| Desmogging a 907 |
By Kiyoshi Hamai
The following article originally ran in the C/R in September of 1981, a time when smog laws were substantially more lenient.
According to the EPA, the CHP, the CIA and who ever else, this article and the adjustments suggested are strictly no-no's for street use! That means don't do 'em and drive on the street!
Okay, now that we've got that out of the way on with it. The Exprit with the stock 907 is a classic case of smogitis. The 907 is choked, strangled and generally made not to run. (Except when it wants to...). For example most every Esprit with the air injectors, the smog pump, the catalytic convertors, the extra plumbing, etc., have a rather embarassing habit of diseling for 10 or so seconds after shut down. In addition there was the traditional grinding away before starting when the engine was hot. More red face.
The engine can be modified in stages, four to be exact, they are: Exhuast, Cam timing, Ignition, and Carburation.
First loosen the pulley retaining bolts on each cam. Set the engine to TDC, check timing not only at the flywheel but see that there are tow dots aligned at the cams. Now, the timing belt can be removed. The air box, the alternator and V-belt must be removed first. Then the belt tension adjustment screw can be slackened off, but not more than ½"! Do not remove the belt just yet, using a suitable nail or drill bit that is roughly 4mm in diameter (5/32") as a locking pin for the spring loaded tensioner, push down on the belt thus pushing the tensioner back and insert your locking pin in the small hole provided in the body of the tensioner. Now the belt is free to be removed from the pulleys.
Now the cams can be timed. On early 907s the blue dots were aligned on U.S. cars. On later 907s the blue dot on the intake and the red dot on the exhaust cam were aligned on U.S. cars. On early 907s remove the intake pulley and flip it over so that the dimple faces into the engine. Note each cam will have a dimple only on one side allow its outer edge. On the exhaust pulley likewise flip it, but the dimple will face out. Now both cams will require slight adjustments to align the red dots. Do not attempt to tighten the retaining bolts unless you have locked the cams in place (it's easy to turn a cam too far, pushing valves into pistons and bending those valves while excuting the tightening). Later 907s do likewise except align the yellow dots.
Now the jacksaft will require retiming. This is done by lining up the red dot (adjecent to the dimple) in line with the centers of the crank and the jackshaft. You can now replace the timing belt. If you do not have access to a belt tension gauge I recommend counting the number of turns that you loosened the tensioneer and additionally measure the deflection in the belt BEFORE having removed the belt. In this way you can refit the belt back to the original settings. Check the belt tension after turning the engine by hand at least one full turn. Replace V-belt, alternator, air box, etc. The ignition will have to be retimed to specs, but more on the next.
Cam Timing - Feb '99 Update from Ken Richie - Atlanta GA
I excerpted Kiyoshi's notes about 907 cam timing. This is the one on converting to "Euro" cam timing. A very lucid
description. + No new parts required!
I changed "dimple" to "pimple" after I realized we were looking for a raised bump on the pulley webs. I uncovered a clever mnemonic (now in bold italics), pulley pimples face [in on intake], [out on exhaust]. Even I can remember it that way... for a while ;-)
I checked my car, and it's set up just as described. The dimple my the distributor's jackshaft pulley is also facing out (i.e. forward, away from the block).
As for belt tensioners, the spring loaded ones are now taboo. Replace with fixed offset idler pulley. The one with the off-center center bearing bush.
Richie Longo recommends twisting the cam belt to get a "feel" for proper tension with motor COLD. Grab it on the long upper run, twisting 45 degrees. Probably not much over that. If the belt is loose enough that you can't get a hum by "thumping" it, it's way too loose. Or... gently lift and then lower the upper stretch to be sure it returns to center. Ultimately, the most accurate way to measure the tension is to check the resonance -- exactly 99Hz. Check and rotate the crank some and check again. That's about the same pitch as a "low G" on a piano counting down just 10 white keys from "middle C."
The only other thing I'd add to this article is to recommend checking the original alignment of the distributor pulley before loosening the cam belt, just in case it was set up 180 deg. "out of phase." The alignment could go either way, as long as the plug wires are routed appropriately into the cap.
To briefly review, the European versions use Dellortos, which are really a British Weber. Whereas the US cars have Strombergs. Recall that the Weber conversion is estimated to cost in excess of $700, therefore it is desirable to be able to modify the Strombergs to a certain degree.
There is a great lot of tubing and plumbing associated witht he Strombergs in the smog state. In principle the Strombergs are variable venturi carburators, the moving air valve is the controlling factor. On the front side of the carbs are two air bleed valves. Refer to diagram, air valve 1 is a diaphram type which opens by depression, ie. vacuum in the intake manifold. The thinking is that at a trailing throttle position the mixutre should be leaned. The problem is that the valve can not tell between a vacuum created by a trailing throttle or by the throttle being mashed open. Leaning is desirable in the former but richening is the most deairable state in the later. Air valve 2, is a heat sensitive one, it bleeds air into the mixture when the engine is warm. Air valve 2 can be recognized by the elogated 'yellowish' plastic cover. The removal of hte cover reveals a bi-metallic strip which deflects as it warms. At the upper most end there is a mounting screw, toward the mid section there is a small nut which adjusts the throw of the strip. And on the lower end the bi-metal strip forks to accomodate the valve itself. to de-activate this, remove the bi-metal strip and reverse it. Thus as the bi-metal strip heats and deflects it will do so inward, pushing the valve deeper into its seat. A hint, count the number of turns required to remove the adjusting nut, write it down, so if ever needed you can adjust the tension on the bi-metal strip back to its original setting. The de-activation of air valve 1 is simple, disconnect the vacuum line leading to it. Remember to plug off the manifold tap.
