Flathead coupe issues

[Chas uk]

Hi guys really hoping you can help me out I have a 48 coupe which I recently purchased, when I bought it it had a standard single ford carb which was leaking badly around the throttle spindle. I replaced the manifold and carb with an offenhauser manifold and twin Stromberg 97s, the car drives very well but my issue is the idle is uneven it fluctuates from 390-450. The air cleaners are very restrictive ( if I remove them the car stalls and with the filters on I have to almost close the idle jets) I have new filters on order so hopefully that will help. The twin carb setup came of an identical engine and judging by the way the vehicle drives I would say are jetted OK. I'm in the UK so flathead knowledge is a little thin on the ground over here so any help great fully appreciated.
Charlie

[JSeery]

What engine is in it and what distributor? Have you checked for vacuum leaks?

[Chas uk]

Oops sorry I forgot to say its a 48 v8 flathead 239 cuin standard vacuum brake distributor, the distributor vacuum is hooked up and it does alter the idle when disconnected and plugged up. As far as I can tell I have no leaks

[JSeery]

Have you checked the engine with a vacuum gauge?

[Chas uk]

Many thanks for your reply. No as I thought an erratic idle wouldn't give me a very accurate reading, any idea what kind of vacuum reading I should expect ?

[Brian]

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[Chas uk]

I'll connect up the vacuum gauge and see what it reveals

[Drbrown]

A vac gauge can reveal a lot about an engine. My stock 59 AB V8, fully warmed-up, with one stock carb reads at about a steady 19.0 to 19.2 on the vac gauge. The reading is going to vary pending condition of engine .... average good condition should show 19 to 21. I do not know effect of having multiple carbs.

Go here for an article about reading vac gauges ...http://www.secondchancegarage.com/public/186.cfm

and forum discussion:
www.fordbarn.com/forum/showthread.php?t=149754

[Chas uk]

Cheers for the information I'm of to hook up a vacuum gauge in an hour or so I'll post my results later

[Chas uk]

OK well hooked up the vacuum gauge and the reading was a needle fluctuating between 14- 20" of vacuum fairly quickly, the idle is still off it sounds like there is a stunble. I drove the car and found it backfired under acceleration, low power and hesitation, as you can tell I'm pretty annoyed and frustrated

[Byron Warwick]

Doesn't backfire equal bad timing?
Byron

[rotorwrench]

Restrictive air filters are a lot like pulling the choke on a normal single carb application. When applying choke makes it run better, it indicates that the fuel air mixture is lean. Less air in the A/F mix makes a lean mixture more rich. Most of the little filters sometimes utilized for multi carb applications are really not designed for the kind of air flow a multi cylinder engine needs. A lot of them were designed for air compressors or small single cylinder engines. I would see if you can get it to run well without the filters first so you can check performance. A good filter will have a negligible affect to the fuel air mixture. You should see no change when you install them.

[ookpik]

I see everybody is guessing, do here is mine. The engine is too lean, vacumn leak or too small a jet.Surge is caused by the power valve opening when the vacumn falls off and closes when the vac comes up. Is your engine bigger than the other mentioned or just in better condition?

[Chas uk]

Hi guys thanks for your help and suggestion's, if I remove the restrictive air filters I can get the car to idle fairly smoothly albeit still with a fluctuating vacuum reading. However the car is undrivable without the air filters popping and banging through the exhaust as soon as the throttle is touched. I'm going to refit the original manifold and try another carb which I know is good at least that way I should be able to discover whether or not the engine has a problem before tearing the carbs apart. Again many thanks and I'll report my findings later

[rotorwrench]

Afterfire or popping in the exhaust is a sign of an over rich mixture or a valve problem. Valve problems could be a sticking valve or cam shaft timing is off. With twin carbs the acceleration & power circuits (accelerator pump & power valve) may be leaky or need adjustment to reduce the fuel output.

[Chas uk]

Original manifold refitted with a new gasket and a new standard carb ordered, due to not being able to run the car at the moment I performed a cold compression test 6 cylinders showed 105 psi 1 cylinder showed 115 psi and 1 showed 120 psi. Hopefully this is ok, the new carb should be here tomorrow so if all goes well I will have it running tomorrow and perform a hot compression test

[rotorwrench]

Those readings aren't bad at all.

[danliveshere]

Backfire can also mean poorly adjusted points. Also possible plug leads in the wrong firing order Have you checked the distributor, vac advance, dist cap and points etc? Have you got a manual to go through these items?

[Chas uk]

Hi the car isn't running points its running pertronix ignition I've been through the plug leads and checked them all for a spark, the vacuum adjustment was loose so not too sure where to set that.

[danliveshere]

Ok is the black box malfunctioning? I would put a timing light on and have a look what's going on. What you're looking for is a steady timing light that advances steadily and not bouncing around or advancing too far. I've just gone through this with my 351 in my ski boat but in my case was idling perfectly but couldn't rev past 1000 rpm when the timing advanced so far it would backfire and snuff out due to advancing too far.
Yours maybe ok but sounds like you need to set your vac advance anyway.