on?

Another question, why can't I get one of the spark plugs out?  Is there something I am missing?  I was able to pull the outside two and one of the middle ones... its not rounded... the problem is that I can't get the socket deep enough to get around it.  Its weird.

Wanted to bump to see if anyone had any advice.  Thanks!

[BrockSamson]

Here is a picture of the plug in question.  I think I need a thinner walled socket... I just can't think of what else it could be...

Also, as promised there are the pictures of the wrecked paint :/.  I should have realized there was a vent on the top.  I will never make that mistake again.

[BrockSamson]

I typed this up and put it in the How-To section and realized that was a mistake as it clearly says not to do that.  I apologize and can a moderator please delete the post?  Thanks!

I keep searching and I don't think I am using the right words to find what I am looking for.  Keep in mind I am a motorcycle and carb engine novice. 

I have a 1977 CB750K with Mikuni 29mm Smoothbore carbs that were recently rebuilt.  There is also a 4 into 1 exhaust on the bike.  The engine is unmodified otherwise.

Series of Events:
1) Carbs get put on the bike after rebuild and after second kick starts up.  Idles a little wonky but eventually settles down and hits a nice ~1400 RPM idle. 
2)  I fiddle with the idle adjuster and it works appropriately.
3)  I walk away from the bike.
4)  Next morning I go to change the spark plugs.  I get them all out but one (this is another issue I am having... I can't fit the socket over the stupid plug... I must need a thinner wall socket ?? idk).  I attach the pod filters so I can start tuning to those or at least see if there is any change to how the bike runs.
**(As I am typing this I just realized I might know what the problem is.  I am going to continue)**
5)  I kick start the bike and the RPMS shoot straight for 4k and there is some popping (assuming this is back firing of the engine).  I freak out and flip the kill switch.
6)  I try a couple more times and the same thing occurs.  I stopped knowing that its not a good thing to rev the engine cold like that.
7)  I yank the pod filters off to make sure the slides are just barely cracked and nothing is stuck open.  Everything looks great.
 I take the idle adjuster and back it all the way out, so the slides are as closed as they will ever be.
9) I back the choke off and try to start the bike, no go. 
10) Go back to full choke, start bike and it again it runs away from me shooting straight for 4k RPM.  I kill the engine.


I am at the point where I am flat out afraid to start the bike up again.  So after being clueless, I think I might know whats wrong... funny how talking something out can help your brain work. 

I was going to buy the $150 perfect fit rubbers from Sudco that are designed to fit the 29 mm smoothies to a 750k.  I already had these hacked up rubber heat resistant engine hose things that were on the bike before and worked.  There was no problem other than looking a little junky.  I figured I would save the money and get the sudco "intake manifolds" another time.  So I go to fit the carbs onto the engine and its a real pain in the ass to get it back on.  I double checked and they seemed to fit all the way around the inlets... I am starting to wonder if one of them slipped off and there is an air leak somewhere.  Does anyone know if that would result in the issues that I am having?  I am going to stop by the garage tomorrow to look at it and see if that is what happened.

Any help, insight, knowledge, experience... anything would be very much appreciated.  I am trying to trouble shoot this on my own, but like I said before I am not very smart about these bikes or carb motorcycle engines at all for that matter. 

Attached is a picture of the carbs and you can see the blue rubbers I am using to mate the carbs to the engine.

Thank you!

[BrockSamson]

Well, I dug into things to figure out how to solve my problems.  After using the picture to determine the actual clearance allowed between the spark plug and that piece of metal I determined that I needed a thinner walled socket.

Sears craftsman 12-point standard deep socket had a clearance that was a millimeter or two better than the sockets I had.  It actually got the job done and I was about to get the spark plug out.

Now to the uncontrollable revving of the engine.  I examined the heat resistant rubber tubing used to mate the carbs to the engine and they are just not getting an ideal seal around the engine intake inlets.  I would have to fiddle with them for hours to get an exact fit.  I realized this is just a cheap fix to fitting the carbs.  Just to see if there was a difference after trying to fit them better I started the bike.  It still idled high but about 1k less.  I am certain I am having a vacuum leak there and I will be ordering the direct fit "intake manifolds".  During this time I noticed a little drop of oil under the bike after kicking it and letting it run for a couple seconds.

