Back Fire Now Won't Start

[d2dave]

I have a friend who has an old 88 Classic. It has backfired and now won't start.

Things I have checked.
It has gas to convertor
Power is at cut off solenoid for a few seconds when ignition is switched on
Power is also there during cranking.

I believe that the problem is in the mixer. I have removed it and all it has is a rubber diaphragm which appeared to be ok.

The mixer is a copy of an Impco 200 like this.

Genuine Impco Model 200 Clean AIR OR Feed Back GAS Mixer | eBay (http://www.ebay.com.au/itm/GENUINE-IMPCO-MODEL-200-CLEAN-AIR-OR-FEED-BACK-GAS-MIXER-/251548107972?pt=AU_Car_Parts_Accessories&hash=item3a916f84c4)

The diaphragm is the same as this

Impco AIR Valve With Sicome Diaphragm Suit 125 225 200 Mixer C A OR F B | eBay (http://www.ebay.com.au/itm/IMPCO-AIR-VALVE-WITH-SICOME-DIAPHRAGM-SUIT-125-225-200-MIXER-C-A-OR-F-B-/251550820510?pt=AU_Car_Parts_Accessories&hash=item3a9198e89e)


When I blow into the gas inlet of the mixer it is like it is blocked. I even put a rag in the hole and then 130psi with compressor and it still would not let air through.

If I give the convertor a good prime and then lift the diaphragm with my finger it will start but I cannot keep it running like this.

When on the car , after a couple of ignition on/off cycles it is building up pressure as I cannot press the primer valve. If I lift mixer diaphragm it releases the pressure and I can the push the convertor primer.

Help please.

As a side note, what is the difference between a clean air and a feed back mixer?

[Scouse]

An '88 has a flapper type air flow meter. A backfire can bend/jam the flap inside resulting in no air flow to the motor.

[bee utey]

A backfire can warp the 5 screw cover plate leading to a loss of vacuum behind the diaphragm and therefore no go. Check it for flatness, also inspect the diaphragm very carefully for fine tears especially around the gasket edge.

The behaviour of the gas pressure and priming button is correct, that's the way they work. It just means the valve seat of the gas diaphragm is sealing correctly.

PM me if you want a spare 200 mixer, I tend to remove them as they are little better than corks in the intake IMHO ;). There are better gas systems.

Oh and if the flapper AFM is undamaged it will run on petrol with the mixer diaphragm removed and the cover replaced.

[d2dave]

This vehicle is gas only, so petrol not applicable.

The cover plate has only four screws not five. Is it then normal that I should not be able to blow into the valve?

Does it rely on engine vacuum during start up to lift the valve and let gas in?

If this being case this would mean no gas can get into engine just from priming. Yes?

Also in my OP I did ask the difference between a clean air and a feedback mixer.

[bee utey]

This vehicle is gas only, so petrol not applicable.
So you have no AFM to bend, good.

The cover plate has only four screws not five. Is it then normal that I should not be able to blow into the valve?
Never seen a 4 screw version, your ebay link shows the 5 screw version. You can't blow into it when the valve is closed. This is to prevent LPG dribbling into the intake after shut down and flooding the engine.

Does it rely on engine vacuum during start up to lift the valve and let gas in?
Yes

If this being case this would mean no gas can get into engine just from priming. Yes?
The button you press on the converter isn't really much use as a primer, a straight gas engine usually has a manual primer consisting of a petrol solenoid screwed to the test pressure port on the converter. It's found between the water connections on a model L converter or similar. You plumb it to the inlet side of the mixer.

Also in my OP I did ask the difference between a clean air and a feedback mixer.
Dunno, I'm not an Impco expert, I suspect the feedback version has an extra port for the vacuum control for connecting a fuel processor to.

[d2dave]

Never seen a 4 screw version, your ebay link shows the 5 screw version. plumb it to the inlet side of the mixer.

It is a five screw. I was thinking of the screws that hold the diaphragm to the bit in the centre (gas inlet valve?). It has four screws.

Thanks for your help.

[d2dave]

So having another look at it there are 4 small holes in the centre of the diaphragm. I assume these are for engine vacuum to pass through to create suction on the other side of the diaphragm to lift it, to then allow gas to flow to engine. Is this correct?

[bee utey]

So having another look at it there are 4 small holes in the centre of the diaphragm. I assume these are for engine vacuum to pass through to create suction on the other side of the diaphragm to lift it, to then allow gas to flow to engine. Is this correct?
yup

[d2dave]

I found the problem. A friend of mine had a mixer off a commodore which was working when removed from vehicle. It was not the correct configuration to be able to use it, but it could be fitted for testing purposes.

After fitting it the vehicle still would not start. It was pressurizing the convertor just like the original was doing. This told me that the diaphragm was not lifting to let in gas.

I then figured there must be a vacuum issue with the motor so removed the mixer and put my hand over the manifold inlet. Upon cranking there was not suction.

I then went investigating and bugger me, around the other side of the plenum box there was a hose, about half inch diameter had blown off.

Refitted and car is now all good.

Thanks everyone for your help.

[d2dave]

Still not fixed. On a cold start today it would not go. I primed and manually lifted the diaphragm and got it going. Once running it was ok and then when switched off it would re start.

I figure that there is not enough vacuum at cranking to lift the diaphragm.

When running it has around 16 inhg at idle which is a tad low. Cranking produces about 5 inhg. How much should a motor have when cranking?

[bee utey]

Still not fixed. On a cold start today it would not go. I primed and manually lifted the diaphragm and got it going. Once running it was ok and then when switched off it would re start.

I figure that there is not enough vacuum at cranking to lift the diaphragm.

When running it has around 16 inhg at idle which is a tad low. Cranking produces about 5 inhg. How much should a motor have when cranking?

1. Fit a proper primer with a button inside.

2. Richen the idle slightly. The model 200 mixer idle screw is an air bypass so screwing it in richens the mixture. This also increases the vacuum to the diaphragm.

3. Use a small amount of throttle for starting. About 1" down on the pedal once cranking has commenced.

4. Make sure the timing is set at 10 degrees BTDC.

5. Set the plug gaps at 0.7mm

6. Make sure you've identified all the vacuum hoses and ensure they are properly fitted.

7. Fit a bigger & better battery and/or starter motor. Early Lucas starters are too slow compared to a good Bosch one.

[d2dave]

Is 5 inhg enough vacuum at cranking? I did a bit of googling and one site I came across said that 3 inhg is ok.

[d2dave]

1. Fit a proper primer with a button inside

This would certainly do the trick. How do I go about it? Switch and wiring in to the car I am ok with, it is just the procedure at the gas end.

[bee utey]

This would certainly do the trick. How do I go about it? Switch and wiring in to the car I am ok with, it is just the procedure at the gas end.

as previously posted by me


a straight gas engine usually has a manual primer consisting of a petrol solenoid screwed to the test pressure port on the converter. It's found between the water connections on a model L converter or similar. You plumb it to the inlet side of the mixer.

as to your other q


Is 5 inhg enough vacuum at cranking? I did a bit of googling and one site I came across said that 3 inhg is ok.

Wouldn't have a clue, however anything that raises the cranking speed will obviously raise the vacuum reading.