Accel stutter on my L-18, dogleg with SU's

[spec-bre]

Gang I'm trying to trouble shoot an increasing issue I am having with my Wagon.

L-18, a87 closed chambered head, matchbox, 38mm SU's (new composite floats, needles-jets, synced, good fuel pressure, 14degs BTDC at 800 rpm's 36degs total at 4,000.

 

Sitting out of gear it responds without issue to a hard blip on the throttle but once in gear and driving it steps and stutters at low RPMs and then comes out of it if I step into the gas. I've checked for any notable vacume leaks and not found any. I have not ran the valves yet or checked plugs. As I said earlier it is coming on in a progression. What should I check next?

[datzenmike]

Damper oil topped up?

 

Try 12 degrees with vacuum advance disconnected on an L series engine. Are you running vacuum advance properly from the SU fitting?

 

Set the valve lash

[metalmonkey47]

What oil are you using? 

I had a similar issue in my 240 when the weather started to change, I turned down another 1/2 and solved the issue. According to the guys at ClassicZCars the SU's need another 1/2-full turn down as the temp starts to drop. 

Just something to consider. 

[spec-bre]

Dash pods are topped off with a light weight 3 in 1 type oil. I'll run the valves next and check the plugs. Thanks for the info. It was 74 degs yesterday when I gave her another goal. Not running the vacuum advance currently but either with and or without issue persists.

[datzenmike]

Should always run vacuum advance as part throttle runs better with added advance. Better performance and mileage... this is why you needed 14 degrees. Try 12 with the advance connected.

 

Thicker oil will delay the opening of the slides and richen the mixture on sudden throttle opening. Maybe too thick? Thin? Maybe  ATF?, try different weights of engine oil.

[spec-bre]

I generally run the VA I was just pulling it to see if I could gain a lil bottom end accel for Solo days.

 

This is the oil I generally run for damping oil. I have ran these ver same SU's dry on SOLO days before. Never had the issue I am having now. I have checked for vacume leaks and not found any.

 

[datzenmike]

Vacuum advance follows manifold vacuum. When talking performance, we're talking WOT (wide open throttle) and manifold vacuum is at it's lowest and so is this advance. Basically there is none. Vacuum advance adds needed ignition advance at part throttle driving where the burn time is longer so the power of the expanding hot gasses are pushing at maximum on the descending piston just after TDC. Without proper advance the expanding gasses miss this point and are wasted chasing a rapidly accelerating piston as it descends ahead of it.

 

The damper oil is selected with it's thickness in mind... anything will do really.  It resists the slide rising too quickly when the throttle is opened suddenly. Too thin with quick opening and the engine leans out and stutters.  Too thick (slow opening) and the engine runs too rich. In effect this is the carburetor equivalent of an accelerator pump.

[spec-bre]

Thanks for all the good input and I am slowly getting around to addressing and trying all suggestions. The dash pods have been drained, cleaned and refilled with some automatic trans fluid, the timing has been set at 12 BTDC, and vacume hooked back up. I haven't gotten to running the valves just yet but I did take her for a spin and no change as of yet but I did remember a lil trick David Levinson told me about once in regards to the SU's when I was having a similar issue. When the stutter/bobble is happening pull the choke and see what happens. It came to life and the stutter disappeared! Felt a bit stronger as well!! Now the issue I trying to remember the fix.. Carbs have new floats, needle and seats, needle and jets, bowel to jet soft lines and are synced and floats set. I know Mikuni/weber Side drafts are allot of people's answers to "fixing SU's" but I kinda like the looks and know when they are working good there rock solid. Does any of this above shit sound familiar??

[datzenmike]

The choke raises the needles allowing more gas in and running it richer. So thicker oil will delay the slides raising too fast and force a rich mixture during the transition from low to higher speed OR larger jets and or smaller needles... but this will richen it across the board.

[wayno]

How old are these SUs?

Did you set them up when you put the new parts in?

How did you center what I would refer to as the seats in the carbs?

How are you setting the fuel mixture, tuning them?

[spec-bre]

 

 

"How old are these SUs?"

Not sure. They lived on a shelf of a fellow auto Xer for many years before I picked them up.

 

"Did you set them up when you put the new parts in?"

Yes. Per the "Just SU's" video from Z Thearapy

 

"How did you center what I would refer to as the seats in the carbs?"

Screwing the jet all the way up (closed) and putting the needle in the Venturi (piston) semi snug/loosely and out (extended) further then required to seat and inserting it into the carb body and jet seating the two together then locked down the needle.

 

"How are you setting the fuel mixture, tuning them?"

Start with 2 full turns down then lifting the Venturi at idle as well as a UNIsyn to sink. I run the jet and listen for the engine to speed up and or slow and try and find that happy spot. Then lift the Venturi to check for a drop in idle (almost dies).

[spec-bre]

All parts are from Z Thearapy as well.