Current Todo List:
-Fit carbs to engine with new rubbers
-Tune
-Drill replacement side cover, sand, prep, paint, install
-Ride the bike.

Still trying to decide what to do:
-Tank paint situation (see pictures a couple posts before this one)

[ZanVooden]

Yes, as you've figured out, it certainly sounds likes a leak at the manifolds. I've also run into the same thing with the spark plugs on a 550 and have found the best thing to use is the factory tool kit. there is a plug wrench in it.

I have a feeling you're not getting many responses to your questions because not many people here know much about the carbs you're using.

Did you paint the tank yourself? did the gas sit on the tank for a long period of time? It appears that the tank has been painted before judging by the blue color. imo you should just plan to repaint the tank, and maybe have it done by someone who has access to automotive or better grade paint. I've had my tanks done with good quality paint and little gas drips dont affect. Something to consider, and well worth the cost (especially if you make friends with a body shop painter) .

[BrockSamson]

Hey ZanVooden, I agree... a combination of the carbs being a bit obscure and actually the manifold air leak is a common issue.  I was just not using the best search words to find it.  Ultimately I shouldn't have stuck with the short cut the PO took when mounting the carbs.

I spent so much time trying to figure out why I couldn't get the socket around that spark plug.  Finally after snaking my cellphone camera in there I snapped a picture and it was obvious.  The craftsman socket did the job so I won't be purchasing the Honda tool.  I wonder why the engineers couldn't knock off another millimeter or two of steel on that #2 plug. 

The tank... The PO had told me "It will need to be repainted.".  Seeing the unimpressive flat black I understood.  What had happened with the tank was a constant slow drip of gas out of the top vent.  This happened because my inexperience and flawed logic led me to storing the tank upside down.  It slowly ate through all those layers over two weeks.  Apparently in the bikes lifetime whoever owned it just blasted another layer of paint over the previous.  Not all that surprising.

My current plan is to strip it down, seal it and blast another flat black layer from a rattle can.  I am doing this because I am an inexperienced rider.  It will take some time getting acquainted to riding and the bike and I know there is a high chance I will drop/lay down the bike.  Putting a lot of work into paint or paying for a pro job is not practical right now.

[BrockSamson]

Ordered the new rubbers.  Also picked up a scorpion closed AGM battery.  I saw some pictures of a frame that had been eaten away by spilled battery acid.

Beginning to realize there will ALWAYS be something else to buy with this bike.  I am ok with that though, I knew I was getting into a handful with a 1970s bike.

[knowsnothing]

Ordered the new rubbers.  Also picked up a scorpion closed AGM battery.  I saw some pictures of a frame that had been eaten away by spilled battery acid.

Beginning to realize there will ALWAYS be something else to buy with this bike.  I am ok with that though, I knew I was getting into a handful with a 1970s bike.

That just keeps it interesting    But yeah i definitely know what you are talking about

[BrockSamson]

I have to say, there is nothing I like better than heading over to my pals garage with a few beers and getting my hands dirty.  When I can finally safely ride this bike down the road it will be worth all the money and effort.  I am hopefully getting the guy at Sudco to send those boots out next day so I can work on the bike this weekend.

[BrockSamson]

I am still having a bit of trouble with the bike.

Can't remember if I mentioned this before or not... but it is NOT leaking oil.  So some good news.  It was also blowing a good bit of white smoke but that turned out to just be condensation in the pipes.

Advice I need:

1)  My starter button has ceased to operate.  Light still dims when pressed but not other noise or mechanical operations are occuring.  I also looked at what I thought was the fuse and it looked intact and not blown.  I literally ran the battery down a couple weeks ago until the battery was completely dead.  I also accidentally left the key in the bike and the battery ran down again.  After that it stopped working.  I got my new AGM scorpion battery today so that will be going in tomorrow.  Where do you guys think I should begin to repair the starter operation?  It still kick starts so this is not a huge deal and I actually like kick starting it over pressing a button.   

