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Discussion Starter · #1 ·
I recently what I thought was finished my carb conversion (still have some work to do with some custom pieces I crafted). I got them as close to pretty good as I could tune wise, thing runs great! But only when the hood is open....

When I close the hood the idle kicks down almost immediately, almost to the point of stalling, the engine starts to sputter and break up anywhere above 2,000rpm, and when I press the brake pedal it almost dies. I presume that last one is because the booster steals vacuum from the already running poorly engine. Why on earth does it do this? It only happens when the hood is closed. When the hood is open and I'm messing with it, there's no issues. It happily revs, idles where it's supposed to, etc. HELP! I'd like to get this stupid thing back on the road this summer and this predicament isn't exactly helping...
 

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Have you checked that the hood is not moving a wire somewhere when it closes?

My car has a under hood light that means wires have to go up and on to the hood. I assume they connect to the loom somewhere and maybe yours has caught on something or has been shortened so when the hood is up the loom is lifted off something, but when down it is able to drop and makes contact on that something.

Over 30 years ago my father had an electrician in desperation bring a Datsun 280Z to his shop. This car would just die randomly and leave you stuck on the side of the road. After a lot of test drives (by a very young me :) ), my father eventually found a wire running down the front of the engine that had lost some of it's insulation ... and yep at points of acceleration that wire would earth on the engine block. Funny thing these cars :D
Pete
 

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Discussion Starter · #3 ·
Have you checked that the hood is not moving a wire somewhere when it closes?

My car has a under hood light that means wires have to go up and on to the hood. I assume they connect to the loom somewhere and maybe yours has caught on something or has been shortened so when the hood is up the loom is lifted off something, but when down it is able to drop and makes contact on that something.

Over 30 years ago my father had an electrician in desperation bring a Datsun 280Z to his shop. This car would just die randomly and leave you stuck on the side of the road. After a lot of test drives (by a very young me :) ), my father eventually found a wire running down the front of the engine that had lost some of it's insulation ... and yep at points of acceleration that wire would earth on the engine block. Funny thing these cars :D
Pete
No wires or anything in the way. I see no reason for it to be happening other than it's sucking in warm air that it doesn't like, but by putting the air cleaner on I removed that factor. It just doesn't make any sense.
 

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OK. I'm going to assume (always a bad idea) we're talking a four cylinder here so it'll be an Alfetta and also, most likely, an ex Spica car. If it's a V6 with carb conversion that's a whole different deal. BUT, either way, it sounds like you're starving the engine of air. Pictures will help immensely here so we can see what kind of set up you have, what kind of air filters you're running, what kind of air box if any, air horns, all that kind of stuff. Lots of variable so lots of possibilities. If it's a four cylinder that's an unusual situation because there's lots of air available in the engine bay and choking out the engine is very unlikely. If it's a carbed V6 then depending in what carbs you have and how close to the hood they are when closed, it's very possible that you're choking them out. Like I said, pictures will help big time.
 

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Discussion Starter · #5 ·
OK. I'm going to assume (always a bad idea) we're talking a four cylinder here so it'll be an Alfetta and also, most likely, an ex Spica car. If it's a V6 with carb conversion that's a whole different deal. BUT, either way, it sounds like you're starving the engine of air. Pictures will help immensely here so we can see what kind of set up you have, what kind of air filters you're running, what kind of air box if any, air horns, all that kind of stuff. Lots of variable so lots of possibilities. If it's a four cylinder that's an unusual situation because there's lots of air available in the engine bay and choking out the engine is very unlikely. If it's a carbed V6 then depending in what carbs you have and how close to the hood they are when closed, it's very possible that you're choking them out. Like I said, pictures will help big time.
Yes, it is a 4 banger 'Fetta. Dellorto DHLA 40 carbs.
 

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A 4 cylinder Alfetta would be similar in period to a Triumph TR6 ... so does the Alfetta have a, I think, fuel cutoff switch. It might be an engine cut off switch? The intention here is if the car is involved in an accident the engine is automatically killed.

I remember once finishing a clutch job, again when very young, on a customer's TR6 and when we had finished my father let the bonnet drop from a little too high and it triggered this switch ... engine died ... Took him a little while to click as to what was going on. So I'm wondering if you have such a device and it is miss behaving.

