4a. Won’t start after full rebuild. Any ideas what to check next pse?

[Steven Whitaker]

1 hour ago, Michel Higuet said:

Adjusting dizzy is a detail see first the important one, how much are the compressions in the 4 cylinder? how much is the reading of the vacuum gauge in the manifold?

Those cost now peanuts and delivered with amazon... 

Would you mind suggesting one I can order from Amazon please?  TIA.

[Steven Whitaker]

8 hours ago, Bfg said:

Another thought, which will make starting difficult is if the main jet is adjusted too high.  I say this because the Brookland's book I have on SU carburettors says "Screw the jet adjusting nut until the jet is flush with the bridge of the carburettor, or fully up if this cannot be obtained."   ..well that is not how mine is . . .

^ This is one of my carbs, and as you can see the top of the main jet is a good 1mm or more below the level of the venturi's bridge.

Admittedly I'm just in the process of re-tuning my carburettors, having previously lost a bottom manifold clamp and so an air leak passed the gasket.  Although on Sunday (70 mile round trip) she was running just a little rich, this jet position is not very far out for the needles fitted and the air filters I'm using.  This is of course with the shoulder of the needle being flush with the bottom face of the piston.

Hope that helps

Pete

Isn't this negated when using the choke?

[Bfg]

Just now, Steven Whitaker said:

Isn't this negated when using the choke?

Not entirely. The choke mechanism does pull the jet housing down, but it is a relative setting, so choke applied to a very lean needle position will bring the fuel-air mix correct for when hot. But the mix needs to be richer still for starting from cold, especially on a tight engine.

[Motorsport Mickey]

9 minutes ago, Steven Whitaker said:

Ok, thanks.  So far as I can see, the inlet mani is sat down, but will look further.  Cheers.

STOP !

You need to use the advice given to you rather than interpret it in your mind.

Banging 5cc of petrol into the chamber bypasses EVERY other problem you have...or may have, it's easy, no petrol or no spark equals no go. Even if your timing is adrift I'd expect a direct feed of petrol into the chamber to remain there long enough with the engine turning over to give at least a 1 cylinder firing solution that works. If it does then the manifold seating is suspect.

Mick Richards 

[Motorsport Mickey]

1 hour ago, Michel Higuet said:

He never says it fired a few times... 

As you posted yourself he said...to precise here your " I tried to start it and no difference, but it did try a little. " doesn't that mean it nearly started ? even if just 1 cylinder part combustion ?

Mick Richards

[Steven Whitaker]

Just checking valve timing.  It seems ok?  All at tdc showing valve positions.  The pic with both valves closed with clearance under tappets is at tdc and rotor arm pointing to cylinder 1 on comp stroke which I think is correct..  Other one at 180 out shows valves in different positions.  So can I rule out the valve timing being wrong?

Edited November 22, 2023 by Steven Whitaker

[Steven Whitaker]

29 minutes ago, Motorsport Mickey said:

STOP !

You need to use the advice given to you rather than interpret it in your mind.

Banging 5cc of petrol into the chamber bypasses EVERY other problem you have...or may have, it's easy, no petrol or no spark equals no go. Even if your timing is adrift I'd expect a direct feed of petrol into the chamber to remain there long enough with the engine turning over to give at least a 1 cylinder firing solution that works. If it does then the manifold seating is suspect.

Mick Richards 

Thanks Mick.  I’m listening, but what does this actually prove?  Be gentle mate, I’m learning.  Tnx.

[keith1948]

1 hour ago, Steven Whitaker said:

I corrected this last night.  Thanks.

Hi Steven for info

Just checked spacers on my car and in your photo the one on the right (rear one) is correct with flat face at top. The carb on the left of the photo (front one) is wrong. The concave faces should be on the sides. At least that is how mine are fixed.

Keith

[Motorsport Mickey]

My suspicion (and I think a few other members) is that is a massive inlet manifold leak. I've had a couple of engines where the manifold has "hung up" enough to give very bad running characteristics, which is even worse than where you appear to be...at least your engine won't run at all !

Commonly what happens is there is a bad leak on one of the Siamesed ports serving 2 cylinders, that gives an abnormally weak mixture over the 2 inlet stacks attached which unknowingly the owner seeks to cure by richening the carb serving the weak cylinders. At a certain range this crudley allows the engine to run and drive, however because it is varied by the suction within the manifold the weakness varies and so the engine will be altered by the owner to run OK at maybe 2500-3000 revs, ie cruising by richening the carb affected. However when the car runs at 1500 revs or stands at tickover the carb which is overrichened VASTLY overfuels the cylinders fueled from that carb, which fools the owner who then starts to weaken off that carb which then won't run at higher revs or even start nicely from standstill. !

