219 replies to this topic

[SA EH]

So not to sound funny but I can't find anything on pros & cons to raising or lowering the float level. My guess is it just helps your engine tune Warren?
Obviously a level set too high will result in overfuellung at idle to some degree but is there any advantage to going against specs? I just can't see it making too much difference.

[LC-GTR-1969]

Thanks heaps Warren! Great info...

 

I have some fuel bowl spacers- I got them from Midel- they raise the height of the bowls by about 5mm- the lid of the bowl now sits about 5mm higher than normal with these spacers. They are just a cylinder cut down to 5mm then I used a gasket on bottom and top. No idea if they are right or not as yet- I should know in a few weeks though... Ill try to get a pic.

 

Cheers again. 

[SA EH]

Like these....?

[SA EH]

I can't see them doing anything but increasing the fuel bowl capacity, much like the hd8 big bowl conversion that Midel offer. There is nothing mentioned about bowl height effecting fuel levels in the jet tube.
I can't get my head around why you would alter the level, when at the end of the day fuel is drawn constantly out of the jet under vacuum & it's limited by a dirty great needle jammed down its guts, so by changing the fuel level 1 or 2mm in the jet is going to alter the way it runs?? I'm happy to hear reasons.

Edited by SA EH, 23 March 2017 - 05:56 PM.

[LC-GTR-1969]

Im no expert, but in a nutshell-by raising the float a little, I gather I am in effect returning the carb to spec...

 

the standard HS8s (just referring to this as these are my carb) are mounted with a 0 degree angle to the carb- ie the jag manifold keeps the carbs horizontal...  and the float bowls horizontal...

 

however, my manifold puts the carb at 30 degrees incline. To correct this, I have used a rubber bush where the float bowl mounts, to change the angle of the float bowl, so the float is now sitting vertical, even though the carb is mounted 30 degrees. As explained to me, this then changes the level of the fuel within the jet tube, hence, MIDEL supplied a spacer for the float bowl which raises the float and lid, to ultimately correct the level of the fuel within the jet tube (so that it is closer to what original spec was). The little washer spacers that sit under the needle seat are only for fine tuning this fuel level. 

 

As said, I have not used these yet so I could be talking out of my arse- but ill know in a few weeks lol

 

EDIT- I am actually starting to get confused now- ive had a few vinos and could be talking out of my arse hahahaha. I am sure others who have much more experience with SUs can clarify 

 

Here is a pic

 

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Edited by LC-GTR-1969, 23 March 2017 - 08:27 PM.

[SA EH]

Ok, so now I see where you're coming from....

 

Clearly it's one of those things that's not on my priority list, maybe it should be.

Anyway, still keen to see the results so you might as well make them as accurate as you can.

Carry on 

[warrenm]

What LC-GTR-1969 said is correct, I don't think I can add to it.

[LC-GTR-1969]

Just a quick update...

 

I have fitted the HS8s- now I just need to finish up a few things, make a few brackets for throttle etc, then test drive and tune.

 

First observations, once I got it started, the exhaust note is quite different- more of a rap rap rap sound than my webers. The throttle response seems excellent, very quick and aggressive, however I have not driven it yet, so cant be sure how it will be under load.

 

I've spent a lot of time dicking around with linkages etc, as ive had to make a lot of the stuff I needed, which chewed up a lot of my time...

 

I was just wondering how others hold the chokes open without a cable? I used a but of wire to hold 2 carbs choke open but that was annoying... ill try to get a pic and post up soon...

[Zook]

My '74 XJ6 Jag has two 2" SUs on it. 4.2L motor. I'm not sure that helps.

[rodomo]

I was just wondering how others hold the chokes open without a cable? I used a but of wire to hold 2 carbs choke open but that was annoying... ill try to get a pic and post up soon...

I had triple 1 3/4s with no choke cable. I used to manually pull down the middle jet and use a clothes peg to hold it down to start it cold. Didn't take long. Those carbs will be ending up on a streeter so my thoughts are to run an after market cable from the dash to just pull the middle jet down.

Edited by rodomo, 14 April 2017 - 10:10 PM.

[LC-GTR-1969]

So here is a before and after- 

 

Before, with webers;

 

And after, with seton manifold and HS8s;

 

BTW- Photobucket seriously sucks balls. 

Edited by LC-GTR-1969, 15 April 2017 - 04:51 PM.

[LC-GTR-1969]

Hmmm, Much as im a SU junkie, the Nitrous on the webbers does diddle my skittle a bit...

Yep- the nitrous looks the part on a weber manifold... The SUs are nice as well, but the webers are very pretty.

[LC-GTR-1969]

I forgot to mention that, in case anyone is interested, the seton manifold seems to have a lesser upward angle than the armours, and so the front carb clears the bonnet by an inch or more in my LC- plenty of room. Also, the length of the seton seems the same as the armours- just over 7 inches along the top face.

Edited by LC-GTR-1969, 16 April 2017 - 08:32 AM.

