Fuel starvation or leaning out at highway speed

[leifanten]

Gents,
I got the car running well. Driving around smaller roads at varying speeds up to 55 mph is a very pleasant experience. Runs at 85 deg C the whole time. However, as soon as I try highway speeds the car suffers lean conditions/fuel starvation.

Symptoms at speeds above 55 mph:
- loss of power - unable to break 65 mph
- will not rev high enough
- heats up to 100 deg C (about a mm from the start of the red marking on the temperature gauge)

Facts:
- new carter fuel pump
- new fuel filter
- new spark plugs NGK bp6
- fresh fuel
- rebuilt strombergs (new needles, diaphragm and the works)
- even 170 psi compression
- starts easily, idles smoothly and runs well up to 55 mph
- valve clearances set within spec
- no undesired knocks or rattles
- new K&N airfilter in the nose of the car
- new Lucas gold coil (grounded body)
- new pertronix dizzy with flame thrower guts (with separate ground wire from body to ground point)
- new plug wires
- ignition advance set by the advance a bit at a time until it knocks, and then back it off 2 degs
- dampers in carbs have oil
- good oil pressure in engine at all revs


I am leaning to one of three potential sources of the problem:

1. needle valve in float chambers set too low - fuel starvation at speed
2. carb jets sitting to high in the body - too lean mixture
3. whatever someone with more experience on this esteemed forum can point me to

How can I tell the difference between 1 and 2 above?


I have reseached the archives but could not find this described.

[69S4]

Gents,

I am leaning to one of three potential sources of the problem:

1. needle valve in float chambers set too low - fuel starvation at speed
2. carb jets sitting to high in the body - too lean mixture
3. whatever someone with more experience on this esteemed forum can point me to

How can I tell the difference between 1 and 2 above?

My S4 has Strombergs and I've had similar twice symptoms in the past - great up to about 60 and then flat with increasing misfiring / detonation after that. First time it was float heights out of spec - they were quite a bit out though, over 4mm, as I'd reset them using misprinted info (pre internet days). Second time it was a bad rotor arm.

Re the carbs, the info I have says that the needle valve height can be adjusted by means of the aluminium washer underneath it and that the std thickness is 1/16". The float height, from the body face to the highest point on the float should be 16-17mm.

My carbs have adjustable needles (B1Y) but a fixed jet. Are yours different or have the jets been pressed out and replaced? I can't find any info on jet height in any of the Stromberg publications I have but mine are off the car at the moment and I can have a look later and see what I can measure.

Stuart

[oldelanman]

My carbs have adjustable needles (B1Y) but a fixed jet. Are yours different or have the jets been pressed out and replaced? I can't find any info on jet height in any of the Stromberg publications I have but mine are off the car at the moment and I can have a look later and see what I can measure.

Here you go.........

Depth of jet below bridge.

Single carburettors: 2,5 ?0,1 mm
(0.0984 ?0.0039 in)
Twin carburettors: 2,3 ?0,1 mm
{0.0906 ?0.0039 in)

Regards,

[69S4]

Just been to measure mine and they're both 2.95mm below the bridge. That's to the hole in the middle. There is a slightly raised ring at the edge and that measures 2.60mm down from the bridge. (FYO float height measure at 16.8mm)

Stuart

[oldelanman]

Hi Stuart,
Your jets do look to be set a bit below the optimum position but I would not worry about it, particularly if your engine is now running OK and you are happy that the mixture is correct.

The setting given probably corresponds with the mid point of available adjustment of the needle so if the jet is set much too low in the carb body the mixture would be too rich and you would not have enough needle adjustment to correct it. I can't remember exactly how much adjustment there is on the needle but I would think it's more than enough to take up the 0,55mm extra jet depth you have.

One thing worth checking with the adjustable needle set up is that the grub screw which locates the needle housing in the air valve has not been overtightened or slightly too long. It should just prevent the needle housing from rotating but should not stop it from sliding. If the adjusting screw does not turn freely with minimal force then the grub screw may be overtight. The adjusting screw is only held in the air valve tube by a star washer and if the grub screw is too tight and you are trying lean out the mixture it's possible that you may push the star washer up the tube instead of moving the needle down towards the jet. So you may think you have adjusted the needle but may not have !

As for the original subject of this post..........
liefanten wrote....

- rebuilt strombergs (new needles, diaphragm and the works)

Did this carburettor rebuild include checking the temperature compensators ?

Regards,

[collins_dan]

I have a similar issue and one suggestion that I had heard was that maybe the mounting o-rings were leaking and that as speed increases they suck in more air through the leak, creating a leaner mix at high speed than any other time. I can't say if this solves the issues as the o-rings are sitting in my 'needs to be installed' pile, but thought it was worthy of consideration. Good Luck, Dan

[bast0n]

Sound silly but I had a similar problem - power drop off as speed increased.

Guess what? Fuel filler cap not letting in enough air.................!

[69S4]

Did this carburettor rebuild include checking the temperature compensators ?

I've been looking at the settings on my temp compensators recently and although I doubt they have much of a part to play with Leif's problem
I would welcome any advice on how to set them up.

So far I've found two sources of info - one, relating to TR6s, saying that they should start to open at 115F and be fully open at 140F and a second, relating to Volvos, saying that they should open at 70-77F and be fully open at 85F. These are quite considerably different settings for what are the same carbs (and the same ones as on the Elan). Anyone know if either / neither of these are right for the Elan?

Stuart

[leifanten]

Gents,
this thread has been invaluable (combined with my thread on the mysteriously high ignition advance)... it is all coming together in a bit of a Eureka moment.

