IMS settings for OEM Vs FCR carbs

[koko64]

When I set the idle mixture on 900 motors with OEM Mikuni BDST38 carbs I nearly always find the settings are different for each cylinder due to the different angle of the manifolds to the ports. I have found about a couple of full turns difference. When I do the same tuning with FCR carbs I usually find the number of turns adjustment for each carb to be much closer, often within half a turn. Does anyone have a theory as to why this is so?

I find it interesting that Dynojet kits for OEM carbs have a guide which suggests using the same 3 1/2 turns out for each carb as a baseline. In practise I have found idle speed fuel requirements of stock carbs to be leaner for the rear cylinder and richer for the front cylinder.
Taking into account valve shim settings, cylinder compression and valve seat health, etc it appears the FCR carbs are not as affected by the manifold/port asymmetry of each cylinder.
What are your experiences and thoughts?

[Speeddog]

Are you checking each cylinder with an exhaust gas analyzer?

[koko64]

Just going for fastest, strongest, most even idle speed and checking with manometer after synching. So going for real world idle quality and matching strongest idle speed with each cylinder.
The FCRs dont appear as affected. I wonder if the much shorter length of the carb (bellmouth to venturi) and slide design of FCRs  means they are not as sensitive to the port/manifold asymmetry.

[Speeddog]

I think the IMSs on the FCRs just aren't very powerful.
I can close them completely on my M750, and a small adjustment of the air screw will have it running just as well.

It would be interesting to see what an exhaust gas analyzer said.

I'll play with with the IMSs next time I'm fiddling.

[Howie]

I think the IMSs on the FCRs just aren't very powerful.
I can close them completely on my M750, and a small adjustment of the air screw will have it running just as well.

It would be interesting to see what an exhaust gas analyzer said.

I'll play with with the IMSs next time I'm fiddling.

If you have one, yes!  You are dating yourself with best idle, most of the young wippersnappers turn the screws out the same on each.  They seem happy.

[koko64]

 

[koko64]

I love these discussions. There's bench racing and bench tuning.

Yeah, often a motor wants an AFR against accepted numbers at various rpms/throttle positions. Numbers are an important guide and measurement tools are very useful for sure. What does Lofgren say "If you dont check your work you can assume it's perfect". . I'm a dinosaur using plug chops and my senses and I plan to buy a Gunston sniffer next year.

I do visit my local dyno and use recorded jetting data for mid range and high speed tuning. It's a capital offence exceeding 60 mph down here.

I think the FCRs rely on the correct slow jet to be precise for idle and the IMS is a very fine tune as Nick said. Like Nick I also use the (worth its weight in gold) adjustable slow/pilot air jet screw to smooth things out. The SAJ screw also influences the character of throttle feel when opening it it from a closed throttle (smooth Vs snappy). Very bloody handy.

FCR range for correct IMS tuning indicating the correct slow jet is between 1-2 turns, the same for BDSTs is a much wider range from what I can gather. Since the BDST has a butterfly and a vacuum slide guiding airflow and is longer bodied (adding to intake tract tuning length)? I was wondering if this exacerbates the effect of the assymetric manifolds.
When tuning FCRs I can get the cylinder to miss 1/2 a turn each side of the correct setting with the correct slow jet and that they are an equal setting or very close. I think we often choose rich pilot jets in FCRs to help starting due to no choke/enrichening circuit. My guess is that this reduces the ability of the IMS to control idle mixture. Then with a too rich off idle mixture we open the SAJ a little to smooth it out. Starting is difficult with the correct slow jet. With the BDSTs I often find one cylinder requires about 2 turns difference on the IMS. The choke is a beautiful thing.

[koko64]

Appreciate your thoughts. I'm home with the flu and enjoy the technical discussion, and since we cant sit down with beers in person, the forum is a great avenue.  My theoretical framework is limited so I really enjoy throwing some ideas around with you blokes.

[pepeducat]

And MIKUNI TDMR40 ?

😉

[koko64]

I have thought about them since they have a starting circuit/choke. How sensitive is the idle mixture adjustment on the TDMR40's?