Lotus Elan

stromberg jet height, what is yours?

PostPost by: davea » Tue Dec 14, 2010 1:28 am

US spec cars with Stromberg heads! This is about those emissions Strombergs.

Many threads mention the Stromberg jet, but few mention the critical issue of the jet height
(the distance from the aluminum bridge of the carb body). The jet is pressed in in the
emissions carbs so it is not easy to change (well, one has to remove the carbs
and use a press, which is moderately time consuming).

I'm using B2AR needles and could not get a sensible air/fuel ratio using the small adjustment built
into the needle holder. The carbs were recently beautifully refurbished (completely rebuilt)
by Joe Curto (New York). New jets, needles, screws, etc.
Anyway, I finally measured the distance he set (bridge to jet)
and it was 0.125 inches (which one site suggested was
a standard, perhaps for Jaguar). In contrast, the stromberg_needles.xls spreadsheet
here on LotusElan.net says the distance should be 0.057 inches!

I measured the Strombergs from a TwinCam
Europa and I find I had set those (years before) at 0.094 inches from the bridge. Which, if
I recall correctly, was about what they were when I bought the Europa. Anyway, back to Elan.

I failed to measure the bridge-to-jet height before I shipped the carbs off to Joe Curto... My bad.

I am measuring air/fuel with an LM-1 and a wide-band oxygen sensor (I had a bung welded
onto the collector of the headers after it does the final 2-into-1). I was very careful to ensure
no air leaks upstream of the oxygen sensor (downstream is another matter).

So I see 11.5/1 or 12/1 AFR at idle and cannot make it leaner (needles are
all the way down to the point the holder is
level with the bottom of the piston, but not so far as to fall off the threads). At WOT 5500 rpm it's about 13/1.
I sure don't like idling at that rich a mixture, I would like to have a choice of settings for the needles!

By the way, one has 2.5 turns from needles-full-up to being down to level with the bottom of the
piston (meaning the needle-holder being level with the bottom of the carb piston). One can go
a bit beyond that, but if you get the holder off the threads you lose control....

The screw thread in the needle-holder is 32/inch.
So 1 turn is 0.03125 inches, 2 is .0625 inches,
2.5 turns is about 0.078 inch

So I am wondering what others find their jet height set to. Comments?

I'm going to move my jets up before year end 2010.
To no more than 0.090 inches below the bridge and possibly less
(have not decided). I'll make a small piece to use as a positive stop
when I press it in further so I can set both equally (relatively easily).

Suggestions? References? Commiseration?
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PostPost by: 69S4 » Tue Dec 14, 2010 7:30 am

I replaced the jets in my UK spec Strombergs about six months ago and measured the old ones at about 2.5mm before I took them out. "about" because one was at 2.5mm and one a bit less - about 2.3mm iirc. I did post about it here but the search function may not have found it. I put the new ones in at a target figure of 2.5mm and the car's been running ok although it could do with a bit of fine tuning.

I did wonder at the time exactly what the effect of moving the jets up or down a bit would be but that information didn't seem to be available easily.
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PostPost by: m750rider » Tue Dec 14, 2010 6:01 pm

I had Joe Curto rebuild mine a few years ago but don't know where they are set right now. I'll have to pull the airbox and look this weekend. However I didn't make any changes to his rebuild. I do know that I did have to set the needles flush with the bottom of the piston to get a good mixture without running too rich and fouling plugs very quickly. I didn't have any tests as you have, just watching plugs over time and making adjustments as needed. Right now it idles very well, plenty of power and no issues with plugs at all.

BTW I think I have B1Y needles.

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PostPost by: redskatejbf » Tue Dec 14, 2010 6:33 pm

I have had my U.K. carbs. rebuilt by Burlen Services here in the U.K. I measured the jet drop before dispatch at .118" but was probably meant to be .125". They returned the totally rebuilt carbs. with a jet drop of .090" and .095", I had asked them to fit BICK needles so the new needles may have had some bearing on this setting.
I have a copy of tuning notes from a few years ago by a `racer` who tried B2AR and found them to very rich at idle but ok at high revs., the jet drop he had was .125".
He tried B1CK and found them `excellent` with the same jet drop, that is why I had them fitted by Burlen, I have yet to try the car on the road or to fine tune the engine. I have only bench run the engine and it ran ok at that time so I am unsure if the .095" will be correct.
I have some info. from the Zenith Stromberg service book and it notes all Elans (inc. +2) emission carbs. should have B1G needles fitted with a air valve return spring with a blue colour code, but no comment on jet drop.
If you have removed all the emission crap I would try a jet drop of around .095" with those needles and if you cannot get a good A/F ratio try the B1CK needles at possibly both settings, a pain in the rectumn I know.
Hope this info. is of some use but sometimes to much is as bad as to little.

Regards John.
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PostPost by: leifanten » Thu Dec 16, 2010 3:33 am

I also run the B1Y (or was it B1G - I went back and forth a couple of times, but they are pretty similar) I think I ended up with about 1mm clearance from the surface of the air piston to the shoulder of the needle. There is a great xls spreadsheet in the archives that compares various needles and deck heights in the archives here. I found that invaluable when working through some similar issues.
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PostPost by: saildrive2001 » Thu Dec 23, 2010 10:37 pm

I know I am a little late with this reply but here is my added info. I have rebuilt my carbs recently & the dim from the bridge to the jet was & is 0.095" This is to the outer dia of the jet, the jet has a slight recess in the centre so be careful where you measure to. I made a tool when I pressed the new jets in that would bottom out on the bridge when the jet was at the correct depth. My car is a UK spec SE engine & the needles were B1Y
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PostPost by: davea » Sun Dec 26, 2010 4:47 pm

The design of the jet-top (with recessed center, and the center is where the
needle does its work) does lead to confusion in measurement. I measure from
the bridge to the jet center. Not to the raised-edge of the jet.

