80cc upgrade on 1E40QMB compression Question

[kevinx]

I would like to know what the compression should be on that engine with the 80cc upgrade. What would be the compression ratio on the motor if with out the upgrade it's like 7:1. Why I'm asking is i have the motor on my new scooter and the first time i road since i put it on the road i it a sharp bump that made it cut out it started again but cut out 2 more times then it sat for an hour then started again i rode it about 7 miles then cut out again 2 more times. Now it won't start so in the process of looking at it i took a compression reading and come up with around 100psi and that is good at 7:1 compression ratio but since it has an 80cc upgrade don't you think it would have a higher compression ratio inturn higher compression reading. I haven't got it to start yet. I think the coil is mot putting out a hot enough spark so i'm going to change the coil and the cdi unit if it starts maybe it don't have the 80cc upgrade and i was ripped by ATV Discounters. Any replies would be appreciated

Thanks,
KevinX

[90GTVert]

Stock is supposed to be around 125-140psi I thought. The compression with a cylinder kit would vary. Some will prob keep around stock compression ratios and others are meant for race gas. It really could be anywhere. Have you tried a wet compression test to see if if there is a big jump? (Wet compression test meaning put a small bit, like a few drops, of oil in the cylinder and then do the compression test. If your piston rings aren't sealing properly the oil will often help them seal and make the psi go up pretty well.)

If it just hit a bump and the jolt killed it, I wouldn't look for an issue with the compression tester first. I'd be trying to figure out what it knocked loose. Check over the whole scoot for loose connections on critical components (coil, kill switch, ignition switch, cdi). Is anything leaking? I came down hard from a wheelie on a little bike once and it was enough to cause a small leak around an intake gasket.

[kevinx]

2 stroke compression is lower than a 4 stroke. you have to take the compression ratio and multiply that by atmopheric pressure(14.7) and you get the the compression spec of the motor. i see the compression ratio of this engine 1E40QMB is 6.9:1 so 6.9x14.7=101.43. and my motor has around 100psi. I was thinking a 80cc would have more but i seen a Honda have 6.8:1 compression ratio. At of all ther two stroke i seen the comperssion ratio are anywere between 6.8:1 to 7.1.1 where 4 stroke 50cc are atleast 9.1:1. i'm going to swap a coil in and see if it starts even thou it has spark it doesn't seem that strong

[90GTVert]

Trying to know your compression ratio from a compression test in the method you describe is not accurate. You can have an engine with 10:1 compression produce different compression readings.

Your case is a little odd because you are trying to determine if you have a larger cylinder kit by use of pressure readings. You can get 100psi from a gigantic big block cylinder or from a tiny little 49cc cylinder. Like I said, you would need to know what the manufacturer claims cylinder pressure should be for a compression test, because you could have a big bore kit with different specs.

If you really want to know if you have a cylinder bore larger than stock you need to remove the head and have a look. If your piston measures around 40mm, you've prob got a 49cc. You can just look at a big bore cylinder and tell by the amount of space between the studs and the bore if you have seen one before.

Here's my 48mm MRP Minarelli cylinder next to my 40mm stock 1E40QMB Minarelli cylinder.

[rallyrrr]

This is slightly off topic, but keep in mind that there are two modes of thought concerning two stroke compression ratios. Asians prefer to use the 'effective compression ratio' method, i.e. volume of piston displacement remaining after the instant the exhaust port closes on the piston upstroke plus combustion chamber volume divided by combustion chamber volume. In the west conventional compression ratio is used. This is measured the same way on two or four strokes, i.e. piston displacement volume at bottom dead center plus combustion chamber volume divided by combustion chamber volume. In reality the actual compression ratio is somewhere in between on expansion chamber equipped two strokes due to the stuffing effect of the baffle section of the pipe. Lots of people are unaware of this.

[rallyrrr]

I should have added this note at the end of my post: If your specs express a compression ratio between 6:1 and 8:1 then you know that the Asian 'effective compression ratio has been used. If it is expressed in the 10:1 to 14:1 range then you know that the conventional method was used.

[Karl]

rally, you get a bone for that great info, my friend! Welcome to the group, btw!

Karl

[90GTVert]

I found the compression check specs for Minarelli engines from Vento scooters. They want... Standard : 198psi, Service Limit : 139psi.

[kevinx]

What size will the piston meaure across for a 80cc. the scooter i bought from them scumbags at ATV Dicounters says 49cc upgraded to 80cc. I need to know so i can get the right paris for the scooter. Everyone was right and i was hoping there was no cylnder damage but 100psi is not enough but since it stills cranks and has atleast 100psi i think i can just put a set of rings and hone out the cylnder and it shound be fine. Any suggesting

[90GTVert]

The bore should be around 51mm to make 80cc. It's hard to say what you'll need to do. Best bet is to get it apart and have a look.

[kevinx]

I took it apart it was wasted and the piston and cylinder are 50cc not 80cc like i was told. I need to rebuild it. Any suggesting. I was thinking of getting a 70cc upgrade but need to know do i need to change anythig else or can i keep all the other parts

[90GTVert]

I'll reply in your 70cc thread.