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Discussion Starter · #1 ·
I have a 95' Mach-1. PTO cylinder is 135psi and the MAG is 162psi. This has been checked on a good working compression tester. Oil in the cylinders increased both cylinders by 10 psi each. I have read 120-130psi on the 670 is normal, why would one cylinder be so high? (I have not run this sled yet either, I bought it with issues I need to fix in the fuel system) Am I missing something here?
 

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Discussion Starter · #2 ·
I have checked the compression on 3 separate occasions and the results don't vary. I also used the same tester on another sled and it reads fine.
 

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Excellent work with triple-checking the tester and checking another sled! Hot vs Cold results will reveal a lot more too. My next step is a Hot vs Cold Cylinder Leakage Test (the one we see used on cars) to diagnose the rings. Exactly like you have been doing, compare mag with pto, and compare engine to engine. This will pole vault your diagnoses for the price of a new tool, maybe $50 USD.
 

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I'd run it and get it up to temp and then shutdown and retest. Could be just some residual oil keeping the rings sealed up better than normal.

Also, make sure both exhaust valves are working properly. If one is stuck out, it could possibly throw the reading off.
 

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Discussion Starter · #5 ·
Thanks for the replies guys. I will check the rave valves and check again after it's warmed up.
 

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Thanks for the replies guys. I will check the rave valves and check again after it's warmed up.
This is a strange one I have seen similar thing before but not that much variation, on a Friends sled

his problem was the engine had one regular 670 jug and one Mach I jug. the differance in the port timing effected the reading!

The more port timing you have the more charge you lose out the exhaust!
 

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Discussion Starter · #7 ·
Does anyone know if the 670's from a 94 Mach-1 (PRS) and a 95' Mach-1 (F-chassis) are the same? Same hp and all that or no?
 

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I just posted info on those 95/96 motors in the vintage forum. Quite different from the standard 670. Problem is we never know what has been done to sleds, I have taken them apart with mismatched pistons etc. Seen one dual ring, one single ring in my MXZ 440. Damaged dome that has been ground to smooth out damage. 135 is alright. Testers can vary a fair amount. The main thing is that both are close, ideally within 5%, some claim as high as 10. The variation on yours seems a bit much.

I would get it running, run it some and re-test. I can't see a RAVE valve making a significant change in compression but good to service them anyway.( This makes me curious enough to try it sometime to see actual change) IF numbers don't even out, seeing you don't know the sled, for the work involved, I would be taking the top end apart to have a look around. You will get to see the condition of the pistons, cylinders, head and also some idea of the rod bearings. I have also seen failures of the base gaskets, so a fresh gasket set is good to have in there. Actually if all looks great, I would do rings and gasket set. Not really a big job at all.
 

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Can't say if a stuck RAVE would make a difference, but I did just recently rebuild a motor and tested compression before and after putting the RAVEs back on and it had between 5-10 PSI difference. (around 8PSI)

Actually surprised me, because I thought there wouldn't be much difference at all.
 

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Discussion Starter · #11 ·
Can't say if a stuck RAVE would make a difference, but I did just recently rebuild a motor and tested compression before and after putting the RAVEs back on and it had between 5-10 PSI difference. (around 8PSI)

Actually surprised me, because I thought there wouldn't be much difference at all.
Good to know.
 

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Discussion Starter · #12 ·
I just posted info on those 95/96 motors in the vintage forum. Quite different from the standard 670. Problem is we never know what has been done to sleds, I have taken them apart with mismatched pistons etc. Seen one dual ring, one single ring in my MXZ 440. Damaged dome that has been ground to smooth out damage. 135 is alright. Testers can vary a fair amount. The main thing is that both are close, ideally within 5%, some claim as high as 10. The variation on yours seems a bit much.

I would get it running, run it some and re-test. I can't see a RAVE valve making a significant change in compression but good to service them anyway.( This makes me curious enough to try it sometime to see actual change) IF numbers don't even out, seeing you don't know the sled, for the work involved, I would be taking the top end apart to have a look around. You will get to see the condition of the pistons, cylinders, head and also some idea of the rod bearings. I have also seen failures of the base gaskets, so a fresh gasket set is good to have in there. Actually if all looks great, I would do rings and gasket set. Not really a big job at all.
I'm not new to internal engine work but I have never honed a 2 stroke before. I've used to a ball hone on my car which I'm assuming wouldn't work because of the ports?
 

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Can't say if a stuck RAVE would make a difference, but I did just recently rebuild a motor and tested compression before and after putting the RAVEs back on and it had between 5-10 PSI difference. (around 8PSI)

Actually surprised me, because I thought there wouldn't be much difference at all.
it is interesting and I will check sometime to know for sure. At first I thought no, it can't affect compression because the ring doesn't seal against the rave valve, because of clearance and space at either end. But, air could get trapped there and possibly make a change. It may be more noticeable at higher rpm's, so tough to say on a static compression test. I would not be surprised to see a couple of PSI difference, but nothing major. There seems to be different schools of thought on this, would be interesting to know for sure.

Sent from my iPhone using Tapatalk
 

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Does anyone know if the 670's from a 94 Mach-1 (PRS) and a 95' Mach-1 (F-chassis) are the same? Same hp and all that or no?
The 1994 Mach I Engine is basically the same engine as the Regular single pipe 670 engine used from 1993 to 1998. The 1995-96 Mach I engine is the twin pipe

engine! although these two engine do look the same the jugs, crank case, head and pipes are all different. The twin pipe was the only stock 670 that came with 501 rotary valve disc and twin pipes, It also came with 44 mm carbs. if a regular 670 jug is installed on a twin pipe engine you will gain compression but you will lose power because the Exhaust port is open longer. This will lose compression out of the port!
 

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Discussion Starter · #15 ·
The 1994 Mach I Engine is basically the same engine as the Regular single pipe 670 engine used from 1993 to 1998. The 1995-96 Mach I engine is the twin pipe

engine! although these two engine do look the same the jugs, crank case, head and pipes are all different. The twin pipe was the only stock 670 that came with 501 rotary valve disc and twin pipes, It also came with 44 mm carbs. if a regular 670 jug is installed on a twin pipe engine you will gain compression but you will lose power because the Exhaust port is open longer. This will lose compression out of the port!
I am reviving this thread due to this reply and my search for an issue. I did the base gaskets on the 670 (because of coolant in the cylinders) and tested compression again. Lowered a bit but only about 10 psi each cylinder, cold. I then decided to look up this info again and reread this thread. Checked the head casting numbers and found out the high psi cylinder is using a "normal" 670 jug.

The high (153) compression jug is casting # 6913524

The low (125) compression jug is casting number #6823194

Thanks again for the replies.

Seems like I need to find myself another 95'-96' Mach-1 jug. Anyone have one?
 
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Discussion Starter · #16 ·
Well I found ONE cylinder on ebay so I'm attempting to buy that. It looks to be in decent shape.
 

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If I’m not mistaken all 670 jugs carry the 194 casting number. Don’t matter if it’s a ho or m1 or regular motor. So you have a straggler jug from a ? Pull head off and see if it has triple exhaust port.
 
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