Topic: Leak-Down Results - Valves??

[Dr. Frankenstein]

Just when you think you have it, you don't  - in spite of my '74 550 being totally rebuilt, I have a no-start problem I think is related to low compression. Granted, I did not perform a leakdown test before I rebuilt the engine b/c  the engine was stuck so I had to take it apart anyway, but after putting it all together again with new gaskets throughout and trying to start it, it wouldn't. I went over every part of the engine, cleaned the valves and got all the carbon off them, lapped them, put in new caps and cleaned the guides meticulously, or so I thought. I didn't see anything wrong with them, but did not put them on machinist's dial (which I don't have anyway), however they slid back in just fine with no lateral movement. I did run a bit of a 'test' on them by squirting some carb cleaner into the exhaust holes while pressing on the valve face itself until no fluid escaped, so I thought they were tight. I re-used the original rings b/c they were sharp, stiff and springy enough and I didn't notice any remarkable difference between those and the new rings I bought for my 500 when I compared them.

My efforts to start it resulted in a small backfire or two through the exhaust. I do have spark on all four plugs and have a slight smell of fuel in the cylinders, but the plugs are not wet. A compression test yielded results in the low 120's with about a 10 percent difference on the low side across all the cylinders, which prompted me to do the leakdown test. My compressor is set at 100lbs and the test yielded readings on 1 and 4 at 26 percent loss, while 2 and 3 were at 16 percent loss. I did however get a noticeable leak 'hiss' from the exhaust pipe, so that tells me the exhaust valves may have been (or are) faulty, yes?

I rechecked the valve lash on all the valves (.002 Intake, .003 Exhaust) and that was correct, but now I may be back to Square One, as it were. I also tried tapping on the valve adjusters to see if that would 'reseat' the valve, but that didn't yield much; it helped a bit on Number 1, but didn't do much for the others. Any ideas or observations...??

[bryanj]

Have to ask if points gap and timing are spot on because of backfire in exhaust

[BenelliSEI]

Have to ask if points gap and timing are spot on because of backfire in exhaust

+1 to above. Don’t panic yet…… Check everything again. With that compression it should run. Get it running, check timing with a light, sync the carbs and put 50 miles on it. Your leak down data will change significantly.

[willbird]

Have to ask if points gap and timing are spot on because of backfire in exhaust

+1 to above. Don’t panic yet…… Check everything again. With that compression it should run. Get it running, check timing with a light, sync the carbs and put 50 miles on it. Your leak down data will change significantly.

Yep I agree it should run. We ought to do a test some time with adding ring gap to a set of used rings where the engine ran fine before tear down until it will not start cold at all, wonder how big the ring gaps would end up ??

[carnivorous chicken]

Or wires to points reversed...

[newday777]

Or wires to points reversed...

That's my thoughts when I first read it.
Swap the blue and yellow wires at the coils.

[bryanj]

Rings broken will still run just smoke

[M 750K6]

Is your cam timed correctly?

[MRieck]

26% and 16% loss is lousy to be honest BUT it should run. You need new valves and a valve job.

[M 750K6]

If as he says, they sealed well on the bench, the only difference is he now has the valves connected to the cam. So, although unlikely, I would check whether the cam was installed correctly. However, would need to be quite a few teeth out to have an exhaust valve ajar at TDC compression stroke.

Easy to check. Pick the worst cylinder, watch the exhaust rocker as you rotate it through the compression stroke. If it's still moving after tdc, there's the answer.

[Dr. Frankenstein]

When I rebuilt the engine, I made sure the camshaft was in its correct position, with the notch in the end parallel to the head surface and set to cylinders 1 and 4. During my efforts to start it, I adjusted the valves at least twice - .002 Intake, .003 exhaust. I have new 5 Ohm coils, with the new wires from the left one (blue) going to cylinders 1 and 4, while wires from the right one (yellow) going to cylinders 2 and 3.

