Alternative to a Amal 397 carb to fit howard 810cc engine.

[vhgmcbuddy]

Hi guys,
First post for me even though I’ve enjoyed learning from others on here for a while now. As I’ve hit my mid forties I’ve found a love of old agricultural machinery, so it’s good to see I’m not alone in this!! I have two rotavators(Norlett and howard) and an Oxford Allen Scythe, all were rather bedraggled examples that had seen better days.
So onto my question……
I have a howard Gem with the howard 810cc 360 degrees engine, it was laid up for 30 years so has needed considerable tlc to get it going.
It is going now intermittently and when going it is prooving to be a great, strong engine. The mag/ spark/ timing are all set and working well. The tank has been cleaned and new filter/fuel lines added.

It has an Amal 397 carb and even though we’ve replaced all the available parts I can from the Amal website, it’s still proving hard to get it to perform well enough.

My question is, does anyone know of an alternative carb to a amal 397 that I could use on this engine?
I have looked but cannot see which others would fit, a sticking point is finding one that has vertical mounting holes to the engines inlet manifold.

Any help is greatly appreciated!
Thank
Brett.

[andyfrost]

The first port of call for me would be to have your magneto properly tested , a wise old saying “90% of fuel problems are ignition related”.

Andy.

[vhgmcbuddy]

Thanks Andy. Yes agreed, I have already sent it to a local light engineering company who specialise in refurbing old machinery similar to this. They have stripped the magneto and set the timing etc to make sure I hadn’t made a mistake, so they are suggesting I find an alternative carb.

[andyfrost]

Brett , have they actually tested it , stripping and re-assembling is pointless if the coil or condenser is failing.

Andy.

[hortiman]

It is really important to make sure that all the jets and airways are totally clear on that carb. Really thorough clean with carb cleaner, then carefully blow out all airways etc. Things can get gummed up after such a long period of non-use.

[trusty-bloke]

I am not familiar with the 397 (so apologies), but from experience with other carbs a worn butterfly spindle can cause additional air leaking around the butterfly and upsetting the mixture such that the engine will rev and the tickover screw becomes redundant.
If you grasp the shaft and see what movement you have up/down/side/side ..etc..
That could a problem …

My experience is with 24T2 Zenith Carbs ..
Where the brass spindle wears more than the ally carb body …

[trusty220]

I’ve found that these carbs do block up readily, probably because they have so many fine drillings in so many different places it doesn’t take much in the way of debris to start a blockage.

Trouble spots are usually confined to the brass main jet and holder which needs to be removed and thoroughly cleaned, including the cross-drillings in the holder, and also the slow-running jets which are drilled into the inside of the venturi of the carburettor alongside the butterfly; these drillings give a progressive feed of fuel to the engine when the throttle opens. If you don’t clean these out you will find that it may not tick-over at all and it will hesitate when the throttle opens. There is a series of galleries on the outside of the carb body behind the slow running mixture screw that will need to be cleaned and the drillings for the slow running jets can be accessed by removing blanking screws that cross this gallery.

Best of luck with it.

[vhgmcbuddy]

Hi All,
Thank you so much for all your replies, i really appreciate them.
Yes, the mag was properly tested once stripped, i had already put in new points and condenser and they have now also bench tested the coil. The spark is now ‘superb’.

These points on the carb airways are where are kind of getting stuck at i suspect, the carb has been ultrasonically cleaned already, it’s coming off the machine again though to double check these airways as there seems to be a theme with these carbs.
I’ll let you guys know how we get on once it goes back on he machine.

Thanks for your informative help so far.

Brett

[trusty220]

We use an ultrasonic cleaner in our workshop but we find that it isn’t foolproof. By all means use it but more often than not we find that we have to clean the carbs the old fashioned way with plenty of clean petrol on hand, an airline and even a small piece of soft fuse wire to poke the holes out in extreme cases.

Don’t think I’m advising you to poke wire into the holes because that’s the easiest way to enlarge them, then it won’t work at all! Better to try all other methods and if all else fails try a piece of wire, but make sure it’s small diameter soft wire that won’t affect the hole size. Certainly don’t use gas nozzle reamers like I’ve seen someone do years ago- the carb was scrap after he’d “cleaned” it!

You may want to have a look at the section that I did on rebuilding a Norton Big Four engine on a Trusty Steed. There are some pictures on there of the carburettor which you may find useful.

Best of luck with it, it sounds like you’re nearly there.

[vhgmcbuddy]

Hello again, thought I’d give you an update. I have the Gem back from the mechanics now. He had done a great job, it fired up first turn from cold, then we let it settle down (had to adjust the carb again) and tested it out in a nearby field, what a beast of a rotavator!

He did warn me that he was still not happy with the mixture, it’s either good at idle or good under load, there’s no real sweet spot that I can leave it at, so I should expect to have to play around with the mixture a bit whenever I use it to get the most out of it. Not ideal.

So I got it home and gave it a go again.
It wasn’t running so smoothly this time. I set the mixture so it ran a smoothly s possible and started turning over some ground and it started hunting and nearly stalling
, the choke was opening and closing on it’s own while this was happening, then it started backfiring quite badly and died on me.
It’s stuck in a field now and won’t start at all….it’s a very expensive gren ornament if I can’t get it started, the Joy’s of old machinery!!

Any further ideas?

Thanks

Brett.

[vhgmcbuddy]

Oh and trusty2020 I forgot to say, the ultrasonic cleaning was not foolproof as you said, soft fuse wire was needed to clear a blockage. Thanks again.

[charlie]

If the choke is operating on its own it sounds as if it is loose, wear around spindle could be allowing air to be sucked in as would wear around throttle spindle. This will alter mixture and make it impossibe to set it correctly.

[trusty220]

The tuning of the carburettor has to be done in sequence and somebody that isn’t used to these carbs may not appreciate that.

Firstly you want to screw in the main jet until you feel it stop, then turn it out one turn. Do the same with the slow-running air screw on the side of the barrel. That should be sufficient to get it running.

When it’s running, set the throttle on a fast idle (around 1500 revs) so that the majority of the fuel that the engine’s running on is going through the main jet. Then wind the main jet out until the engine starts to run rough; turn it back in again until it runs rough again, then turn it out half the distance between these limits. This should then be the ideal setting for the main jet.

For tick-over you want to close the throttle and repeat the process with the airscrew on the side of the barrel, adjusting the slow-running screw on the throttle spindle to achieve a sensible tick-over speed.

Best of luck- as Charlie says, if the choke is opening and closing by itself you really want to stop it. It’s not like a car choke, it is purely a cold-starting aid to richen the mixture and the engine will not run with the choke on when it’s warm.

[vhgmcbuddy]

Thanks a lot guys, all great advice, i’ll try this all and will keep you updated…..with good or bad news!
Thanks
Brett

[vhgmcbuddy]

the Service Facts book 1969 lists 3 different carbs on the inline twin engines, you have the ‘latest and greatest’ so i would not be in a hurry to find a different one… however if you happen to have one of the others close handy then it might be worth a try. cheers Rod.

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