Help me become one with the SU piston lift pin....

[eschneider]

So when setting the mixture on SU's, you lift the piston with the pin.

If the RPM goes up, that means a rich mixture.
If the car starts to die, you're lean.
If the RPM goes up and returns to normal, you're good.

Normally I gradually adjust the jets until I achieve a lean condition, then go back a flat.

So how long do you hold the piston up?

I was tinkering with a +4 with H6's, and set both carbs to the point where, when the piston was raised for a second or 2, the idle bumped up slightly, then settled back.

But I noticed that on one carb, when I continued to hold the pin, the idle was relatively unaffected. On the other carb, the idle degraded the longer I held the pin and would kill the engine after around 8-10 seconds.

Yes, the carbs were balanced.....

My thought was that it's the instantaneous reaction to lifting the piston that matters most. But now I'm scratching my head.

 

[martx-5]

I've always just held the pin up for a second or two to get a reaction. Never experimented past that point. I also have never seen any info on how long they should be held up. I just always assumed that whatever reaction was going to occur would happen right away. /bcforum/images/%%GRAEMLIN_URL%%/smile.gif

I'll be installing an O2 bung in my header on the TR3 resto, and hopefully, that will guide me in setting the mixture. The nice thing about that is that you can see the mixture (Air Fuel Ratio) while driving. I like the idea of sprinkling some modern tech into these boiler plate cars. /bcforum/images/%%GRAEMLIN_URL%%/cheers.gif

 

[NutmegCT]

and how high are you lifting the pistons?

T.

 

[Tinster]

Re: Help me become one with the SU piston lift pin

I've always just held the pin up for a second or two to get a reaction. Never experimented past that point. I also have never seen any info on how long they should be held up. I just always assumed that whatever reaction was going to occur would happen right away.

Well, as the stupid kid on the block, my take is the same as
Art's. You raise the piston just a hair and instantly get a
reaction. I don't think time duration of lifted piston is a factor in this one.

tinster

 

[TR3driver]

Re: Help me become one with the SU piston lift pin

Well, I have held the piston up longer, and it's my belief the engine should continue to idle at about the same rpm as before for as long as you hold the piston up. It will be a little rougher since the mixture is lean but certainly shouldn't die.

Obviously you have something different between the two carbs; and I'm going to guess that either you weren't lifting the pistons the same amount or it's something outside the carb like ignition or valve lash.

Hmmm, never looked into what a difference in float level would do to this test, but since lifting the piston weakens the depression that lifts the fuel out of the jet, maybe ??

 

[KVH]

I've read about and tried the "piston lift" technique many, many times over the 30 years I've played with TRs. Granted, I may have my own problems, but from my experience, I'd say to forget the technique. Just get the air flow reasonably balanced, screw the adjusting nuts all the way up, then back them off 12 flats, and leave it there unless it's clearly too rich. I know others have had certain experiences, and I'm not among the best mechanics on this board, but the piston lift technique for me has been totally worthless--for all the reasons given, plus vacuum advance problems, idle speed problems, air leak problems at the bushings, length of lift, etc. I don't even think of that method anymore. Good luck.

 

[eschneider]

Re: Help me become one with the SU piston lift pin

and how high are you lifting the pistons?

I'm pushing them all the way up as far as the piston lift pin will allow - 1/8"? Although I didn't verify with a caliper, the pistons were being lifted the same amount in the 2 carbs.

 

[eschneider]

Re: Help me become one with the SU piston lift pin

Well, I have held the piston up longer, and it's my belief the engine should continue to idle at about the same rpm as before for as long as you hold the piston up. It will be a little rougher since the mixture is lean but certainly shouldn't die.

Obviously you have something different between the two carbs; and I'm going to guess that either you weren't lifting the pistons the same amount or it's something outside the carb like ignition or valve lash.

Hmmm, never looked into what a difference in float level would do to this test, but since lifting the piston weakens the depression that lifts the fuel out of the jet, maybe ??

Thanks for sharing your experience. I'll have to do some further trial-and-errors out of curiosity. Seems like if the cause is float level or something external, this could be a diagnostic tool.....

 

[Dave Russell]

Re: Help me become one with the SU piston lift pin

and how high are you lifting the pistons?

