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**stagmuffin** · 20 March 2009, 12:26

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hi peter what year is your car and what system of inlet have you opted for .ie manifold adaptor or modded inlet manifold if adaptor is it angled or not lastly what were the readings you got on emissions..............steve............

**V Mad** · 20 March 2009, 12:41

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Hi Peter
Is your Holley carb new, and was it set up for a Stag already?

Flat spots are often caused by weak mixture, due to insufficient accelerator pump delivery. Try adjusting the pump cam to increase delivery when opening the throttle.

Also make sure your secondaries are not opening too soon causing a flat spot (try jamming them shut).

To check vacuum advance you should use a strobe lamp. This will show quite clearly the change in timing when you disconnect the vacuum advance.

**engineman** · 20 March 2009, 15:15

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Steve, The car is a 1975 model, the carb has an adaptor plate with is at at 7 degrees. unfortunately I did not keep the test data.

Peter

**engineman** · 20 March 2009, 15:24

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The 4150 carb is only 1-year old and was not specifically set up for a stag, I have coverted the primary and secondaryjets to 0.47.I have adjusted the pump cam so that as soon as the throttle is pushed it pumps out fuel in an even spray, how should I use the strobe lamp to check the vacuum advance, I will try jamming the secondaries this weekend.

Regards

Peter

**V Mad** · 20 March 2009, 18:20

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There are two adjustments on the accelerator pump. One sets the stroke of the pump by moving a screw into one of three positions. Try setting this to the maximum stroke position. If thats not enough fit a larger accelerator jet.

If you put the strobe on, then when you disconnect the vacuum pipe (block the end off as well) you should see the ignition retard by a few degrees.

The MOT test only really checks idle mixture I think (depends on which year/emission level) so it is not an indication of correct jetting. The main jet mainly affects the full speed mixture, so will not have much effect on low/medium speed running.

**Stagdad** · 21 March 2009, 04:17

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V Mad wrote:

Flat spots are often caused by weak mixture, due to insufficient accelerator pump delivery. Try adjusting the pump cam to increase delivery when opening the throttle.

You can also get diffrent size squirter nozzles to tune the accelerator pump.

Steve

**V Mad** · 21 March 2009, 05:10

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Yes, 'squirter nozzle' thats what I meant by larger accelerator jet Steve. Not sure which is correct terminology, but yours is more descriptive!!

**Stagdad** · 21 March 2009, 15:04

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V Mad wrote:

Yes, 'squirter nozzle' thats what I meant by larger accelerator jet Steve. Not sure which is correct terminology, but yours is more descriptive!!

Sorry Chris ,missed that bit of your post ,Squirter nozzle is what the guys on the Holley video call them,must be a yank term

You can also get higher cams and larger capacity pump bowls .

Cheers Steve

**engineman** · 22 March 2009, 15:54

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V Mad,

Myaccelerator pump cam is set to the number-2 position on the throttle spindle, do you know if moving it to the number-1 position increases/decreases the quantity of fuel? it looks to me to increase it,the rotationaldifference between the two positions is very small probably not more than 10 degrees. I presume that after changing the cam position the adjustment will need to be made on the accelerator pump arm.

Peter

**engineman** · 30 March 2009, 10:39

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Gentlemen,

I have now performed one at a time the following aqdditionalactions to address my flat spot without success,

- Jammed shut the secondary side of the carburetor.

- Increased the size of the primary squirter jetfrom 0.025 to 0.028.

- Fitted every acceleratorcam in the holley kit in the1 & 2 positions.

- Purchased a new electronic distributor from EJ Ward.

- Ran the car without a filter.

The installation of the different accelerator cams made the characteristics of theflat spotchange veryslightly. I had a long conversation todaywith Mick atEJ Ward who is suffering with a similar problem to me in his Triumph 2.5 with a Stag engine fitted, he has tried similar actions to myself without success, he is now thinking that the short stroke engine may be the contributing factor, the Rover 3.5 engine has a longer stroke and may havegreater inlet volume velocity?? He recently worked on a stag fittedwith the rover engine and it did not suffer with this problem, thought provoking!!