2)  I got the new Sudco "intake manifolds" or the boots that mate the engine to the carb bodies.  They worked GREAT, and are much easier to get on than my previous solution.  Worth every penny.  Also, I believe there are no more air leaks and as soon as the bike is running consistently I will do the standard tests.  I started up the bike after getting the boots on and it ran a little high on RPMs but after some time (3 minutes?) and fiddling with the Idle adjuster it got down to the 1000-1400 RPM range.  Then it kept dropping RPM until it died.  I had full choke the entire time, did not have the pod filters on as I was trying to adjust the idle and wanted to watch the slides move.  I am going to attempt to start the bike tomorrow and if I can't get it going what should I do?  My plan was to pull the carbs off and inspect them. 

I am really excited because I go to the motorcycle training classes and then get my full license at the end of the month.  I am dying to have this bike running in time for that. 

As always, thank you guys very much for taking the time to help me out!

[mrrch]

I think with 3 minutes of full choke you fouled your sparkplugs.

[BrockSamson]

Really with only 3 minutes it will foul that quickly?

Dumb question:  How long should I normally be at full choke?

[BrockSamson]

I wanted to bump this for the questions in my last couple of posts.  Thanks!

[mrrch]

I try to get mine off the choke as soon as possible (usually 10-15 seconds) but I do have to play with the throttle some to get it warmed up.

[BrockSamson]

It will not run without choke after 30 seconds...

I might have one of the following wrong, correct?
-Clogged Pilot Jets
-Air leak in boots
-Should put my pod filters back on instead of trying to run without them

When I finally get a chance to sit down and work on the bike I will look at those three things and pull the plugs to make sure they are not fouled.

[mrrch]

Check the jets and the fuel supply (drain the bowls and check for crud)
Leave the pods off for now until you can get it to idle at least.
You will know your fuel system inside and out in no time.

[BrockSamson]

Alright, great news.  It seems that the boots still were not on all the way and also the air/fuel screws needed adjusting. The bike now idles pretty well.  I do get a bit of air being pushed out the carbs on the air intake side, it like burps.  It seems to come and go as I tune the screws.  I was able to get it pretty happy and hit the throttle and it was super responsive.

I did notice some smoke coming out of my crankcase filter, but this seems to be common after checking to forums.  It was smoking a lot, like a cigar.  I am going to attach a picture of it just in case someone things its a big deal. 

I also can not figure out where the hell the damn fuse for the starter is.  My button still doesn't start the bike and only the kickstart is working.  I am attaching a picture of the fuses I am looking at also.  Can anyone tell me if I am looking at the correct location?

Thanks everyone.

[BrockSamson]

Bump 

[BrockSamson]

Update:

I wanted to thank everyone again who provided information in the thread I created about my VM29 Smoothies and in this thread.  I would be lost without this forum.

I have the original PD carbs back on the bike.  Stock airbox with new K&N filter.  I rode the bike for the first time yesterday!  I haven't stopped smiling!   

Does anyone know if the search is broken right now?  I have a couple of issues I wanted to get answers for and I really don't want to be the guy who didn't take the time to look up the answers himself.  It just is not producing results for me at all.  Before I would always go to the home page and search from there...

[BrockSamson]

Google search works.  I should have scrolled down to those other sections I never look at. 

I am currently researching why I can not get the RPMs over 4500-5000 in every gear.  It stutters and bogs down.  From what I have read there isn't a quick answer. 

The mechanic that worked on my bike slapped a fuel filter on the line from the tank to the carbs.  I haven't heard of too many people doing that here.  The inside of the tank is spotless... no rust or anything.  Fuel deprivation? 

Either way I am going to run the bike for a couple tanks of gas and run BG44K through them to see if that  makes the (valves?) happier.  Given that the bike sat awhile I don't want to run around taking shots in the dark.  Next thing I guess is to put in new plugs and get it up to that stutter RPM than cut it off and check out the plugs. 

The bike pulls great through the early range of RPMs and then just won't go above that RPM.

[BrockSamson]

Read TT's responses to an inline fuel filter thread.  Every time I come across his stuff I am floored.  The combination of reason, facts and thoughts that are always supported by scientific methodology... I am an engineer by trade and he is still on another level.  Awesome.  So that part of my question has been answered .

[seanbarney41]

Google search works.  I should have scrolled down to those other sections I never look at. 

I am currently researching why I can not get the RPMs over 4500-5000 in every gear.  It stutters and bogs down.  From what I have read there isn't a quick answer. 

