Lotus Elan

Stromberg carburetted s4 bogs at acceleration

PostPost by: Greg Foster » Sun Nov 03, 2013 11:35 pm

Back story. Car ran decently before sitting about 4 months. When I tried to start it, gas poured from the carbs. So went throught the process below;
Just rebuilt the stromberg 175 cd 2 s on an elan S4. I also replaced the points with pertronics in a 43d lucas distributor (distributor was a newer replacement not done by me). All intake gaskets were replaced and manifold was properly tightened. New soft mount o rings and rubber retainers for the carbs. Put it all back together and it fired right up on the first try! Warm it up and drove it .
I took it out for a spin and there was a slight hesitation on acceleration but the car idled fine and ran at speed just fine. Distributor advance was set to 12degrees btdc, new rotor , cap was new. Wires are copper and in good shape(checked resistance). Plug gap set at .025 plugs were clean and almost new. Ran it for about an hour and parked it.
So everything was good...just needed a little fettling. Well today I fettled and lost.....I advanced the distributor a little less to 10 degrees and the carbs have adjustable needles( the crossover tubes have been deleted, and the carbs are of the earlier design...Ive been told). I leaned out the needles and turned them 3 turns back rich to see how it would do.
Now as I try to accellerate, the car has NO POWER AT ALL It won't pull. It idles fine starts fine, warms up fine... I really haven't spent too much time with the adjustments but thought I would ask for considerations before bodging something. Strombergs seem simple but I don't have much experience with them.
Should I advance the distributor to 12? Lean it out some?

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PostPost by: Greg Foster » Sun Nov 03, 2013 11:37 pm

To clarify, I did use a timing light to set the timing at around 700rpms.
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PostPost by: RogerFrench » Mon Nov 04, 2013 5:20 am

First thing - when you "leaned out" the needles, did you do that with the air valves and needles in the carbs or out? If they were in, are you sure you didn't screw them all the way out of the bottom of the air valves? I'm a bit surprised you could get as much as 3 turns of enrichment. I'm guessing you have maximum lean and the screws may no longer be engaging in the needles.

Second thing - how about the air valve dampers? Do the valves show marked resistance to being lifted by hand, or rather by finger? Do they drop with a click? If they lift very easily, you may have too thin a fluid in the dampers. Lotus recommend 20/50.

As for your suggested cures - normally, if engines bog at low revs, and the problem is carburation, it's usually because they're too lean. So setting them leaner will make it worse.

Does your distributor have the vacuum retard fitted? If so, is it still working? There are more models of 43D than I care to think. If you have one of the newer 43D distributors as sold by our favourite suppliers it probably has no vacuum. I don't believe that 2 degrees more or less of static advance or retard will give the symptoms you describe - it certainly wouldn't on my Europa TC which has Strombergs and a newer 43D with Pertronix.
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PostPost by: oldelanman » Mon Nov 04, 2013 9:10 am

Hi Greg,

I think Roger is right at 3 turns you have probably wound the adjuster screw right out of the needle carrier.

I did some measurements when I had my carbs apart and there are only 3.75 turns of adjuster thread engagement in total, that's around 1.5 turns either way from the nominal setting where the shoulder of the needle is flush with the base of the air valve.

If you have screwed it right out you may want to check that the adjuster screw is still seated in the bottom of the air valve tube and that the star washer which holds it in place is pushed fully home before you reassemble it.

If my measurements are of any interest let me know and I will PM you or post them on here.
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PostPost by: Greg Foster » Mon Nov 04, 2013 12:41 pm

Thanks for the response.
I will recheck this afternoon if I can. I need to reread the manual to be completely familiarized with the carbs and vocabulary.
The distributor is a mechanical advance and that is all I know. I could see if there are pertenate numbers or markings for reference. What should the advance be?
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PostPost by: Greg Foster » Mon Nov 04, 2013 12:48 pm

So a little more information.
I completely rebuilt the carburettors, cleaned, new seals on throttle shafts, seal at needle mount on the bottom of the piston, new fuel valve at float bowl and float adjusted to what I believe is correct from the lotus wsm....
More research is needed.
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PostPost by: 69S4 » Mon Nov 04, 2013 1:26 pm

Have you checked the diaphragms? A hole in one would give symptoms similar to yours - good idle but no power - as the piston won't rise.
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PostPost by: Greg Foster » Mon Nov 04, 2013 10:54 pm

Diaphragms in good shape.....was running decent before I messed with it.
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PostPost by: collins_dan » Tue Nov 05, 2013 4:43 pm

I tried to reduce advance on my S4 and had a similar problem. I would go back to 12. I think the manual calls for 12.

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PostPost by: oldelanman » Tue Nov 05, 2013 5:24 pm

My manual says 9 deg advance for Strombergs and that works with mine.............5 deg with the exhaust emission setup
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PostPost by: collins_dan » Tue Nov 05, 2013 5:43 pm

Not sure why different degrees of advance seem to work better for what are seemingly identical set ups.
Here is a post that influenced my set up when I had L2 cams. I now have Sprint cams, but am still using the same 12 degree advance.
lotus-electrical-f38/how-much-advance-t18100.html
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PostPost by: oldelanman » Tue Nov 05, 2013 6:33 pm

collins_dan wrote:Not sure why different degrees of advance seem to work better for what are seemingly identical set ups.
Here is a post that influenced my set up when I had L2 cams. I now have Sprint cams, but am still using the same 12 degree advance.
lotus-electrical-f38/how-much-advance-t18100.html
Sorry I should have said.....my engine is completely standard hence the "standard" 9 deg timing which still seems to work despite the modern low octane lead free fuel, fitting Sprint cams will obviously make a difference.
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PostPost by: Greg Foster » Wed Nov 06, 2013 1:44 am

I checked the timing this evening and found it was set high , 15deg. So I reset timing to 9 deg btdc. I then went back to the carbs and checked the needles were still seated and turned them all the way up (rich) and backed them down 1 full turn to start. Meanwhile the car started and idled great....idle set at 750 rpm. After warm up of about 15 minutes running, it seemed to rev evenly enough to take it for a drive. NO GO! The engine seemed starved for fuel.... I parked it and will wait until the morning to start with the needles full rich.
Quite puzzling since it ran decently( but still had the hesitation) in the beginning ........ could I have mistakenly set the float levels incorrectly? 16mm from base to bottom of float at the highest point... I am puzzled as this probably is some simple thing that is being overlooked.
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PostPost by: 69S4 » Wed Nov 06, 2013 7:54 am

Book says 16 to 17mm for float height. I've got mine set to 17mm at the moment but I've had it at 16mm in the past and not found it that critical until it's out by about 4-5mm (don't ask how I got it out by that much!). Then the symptoms were similar to yours - fine low down but nothing higher up - but with lots of tinkling noises from the engine on acceleration.

I had something similar a couple of years ago when the zig zag clamp thing joining the carbs together came loose on one side and only one carb was opening. That was fairly easy to spot though.

In my experience a few degrees of static timing one way or the other won't affect the top end - idle and ease of starting maybe, but I did notice a significant reduction in top end performance when a new distributor had 7 degree max advance bob weights fitted (roughly 23-4 degrees total advance). Yours are not stuck are they?
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PostPost by: Greg Foster » Wed Nov 06, 2013 9:57 pm

I just took the car out for a drive after moving the needles as high as they would go. Again the car starts and Idles fine, but on the road I need to have the choke out while accelerating and driving at normal speed... Fuel problem for sure but what am I doing wrong?
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