Engine hesitation when increasing speed from idle

I have a carburation problem on my 1970 Series 2 OTS which I am hoping that someone may have ideas on. I needed to overhaul the Stromberg Carbs because the needle valves in the float chambers were sticking closed on occasion and the engine would stop from lack of fuel.
I removed the carbs and overhauled them fitting new metering needles and the Joe Curto adjustable main jets. I have overhauled several Stromberg’s in the past so I am not a novice at this. I set up the temperature compensators but decided to inhibit their operation by blanking off the air feed into them. I also overhauled and set up the bypass valves but again decided to render them ineffective by blocking off their exit passage (the one that bypasses the primary throttle butterfly). Since emissions testing is not required on pre-1974 cars in California I figured that this would make the carbs easier to set up vis a vis the mixture strength. I also decided to remove the secondary throttle butterfly valves and their operating mechanisms and just blank of the passages that the throttle shafts went through.
Now to the problem. When the engine is fully warmed up and idling at about 850 rpm if I press the accelerator to increase the engine speed (say when setting off from stop lights) the engine virtually dies and it sounds like it is not getting enough fuel. If I press the accelerator very slowly the engine will speed up without a problem but this takes a few seconds to do which delays getting away from stop lights. If I use the choke to speed up the idling speed to say 1200 rpm them this hesitation does not occur. The engine runs fine when the car is moving and there is no hesitation or flat spots as the speed is increased or decreased. Hence the problem is just when the engine is asked to accelerate from idle prior to letting the clutch out.
The carb piston dampers are full to the correct level with 20-50 engine oil so I don’t think that pistons are rising too fast when the primary throttle butterfly is opened. The mixture strength at idle seems OK based on listening to the exhaust note. I set the float heights in the float chamber to 16.5mm as per the ROM and the floats are new so I cannot see the fuel level being the issue.
The question is what is the problem? I am thinking that the removal of the secondary throttles may be causing this problem. For small primary throttle openings, the secondary throttles remain closed so maybe the suction at the main jet position is less than when there are no secondary throttles. Maybe removal of the secondary throttle requires changing the size of the main needles. I cannot think of any other issues that would cause this problem. If anyone can suggest what might be the cause I will be grateful. Has anyone who has removed the secondary throttles experienced anything like this?

not doubting you ability to work on carbs, i have comprehensive tuning and service info on my library (free) worth a read, there maybe something you’ve done or haven’t done.

Thanks I will read your stuff. Nobody is immune to screwing up no matter how many times you have done the job, particular as you cross the 80 years old mark as I have done

Stuart A Greenwood

not far behind you

I removed the secondaries also , seemed a lot better throttle response , but …one or two slight problems - Slight misfire starting from cold , and a tendency to stall setting off from rest .
I took a close look at all the places air might be leaking in ( secondary throttle shaft sealing , bypass valves , temp compensators , and the blanking plate for the cross-over pipe ) and tightened them / applied sealer .
Now it runs ok , so must have been air getting in somewhere .
Maybe it would be an idea to check the temp compensators - there is a tiny sealing ring in there which can easily get lost - I sealed up the entrance ports and silicone sealed around the covers just to make sure .


Do you still have the original 41322 distributor fitted with the vacuum retard module piped to the front carb?


This surprises me. California…?.

Hi Stuart,

You say the piston damper levels are correct which is good but are the dampers actually working. If you unscrew each damper and pull them up and inch and then push them down, do you feel resistance caused by the oil. (I have never run across a failed piston damper but thought to ask regardless!)

I would suggest you pull the spark plugs and look at their colour to see if there is a difference between the front three and the rear three cylinders. Do post photo if you do that!

Also an idea is as follows. Drop just the front carb adjustable jet a turn. And see if problem goes away. Then do the same on the rear carb. (I have SUs and do not know the effect of one turn, others reading this will know how much turn will give a decent enrichening)

I don’t think you have an ignition problem as the problem does go away if you use the choke.

