CB77 High Idle When Hot

[Merz64cb77]

I just came back from a 30 min ride on my CB77 and for some reason after downshifting from third and while in first with the clutch in the revs starting climbing to about 5000 rpm. The revs drop after I release the clutch though. So I pulled over and quickly checked the throttle cable and it was ok so the only thing I can think of is the slide idle screw. When I started the bike and left my house for the ride the idle was fine. Any ideas?

Thanks

[DJM]

I had similar problem a few years ago with my restored CL77 which had never really run as well as I hoped it would.

The story is much too long to retell here but it eventually turned out that the problem was the ignition advance - retard unit.

When I replaced that the bike was transformed, went the best it ever had and finally lived up to my expectations.

Certainly worth checking out your ignition timing, static with a simple bulb on a wire or continuity tester then fully advanced at around 3500 rpm with a strobe.

[Merz64cb77]

Hi DJM

What exactly is the ignition advance - retard unit on the cb77? Do you have a part number?

I’ll check the static timing but I think it’s ok as I set it only a couple miles ago

Thanks for the reply

[DJM]

The ignition advance retard unit on the 72/77 Series of engines is incorporated in to the camshaft drive sprocket tucked away nicely in the centre of the cylinder head! Part number is 14320 268 000

Before you even think about changing it I would verify with a strobe light that it is definitely faulty, I suggested it only as a 'possibility'.

The units aren't the most reliable of components, especially if the engine has done a fair mileage, replacement, needing the engine out and usually the cylinder head off, is a pain even if you can find a good replacement unit.

Not one of Honda's finest design achievements, that said, Honda stuck with it right through the production run from 1961 to 1967 and made hundreds of thousands of the engines so they must have been reasonably happy with it.

However, as far as I know, this design was not used on ANY other Honda model, or one from any other manufacturer so perhaps 'lesson learned'

[Merz64cb77]

I really hope I don’t have to change that part and do more engine work. I had the top end rebuilt about 50 miles ago and removing the engine at this point would be a set back but what can you do. I had the timing set with a light at a vintage repair shop about 6 months ago but if the problem persists I’ll bring it back in and get it checked out.

As for mileage the bike has 17000 miles on it and has generally been running well up to this point and didn’t present itself until yesterday.

I’m going to take it for another ride tomorrow and will see how it goes.

[DJM]

These old carbs are notorious for sticking slides, particularly if the flanges become distorted which can easily happen if the fixing nuts are overtightened, they only need to be tight enough to pull the carb snug up against 'O' ring . If you 'fully' tighten the nuts, say in the same way you would the exhaust fixing flange, you are asking for trouble. You could try loosening the fixing nuts half a flat at a time to see if that helps.

I run a CB77 and a CL77 and at times both have suffered from sticking slides, I run them with the carb fixing nuts only just tight and held with a spot of Locktite, the High Tension wire clips are still loose enough to move easily but not loose enough to flop about.

Does your CB77 simply stay running at high(er) revs when you close the throttle or do the revs actually rise from tickover at random times? To me at least the latter might point towards the advance retard unit.

[Merz64cb77]

I checked the nuts and they were a little tight so I backed them off a little bit. Newbie mistake as I didn’t know about the over tightening. I’ll go for a ride tomorrow and see if I get any sticking slides. I took the air filters and boots off so I can get a better look. I think one of them must be sticking because when cold the bike idles perfectly.

In my case the revs simply stay when I close the throttle. Nothing odd at random times.

Thanks again for the help.