how high is too high? Compression issues

[Trafficjamz]

how high is too high for a race stoker setup that runs on 110 octane fuel?

took a compression test today and it was around 180psi.

seems to run ok but stumbles on the start........could this be from too high compression?

[Wheelman-111]

Greetings:

Yikes! That's over 12:1 static. There's a pressure-wave factor when it comes on the pipe too.
Race gas: a Very Good Idea.

What temps you seeing? Have you tried raising the needle a click?
If it "runs OK" everywhere else, the Handoff from pilot to main may be lean.

[Bear45-70]

When I ran 12.5 to 1 (in a four stroke) I ran 114 octane gas.

115/145 purple AV gas, cut 50/50 with 98 octane unleaded gas.

Never had a problem. So I think you should be good.

The stumble is usually a lean issue with the idle and/or the pilot circuits.

Since there is no accelerator pump on these carbs, there has to be a fairly

rich idle mixture to have enough fuel to transition to the main jet circuit.

[Wheelman-111]

Greetings:

There are two inter-related numbers to consider. Compression Ratio and Cylinder Pressure. CR is a calculated value obtained by dividing the volume of the swept cylinder by the volume of the combustion chamber.
The value obtained by multiplying the Nominal CR by Atmospheric pressure (14.7 PSI) SHOULD predict Cylinder Pressure in theory. In practice, in 2-Strokes and modern 4-Stoke engines, that doesn't happen at the cranking speed at which we measure.

This is because of valve (or Port) Overlap. Clever engineers factor in fluid column pressure wave effects to enable higher-RPM and power. The ports (or late-closing valves) let a good part of that pressure out of the cylinder before closing.
Modern SportBikes claim Nominal CRs at Diesel-like values higher than 13 and run pump gas. The only way this is possible is because of that overlap factor.

If you measured 180, I suspect that your Nominal CR is a LOT higher than 12:1, and that no head mods (or added gasket space) were done to compensate for the considerably larger swept cylinder volume.
It may survive the Eighth-mile drags, but I'd be concerned about longer rides.

[Trafficjamz]

It may survive the Eighth-mile drags, but I'd be concerned about longer rides.

This engine is 1/8th mile only and will never see a long ride.

So jetting, and not the high comp ratio, is most likely to blame for the stuttering take off?

[bakaracer]

are you using a higher voltage coil? or a wire like nology? whats your plug gap at?

[Wheelman-111]

Greetings:

I agree with The Bear. The intake charge gets a lot of air before the fuel "catches up" whenever you crack the throttle. It could see 50 lean puffs in 1/2 second at 6K RPM.
It's tough to push that piston up and over with a wimpy combustion event and no flywheel mass.

Now Baka seems to imply that it's hard to make a decent spark under extreme compression conditions. Judging from the universal use of hotter ignition systems bu racers, I suspect he may be correct.

A bigger pilot and hotter spark should resolve your issue. Maybe confirm by raising the needle?

[bakaracer]

how high is too high for a race stoker setup that runs on 110 octane fuel?

took a compression test today and it was around 180psi.

seems to run ok but stumbles on the start........could this be from too high compression?

what does the engine do when it stumbles? what size carb and jets being used?

[Trafficjamz]

this is spremadness520's 130cc stroker

oko 30mm carb jetting was aroud 55 pilot and 150ish main (we changed it alot in the past few weeks)

it bogs like a spree with no air box lean ?

spreemadness has video I'll get him to post it.

[Trafficjamz]

are you using a higher voltage coil? or a wire like nology? whats your plug gap at?

Funny you mention that. I just gave him a high voltage coil to try out......not word on if he tried it out yet.

No high volt coil was being used...... just a good ole spree coil

[bakaracer]

what cdi is being used? have him try that coil you gave him or find a 125 or 250 dirt bike one

[Trafficjamz]

the cdi is stock. is there a differnce between the spree cdi and the elite?

I swapped my cdi to his for testing. there was no difference. cdi was not the problem.

[bakaracer]

this is spremadness520's 130cc stroker

oko 30mm carb jetting was aroud 55 pilot and 150ish main (we changed it alot in the past few weeks)

it bogs like a spree with no air box lean ?

spreemadness has video I'll get him to post it.

so 55/150 was the best setting? or did you try other jets? whats the needle position?

[Trafficjamz]

got it running right.

the timing was off.

[evilone]

No woodruff key?