Jetting 1977 CB550K

[fourguy]

I'm new on this forum,living in Belgium so my english will be spitting.The past 6 monts i've restord a CB550K 1977..engine,frame the hole storie,is not complete yet (still needs to be painted,seat,spokes)but I already riding the thing.Bike came to me with 4 into 1 Marshall,end still using this exhaust.Ignition,carbs all rebuild,stock airbox..everything is stock.Bikes is running not so smooth,engine feels lazy in low rpm's.I think the problem is the exhaust... mixture to lean with 4 into 1.I did some research in old honda manuels,so a 77' CB550F came with stock 4 into 1 pipes,jets 98 but diffrent carbs to (bigger venturi).My bike has Keihin's PD46C carbs with stock jets 90,so i think i've have to put in also 98 jets.Anybody some experience with this...a good jetting tip with 4 into 1 Marshall.Ore will 98 jets be to big for my cabs?

[SteveD CB500K0]

Hi and welcome.

The 550K had 4-4 as standard and you are correct - the PD46s were completely different from the 500 and 550F carbs (which were 627B)

The Marshall exhaust will breathe far more easily than the stock system and will cause the engine to run very lean - you'll have massive flat spots in the performance. You will need to increase the size of the main jets to get flat out performance but will probably have to increase the pilot jets as well (and fiddle with the air screw).

It is not easy and can only be done by trial and error.

Go to www.sohc4.net and look up a member called "Two Tired", he is generally accepted as the "global" expert on rejetting a 550. There are loads of threads on there about this.

Steve
(use the same name when you register so that we recognise you!)

[fourguy]

O.K. thanks for the reaction,i will take a look there.

[Yoshi823]

Hi and welcome.

I have had quite a few 550 Hondas over the years,from the F1 to the F2 to the K3.I have had a variety of exhausts on all of the bikes,from a Motad 4-1 on the K3 to a Piper on my commutor F1.The exhaust on the F2 was the standard Sankei.

But I did not need to rejet as much as a 90 to a 98 main on any of them.

But I did have a problem on the F2 at one time.The tickover was bad and no amount of carb balancing would get it right.But what I did notice while using the carb gauges was that the readings were pretty unsteady.What I used was some WD40 (or Easy Start...whatever is to hand) to spray around the inlet manifolds.When the engine revs picked up I had a good idea then that the inlet casting to cylinder head seals had a problem.So the carbs came off anf the inlet manifolds removed to find that the seals had indeed perished.
With new seals in place it was amazing the differance.The carbs balanced easily,and the throttle pickup was much much smoother.Unforunately the exhaust headers had discoloured by this time as the mixture was running too lean,but at least the fault was cleared.

[fourguy]

Thanks for the tips,but all seals,o-rings,inlet rubbers are new on my bike.I'm gonna do some simple test now by blocking half the airinlet on the airbox,see how it response.