Camshaft worn?

[erictharg]

New distributor fitted and working fine. So we now forgive and forget Aldon's cock up. Timed to within a couple of degrees of where it needs to be and road tested. Pulls well until bloody "valve float" misfire back again at around 6200...maybe it never went away, what with all the ignition bother the last two events.
So, I've an all new ignition system. The misfire is speed related and too consistent for fuelling. Happens in 2nd and 3rd, and also in neutral with no load. Has to be something to do with the cam or followers - everything else has been checked or replaced.
Checked valve lift and No.1 way down on what it should be. Should have c.0.400" valve lift + another 15% from the 1.5 rockers (say 0.460" less clearance). Actually measured at 0.350" on No. 1 Exhaust. Ooops. No. 8 by comparison is better at 0.415" (including 016" clearance) but still below spec.
So, seriously worn camshaft. Still not sure how that is causing the misfire though. Whilst some 0.060" worn it should still be plenty of lift to run OK even at those revs.
Either way it's engine out and have a look. Hoping the follower bores are not scored, as that means compete strip and ream out for oversize followers. Otherwise it's a new cam, followers and timing chain
It was a reprofiled Piper / MED XTR cam, but fitted with new followers, all coated in cam lube and run at 2000 rpm for the first 10 mins of running when new.
The replacement will be a billet cam. Piper 649+ or Swiftune SW23 is the question.
Jan - any experience of early wear on cams before? I believe you run a 310 deg scatter cam - is that with 1.5 rockers?
Less than 4 weeks to Anglesey...

[jonclancy]

Bad luck, Charles!! Hope you get it sorted.

[Westfield 129]

I have never had a problem with cam wear. Ever. Lots of miles, 100s of1000s in fact.

One of the things I do is to run oils that are good for flat tappet cams. These usually have ZDDP. Most of the "Racing" mineral oils have ZDDP. I use Valvoline, and have for over 40 years with excellent luck.

With my new engine, I ran 20 minutes for cam break in, using the Valvoline, and an additional ZDDP break in additive.

My current engine(s) uses a scatter pattern cam with 1.5 rockers. Over the last 15,000 miles, I have not had significant change in valve adjustment.

The setup uses Swiftune rockers, Iskendarian valve springs and retainers and the drilled Iskendarian lifters. I checked the valve train geometry to make sure that everything cleared properly, and the spring seat pressures were set and checked by the fellow who does my engine machining. He is very experienced with racing A series engines. Other than the head, I do my own assembly.

While you might think that you have a problem with the cam, I suggest looking at the cylinder head, as you might have hammered a valve seat or two (or a valve), which would also cause your problems with valve adjustments. This sort of damage is not unusual when valve float is present. The cam may still be fine. Since you have to remove the cylinder head anyway... Fortunately, this is about a 40 minute job on the XI.

Have you done a cylinder leak down test?

The miss could still be a problem with the carburetion. Are you sure that the ignition is spot on? Perhaps you have cross firing between cylinders in the cap, or the wires are a bit old. Maybe it is a voltage drop (bad alternator or generator) causing a problem with your spark amplifier, if you have one. Luminition, MSD can have problems if the voltage is not spot on. Some magnetic triggers are also sensitive to high or low voltage.

Worn cams may cause problems with power loss, but missing??? It would be a very damaged cam to cause that. A consistent miss could very well be a fueling problem. Anything from an improperly adjusted carb float to an air leak.

But you know that you have valve float, so 'better fix that first.

But, with your valve float problem at only 6200 (my engines go to 7200 and no valve problems), I suggest removing the cylinder head and taking a good look at the valves and seats, as well as replacing the valve springs with high quality products from Iskendarian, or one of the better UK suppliers (My Longman heads came with excellent valve springs, so I know that you can get good ones locally). You can also get the chilled iron/drilled lifters (Also from Iskendarian) as well. These improve lubrication, and will promote cam life.

'Sounds like you have a fun summer ahead.

