How to tell DR 750 and 800 engine apart?

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How to tell DR 750 and 800 engine apart?

Postby FRQ » Thu Nov 16, 2017 7:24 pm

After buying an SR42 and being bitten by the DR bug about a year ago, I just purchased three project bikes.

All have an SR41 frame and manual decomp levers, but the seller said one of them had an 800 engine.

Is there a surefire way to determine whether an engine is a 750 or 800? Differences in the casting, exhaust/inlet port diameter/engine code stamp, something like that?

I take it the SR43 has a physically different auto decomp system? Would that be a good place to start or does the manual setup just bolt on? What about the SR42?
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Re: How to tell DR 750 and 800 engine apart?

Postby nug » Thu Nov 16, 2017 10:39 pm

It will say the capacity on the barrel. 727cc or 779cc.
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Re: How to tell DR 750 and 800 engine apart?

Postby FRQ » Thu Nov 16, 2017 11:01 pm

That's... so simple I feel somewhat stupid. :D Any idea where these markings are located exactly on the barrel? Some of the engines are covered in multiple layers of paint and dirt. Worst case scenario I'll have to do some pressure washer archaeology.
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Re: How to tell DR 750 and 800 engine apart?

Postby O.R. » Fri Nov 17, 2017 9:49 am

You're right when looking @ the barrel from a *stock* engine.

Be aware that some engines are not stock. Found that out myself some years ago. Then I bought a SR42 (800cc). When repainting the clutch cover, I found orange paint of the DR750 (SR41) while the magneto cover origined from a SR43 (800cc). A fellow bigger around here, uses both short stroke crankshafts (DR750) and long stroke crankshafts (DR800) in his engines for the specific engine behaviour. Engine number reveals only the secret of original displacement. Concerning the manual decomp levers....my actual bike (SR43) has a manual decomp lever although it used to have an solenoid (engine safety mod).

What I'm saying is: several parts of SR41-SR42-SR43 are interchangable and the only way to determane the piston displacement, is to measure both stroke and bore.
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Re: How to tell DR 750 and 800 engine apart?

Postby dave#22 » Fri Nov 17, 2017 9:51 am

............I take it the SR43 has a physically different auto decomp system? Would that be a good place to start or does the manual setup just bolt on? What about the SR42?........

Apart from the capacity indication on the RH barrel base, the SR43 does not have the drilled hole on the rocker cover for the decomp bracket and the LH crankcase cover has a hump which contains a friction drive unit to protect the starter gears....and a few internal mods as well.
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Re: How to tell DR 750 and 800 engine apart?

Postby O.R. » Fri Nov 17, 2017 7:59 pm

dave#22 wrote:............I take it the SR43 has a physically different auto decomp system?


The decomp system is basicly a decomp shaft, decomp shaft lever and actuator.
1.png

The decomp shaft (part #11) has a "flat" side that most of the time does not have contact with the rocker arm (#6) of the EX valves.
The decomp shaft lever (#13) is attatched to the shaft with a nut. When the shaft turns, the "cylindrical" part of the shaft pushes the EX valves open a little bit.
That's why the compression is a lot smaller, and the engine turns a lot faster during starting. When the camshaft (#1) operates the EX valves, the decomp shaft turns back because of the spring (#14) and so the compression will become normal again.

The actuator can be manual or electrical. The SR41/42 has a handle lever on the handle bar, a bowden cable and a cable support on the rocker cover. The SR43 has a electrical solenoid that is mounted directly on the frame. That's why the SR43 does not have the drilled hole on the rocker cover.

A picture of a SR43 magneto cover:
SR43SG.jpg
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Re: How to tell DR 750 and 800 engine apart?

Postby nug » Sun Nov 19, 2017 12:53 am

Many earlier models had the later SR43 type covers replaced after a failure so be aware of that.
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