Different compression readings on each bank

[reneauew]

First off I'd like to say this is a Great Forum! I've been reading through the threads in my search results on "compression" and can't find any hard info on my situation so here's the issue:

I've got an '89 Rude 150 XP (E150STLCEM), 22p Raker, on a 18ft bass boat that had carb issues earlier this year (missed at WOT). I rebuilt all 3 carbs and runs smooth now. Also ran a gallon mix of seafoam through the motor as well. Link and sync performed (only had a Clymer manual, doesn't go into great detail). I don't have a test wheel so I tried checking the timing statically (pull plugs, gnd wires, and set 4 degrees below specs) as mentioned in other threads but I couldn't get a good reading.

Performed compression check; starboard bank read #1-100, #3-102, # 5-101 Port bank read #2-91, #4-91, #6-92. Could this be an indication of a leaking head gasket on the port side?

It's running 5500 and 55mph (GPS) in 95 degree temps but was hitting 5900-6100 and 60mph (GPS) in 70-75 degree temps before the carb problem. I don't what the compression readings were before now.

The difference in the reading between the banks seems unusual to me. Any ideas? Thanks in advance!

 

[Nick1818]

Re: Different compression readings on each bank

I was always told no more than 10% variance b/w each hole. Not 100% on that though. Also it seems to be slightly low at 90ish just a thought without specs to refer to.

 

[Dhadley]

Re: Different compression readings on each bank

Look at the casting numbers on the heads. Lets start by making sure we have a matching set. The port head should read 1 number lower than the starboard. IE -- port 123456 vs starboard 123457. That would be a matching set.

Then look at the head gaskets. Make sure they're both the same thickness. They probably are but we always start with the basics.

 

[reneauew]

Re: Different compression readings on each bank

Thanks folks!

Dhadley, I didn't even think of either one of those. I'll check this afternoon when I get home and post the results with the head #s.

Overall the motor doesn't look like anyone has ever been in it. But you're right, better to know every detail.

 

[Dhadley]

Re: Different compression readings on each bank

If the motor has never been apart most likely the heads are a matching set. But you never know.....I had a new 200 XP once that had two different heads from the factory. And neither one was an XP head. Go figure.

On the other hand if the heads have never been off the deflectors are way over due for replacement.

 

[reneauew]

Re: Different compression readings on each bank

Deflectors?? Lost me on this one. Parts breakdown on the BRP site says water deflectors, is this what you're refering to? Can you elaborate?

Cylinder heads are 332544 Port and 332545 Stbrd. Gaskets look exactly the same.

 

[Dhadley]

Re: Different compression readings on each bank

Ok, great. That's the right heads and gaskets.

The water deflectors need to be changed from time to time. Whenever a water pump is changed is a good time. With age they "pooch out" and can actually slow down the water flow rather than simply divert the water flow. The part number is 322411 I believe. They are actually small pieces of rubber hose.

 

[Dhadley]

Re: Different compression readings on each bank

Sorry about that! Yes, they need to be changed at some point but they most likely are not the cause of the difference. By any chance is the port side very cool compared to the starboard head when running at idle?

If you haven't done a decarb in a while, let's see about doing that now.

 

[reneauew]

Re: Different compression readings on each bank

According to the exploded view I printed it looks like there are six diverters and they are installed between the head and the block, correct?

Are you saying these are responsible for the low compression on the port bank or that they just need to be changed while the head is off changing the gasket (which might be the problem)?

Kinda lost me on your thought process there.

 

[reneauew]

Re: Different compression readings on each bank

Haven't checked the head temps but I can do that.

Decarb (running 1 gal. gas/ 1 can seafoam mix) was accomplished this spring.

 

[reneauew]

Re: Different compression readings on each bank

Just ran the motor on muffs for 5-10 minutes. Both heads get warm, you can only hold your fingers on them for a few seconds. So it looks like the t-stats are working properly, correct?

Decarb c/w (see previous post).

 

[Dhadley]

Re: Different compression readings on each bank

Yep, that means the T-stats are working.

I really don't think there's much to worry about but we might as well plan on pulling the heads.

 

[reneauew]

Re: Different compression readings on each bank

OK. Head gasket seems pretty straight forward and fairly inexpensive.

So just to set me straight, pull the head, check for scoring/ broke rings/ anything not looking right, change the deflectors (3 on the port side) while I'm in there, reassemble, and recheck CR.

If it's the same afterwards, just write it off as an old motor that can still get up and go? Or, look at a re-ring to get optimum performance back?

Thanks for your help Dhadley!

 

[emdsapmgr]

Re: Different compression readings on each bank

The variance does seem unusual. Remotely possible that an earlier owner had to plane a head and that it may have more compression as a result. I have seen variances like this in engines-years later, the compression is still the same odd, unexplaned variance. Engine idled fine, good performance. Would be interesting to know the operating temp differential between heads.

 

[reneauew]

Re: Different compression readings on each bank

emdsapmgr, I don't have calibrated fingers but both head temps were similar as I could only hold on for 5-10 secs on each one. The motor doesn't look like anyone has ever been in it. Paint hasn't been broken anywhere, even the bolts look like a wrench has never touched them.

Dhadley, I'd like to hear your thoughts on my previous post.

 

[reneauew]

Re: Different compression readings on each bank

BUMP for Dhadley.

 

[Dhadley]

Re: Different compression readings on each bank

We've seen the side-to-side variances too. I'd plan on pulling both heads and replacing the deflectors just as preventative maintence and looking for anything visually. Then if everything is still OK I'd do a decarb and concentrate on the port bank.

If it still idles OK, I'd bet you're all right. The first sign of trouble in a 2 stroke is a weak idle.

 

[reneauew]

Re: Different compression readings on each bank

Thanks Dhadley! Parts are on order.

 

[MASTER Brian]

Re: Different compression readings on each bank

delete.

 

[reneauew]

Re: Different compression readings on each bank

Mods, please delete the previous post by Master Brian. For some odd reason I tried to post this entry and it posted under his user name.

Update- Changed the head gasket and deflectors, no difference in the compression test. The cylinders were not scored nor did the old gasket show any signs of leaking. Actually could still see some cross-hatching in the cylinders. The pistons and ports didn't show any carbon build-up either. Haven't had a chance to put it back on the water but i don't expect to see any changes.