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Discussion Starter #1
Hello

I have a Suzuki SC100GX with a 970cc F10A engine and a 4 speed manual gear box, which has some trouble getting up to speed.

It has trouble getting past 110 km/h and only sometimes reaches 120 km/h.
It easily goes to 6000 rpm in first gear, has trouble getting there in second, stops at 5000 rpm in third and reaches 3500/4000 rpm in forth gear. It is just impossible to go faster. During shifting it somehow feels like you get a push in your back when you shift up.

I've completely renewed all the ignition parts (spark plugs, cables, rotor, electronic ignition timing and ignition coil) checked the timing which is at 10° and goes above 20° (last shown number on the flywheel) so I know the centrifugal advancer works.

I've also installed a wideband lambda to check the air fuel ratio, which is between 11 and 12 during full throttle. This is a rich mixture which should indicate that the engine is getting it's fuel.

The compression of the engine is 145 psi (10 bar) for three cylinders and 122 psi (8 bar) for one cylinder. I found in the service manual that the pressure should be 199 psi (14 bar) for a engine in new condition. But I've spoken to someone with the same car which also has a compression of around 10 bar and he can easily reach 140 km/h.

Does anyone know what I should look at next? or is this really a compression problem?
 

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Did you resolve this? compression tester can show a 20% difference from one to another. I would investigate this, you may need an engine overhaul. Have you checked that the exhaust isn't blocked? have you got over-sized tyres fitted?
 

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Discussion Starter #3
I haven't resolved the problem yet. It could be that the meter has some tolerance in measuring the compression. But I indeed have oversized tyres.
I will try to test it again with the original size of tyres
 

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With compression readings like that my first suspicion would be a worn engine - but - first I'd like to know that the compression test was done correctly - engine warm, battery fully charged, all plugs removed, and the throttle blocked wide open - I'd also like to see wet & dry compression numbers.
 

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Discussion Starter #5
The engine had run but was cooled down a bit, so between cold and warm.
Battery was good, all sparks removed, inlet manifold was removed so no need to open the throttle body.

But I had some trouble later on with the compression meter. The valve which keeps the pressure on the meter was leaking. When I put a compressor on it with 5 bar of pressure the meter only showed 3 bar, so i'm going to retest the compression values with a warm engine after driving a few miles with smaller tyres.
 

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Discussion Starter #6
I tested the car again with the original tyre size, but there was no difference.
Still couldn't get high in rpm in third and fourth gear.

New compression test, now with a warm engine, showed 175 PSI (12 bar)for each cylinder. One or two were a little bit higher (2 or 3 PSI) but all above the 175 PSI.

I really have no idea why the engine refuses to go faster..
Could it be something with the valves or valve springs?
The engine had, before I bought it, been standing still for more than 10 years.
 

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Discussion Starter #7
Is it possible that this problem could be caused by the timing belt?
I've been searching the internet and found that if the camshaft gear is one tooth off, the engine could run but with less power?
 

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Discussion Starter #8
Found the problem, it is the exhaust.
Removing the lambda from the exhaust (creating an extra hole) increased the speed from 110 to 120 km/h and allowed for higher revs in every gear.
 
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