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 Spark plugs- hotter baby?
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Jimmys old Saab
V4 Beginner

United Kingdom
98 Posts

Posted - 04 Sep 2020 :  19:22:25 Show Profile Reply with Quote
My 96 is running OK with the large Nikki twin choke; however it is reluctant to start, especially when hot; I'm pretty sure it's overfuelling.
Now, the Spark plugs are near as old as the car so I was going to change them anyway. I was wondering if going to a hotter plug might help reduce any fouling owing to running an oversized carb at low engine speeds. Importantly is a hotter NGK (they are what I tend to use) going to foul the pistons?

Saab 96 1971
Datsun 120y coupe 1975
Saab 9000 CSE turbo 1993
VW T5 LWB 2012

melle
V4 Guru

United Kingdom
3342 Posts

Posted - 04 Sep 2020 :  20:47:07 Show Profile Reply with Quote
What makes you think it's overfuelling? If you think you have an "oversized" carb I would look at the jetting first. BP(R)6ES plugs should be just fine if the rest of your engine is standard.

www.saabv4.com
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Jimmys old Saab
V4 Beginner

United Kingdom
98 Posts

Posted - 04 Sep 2020 :  21:32:29 Show Profile Reply with Quote
I don't think there's anything wrong with the idle jet, or the operation of the accelerator pump; it's a brand new old stock carb, likely for a car of similar capacity, a Datsun 160 or Toyota Corona or similar. I am searching for reasons for the reluctance to start from cold but most especially from hot.
Maybe the coil is on it's way out, the plugs are old but they look low mileage so could just be a rather poor spark. When left to idle sometimes dies too; the idle is not as low as I would like but I don't think the Taunus will idle that slow (like 600 rpm) will it? Having said that it revs very freely and feels surprisingly likely for what is a rather pedestrian engine.

Saab 96 1971
Datsun 120y coupe 1975
Saab 9000 CSE turbo 1993
VW T5 LWB 2012
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melle
V4 Guru

United Kingdom
3342 Posts

Posted - 04 Sep 2020 :  21:38:08 Show Profile Reply with Quote
Idle should be in the 8900rpm range.

www.saabv4.com
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Iain G
V4 Fan

United Kingdom
138 Posts

Posted - 05 Sep 2020 :  08:05:25 Show Profile Reply with Quote
I dont know Nikki carbs but on a Weber I would be looking at....
1. Air leaks. Spray carb cleaner around base and listen for running engine revs changing.
2. Webers like about 10 deg advance so then you can screw out idle speed screw and throttle is more closed at idle
3. Fuel pressure may be too much. Fit in line regulator flat disc dial type Sytec. 2 or 3 psi ample. Or fuel needle valve sticking open.
4. Idle jet size
5. IMHO modern fuel seems to need a VERY strong spark. New leads, rotor, dizzy cap, electronic ignition.
Hope thats useful?


Iain G
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melle
V4 Guru

United Kingdom
3342 Posts

Posted - 05 Sep 2020 :  12:46:07 Show Profile Reply with Quote
Some good ideas there Iain. I have a Nikki here, but I've never used it.

www.saabv4.com
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TTruckie
V4 Fanatic

United Kingdom
315 Posts

Posted - 07 Sep 2020 :  18:20:07 Show Profile Reply with Quote
do you have a thick insulating plate between the carb and the inlet manifold?? If not it could be fuel evaporation. I had this on a big carb set up. It worked fine but would never start when hot - fitted the insulator and solved the problem.

just a thought
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Iain G
V4 Fan

United Kingdom
138 Posts

Posted - 07 Sep 2020 :  18:56:00 Show Profile Reply with Quote
Another good idea. I have insulators on all of mine.

Iain G
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Derek
V4 Guru

United Kingdom
2018 Posts

Posted - 08 Sep 2020 :  12:16:58 Show Profile Reply with Quote
If your Nikki is one with the window for the float chamber, does the fuel level correspond accurately with the marked level line? Too high and fuel may just flow into the inlet then be far too rich for easy starting. Leaving the car engine off for half an hour should allow the fuel to evaporate and then it will start normally. It can then flood out if you have to freewheel on and you run down a long hill off the throttle. This is the same problem I had when I had a holed float (FoMoCo) which took in fuel and made it heavier. This caused the carb to flood the manifold.
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v4admin
Forum Admin

United Kingdom
467 Posts

Posted - 13 Sep 2020 :  09:29:13 Show Profile Reply with Quote
interesting thought about spark advance, always kept mine dead on 6 degrees but would be interested to know what others have found
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Borstlap
V4 Fanatic

Norway
301 Posts

Posted - 15 Sep 2020 :  16:46:37 Show Profile Reply with Quote
quote:
Originally posted by v4admin
but would be interested to know what others have found



I found a spark plug where the electrode and ground were welded together. This happened a good 20 years ago when I was still largely car technically ignorant. I found out that the engine ran much nicer at idle when I started to turn the dizzy. Then I wanted to see what the result would be at motor way speed. It went nice for as long as it lasted, which was maybe 500 meters. The clue of the story: don't muck about too much with mechanical advance at idle unless you are able to adjust max advance.