[Nuclear Water Boy]

All these new Ztherapy parts are great but mean nothing. I ran a set of their Z carbs on an LZ22. I had the stumble and when reading Just SU's book and watching the same video they talked about this stumble and how to get rid of it. The fix was profiling the needles,,,,turning down the stage in the needle (outer diameter to richen that circuit) where the stumble was occuring. I'll tell you what, it worked beautifully and made a world of difference in everything. It's a bit time consuming but to get things right and feeling the driveability difference, the 6-pak it took to get it right was well worth it. But then I'd a drank the sixer anyway 8>)

[Nuclear Water Boy]

Dash pods are topped off with a light weight 3 in 1 type oil. I'll run the valves next and check the plugs. Thanks for the info. It was 74 degs yesterday when I gave her another goal. Not running the vacuum advance currently but either with and or without issue persists.

What do you mean Topped Off? To me this means filled to the top which is WRONG!!!!!! You have graduated lines on the dashpods rod for oil correctness,,,

[spec-bre]

Sorry, "topped off" was in reference to filled to the line. Not completely full..

 

What's the process on the needle "tuning"? I don't recall that portion on the video..

[wayno]

OK, I just was curious if you knew what you were doing.

I likely have the same stutter as you do, I just don't notice it anymore.

My SUs are in my work truck which hauls around a trailer, so I don't start out fast, I just get it moving the best I can.

I top off my SUs the wrong way, and have for over 15 years now, mine seem to self level themselves, all I can say is they run/act the same all the time, no surprises, no back firing thru the carbs, and they hardly ever need adjusting once I have them set.

I set them up exactly the way you described on first start up, but then I tune them by the spark plug color after that.

"How old are these SUs?"
Not sure. They lived on a shelf of a fellow auto Xer for many years before I picked them up.

"Did you set them up when you put the new parts in?"
Yes. Per the "Just SU's" video from Z Thearapy

"How did you center what I would refer to as the seats in the carbs?"
Screwing the jet all the way up (closed) and putting the needle in the Venturi (piston) semi snug/loosely and out (extended) further then required to seat and inserting it into the carb body and jet seating the two together then locked down the needle.

"How are you setting the fuel mixture, tuning them?"
Start with 2 full turns down then lifting the Venturi at idle as well as a UNIsyn to sink. I run the jet and listen for the engine to speed up and or slow and try and find that happy spot. Then lift the Venturi to check for a drop in idle (almost dies).

[Sealik]

A wide band would tell you what your A/F ratio is when the engine stutters 

I had no stutter....but my A/F numbers were not good, rich at idle/WOT and dangerously lean at cruise

Sooo...I grabbed another set of needles (fatter) and started profiling, but first I installed them and noted the numbers in different throttle positions. Then.........remove, buff and re-install....go for drive and check numbers......remove....repeat....

46mm SUs on LZ23........filled with 3cc of ATF

 

The Dime, Quarterly..... Volume 4 issue 1

 

Stages of the needle

 

[spec-bre]

So by using dial calipers and measuring in incrments (how many and what length/gap?) you document the new needle then using the drill and sand paper (grit?) to start fine tuning the needle. Looks straight forward enough. Do I need to pull a base measurement on my current needle to give Z therapy (Bruce) before ordering a set to fine tune?

 

A wide band would tell you what your A/F ratio is when the engine stutters 

I had no stutter....but my A/F numbers were not good, rich at idle/WOT and dangerously lean at cruise

Sooo...I grabbed another set of needles (fatter) and started profiling, but first I installed them and noted the numbers in different throttle positions. Then.........remove, buff and re-install....go for drive and check numbers......remove....repeat....

46mm SUs on LZ23........filled with 3cc of ATF

 

The Dime, Quarterly..... Volume 4 issue 1

 

Stages of the needle

[Sealik]

So by using dial calipers and measuring in incrments (how many and what length/gap?) you document the new needle then using the drill and sand paper (grit?) to start fine tuning the needle. Looks straight forward enough. Do I need to pull a base measurement on my current needle to give Z therapy (Bruce) before ordering a set to fine tune?

 

 

14 stages and sanded with 240 grit.

I believe I used some worn 150-180 to remove material a little quicker and then finished with some 240. Remember you can't add material to the needle, so if you have rich readings on the wide band you'll most likely need different needles Whereas if you have lean conditions the needles can be sanded to richen the mixture up. But first......you need to install a good wide band and then read that article I linked.

 

I bought this...about the best bang for the buck

 

http://www.ebay.ca/itm/Innovate-MTX-L-Wideband-Controller-Digital-O2-Air-Fuel-Ratio-Gauge-Kit-UEGO-3844-/261264095297?pt=Motors_Car_Truck_Parts_Accessories&hash=item3cd48db841&vxp=mtr

 

I was running low 9s at WOT and 20 at cruise....and now after profiling a different set I'm 12.5/WOT and 14-16 at cruise, dependent on throttle position.