Bad misfire on rear cylinder

[Neophyte]

I also posted this in the Bike Chat forumn but the Workshop is obviously better:

I've got quite a frustrating problem now after I partially rebuilt my '98 Storm's motor.

Some history first: About 2 months ago the rear CCT broke while I was on the freeway. As you can imagine it caused quite some damaged. It bent all 4 valves to the point where one exhaust valve head actually broke off. That caused the piston to get damaged and the con-rod to get bent.

Now that I'm trying to get it running I'm getting a total misfire on the rear cylinder, as if the timing is out. If I disconnect the rear plug then it's a "smooth" sound with just the front running but with the rear plug it sounds very uneven with backfiring.

There is definitely spark on the rear cylinder but the plug (new) doesn't change color while the front has already started changing color.

When I striped the motor I didn't take the timing sprocket off from the crank so it can't be that. Am I correct in understanding that it doesn't just fire at once in each cycle but multiple times?

All pointers and help will really be appreciated.

I had the following reply:

Could be a number of things if you do a search under workshop and for cct's.

Things that spring to mind;

Cam chain timing out by a tooth or 2 (very easily done!)

Valves not ground in properly (sounds strange but there have been reports)

Bent big end causing timing pick up miss when to fire.

On the bright side, as it's the rear cyclinder, access is a lot easier!

I've checked and double checked my valve timing even thinking I might have had it out by 180 degr.

Valves not ground properly is a possibility but I don't know if I want to go there.

Bent big end? What exactly is meant by this? Is it referring to the crank?

What I did find when I had the carbs off is that the front gives fairly good suction when I place my hand over the manifold but the rear gives nearly a pumping action instead of suction. If I spray a very small squirt of starting fluid down the throats the front wants to fire while the rear does nothing and now and then just pops loudly.

I do have a compression tester but not with adapters for testing this motor so I can't do a compression test yet.

[sirch345]

What I did find when I had the carbs off is that the front gives fairly good suction when I place my hand over the manifold but the rear gives nearly a pumping action instead of suction. If I spray a very small squirt of starting fluid down the throats the front wants to fire while the rear does nothing and now and then just pops loudly.

I do have a compression tester but not with adapters for testing this motor so I can't do a compression test yet.

As you explained above you should have suction on the rear manifold the same as the front, possible causes would be as Baldzone said valves not sealing properly when closed (when new were fitted they should have been lapped in to get a good seal) piston rings broken or cylinder head gasket not sealing properly
I take it you have checked valve clearances

Just another thought have you re-set the timing correctly on the rear in conjunction with the front cylinder also the RI and RE marks on the cam sprockets are facing away from each other when you align them up with the top of the cylinderhead NOT facing each other, (at this point the cam lobes should be facing each other) and you put the camshafts back in their rightful places

Chris.

[Neophyte]

Thanks for the reply.

The valves were lapped but I don't know how good a job was done.

Firstly I'll go with replacing the head gasket. I must humbly admit that I did re-use the old one. Could that cause such a bad leak?

No, I didn't check the valve clearances. Will do that later today.

[baldzone]

New valves could require new spacers.

From what you say about the rear creating a pumping action, it does sound as though your valve timing is out.

To set the timing properly you really need to set up one cylinder, then rotate the crank 1 and a quarter turns (best check as this is from memory), then set up the other cylinder.

For a simple check, take the cam cover off, put a rod down the spark plug hole (not something that will break or get lost), rotate the engine manually (anti-clock) and check the inlets open when the piston goes down, close as its coming up, all stay closed on down, then exhausts open on the way up. This should tell you if the timing is badly out.

Never use an old head gasket. For the sake of £20 replace it!

[Neophyte]

I have checked and rechecked valve timing, first setting up the front then rotating to check the rear. I also checked in reverse with same results.

Unfortunately in our currency (South African Rand) that 20 pounds relates to quite a bit of money. But yes, I agree with what your saying.

What I have found though, and I think this might be the source of the leakage, is that the valve clearance are now way out with the new valves:

The left intake with a 1.77mm shim gives me 0.63mm clearance vs 0.16mm spec. The right intake has 0 clearance without bolting the cam down but just pushing down on the cam. The left exhaust with the same 1.77mm shim has a clearance of only 0.1mm vs 0.31mm spec. The right exhaust also has 0 clearance.