What size jets are you Holley folks using assuming that you are notsuffering with Flaty spots.

Look forward to hearing from you.

Peter

**V Mad** · 30 March 2009, 15:59

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Peter
You have clearly tried a lot of the things that should improve the response, but it seems you still have the problem. I wonder if there is a basic fault with the carb? I can only hazard a guess that the accelerator pump itself may be faulty (diaphragm, gasket leak etc), or some other fault like a blown power valve (was common on older Holleys until they fitted protection agaist this).

Unfortunately your story is only too common; many guys in the USA have had the same experience and have ditched the Holley in favour of a Weber, and have never looked back since.

One final sugestion is use a rolling road and an exhaust probe to detect what is wrong. This allows you to monitor the mixture under load.

Holleys are rarely give trouble once they are set up OK but are a real pig to fault find and/or tune.

I can only wish you luck.

**Kevin Garrod** · 30 March 2009, 16:10

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V Mad wrote:

many guys in the USA have had the same experience and have ditched the Holley in favour of a Weber, and have never looked back since.

And one or two over here.....

**Mpower738** · 2 April 2009, 16:54

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engineman wrote:

Gentlemen,

I have now performed one at a time the following aqdditionalactions to address my flat spot without success,

- Jammed shut the secondary side of the carburetor.

- Increased the size of the primary squirter jetfrom 0.025 to 0.028.

- Fitted every acceleratorcam in the holley kit in the1 & 2 positions.

- Purchased a new electronic distributor from EJ Ward.

- Ran the car without a filter.

The installation of the different accelerator cams made the characteristics of theflat spotchange veryslightly. I had a long conversation todaywith Mick atEJ Ward who is suffering with a similar problem to me in his Triumph 2.5 with a Stag engine fitted, he has tried similar actions to myself without success, he is now thinking that the short stroke engine may be the contributing factor, the Rover 3.5 engine has a longer stroke and may havegreater inlet volume velocity?? He recently worked on a stag fittedwith the rover engine and it did not suffer with this problem, thought provoking!!

What size jets are you Holley folks using assuming that you are notsuffering with Flaty spots.

Look forward to hearing from you.

Peter

I've had exactly the same problem ever since I fitted the Holley, and have tried various cams and jets in the accelerator pump circuit.

I also had the vacuum advance pipe on the wrong connection point on the carb, though when I corrected it there was no difference.

Another theory was that the SU pump cannot supply correct fuel pressure for the Holley, which has a spec of something like 5-7 psi. The supposed solution was an American fuel pump giving 6 psi, which I haven't got round to fitting yet. I think the SU only gives about 2.

I'm using the jets recommended by Monarch, the jets that were in the carb when I got it were larger, and larger than standard I think. It was originally supplied by HRS, I'll have to dig the old jets out to get numbers off them.

I was chatting to the guy who brought the stag engined TR5 to national day, and he said he overcame it by drilling small holes in the butterflies to let more air through at idle, and then opening up the idle jet screwsto allow more fuel aswell. I don't remember how he didn't end up with the thing idling at 1500 rpm!! He went to EFI in the end so he obviously got fed up with it.

I think his theory was that there is just not enough fuel below the jets when the butterflies open to maintain the mixture, and the accelerator pump is not quick enough to compensate.

I'm not sure where to go with it, certainly not back to Strombergs.

I'd certainly be interested to know if you manage to solve it.

Cheers

Doug.

**Stagdad** · 2 April 2009, 17:19

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Mpower738

I'm using the jets recommended by Monarch, the jets that were in the carb when I got it were larger, and larger than standard I think. It was originally supplied by HRS, I'll have to dig the old jets out to get numbers off them.

I'd certainly be interested to know if you manage to solve it.

Cheers

Doug.

That could be your problem Doug,Ian at Monarchfit's 48's but the holley is supplied with 52's ,i tried 48's in mine and it was horrible ,massive flat spot ,i put the 52's back in and the flat spot has all but gone ,i may tryeven larger jets and see how it is.I found the idle mix to make a big difference as well.

I got a good smooth power curve at the power challenge with the 52's

Cheers Steve

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