The other system of hoses attached to the carbs is the fuel recovery system. The system is designed to close the fuel system so no fumes are vented off into the atmosphere. This system does not affect performance, thus you might as well save it.
You will probably find that the engine will run a bit richer, which is fine as I believe it is the excessively lean state which causes the running-on and hard starting. Mike McHugh's Esprit is currently running lean, so de-activating the air valves will help to richen the mixture and hopefully reduce the run-on problem. My Esprit on the other hand was running slightly rich before I de-activated the air valves, so I ended up richening an already reich mixture. The remedy was to adjust the needles in the air valves. What is required is a needle adjustment tool. This can be had from most foreign auto parts shops, or purchase along handled 1/8 inch Allen wrench. what is entailed to adjust the needle/jet is to unscrew the damper (the black knob on the top of the carb) and then insert the adjusting tool down into the carb. Turning the tool clockwise raises the needle and enriches the mixture, turning counter clockwise lowers he needle and weakens the mixture. Be sure to hold the outer barrel steady as the diaphram can be easily damaged. Do not turn the needle any more than one turn in either direction. Adjust for the smoothest idle. It is a good idea to check youcarb synchronization after doing this adjustment.
Added Feb 7, 2005
From Tim Engel on Esprit List
The Federal 907 carbs are Zenith Stromberg 175 CD 2SE's. However, not all of those carbs are non-adjustable. Some were built application-specific and non-adjustable (a relative few), some (earlier) have adjustable jets that screw up and down and some (later) have adjustable needles that screw in and out. I do not know with certainty that all Lotus 907's with 175 CD 2SE's are adjustable, but I have yet to personally touch a set that were not. At the moment I have three sets of 907 ZS carbs in boxes (all replaced by Dellortos... one Esprit, two Eclats and one Elite), and all three sets have adjustable needles.
To adjust the idle mixture, adjust the depth of engagement between the jet and the tapered needle. Whether you move the jet or the needle isn't important since it's the relative positions that count.
To adjust the mixture over the engine's full range of speeds and loads, you change needles. The variable taper of the needles is like mechanical programming. Changing needles is like chipping a modern car.
If your carbs are stock, original Lotus carbs that have just been rebuilt and now run rich... then it's probably just the idle mixture adjustment that needs to be set.
All of my Lotus ZS carbs have adjustable needles (just an observation). Remove the black cap at the top of the carb, where you put the damper oil. Insert a special tool that consists of an Allen wrench slipped through a pin-spanner made from a piece of pipe about 7-8 inches long. The Allen wrench is used to turn the needle. However, just turning the needle would twist, distort and tear the rubber diaphragm. So there are notches in the diaphragm's hub that can be engaged by the pins on the end of the steel tube that slips over the Allen wrench. Engage the notches and hold the tube firmly to prevent the diaphragm from turning. Then use the Allen to adjust the needle.
Turning the needle counter-clockwise raises it, pulling the taper up out of the jet. That leaves a thinner part of the taper in the jet, opening the orifice more so it can pass more fuel into the throat. CCW = richer. Go CCW far enough, and the jet will hit a stop... full rich.
Turning the needle clockwise lowers it, pushing the taper deeper into the jet. That further restricts the orifice so it flows less fuel. CW =leaner. From full up, max-rich to full down, falling out the bottom lean is about 4 1/2 turns.
And the needle will fall out the bottom of the piston. If you go too far, then you need to stick a finger into the carb's throat to support the needle up in the piston while you screw it back in with the Allen wrench. There's no need to get close to the needle falling out since that's beyond the normal range of adjustment. If you need to approach 4 turns to get the mixture close to right, then something else more important is wrong. Deal with it first, then set the idle mixture.
When I ran the ZS carbs, I found the Fed 907's like to run at around 1 to 1 1/2 turns down from full up / CCW / full rich. For emissions checks they usually had to be adjusted to something between 3 and 3 1/2 turns down from full rich. Go up CCW until you hit the stop, then down CW counting turns as you go.
Adjustable jets work about the same way, except that access it through a removable plug in the bottom of the float bowl. Much less convenient, but the idea is the same.
If you actually have non-adjustable carbs, then your options are limited. If the carbs worked well before cleaning and now run too rich, make sure everything was properly installed. Especially the needles. Make sure they are installed the to the correct depth and centered before tightening them.
If the needles were replaced, make sure the correct ones were installed. If the old needles were re-used, they may be worn. There can be some contact between the needle and jet and over time they can wear. As they wear, the mixture goes rich. In that case the only good solution is to replace the worn parts.
If the carb itself is functioning okay, then there are a couple of add-on devices that can cause the mixture to go rich... the Temperature Compensator and the Throttle ByPass Valve. Both tend to go bad over time and they are "officially" not serviceable. You just replace them.
However, there is some grassroots knowledge on how to breathe new life into them. Go to vtr.org (Vintage Triumph Registry) and look through the tech articles. Both are basically valves and sometimes they get dirty or corroded and stick. Clean them and make sure they work freely. If they're beyond help, replace them. Or...
The Throttle ByPass Valve is the least likely of the two culprits, but it's possible. You can try cleaning everything or replacing it. Or, the engine will run better without the ByPass. Just install the old bits with a hand-cut, solid gasket to blank it off.
Temp Compensator is only worth something if you drive the car in cold weather... and I don't mean Winter in Florida. If the car goes into Winter storage or otherwise never sees real cold, then the Compensator is of marginal value. One option other than "making it right" is to defeat it. Remove the cover and turn the adjustment until the little piston is forced home so tight it will never open. Then adjust the carb's mixture for the normal driving season's predominant weather conditions and don't worry about Winter. I'm not suggesting a kluge... just saying here's an option. Use your own judgement.
The best fix... install a set of Dellortos. Don't spend money on a set of SU's when you can put the same money toward some real carbs. ;-)