Don Terrill’s Speed-Talk

I have the borrowed AEM air/fuel meter installed on my vintage Triumph 750..This bike is two cylinders, 45 cu in,about 50-55hp...The bike has a two into one short primary header,the sensor is located on the collector,about 10 inches aft of the pipe merge ,32 inches total from the head ports..
The carbs are 32MM Mikuni TM flat slides, a smaller version of what used on my race bikes.These are modern popular carbs for performance..Car guys might call these variable venturi, fuel delivery past 1/8 throttle is by a slide and metering rod directly controled by the rider's right hand on the twist grip throttle....this is an old design air cooled engine and needs to be slightly rich under load .....
The gauge seems accurate enough, cold engine with chokes off is near 17-1 with the engine running lean rough at part throttle,,, a few miles it warms up and running smoothly, the A/F is around 14-1 at a steady 55 mph, 3500 rpm.....Full throttle in the first three gears (about 75-80 MPH) the gauge drops to 12-1 and stays there as the engine winds out.It feels strong..However in 4th and 5th it obviously takes longer to wind out and the A/F drops to 10.5 and stays there until the engine gets near power peak, 6500 RPM...Engine feels a bit soft from a rich mixture...
Question, why would the A/F ratio be richer in the higher gears?

Remove the air filter TEMPORARILY and see if the same pattern shows up. If not, then your air filter setup is too restrictive. If it doesn't make a difference... not sure what the next step should be.

My experience with the TM flat-slides was from when they were the OEM carbs on most Japanese motocross bikes that didn't use Keihin(?).

I am running easily removed pod K&N types....I will try that..it's also possible the 32mm flatside is a bit large for a 30HP cylinder at 6500 rpm...But I'm thinking to much carb will go lean?

What's the cam and overlap? At the lower RPM, wouldn't overlap have more effect, even with IR carbs.

The sensors need to be right outside the ports in the headpipes. there is air backflow at the current location

PackardV8 wrote: ↑Tue Jun 11, 2019 1:16 pm What's the cam and overlap? At the lower RPM, wouldn't overlap have more effect, even with IR carbs.

it's stock cams ...About 245 degrees duration at .050 lift...The intake cam timing at .050 is about 30 IO and 60 IC, the exhaust oppsite and slight less...A Triumph tuned like this one shouldn't have problems from overlap...But maybe lol

by HDBD »
The sensors need to be right outside the ports in the headpipes. there is air backflow at the current location

Ok but why is there a big difference reading between lower and upper gears? .... I'm not so concerned with actual numbers so much as the difference in numbers..

Seems to me if you are fat at WOT you need less main let and a richer needle to keep the mid fuel curve in shape

I agree with the previous air filter comment. I've run a dry paper filter and an oiled foam filter and both ran the same except the top end. Oiled one was a bit richer up top. I figured it was just a bit more restrictive. I preferred the oiled one so I just jetted accordingly. This was on a Mikuni VM.

Where's the needle set?
1 run set rich & 2nd set lean, then if not close, go to the Mik catalog for the needle dimensions with a leaner top end then yours.
SU's different, a CV, constant velocity carb, and they've got dozens, if not hundreds of needles, drive you F'n crazy,
Good luck.

http://www.mikuni.com/fs-tuning_guide.html

This is a wide open throttle situation above 4500 rpm where the A/F in the upper gears appears richer than the lower gears ...

Truckedup wrote: ↑Tue Jun 11, 2019 3:26 pm This is a wide open throttle situation above 4500 rpm where the A/F in the upper gears appears richer than the lower gears ...

Well, the only apparent differences are the rate that it builds revs, and the load on the engine. In the taller gears the rev rate is slower and the load is higher. What follows is pure speculation.

IF the higher loading is raising the EGT for some reason, the exhaust scavenging effect could be affected, and this can have an effect on mixture. No clue if that is right, or even reasonable to assert.

GRTfast wrote: ↑Tue Jun 11, 2019 3:42 pm

Truckedup wrote: ↑Tue Jun 11, 2019 3:26 pm This is a wide open throttle situation above 4500 rpm where the A/F in the upper gears appears richer than the lower gears ...

Well, the only apparent differences are the rate that it builds revs, and the load on the engine. In the taller gears the rev rate is slower and the load is higher. What follows is pure speculation.

IF the higher loading is raising the EGT for some reason, the exhaust scavenging effect could be affected, and this can have an effect on mixture. No clue if that is right, or even reasonable to assert.

I call it reversion...The exhaust tuning, or lack of tuning cause air to move in and out the carb mouth during certain rpm ranges causing a rich situation...it is worse in upper gears...But on bikes where I expereince it ,it was there in lower gears..The way this engine is tuned, it shouldn'be be..but could be

Truckedup wrote: ↑Tue Jun 11, 2019 3:54 pm

GRTfast wrote: ↑Tue Jun 11, 2019 3:42 pm

Truckedup wrote: ↑Tue Jun 11, 2019 3:26 pm This is a wide open throttle situation above 4500 rpm where the A/F in the upper gears appears richer than the lower gears ...

Well, the only apparent differences are the rate that it builds revs, and the load on the engine. In the taller gears the rev rate is slower and the load is higher. What follows is pure speculation.

IF the higher loading is raising the EGT for some reason, the exhaust scavenging effect could be affected, and this can have an effect on mixture. No clue if that is right, or even reasonable to assert.

I call it reversion...The exhaust tuning, or lack of tuning cause air to move in and out the carb mouth during certain rpm ranges causing a rich situation...it is worse in upper gears...But on bikes where I expereince it ,it was there in lower gears..The way this engine is tuned, it shouldn'be be..but could be

Also, it could be that in the higher gears with a lot of air moving around the bike that you are changing the ambient conditions around the intake and that is having some effect.

I've been tuning carbs for more than fifty years and it wasn't until I began using the wideband O2 that I learned how much I didn't know about carbs.

Open carbs with no airbox (pod filters = same thing) ... It is fairly likely that air circulation around and past the bike is affecting the vacuum signal in the float bowls and/or disrupting airflow into the air filters themselves. It's not exactly clean undisturbed airflow ...

Where are the float bowl vents connected to?