I think your distributor has vacuum retard which retards spark to cause lower emissions. It’s been a while but I think some owners do make that inoperative. But I can’t this contributing to your problem

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Thanks to all who replied.
davidxk. I don’t know the model number of the distributor.Looks like you have to remove it so see the marking. The distributor does have the vaccum retard but it is disconnected and the feed to it from under the front carb is plugged off. But I will check that this plug hasn’t fallen off.
Dennismo. You can feel resistance on the dampers when you push them in. I had also wondered about richening up the carbs as you suggested. I’ve richened up both carbs by a quarter of a turn on the Curto jets and will take the car out and see what happens. After this run I will pull the spark plugs.
Rockingjag69. In California all pre 1974 model year cars are exempted from emission testing. But there are some lawmakers who want to abolish this. Considering that our old cars don’t do many miles per year they don’t contribute much emissions
Persimon. Seems like you had the same problem as I have. I will spray a bit of carb cleaner on each of the air joints one at a time and if the engine speeds this would indicate an air leak.
If you put your hand about a foot away from the exhaust tail pipe when the engine is warmed up and idling at 850 rpm you can feel an occassional “puff”, say every 3 to 5 seconds, which suggests that the mixture is a little weak. It is principally from the front 3 cylinders. When I richened the carbs up a quarter of a turn this made no difference to the “puffing” so maybe I have to be a bit more aggressive on the adjustable jets. I’m a bit loathed to richen the carbs since the gas consumption is pretty appalling considering I never agressively accelerate, 15 miles per US gallon at best , can go as low as 12mpg.

15 mpg sounds a bit low - is that on a long run , are the plugs black ?


How did you adjust the adjustable mixture needles. Have you tried to adjust each of them up until you get highest idle speed?

I once found a big variation in the manufacture of SU needles. The profile was correct, but the ridge (that one installs flush with the piston) was actually machined too high.

This meant that the initial setting of the jets 2 1/2 turns down for the SU was way off and the car would hardly idle. It turned out the jet had to be turned another 3/4 plus down which made a world of difference.


I set the main jets three turns done from flush with the bridge as a starting point. Then start the engine and warm it up. Set the idle at about 850 rpm and balance the air flow. Listen to the exhaust note to see if it is reasonably even from the front and back carbs. Lift the pistons on each carb in turn by about 1/8 ins and see if the engine falters or speeds up. Generally I finish up with four turns down on each carb. Go for a 40 mile run and look at the plugs . If they are a light brown to off white I consider this OK.
The above sounds as though the procedure is quite predictable but that is not really the case. Turning the main jets up or down by half a turn doesn’t make that much difference and lifting the pistons doens’t always respond the way it should.
The exhaust note is never absolutely even. There is always evidence of a regular miss fire ( I don’t know what to really call it. You can best detect it by holding your hand about 6 ins from the exhaust and it occurs say every 3 seconds). This is undsteadiness is on the front carb, the rear carb is pretty even. Richening up the front carb ,say in quarter turn increments on the main jets doesn’t cure the uneveness and I am loathed to keep richening up the carb because, what I consider to be, the appaling gas consumtion

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You are implying that you get better than 15 mpg, if so that’s bloody marvelous. My test run is about 40 miles and the plugs are never black just a light shade brown going to white on the insulators. My Toyota Tacoma which has a V6 4.2 litre engine and modern fuel injection gets about 18 to 20 mpg so I surmise that a 4.2 Jag with carbs cannot be capable of bettering the Tacoma.

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Fuel consumption will vary dependent on lots of factors , such as axle ratio and driving style and conditions . Going by the plug colour , it looks like the mixture is ok . The slight missing at the exhaust pipe does sound like an air leak though - that`s how mine sounded before I sealed all the potential leak points .

Assuming your ignition system is solid… and that situation is exactly where a good spark is needed…

That sounds like its not getting enough fuel (lean) or too much fuel (overly rich) or not enough air (which would also be overly rich) or some combination. This is the acceleration enrichment situation.

