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Tuning Stromberg's  Rating:  Rating
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 Posted: 05-07-2005 02:29 am
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Gary Martin JH 15371
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I just got the engine running after 19 years out of service in JH 15371. I did not rebuild the lower end as the car only had 55,000 miles when it had a carb fire in 1986. Head is rebuilt, all new seals, carbs rebuilt. Engine started on first try even with the timing off quite a bit. Sounds fine, oil pressure over 50 psi. Still using points in dist for now.

Once running I set timing (aprox 7 deg BTDC). I then used a carb synchronizer to equalize carbs. Engine idles, a bit of black smoke out the tail pipe, and idle "Hunts" a bit. After engine is warmed up, if I rev it up, when it returns to idle it almost dies and has trouble getting back to idle again.

There are no vacuum leaks. Bypass valves rebuilt, but caped off as my vacuum switch valve is toast (leaks vacuum to bypass valves all the time). Mixture needles initially set to level with bottom of valve body as instructed in rebuild kit. I have tried leaning mixture about 3 quarters of a turn CCW. Did not seem to change anything much. Idle still hunts around 200 or 300 RPM.

I also notice gas drips from bottom of Choke assembly on front carb. Seems to drip one or two drops after car is shut off. The carb kit did not seem to come with parts to rebuild choke.

Any advice on tuning, and why the choke drips gas ? Don't want it to burn again.

Thanks, Gary

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 Posted: 05-07-2005 05:27 am
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Jensen Healey
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Remember that 90% of carbureter problems are ignition. Go ahead and check the floats (with the big hammer) and "read" the plugs. I'll bet there's oil on them. If so, installing a hotter plug will help.

Let us know,

Kurt

 

Last edited on 05-07-2005 02:13 pm by Jensen Healey

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 Posted: 05-07-2005 05:52 pm
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Mark Rosenbaum
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Congratulations on waking up your car after its long sleep.  Sounds like you have the usual sort of teething problems, all of which can be fixed (eventually).

The fuel leak from the choke implies that either the front carb's fuel inlet valve is not sealing properly, or the float level is set too high, or perhaps both.  Over-tightening the fuel inlet valve may distort it to the point that it won't seal properly.  Probably you'll have to pull the carb to find out what's wrong.  Whatever it is, is also likely to be the cause of your black smoke.

The rebuild kit should have included the choke-to-carb and choke-internal gaskets.  See the attached photo.  If your kits were missing these, and if the old gaskets are not reusable, contact me directly as I've doubtless got some left-overs I could send you.

Unless the temp compensators on both carbs are disabled, carb balance needs to be set after the engine is fully warmed up.  From your post it isn't clear if you set the balance before that.

As far as mixture goes, it's hard or impossible to set the mixture properly until everything else is taken care of, specifically including the elimination of all fuel leaks and incorrect float level settings, so I advise returning the mixture needles to their 'datum' positions for now.

Don't know if you've read the Stromberg rebuild article that's currently appearing in the JHPS magazine but I do comment there on rebuilding the vacuum switch, which may be possible in your case.  This switch, and the throttle bypass valves, allow the engine to return to idle relatively slowly when the throttle is closed.  The fact that these are currently disabled is quite possibly contributing to your near-stall upon returning to idle, though there may be other factors involved as well.

Normal idle speed drift on a stock 907 engine in good condition, with stock cams, seems to be about 25-30 rpm on either side of the nominal 975 rpm setpoint.  This is actually pretty stable for an engine with cams as radical as the stock grind.  The 200-300 rpm you observe is a bit too much.  Unbalanced carburetion, too lean a mixture, and low manifold vacuum can often cause such drift, but as Kurt points out, ignition is often the true culprit for an alleged carburetion problem.  Things to check include:

(a)  Is your vacuum retard mechanism hooked up?  If so, disable the thing while you troubleshoot.  If the internal diaphragm is damaged and has a leak that varies with diaphragm position, it can cause the hunting idle you describe.  (For that matter, so can excess space between the carb piston [vacuum valve] and the carb top, or a lack of damper oil.)

