Sometimes, it's the little things...
David Jenkins - 19/10/20 at 01:25 PM
As you've probably read elsewhere, I've swapped from my bike carbs back to a Weber DGV 32/36 downdraft carb (it's been a bit of a
saga).
Went out for a long run this morning, the first time after the change-over, and while OK at full throttle it was a bit of a pig at low revs -
wouldn't pick up when I put my foot down, and generally sounding rough. I also noticed that my fuel gauge was dropping a bit quicker than
normal. Although this carb has been professionally tuned, I hadn't used it much in ordinary driving and it had been sitting in a box for many
months. So I decided to take a plug out and have a look at its condition, just for an initial check - and it was fine, slightly darker than I would
choose, but perfectly within limits.
The penny dropped when I went to replace the plug - all I could see on the top was the threaded portion, as the screw-on metal bit was still in the
plug cap. So I'd been driving around getting just a partial or disrupted spark on No. 1 cylinder... no wonder it wasn't behaving itself!
Wound the plug back into the cap, pulled it out with the metal bit now attached, made sure the top bit was firmly screwed on, then put everything back
together. Went out again just now, and it's running a lot better. In fact, it's pulling like a train at full throttle. 
[Edited on 19/10/20 by David Jenkins]
Deckman001 - 19/10/20 at 01:53 PM
Well done, just a thought, what bike carbs did you use, and why did you change from them ?
Jason
Mr Whippy - 19/10/20 at 02:03 PM
Yeah those do tend to slacken off, I prefer the plugs that they are one piece, I mean when was the last time that even served a purpose on a car
unless it was like absolutely ancient with a copper wire for the HT lead.
David Jenkins - 19/10/20 at 02:18 PM
quote:
Originally posted by Deckman001
Well done, just a thought, what bike carbs did you use, and why did you change from them ?
Jason
They were Honda CBR600F-X - I go into some detail on how I fitted on my website (see WWW link below).
They worked well enough, but they weren't perfectly tuned - there was a bit of a flat spot in the middle of the rpm range, with some pinking. I
thought about getting them tuned on a rolling road, but most places I spoke to weren't over-keen to do them: too much faffing about dismantling 4
carbs every time they wanted to change jets, etc. The most sensible conversation I had was with a tuner in Southend (who's name escapes me, but
he used to be on this forum) and he predicted a minimum of 2 hours, probably 3, at £100 per hour, plus the cost of any parts required at the time.
These carbs were also amazingly noisy - the intake roar at WOT was at least 3 or 4 times louder than the exhaust noise! I could have fitted an airbox
with a remote filter and that would have reduced the noise massively, but there was little space for that under my bonnet.
When I first built my car I had fitted a Weber DGV 32/36 and it worked well enough, except that it was old and worn out. This was why I originally
fitted the bike carbs (and I enjoyed the building challenge). The thing is that the Weber works really well on a modestly tuned x-flow, with
respectable economy if you are gentle on the pedal, but with loads of power if you boot it. When this carb was tuned I got somewhere around 100BHP,
and I don't think the bike carbs were much different in the state I had them. Certainly the acceleration was much the same, if this
afternoon's experience was anything to go by.
Deckman001 - 19/10/20 at 02:31 PM
Thanks for the heads up, mine are GSXR600SRAD ones, with just an idle jet to clean sometimes, and a fixed, sealed in main jet. I failed my IVA test
with a hc reading of 1388 instead of below 1200 ppm, Have heard that changing the timing can help with emissions, have you ever come across that ?
I'd really like to keep mine as they sound great and it pulls like a train, but was very lumpy at the test for some reason 
Jason
coyoteboy - 19/10/20 at 02:41 PM
quote:
Originally posted by Deckman001
Have heard that changing the timing can help with emissions, have you ever come across that ?
Jason
Timing can have a massive impact on emissions, because if you don't have enough time to fully combust your charge you're going to spit out
poorly combusted gases. If it's tuned right though, you should be fine - normally you'll see it lean out as you advance spark if your
starting point is such that you have insufficient time to combust.
Deckman001 - 19/10/20 at 02:45 PM
quote:
Originally posted by coyoteboy
quote:
Originally posted by Deckman001
Have heard that changing the timing can help with emissions, have you ever come across that ?