Not enough fuel: Could come from dampers rising too slowly. 20W50 seems too thick to me. Are the damper springs correct, not too stiff?

Too much fuel: Dampers rising too fast? Are the damper springs correct, not too floppy? Getting too much fuel should cause a brief expulsion of black smoke out the exhaust.

Not enough air could also result from the dampers not rising fast enough, when they would effectively choke the airflow.

Something I would try, just to have an idea of what is going on, is to set the idle mixture as rich as you dare. If the situation improves, it kinda suggest that she is indeed going lean on tip in.

If not, I recon it points to overfuelling or not enough air. Then you are “killing the fire with fuel”. Then I would set the idle mixture as lean as possible but still have it running, again just to see what happens.

This should at least tell you something…?

I can only assume that the springs are original. They are certainly not too stiff. I fitted a new ignition coil, condenser and points sometime ago so I think that the ignition is OK. The timing is set at 4 degs before TDC when at 1000 rpm which is a lttle retarded but this was recommended to me because the vacuum retard is disconnected. I think your suggestion of setting the carbs deliberately rich and then lean and seeing what happens is a good idea. At the moment I don’t have time to work on the car because of other things but I’ll get around to it as soon as I ca

Interesting you say that. The front carb is the one with the slight missing so , as I said before, I 'll test for air leakage particularly where I blanked off the secondary throttle bushing

Quote " the timing is set at 4 * BTDC " . I wonder if this is part of the problem ? . I`m not qualified to give advice , but mine seems to run better now its set to 10 * BTDC . From what I could observe , the ignition retarded setting made the engine run hotter and it seemed a bit sluggish . I plotted the ignition advance curve for the two settings , and 10 * looked a lot better .Mine had no capsule fitted , but now has ignition advance at light throttle / cruising speed , which gives better fuel economy .

Vacuum retard is the worst thing ever, I have ported vacuum advance with >10° static timing.

They always say that most carburetor issues are igition!

Personally I decided that I‘m not (yet) capable of setting the strombergs perfectly.

Here is an update to the hesitation issue and I might have found a reason.
I richened the carbs up by 1/4 of a turn as I said previously and the idle sounded really good. I ran the car for 18 miles and never had hestitation issues at stop lights ( but I’ve had this before). Back in the garage I let the engine idle for a few minutes and then opened the throttle quickly and it hesitated just like before. When idling in the garage the coolant gauge show a little hotter ( just in the normal range whereas on the run it is slightly below). I adjusted the main jets richer and it made no difference., It still hesitated upon throttle opening quckly but OK it opened slowly. I let the engine idle some more and low and behold the engine stopped, clearly because of lack of fuel. I tried this a few times with the same result.
So here’s my theory. The needle valves in the carbs are sticking closed or nearly closed. If you let it idle for only a short time and open the throttle it just hesitates but if you leave it idling it dies. How can an opening of the throttle unstick and stuck needle valve? I don’t think that it is a fuel pump problem since the fuel filter remains full of fuel and the pump is a new one ( not the SU type but the one from New Zealand which I have used on other cars with no problems).
The needle valves are new from Moss. I went on the SNG Barratt web site and noticed that they offered the same valve as Moss at basically the same price but they also offered one that look really original ( gauze filter upstream of the valve seat and the needle itself looked like it was plastic or nylon where it contacts the valve , the rest being metal). This was twice the cost of the cheaper valve. So I have ordered this, hopefully, better valves. I do remember when refurbishing Strombergs in the past you can get the valves to stick closed when you are messing about setting the float levels. I can’t believe that it’s an incorrect float level or a bad float since I used new Nitrophyl floats and I’m fastidious about setting the height.
Of course I maybe completely wrong since I’ve never had the engine stop before while idling and I’m pretty sure that in the past I must have left it idling for quite some time. I’ll take the carbs off replace the needle valves and generally check the carbs over.
Note that I have used Gross jets on the past but I’m not a fan of them