Low cranking vacuum

I have rebuilt the engine (new valves and rings) in my 1968 Series 1.5 E Type. I have never seen the engine run before and the car I bought had a lot of engine parts missing (carbs, distributor, alternator etc.).

It had good cylinder pressure when I tested it. All.The parts I added are either new or in very good condition. But I.have been unable to get it started or even fire.

I think the problem is very low or no vacuum when engine is cranking. By contrast, the green Jag manual says cranking vacuum should be 16-18 inches mercury.

My list of possible causes for the lo/no cranking vacuum are:

  • Vacuum leaks but I can’t find any
  • Hole in Stromberg carb diaphragm but again I can’t find any
  • Incorrect crank, can and valve synchronization but I was very careful when I assembled the engine and good compression tests suggest this is not the problem

So I have 2 or 3 questions:

  • Is it correct that cranking vacuum should be similar to when engine is running?
  • Which item in my list of possible faults in list above is likely to be my problem?
  • What are other possible causes that I have not listed

And thanks for your patience if you have managed to get to the end of my long story.

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If you have good compression, I would not suspect a mechanical problem. Have you tried spraying in a shot of staring fluid. If it fires on that you can discount a ignition/ timing problem. If nothing happens with the starting fluid, I would assume an ignition problem. Check for spark at your plugs. Good spark? You have a timing problem. No spark at plugs, check for spark at coil. Good spark at coil? Check your distributer and rotor. No spark at coil? Check for 12v at the low voltage side of coil. No 12v at coil? You might have an ignition switch problem. Try a direct wire from the battery. Have good 12v to the coil? You have a points problem or a bad coil.

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I would start at the basics, are the plug wires going to the correct spark plugs and do you have a good strong spark?

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Thanks for the advice but can others focus on the specific questions I ask about a sense of manifold vacuum.

I have already checked other issue such as spark and all is ok.

To add to the evidence of low manifold vacuum, I have observed that the piston do not rise off the base with my Stromberg carbs when cranking the engine. If there was a good vacuum, the piston would be higher.

Check the Exhuast pipe , make sure it’s not sucking :roll_eyes:


I must admit I have never checked vacuum while cranking an engine, I wouldn’t imagine there would be much as the engine revs are very low. When the manual says it should be 16-18 inches at what revs does it say that should be measured?



Also does that engine have a pcv valve going to the manifold, or any other devices such as brake booster, etc. Block every hose going to the manifold as well.


After a bit of googling I found that cranking vacuum could be as low as 3" and even lower if you are not at sea level.


I suppose the cams are on the correct sides? My brother had a head reassembled years ago and that happened. It didn’t run either.

I agree with Ian. It must be sucking somewhere. find out where.

If you find suction on the intake then try a little carb cleaner spray. (Better for your engine than ether starting fluid.)

Thanks for thoughtful comments. My replies:

Ian - what do you mean by exhaust pipe sucking?

Matt- page 750 in the big green manual is explicit that “cranking vacuum” should be “16-18 inches Hg”. Google may be right about 3" but I find jag manual is trustworthy .
There has to be a good vacuum or the ZS carb won’t work and air and fuel won’t flow into the cylinders.

Robin- Frightening thought but my good compression tests suggest this is not the case. But I will double check.

May I ask anyone else who wants to comment to please assume that my cranking vacuum is lower than it should be and then speculate on what is causing this. Thanks

If the Engine has proper compression, and you don’t have proper vacuum at the carburetors there can only be one reason: a massive vacuum leak somewhere.

Look for a crooked manifold, or a missing pipe on the intake such as the takeoff for the vacuum brake assist.

Do you have the large crossover plenum intake system that goes over and back to the exhaust side? Replaced all the gaskets in that and carefully tightened the flanges?

Years ago a friend replaced a starter motor , and a few other parts , car would not start ,
Took him and a m8 2 days to find the fault , the starter motor was the wrong polarity , was turning the engine the wrong way :scream: :rofl:
Was not a jag mind !

Throttle plates open instead of closed?

I too am having trouble overlooking my prejudice that the vacuum target range quoted looks like “idle” rather than “cranking”. A search of the broader interweb provides some support for this, but I’m having trouble overlooking that any view can get support there…

Several days have passed and I have made more tests including pressure testing to check for leak s - and found none.

I now accept that vacuum of around 3 inches is ok and jag manual is misleading But still engine won’t start. I have double checked ignition and it seems ok. Injecting starter fluid in the carbs gives strange reaction but no cylinders firing. Instead, there seems to be back firing on the exhaust and some back flow of air from the carbs. Again cylinder pressure seems good.

Given all this, what else should I check, especially re the Stromberg carbs?

Your timing is off, and/or the plugs miswired.

When you state “compression pressure is good,” what is “good?”

I did a test on my 3.4 XK150 to see if the brake servo was leaking. Vacuum at idle was 13. I plugged the servo line and it remained unchanged. If you are a 4 you have a leak.

Pat, the problem is low CRANKING vacuum.

I suspect Wiggles is right - I good-looking and will check ignition timing

I concur.
I had vacuum gauges in many of my cars, including my V12, never saw more than 3 - 5 inches vacuum while cranking.
18 - 20 inches is on idle, on a perfectly tuned and fully warmed engine, with A/C off and no electric loads.

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