Sticking the fuel directly into the engine bypasses this, and hopefully proves the point that fuel plus spark and compression will at least give a bang ! If it does I would then definitely check for an air leak on the manifold. Mask the carb fronts around the air cleaners so it can't draw air in from the back and spray the Start Pilot or even brake cleaner around the inlet manifold to head fitments, if there is a leak there the engine will pull it in and Bang ! proving the point. Then you can refit the inlets confident you are remaking a bad area.

Mick Richards 

 

[Steven Whitaker]

22 minutes ago, Motorsport Mickey said:

My suspicion (and I think a few other members) is that is a massive inlet manifold leak. I've had a couple of engines where the manifold has "hung up" enough to give very bad running characteristics, which is even worse than where you appear to be...at least your engine won't run at all !

Commonly what happens is there is a bad leak on one of the Siamesed ports serving 2 cylinders, that gives an abnormally weak mixture over the 2 inlet stacks attached which unknowingly the owner seeks to cure by richening the carb serving the weak cylinders. At a certain range this crudley allows the engine to run and drive, however because it is varied by the suction within the manifold the weakness varies and so the engine will be altered by the owner to run OK at maybe 2500-3000 revs, ie cruising by richening the carb affected. However when the car runs at 1500 revs or stands at tickover the carb which is overrichened VASTLY overfuels the cylinders fueled from that carb, which fools the owner who then starts to weaken off that carb which then won't run at higher revs or even start nicely from standstill. !

Sticking the fuel directly into the engine bypasses this, and hopefully proves the point that fuel plus spark and compression will at least give a bang ! If it does I would then definitely check for an air leak on the manifold. Mask the carb fronts around the air cleaners so it can't draw air in from the back and spray the Start Pilot or even brake cleaner around the inlet manifold to head fitments, if there is a leak there the engine will pull it in and Bang ! proving the point. Then you can refit the inlets confident you are remaking a bad area.

Mick Richards 

 

Thanks Mike.  I’ve just removed the inlet manifold and found that the mating face is not flat as per the photos.  Would this be enough for it not to run.  I’m thinking that the draw / pull on the vacuum stroke will pull air from least path of resistance, which could be the gaps as shown?  I shone a torch from behind and can see a millimetre gap.  Tnx.

 

 

[Motorsport Mickey]

If that isn't enough of a gap for a massive air leak it will do until a HUGHER one comes along. Deserves to be corrected before you try again and see what happens, always good when you find something wrong.

Mick Richards

[keith1948]

Coming back to an earlier observation I made, the vacuum advance isn't connected so there is an open tube at the carbs which will allow air in so suggest again that you connect up the vacuum advance tube or block up the tube on the carb where it connects. Whether that is enough of a leak of air remains to be seen. 

Bit of a mystery this one.

Keith

[twostrokekid]

I can't see where you've connected the choke cable. There doesn't appear to be a choke cable fitted in the usual position, the brass insert on the rear carburettor. The jets should pull downwards about a quarter of an inch on full choke and the idling throttle stop is slightly nudged open. You may have improvised a choke connection but I can't see it.

[roy53]

Why are you not trying the direct fuel into the carbs as suggested ????

[Steven Whitaker]

20 minutes ago, keith1948 said:

Coming back to an earlier observation I made, the vacuum advance isn't connected so there is an open tube at the carbs which will allow air in so suggest again that you connect up the vacuum advance tube or block up the tube on the carb where it connects. Whether that is enough of a leak of air remains to be seen. 

Bit of a mystery this one.

Keith

Thanks Keith.  Will try that thanks.

[Steven Whitaker]

1 minute ago, roy53 said:

Why are you not trying the direct fuel into the carbs as suggested ????

Just done that.  Absolutely nothing!

[Steven Whitaker]

Inlet manifold now perfectly flat and all back on snug.

Valve timing checked and appears ok.

Still little pull at carb mouths.

Injected fuel straight into the pots and refitted plugs quickly, and not a murmur!

Seriously losing the will now.  Another night getting nowhere.

Wont get time on it now till Saturday.

At my wits end….

Think I’ll fit a K series Honda in it.   Grrrrrr.

 

 

 

[Steven Whitaker]

40 minutes ago, twostrokekid said:

I can't see where you've connected the choke cable. There doesn't appear to be a choke cable fitted in the usual position, the brass insert on the rear carburettor. The jets should pull downwards about a quarter of an inch on full choke and the idling throttle stop is slightly nudged open. You may have improvised a choke connection but I can't see it.

