XJ6 & XJ12 Series I, II & III 1968-1992

XJ6C starts but won't rev past ~1000 rpm

 
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Old 05-04-2014, 03:58 PM
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Default XJ6C starts but won't rev past ~1000 rpm

Hello again,

I just replaced both pumps (submersible type) on my XJ6C. I had a problem with the oil pressure switch cutting off the fuel pumps, so I ground the pressure switch lead (while I wait for a new switch) and now have fuel delivery to the carbs (thanks to all for the help on that issue). Now that I have fuel, she will kick over and run at an idle but will not rev. I measured the pump pressure at the front carb input and only get 4.5 psi on the driver side tank and 3 psi on the other . I thought this was kind of low, so I found another gage and it read 4 psi and 3 psi respectively. Is this to low? I was expecting closer to 10 psi.

Thanks,

Steve

76 XJ6C
 
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Old 05-05-2014, 06:41 AM
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I am not aware of the PS spec for these engines. But, the last carb'd engine I messed with was the dual and sometimes triple carb's Ford V8 flat head in my roadster project.
Those numbers were OK for those primitive carbs and the engine would scream. Actually I had a regulator to keep the PS down as much more would overcome the float needles and cause a flood condition.

Kind of a guess, it might be that PS is good, but volumn is low. Partialy occluded filters is a popular cause for no revs in a lot of critters. Did you clean the tanks and purge the lines. Crud might have been the cause of the demise of you pumps!!

Keep grinding and you will get things right.

My leaf blower is suffering a similar malady???

Carl
 
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Old 05-05-2014, 06:02 PM
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Carl,

More in common than you might think, lots of flatheads were topped with Strombergs as is my XJ6C, although mine are 175 CD2 side drafts. However, the needle and seat arrangements are probably similar. Too much psi might overflow but I'm afraid too little will starve my 4.2. The manufacturer of the pump specs it at 5.1 to 7.3 lbs. Is it common to lose a few lbs. between the tank and the carbs? Or should I see 5.1 to 7.3 at the carb?

Thanks,

Steve
 
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Old 05-06-2014, 03:25 AM
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Pump specs have always been a tad iffy, and the pressure you are seeing is OK for the Jag in my opinion.

Flow is more critical than pressure on the carby cars, and as said, the filter/s, supply pipe damage under the car, etc, all play a part in this.

The Stromberg CD's were notorious for pin holes in the diaphragm, and that always killed revs, hence very few of those carbs down here now. SU replacement are "standard".
 
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Old 05-06-2014, 11:53 AM
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Grant,

I popped of the fuel line, stuck it into an old soda bottle and turned the key for a few seconds. Got about 5 or 6 ounces so thinking an ounce to 2 per second which calculates out to about 40 to 60 GPH. So probably not fuel delivery? Thinking carbs? The pistons seem to move easily. What would keep them from rising?

Thanks,

Steve
 
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Old 05-07-2014, 03:23 AM
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Yep, that flow is more than adequate.

The diaphragms are under the top "lid" of the carby, and as I said we used to sell those diaphragms by the "bucket load" back in the day.

It has been 30+ years since I replaced one, and the details simply will not come to the front of the brain, but it is NOT rocket science I am sure.

Basically remove the damper plunger, then the screws securing the "top lid", lift off that lid, retrieve the "loooooong" spring, and there is the diaphragm basically "in your face", then the fuzzies kick in, sorry.
 
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Old 05-07-2014, 06:26 PM
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Popped the "hats" of the strombergs and the diaphragms look brand new. Actually stretched them a little to see if I could see any pin holes. Look fine.
hooked a vacuum gage up and while running around 1000 rpm, getting fluctuation between 15 and 22 inches. Ignition not advancing? This is a puzzler....
 
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Old 05-07-2014, 10:00 PM
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Mmmmmm.

I am going on memory now, and that is not good anymore.

I have never seen the USA Stromberg set up, BUT, I do remember when I was Parts Manager for Jaguar here, seeing a parts diagram of the set up. It appears as if there is a "secondary" inlet butterfly arrangement between the carbies and the actual inlet manifold. These butterflies would be controlled by a linkage arrangement in synchronisation with the carbie butterflies.

Soooo, if that linkage has broken/fell off/whatever, the engine could not rev, and vac readings would be all over the shop.

I have looked thru my books etc in the shed and cannot find a diagram of the USA Stromberg set up, but that extra butterfly manifold is real clear in my head.
 
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Old 05-10-2014, 02:07 PM
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Had business last couple of days, so couldn't fiddle with the coupe. This morning pulled all the vaccuum lines and plugged them all with vacuum "nipples". Started her up and VROOOMMMM!!!!. So now its easy. Replace the old vacuum lines. Puzzling because it was pulling 15 to 20 inches, although eratically. Must be a pinhole so small.....

Thanks to everyone for there help. Sometimes just talking through it jogs the brain.....

Steve
 
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