RD high compression?

[car91r]

Finally finishing up my '79 RD400 Daytona project. Did final check on everything yesterday in preparation for its first start up. When I tightened down the spark plugs and tried to kick it through (no gas), I thought something was stuck. I removed the plugs and tried again. No problem, kicked through tight (as I would expect) but no detected interference. Reinstalled the plugs and tried again. Compression seems unusually high. Practically have to stand on the kick starter to kick it through. I'm using the stock foil type head gasket, new .020 over bores with modified Wiseco RZ pistons. I tried a compression test and got 90 PSI on both cylinders. Am I missing something? Perhaps head had been previously shaved for a compression bump. I did notice very little (almost no) lip between the gasket surface and combustion chamber. Hate to have to take the head back off. Gaskets aren't cheap. May need to cc combustion chamber. Anyone know what stock value would be?

[SoCal250]

90 psi is very low, especially for a fresh top end. What clearance did you use, and how was it measured? What's the ring gap?

Regarding the engine not kicking over when the plugs are installed, there's definitely something wrong there.
-Are you using the stock 2V0 Daytona head? The stock head gasket for the Daytona is thicker than the 76-78 models. 0.055" vs 0.020"
-Which head gasket, the OE Daytona 1-piece?
-After kicking it with the plugs in where it got "stuck", have you examined your plug gap to see if they are contacting the pistons?
-Have you checked squish?

[1976RD400C]

Did you happen to pour some oil in there before the spark plugs and trying to kick it?

[sav0r (CL MotoTech)]

Wrong length plugs?

[car91r]

No contact with plugs, did not pour oil in cylinders, aftermarket stock Daytona head gasket. Bored to pistons by Bill Bune. Ring gaps set to correct spec.

Maybe my old leg is just getting wimpy.

[car91r]

It will kick over with the plugs in, just seems harder than any RD I've ever owned.

[m in sc]

run it.

[2steve]

My '77 is harder to kick. A few factors including Chuck Q plugged the little cyl to exhaust holes.

Gotta be deliberate with it and watch the foot placement. It can kick back hard when it lights up before kicking through the stroke.

[m in sc]

it might loosen up, and it should, after its run the rings in a bit.

I have bikes with 145 psi cranking psi, so, 90 isnt shit. as long as you are sure your compression gauge is accurate.

[quocle603]

i have always tested with another gauge to confirm the compression.

[Dvsrd]

Did you kick it with fully open throttle when getting the 90 psi reading? And is there a long hose on your compression gauge?

(When you consider the effective volume above the exhaust port is maybe 125 cc, and combustion chamber volume is maybe 22 cc on an RD400, the added volume from a hose will reduce pressure reading significantly)

[m in sc]

if the schraeder valve on the compression tester is on the end that goes into the plug hole, as it should be, length or volume of the hose doesn't  matter. just keep kicking it until the needle on the gauge stops moving and thats your reading.

[car91r]

I made the attempt to start it. After 6 strong kicks, it started. Music to my ears! After a little warm up, opened the chokes, and it settled into a nice idle. Checked the timing on the Vape ignition, dead nuts on. Thanks Supertune! Did a couple warm up cool down sessions in the garage. Easier to kick over after a little run time. The oil injection must have purged and started feeding oil. Looks like the plugs got oil fouled and gave up. (Running 32:1 pre mix for start up) Need to get new plugs and put some straight gas in the tank so I can give it a shake down run.

[X-Man]

Awesome! You must be super stoked to have it up and running.    I wonder what the compression is now after its ran a few times?

[quocle603]

Cool