lpg problems in xy

[Ext User(SB)]

I haven't driven my XY (351 cleveland 650 vac sec holley) for about four
months, and she starts up fine on petrol.
I was in traffic and switched her over plg and ran fine until I took my foot
off the accelerator. She died, and wouldn't start up again without
switching her back to petrol.

So from what I have tested, she won't start on lpg but she'll switch to it
from petrol as long as you've got your foot planted and leave it there. The
second you back off the throttle she dies.

I hadn't had this problem before I parked her.

Obviously it's a lpg supply problem, I've read that the converter can get
clogged and if you pour hot water over it the gum can break up and
eventually flush itself out.

Any thoughts?

[Ext User(John McKenzie)]

SB wrote:
>
> I haven't driven my XY (351 cleveland 650 vac sec holley) for about four
> months, and she starts up fine on petrol.
> I was in traffic and switched her over plg and ran fine until I took my foot
> off the accelerator. She died, and wouldn't start up again without
> switching her back to petrol.
>
> So from what I have tested, she won't start on lpg but she'll switch to it
> from petrol as long as you've got your foot planted and leave it there. The
> second you back off the throttle she dies.
>
> I hadn't had this problem before I parked her.
>
> Obviously it's a lpg supply problem, I've read that the converter can get
> clogged and if you pour hot water over it the gum can break up and
> eventually flush itself out.
>
> Any thoughts?


I'm assuming it's an impco 300a (or a-50) mixer on top of the holley via
a snorkel.

It's possible that either the convertor has accumulated some corrosion
and or the mixer diaphragm has degraded (they can both stretch, and or
rupture, theoretically) or it's just sticking in the closed position.

Either of these would lead to the possibility of either a very rich
(stuck mixer air valve, I think is the correct term - I could point to
the item in a heartbeat) or very lean condition at idle, and the
complication (or complete impossibility) when starting.

Both of which could be tended to (afaik rebuild kits are available for
both the mixer and convertor if required) but as to what to look for and
how to do it, it's not something I'd confidently be able to explain in
text adequately. It might _not_ need any bits from a rebuild kit, it'd
be a case of inspecting it and moving forward from there.

I'd also suggest that if it currently doesn't have hei ignition, and one
can be adapted (didn't the last of the clevelands have hei bosch dizzy
from the factory?) it'd be a good start. due to a couple of
idiosynchrasies of lpg metering in general, the mixture homogenity at
idle is slightly inferior to 99% of petrol carbies. Meaning a stronger
spark goes a long way, and conversely that a borderline spark (say the
points are in need of replacing) will give grief starting and idling on
lpg _well_ before it even shows any hint of trouble on petrol.

In your shoes, I'd go the distributor first and foremost (or at the very
least fit new points, check the plugs, plug leads, cap coil and rotor
and replace as necessary) - as there's no downside.

I'd then take stock of what it's doing (or not doing) with an ignition
system in good working order. Next I'd look at the mixer and lastly the
convertor. Basically that's it in order of importance/likelihood.

I'd also double check the air cleaner attachment (perhaps do this first
since it's dirt cheap) - and that the stud going into the centre of the
carb holding the snorkel on there is ok. sometimes they strip the
threads out, and any loose fit of the snorkel is an air leak that then
prevents enough signal strength getting to the mixer (and then to the
convertor) to initiate flow at starting and maintaining it at idle. If
that were the case, you'd need to look at repairing the thread (or going
one size up, or whatever needs to be done).

Other than that the only thing which occasionally comes up is the choke.
If it has a choke fitted, esp a manual choke, NO MATTER WHAT do _not_
use it to try and start on lpg. It only 'works' on a petrol carb. It's
location on the petrol carb relative to the lpg mixer means the choke
plate will do nothing good to help starting. In fact if anything, it
will obstruct flow/signal and work against it starting and idling on
lpg.



--
John McKenzie

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