421 Cub Compression Low On One Side - Part II

[doug]

Hi Guys,

 

I've got a low compression issue on my 421 cub motor. (approx. 100psi right, 150psi on the left) Sorry for the long post, I'll try and keep this short as possible.

 

The motor is out of the bike and I have identified the following parts:

 

Crank (HR204 found on crank itself)

Rods (CK 158 found on rods)

Hot Rods 4mm Crank/Stroker Kit?

 

Pistons (7842f on upper top of piston, 573m06800 on lower top of piston)

Wiseco Pistons?

 

Doms (20 found on dome)

20cc domes?

 

Cool Head

 

I have driven the bike for no more than 15 minutes. Apparently, the motor had been recently rebuilt (before I purchased it) and this appears to be true. (cross hatching appearing on cyclinder walls, pistons, bearings all look as though they have very little use.). The bike was always ran on Klotz at 35:1 with 91 octane pump. While I was riding it there was always heavy smoke (too much) and the plugs where running quite dark, so I don't think this occurred during my 15 minutes of riding.

 

From my limited knowledge of this stuff it appears as though this might be a heat seize.

1. What do you guys think caused this?

2. Is this is a mismatch of parts? (too high compression for example?)

3. Do you think the play in the bottom end is something to be worried about? (see video)

4. Best option for repair? (Re-plate the cylinder, new pistons, gaskets, domes and good to go?)

 

Photos/video link:

 

Picasa Album

 

Some closing thoughts:

 

-Rad fluid is blue. (never seen this, any good?)

-Temp. gauge on rad hoze was never near red.

- always warmed bike up properly.

- ran very strong in high rpms.

 

Really appreciate the help. Feeling like a wet dog right now.

Edited April 17, 2010 by doug

[banshee_terror_]

Hi Guys,

 

I've got a low compression issue on my 421 cub motor. (approx. 100psi right, 150psi on the left) Sorry for the long post, I'll try and keep this short as possible.

 

The motor is out of the bike and I have identified the following parts:

 

Crank (HR204 found on crank itself)

Rods (CK 158 found on rods)

Hot Rods 4mm Crank/Stroker Kit?

 

Pistons (7842f on upper top of piston, 573m06800 on lower top of piston)

Wiseco Pistons?

 

Doms (20 found on dome)

20cc domes?

 

Cool Head

 

I have driven the bike for no more than 15 minutes. Apparently, the motor had been recently rebuilt (before I purchased it) and this appears to be true. (cross hatching appearing on cyclinder walls, pistons, bearings all look as though they have very little use.). The bike was always ran on Klotz at 35:1 with 91 octane pump. While I was riding it there was always heavy smoke (too much) and the plugs where running quite dark, so I don't think this occurred during my 15 minutes of riding.

 

From my limited knowledge of this stuff it appears as though this might be a heat seize.

1. What do you guys think caused this?

2. Is this is a mismatch of parts? (too high compression for example?)

3. Do you think the play in the bottom end is something to be worried about? (see video)

4. Best option for repair? (Re-plate the cylinder, new pistons, gaskets, domes and good to go?)

 

Photos/video link:

 

Picasa Album

 

Some closing thoughts:

 

-Rad fluid is blue. (never seen this, any good?)

-Temp. gauge on rad hoze was never near red.

- always warmed bike up properly.

- ran very strong in high rpms.

 

Really appreciate the help. Feeling like a wet dog right now.

 

 

dam yah look at that plating chip.. thats why u lost compression. idk how it chipped but it needs to be replated.

[doug]

Yepper, needs to be replated which sucks, but I need to find out what caused this so I can avoid it after the rebuild....

[doug]

Anything gang?

 

Any advice is appreciated.

 

I'd love to start getting this back together ASAP.

[DarrenB]

yep i agree,

it looks like your in the same pile of shit im standing in.

 

just so your aware all the missing nickasil is probally in your crank bearings, so you will at a minimum have to spliit the cases and clean it all out thoroughly,

preferably new crank bearings.

[blowit]

I would suspect a simple adhesion problem on the plating due to the location. The size of that flaking along with the relatively low load area point me in that direction. You need to make sure all the bearings are free from plate contaminate. That stuff will destroy a bearing in seconds. Might not be a bad idea to show your retailer either because that just is not normal.

