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Burney

Bike Carb trouble

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Hi

 

Help needed

I bought a set of CBR600 bike carbs with manifold off ebay off a running pinto

Fitted them to mine with electric fuel pump and fired up idleing smooth.

Runs around town ok but putting foot down spluttery accelleration.

Assumed fuel pump no good refitted mechanical pump in line.

Fired engine up still spluterring on accelleration. 

Twisted middle screw clockwise which inctrased revs and got rid of splutter.

Reduced idle with idle adjustment screw.

Left running a little engine stopped and now wont restart.

Q1. Have I unbalanced the carbs turning the middle screw?

Q2. Am i now getting too much fuel?

Q3. Is a regulator necessary?

Q4. What is the pipe on top of carbs? See photo. This was spitting out fuel so I bunged it up.

20200802_104613.jpg

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I reckon you're over fueling with your standard car pump. Get any cheap second hand bike pump. Or an adjustable facet type one. No more than 3psi for bike carbs. 

Bike pump is the best way to go if you ask me 

 

Andi 

Edited by Andi

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I just use the pinto pump with a pressure regulator set at 1.5 psi, ,  did you sort the vacuum advance on the distributor?  have you altered the timing at all?   Definite over fueled first step get the regulator fitted. 

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Thanks folks can you advise of supplier for regulator?

Got fired up again and my father in law a good mechanic advised two were running lean and two rich so imbalanced.

I think I know how to balance properly with a vaccum guage but how do you adjust mixture?

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I just searched Ebay, I can't remember where I got it from.

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Ordered a regulator

Yes I have distributer vaccum and brake vaccum T'd into inlet manifold.

Borrowed a vaccum guage so I can balance properly

I've read that the mixture on these carbs are factory set and the screws are covered with welch plugs. Does this mean I should just leave them be after balancing carbs properly?

I havent done anything with the timing as far as I'm aware. How would I check this?

I'll report back when I've fitted regulator and balanced carbs

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The pipe on top pf the carbs is a breather from the float chambers.  Don't block it off or back pressure will stop the chamber from filling.  If fuel was coming out that pipe you have a float that is sticking causing the float chamber to flood (and the engine too) and fuel overflow from the breather.

These carbs are really very simple devices and work very well.  Make sure you have the correct fuel jet size in them.  For a 2 litre you are looking at a 1.5mm jet.  Check DanST Engineering as they sell all the jets and have lots of info on fitting bike carbs to Pintos and other engines.  I still use the original bike fuel pump on mine.  I expect it to give up at some point but it still seems to keep up with fuel demand.

Balancing the carbs really only checks that each cylinder has the same air flow into it so in theory each cylinder offers the same power output.   If you have two cylinders running rich it is more likely because of the sticking float(s)

If you have to change mixture settings on these bike carbs it is done by adjusting the needle setting on the sliders in the venturi.  Actually adjusting the needle doesn't change the fuel as all you are doing is raising or lowering the start position of the slider increasing or decreasing the air flow.  The needle always sits at rest at its lowest point because it is returned to that point by the spring in the vacuum chamber under the black top cap.

The vacuum which operates the slider comes from the air flow through the carb itself.  The rear of the slider is chamfered to create a low pressure area behind it sucking air out of the vacuum chamber so raising the needle.  It can be worth checking the rubber diaphragms - they are very thin and even a pin hole can make a difference.  A loss of vacuum however would make a cylinder run lean as the needle wont lift as far as it should or perhaps not at all.

If your manifold has a vacuum connection is is likely to be from just one runner which is fine for the brake servo, but not so good for running your distributor vacuum advance.  It pulses too much to give a smooth response.  A vacuum advance would typically have been connected to  an inlet manifold plenum chamber where the vacuum is more consistent across all four cylinders.  I use a centrifugal (spring) advance distributor on my Pinto which requires no vacuum and Mikuni (Yahama R1) bike carbs with original bike fuel pump.

Lastly I might say that these alloy inlet manifolds benefit from a chunky gasket like 1mm thick, again DanST Engineering can probably supply.  Air leaks at the manifold cause all kinds of problems with mixture!

Hope the above helps

 

Kevin

 

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What a fabulously concise write-up on bike carbs, Kevin; was a riveting read. Got me thinking about getting some for my Pinto. 👍

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Thanks folks.

I really should have started by stripping them down and giving them a good clean!

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