The reason for the need for the excessive advance is too lean mixture. I pulled the carbs apart and guess what, the "professional" rebuilder I paid $400 to do the overhaul had done three things that together means waaaay to lean mixture

1. Needle is B1G instead of B1Y - leaner mixture
2. The jet is at 2mm and not 2.6mm - leaner mixture
3. He had left a small plastic ring around the needle (not even sure what it was intended for - if I knew how to post pictures I would show you. But in essence it is a little plastic ring that rests on the top of the jet, and with the needle being biased towards the engine side, the plastic ring would always take the position of covering up most of the open area on the air filter side of the jet, plus it removed another 0.4mm of deck height from the jet to the needle ----- way leaner mixture

The compounded effect of this was that in order to make the car run the advance needed to be set at 35deg btdc, and under heavy load that would not give anywhere near the right mixture.

Thank you all for the excellent contributions. The discussions on these two threads helped me take one step back and point me in the direction that they were connected. The question it made me ask was: What conditions would allow the car to run with this kind of advance TOO LEAN AIR/FUEL RATIO which I had already established that I had....



I now need to see if theory matches reality and order a pair of B1Y needles, lower the jet and throw the little plastic rings so far away that I never have to see them again.....

Final question: how hard is it to lower the jet? Is that done by screwing, pushing or carefully knocking it lower??

[oldelanman]

Hi Leif,
Good to hear that you are making progress - I think I would be asking for my money back from your carb "expert".

Assuming that you have the adjustable needle type carburettor the jet is just tapped in and you should be able to reposition it by gently tapping it down with a suitable drift - it needs to have a hole in the centre so that you don't damage the jet orifice. However, as I said in my earlier post to Stuart, I don't think you necessarily need to reposition the jet - as it is only around 0.5mm above the optimum position you should have ample adjustment on the needle to set the correct mixture from there. It seem likely that the plastic ring is the real culprit. Have a look at this link (I think miked found it originally if I remember correctly).....you may find it helpful.

http://faq.saabnet.ru/saab_carbs/6_misc.pdf

Stuart,
This article also talks about checking/adjusting the temperature compensator. Note that it is a Saab website so the setting may be different from the Elan although I don't really see why it should be. I did not attempt to check the opening/closing temps of mine - just made sure they moved freely and were fully closed when cold and fully open when hot.

Regards,

[69S4]

1. Needle is B1G instead of B1Y - leaner mixture
2. The jet is at 2mm and not 2.6mm - leaner mixture
3. He had left a small plastic ring around the needle (not even sure what it was intended for - if I knew how to post pictures I would show you. But in essence it is a little plastic ring that rests on the top of the jet, and with the needle being biased towards the engine side, the plastic ring would always take the position of covering up most of the open area on the air filter side of the jet, plus it removed another 0.4mm of deck height from the jet to the needle ----- way leaner mixture

I now need to see if theory matches reality and order a pair of B1Y needles, lower the jet and throw the little plastic rings so far away that I never have to see them again.....

The B1G needle is slightly richer at idle and slightly weaker mid range / full throttle than the B1Y. It would be tricky to tune but I wouldn't have thought it would cause a huge problem. The small plastic ring shows up in a lot of Stromberg diagrams (called a delrin washer) so it's obviously meant to be there. I've no idea what its function is and they're missing on my needles but it's not part of the packaging that's been left behind

The starting point for setting the mixture is to set the needle shoulder level with the bottom of the piston. I've found that mine works best about 0.5- 1mm lower than that and as my jets seem to be 0.7 mm lower than they should be that would make sense. With your jets at 2.00mm you'll probably need to compensate by raising the needle about 1/2 turn and then a bit more for the weaker B1G needle. The characteristics of the B1G are such though that to get them right at full throttle they'll be rich at idle, which might make initial tuning tricky.

Stuart

[carrot]

I too had similar symptoms but on opening the bonnet the carbs were covered in ice from the air flow at high speed, it would happen when ever the atmospheric conditions were similar and more often in winter. I had to stop every 1/2 hour to let them thaw out.! (it was a non standard air box)

[oldelanman]

The small plastic ring shows up in a lot of Stromberg diagrams (called a delrin washer) so it's obviously meant to be there. I've no idea what its function is and they're missing on my needles but it's not part of the packaging that's been left behind

Curious about these plastic rings I made a couple of enquiries. This is what Gower and Lee said.......

"The plastic washers were once fitted on the bias metering needles to stop fuel entering up into the needle carrier. I don't think it's really essential as they haven't been used on these needles for some time."

Still awaiting a reply from the other supplier.

Update: Got a reply today from Burlen ......

Reference to your enquiry, I have checked the parts list but cannot find the part listed in any of our book?s so i am unable to comment on this part.

Leif, looks like you can safely throw yours away and the rest of us can stop worrying because we don't have them!

Regards,

[leifanten]

I removed the delerium washers, and the car runs with normal ignition advance again and is much(!) more powerful. What a joy to drive it is. No ticks, no a rattles, not a beat lost and lots of smooth torque throughout the rev range

I can see how these washers would do no harm if they had been fixed around the needle on the surface of the air valve (i.e moving up and down with the air valve), but mine were actually resting on the jet, thereby restricting fuel flow because of the spring loaded needle pushing into a concentric position to the jet. Effectively blocking off most of the open area of the jet.

It would be interesting to know what their purpose is I can see that they could potentially serve as a soft landing / damper for the return of the air valve, but only if they moved up and down with the air valve. Left on the jet surface it provides an inconsistent and moving object in the way of fuel flow.

I have ordered a set of B1Y jets as well, but as I have learned here and else where the B1G is what came standard on the federal strombergs. And they work like a charm now that the mysterious delerium washers in there.

[oldelanman]

It would be interesting to know what their purpose is

Leif, see my previous post.

Glad you've got it sorted at last.

All the best