I’m holding the choke fully open when I try to start it.

[Steven Whitaker]

Could it be a dodgy coil.  I get 8.2 ohms across the terminals.  Is that right?

[RobH]

Much too high. The primary should be around 3 Ohms.   What does the meter read with the probes shorted together?

Edited November 22, 2023 by RobH

[Steven Whitaker]

14 minutes ago, RobH said:

Much too high. The primary should be around 3 Ohms.   What does the meter read with the probes shorted together?

Just checked.  I think my meter is rubbish at it’s saying 5.3 now.  And 1.2 shorted out, but it’s different each time I check…..

I might try a different coil.

3 hours ago, Motorsport Mickey said:

My suspicion (and I think a few other members) is that is a massive inlet manifold leak. I've had a couple of engines where the manifold has "hung up" enough to give very bad running characteristics, which is even worse than where you appear to be...at least your engine won't run at all !

Commonly what happens is there is a bad leak on one of the Siamesed ports serving 2 cylinders, that gives an abnormally weak mixture over the 2 inlet stacks attached which unknowingly the owner seeks to cure by richening the carb serving the weak cylinders. At a certain range this crudley allows the engine to run and drive, however because it is varied by the suction within the manifold the weakness varies and so the engine will be altered by the owner to run OK at maybe 2500-3000 revs, ie cruising by richening the carb affected. However when the car runs at 1500 revs or stands at tickover the carb which is overrichened VASTLY overfuels the cylinders fueled from that carb, which fools the owner who then starts to weaken off that carb which then won't run at higher revs or even start nicely from standstill. !

Sticking the fuel directly into the engine bypasses this, and hopefully proves the point that fuel plus spark and compression will at least give a bang ! If it does I would then definitely check for an air leak on the manifold. Mask the carb fronts around the air cleaners so it can't draw air in from the back and spray the Start Pilot or even brake cleaner around the inlet manifold to head fitments, if there is a leak there the engine will pull it in and Bang ! proving the point. Then you can refit the inlets confident you are remaking a bad area.

Mick Richards 

 

 

[twostrokekid]

I would say fuel starvation. The mechanical fuel pump would run at a pressure of at least 2.5psi (probably more) which in pressure head terms would need the suspended fuel tank to be at least 7.5 feet high, probably even more with the added pressure drop of the non standard in-line fuel filter. I recall from my test bed days that a yard of paraffin is 1 psi static head and petrol would be around the same value. I'd guess you're filling the float chambers before you attempt a start but then very rapidly dropping the fuel levels as you draw the first droplets into the inlet manifold as there is insufficient pressure to drive an adequate flow across the needle valves in the float chambers. I would direct the flow from the dummy fuel tank into the fuel pump inlet rather than straight into the carbs.

[Michel Higuet]

11 hours ago, Steven Whitaker said:

Just checking valve timing.  It seems ok?  All at tdc showing valve positions.  The pic with both valves closed with clearance under tappets is at tdc and rotor arm pointing to cylinder 1 on comp stroke which I think is correct..  Other one at 180 out shows valves in different positions.  So can I rule out the valve timing being wrong?

Pity he pic is not horizontal but on pic 1 as I already says previously the pushrod of inlet cyl 2 is/seems lower than those on cyl  1 but I measure from the cork as reference and maybe it is more compress in front of cylinder 1... 

[Michel Higuet]

12 hours ago, Steven Whitaker said:

Would you mind suggesting one I can order from Amazon please?  TIA.

https://www.amazon.co.uk/MRCARTOOL-Pressure-Vacuum-Carburetor-Diagnostic/dp/B08HCXLT94/ref=sr_1_9?keywords=vacuum+gauge&sr=8-9

 

https://www.amazon.co.uk/Alight-Tools-Compression-Cylinder-Automotive/dp/B08C5J3GLP/ref=sr_1_5?keywords=compression+gauge&sr=8-5

 

 

[Kiwifrog]

Was the engine in the car when it was painted?

The reason I ask is I had the same problem with my 3a after it was painted, I had good compression, I double checked the timing, I squirted start pilot into the carbs still no go. Pulled the plugs and got spark, drove me mad. I accidentally dropped a spark plug on the floor and had to replace it.when I tried again it fired on one cylinder where I had replaced the plug so I replaced all the plugs with new ones and it ran perfectly. 
I could only assume that the thinners in the paint had affected the plugs somehow.

try replacing the plugs for new ones or known good used ones

cheers Alan