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| T O P I C R E V I E W | | EdinburghJoe | Posted - 14 Sep 2023 : 10:22:11
Morning all,
I've had my 96 (with FoMoCo carb) since 2016, and in the early days had a recurring problem with an air leak (engine wouldn't idle, really 'lumpy' when cruising, but totally fine when pulling hard). I've fixed this twice by taking off the carb and intermediate plates, cleaning everything, making a new cork gasket and reseating everything with hylomar blue.
It has been fine for several years now, but I had something similar crop up again last week. It wasn't as serious as before (previously the car had been undriveable), but I think remaking this joint would be a good idea.
I've read in a few places that the carb base plate can get distorted making it hard to make a good seal. This is never something I've checked before, so I wanted to ask:
1) I've read that you can check the 'flatness' of the base plate with a square of glass, which I have. I'd intend smearing it with oil and pressing it against the base plate and look for gaps. Sound OK?
2) If it isn't flat, I have wet dry sandpaper, but how to use it? My guess would be lie the paper on a flat surface, wet it, rub the carb base against it for a bit, then recheck against the glass. Reason for doing it this way is to reduce the chance of debris falling into the carb if it was inverted.
That's the best plan I can come up with but sounds a bit agricultural. I'm concerned that I'll introduce a load of metallic debris into the carb which will create more problems. Does anyone have any advice?
Thanks,
Joe
| | 11 L A T E S T R E P L I E S (Newest First) | | melle | Posted - 22 Sep 2023 : 09:07:42
The metal gasket between the manifold and the vacuum intermediate plate was factory fit, it should have a compressible ring that acts as a seal, and no other gasket should be required here. If no longer usable, a paper gasket should suffice. There is a paper gasket between the vac plate and the carb base, 0.5mm should do just fine for both gaskets, if you need thicker ones and/ or sealant to get things to seal there is an issue that needs sorting first.
www.saabv4.com | | EdinburghJoe | Posted - 21 Sep 2023 : 17:44:05
Thanks, I'll order some 1mm paper and next time I have a problem I'll use it and see how that works out.
But I'm pretty sure I've read somewhere on this forum a recommendation to use one thick cork gasket to reduce heat transfer off the top of the engine (and the other one thinner). Have I just mistaken this?.
And what about that thin metal gasket I mentioned that sits between the top of the engine and the vacuum take-off plate? I've never seen the point of that (in fact it always looked a potential problem to me because it's metal-on-metal contact with the top of the manifold). Any advice on that? Should it not be there? | | melle | Posted - 21 Sep 2023 : 14:26:17
Yes, Stanley knife, template on my website if required: http://www.saabv4.com/index.php/fomoco/
www.saabv4.com | | EdinburghJoe | Posted - 21 Sep 2023 : 12:54:51
I guess that could be the reason it works for a few years and then develops another leak.
Maybe next time I'll just cut new gaskets from gasket paper. They're a simple construction so I'd imagine just cutting one from a template with a stanley knife? Does that sound sensible? | | christophe | Posted - 18 Sep 2023 : 11:39:04
You don't have to use any cork or rubber there. They're too soft for the job. Just google fomoco 1v carb repair kit to have a look at the correct seal. | | EdinburghJoe | Posted - 18 Sep 2023 : 10:24:15
Thanks all for the suggestions. I took the carb off and as Derek suggested I used a steel rule and found the base plate to be perfectly flat, so I didn't need to do any sanding. Instead I re-did what I've done twice before (last time about 3 or 4 years ago, so it did work reasonably well), which was to clean everything, make a new gasket, and re-assemble with hylomar blue. I've not done a proper test drive yet (too wet), but it did start and would idle when warm, so promising.
I wanted to ask if my set up sounds right: taking it from the top of the inlet manifold upwards, I have:
- a thin metal gasket (never been sure about this, seems a bit pointless)
- a cork gasket - this is what I remake each time from 3mm gasket cork (I've read it's good to have a thick cork layer to reduce heat conduction)
- the vacuum take-off plate (for brake servo and PCV valve)
- a thin rubber gasket (needs to be thin - if I made it from 3mm cork then I think the cork would bulge into the narrow channels within the baseplate and obstruct them)
- the carb baseplate (it's the original FoMoCo carb)
Is this the right parts in the right order? Is there anything that seems wrong?
Joe | | stevebod | Posted - 15 Sep 2023 : 10:01:53
Another idea.
Scribble on the face you are working on with a permanent marker. Then lap the surface like you suggest, until all the marker has been removed.
The idea is that the marker shows any low spots in the surface.
I would use a piece of glass/mirror for the lapping process as well. Glue the grit paper down with paper glue or tape round the edge.
A figure-of-eight lapping pattern is usually recommended for the most even finish. | | Fuely | Posted - 14 Sep 2023 : 20:20:18
Ideally put the sheet of 60/80grit carborundum paper (dry) on either a sheet of glass or a surface plate and rub the flange in a circle of eight configuration. The low and high spots will be revealed pretty quickly.
P | | christophe | Posted - 14 Sep 2023 : 13:09:46
I tried that. It is very difficult to stay perfectly horizontal when rubbing the base of the carb on the sand paper. If I had to do it again, I would put it upside down on a perfectly flat and level surface, then use a standing pad very gently while checking regularly that the base stays parallel. Don't forget that for a slight distortion, à more compilant or thicker gasket might do the trick. | | Derek | Posted - 14 Sep 2023 : 13:04:27
A steel rule can be used as a quick check. An old thick mirror is fine for a sheet of glass. Should be obvious if the carb base is bowed. Do it dry. You can check with a small torch. Feeler gauges can be used for a basic yes/no answer. Use a new sheet of wet 'n dry wet on top of the mirror. A few rubs of the carb should show if there are any high spots/areas. If checking the carb nuts has been a habit you may have bowed the base so the ends with the holes will show up shiny. Carry on until flat. With the carb vertical you won't get any metal dust anywhere but you can wash the paper now and then. Clean down with carb cleaner afterwards. | | EdinburghJoe | Posted - 14 Sep 2023 : 12:41:51
Just been googling about this and saw somewhere the idea of using feeler gauges against the glass to look for gaps. Sounds sensible, but the rest of my question still remains... |
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