I do have spark on all four, but it's more of a 'spark' rather than a strong, blue one. I think it could be stronger, even though I have the Negative lead from the new battery grounded to the frame. Does it possibly need a stronger ground? I keep my batteries on a tender, so they all register at least 13 volts when I put them in.

Thinking about it, I may try switching the coil wires as was mentioned above; I had the same experience with my Goldwing, and when I switched them out it started right up, so that could be a possibility - but with spark wires on coil #1 (left/blue) going to cylinders 1 and 4 and the other coil (right/yellow) going to cylinders 2 and 3, I believe I have it set up correctly.

This could be a clue too - when I rotated the engine through the compression stroke, I set the engine to "F" as seen through the little points window and visually checked through the spark plug holes, and the pistons were seen to be at TDC. The points are also set at the specified gap (I don't recall what that is right off the top of my head, but I'm sure I set them to it), but when I tested them with a light I had to adjust the mounting plate almost all the way to the right to get the light to come on as I rotated the engine, as per the manual - I think that's a little strange to have it adjusted that far over, so may just start all over again. I still feel that I missed something (obviously), but don't know what that might be.   

 

[bryanj]

Points 0.35mm or 14 thou BUT i guarantee when you rotate the plate the gap will change and, of course, when you alter yhe gap the timing changes.

It takes a lot of patience AND practice to get BOTH gap and timing spot on at the same time

[Dr. Frankenstein]

Makes sense; I'm just following the manual, but will re-check it again.

[bryanj]

Yes, follow the manual but recheck everything at every adjustment, when servicing Hondas for a living i could swap points and reset in less than 15 minutes, now takes about 45!
Having said that it is work doing it 100% right the first time and from then on at service you clean the points and set gap but timing stays put.
It does depend on the make of points you have fitted as well.

[Dr. Frankenstein]

An SOP for the forum on how to do it right/your way might be a good idea, if you have a better way to do it, or if there's something the manual overlooks...? Just an idea...

I'll reset everything and start from scratch and see what happens.

So here's what I have happening: I set the gap on 1 and 4 and 2 and 3 and attempt to do the timing with a timing light; I set cylinders 1 and 4 to the "F" mark, the light comes on. I rotate past the 1/4 mark and the light comes on for 2/3, but stays lit as I rotate it to the 1/4 mark, even though I can see the points on 2/3 opening...what am I missing?
 

[bryanj]

OK,
Just concentate on 1 & 4,
Set the gap to 0.35mm when points heel on highest point of cam
Now in the manual it shows a timing light made from a buld in holder with the 2 leads( ground and power) having small crocodile clips on the end, i make these from old pilot light holders.
Connect one croc clip to the alloy engine casing and the other to either the screwholding lead to coil OR the spring leaf going to moving point MAKING SURE IT DOES NOT CONTACT ANY GROUND AT SAME TIME.
With ignition and kill switch on the bulb will light at the moment the points open.
NOTE this is not normaly what is known as a timing light which to me is a strobe light to check dynamic timing.
Every time you move the big plate the points gap will alter but with time, patience and perseverance(or just luck!) you willget both gap spot on and bulb lighting exactly on F mark.

Have probably spent quite some time, beers, cussing and anoyance getting tis 100% correct you nowdo the same with the 2 & 3 points on the small plate.

There is no point whatsoever in writing an sop as it is simply practice and patience and that only comes with doing not reading i am afraid

[Dr. Frankenstein]

Yup, that's exactly the way I'm doing it. I've got the little light w/ the alligator clips - I even made a buzz box out of an old Altoids container that I used on my Dreams that only buzzes when I get continuity through the points/when they're closed, but that might be a little more definitive as far as telling me when the contacts are opening. 1 and 4 seem to be okay, but it looks like I have to pay a little more attention to 2 and 4 to get those exactly right. It's funny how some bikes are easy to time, and others make you work for it.

[rotortiller]

A compression test yielded results in the low 120's

The bike should start and run with that. It also suggests the cam timing is likely correct. When you check the ignition timing for points opening make sure the respective 2 pistons are actually at 'F' TDC and representative of the marks on the advance unit. That should help evaluate further if need be.