[erictharg]

All I can say about oil is that I've seen no significant valve adjustment changes this year so supect the damage (if that's what it is) occured last year. When running VR-1 20-50 Mineral Racing oil.
But seriously, I don't believe that running a mineral vs a synthetic race oil can have that much bearing on this (no pun intended!).
As MED and myself checked the head before Spa I'm pretty happy it's not seat sinkage - and over 1mm of sinkage is pretty grim.
All your suggestions on ignition are valid but I've replaced literally everything in the ignition system over the past month.
Fuelling I've ruled out as it is speed dependent and happens both under load and no load.
Either way I need to check the apparent difference in valve lift.
So engine out and partial strip this weekend. Hopefully no follower bore damage and it's "just" cam, new followers and timing chain (always worth changing). Hopefully leaving the crank and bores undisturbed.
Kent offer the scatter cam grinds (296 and 310 deg), but I'm inclined to stick with either the Swiftune SW23 or MED's 649+ profile.
Let's see how it looks once it's out and head and sump off.
Last time I take the mickey out of Cheng checking his valve clearances after every run (702 HP "small" block Cobra rep). Perhaps I should learn from him!

[adamwilkinson]

Charles, do check all your fuel system - we had a similar issue at Snett a few years ago and the rest of that season due to fuelling issues at high rpm.

BUT having heard it at Spa its a bit strange as even with a dodgy fuel system you were getting missing soon after starting the engine where there would be lots of fuel still in the carb.

Have you tried googling the issue?

[erictharg]

I will likely replace the fuel pump for good measure, and clean out the carb but the fact that it happens regardless of load rules out fuel problems to me. Plus, there's no getting around the missing valve lift so whatever else might be amiss the cam needs sorting.
Hey ho - it's an opportunity to upgrade and hopefully squeeze a little more out of the engine!

[Westfield 129]

I firmly believe in synthetic oils, but flat tappet cams do work well with ZDDP oils. My engines all do well on mineral oil, or synthetics. I just run the early tech engines on mineral oil with ZDDP, as they tend to ooz a little with the thinner oil.

I don't break in engines on synthetics unless the rings and cylinder bores have the appropriate finish and materials. I use mineral oil, with ZDDP.

You could check the lift at each pushrod, but I still suspect the cylinder head and the valve springs. Hammering a valve seat is more likely than flattening out a cam lobe.

My cam is a 296º. I do drive the thing on the road, and that cam has plenty of bottom end (and 125 HP on top). I might try the longer duration race cam on the next version.

[adamwilkinson]

Charles,

Are you running a ballast resistor in your ignition circuit?

[erictharg]

Adam - no ballast resistor. 12V all the way through.

Jan - If it was valve seats sinking and I have the correct rocker to valve clearance, I'd still clock full valve lift, assuming the cam to be good. Plus I'd see the top of the offending valves out of line with the others (we are talking over .060" here).
Head is now off and all the valves are fine. As they were two hours running ago when I last checked them. It's the cam.

[erictharg]

Stripped the engine today (left the crank and pistons in place) and pulled the camshaft. Absolutely knackered. Horrible. As were all 8 followers. No. 1 Inlet was the worst, but all the lobes on the front two were worse than the rear two. Makes me cringe to think I was racing it like that. As I've seen no significant change in valve clearance this season it must have happened at some point last season. So, despite covering the lobes and followers with build lubricant and keeping to 2000 rpm for the first 10 mins of running, I'm guessing they started to wear pretty early on in the life of the engine. New billet 649+ ordered along with followers and timing chain. Will set the cam timing this time before the head goes on (DTI on a pushrod to measure lift) to minimise rubbing of cam to follower before start up. Also use even more cam lubricant this time. On the positive side I should see a very signficant uplift on power compared to the worn old cam. Hopefully have the new parts by Tuesday. Back together and running for next weekend. Might even have time for another (hopefully successful this time) visit to Northampton Motorpsort's rolling road before Anglesey...