Jason
Timing can have a massive impact on emissions, because if you don't have enough time to fully combust your charge you're going to spit out
poorly combusted gases. If it's tuned right though, you should be fine - normally you'll see it lean out as you advance spark if your
starting point is such that you have insufficient time to combust.
Thanks for confirming that for me, I'll investigate the rest of the options too just in case.
gremlin1234 - 19/10/20 at 04:58 PM
quote:
Originally posted by David Jenkins
The penny dropped when I went to replace the plug - all I could see on the top was the threaded portion, as the screw-on metal bit was still in the
plug cap. So I'd been driving around getting just a partial or disrupted spark on No. 1 cylinder... no wonder it wasn't behaving
itself!
I guess you have checked all the other plug/top-cap/lead connections
steve m - 19/10/20 at 05:39 PM
Jason,
Not sure if your running any electronic ignition, but i ran accuspark module in the distributor, and there coil to suit, and the spark was like a
welding flame!!
If your still running points and condensers stuff, it will never burn the fuel efficiently
steve
Mr Whippy - 19/10/20 at 07:03 PM
quote:
Originally posted by steve m
Jason,
Not sure if your running any electronic ignition, but i ran accuspark module in the distributor, and there coil to suit, and the spark was like a
welding flame!!
If your still running points and condensers stuff, it will never burn the fuel efficiently
steve
Oh yeah electronic over points, chalk and cheese. Best upgrade you could give your engine.
rusty nuts - 19/10/20 at 08:54 PM
High HC is caused by unburnt fuel which can be caused by a number of things , start by checking and adjusting the valve clearances, tight valves will
cause high HC and are often overlooked. Check points gap making sure you use the correct gap for whatever make distributor is fitted, lubricate the
distributor spindle and check the cam can be rotated( timing advance, some distributors can and do seize ) check vacuum advance works by sucking on
the vacuum pipe , check and adjust the ignition timing again make sure you use the correct setting. check rotor arm, distributor cap , HT leads and
plugs are all in good condition. A compression test with the engine hot is a good idea as is an oil change ( surprising how often the oil can be
contaminated with fuel) Check the crankcase breather system is working and finally check and adjust the CO and idle. If you can find one a
distributor, coil and wiring from a Valencia engined Fiesta is an easy , reliable conversion
Deckman001 - 19/10/20 at 09:11 PM
quote:
Originally posted by rusty nuts
High HC is caused by unburnt fuel which can be caused by a number of things , start by checking and adjusting the valve clearances, tight valves will
cause high HC and are often overlooked. Check points gap making sure you use the correct gap for whatever make distributor is fitted, lubricate the
distributor spindle and check the cam can be rotated( timing advance, some distributors can and do seize ) check vacuum advance works by sucking on
the vacuum pipe , check and adjust the ignition timing again make sure you use the correct setting. check rotor arm, distributor cap , HT leads and
plugs are all in good condition. A compression test with the engine hot is a good idea as is an oil change ( surprising how often the oil can be
contaminated with fuel) Check the crankcase breather system is working and finally check and adjust the CO and idle. If you can find one a
distributor, coil and wiring from a Valencia engined Fiesta is an easy , reliable conversion
Cheers for the above, I already have an electronic dizzy from Spark right, along with the correct arm and top cap, new leads, plugs and uprated coil,
so am fingers crossed that all those are correct and so I can hopefully forget about them for a little while. The plugs were roughly gapped, so I had
better check those too, and while out, I will check their colour, but i do remember that the old plugs were a nice light brown so should be seeing
good mixtures. hence why I am also now hoping that altering the Dizzy will get me less hc. Crank case breather might need a look over, thanks for that
head up too, I have a new catch tank, but the plumbing to it is original, so maybe about 40 years old , eek !!
Jason
MikeR - 19/10/20 at 09:33 PM
Its just clicked. Different cars like different advance. What do you have set up at tick over and how much does it change at different rpm? Is it
matched to the engine?
David Jenkins - 20/10/20 at 01:18 PM
quote:
Originally posted by gremlin1234
I guess you have checked all the other plug/top-cap/lead connections
Two others were a little slack, but still connected OK - now fully tightened.