Topic: DOHC 750 fuel coming out overflow while parked sometimes (float is correct)

[ct_racer]

Subject probably sums up the issue, but in detail...  It's a 79 CB750, and pretty randomly when I park it will drip fuel out of the #2 cylinder overflow tube.  It's enough fuel to leave a pretty decent size spot on the ground when it happens, but it only happens once every couple weeks maybe.  It's been happening the 3 times I parked on my lunch today, but hasn't happened in a couple weeks (I've ridden probably a half dozen times since the last time it happened.

I know the float is correct, I checked it when I first got the bike back in September.  The #2 is the one with the vacuum pump assembly attached to it so I am wondering if a leak might be causing the bowl to overfill or something.  My only other thought is possibly a worn needle valve, but it seemed like it was in fine enough shape when I cleaned out the bowl area (was fairly clean but not perfect).  First time with these CV carbs, so I don't know much about them.

Any thoughts on what I should look for as to the overflow issue?

[Cougars750F0]

Ive had this problem before myself. I think you're correct on the worn needle valve or possibly a small piece of crud stuck to the seat for the needle value. Thats why its always a good idea to turn the gas off at the pit cock.

[ekpent]

I think the 79 DOHC has a vacuum operated petcock that you do not turn off like our older bikes. Maybe the problem lies in that circuit. Old Yammie 750 triples are notorious  for having  issues also with their vacuum petcocks. Other than that a bad needle and seat sounds good.

[KayOne]

'79 CB750F DOHC had a standard petcock.

[ekpent]

'79 CB750F DOHC had a standard petcock.

Yep I stand corrected,the 79 is different than my 1981. Sorry and carry on gents. 

[ct_racer]

Yes, its a standard petcock.  It does turn out it happens when the petcock is open, even when running, but again not all the time.  Its not terrible, and since I've only got a couple weeks of riding left here in Michigan I think I'm just going to live with it until I pull the carbs this winter and check it out.

Fortunately its a DOHC and parts are plentiful.  I'll have to see of its the seat or the valve, I think I'm also going to replace the accelerater pump while I'm at it.  The one in their was stuck in its seat, and I'm afraid I'll tear it if I remove it.  Oh well they're real cheap.  What isn't cheap is the vacuum leak I believe I have, meaning new carb holders are needed.  At least I can get em and they're not as bad as the SOHC ones.  Glad I never needed those for my 500.

[ct_racer]

Oh, and I do always turn off the petcock.  I added an inline filter so the fuel line now holds a lot more after the petcock than before (though it did this before I added the filter).  But the tank that came on the bike was rusted pretty bad, so I've probably got a rust flake somewhere hosing everything up.

I'm glad I put that filter on though, when I tried to flip to reserve with the old tank nothing but rust came out...  Not a fun way to get stranded.  Fortunately the bike came with a clean, all be it ugly, tank that is working great.

When I inspected the bowls after I bought it there was some rust flakes in there, but not bad.  I'm sure I didn't quite get all of it which is probably what's causing the issue.

[grcamna2]

+1 on having a clean tank.I re-line most of my older tanks w/ either Redkote or a similar product;Prep is Everything.The tank'll never have any more issues when it's relined.
It'd also be good to check the little brass overflow tube inside the bowl to see if it's cracked.

[ct_racer]

I'm going to try giving the tank the works and water acid bath this winder to try to eat up the rust.  Then actually stay up on maintaining it to keep it from rusting again.  But thats just on the list of winter projects for this one.

[grcamna2]

I've tried that solution, & a few of my customers have also,but the rust Always comes back;I'd re-line it once & be done w/ it forever.

[ct_racer]

See, I've heard terrible things about red-kote not taking and clogging up carbs, filters, fuel lines etc.  And staining gas a red color which then get's into carbs, filters, intake and all that.  And that once it starts deteriorating there isn't really anything you can do and it will just continue to slowly deteriorate and make a mess of things.  To be honest red-kote scares the crap out of me far more than a chemical bath.

From the research I've done the trick is to use an oil of some sort to coat the inside of the tank after the acid wash.  My thought is to run a tank of 2 stroke through it to get a nice oil sheen through to prevent rust, hopefully even in the exhaust.  Also, come winter if I store it with 2-stroke  it will also act as a fuel stabilizer due to the fact that they now put stabilizer additive in 2 stroke.  Obviously she's gunna smoke for that first tank, and I'll have to run her hard to keep the excess carbon out of the chamber.

I've also read to use ATF and swirl it around in there to get a sheen over the freshly bare steel.

[grcamna2]

I've never heard bad press about Redkote yet..,your the first.I always gave it to my radiator guy & he did hundreds of MC tanks for me w/ no problems.I know that the tank has to be correctly prepped and FULLY dried out "to the bone" before the application will take to it & adhere well.The tank should be rinsed w/ MEK or similar solvent before drying out, which can be a real pain... Al at the radiator shop told me he thins out the Redkote w/ MEK;I'd always let him do it because that stuff makes me sick.
I think that the special bluecoat type of sealer is even better.I contract w/ a man in a Radiator shop in Lavonia,GA. now who'll do a tank from start to finish $50 complete & I love the results.
I'd sure never use that Kreem stuff though after a few bad experiences with it.