Compression loss on number 1and 2 cylinders

Have you run a compression check on all 4 cylinders???   Do that first....   Do a visual check of the valve train on both sides ( per oldyeller ) and go over the exhaust system on the "noisy" side...  It may be a loose head indeed, but with today's thread saver cases, it would be rare....

If it runs OK over 2500 RPM or so it could be your idle jet circuit,clogged circuit.Symptoms would be some smoking out the 1/2 side, possibly a popping in the exhaust as in a lean condition. If you have a laser temp gauge you will also find that if you check the exhaust outlet at the head while running, they are running a lot cooler than the 3/4 side.

My two cents,Michael

Wondering how you checked cylinder compression "while running at other speeds." This is with a normal cylinder compression gauge that screws into the spark plug hole?

Yeah......

 

I was wondering that, too.

I used a compression gauge with a long reach hose and screwed it into the plug hole for eat cylinder and ran the engine. I did cranking I compression, idle compression and at 2500 rpm. The cylinders 3 and 4 showed cranking compression at 120 and at idle And 2500 rpm was 115. Doing the same for cylinders 1 and 2 showed cranking at 120, idle 50 and @ 2500 rpm 115. 

 

I am figuring I either have valve seating issues or valve stems leaking.  keeping the idle rpm at 1100 engine runs runs great on the road. I may just keep idle up amid run it till next winter and tear it down.

 

renfrance

Are you running dual carbs? Are they synchronized properly? Could be the 3/4 side is open farther at idle than 1/2, or as MikelB mentioned clogged idle passages on 1/2, or 3/4 has a vacuum leak.

 

I'd recommend doing a leak down test.

This is a realy unusual situation to me......   I've never seen  compression drop as the RPM increases ( idles with 50 ) and then regain its self at still higher revs. ( 2500 rpm )....

 

Normaly all cylinders will read with in limits when things are right, and if bad, the cylinder(s) under question show an unacceptable reading at all speeds tested....

 

Please give a follow up on this one....

I dont do them with it running. a leekdown would be helpful, a valve adjustment may also be needed, head retorque may be in order, check the intakes to see if there tight at the head.

I would do a leakdown test, too.....dry then wet.  That would tell you more than anything else about now.

Justin, marksburg  --  I don't see what effect carb balance or intake manifold seal has to do with compression.  Help me. I think you should get the same compression readings no matter if the carbs / intake manifold are perfect, or if they are completely removed.

 

I'd go with the previously mentioned:

1.  Check head stud torque (pulled or broken stud?)

2.  Check valve adjustment (valve too tight, not closing?).  In fact, you can check compression with the rocker arms removed.

3.  Because it is easiest: Compression test (on starter motor)

4.  If still not convinced: Verify with leak-down test.

If the engine can't pull in air (throttle closed, jets plugged) compression will be lower. If everything is working correctly it generally won't make a difference if the throttle's closed or not, but not always.

Thanks Justin.  I'm not all the way in with you yet on the "jets" having anything to do with compression.  But I do agree if the throttle is sealed shut it might restrict cylinder intake maybe resulting in low compression reading (you can't compress a vacuum).  With that in mind I need to correct one of my suggestions to Renfrance.

 

Renfrance  --  Forget that part I said about Compression Testing with the rocker arms removed.  You need to open the intake valve to let air in before you can compress it.  I should have said for Leak Down Test you don't need rocker arms.

 

Thanks to all of you who responded.itlooks like my issue is a dropped seat in the valves on one and two cylinders. The steel seats have moved in the aluminum head causing the compression loss at idle. At higher revs the loss goes away due to the rpms  not needing to seat for a long time. Now comes the $$$ to rebuild the heads.
Thank you

Renfrance

Thanks to all of you who responded.itlooks like my issue is a dropped seat in the valves on one and two cylinders. The steel seats have moved in the aluminum head causing the compression loss at idle. At higher revs the loss goes away due to the rpms  not needing to seat for a long time. Now comes the $$$ to rebuild the heads.
Thank you

Renfrance

Sorry, that was rather incorrect of me, I really meant the whole idle circuit that bypasses the throttle plate with air and fuel. If it's plugged you'll have a hard time getting air into the engine at idle. Of course, not all carbs are the same in this regard.

 

And unfortunately, it turns out that wasn't the issue anyway.

Thanks for the follow up....  The only dropped valve seat I ever encountered litteraly locked up the engine when it cocked in the head and snapped a valve ....  Hope it works out for yoy...

Leon...(or anyone)

 

''...cocked in the head and snapped a valve''...

 

My 'Learning Curve' question is: Can that snapped valve be replaced with just removing the head? or does the case have to be cracked to fish out the broken valve stem?

If the valve snaps, the spring(s) lose tension and the retainer and keepers could possibly fall and make their way down the pushrod tubes and into the sump but the rocker arm could keep that from happening but it is a possibility. It's best to make sure you've found every piece of the assembly but if you've got the engine out, splitting the case isn't a big deal.

 

I dropped a valve seat about 2 years ago on a set of new heads and luckily didn't cause any real damage. The valve itself was still intact and the vendor took care of the problem for me so luckily I was out no money (though I did upgrade to the VW650 springs when they repaired the heads) and I was back on the road in about 2 weeks. (Mostly due to shipping time)

 

If you have a good machine shop in your area, you can be back on the road in no time.

 

Renfrance - did you just suddenly notice the loss in compression or had you made any recent changes to the engine or timing or fuel? Dropping a seat isn't uncommon but it's nice to know if there was a reason it dropped or if it just got tired of hanging on. High head temps would be my concern.

 

Have a good weekend!

 

Jimmy

I am not sure what caused it. It apparently was masked by the previous owner by bypassing the idle circuit and keeping the revs up.

The engine involved was a major disaster....   Head was chewed up, and when inspected, the crank had a "flat" spot on it....  Reground the crank and replaced the head...  I don't think this was typical as the owner drove the car for a couple of days before giving in to the situation...

 Possible causes!!! The wrong exhaust will cause it,( Cure a good sidewinder exhaust ) If 1 and 2  have  too little back pressure in the exhaust. they can drop the seats  

 

(2)a too rich mixture could have chilled the seats   (cure Proper carb rebuild and tuning).

 

 Or if they were not install right to begin with.( Cure Find a better head builder.)   That is what I found to be the common boo boo's

sorry old yeler those wont cause it execpt for the loose instaled seats. and back pressure can cause chamber over heating and cause it to drop seats.You dont want back pressure you want a free flowing properly sized system. and the sidewinder is just a typical merged header with the collector orinted 90 degrees.though there are many "sidewinders out there some have very diferent length tubes some are much closer to equal length.