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Bike carb main jet sizing question
Mopple - 2/4/22 at 05:41 PM

Due to various ways I have a 2 liter Pinto engine with custom intake currently installed on a 2-door Sierra. The engine itself is standard Pinto, but head was 8-9 years ago ordered from UK and has a Kent FR32 cam and R1 carbs on Bogg Brothers alu manifold. Ignition is managed by NoDiz and TPS combo.
Hence the whole head setup was initially ordered for Lotus 7 clone mainly for track-days, therefore the carbs are over fueling the engine on normal road driving. The AFR reading is taken by wideband clock and is up to 3000 revs too rich (9-12).
According to my understanding the best option for good normal drivability would be to change the main jets of the carbs to smaller size. Am I correct or are there some other thing to change? Aim is to get a good road driving setup.

robertst - 2/4/22 at 10:55 PM

When I was tinkering with my R1 carbs on my Pinto it too me some time to figure out that the overfueling issue I had was mainly due to three things:
1: Carbs were not balanced properly
2: the float settings were way off, they were letting too much fuel in.
3: the air corrector holes on the R1 carbs were blocked.

Once I solved both those issues, my Pinto ran perfectly fine with the standard R1 main jets (by standard i mean the jets that came with the carbs when I bought them IIRC they're 130 jets).

snapper - 3/4/22 at 02:41 AM

http://www.nightrider.com/bt30/carb_jet_ranges.htm

Main jets mostly affect a higher rpm range than where your problem is.
If you have adjustable needles I would drop them down a couple of notches, if not I would buy adjustable needles.
Don’t buy dynojet needles just adjustable standard profile ones, I had an absolute nightmare with a set of carbs fitted with a dynojet kit.

The link above has a diagram with what part of a bike carb does what where in the rpm range and how to set it up but for now adjustable needles should get you closer than you are, if adjustable needles sorts the 3k rpm problem but you go lean at high rpm you can go up in main jet size and adjust needle clip position for mid range.

You drop the needle for lean and raise the needle to go richer

jacko - 4/4/22 at 10:58 AM

Hi
I had my zx9 carbs set up on Boggs r/ road if you search under my name on here you may find info that will help you
Jacko

Mopple - 4/4/22 at 05:23 PM

Thanks for all the replies.
I forgot to mention on the initial post that the main jet needles are adjustable and if I rise the needle by one step then the engine becomes strangled and on the next lower one the car is drivable, but not good by any means (high gas mileage, dark black and wet sparkplugs)
The carbs are balanced with external vacuum gauges on all runners.
Jacko - Will try to find your tread about r/r setup. Thanks

wombat258 - 4/4/22 at 09:27 PM

All motorcycle carbs are designed to be gravity fed, so your fuel pressure may be too high, resulting in the carbs constantly flooding. Can be solved with a good low pressure regulator set to 0.5 to 1 psi, and preferably with a small orifice in the return line.

40inches - 4/4/22 at 09:39 PM

quote:
Originally posted by wombat258
All motorcycle carbs are designed to be gravity fed, so your fuel pressure may be too high, resulting in the carbs constantly flooding. Can be solved with a good low pressure regulator set to 0.5 to 1 psi, and preferably with a small orifice in the return line.

Or use a carbed engine fuel pump, designed for the job and not expensive

Mopple - 6/4/22 at 08:16 AM

There's currently a pressure regulator installed before the carbs. Fuel pump is for car, can't remember the exact type at the moment tough.
Will organize a pressure gauge to measure and adjust the fuel pressure into carbs.

SJ - 6/4/22 at 12:01 PM

quote:

There's currently a pressure regulator installed before the carbs. Fuel pump is for car, can't remember the exact type at the moment tough.
Will organize a pressure gauge to measure and adjust the fuel pressure into carbs.


I would just dump it and spend £20 on a proper bike pump.

Contrary to what was said before main jets will affect low rpm and high rpm mixtures. Ultimately mixture is a blend of the effects of the float height, pilot jet, main jet, needle profile, needle position and idle mixture screw. Throttle position and RPM all affect how the above contribute to your AFR.

Best approach is to just change one element and see what effect it has. I have 180 mains in my 2.0 Zetec (standard zx6 was 130 IIRC). I can run 190s with the needle in a different position and get similar AFR at WOT.

I started with the needle as high as it would go and sized the mains accordingly (160) but then to get AFR right at cruise had to drop the needle down so needed to go up with the mains.

Getting the float height right is key before you start. For me I had to increase the pilot jets a lot (0.12 to 0.2) to get the idle mixture right.

It takes time but with a bit of trial and error you can get it right without a rolling road.

Mopple - 7/4/22 at 07:10 AM

I have ordered a used R1 pump. Will start dealing with the Sierra when snow melts
Thank you all for your input!