Alex
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Jimmys old Saab
V4 Beginner

United Kingdom
98 Posts

Posted - 18 Sep 2020 :  13:32:29 Show Profile Reply with Quote
Some very useful advice there-
No there is not an insulation plate, only the intermediate plate with the ports for vacuum/fumes. Next time I shall see how hot the body of the carb gets.
Vacuum leaks; I don't think there's a problem there, the car idles (as Melle correctly says should be 800rpm or more) at higher rpms and revs very well. Its just starting and esp hot starting that's the issue.
Nikki float level is on the line and it's a brand new carb. I will add a couple of degrees more advance, on the V4 engine I doubt that's will lead to any melted plugs or pistons. In my admittedly limited experience Weber and similar mechanical carbs, whilst great for improving throttle response and top end power are thirsty and smelly. The twin cam fiats I had were stinky creatures- and always goung out of tune!
Electrical gear, YES, it's all rather.....elderly. I shall replace the plugs/points/cap/arm and the coil before I fiddle any more; I think that's where the problem lies- Any recommendations re coils? I would usually plump for a Bosch but they have gone Chinese very often these days...


Saab 96 1971
Datsun 120y coupe 1975
Saab 9000 CSE turbo 1993
VW T5 LWB 2012

Edited by - Jimmys old Saab on 18 Sep 2020 23:47:35
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melle
V4 Guru

United Kingdom
3342 Posts

Posted - 18 Sep 2020 :  15:00:03 Show Profile Reply with Quote
quote:
Originally posted by Jimmys old Saab
I will add a couple of degrees more advice
How many degrees are you currently running?

quote:
Originally posted by Jimmys old Saab
In my admittedly limited experience Weber and similar mechanical carbs, whilst great for improving throttle response and top end power are thirsty and smelly.
If we're talking "tunable" Webers like DCD or DGV family (not OEM ones where you can't swap out parts) you've been doing something horribly wrong. Read John Passini's excellent Weber books to learn how Webers should be set up.

quote:
Originally posted by Jimmys old Saab
I shall replace the plugs/points/cap/arm and the coil before I fiddle any more; I think that's where the problem lies- Any recommendations re coils? I would usually plump for a Bosch but they have gone Chinese very often these days...
I've never had any issues with genuine Bosch ignition parts, so that's the only thing I use on all my cars; part numbers here: http://www.saab-v4.co.uk/speedball/topic.asp?topic_id=16498 Winter ignition improvement project here if you're interested: http://www.saabv4.com/index.php/project/hall-ignition/

www.saabv4.com
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Jimmys old Saab
V4 Beginner

United Kingdom
98 Posts

Posted - 23 Sep 2020 :  15:56:23 Show Profile Reply with Quote
OK, I got a spare couple of hours on Monday so I replaced the distributor cap, cleaned up the points, I had put the battery on charge already. Did not want to start, the float bowl filled up fast to the line, there was a good spark too. So I liberally sprayed some carb cleaner down the carb and that did the trick.
I set the idle rather higher, about 800-1000 rpm and set the timing to 6 deg with vacuum advance plugged. Ran very nicely until I switched off and went to start again, needed a good squirt of carb cleaner. So is there a problem getting enough fuel to start? Screwing the idle Screw in and out seems to make little diferrence but am I wrong here? Looking down the primary barrel of the carb whilst the engine is running I can clearly see fuel dribbling out of an orifice in the centre of the venturi and when the throttle lever is moved that dribble increases to a stream.

Saab 96 1971
Datsun 120y coupe 1975
Saab 9000 CSE turbo 1993
VW T5 LWB 2012
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Jimmys old Saab
V4 Beginner

United Kingdom
98 Posts

Posted - 23 Sep 2020 :  19:38:38 Show Profile Reply with Quote
I started her up again this evening, needed a good puff of carb cleaner; now, in the past with stock Weber or similar carbs I've set up the idle by turning the idle valve screw in until the engine begins to stumble and then backed off half a turn. I turned the idle screw out another two full turns and she started fine. I think now that the carb may be a little large and so the speed of air may be too low going into the venturi so not getting enough fuel to start easily but fine at idle. Now I can visibly see more fuel going into the carb at idle. Now that she starts OK, I'll run for a while and see what state the plugs get into, It may be that at higher rpms and greater velocity she will lean out. I'll get the colourtune out and have a look through that too.
I would be interested to know what amount of air the standard FOMOCO moves versus the twin choke Weber and my Nikki, the Nikki looks to have a similar or slightly smaller area to the venturi but that will not give a very accurate indication of the air it will consume.

Saab 96 1971
Datsun 120y coupe 1975
Saab 9000 CSE turbo 1993
VW T5 LWB 2012

Edited by - Jimmys old Saab on 23 Sep 2020 19:42:13
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melle
V4 Guru

United Kingdom
3342 Posts

Posted - 24 Sep 2020 :  09:43:33 Show Profile Reply with Quote
The fact that it starts on carb cleaner doesn't necessarily mean it's not getting enough fuel. Carb cleaner ignites much easier than the mixture your carb produces, so it's great to hide vacuum leaks or ignition issues. Fuel dribbling out of an orifice doesn't sound good to me; you say it's in the centre of the venturi, which I assume means the orifice is some kind of booster jet? I'll dig out my Nikki (which has been used on a V4) this weekend if I have time, be interesting to learn more about them.

www.saabv4.com
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