SOHC.co.uk Forums > CB750

F2 carb issues

<< < (2/3) > >>

Bryanj:
Dont think so Ted, i have heard of people fitting earlier 750 carbs but you need airbox complete and the rubber inlet manifolds plus clamps so it gets expensive, they were designed for the yanks requiring lean burn mix to reduce emissions, says a lot when you needed about 10 mins of choke before they would run ok.
On a 550, which does not have the extra complication of an accelerater pump i have spent days trying to get a set running right only for it to stand 4 weeks and be back to stage 1

Cb750r:
The idle jets on these carbs are very small and lean, I’ve been through my PDs and made many jetting changes as I’ve owned my bike, to suit different air filters, and exhausts. I can take reference photos of my carbs if you’re going to work on them yourself. I’m in Canada so can’t help much beyond that.

I’d suggest start with the idle jets and confirm float heights if you’re going to work on them.

K2-K6:

--- Quote from: McCabe-Thiele (Ted) on April 02, 2024, 12:26:19 AM ---
--- Quote from: Bryanj on April 01, 2024, 11:29:55 PM ---PD carbs are notorious for clogging and need complete and thourough cleaning by somebody who understands them, i wont touch them at all

--- End quote ---

Are there later carbs available that will fit to replace the PD carbs Bryan - they must be bad for you not to touch them?

--- End quote ---

There's general record of people's dislike for working on them throughout the forum, being very fiddly the primary observation. But then so are many intricate device in reality.

They are though an absolutely excellent carburettor design in metering and accuracy terms.  Viewed as "lean" running, well that's only when referenced to their predecessor types which are technically too "rich" by design and common faults in that fuelling strategy.

These are one of the nearest to fuel injection in the optimisation of air fuel ratio from that era in slide carbs on Honda bike, in my view.

They run close to optimum for A/F ratio for constant demand to give more ideal fuel burn without much in the way of excess .... the excess inherent in predecessor being primary reason for short oil change interval by running mixture sufficient to accelerate all the time, whether needed or not. These PD 41/42 series run much more optimum steady state mixture, supplemented by accelerator pump only when required to stoke it into fast response ..... exactly what a fuel injection system does.

Cold blooded, yep, that too until they get combustion up to temp, facilitated by convenient choke pull up by instrument console with full rpm holding range to give competent low temp, running.
This is why FI stuff ordinarily use 88C thermostat in water-cooled installation, to bring the engine temp up to target as fast as possible, then to reach normal (non enriched ) mapping and run near to stoichiomeetric combustion.

Its all there in these carb, highly advanced when you look at them now. The design is no excuse to having them work competently, there's no inherent "fault" with them.

Cleanliness and diligence absolutely required, but they do work well if clean and set properly.

Bryanj:
I agree they are good carbs but there is one serious inherant problem in that they hate being stood doing nothing.
I was at Meads when the first dohc's came in and they were as bad.
If left on the showroom floor for over a month they were all a swine to get going, and that was before ethanol.
There are loads of tiny bits and pieces in there and i find them awkward and frustrating to assemble so not worth my time to do as people dont want to pay for the time required

K2-K6:

Practical advice for OP to check nothing simple is catching you out on this set.

No carbs should "hang" on the throttle pull cable at closed position. This is ONLY facilitated by throttle stop screw next to cylinder #4 in providing absolute slide base position.  If there's any tightness in pull  cable at this position, then it needs adjustment to provide slack such that it can't interfere with complete "drop" onto that stop when you just let the twistgrip go.
Make absolutely sure there's no conflict with secondary cable throughout full twistgrip movement.  There's usually only adjustment on pull, with return being fixed.
You can remove the return cable while you try to diagnose fault, but advise to use when ultimately riding it though.

Idle mixture screws are set from a start point of 1 & 3/4 turns out from fully closed on the F carb set. Anything significant below that will likely give the symptoms described, effectively lean idle, with revs slowly drifting down and stalling. Make sure they are in the starting position if you don't know how they've been set.

Worth verification of these points to see if there's anything basic running you ragged.

Navigation

[0] Message Index

[#] Next page

[*] Previous page