Old school technology - wierd issue

[DodgeRover]

I'm running a transit with a 2l pinto. I have a BLOS gas carb with cone filter as the mixer and a Bigas M84 vapouriser. I'm trying to tune roughly by ear and using a narrow band lambda and a cheapy chinese air/ fuel gauge.
Most of the time it will run fine with no issues, however on the motorway it will always show lean at cruise (55mph, 3500rpm) no matter how I have things set - if I get to a hill on very light throttle it will pink - last time it did it i noticed for a fraction of a second it read rich before going lean again.
No amount of playing with the ignition timing seems to make a difference - I've changed the distributor but its made no difference, what has helped slightly all round is disconnecting the vacuum advance.
The BLOS equivalent of the mixture screw is set almost fully out as is the idle mixture adjustment screw on the blos. Idle bypass screw on the mixer is fully shut.

I'm wondering if I've got some sort of ram air effect going on which is messing things up - however doing some trial runs in 2nd and 3rd if I wind it up it will suddenly go lean at about 3500 - 4000 rpm altering the vapouriser doesn't change this?

[DodgeRover]

Thinking logically, could I have a partially collapsed feed pipe somewhere, I'm running twin tanks connected via a non return valve

[LPGC]

The temporary rich mixture isn't being caused by some factor such as remaining fuel in the carb's acceleration/throttle pump is it?

Were both LPG tanks full during the tests?

[DodgeRover]

The temporary rich mixture isn't being caused by some factor such as remaining fuel in the carb's acceleration/throttle pump is it?

Were both LPG tanks full during the tests?

Both tanks are full, definitely not dual fuelling on petrol& lpg at the same time. Took the feed pipe off the vapouriser and powered up the gas system and got a good blast out so I don't think there's a restriction in the feed.

Could it be the cone filter that's not providing enough restriction to get the BLOS to open easily at lower throttle openings? I might try putting a few turns of duct tape around it tomorrow to see if it improves things.

Fuel economy is pretty poor around 15mpg on lpg - 18 on petrol

[DodgeRover]

Duct taping 1/3 of the filter doesn't appear to have made any difference.

[LPGC]

Could still be not enough restriction with the cone filter, not sure if the Blos would be any less sensitive to that than a standard mixer. Usually too little restriction from the air filter would manifest as not being able to get rich enough mixture at idle. Could be ram air.

Why have you been playing with ignition timing to try to get around a mixture problem? If you get pinking on gas, would expect either a very lean mixture on gas or for timing to be so far advanced it would pink a lot more on petrol.

Could even be a faulty reducer that won't supply enough gas at the high load end (known it before) but before going that route it's worth going over the basics again - all the reducer / Blos settings (but keep the idle bypass screw on the reducer closed) and making sure the reducer stays hot when cruising on the motorway...

When you're just driving around in (say) town, have a feel of the temp of the tank and reducer solenoid and check mixture, then check mixture when you start driving on the motorway, then check mixture when you've been driving for some time on the motorway (has it gone leaner) and check tank temp / reducer solenoid temp again. Is there some connection between tank temp and mixture or is mixture always lean at motorway speeds? What's mixture like at low speed but working the engine hard going up hill or riding the brakes for a short time (similar amount of work and rpm to motorway cruising)?

Simon

[DodgeRover]

Hi Simon, thanks for the help, the manual with the BLOS recommended fitting a cone filter, I thought it was odd as it's the opposite of what I would normally think.
It had run fine previously for a good year after fitting the new vapouriser without the pinking issue so I thought it was timing related or a massive air leak that was appearing randomly when up to temperature on the motorway.
Fitted A/F gauge trying to figure out what was going on. Seeing it going rich when pinking threw me as I always thought too much advance or too lean hence changing the distributor and disconnecting vac advance.

Anyway had a fiddle after work today, screwed the manual shut off valves in then back out on the multivalves,
Screwed in the main adjuster on the vapouriser then out then back to it's starting position 4 turns out.

It revved out to above 5000rpm in first without leaning out, however once I got going through the gears it was back to going lean at above 3500rpm BUT after about 10 miles of steady dual carriageway driving where it was indicating rich I hit the motorway and it's now not going lean and not pinking. I have no idea what I've done. I've got the same journey tomorrow so I'll find out if it's miraculously cured itself.

I stopped tonight after a roundabout on the dual carriageway section to check if it was icing the vapouriser as that thought randomly occurred to me but it seemed a reasonable temperature, there was no frost visible on anything.

Morning edit no it's just the same as before showing lean on cruise this morning.

[LPGC]

It was reading as mostly good until I got to the last line lol!

What I was getting at (above) is that tank/gas/line temperature has the potential for adjusting mixture a bit on an open loop mixer system, and reducer temp does. See if there's some correlation between any of those and changes in mixture, or if the mixture problem is there all along. Maybe try going about things the opposite way - set mixture for motorway conditions, see if remains OK for motorway conditions and then see if mixture remains OK for none motorway conditions, especially if in the past you've fired the car up and tried setting mixture when the engine hasn't been running for long. I know you said you'd adjusted the Blos mixture screw to the max, there's been no mention of the reducer sensitivity screw though, try adjusting that to a more signal sensitive (richer running) position.

Simon

[DodgeRover]

Update time, screwed the adjuster on the vapouriser out until I began to see an o ring then turned it back half a turn, result is the lambda is now showing between 14 & 15 at cruise on the motorway and no pinking.
No idea why it should suddenly need to be that far out though, it was 4 turns out previously and set up on the mot emissions tester machine.
It's showing rich at lower rpms though I'm not really worried about it - although the mot man might be!

[Gilbertd]

If you've not seen it before, have a read of this http://www.diy-lpg.co.uk/articles/files ... -loop.html. It may not be completely the same as you have the Blos in there but it should give you an idea of what is happening and what the various tweaks do.

[LPGC]

As Gilbert says...
Probably a bit different with the Blosw though, since the Blos has it's own idle screw and it's now running current mix cruising but rich near idle you might want to adjust the Blos idle screw.

[DodgeRover]

More strangeness, it's run fine if richly for 3 days, it ran low on gas 100 miles early and started with the going weak symptoms. Refuelled and its running weak again above 3k rpm. I'm going to clean the last gas cut off solenoid and see if I can check the one on the vapouriser this weekend.

Not looking forwards to my hands stinking of gas for the next few days.

[Gilbertd]

Latex gloves, £4.99 for a pack of 100 at my local motor factors......

[classicswede]

Have you cleaned the heavy ends out of the vapouriser?

[DodgeRover]

Bit of a delay in following up, yes have had the cover off or plug out (can't remember which) to drain the heavy ends, there were only a few drops.

Both gas solenoids at the front end appear to be behaving normally.

However I think I might have found the problem there is an O ring seal on the gas pipe elbow connector that comes out of the top of the BLOS. I had previously dismissed this as I had gone round checking for air leaks with a can of carb cleaner while it was running and found no difference in engine note when I sprayed it. Out of desperation I smeared grease round this last night not really expecting any difference. It's been fine today even had it upto the heady heights of 65mph on the motorway with no sign of it suddenly leaning out and it's still showing rich on full throttle winding it up as much as I dare through the gears.

Will report back in a few more days if it's is still behaving!