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Vacuum switch question  Rating:  Rating
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 Posted: 12-29-2013 11:53 pm
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bora450
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Hello all, this is my first post about my new 1974 J-H 5. Carburetors are Stromberg 175CD-2SE. I know nothing about these carburetors yet and have already ordered a carburetor manual to learn. So I need your help.

Today, the vacuum switch behind the rear carburetor is leaking fuel out of the hole in the black cap. I removed it and plugged the vacuum hose with my finger and left the carburetor hose open. At idle, fuel is dripping out of the hose attached to the carburetors. Is that correct? Is fuel always present there?

Thanks,
Claus

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 Posted: 12-30-2013 12:07 am
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bora450
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That vacuum switch has a hole in the diaphram but I am lucky enough to have another vacuum switch that is good.

So I need two answers. Is it OK for fuel to be in the carburetor side hose? Second, the vacuum switch has two fittings, one larger than the other. Which vacuum switch fitting is toward the carburetors?

Thanks again,
Claus

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 Posted: 12-30-2013 08:15 pm
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bora450
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I have determined that fuel is dribbling out of the rear Stromberg emissions fitting that mark Rosenbaum labeled "Calibrated Leak" in his sketch. Can anyone pull the vacuum tube off of the fitting and let me know if it is wet or dry?

All of a sudden, the engine idles rough until it dies. The front two spark plugs are white while the rear two are tan. I'm trying to figure out if this has anything to do with it.

Thanks, Claus

Attachment: JH Vacuum Lines.gif (Downloaded 106 times)

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 Posted: 12-31-2013 02:30 pm
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Brett Gibson JH5 20497
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My 2 cents, get rid of it, remove the light blue tubing and plug it off at the source, run the green line just to the dizzy, put a full face gasket on the bypass valves on each carb, and then screw down the temp compensator's nut's to shut that off as well, your car will thank you for it by running much better.

Brett

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 Posted: 01-06-2014 09:52 pm
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Esprit2
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> Is it OK for fuel to be in the carburetor side hose?

No, that's not normal. However, it's most likely the hole in the vacuum switch's diaphragm that is the root cause problem. By allowing a full time manifold vacuum to exist in the hose, where a vacuum shouldn't normally be, it's drawing fuel out of the carb, rather than fuel forcing it's way into the hose. Fix the diaphragm hole/ vacuum leak and see if the fuel leak goes away as well.

> Second, the vacuum switch has two fittings, one larger
> than the other. Which vacuum switch fitting is toward
> the carburetors?

Sorry, I'm really not trying to make light of your question, but there are also two mating hose diameters. The large one goes on the large spigot, and the small one goes on the small spigot.

But without beating up on that any further, +1 on Brett's comments.

"IF" you don't have to pass an annual emissions inspection (and in must States, cars of this vintage don't), then just remove/ blank off all the bits in the blue/aqua Throttle Bypass circuit. Remove the vacuum switch and hoses, and plug all the ports. Remove the Throttle Bypass Valves from the carbs, hand-cut solid gaskets (no cut-outs or holes in the center), and re-install the valves. The solid gaskets effectively eliminate the valves, while re-installing the valves plugs the hole/ scar in the side of the carb and 'looks' right.

The Throttle Bypass is an emissions thing to minimize the spike in oxides of nitrogen during closed throttle over-run, and the engine will run better without it. On over-run, the resulting strong manifold vacuum causes the vacuum switch to open and admit vacuum into the blue hose circuit, causing the bypass valves to open. That provides a Air/Fuel flow path around the butterfly, keeping the idle rpm up, and weakening the manifold vacuum until the over-run period ends. That really messes with throttle response and a consistent idle. Deleting the system makes the engine more responsive and pleasant to drive. No more or less power, just more pleasant to drive.

The Thermal Compensator is supposed to make a slight mixture adjustment for cold weather operation. If it's working well, it's really not a problem. Most aren't working well any longer, and cause more small problems than they solve. Most Jensen-Healeys don't get driven in real 'Winter' conditions, so the Temp Compensator's value is further diminished. IMHO, remove the plastic cover, turn the screw to permanently, firmly seat the plunger/ needle at the end of the bi-metallic strip, and replace the cover.

If anyone living in the Northern tier is still driving their J-H in Dec-Feb, then it's easier to make a small tweak to the idle mixture twice a year... cold & warm. If yours is a fair weather car, then just tune the carb for the best idle and performance during your prime driving season, and don't sweat the storage season. If it doesn't get cold enough where you live to warrant seasonal storage, then you don't really need the Thermal Compensator... IMHO.

The green hose to the distributor is vacuum RETARD. It retards the ignition timing at closed-throttle idle, and goes away once the throttle is cracked open... it's not a factor in normal driving. Beyond emissions effects, vac retard gives a more benign idle. On the other hand, it also slows off-idle throttle response. You'll get alternate opinions on whether to leave it, or remove it and plug the ports. For daily driving, you might want to leave it. For spirited driving or motorsports (autocross), delete it and plug the ports, then adjust the ignition's idle advance to 10-12 degrees BTDC and the carb for strongest idle/ best running. Set the idle rpm to 1000 rpm.

Enjoy your car...

Last edited on 01-06-2014 09:57 pm by Esprit2

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 Posted: 01-06-2014 10:03 pm
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Esprit2
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bora450 wrote:
I have determined that fuel is dribbling out of the rear Stromberg emissions fitting that Mark Rosenbaum labeled "Calibrated Leak" in his sketch.Good, you're reading Mark's posts. He was a great resource. To get up to speed about the JH with good sound advice, search for all his posts and start reading.

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 Posted: 01-06-2014 10:33 pm
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bora450
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I now have answers to what happened. The vacuum switch did have a crack in the diaphram. The carburetor fuel bowl needle injested debris and stopped closing. Fuel filled the carburetor until it exited near the top, into the air cleaner housing, dripping onto the starter, and through the tube leading to, and out of, the vacuum switch onto the engine. I'm really glas that I saw it before heading out for another test drive.

The needle and seat from one of the Strombergs removed from my TR6 (Webers installed) has an inlet screen incorporated into it and after installing it, the carburetor is back to normal. So even though I installed a new fuel filter, I had better clean out the line to the carbs.

I'll take your suggestions and remove the vacuum curcuit, also capping the distributor retard and retiming.

I have been working my way through the forum, reading every post in every thread. This is the fastest way to learn about the car.

Thank you both for your input.
Claus

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