[SA EH]

The webers definitely make the engine look a bit more "modern".
I think you'll find they'll start relatively easy without a choke once they're tuned nice. I don't run any choke on mine & it kicks over after a half dozen turns.

[LC-GTR-1969]

Ok, so I have been syncing the carbs and setting the AFR at idle... 

 

Firstly, it seems a little rich at idle- my INNOVATE wideband is telling me 12.5:1 and my colourtune confirms this. I have the jet wound back as much as possible and this is the reading... it then appears a little lean above 4000- dips to 14:1. The lean above 4000 could possible be a small vacuum leak- so I am going to test that and fix any vacuum leaks if there are any.

 

Running .125 jets and UP needles. The spings are red at one and and green at the other- these are just what Midel gave me... I have no idea if the springs are ideal, but I still have the original JAG springs that came in the HS8s. The original feel lighter but could be my imagination.

 

Secondly- it doesnt like idling below about 1400- but that could have a bit to do with the rich condition at idle I suppose. Carbs are synced but she gets all rattly at 1000rpm, and the SUs are shaking like an ice addict on the morning after. Settles and smooths nicely at 1300 or so rpm. My webers idled very happily at 850rpm but I still have tuning to do so not too worried at this stage. 

[SA EH]

If you want to compare piston spring pressures, grab the electronic kitchen scales off the missus & push down on the spring until you compress it to say 2", then measure the grams. Do that to the other set & see the difference. More grams equals more stronger spring.
I'm guessing the afr's you got were stationary in the shed?? Maybe see if it's too rich on the road before grabbing some other needles, don't forget some oil in the dashpots.

[LC-GTR-1969]

Cheers mate, 

 

I just took the old girl for a spin and it is a little rich at idle- 11.9 - 12.1:1, but when it comes on the cam it jumps up to 13.5:1- this is at about 4500-5000rpm... 

 

I cant really richen up much with the adjuster screws as it starts to stumble and want to die from being too rich at idle. 

 

I will try the other set of springs and see if that helps...

 

Also, yes, I am running oil in the dashpots, quick snap of throttle seems ok, no lean bog. Genuine SU oil. 

 

Also, the chappers just pulled me over lol- they said they had reports of me driving like an absolute maniac- which was not true, I hardly got out of first gear and would not rev it past 4500ish rpm as it was too lean.. . Coppers were very nice though, they just said to drive my normal car as its so loud that everyone will think that I am flying... At least they were pleasant and did not hassle me. 

[LC-GTR-1969]

Ive been tinkering again this morning, and have it idling better- the AF is now about 13.5-13.8:1 at idle, which is quite crisp- pretty much what I ran with the webers. The idle is now 1200rpm where it seems happy- once I got the idle AFR more ideal it idles cleaner obviously... but still doesnt like it lower than that though.

 

Problem is she is still leaning out when coming on the cam- so I suspect either stiffer springs or needles with more taper in the mid/ bottom half than the UP needles? Any recommendations?

[SA EH]

Maybe try some heavier oil in the dashpots. I have no idea what weight the genuine SU oil is but possibly atf or (I remember someone telling me to use straight 30 weight or something).

[LC-GTR-1969]

Ok, so I seem to have them dialled in quite well now- just need to drop the idle a bit more, but otherwise she is running very clean.

 

Just some obervations;

 

The good (compared to webers);

 

- Much easier to drive than webers- seems smoother just above idle

 

- Much better low down torque, runs crisper from 2000-5000rpm. This may have more to do with the manifold but whatever...

 

- Pulls out of corners easier

 

- Car sounds more aggressive at idle (I didnt think it was possible but it does)

 

- Revs quicker, much better response. Violent response would be a good way to describe it.

 

The bad (compared to webers);

 

- Harder to start, but no big deal, seems to start most times without choke but only when weather warmish.

 

- Does not like idling very low- the webers could idle at 650rpm without too much trouble, but the SUs start rattling bad below 1000rpm. This is a big cam though, with lots of overlap (266@50 thou).

 

- I do much prefer the look of webers

 

- I actually find the webers more pleasant to work on, but thats just me..

 

- Did I say I prefer the look? (the SUs look good, but just not the same)

 

Neutral observations;

 

- The webers seem to bring the cam on harder- has more 'hit', whereas the SUs seem to spread out the torque curve quite a bit (which softens the hit).

 

- The sound of the car is very similar when on the boil- cant say the webers sound better on full song, seems the same above 5000rpm.

 

 

 

From a standstill, I would say the SUs would win the drag race due to better response and better torque down low to pull it out of the hole.

 

However, my guess is the peak HP would be the same or very similar. On a circuit track, where the revs would not drop below 4000rpm, I would say their are not many advantages to the SUs, they would basically be very similar to webers. But on the street, or for drag racing, the improved torque and low down response really wakes it up. I can see why so many drag racers seem to prefer 2 inch SUs and circuit seem to prefer webers.

 

At some point I will try to get the car dynoed at my local dyno place, but the operator has broken his back (poor bigger), so it may be a while from now.