Throbbing engine causes?

My 68 4.2l engine has been rebuilt with new valves and rings plus timing chain. Compression very good on all cylinders

The two zenith stromberg carbs are clean and seem to be well adjusted

Petronix ignition with new wires and plugs. Timing at about 10 degrees btdc.

Engine starts quickly cold or hot

All seems great except engine throbs in a cycle like my pulse after running up strairs. And I can’t get it to idle under about 1000 rpm.

I suspect I have a vavuum.leak although I have sprayed starter fluid on the joints and did a half ass smoke test with no leak indications.

If you have read all this, I ask you engine pros for suggestions on the possible sources for my problem.

Could it be that I didn’t get the cams adjusted correctly? Where to focus next: carbs, ignition, cams, vacuum or otherwise.

Oh I should mention that the engine runs with a vacuum about 40 in Hg, just at the red/green interface on my gauge

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If you can’t drop the idle below 1000rpm and there’s no vacuum leak one possibilty with Strombergs is deteriorated by pass valves. They are supposed to open only on overrun but if the diaphragms inside are damaged it will allow unmetered air past the butterflies. This is a pretty common issue with Strombergs.

Also regarding the pulsation issue, check that the dampers are full of oil. Stromberg dampers have an O-ring at the bottom that can fail allowing the oil to leak out.

My by pass valves are plugged shut and oil in carbs ok. So hope someone can make another good suggestion. - or more

40 in. of Hg can’t be right. 29.92 in. of Hg is a perfect vacuum. Do you mean 40 kPa, or perhaps 40 cm of Hg? Is this measurement at your 1000rpm idle?


Do you mean it won’t run below 1k revs - as in it will die or that even with the butterflies completely closed by backing off the stop screws it will stay running at 1k?

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Do a piston lift test and report back what happens when you lift either by 1-2mm

Apologies for delayed replies to several:
For davidxk. Should have said 40 cm Hg or about 15 inches at 1000 rpm idle… Is this on low side?

For rustfreeMike. I unscrew idle adjustment screws until they off the tab below. Still around 1000 rpm and “throbbing”

For davidxj6. I did the lift test and engine neither speeds nor slow so I guess my mix is correct.

Does most of this information suggest an undetected vacuum leak, or something else?

I greatly appreciate your comments and questions.

Sounds a bit low to me, but I don’t have direct numbers from your setup to compare it with.

Do you still have the secondary throttles in place? Have you tried testing for vacuum leaks all along the secondary manifold, and cross-over to the exhaust heat exchanger? Other members have reported vacuum leaks though the gaskets on the cross-over.

If the butterflies are completely closed as you describe and the engine still runs at all - let alone at 1000rpm! -you definitely have unmetered air entering somewhere (a vacuum leak).

What if they’re not? And with a leak why does it pass the lift test? The engine did react to lifting further right?

The OP says that they are and he also said earlier that his bypass passages on the Strombergs are plugged which is one of the first things I’d think of in this case. Also how were they plugged? Depending how it was done that can also fail.

Anything is possible but IF the idle screws are off the stops and the linkage isn’t holding the butterflies open there has to be air entering somewhere. I think more info is needed.

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Some replies:

  • secondary throttles are removed and manifold openings plugged. There is no crossover

  • I have sprayed starter fluid all around manifold and joints and find no effect on engine speed.

  • completely unscrewed idle adjustment does not mean, in my humble opinion, that throttles are completely closed. Otherwise no fuel would get to cylinders even though most of the air could get there via manifold/joint leak.

  • re an earlier question, my carb diaphrams are new

Please remember there are two strange
Phenomona (sp?). Engine is running fast with virtually closed throttles and it throbs. Could an air leak cause both or is there another explanation?

Please advise what more informationI can provide.

No further theories in past day so I will ask another that is unrelated to the high rpm/possible air leak issue

What can cause what I call throbbing or uneven engine speed. Could it be that one plug or cylinder is not firing? Or something else? How do I find the cause?

Hi Phillip,

Also called engine hunting. There could be a vacuum leak in the brake servo system. Try removing the vacuum take-off from the inlet manifold and plug the manifold to see if that cures it.


In my experience of troubleshooting problems reported via Jag-Lovers, problems that linger without resolution or new suggestions usually turn out to be due to lack of key bits of information. This is particularly true when the car in question has been modified from stock. To this end, I would suggests that:

  1. Record and post a video of the “throbbing” noise that you observe. We all probably think we know what it sounds like, but could be wrong.

  2. SInce you say that you have removed the secondary throttles and plugged the holes, please submit photos of what you did.