From this I feel it's safe to assume that the right intake and exhaust valves are slightly open when the cams are bolted down.

I'll call one of the local Honda dealers to order some new shims. I'll keep you guys posted with the outcome of that.

[sirch345]

I have checked and rechecked valve timing, first setting up the front then rotating to check the rear. I also checked in reverse with same results.

Unfortunately in our currency (South African Rand) that 20 pounds relates to quite a bit of money. But yes, I agree with what your saying.

What I have found though, and I think this might be the source of the leakage, is that the valve clearance are now way out with the new valves:

The left intake with a 1.77mm shim gives me 0.63mm clearance vs 0.16mm spec. The right intake has 0 clearance without bolting the cam down but just pushing down on the cam. The left exhaust with the same 1.77mm shim has a clearance of only 0.1mm vs 0.31mm spec. The right exhaust also has 0 clearance.

From this I feel it's safe to assume that the right intake and exhaust valves are slightly open when the cams are bolted down.

I'll call one of the local Honda dealers to order some new shims. I'll keep you guys posted with the outcome of that.

That sounds quite promising Neophyte hopefully when you get your new shims she'll run a lot better

[chaz]

is the head torqued down square?
did you check valve guides for damage or replace them automatically if fitted or have you inspected the bores of them for damage if not fitted, sorry Im not into the motor that serious yet.
is there a crack in the casting?

put the plug in and spray some soapy water round the gasket join and spin the engine over to see if there is a leak.

[Neophyte]

Thanks for the confidence booster sirch.

Chaz, I feel I did torque the head squarely. I used a criss-cross pattern as follows: Left Centre, Right Center, Left Rear, Right Front, Right Rear, Left Front. I also did this in three separate steps.

I had all 4 valve guides replaced with the valves.

There are no obvious cracks visible.

Thanks for the tip on checking for a leak. But, will the water jacket around the cylinder not catch any such leaks and stop them from being visible outside the head?

At this time I feel quite confident that it's the valve clearances (or lack thereof) causing the major leak. The fact that the one intake valve is being pushed open would also explain the slight pumping action I get when I hold my hand over the intake manifold when cranking. The front cylinder actually has a distinct suction when holding my hand over the manifold.

I have ordered shims from Honda and will pick them up on the way home this afternoon which means I'll know the results this evening.

If that fixes it I can give my wife's 125cc Jonway scooter back to her later this week once all has been assembled. At least the scooter gets me to work albeit quite slowly. The scooter averages about 30 km/liter whereas my 3.0 V6 pick-up only gets 7.5 km/liter.

[Neophyte]

I take it you have checked valve clearances

Chris.

Thanks for pointing me in the right direction Chris. The motor fired up just like it should have in the first place.

The two shims I got for the valves with no clearance are a touch too thin but the one I put on the exhaust valve will be right for the intake valve, so I just need a new one for that exhaust valve.

Now, I still need to properly fill the water, put the airbox back, tighten up the rear wheel, put my front brake discs back after having them skimmed and a few other things. With the little time available in the evenings I should be mobile again by the weekend. Yippeeeee!!!!!

[sirch345]

I take it you have checked valve clearances

Chris.

Thanks for pointing me in the right direction Chris. The motor fired up just like it should have in the first place.

The two shims I got for the valves with no clearance are a touch too thin but the one I put on the exhaust valve will be right for the intake valve, so I just need a new one for that exhaust valve.

Now, I still need to properly fill the water, put the airbox back, tighten up the rear wheel, put my front brake discs back after having them skimmed and a few other things. With the little time available in the evenings I should be mobile again by the weekend. Yippeeeee!!!!!

I'm very pleased to hear that Neophyte I'm glad we were able to have helped it could have been worse. I appreciate you keeping us up to date

Cheers,

Chris.

[Neophyte]

I went for a test ride tonight around the block and it felt great. The riding position felt a little weird because of getting used to the scooter. So, by Thursday or Friday I can give my wife's scooter back.

[sirch345]

I went for a test ride tonight around the block and it felt great. The riding position felt a little weird because of getting used to the scooter. So, by Thursday or Friday I can give my wife's scooter back.

That's good news Perhaps your wife would prefer your VTR