(b) Have you checked the distributor and made sure that its advance springs are okay and that the advance weights move freely?  If these are intermittently sticky, the ignition advance will vary erratically and can cause all sorts of odd problems at idle and elsewhere.

(c) Are the wires connecting the points to the blade connector, and the rotating plate to the body of the distributor, properly limp?  Stiff wires here can do really odd things to the ignition advance/retard.  Things can get quite misleading if the wires' flexibility varies with temperature.

(d) What sort of intake manifold vacuum are you seeing?  With a somewhat retarded spark such as the 7^ BTDC that you mention, you probably have only 12-13" Hg of vacuum.  Try advancing the timing to about 12^ BTDC while watching the vacuum gauge.  With stock cams, the manifold vacuum should climb to 15" Hg or slightly more, and you should see upwards of 18" Hg momentarily when closing the throttle.

And finally, if you're getting lots of oil in the combustion chambers, then the car won't ever run perfectly.  As Kurt notes, going one step hotter on the plugs may help, if the oiling isn't too bad, but going too far on the plug heat range can result in major engine damage.

Attachment: Stromberg Choke Assy Gaskets.jpg (Downloaded 363 times)

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 Posted: 05-07-2005 09:23 pm
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Gary Martin JH 15371
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OK, I have many things to check now. I don't think I overly tightened the inlet valve. I did use the thicker Aluminum seal as this is what carbs had before. By the way these are not the burned carbs, but a used set off an "un-burnt" JH. The float level was set to spec listed in rebuild kit. The carb kit did come with one choke gasket, the internal one. But not the one that seals choke to side of carb. That gasket is still good and I can re-use. I was not sure if there were other gaskets or seals internal to choke, evidently just the two gaskets.

I have been reading your articles in JHPS monthly mag.  I have been working to rebuild the vacuum switch. I used a counter sink and made a new beveled seat. The rounded seal was scared, and I have tried to make it seal against the seat using a fine valve lapping compound. I think the main problem is the diaphram. It is old and dried out. It causes the internal parts to lean over to one side when actuated. It does not open/close in a centered position, but tilts over to side. So for now I decided to not use and capped of the bypass valves. My vacuum switch did have a gasket on lower part (you mentioned you had never seen one there).

Carb Sync was done after it was warmed up. I did have a question about the Temp Compensators. Jim at Delta told me that when cold, they should just barly close. And to make sure they open when warmed up. They do open when warm. The tension on the bi-metal spring seems the same on both carbs.

My distributer is rebuilt. The advance springs and weights move freely and are cleaned and lubed.  New points, condensor, rotor, cap, wires, plugs. The Vac advance diaphram does work, but if you check with hand vac pump it will not hold vacuum for more than a few seconds, so this may have an internal leak. I will disconnect for now and see if this improves things. The internal wires are in good shape, but are a bit stiff as you explain. I installed a new power lead and this is particularly stiff.

I have not checked manifold vaccum yet. I will do so shortly and advance the timing to 12^tdc. I will also check the plugs to see how they are doing.

Almost forgot. The throttle shaft on rear carb is loose a bit. This could also be a problem. Can these be re-bushed ?

And last, I have another set of 175 Stromberg's off a early 70's Volvo. They are very similar. Both carbs have a choke which seems good. But there is one difference, the Volvo carbs have an extra linkage that operates a valve internal to the carb body. Has anyone used Volvo carbs on a JH ?  

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 Posted: 05-07-2005 11:09 pm
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Mark Rosenbaum
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The 'choke' (actually a starting enrichment circuit) works by applying venturi-created vacuum through tubes to the fuel in the front carb's float bowl.  The amount of additional fuel provided is regulated by the size of the selected passages in the rotating disk that sits within the mechanism.  This means that raw fuel passes into the choke mechanism and thence to the venturis of the two carburetors.  Note that unlike conventional chokes, this method adds no restriction to the air intake path. 

If the rotating disk is installed incorrectly, the choke may not function, or may never shut off.  In the former case the car will be hard to start when cold.  In the latter case, you'll have an excessively rich mixture at all times.  This is another possible cause of the excess carbon in your exhaust.

