carburettor jet size for modern fuel

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trevor
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carburettor jet size for modern fuel

Post by trevor » Tue Jul 14, 2020 12:57 am

My 1968 Coupe V6 Essex engine started first time (almost) after the rebuild. It starts easily and runs, however the tick over is uneven, as if a cylinder is not pulling it weight. I need to sort that before the CO2 and tick over can be set.
The engine has a fast road cam. Unleaded D shaped heads and a Weber DGAS38 carb.
I have checked the compression, valve timing and tappet clearances. All OK
Next the ignition timing condenser, points and dwell. Finally the cap and plug leads and plugs. There is a new AC fuel pump. All of the above is spot on.
With those checks complete it is finally time to consider the carburettor. which has been stripped cleaned, diaphragms replaced, fuel delivery and level checked.
The car has not run for 8 – 10 years when it ran well on the fuel available at the time. Yet it now has a lumpy tick over.
What are peoples thoughts and experiences on carb jet sizes for the modern fuels?
Trevor S



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Post by willholderogri » Tue Jul 14, 2020 6:43 am

No change on jet sizes needed


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AJL Electronics
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Post by AJL Electronics » Tue Jul 14, 2020 8:18 am

No changes should be required. If the cam is non standard, sometimes it is an advantage to go to 50 idles. However your problem is likely to be the age old one of oval holes at the butterfly spindles letting in air unwanted. Only cure for that is reaming out and fitting real bushes, or replacement.


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trevor
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carburettor jet size for modern fuel

Post by trevor » Tue Jul 14, 2020 12:04 pm

Hi AJL,
The car spent most of it life in store with only 70,000 miles on the clock, I'm not sure that would effect the carb, however I take your point and will check.
I have confirmed and eliminated problems elsewhere so its just down to carburation - which is always the last thing to pay with.
Trevor S



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AJL Electronics
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carburettor jet size for modern fuel

Post by AJL Electronics » Tue Jul 14, 2020 1:59 pm

trevor wrote:
Tue Jul 14, 2020 12:04 pm
Hi AJL,
The car spent most of it life in store with only 70,000 miles on the clock, I'm not sure that would affect the carb,

70K will have the spindle holes looking like eyes!


Yes, we can mend your Scimitar!
Order your Scimitar parts any hour of the day at http://www.classicmicrocars.com (member's discounts).

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peter freeman
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carburettor jet size for modern fuel

Post by peter freeman » Tue Jul 14, 2020 2:38 pm

That 70 could be 170 or 270 - I think my Coupe has done over 300 on the original engine which has been re-conned by the previous oner - fitted the wrong tappets which did not help me.



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adamwilkes
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carburettor jet size for modern fuel

Post by adamwilkes » Tue Jul 14, 2020 3:14 pm

No irregular tapping noises when running? Just wonder if one valve is sometimes not shutting fully, which would leave a tappet clearance large for a moment.

I'll place a bet on the cap or one HT lead being dodgy though :wink:


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carburettor jet size for modern fuel

Post by Scim8tar » Tue Jul 14, 2020 3:52 pm

With aftermarket cam's of the longer duration type, at idle & low revs the ignition system has to try and burn the mixture at lower combustion pressure's than a standard cam so a good electronic conversion (variable dwell %) is preferable for max coil on time and big sparks, you may also need more static advance on the timing if that couldn't be done without exceeding total advance parameters then we had to change dizzy springs to achieve a good curve back in the day.
when you have sorted the carb as Andy AJL suggested and moved on to mixture set up, i would say to anyone running a tuned engine and wanting to tune or jet your own carb, if your budget will run to the price of a 4.9 lsu wideband AFR kit, lightning fast response to carb adjustments and if you make a sniffer pipe as pictured you won't have to weld a union in your down pipe and you can use it on other cars.
Attachments
IMG_2199.JPG
mk4 sniffer
IMG_2199.JPG (209.29 KiB) Viewed 690 times



trevor
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carburettor jet size for modern fuel

Post by trevor » Wed Jul 15, 2020 7:21 pm

This is good!
Lots of things to think about, and maybe encouragement to re visit areas - just to be sure.
I wonder if a fairly soft "fast road" cam would make a rough idle. like you would expect from a more top end performance type?
I will check the inlet manifold for leaks first though, I have not done that. Never happy with the gasket construction or fit.

Present milage is 76,000 This car was taken off the road in 1979, dismantled for a respray but the owner moved abroad and it didn't get completed.
Dry stored at first it finally ended up still with no glass out in the open. That is how it I got it in 1998. Still with masking tape and paint all over the wheels.
Mice in the headlining and boot, with pond life everywhere else. The paint fell off in chunks - it became known as "Scabby".
I put it back together and ran it until 2003 when low oil pressure put it out of use. it has stayed out of use until it was decided to break it for parts or restore it.
It now looks good, and will soon be running well.

The previous owner (who I never met) was a club member, Mr N Kennesion of Merstram, Surrey - anyone remember him?

Happiness would be an old SUN engine tuner!!



Scim8tar
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carburettor jet size for modern fuel

Post by Scim8tar » Wed Jul 15, 2020 10:56 pm

All good fun Trevor!
Some of those fast road cams have a fair old bit of inlet duration, the rally profile has a touch more plus more lift,
Not wanting to knock standard points ignition's but they are not the best thing for performance engine's and if you have even a slight misfire it messes with your readings if you're using an exhaust gas analyser to set your carb up,

Happiness is an old sun, crypton or Kal engine tuner, i wouldn't be without one !


Paul



trevor
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carburettor jet size for modern fuel

Post by trevor » Thu Aug 27, 2020 8:18 pm

Following up on some of the advice and clues on this forum:
I found the inlet manifold leaking. The gasket has now been replaced - what a difference.
Still having difficulty getting the CO2 where I want it. Probably corrosion and wear and muck in the carb. To be sorted.
Trevor S



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philhoward
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carburettor jet size for modern fuel

Post by philhoward » Thu Aug 27, 2020 8:19 pm

Good to hear the progress is in the right direction 8)


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adamwilkes
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carburettor jet size for modern fuel

Post by adamwilkes » Wed Sep 16, 2020 8:39 pm

Scim8tar wrote:
Tue Jul 14, 2020 3:52 pm
wideband AFR kit, lightning fast response to carb adjustments and if you make a sniffer pipe as pictured you won't have to weld a union in your down pipe and you can use it on other cars.
Cheaper wideband air/fuel ratio measuring kits are available - would these be any good for setting the mixture (especially when parked) on a standard V6? eg: https://www.ebay.co.uk/itm/Air-Fuel-Rat ... SwaTtd5g0Q


Adam Wilkes
Scimitar GTE 5a 1973, kinda Highland Purple; after months of work, still a ruin!
I can't help myself - I have to ask questions and find out why. Why ? Dunno :shock:

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philhoward
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carburettor jet size for modern fuel

Post by philhoward » Wed Sep 16, 2020 9:08 pm

Looks like a Chinese copy of the grams one I’ve got which works pretty well: https://www.tegiwaimports.com/mtx-l-wid ... gauge.html

Mine came with a genuine Bosch sensor and when gauge started playing up occasionally I got a replacement pretty quickly (in fact, still got it). As for the copy? Your call.

Used it on a carb engine (1600 CVH) and it’s response is very good. You will probably be able to get the idle jet setting right on the drive but any changes to the mains will need a dyno run to get the engine under enough load to actually use them.


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