1981 CB750C - Hard Start after sitting 3 weeks

[cyber104]

Hi All - first post here.

I restore and repair motorcycles as a side business but mostly Honda XL's and old Can Am's.

I was recently given a 1981 CB750C in near showroom condition. It runs just fine but I have 2 issues I was hoping for some guidance on.

1) If I start it everyday it fires off like it is fuel injected - meaning right away - needs no choke. If I wait 3 weeks or so and try to start it - it takes a while. The fuel system is original and I simply installed a new inline filter in place of the old one. I was using the choke in this case and perhaps I didn't need to and it is just flooding? I'll try again with no choke after I fix issue #2.

2) The starter would only catch intermittently - sometimes turning the engine over and other times just making a grinding noise. I pulled the starter clutch and the only issue I see is that the spring seat cones which sit in the spring recession holes are stuck and at an angle. In one case it looks like something tried to actually push the cone out the back of the clutch. I can't imagine what would have enough force to do that? Has anyone ever seen something like that before? I have a used clutch on the way.

Thanks you,
Chris

 

[brettp]

My 1981 750K will take a few tries if I don't ride for a week or more. I'm wondering if the fuel evaporates somehow. In any case, i have the same thing happening.

 

[cyber104]

My 1981 750K will take a few tries if I don't ride for a week or more. I'm wondering if the fuel evaporates somehow. In any case, i have the same thing happening.

Thanks for the reply! Are you using the choke? And if so and the bike floods how do you clear it out? On a single cylinder like an XL600 you just turn the ignition off - open the throttle all the way - kick a few times and you're good to go. Don't have that option on the 750 LOL.

 

[Loose Chain]

Does the bike still have the vacuum operated fuel shut off on it?

 

[cyber104]

Does the bike still have the vacuum operated fuel shut off on it?

Yes it does. I've read that that system may be problematic but I'd be interested in hearing why exactly before I make any changes to a bike that is showroom stock.

 

[brettp]

I removed that thing when I first got it. Went gravity feed. I don't have an issue with the carbs flooding, but if I did I'd be taking that carb off to clean the inlet and float valve.

 

[cyber104]

I removed that thing when I first got it. Went gravity feed. I don't have an issue with the carbs flooding, but if I did I'd be taking that carb off to clean the inlet and float valve.

Understood but you'd have to clean/rebuild all 4 because it's not obvious which carb it is

 

[brettp]

When it overfills, the overflow bike at the bottom of the bowl should be leaking. Then you know which one it is

 

[Loose Chain]

Yes it does. I've read that that system may be problematic but I'd be interested in hearing why exactly before I make any changes to a bike that is showroom stock.

Yes they do give trouble, that is why I asked if yours had one. It is easy to just run the hose direct from the tank to the carbs as a test to see if that cures the problem. If that is the problem you have choices, rebuild it, eliminate it, or leave it on the bike and bypass it.

 

[cyber104]

Yes they do give trouble, that is why I asked if yours had one. It is easy to just run the hose direct from the tank to the carbs as a test to see if that cures the problem. If that is the problem you have choices, rebuild it, eliminate it, or leave it on the bike and bypass it.

I'll give it a try - thank you

 

[cyber104]

Hi All - just a follow up................

I sourced a nice used starter clutch as mine was damaged and not worth a rebuild. It works great now - probably the easiest repair I'm ever done on a starter.

I also bypassed the vacuum activated fuel valve - the jury is still out because I haven't had a chance to let the bike sit for an extended period of time - but at this point it looks promising.

 

[brettp]

Turn off the petcock after you stop, every time. If the float valves fail, the petcock is a good backup to prevent your entire tank from emptying.

 

[cyber104]

I've been there with all the old Can Am Qualifiers and Honda XL's I have! It sucks coming out from a nice breakfast into 90 degree heat only to find your bike is flooded and there's a huge stream of fuel running down the street.

 

[cyber104]

I just wanted to follow up on this post as I feel it's the responsible thing to do to provide the outcome/solution

Bypassing the vacuum activated fuel valve and going straight from the tank to the fuel rail worked. Starts easy even after sitting 2 weeks.

I'm also running an inline filter - just wanted to mention that because that seems to be a controversial subject based on the history I read on a few forums.

 

[Loose Chain]

I'm also running an inline filter - just wanted to mention that because that seems to be a controversial subject based on the history I read on a few forums.

Tell us more about that. Never heard of a filter being "Controversial".

 

[cyber104]

Some of that controversy is discussed in this thread.............

http://forums.sohc4.net/index.php/topic,87550.25.html

I also found this statement: The fuel filters might be the problem. I have had them cause problems with fuel flow when there was an air pocket in them. try removing them and see if your problem goes away. If it does and you want to have a filter install it vertically where the fuel feeds into the filter from the bottom and exits from the top.

 

[brettp]

Some filters do not let fuel go through them when there is air in the line and they get an air lock. I've had this happen to me and changing to a different filter (I wish I remembered the band) cured it.
Some filters are also so restrictive (10 microns) the fuel cannot pass through easily.