Triple Oer Carburetor Troubleshooting - Help

[adam.]

Update:

I have received #55 and #57.5 idle jets, the #55s are still too lean to even accelerate, while the #57.5s have a cruise AFR of 12.5 and acceleration is in the 13s - 14/15 which cause stuttering (around 2.3-3krpms). Idle is about 13.

Any ideas what could cause this lean spot causing the stutter? I cannot get my head around it. I have tried checking timing and best I can do is 15 degrees at idle.

It seems like I can't get cruise any leaner without causing hesitation on acceleration.

The #57.5 are 1.5 full 360 degree turns out from full lean.

Really struggling.. I think it's that dreaded transition lean.

Edited March 14, 2015 by adam.

[Travel'n Man]

Not that this is your problem - but a lot of folks think "bigger is better".  With a stock engine - running stock numbers - your carbs could be a little oversized for your engine output, producing your running enviroment.

 

You may want to check and note the RPM that you are runing (along with the AFR) at each of these above marks you are referring to.  Have you developed a spreadsheet to track these settings for reference?

[240260280z]

How does the A/F Ratio track as you gently go from 1200 rpm to 4000 rpm while gently accelerating in 2nd and 3rd gear?

How does A/F Ratio track going from 1200 to 4000 rpm while hammering it with WOT.

For your mid rpm problem, if it occurs when gently accelerating, you can raise your fuel levels 2mm at a time from their current 31mm level and the leaner mid will go away but you may have to use a leaner idle jets or turn in the idle enrichment screws as the higher fuel level will also affect the idle A/F ratio.

For your mid rpm problem, if it occurs when aggressively accelerating, you will have to address the pump jet size.

Play with fuel level and report back the data and we can help more.

Edited March 14, 2015 by Blue

[adam.]

Ok I have had a chance to play around with it again today. Once again, just the idle jets were installed, I first trialed it with all mixture screws fixed at 2 turns out from full lean.

 

This is at 31mm float levels, #57.5 idles. Graph is from take off in first until 3krpms and then shifting into 2nd, and again driving it until 3krpms with light acceleration. As if you would accelerate to a 35mph (60kmh) speed limit.

 

 

As you can see, it slowly gets lean as it approaches 2.5krpms until 3kpms at gear change.

 

Idle AFRs sat at around 12:1, cruise it is at ~12-12.5:1.

 

 

I then adjusted float levels to 29mm. Everything else remained the same. Once again, take off from stand still in first until 3kprms where a gear change into 2nd is made, and then again until 3kprms at light acceleration, as you would to 35mph.

 

 

As you can see, there's a slight improvement up and around the 2.5krpms range but it still leans out a little too much causing coughs and spluttering.

 

Cruise is still very similar at around 12-12.5:1 AFR.

 

I then thought I would install the #135 mains with #190 airs. Unfortunately don't have a graph for this, but slow acceleration felt like it improved a little bit more again. Hesitation reduced but was still there and annoying. WOT was good from above 3krpms, AFRs in the 12.5-13 range until top end where it richened up to about 12.2:1.

 

Idle and cruise mixtured stayed the same.

 

And then I kept the above setup and installed #55 idles to see how it would drive.

 

 

Same procedure as before, slow take off in first until 3krpms, shift into second until 3krpms. Ignore the second half of the graph.

 

On take off, AFRs are around 13 before 2.2krpms where it leans out and stumbles.

 

Cruise is now around 14:1 which is nice but it is jerky and pops quite frequently at ~35mph (60kmh). At higher speeds it is quite comfortable.

 

Idle AFRs are floating around 13:1.

 

And that is currently where I am at the moment. I have noticed that the stumble is not noticeable when I accelerate a little harder, as if I was going to 80kmh but any lighter and there's hesitation. I cannot raise the float level any higher than it is at the moment (29mm).

 

 

Any help would be much appreciated!

Edited March 15, 2015 by adam.

[bartsscooterservice]

If they are brand new, did you change anything to the stock factory float height and jetting ?

[240260280z]

Try raising the fuel to 27mm then make a run with idle enrichment at 2 turns. Then turn the idle enrichment screws in until the idle air fuel is 13.5.

Here is a graph of how enrichment screw affects idle and cruise.

[bartsscooterservice]

 I am running 98 octane fuel. Timing is at 15 BTDC with no vacuum advance connected.

 

Use a timing light, and set the timing around 5 BTDC with the vacuum advance disconnected. Then connect it, and hit the thottle and watch the timing marks advance on the pulley. It should read somewhere around 15 to 17 advanced.

