Ride on

[Ceeby-jeebies]

Hi to everyone. Just acquired a 78 CB400 F2 with a believed genuine 15,000 miles on the clock. Rode it 100 miles home, sadly far too much on the intimidating M25. Seems to run well apart from a slight hesitation when opening the throttle from a steady speed, and tending to cut out when changing down at slow speed. Any ideas? Has had a recent carb strip and clean apparently. I have just fitted new air filter and plug caps but no difference. Used to have an identical candy antares red F2 in the late 70s (VLP267S, where are you now?) which took me on an epic 5,500 mile tour of Europe in 1979, so the new bike is a bit of a trip down memory lane…

[Nurse Julie]

Welcome to the Forum. That's a great looking bike. It's probably the carbs, most likely idle jets partially blocked. Do you know if the carbs had a complete strip and clean and was the original brass ware re fitted, were they bench synced? What are the air screws set at? Also, is the tank nice and clean inside?

[smoothoperator]

Welcome, lovely looking bike! I'd start with the simple stuff first and take a look at the plugs, that should give you some indication on where the fault might be, especially as you have had a recent long trip.

[Ceeby-jeebies]

Thanks for the replies. I don’t have the detail on what was done when the carbs were stripped and cleaned, only that it was last November and the bike has not done many miles since - probably only a couple of hundred. The tank looks very clean inside though. I’ll take a look at the spark plugs tomorrow - the seller said they had recently been replaced as well.

[Ceeby-jeebies]

I should add that the only other thing I have noticed on the bike is that the airbox manifold chamber has a small split on both sides - I don’t know how significant that is.

[McCabe-Thiele (Ted)]

At that low mileage I  would dump the fuel give the tank a wash, fill with fresh SUL petrol, fit an inline filter and as has been said clean the jets out.

Old petrol sets like a laquer when a bike is used infrequently then when you run it jets get blocked.

[philward]

Welcome to the forum. Sounds like fuel and agree with previous post but additionally, check the fuel tap flow - I had recent problems with hesitancy and had a problem with the petrol tank cap not venting AND separately petrol tap not free flowing. As a first check, worth a go prior to carb strip - just take tank off and switch tap on to flow into a bowl - lot easier than a carb strip if subsequently not required.

[Ceeby-jeebies]

Thanks for the suggestions. Removed the plugs and they are all blackened, so it looks like it is running rich. How is this best fixed? I’m not much of a mechanic myself…

[Nurse Julie]

Middle 2 look not bad, a little lean for my personal liking, the outer 2  are only a tad rich. There is a air/ fuel mixture screw on each carb, you need to see how many turns out each screw is at and then adjust accordingly. It is always better to run slightly on the rich side than lean side.

[Nurse Julie]

Resistor plugs with resistor caps, not a great idea. Soot on plugs could also be because you've been using the choke a lot to get it started. Take it for a decent run and test them then.

Good spot Ken, I hadn't noticed the plugs were resisted, which is pretty amazing. I don't suffer with OCD but resisted plugs and resisted caps gets me up to borderline anxiety levels 😁😁😁😁