I cannot see how the bonnet can change the air flow characteristics to affect the running performance of the engine, hence I'm looking for other weird and wonderful possibilities ... but I think I'm all out now, unless there is a wire near the bonnet latch ... :eek:
Pete
 

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A bad booster will cause an engine to run like crap in some cases, brakes would not be so hot either.
 

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I’m with those who say it’s a mechanical interference thing.
Also, did the brake-related stalling start to occur at exactly the same time? if so, too much of a coincidence... remove the booster hose and cap/plug the one-way valve at the manifold (easier to plug at the other end, but your hose might be cracked too). Any change?
 

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Discussion Starter · #9 ·
I’m with those who say it’s a mechanical interference thing.
Also, did the brake-related stalling start to occur at exactly the same time? if so, too much of a coincidence... remove the booster hose and cap/plug the one-way valve at the manifold (easier to plug at the other end, but your hose might be cracked too). Any change?
Actually when the hood is open, and when it had the SPICA, when I pressed on the brakes it actually kicks the idle up a couple hundred RPM. I did buy some vacuum caps for some other stuff, so I can certainly give that a try. And @PSk it does have the fuel inertia switch, but that's working just fine. No other wires under the hood except for the ones for my driving lights that haven't been wired in yet. I do have some dangling ones in the engine bay for SPICA sensors that I obviously no longer have. I also have a few loose grounds that I totally forgot about; all the ones underneath the ground for the fan. I can tighten that up tomorrow and see if it does anything. Does anyone know if Jethro would have any good insight on this?
 

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It's a stretch, but how old are your plug wires? When they get old they can leak current, and maybe they're grounding to the hood or something.

Try running the car in the dark, if the wires are bad you may see them arcing.
 

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Discussion Starter · #11 ·
It's a stretch, but how old are your plug wires? When they get old they can leak current, and maybe they're grounding to the hood or something.

Try running the car in the dark, if the wires are bad you may see them arcing.
They're brand new; the ones from Centerline.
 

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Interesting. Where is the coil mounted? I ask because factory carb cars have the coil high up on the inner guard. Its possible the hood frame is touching the coil?
 

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Discussion Starter · #13 ·
Interesting. Where is the coil mounted? I ask because factory carb cars have the coil high up on the inner guard. Its possible the hood frame is touching the coil?
It's in the factory position, in the very front of the top of the fender. Tucked in that little corner where the front structure and the inner fender meet.
 

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Hood light switch acting as a ground with the hood up, and losing the ground when the hood closes?
 
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Are the carb/air cleaner whatever, clearing the hood when it is closed? The hood isn't coming down on the tops of them and moving them changing the linkage settings, or possibly opening up an air leak in a rubber mount maybe?

I would SLOWLY lower the hood, with the engine running, not just dropping it quickly as we normally would, until it just stops without latching, and note the engine performance. If it is still running ok, then push the hood down to set the latch, noting what happens to the engine. This should give you some clues.

I'm sure the engine is not starving for air.
 

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Del just stole my thunder, and I think he's onto something. Check the hood clearance to all possible contact points on the carbs, air cleaners, flanges, etc. I also think you may have a split rubber carb mount, and with any interference with the hood closed, you have a vacuum leak. The booster would only make it worse, as you describe. Dumb question-- is the manifold bolted up tight?
 
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Discussion Starter · #17 ·
Del just stole my thunder, and I think he's onto something. Check the hood clearance to all possible contact points on the carbs, air cleaners, flanges, etc. I also think you may have a split rubber carb mount, and with any interference with the hood closed, you have a vacuum leak. The booster would only make it worse, as you describe. Dumb question-- is the manifold bolted up tight?
No contacts, no interference, no cracked rubber mounts, all brand new. I disconnected the booster line. It seems to be a bit happier once it gets a little temp into it.
 

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So, if you removed the hood and drove the car around, you think it would run okay?
 
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Maybe the carbs are just not tuned correctly.
 

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Discussion Starter · #20 ·
Switching the 135 jets out for the 132's to see if that helps any. My guess is that it runs too rich. Once you get moving and it gets that nice airflow, it's fine.
 
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