503 Engine Questons

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503 Engine Questons

Postby ed_hall1 » Sat Dec 15, 2018 9:44 am

I have a 502, but years ago after my dad passed I found a 503 engine with the electric start lying around in his garage. No carburetor but overall it appears to be in good condition and the starter relay even clicked when I applied voltage. Here are my questions.

1. Can I mount the 503 in my 502? That will allow me to restore the 503 and then drop in when ready.
2. What are the specs on the 503? Compression, timing, plug and point gaps, etc?
3. I do not have the carburetor for the 503. Is it interchangeable with the 502? If not where can I get a carburetor?
4. I did get 65psi of compression but just turning the engine by hand. I suspect it will do better once I get the starter working and can spin the engine faster. Does that sound correct?

All help and suggestions would be great. We received 14" of snow in Virginia this past weekend and has got me in the mood once again.

Thank you.
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Re: 503 Engine Questons

Postby ed_hall1 » Sun Dec 16, 2018 2:01 pm

Update. I got the starter to freely spin and am now getting 190lbs of compression. Spark is there but minimal.
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Re: 503 Engine Questons

Postby seabeeman » Sun Dec 16, 2018 8:50 pm

1 - I kind of doubt it. The 503 had a bigger engine and a third engine mount. You can make anything work, but it isn't a direct drop in. 502 had a Hirth 54R and the 503 had a Hirth 191 with electric start.

2-3 - Carbs might be, they sure look alike. I'd contact Bruce through the site and see what he says, he can also help you with specs.

4 - You won't get full compression without pulling it or spinning it, but I'd have thought you'd get more than that. If the 191 is anything like the 54R, they are beautifully simple engines, Even if compression is low, a refresh is no big deal.

I'm jealous of your snow. We were at nearly 40 the last few days and the only snow is on the trails in the woods. I'm getting projects done I thought I'd be trudging through snow to do, but I'd rather be riding.
Last edited by seabeeman on Mon Dec 17, 2018 9:56 am, edited 1 time in total.
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Re: 503 Engine Questons

Postby Schoobytwo (Bruce) » Sun Dec 16, 2018 9:38 pm

Hello Ed,

I can try and answer some of your questions and add to what seabeeman has to offer. First off, I suggest to jump over to the topic below and scroll through the couple of pages there. This is the post were we turned a 502 into a 503 and it might answer some of your questions. You should look close at what motor you have. The Diablo 503 used a Hirth 190R. It is special like the motor that is in your 502 in that the taper output shaft is unusually small. Like seebeeman said though, a much more common motor used in other sleds is a 191R. They are very similar. If you are using a 191R, the clutch off your 502 won't fit.


More specific to your questions:

1 - You can mount the motor with some modifications as noted in the topic above. The top and bottom motor mounts will be different, need to add that 3rd bottom motor mount along with the bumper and need to modify the handlebars. I have motor mounts here if you need them.

2 - The 190R is basically the same in size as the 54R in your 502 - 75mm piston, 10.2:1 compression ratio and 16 HP. The static timing is 4 to 7 degrees BTDC and I use Champion D-14 plugs gapped at 0.020" and gap the points at 0.016".

3 - All model Diablos regardless of the motor used a Tillotson HR3A or HR14A. They are the same carb just made in different years.

4 - It sounds like you have good compression, but I would suggest replacing the seals and gaskets in the motor. If the crank seals are original, its a good bet they will leak and leaky crank seals just gives you problems long term trying to get the motor to run right and sets you up for a motor melt down when it runs hot.

Hope that helps,


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Re: 503 Engine Questons

Postby ed_hall1 » Mon Dec 17, 2018 4:02 pm

Here some pictures of what I have. I did confirm that the engine is a 190R3E. It does have the two motor mounts on the engine, so I am unsure of where or what the third motor mount looks like. If anyone has a picture that would be helpful.

Seabeeman: The snow is gone already. In Virginia it last only a week at best this time of the year. 50degees here today.

Bruce: I am putting together a list of parts. Will give you a call sometime this week.

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