 

 

 

B

[rubberneck]

A 421 cub with 20 cc domes on 91 octane fuel is a big no no. That motor needs 110. That could very well be the root cause of your problems. I know you said you only rode it 15 min, but the previous owner could very well have cooked it.

[okbeast]

A 421 cub with 20 cc domes on 91 octane fuel is a big no no. That motor needs 110. That could very well be the root cause of your problems. I know you said you only rode it 15 min, but the previous owner could very well have cooked it.

 

Not necessarily. Depending on squish and if any porting or clean-up was done he could be fine with those domes. I have a .020 base gasket on mine with 19cc domes and am barely at 155 psi cranking compression. I run 91 with 20 oz. of methanol just for a little peace of mind but I'm sure straight 91 would be fine.

[blowit]

Not necessarily. Depending on squish and if any porting or clean-up was done he could be fine with those domes. I have a .020 base gasket on mine with 19cc domes and am barely at 155 psi cranking compression. I run 91 with 20 oz. of methanol just for a little peace of mind but I'm sure straight 91 would be fine.

 

 

 

You can get Methanol and Gasoline to mix??

 

Also, remember that basing octane requirements on cranking compression numbers can you in trouble.

 

Just FYI

[live2beel]

I bet Millenium plated those when they were new. If so they will prob. fix them for free, give them a call. I have heard this was a prob. on some.

[blowit]

I bet Millenium plated those when they were new. If so they will prob. fix them for free, give them a call. I have heard this was a prob. on some.

 

 

Yeah, I coating should never peel like that. That is a huge failure in that one area. May be some contamination in the barrel retarding adhesion.

 

 

B

[okbeast]

You can get Methanol and Gasoline to mix??

 

Also, remember that basing octane requirements on cranking compression numbers can you in trouble.

 

Just FYI

 

Eh we don't really know, it's for my own peace of mind and I've been doing it for 2 years with no problems. And yes I know the 2nd part also but we don't have a better way of checking it so that's what we go off of.

 

Edit: What would you suggest as the 'best' method?

Edited April 21, 2010 by okbeast

[blowit]

Eh we don't really know, it's for my own peace of mind and I've been doing it for 2 years with no problems. And yes I know the 2nd part also but we don't have a better way of checking it so that's what we go off of.

 

Edit: What would you suggest as the 'best' method?

 

 

 

Gathering the numbers for swept volume calcs would be best. You know your bore, stroke, and head displacement. The only thing you need to do is determine the piston dome volume. For a ballpark, I suggest 4.5cc for the calc.

[doug]

Hey guys - thanks for the info. So, sounds like:

 

a) split the cases and check the bearings

re-plate jugs (I'm on the hook for this as I bought the bike like this and didn't get receipts - yes, I know I'm an idiot). Millenium seems to be the place of choice...

c) and do some more research on compression with respect to fuel being used.

 

The original owner told me that the motor had been recently rebuilt (which it had - pictures show this). He also told me that it runs pump....this is what is so confusing...why would someone go through the pain of building the motor (hot rods 4mm crank, cub jugs, Donny Rawls porting, etc., etc and then run the wrong fuel in it...weird.

 

Thanks again.

[rubberneck]

Not necessarily. Depending on squish and if any porting or clean-up was done he could be fine with those domes. I have a .020 base gasket on mine with 19cc domes and am barely at 155 psi cranking compression. I run 91 with 20 oz. of methanol just for a little peace of mind but I'm sure straight 91 would be fine.

 

 

Here is the deal. A 421 with .050 thou squish and 20cc domes using blaster profile pistons gives you an UCCR of around 14:1. Im not sure you really want to run less than 110 octane on that. I dont. My bike with the 20 cc domes kicks 180 psi at sea level on a know accurate Snap-on gauge. It is a run of the mill cleaned up 421 cub. Now lets say i take the motor send it off and have a full blown drag port done and change nothing else. Assuming they change the duration fo the exhuast i will more than likely see a drop in kicking compression. Does that mean i can now run lower octane fuel? I dont believe so. Im not saying you are wrong, that has just been my experience. if what you are doing is working cool. To the OP, i would consider better fuel if i were paying the bills.