  3. Since you say that you have removed the cross-over. please submit photos of what you did.

The latter two go back to comment made by @RustfreeMike. As he says, if the carb throttle plates are fully closed and the engine still runs, air is leaking in somewhere. Furthermore, that air stream must be passing over the fuel jet, or the engine still wouldn’t run. (Edited out a section I wrote which was nonsense)

These are just some ideas I had, but the bottom line is that the folks on the forum are currently working deaf and blind, so it’s not a huge surprise that progress is slow. Good luck.

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No response because no clue.

Your engine seems fine but it has a high idle and very regular miss. As if it was very rich (but the lift test) or missing a spark plug (compression is good). Nothing improves when you lift the pistons.

Do post a video and look at the plugs just after a short run.

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Another simple test. With the engine idling and the idle screws wound back so the carb butterflies are both closed and air filter plenum removed, simply place the palms of your hands over the carb trumpets to seal them. If the engine stops running you can be pretty sure the carb butterflies are not sealing, or the bypass valve is not closed and allowing mixture past the butterfly on that carb (I presume that you only have a bypass on the front carb). Repeat the test using the palm of your hand seal on carbs one at a time to narrow down whether one or both carbs are passing air.


When I block either carb with my hand, the engine dies. Why does this mean the butterflies are not sealing? I don’t understand so please help me

But assuming you are correct, could this cause my engine to throb?

Re by-pass valves, I have two. When I place my finger over the carb flange hole which leads to the by pass valves, nothing happens, confirming my belief that these valves are plugged closed.

You have told us that both throttle screws are backed off so that the carb butterflies are completely closed, and that the bypass valves are closed. If this were true, then cutting off the air flow into the carb mouth should have no effect as any air entering the carb mouth can only reach the engine via (a) the throttle plate, or (b) the bypass valve (which bypasses the throttle plate). But, your engine stops when you close off either carb mouth. This implies to me that there is enough fuel/air mixture getting past the “closed” throttle plates or “closed” bypass valve(s) to idle the engine at 1000rpm. The fact that the engine dies when EITHER carb mouth is blocked implies to me that the leakage is taking place on both carbs, but is not of sufficient volume to allow the engine to run on only 3 cylinders.

Maybe, but since you haven’t provided the requested video with the sound of the “throb” (or the photos I requested of the mods to the secondary throttles and secondary manifold crossover either), I really can’t say. We are still woking blind and deaf here.

That surprised me. I thought the S1.5s only had the bypass valve on the front carb. This is the bypass valve:

Note that on S1.5 cars (as far as I’m aware), it senses inlet manifold vacuum by the external brass pipe in the top LH side of the photo. If manifold vacuum exceeds a certain amount (as determined by the screw agjustment on the spring), the vale opens and air/fuel mixture is drawn from the mixing chamber inboard of the jet and through a passgeway that bypasses the closed throttle plate directly into the manifold opening:

Do you really have one of these on each carb?

There is no hole in the intake carb flange that leads to the bypass valve. There is, however, one that leads to the Temperature Compensator which is another device altogether - it is the one with the white plastic cover which is fitted on both carbs. Are you perhaps getting them confused? If so, your bypass valve(s) may not be closed. Do yours have an external screw adjustment as shown in my photo. If you turn the screw all the way in one direction it will close the valve - unfortunately, I can’t recall which direction, but maybe someone else reading this can?

Thanks for many good comments. My apologies that I haven’t sent video but sound part in not working on my phone. I will borrow wife’s and try again.

About internal baffles in secondary manifold, I am not sure what more I can say or photo. I simply took out the butterflies and shaft and plugged holes where shafts were. I do not think the plugs are leaking…

In.interim until you can see video with sound, here is more background info.
-my car had no carbs when I bought it so I purchased a two zs carbs with manifolds off ebay. Not sure they ate 68 or not but:

  • both have by pass valves which I have plugged internally.
  • neither has the brass pipe shown in your picture. Air gets to by pass valve from hole in mixing chamber. Obviously I was wrong about air from.hole in flange.
  • the cross over did not exist and was plugged in standard way.

For sure the main butterflies leak as engine gets fuel with idle adjustment fully unscrewed. Question: how do I fix leaking butterflies?

Evidence ( high rpm and low manifold vacuum)/suggests I have a manifold leak but I am unable to.find it. Have blocked hose to brake system and this changes nothing. Extensive spraying od starter fluid around all joints does not have an effect on engine. I did an ameteur smoke test with no smoke coming out where it shouldn’t. Where else could leak be?

Is there anymore info I can provide when I an able to send good video?