Air leak, bad carb, or something else?

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937carrera
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Re: Air leak, bad carb, or something else?

Post by 937carrera »

On the idle I would wind the mixture screw out until it's reading 2-3% on the Gunson, and allow the idle to rise, then bring it down. Set the carb so it's running sweet, your ear will tell you and use the Gunson to just make sure it's not over rich for now.

Those low readings might be indicative of other gases resulting from a weak mixture. Weak mixture = high combustion temperatures.
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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Re: Air leak, bad carb, or something else?

Post by bustrucket »

OK, another round of testing done! With the fan running from startup, the temp gauge stabilised at about 2/3rds while idling.

I managed to borrow a combustion gas detector, checked the expansion tank with it and it didn't show any signs of CO2 in the coolant. Tried it in the coolant tank as well once pressure was off, there was very little air in there but thought it was worth trying - still nothing. I'm 100% sure there are no holes in the heads, even pinholes, but obviously I can't guarantee the compression rings are good.

Managed to get the idle closer to spec, now sits between 900-950rpm when hot and it sounded pretty good to my uneducated ear at least! Vac a little healthier too, 16-18 in/Hg. CO is still low according to the Gunson at this point, between 0.5 and 1%, although I don't entirely trust its accuracy. The only way I could get CO up above that was by lowering the idle down to 700-800rpm and it sounded like it was going to die any second, not healthy at all. And as soon as I wind the idle screw in to compensate, the CO drops away again!

Even though the Gunson says the CO is lower than spec, when I checked the plugs they were black and sooty, as though the mixture was too rich! I half expected this anyway since the motor has really only really been run at idle since it was put in...
Even the plastics gone rusty!

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Re: Air leak, bad carb, or something else?

Post by kevtherev »

Plug colour should only really be checked after a high speed run and shutdown
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937carrera
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Re: Air leak, bad carb, or something else?

Post by 937carrera »

What's the exhaust like, sooty, light brown or very clean ?
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
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Re: Air leak, bad carb, or something else?

Post by NicBeeee »

Is this issue possibly caused by a blocked bleed ring.
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937carrera
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Re: Air leak, bad carb, or something else?

Post by 937carrera »

Time for a little step back I think.

Why did you rebuild the engine - just a refresh, or was there some specific problem.

Is this the same carb you used before, was it working satisfactorily before

And now a couple of dumb questions, recognising you have a decent amount of knowledge, but we all make mistakes. Have you properly calibrated the Gunson meter to 2% in clean air (preferably using a separate battery), and are you sure you've correctly identified the mixture and idle speed screws. There's a great page at https://vwt25.blog/2016/09/15/pierburg- ... justments/ which explains how to rebuild the carb and has base settings on page 10 of the CJH pdf document, it might be worth resetting them.
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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Re: Air leak, bad carb, or something else?

Post by bustrucket »

Don't worry about dumb questions, I'm definitely no expert! Never seen the flat-4 before this van, although I've rebuilt a couple of other engines in the past.

Exhaust is very clean, no visible smoke.

Sadly, the engine I'm using is not the original, the van was a DF from the factory and it was pretty well knackered, so I picked up a non-running DG and rebuilt that. The seller told me it had a compression problem, and sure enough the heads and pistons were pretty chewed up on one side, I think a plug electrode had broken off, and the other side was cracked between valves.

Totally stripped and cleaned the case, replaced the bearings, cam, conrods, pistons, liners, heads, and valves. Ended up the only parts I think I re-used was the case and stuff like the valve gear, cooling system/exhaust, that sort of thing.

Carb - same as above, got it 2nd hand and never saw it working. Stripped and rebuilt with Brickwerks kit. Following the guide, I'm 99% sure it's the right screw I'm adjusting, I can only get at 2 screws, the warm idle and the mixture, the fast idle screw is a bit less accessible and still has the tamperproof cap on it. During the last test I got the idle sounding "best" with the mixture screw close to the suggested 4 - 4.5 turns out setting.

Gunson is set up as per the instructions, calibrated to 2% in air using a separate battery, engine warmed up before sticking the probe in. Meter is used but seems to be in good condition.
Even the plastics gone rusty!

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Re: Air leak, bad carb, or something else?

Post by bustrucket »

NicBeeee wrote:Is this issue possibly caused by a blocked bleed ring.

This van still has the early cooling system, no bleed ring.
Even the plastics gone rusty!

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Re: Air leak, bad carb, or something else?

Post by 937carrera »

My question on the exhaust was badly phrased - I meant the colour of the tailpipe, rather than if it was smoky.

I would reset the mixture to the recommended turns and adjust idle to around 900 rpm, double check the gas composition is something sensible and then force the engine to tickover at 1500 - 2000 rpm. Weight on the accelerator /wedge against the throttle stop should do the job. I would then leave it running for at least 20 minutes and see what happens to the water temperature (fans normal or bridged, your call).

After that I would then check the water levels, see if they have dropped. If they have it could be air lock being cleared, or water being consumed.

Can you validate the Gunson readings against another car ?
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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937carrera
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Re: Air leak, bad carb, or something else?

Post by 937carrera »

Itchyfeet has just reposted his mark up photo of a DG carb

http://forum.club8090.co.uk/viewtopic.php?f=37&t=164578
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

bustrucket
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Re: Air leak, bad carb, or something else?

Post by bustrucket »

Ah OK fair enough, in that case the tailpipe is black, and I am definitely adjusting the correct screw on the carb. I don't have anything else to test the meter against unfortunately, my daily driver is a diesel so that's no use :(

I'll have another go at adjusting it again at the weekend, and I'll do as you suggest to see what the temp does.

Out of curiosity, the Gunson manual says that it's not unusual for the CO levels to fluctuate by 0.5% during idle, would you expect the reading to vary like this or to be more or less steady? Also, the exhaust gas gets pretty hot, hot enough that I can't hold my hand in the flow for more than a few seconds. Does that seem normal?
Even the plastics gone rusty!

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937carrera
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Re: Air leak, bad carb, or something else?

Post by 937carrera »

Where's the head scratching smiley ?

Is the tailpipe sooty solely from your rebuilt engine, or was it sooty before. That is indicative of a rich mixture, as are the black spark plugs, but you've probably been on choke a lot of the time too.

On the other hand hotter than normal exhaust gases are indicative of a weak mixture or ignition timing being retarded

I haven't seen much fluctuation when using my Gunson, but I do tend to leave it for 15-20 minutes before doing the calibration.

I'd also suggest you disconnect the vacuum advance and set the timing at something like 5-7 degrees BTDC, idle plugs connected together. Leave the vacuum pipe disconnected then you are sure the vacuum advance isn't messing with your idle timing. Probably worthwhile whipping the plugs out and cleaning them before the test, then you'll know how they are operating in these new running conditions.

If you want to understand ignition timing better have a read of this http://forum.club8090.co.uk/viewtopic.php?f=37&t=163700
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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Re: Air leak, bad carb, or something else?

Post by bustrucket »

Good point, I should have said it was a new silencer and tailpipe, so there was no soot on it at all before. Do you reckon I'm better setting hot idle and mixture with the vac disconnected then?
Even the plastics gone rusty!

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937carrera
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Re: Air leak, bad carb, or something else?

Post by 937carrera »

bustrucket wrote:Good point, I should have said it was a new silencer and tailpipe, so there was no soot on it at all before. Do you reckon I'm better setting hot idle and mixture with the vac disconnected then?

Yep, the mixture won't be affected but one ignition timing variable eliminated.
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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Re: Air leak, bad carb, or something else?

Post by ajsimmo »

Any update on this thread? If it's solved, what did you find?

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