I'm pushing them all the way up as far as the piston lift pin will allow - 1/8"? Although I didn't verify with a caliper, the pistons were being lifted the same amount in the 2 carbs.

The SU carb manual says to lift the carb air valve (slide/piston) 1/32 inch, after the free movement has been taken up, for this test.

Since the lifting pin itself is spring loaded, the pin must first be pushed up far enough for it to contact the piston (there is some slack, free movement) & then raised an additional 1/32 inch.

It's a little hard to feel the actual pin to piston contact point the first few times that you do it. With the air filter off, raise the pin until piston itself raises 1/32 inch. After watching & lifting a few times you will be able to recognize feel of the additional resistance of the piston top return spring as the piston starts to lift. Then raise an additional 1/32 inch.

1/8 inch of actual piston lift would be too much to get a sensitive reading.
D

 

[glemon]

Re: Help me become one with the SU piston lift pin

The thing I don't like about the piston test is just that, the 1/32 of an inch thing, I am sorry, but my fingers are not sensitive enough to move something up 1/32 of an inch especially on a hot vibrating motor.

I still use the test for tuning, but don't like it very much.

I test by other means as well, assuming all else is good should run pretty smooth with little or no hesitation in throttle resonse on a warm day, even with little or no warm up. In cooler temps (40-50F)may need a little choke or warm up before it takes throttle whithout hesitation or a miss. Some engines are more finicky about this than others, and tend to need more choke when cold, so not the ultimate test. If it develops a miss when warming or warmed up it may be too rich.

Lastly you can read the plugs, nearly every manual has color pictures of how your plugs should look, if you are not burning oil because of a worn motor this should be a reliable test. Better to look at the picture than me to try to describe. If it is running right and plugs look good you should be OK.

 

[TR3driver]

Re: Help me become one with the SU piston lift pin

One of the things I really like about the LTP test is that it's very sensitive, only when everything else is just right. If you've found the point where the idle just rises and falls back to where it started, then turning the mixture nut one flat (1/6 turn) in either direction should make a very definite change (and 1/2 flat is noticeable). If not, then there is something else wrong (timing, valves, etc.).

BTW, throttle response depends heavily on what's in the dashpots.

And, IMO, if you don't need the choke with the engine below 80F or so, then your mixture is too rich. Lots of folks run that way though, and it doesn't really hurt anything except to burn more fuel and emit more CO.

 

[martx-5]

Re: Help me become one with the SU piston lift pin

The thing I don't like about the piston test is just that, the 1/32 of an inch thing, I am sorry, but my fingers are not sensitive enough to move something up 1/32 of an inch especially on a hot vibrating motor...

I just checked the pin-lift gizmo on my rebuilt cars. There is a snap ring on the lifting pin that will only allow the piston to go up so far. I measured that lift while pushing the pin to the end, and in the case of both carbs, the piston lifted 1/8" within 0.005". Of course, with the engine running, the pistons will already be up a certain amount. It would seem to me, that if the carbs are both first balanced for air flow at the proper idle speed, you would think that the SU engineers most likely figured out what the length of that pin should be to lift an addtional 1/32". Or did they? Probably too many variables come into the equation here, but it is an elegant thought. Just push up on the pin to the end and the piston goes up the correct amount.

At best, the lift-the-piston check will get the mixture pretty close. With worn carbs, it's most likely a crap shoot. This is why I'll being installing an O2 sensor. It's one thing to have the correct mixture at idle, it's another to have it correct under acceleration and while cruising. Actually two sensors would be better.

 

[Don Elliott]

Re: Help me become one with the SU piston lift pin

Randall - FWIW - Another point to consider if the carbs are set too rich. This was explained to me by the owner of the engine re-build shop where I had the liners for my TR3A honed this past spring when I was re-building my engine. He looked at everything and measured it all for me and told me everything was fine (after 94,000 miles) except that the rings were finished because it was easy for him to see that my carbs had been running too rich for a long time and that's why I had low compression and power. After honing the bores and with new rings plus all the other things I replaced because it was all apart, the power is back. I have adjusted my carbs so the color of the plugs is correct.

 

[TR3driver]

Re: Help me become one with the SU piston lift pin

Good point, Don. But most folks (unlike you and I) aren't likely to ever have to worry about wearing out the rings in 'only' 94,000 miles /bcforum/images/%%GRAEMLIN_URL%%/grin.gif