 

Now you are running 15 without the vacuum advance, it's advancing way to much imo.

[adam.]

 

If they are brand new, did you change anything to the stock factory float height and jetting ?

 

 

The carbs are brand new, yes, and I have adjusted the float and jettings from standard. Standard venturis were 36mm and I have reduced them to 34mm with 'ideal' jets to suit.

 

 

Try raising the fuel to 27mm then make a run with idle enrichment at 2 turns. Then turn the idle enrichment screws in until the idle air fuel is 13.5.

Here is a graph of how enrichment screw affects idle and cruise.

 

I can't raise the fuel float level any higher I'm afraid. The middle carb's float adjusting screw bottoms out at the current setting which is showing 29mm on the glass measuring level I have. I don't think I should force it either.

 

That's an awesome graph you have there though, I will have to play around with the mixture screws and see if it makes a difference to the driveability, even if it means sacrificing idle AFR mixtures. 

 

Use a timing light, and set the timing around 5 BTDC with the vacuum advance disconnected. Then connect it, and hit the thottle and watch the timing marks advance on the pulley. It should read somewhere around 15 to 17 advanced.

 

Now you are running 15 without the vacuum advance, it's advancing way to much imo.

 

Unfortunately there is no port on the new manifold or carb to get a vacuum reading for the distributor advance. I have thought about the timing being off, but I have not heard any visible knocking yet which I guess is always a good sign.. I will tone it down a little bit if the above tips from Blue fail and see what it does.

 

 

Please keep any other ideas you may have coming!

 

Would increasing fuel pressure help with the lean? Is it a good idea to use fuel pressure to tune?

[bartsscooterservice]

Should have tested the factory float and jet settings first. Now it's hard to have a reference point.

[240260280z]

I had a closer look at the data. Here are some observations:

The time you are driving from 1000 to 3000 is only ~4 to 5 seconds. It looks like the accelerator pump is coming in and driving the A/F ratio towards 10. (This same "V" shaped dip signature [circled] is evident in all plots after all gear changes...It seems to last ~2 seconds). To remove this extra circuit when collecting data, try idling for 5sec then driving for 5sec at 1000, 5 sec at 1500, 5 seconds at 2000, 5 seconds at 2500, then 5 seconds at 3000 (you will need a nice straight). So no big pushes on the accelerator and slow smooth acceleration. You can also replace the spill jets with bolts to eliminate the acceleration circuit altogether. This will make the data less cloudy and help focus on the right idle jet and fuel level. FYI The 2 second blast low A/F ratio after a shift may be causing your transition bog problem (i.e. a false lean) so it is good to note. The extra fuel squirting on the intake wall (thx to Bryan Little!) from the accelerator pump may cause intake popping if it ignites.

Going from 31mm to 29mm seemed go give you a point lower of A/F ratio at cruise (from 13.3 to 12.2). However the idle seems to be richer at 31mm (11.1 vs 12.5)? Were the idle enrichment screws the same for both of these runs or was the throttle pumped at idle? This is a bit confusing?

With respect to your timing, keep it at 15 degrees at idle while tuning. triples like lots of initial advance at idle.

Your Emulsion tube and main jet will also affect cruise but focus on the tuning with these tubes removed first as you are doing. Also try to disable the accelerator circuit by replacing the spill jets with bolts at the bottom of each fuel bowl.

Bottom line:

It looks like 31mm fuel level and the 57.5 idle jet can give you a nice fuel average distribution ~ 13.5. Just get rid of the rich-dip to evaluate better.

If you want to tune to 12.5 A/F, the 29mm fuel level and 57.5 idle jet will be the ticket.

Edited March 15, 2015 by Blue

[adam.]

Once again, awesome feedback Blue. Much appreciated.

Removing the pump jets is one thing I really need to do to eliminate the possibility of it richening my mixtures, I will have to source a bolt that fits.

And going from 31-29mm float levels, I kept all mixture screws and settings the same. It is honestly quite hard to tell idle AFRs though, bounces around quite a bit. I have been getting average figures for them.

I will give it another go without the mains installed and remove the pump jets. Worse case, holding the car at 1, 2, and 3krpms.

Thanks!

[240260280z]

Glad to help Adam, stepping back a bit:

How well do you think the linkage is balanced between the 3 carbs such that each barrel flows the same air at idle and off idle? This is very important before any jet changes are done.

Do you have a tool for measuring air flow into each carb?

For the required bolt, a small metric is all that is needed. I can't recall the size but just pull one and go to your hardware store.