If you didn't have a gasket between choke and carb body, any left-over fuel in the choke may have been the cause of the fuel dripping you noted.  If so, then all you need do is install the gasket.  If the old gasket doesn't work out, I can send you a left-over new one, or you can make one yourself from a piece of thin cardboard.

AFAIK the vacuum switch diaphragm is just plain unavailable and the price of a replacement vacuum switch is absurdly high.  I've made some attempts to come up with a means of replacing bad diaphragms but so far have not been successful.  Capping things off seems to be the only option in cases like yours.  Yours is the first switch I've heard of that has a gasket at the lower joint.

Jim Medland's comments about the temperature compensators are of course correct.  They have little effect save to slightly lean out the fuel-air mixture at idle once the engine warms up.  Basic functionality can be determined on the car by removing the plastic cover, applying hot air from a hair dryer, and seeing if the arm moves, but if re-calibration is ever needed they should be adjusted in pairs in a hot water bath.

The vacuum retard (not advance) capsule on the distributor should hold vacuum for much longer than just a few seconds, so I'm pretty sure yours is leaking.  You can just disconnect and plug the vacuum line leading to it -- but do save the parts in case you ever sell the car and it needs another emissions test.  As for the somewhat stiff wires inside the distributor, if these don't hinder the smooth operation of the centrifugal advance, coming or going, they are acceptable.  Problems in this area are often apparent when exercising the throttle while checking the timing with a timing light.

Strombergs, like SUs, can be re-bushed but this is rather costly and it's generally less expensive to buy a replacement carb through eBay.  However, with Strombergs the brass throttle shaft seems to suffer the majority of the wear.  Shaft life can sometimes be extended by polishing out any steps in the shaft, but this is limited as eventually the shaft seals will no longer fit, the shaft will wobble too much, or both.  Fortunately new shafts aren't too expensive.  Delta generally has both types in stock, and they should be available from a variety of other sources if Delta happens to be out.

Strombergs from Volvos work perfectly well in a JH, provided the correct mixture needle is installed; no other change is necessary.  In the JH-specific carb, the bowl is always vented directly to the air box.  In the Volvo carb, the extra linkage you mention operates a valve that vents or seals the float bowl depending on whether or not the throttle is open.  This is just another early emissions system which IIRC allowed Volvo to avoid using a carbon canister.  The whole thing can be converted over to the JH system if desired, but it's probably not worth bothering with.

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 Posted: 05-09-2005 08:11 pm
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Gary Martin JH 15371
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OK, yesterday I took the choke off. I dissasembled and cleaned it up. I replaced the internal gasket (only one my kit has). Put it back on using the old gasket to side of carb using a very light coat of Wellseal. This fixed the leak problem. However, I am suprised that there is no seal on the shaft end of the choke by the nut. It looks to me like gas can get past the rotating disk and into the outer chamber. It could then leak out the shaft by the nut. I think is where mine was leaking before.

Tuning update. My vacuum retard unit was leaking internally. I disconnected it. Set idle mixture back to datum. Started it up and let it warm up. With timing set at 12^btdc manifold vacuum is 15 Hg. When I blip the throttle it drops and then goes up over 22 Hg as it returns to idle. Seems to run fine now. Idle is fairly steady at 950 to 1000 RPM by the tach on dash. No more smoke out the tailpipe, but it does smell a bit rich. I can now detect the somewhat lumpy idle of the stock cams. Sounds pretty good.

Dwell and idle speed question. To get even close to the recomended 60 deg dwell I had to close my points to about .010 inch. If I set points to .014 to .016 my dwell is in 50 range. These are new points. But I'm now an expert at removing and installing the distributor! Kind of a pain. I think I'll get that Pertronix system soon. Also with my diagnostic meter, the RMP reads 700 RPM while the dash tach reads more like 1000 RPM. How acurate is the stock tach ?

Oil pressure on startup. I have noticed when starting it up cold, I get a couple seconds of knocking before oil pressure comes up. Is this normal on this car ? Right now I have the oil cooler disconnected by removing the adaptor between oil filter and auxilury housing. Using a WIX filter at moment. I have the K&N filter from Delta, just have not put it on yet. Oil pressure is well over 50 psi when cold and about 25 psi when warmed up. Running Castrol GTX 20-50 oil. Gary

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 Posted: 05-09-2005 10:00 pm
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Mark Rosenbaum
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Gary, it sounds like you've made major progress.