The mechanic that worked on my bike slapped a fuel filter on the line from the tank to the carbs.  I haven't heard of too many people doing that here.  The inside of the tank is spotless... no rust or anything.  Fuel deprivation? 

Either way I am going to run the bike for a couple tanks of gas and run BG44K through them to see if that  makes the (valves?) happier.  Given that the bike sat awhile I don't want to run around taking shots in the dark.  Next thing I guess is to put in new plugs and get it up to that stutter RPM than cut it off and check out the plugs. 

The bike pulls great through the early range of RPMs and then just won't go above that RPM.

...bet it wasn't the fuel filter though...next up, check your float levels with the clear tube method, you need the fuel level in the bowl to be 3 or 4 mm below the gasket, next thing, is your ignition advancer working?...next, change to a smaller main jet...what you got in there now?

[BrockSamson]

seanbarney, thanks for the advice.

My only thought about the fuel filter was if it was starving the engine at higher RPMs.  I really do not know much about gravity fed carb fuel systems but it must be demanding the most fuel flow at those higher RPMs.  I also have about 1/5-1/6 a tank of gas and after reading some threads I am going to question the petcock. 

I am going to fill up the tank, put some seafoam in and go from there.  After that I am going to check the floats and then get some help checking to make sure I am getting timing advance. 

They are stock PD41A carbs.  Only mod is 4-1 exhaust and a new drop in K&N.

I figured it wasn't jetting because it pulls hard up until nearly 5k.  It pulls really well... so doesn't the carb get out of the idle circuit based on throttle?  If so I would say the main jet circuit is happy.  But I could see how the higher fuel demands at higher RPM are obviously dependent on the main jets. 

I need to pull out my hondaman book and do some more reading...

[BrockSamson]

Update:

I looked at the fuel filter and it is clean.  I was skeptical about the usefulness then I thought about what happened to my tank.  If you go a page back you will see pictures of my tank paint completely wrecked.  That paint started to flake and crumble and it is very possible that until I fix the tank paint that some could get into the tank and clog the petcock or even the carbs.  So I am going to leave the fuel filter to protect the carbs. 

So yesterday I hop on the bike and plan on filling the tank with gas.  I switch the petcock to "ON" and go on my way.  Two left turns later the bike dies.  I flip it to "RES" and I am off again.  I guess the gas was much lower than I thought.  I get to the gas station fill the tank and dump 1/3 of the seafoam into the tank and flip the petcock back to "ON".  Ride it back and it drove better than it had ever before.  Even hit higher RPMs.  Well my joy and elation was short lived. 

I pull outside my buddies house, flip the kickstand down and hop off the bike.  I am walking up the driveway when I hear drizzling liquid.  I turn around and the bike is hemorrhaging fuel.  I had not flipped the fuel back into the "OFF" position because my buddy was going to ride it.  The fuel was pouring out of the #1 carb drain.  So I roll the bike into the garage, remove the tank, air box, cables, and the carbs.  I open up the #1 bowl and all looks great.  No problems at all.  I open the #4 bowl and it looked identical but I do notice one difference... the clip that holds the float needle pin thing was backwards on the #1 carb.  I flip it around and hook the carbs up to the fuel tank and flip the petcock switch.  No leak.  Put everything back on the bike and start her up.  Problem solved.  I can't imagine that the clip being backwards mattered and it was probably just a stuck float.  Next time I am just going to give it a tap with my foot before I tear everything apart.  Although I did notice that the mechanic forgot a clamp on the engine side rubber manifolds and a clamp on the air box side manifold stacks.  Which infuriates me because I just fought air leaks.  I fixed that potential issue.

So by the time I was done working on the bike it was night.  I was determined to ease my mind and drive around the neighborhood.  This would be my first night ride, so I kept it very close to home.  There is a road where I can do 45mph.  This brings me to two questions:

1)  I hopped on the bike and flipped the key into the "ON" position and started the bike up.  The gauges did not light up AT ALL.  I could not see RPM or Speed.  Are these supposed to light up?

2)  I put the key into the "ON" position and I was able to pull the key out with it in the "ON" position.  Is this normal? 

That is about it.  It drives great and I am having a blast riding!  Despite all the work yesterday I went home with a huge smile.