I hope to soon make
a tiny tool to place on the jet so I can just press the jets up a little till they contact
the tool (well with the carbs removed I'll actually have the carbs upside down and press down!).
The tiny tool will be touching the jet edge, not the center area. So its depth will
have to be less than my target 0.095 inches for the jet-center-to-the-bridge. A little tool
like that came with a Stromberg adjustment tool set I bought years ago, but somehow
I lost that little bit of the tool set.

The reports above on B2AR and B1Y and B1CK tuning are helpful, thanks.
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PostPost by: davea » Sun Dec 26, 2010 4:52 pm

On rereading this thread it's clear I should be trying B1CK and B1G before moving the jet itself.
I'll order them, hoping to have some results in January 2011.
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PostPost by: davea » Sun Dec 26, 2010 6:02 pm

Here are some specifics using my LM-1 data.
A/F ratios. These are my best estimates
given I have the LM-1 logging rpm (inductive pickup on #4 plug wire) and A/F.
No throttle position sensor yet.
I believe I have no air leaks between the sensor and the head.
B2AR needles set with holder even with bottom of piston..
At idle, even air flow thru carbs. On lifting one piston a little
the engine speeds up (consistent with too-rich idle), both
carbs behave identically in this regime as best I can tell by ear.
jet height 0.125 inches (bridge to jet center).

RPM A/F comment
1000rpm 10-12/1 idle
4000rpm 12-13/1 freeway cruise
3000-5000+ 12.4-13.6/1 Full throttle accel in 3rd up gentle hill (freeway on ramp).

For some reason the tachometer is going crazy as I get to 5000rpm.

My web site has the LM-1 log ( reality.sgiweb.org/davea/elan.html#strombergs ) as this forum's attachment
would not let me add a file ending in .log.
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PostPost by: davea » Wed Jan 05, 2011 11:23 pm

I installed B1Y needles and the mixture strength at idle with the needle holder flush with the
bottom of the piston is 10.7/1 approximately.
Both jets set at 0.122 (my best estimate of the true measurement) and the edge
of the jet set at 0.092 inches.

This is too rich.
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PostPost by: davea » Wed Jan 05, 2011 11:36 pm

The book says float height 16/17 mm, mine are set at 16mm.
(highest point of float from body edge, carbs upside down on bench).
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PostPost by: davea » Thu Jan 06, 2011 12:12 am

Ok, I moved the jets up.
Front carb: jet center 0.092, edge 0.063 inches.
Rear carb: jet center 0.090 edge 0.064.

None of these measurements can really be guaranteed to .001 inches, but
within 0.002 I can believe.
Now I need to reassemble and test.
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PostPost by: davea » Thu Jan 06, 2011 6:57 pm

Overnight I realized I could just not stand it with the jets not the same height!
Now both front and rear are at 0.092 inches from the bridge (measured at
jet center).
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PostPost by: Dennis 45/9760 » Thu Jan 20, 2011 1:28 am

At the risk of hijacking the thread:

Having seen the discussion regarding Zenith-Stromberg jets I thought I might add the following to the discussion.

In the US older Volvos (I believe pre 1973) were not fuel injected and used Z-S carbs with fixed jets. We have owned Volvos since 1973 (and have had them serviced exclusively at an un-authorized Volvo repair shop). We have become good friends with the shop owner. I related to him the problem of ?jet wear? in the Z-S carbs as used in some of the Elans.

He proceed to walk back to his ?genuine Volvo tool? rack and brought out 4 tools which I think Z-S Elan owners might be interested. I?ll describe 3 of them, and if possible, I?ll include photos. (As you can tell, I?m a relatively new poster)

1. Photo 100_2655 is a ?T-handled? tool (Volvo number SVO 2895) which is used in league with the ?Dumbell? looking tool described last, to press the jet downward (richer) in the carb. It screws into the top of the carb after removing the dashpot fill plug.

2. Photo 100_2656 is a ?Hex? tool (Volvo SVO 2896) which is used to raise the jet. It screws into the lower float chamber casting after removing the drain plug.

3. Photo 100_2661 is the ?Dumbell tool (Volvo 2897) which is really 2 gauges used to set the proper jet height for Volvo B20A and B30 engines (about 0.80?) or Volvo B20B engines (about 0.125?). It is inserted into the carb throat (after raising the piston) and surrounds the needle. The lower surface of the tool rests on the jet, and the upper surface of the tool rests on the bottom of the piston. By pressing down on the piston (using the ?T-handled tool) the ?Dumbell tool? presses the jet down into the carb body.

I have since returned the tools to my Volvo friend, but was able to find a set on the usual auction site. I have used them, and they do work for removing, installing and adjusting the brass jets.

Any question . . . I?ll try to answer.
Attachments
100_2655.JPG and
100_2656.JPG and
100_2661.JPG and
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PostPost by: 69S4 » Thu Jan 20, 2011 12:10 pm

They look like they'd make the job a bit easier than the "hit a drift with a hammer" approach that was all I'd come across before. I suspect the carbs were a bit easier to get to in Volvos than in Elans though. The carbs would need to come off to use the one that moves the jet up.

Davea - I've been following your jet height saga with interest. Any progress on seeing how the raised jet height has performed?
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