Choke -- You are correct, there is no seal on the choke shaft and it's possible for fuel to leak there.  However, in normal operation, any fuel in the choke is drawn in then out by suction from the two carbs.  I would expect that normally any leakage past the shaft would be quite low.  I suppose it might be possible to fit an o-ring, but that would probably require machining something, and might stiffen up the mechanism so much that a solid rather than stranded cable would be required.  It would probably be far easier just to find a choke mechanism that didn't leak. 

Tuning -- What you describe is normal for a JH.  The slightly rich scent of the exhaust is probably nothing more than an indication that the carbs are set a bit rich at present.  This is normally dealt with when the mixture is being set.

Dwell -- When the points gap gets down to 0.010" or thereabouts the engine becomes hard to start, is prone to missing particularly under load, and tends to have less power, all caused by arcing across the points.  Dwell should be as specified when the gap is in spec.  If not, either the dwell meter is itself inaccurate (this is almost guaranteed if it's an inexpensive unit), or the points cam in the distributor is worn out.  If it were my car, I'd set the points gap to spec, ignore the dwell reading, and wouldn't plan for a Pertronix device unless the car ran poorly.

Tach accuracy -- Guessing here, as AFAIK there are no specs.  Back in the 1970s, I would have expected readings to be within 6% of actual up to 4500 RPM or so, and within 10% of actual above that.  The meter movement itself is probably accurate to 2% or 2.5% and this won't change significantly as long as its bearings remain clean.  The electronics will change as they age, and after 30 years, one might be fortunate if the instrument were to read within 20% of actual RPM.  As long as none of the electronics components have actually failed, the calibration adjustment inside the tach should allow compensation for age-related changes.  Calibration, however, requires knowing the actual RPM.  I would not trust an inexpensive tach/dwell meter for this.

Oil Pressure Rise Time -- What you report sounds quite normal.  A 907 in good condition that last ran within a day or so ought to show oil pressure quite suddenly about 3 to 6 seconds after the engine has started, or within 10 seconds if it's sat for a month.  If you monitor this time (by mentally counting seconds) at every engine start, you can spot potential problems before they get to cause any damage.

For example, if the rise time increases by 3 to 6 seconds after a filter change, the filter has a leaky anti-drainback valve and should be replaced.  This might occur perhaps 1% of the time with any filter brand, and can be more frequent in low-cost filters.  The problem is exacerbated in the 907 engine as the filter is horizontal and will inevitably drain somewhat after engine shutdown.

If the rise time increases to 10-15 seconds, your oil pump is in the process of failing.  The pump typically fails by suddenly losing the ability to prime itself; only in really bad cases will it quit providing pressure while the engine is running.

Oil pressure -- 25 psi when the engine is fully warmed up is pretty good, particularly if the oil cooler is not being used.  You should expect a drop of 5-10 psi (or perhaps far more without the oil cooler) if you should ever have to use 10W40 oil.  As long as you have at least 5 psi pressure at 1000 rpm idle, and at least 30-40 psi at freeway speeds, you should be all right.  OTOH, for racing you probably need at least 10 psi per 1000 rpm.

 

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 Posted: 11-02-2005 03:02 pm
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LambandAndy
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Mark,

Previously in this string you talked about "capping off" the vacuum switch.

I have just bought a '74 Jensen Healey (S/N 15223) which has stock motor and Strombergs but the vacuum switch is missing.  Would you recommend still teeing the distributor vacuum line into the carburator vacuum lines or just run the vacuum retard directly off the inlet manifold port?

Can anyone tell me exactly how the vacuum switch works (i.e. opening/closing pressures etc.)?

It is also missing the carbon canister so does anyone out there have any alternative methods for venting the gas tank?

As a new member I would like to say to all that contribute that I think this web site is great.  I owned a JH when I was young and dumb and stripped it to the crank and rebuilt it.  I sure do wish there was something like this back then.  (Now that I'm old and dumb I went and bought another one).

Andy

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 Posted: 11-02-2005 07:23 pm
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Mark Rosenbaum
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Andy,
Welcome, or welcome back, as you prefer.  The phrase 'old and dumb' hardly seems applicable to someone who's sufficiently 'experienced and wise' to buy a JH.   :^}

Vacuum Switch.
Used only on cars with Strombergs.  The vacuum switch is just a valve intended to open and close at specific vacuum levels.  It should open ('pass vacuum') at 17.5" Hg to 20.0" Hg, and should just close at 15.5" Hg to 16.5" Hg.  The test procedure, such as it is, is contained in section RM55 of the shop manual.

Replacement switches are listed in the Delta Motorsports catalog, but I don't know if they remain available.  However, I've seen quite a few switches attached to carburetors being sold on eBay.  Such switches have a good chance of being defective, of course, but may be repairable.  Some repair information is present in the multii-part carburetor article that appeared earlier this year in the JHPS magazine. 

The vacuum switch and throttle bypass valve system prevents engine stumble during the latter part of a rapid open-throttle / closed-throttle / full-throttle transition, and provides a decreased likelyhood of engine stalling or stumbling during sudden transitions from open to closed throttle.  The arrangement also supposedly ameliorates a number of emissions issues related to these driving activities.  The vented spigot that screws to the engine side of the rear carb's inlet flange, that I call a 'calibrated leak', determines how long the throttle bypass valves remain open once the vacuum switch closes in response to decreasing manifold vacuum.

Since the normal manifold vacuum of a 907 engine is about 14" Hg to 15" Hg, the vacuum switch in conjunction with the calibrated leak allows extending the hold-open period of the throttle bypass valves to a duration suitable to the engine's needs.  The arrangement would also prevent 'throttle flutter' which could otherwise occur in response to manifold vacuum variations that resulted from the opening or closing of the throttle bypass valves.  (This is a 'positive feedback' situation which at best is very annoying.  If carried to an extreme that's unlikely to be possible with Strombergs, it could result in a complete loss of the driver's control of the throttle, with potentially lethal results.)

As the system does provide a net benefit, my recommendation is to ensure that it remains functional.  If this is not practical for some reason, the manifold vacuum line can be run solely to the distributor.  However, I don't favor using the vacuum retard function at all, so you might be better off just capping the manifold vacuum port.

Carbon Canister.
Used only on cars with Strombergs.  The fuel tank is normally sealed.  A small diameter pipe runs from the top of the fuel tank to the engine bay and is connected to one of the spigots on the carbon canister.  A second canister spigot is plumbed to the canister ports of the carburetors.  The canister's third spigot is capped off.

Expanding vapors from the fuel tank pass through the pipe and are absorbed by the carbon granules in the canister.  Air to replace consumed fuel passes back through the pipe and into the fuel tank.  When the engine is idling with a nearly closed throttle, a vacuum occurs at the carbs' canister ports.  This causes air to flow through the canister and into the carburetors.  Absorbed fuel vapors mix with this air and are consumed.

The canister's rather elevated position is related to the height and location of the fuel tank, and is one of those unfortunate design tradeoffs that must be made from time to time.  If liquid fuel should somehow reach the canister, it will drain out and if ignited should provide fiery thrills.  A lower canister location would have decreased the likelyhood of liquid fuel sloshing around the engine bay, but would have increased the likelyhood of liquid fuel leaking out, particularly if the car should be parked nose-downhill and with a full fuel tank. 

Rather surprisingly for an early emissions system, the carbon canister actually works more or less as intended, and does not adversely affect performance or idle if the carbs are in good shape, so it may be prudent to keep things in working order.  However, there's nothing at all peculiar about the canister used in the JH, and if an exact replacement is too expensive, you may wish to check the local wrecking yard for something that can be made to fit (starting perhaps with mid-1970s to mid-1980s GM cars).  Alternatively, you might replace the existing pipe with one that vents under the car, as was commonly done prior to emissions controls.

I advise ensuring that the fuel filler cap seal remains airtight, as it is highly undesirable to have fuel gushing out so near to the occupant(s) in the event that the car becomes inverted due to mishap or misadventure.

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